Citation
Studies related to the Charleston, South Carolina earthquake of 1886--tectonics and seismicity

Material Information

Title:
Studies related to the Charleston, South Carolina earthquake of 1886--tectonics and seismicity
Series Title:
U.S. Geological Survey professional paper ;
Creator:
Gohn, Gregory S
U.S. Nuclear Regulatory Commission
Place of Publication:
Washington, D.C.
Publisher:
U.S. G.P.O.
Publication Date:
Language:
English
Physical Description:
1 portfolio ([471] pages, [8] folded leaves of plates) : illustrations, maps ; 29 cm.

Subjects

Subjects / Keywords:
Charleston Earthquake, S.C., 1886 ( lcsh )
Earthquakes -- South Carolina -- Charleston ( lcsh )
Geology, Structural -- South Carolina -- Charleston ( lcsh )
Earthquake, 1886 -- Charleston (S.C.) ( lcsh )
Genre:
bibliography ( marcgt )
federal government publication ( marcgt )
non-fiction ( marcgt )

Notes

Bibliography:
Includes bibliographical references.
General Note:
Prepared in cooperation with the U.S. Nuclear Regulatory Commission.
Statement of Responsibility:
edited by Gregory S. Gohn.

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
This item is a work of the U.S. federal government and not protected by copyright pursuant to 17 U.S.C. §105.
Resource Identifier:
09219559 ( OCLC )
83600005 ( LCCN )
ocm09219559
Classification:
QE535.2.U6 S825 1983 ( lcc )
551.2/2/0975791 ( ddc )
13 ( ssgn )

Full Text

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Studies Related to the Charleston, South Carolina, Earthquake of 1886-


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Studies Related to the

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Studies Related to the Charleston, South Carolina, Earthquake of 1886-Tectonics and Seismicity

Edited by Gregory S. Gohn




GEOLOGICAL SURVEY PROFESSIONAL PAPER 1313


Prepared in cooperation with the U.S. Nuclear Regulatory Commission


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UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:1983











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UNITED STATES DEPARTMENT OF THE INTERIOR

JAMES G. WATT, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director


Library of Congress Cataloging in Publication Data Main entry under title:
Studies related to the Charleston, South Carolina earthquake of 1886- tectonics and seismicity.
(U.S. Geological Survey professional paper ; 1313) Prepared in cooperation with the U.S. Nuclear Regulatory Commission. Bib.iography: p.
Supt. of Docs. no: I 19.16:1313
1. Charleston (S.C.)-Earthquake, 1886. 2. Geology, Structural. I. Gohn, Gregory S. II. U.S. Nuclear Regulatory Commission. III. Series: Geological Survey professional paper ; 1313. QE535.2.U6S825 1983 551.2'2'0975791 83-600005
For sale by the Distribution Branch, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304

















CONTENTS



Page
Preface, by Gregory S. Gohn ------------------------------------------------ VII
Studies of the Clubhouse Crossroads test holes:
(A) Geochemistry and tectonic significance of subsurface basalts near Charleston, South Carolina: Clubhouse Crossroads test holes #2 and #3, by David Gottfried, C. S. Annell,
and G. R. Byerly --------------------------------------------------- Al
(B) '0Ar/39Ar ages of basalt from Clubhouse Crossroads test hole #2, near Charleston, South Carolina, by Marvin A. Lanphere ---------------------------------------- B1
(C) Paleomagnetic investigations of the Clubhouse Crossroads basalt, by Jeffrey D. Phillips - C1
(D) Geology of the lower Mesozoic(?) sedimentary rocks in Clubhouse Crossroads test hole #3, near Charleston, South Carolina, by Gregory S. Gohn, Brenda B. Houser, and Ray R.
Schneider -------------------------------------------------------- Dl
(E) Geology of the basement rocks near Charleston, South Carolina-Data from detrital rock fragments in lower Mesozoic(?) rocks in Clubhouse Crossroads test hole #3, by
Gregory S. Gohn --------------------------------------------------- El
Geophysical surveys:
(F) Seismic-refraction study in the area of the Charleston, South Carolina, 1886 earthquake, by Hans D. Ackermann ---------------------------------------------- F1
(G) A reflection seismic study near Charleston, South Carolina, by B. R. Yantis, J. K. Costain, and Hans D. Ackermann ---------------------------------------------- GI
(H) Subsurface structure near Charleston, South Carolina: Results of COCORP reflection profiling in the Atlantic Coastal Plain, by F. Steve Schilt, Larry D. Brown, Jack E. Oliver,
and Sidney Kaufman ------------------------------------------------H1
(I) Land multichannel seismic-reflection evidence for tectonic features near Charleston, South Carolina, by Robert M. Hamilton, John C. Behrendt, and Hans D. Ackermann - Ii
(J) Marine multichannel seismic-reflection evidence for Cenozoic faulting and deep crustal structure near Charleston, South Carolina, by John C. Behrendt, Robert M. Hamilton,
Hans D. Ackermann, V. James Henry, and Kenneth C. Bayer -------------------- Ji
Regional studies:
(K) Distribution of subsurface lower Mesozoic rocks in the Southeastern United States as interpreted from regional aeromagnetic and gravity maps, by David L. Daniels, Isidore
Zietz, and Peter Popenoe --------------------------------------------- K1
(L) Pre-Cretaceous rocks beneath the Georgia Coastal Plain- Regional implications, by T. M.
Chowns and C. T. Williams -------------------------------------------- Ll
(M) Potassium-argon relations in diabase dikes of Georgia: the influence of excess 40Ar on the geochronology of early Mesozoic igneous and tectonic events, by Robert E. Dooley and
J. M. Wampler ---------------------------------------------------- M1
(N) Mesozoic development and structure of the continental margin off South Carolina, by
William P. Dillon, Kim D. Klitgord, and Charles K. Paull ----------------------- N1
(0) Basement structure indicated by seismic-refraction measurements offshore from South
Carolina and adjacent areas, by William P. Dillon and Lyle D. McGinnis ------------ 01
JP) Mesozoic tectonics of the Southeastern United States Coastal Plain and continental
margin, by Kim D. Klitgord, William P. Dillon, and Peter Popenoe ---------------- P1
Seismological studies:
(Q) Relocation of instrumentally recorded pre-1974 earthquakes in the South Carolina region,
by James W. Dewey ------------------------------------------------ Q1
(R) Seismicity near Charleston, South Carolina, March 1973 to December 1979, by Arthur
C. Tarr and Susan Rhea ---------------------------------------------- R
(S) Regenerate faults of small Cenozoic offset: probable earthquake sources in the Southeastern United States, by Carl M. Wentworth and Marcia Mergner-Keefer --------- S
(T) Speculations on the nature of seismicity at Charleston, South Carolina, by G. A.
Bollinger -------------------------------------------------------- T1
V

















EDITOR'S PREFACE


Since 1973, the U.S. Geological Survey (USGS), with support from the Nuclear Regulatory Commission, has conducted extensive investigations of the tectonic and seismic history of the Charleston, S. C., earthquake zone and surrounding areas. The goal of these investigations has been to discover the cause of the large intraplate Charleston earthquake of 1886, which dominates the record of seismicity in the Southeastern United States, through an understanding of the historic and modern seismicity at Charleston and of the tectonic setting of the seismicity. This goal is being pursued to evaluate the potential for additional large earthquakes in the Charleston area and surrounding regions and to determine whether the Charleston area differs tectonically in any significant fashion from other parts of the Southeastern United States. An understanding of the specific cause for the 1886 event and of the regional distribution of any structures that are generically related to or geometrically and mechanically similar to the source structure is essential for evaluation of seismic hazards throughout the Southeast.
Investigations by the USGS began with the installation of a temporary seismograph network in March 1973, followed by the installation of the permanent South Carolina seismograph network in May 1974. Major geological and geophysical field investigations began in 1975 with drilling of the first of three deep stratigraphic test holes northwest of Charleston, at Clubhouse Crossroads in Dorchester County, and the initiation of several seismic-refraction and electricalresistivity surveys. Preliminary results of these and other seismological, geophysical, and geological studies were given in 1977 in USGS Professional Paper 1028 (edited by D. W. Rankin) and in other reports and abstracts in the geological literature.
Since the publication of Professional Paper 1028, investigations by USGS scientists and by affiliated and nonaffiliated scientists in other institutions have continued in the Charleston area and throughout the Southeast. Important among the recent investigations have been multidisciplinary studies of the material recovered from the three Clubhouse Crossroads test holes; seismic-reflection and seismic-refraction surveys in the Charleston area and on the Continental Shelf off-


shore from South Carolina; regional studies of radiometric, aeromagnetic, gravity, and deep-well data; and continued monitoring and analysis of the seismicity in the greater Charleston area. This volume presents the results of 20 of these investigations.
The results given herein represent significant progress toward understanding the tectonic setting of the Charleston-area seismicity. Several chapters in the volume address the distribution and origin of preCretaceous rocks and structures beneath Coastal Plain sediments in the Charleston area and regionally beneath the southern Atlantic Coastal Plain and adjacent Continental Shelf. The modern seismicity at Charleston is occurring at depths equal to or greater than the known extent of these older structures, and rejuvenation of an older fault in the modern stress field is a possible cause of the seismicity. Accordingly, several chapters discuss the possible relationships of the various pre-Cretaceous structures to faults identified near Charleston that have a known Cretaceous and Cenozoic movement history and to the historic and instrumentally recorded seismicity. However, at the present time, none of the young structures can be related unequivocally to the seismicity because earthquake fault-plane solutions and hypocenter distributions do not agree with the locations and orientations of these structures. Therefore, a major emphasis of continuing USGS investigations near Charleston will be to identify additional faults, if any exist, to delineate fault movement histories, and to further refine earthquake locations, focal mechanisms, and related seismological interpretations. The editor is grateful to the authors, technical editors, and many other individuals and institutions that contributed to the completion of the scientific investigations reported herein and to the preparation of the report itself. The U.S. Nuclear Regulatory Commission, Office ofNuclear Research, principally funded [Agreement No. AT(49-25)-1000] the seismograph network and the deep drilling, several of the seismic surveys, and other related investigations conducted by the USGS in the Charleston area. The USGS is grateful to the Westvaco Company for the use of their land for deep drilling and many of the geophysical surveys.
VII
















































































































































































































































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Geochemistry and Tectonic Significance of Subsurface Basalts Near Charleston, South Carolina: Clubhouse Crossroads Test Holes #2 and #3


By DAVID GOTTFRIED, C. S. ANNELL, and G. R. BYERLY STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-TECTONICS AND SEISMICITY



GEOLOGICAL SURVEY PROFESSIONAL PAPER 1313-A




0

34


UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:1983












































































































































































































































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CONTENTS


Abstract --------Introduction -_____Description of basalt flows
CC#1 basalt ___Flow 1 __-_ _ Flow 2 ___ _-
CC#2 basalt
Flow 1
Flow 2 -------Flow 3 -------CC#3 basalt ------Flows 1, 2, and 3 Flow 4 -------Sedimentary bed -.


Page Al
1 3 3 3 3
4 4 4
4
5 5 5
5


Description of basalt flows -Continued
CC#3 basalt-Continued
Flow 5 ----------------------_____ ______F low 6 _ _- _ _ _ _ _ _ _ _ __-_______ __-_-_Flow 7
CC#2 -------------- - -----------------Major-element chemistry --------------Trace-element chemistry ------------_______C C #3 -_ _ ___ _ _ _ _ _- _ -_ _ _ _ _ _ _ __ _ _ _ _ _-Major-element chemistry ----------______Trace-element chemistry -_ ____- - _ __ _ _ _ __Tectonic setting and magmatic history -________--References cited ___- ______--------- _ _


ILLUSTRATIONS


Page


FIGURE 1. Generalized sections of CC#1, CC#2, and CC#3 near Charleston, S. C., showing lower Mesozoic mafic volcanic rocks and other
lith o lo g ic u n its _ _ _- _ _ _ _ _ __- _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __- _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _- _ _ _ _ _ _ _ _ _-
2. Normative mineralogy of basalts from CC#2 plotted on diopside-hypersthene-olivine-nepheline-quartz diagram _--______-3. Graph showing average abundances of REE in basalts from CC#2 __________________- ________________4. Normative mineralogy of basalts from CC#3 plotted on diopside-hypersthene-olivine-nepheline-quartz diagram - ____5. Graph showing average abundances of REE in basalts from CC#3 ______-_- ____- ________________- ______6. Samples of test-hole basalts, ocean-ridge basalt, and continental tholeiitic diabase plotted on tectonomagmatic discrimination diagram of W ood (1980) _- _ ______- ___________- ____________- _ ____ _ _ _ _______ _ _______- ______7. Graph showing comparison of average abundances of REE in Mesozoic tholeiitic basalts of eastern North America and subsurface basalts from Clubhouse Crossroads test holes __ _ __-_- ___ _ - _________- _______- ____________-


A2
7 10
12 14

16

17


TABLES


Page


Major-oxide and normative compositions, in weight percent, of basalt from CC#2, near Charleston, S. C. ------ ----Trace-element abundances, in parts per million, in basalt from CC#2, near Charleston, S. C- -----------------Major-oxide and normative mineral compositions, in weight percent, of basalt from CC#3, near Charleston, S. C. _____-_Trace-element abundances, in parts per million, in basalt from CC#3, near Charleston, S. C. --- -----------Average chemical compositions, in weight percent, of subsurface basalt near Charleston, S. C. _____-_____-____- __Average trace-element abundances, in parts per million, in subsurface basalts near Charleston, S. C. --- --------III


A6
8 11 13 17 18


Page


A5
5
5
6
6
7
8
8 10 15 19


TABLE 1.
2.
3.
4.
5.
6.


_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - - - - -
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - - - - -







-----------------------



































































































































































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STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886TECTONICS AND SEISMICITY



GEOCHEMISTRY AND TECTONIC SIGNIFICANCE OF SUBSURFACE
BASALTS FROM CHARLESTON, SOUTH CAROLINA: CLUBHOUSE CROSSROADS TEST HOLES #2 AND #3




By DAVID GOTTFRIED, C. S. ANNELL, and G. R. BYERLY'


ABSTRACT
Major-, minor-, and trace-element compositions of lower Mesozoic basalts from three deep test holes near Charleston, S. C., are used to characterize magma type and to determine the tectonic setting of the volcanic rocks at the time of their eruption. Chemical and petrographic evidence indicates that slight to extreme oxidation and hydration in nearly all samples has caused widespread mobility of K, Na, and related trace elements. The minor elements P and Ti, the trace elements Th, Nb, Ta, Zr, and Hf, and the rare-earth elements (REE) show little or no variation regardless of the degree of alteration. The contents of these stable elements and the patterns of light-REE enrichment in basalts from the deepest of the test holes clearly show the presence of two chemical types that are strikingly similar to lower Mesozoic high-Ti, quartz-normative tholeiltes and lower Mesozoic olivine-normative tholeiites exposed in eastern North America. The olivine-normative basalt is stratigraphically intercalated within a sequence of quartz-normative tholelitic basalts. Quartz-normative basalts above and below the olivine basalt have nearly identical contents of most of the stable minor and trace elements, but the lower basalts have significantly more Cu and Ni and higher Ni/Co ratios. These differences are ascribed to preeruption separation of an immiscible sulfide melt into which Cu and Ni were strongly partitioned. The new chemical data clearly show that the olivine tholeiitic magma type does not necessarily represent the earliest stage of volcanism in the eastern North American, early Mesozoic tholeiite province and that the spatial distribution of olivine-normative magma types in the province is not related to any significant change in tectonic environment.
INTRODUCTION
As part of a multidisciplinary study of modern seismicity near Charleston, S. C., three deep test holes were drilled in the meizoseismal area of the 1886 earthquake. The recovery from these holes of core samples of pre-Upper Cretaceous mafic volcanic rocks provided an

'Louisiana State University, Baton Rouge, La.


excellent opportunity for conducting geochemical and geochronological studies for the purpose of characterizing the magmatic affinities and tectonic environment of these rocks. The three test holes were drilled near Clubhouse Crossroads in southern Dorchester County, S. C., between Summerville and the Edisto River (Gohn and others, 1983, fig. 1). Clubhouse Crossroads #1 (CC#1) and Clubhouse Crossroads #2 (CC#2) penetrated a 750- to 776-m-thick Coastal Plain section of Upper Cretaceous and Cenozoic sediments and bottomed in a sequence of subaerial basalt flows. Clubhouse Crossroads #3 (CC#3) also penetrated the Coastal Plain section and bottomed in a sequence of lower Mesozoic(?) sedimentary red beds (Gohn and others, 1983). A generalized section of the cored intervals of lower Mesozoic volcanic rocks and associated sedimentary rocks is shown in figure 1.
The results of a geochemical study of eleven samples of basalt from two flows in CC#1 were discussed in detail in a previous report (Gottfried and others, 1977) and are briefly summarized here. Petrographic observations and major-, minor-, and trace-element data indicated that the two flows were affected by postcrystallization processes, including hydration, oxidation, and hydrothermal alteration, that modified the original magmatic chemistry to varying degrees. However, the minor elements P and Ti, the trace elements Th, Nb, Ta, Zr, and Hf, and the rare-earth elements (REE) were essentially stable and showed that the two flows were identical in composition. The major-element composition of the least altered basalts and the stable trace elements showed that the basalts are of the quartz-normative tholeiitic magma type and are remarkably similar to the Al









STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


SOUTHWEST


NORTHEAST


CC#1


CC#3


CC#2


Conglomerate Extremely weathered Highly vesicular flow top

Aphyric

Highly vesicula top; aphyric


ar,


Highly
vesicular,
3 aphyric flows
r


Many fractures, some
mineralized


Conglomerate Vesicular flowtop, aphyric Highly vesicular top



Aphyric


2 --------- Sheet of vesicles
within flow


3


4 - Aphyric, massive,
sandstone
5 Pillowed flowtop,
10-20% olivine,
phyric


1000 [-


6





7


Many fractures, some mineralized Base of flow has 2-4 percent olivine-plagioclaseclinopyroxene phenocrysts and clots

Highly vesicular flow top; aphyric; irregular distribution of vesicles
and amygdules Many fractures, some
mineralized Base of flow has 2-5 percent olivineplagioclaseclinopyroxene in unevenly distributed phenocrysts and clots


Massive, aphyric




) Vesicular top
Aphyric
Conglomerate and sandstone


FIGURE 1.-Generalized sections of CC#1, CC#2, and CC#3, near Charleston, S. C., showing lower Mesozoic mafic
volcanic rocks and other lithologic units. Circled numbers beside sections correspond to numbered basalt flows
described in text.


A2


750


1




2


800 [-


850 H


ul


Z
a
9u

0


950 r


1







GEOCHEMISTRY OF SUBSURFACE BASALTS NEAR CHARLESTON


Upper Triassic or Lower Jurassic high-Ti, quartznormative tholeiitic rocks of eastern North America. Whole-rock potassium-argon measurements of two samples from CC#1, however, gave ages of 94.8 and 109 m.y. (million years) (middle Cretaceous), which were considered minimum ages and were ascribed to the effects of secondary processes (Gottfried and others, 1977). Subsequently conventional whole-rock potassiumargon analysis of three samples of basalt from CC#2 yielded ages of 204, 162, and 186 m.y. (Gohn and others, 1978), which are in the range of ages determined for other Upper Triassic-Lower Jurassic basalts and diabase of eastern North America. However, these three ages are out of order with regard to the relative stratigraphic position of the samples in the drill hole, and this lack of vertical age continuity is attributed to the somewhat altered nature of the samples. Lanphere (1983) carried out incremental heating and total fusion 40Ar/39Ar experiments on the three samples and found that only one sample was suitable for establishing a meaningful crystallization age. He concluded that the most reliable age for the basaltic volcanism is 184 3.3 m.y. (Early Jurassic).
This paper is a continuation of our geochemical study of the subsurface basalts from the Clubhouse Crossroads drill holes. Major-, minor-, and trace-element data are presented for samples representing 131 m of basalt from CC#2 and 256 m of basalt from CC#3. Analytical methods. -Major, minor, and trace elements in CC#2 and CC#3 samples were determined by means of the same methods used for the CC#1 samples (Gottfried and others, 1977). In addition to thermal radiation for instrumental neutron activation analysis (INAA), epithermal neutron activation analysis (ENAA) was used for several selected elements from CC#3 samples because of its increased accuracy and ability to detect small amounts (Baedecker and others, 1977). Th, U, Hf, and Ta are reported by this method, along with the rare-earth elements La, Ce, Sm, Tb, and Ho. The U, Sm, and Ho were counted by means of lowenergy photon detectors, and the other elements by means of Ge(Li) detectors.
Acknowledgments. -All samples for ENAA were irradiated in Norway at the Institutt for Atomenergi, Isotope Laboratories, Kjeller. We are grateful to E. Steinnes for making this technique available to us for supplementing and substantiating data obtained from thermal irradiation at the National Bureau of Standards reactor, Gaithersburg, Md. Deep drilling and related investigations by the U.S. Geological Survey in the Charleston, S. C., area are supported by the U.S. Nuclear Regulatory Commission, Office of Nuclear Research, under Agreement No. AT(49-25)-1000.


DESCRIPTION OF BASALT FLOWS CC#1 Basalt
At a depth of 750 m, basalt was encountered beneath a conglomeratic sand. A total thickness of 42 m of basalt was recovered before drilling was terminated. Two flow units are recognized, which do not necessarily correlate with flows having the same numbers in the two nearby holes.
FLOW 1
The top of flow 1 occurs at 750 m and the base is at 785 m; hence, this flow has a thickness of 35 m. Much of the flow is highly vesicular; the upper 32 m contain up to 20 percent vesicles and amygdules. The basalt is fine grained and nearly aphyric. Alteration is very intense within the upper 3 m of the flow and gives the basalt a greasy appearance and feel. The basalt is bright red in the highly altered top but grades to gray with depth. These color changes also correspond to a decrease in vesicularity and an increase in grain size. The groundmass texture ranges from hyalophitic to intersertal. At 759 m, a few 2- to 3-mm olivine phenocrysts are replaced by smectite pseudomorphs. Within several meters of the base, gash veins occur and contain coarser plagioclase crystals and chalcedony. These appear to be segregation veins formed during late stages of crystallization.
The uppermost portion of this flow contains the most altered basalt cored in the Clubhouse Crossroads holes. In the upper 3 m, large amygdules contain white expandable clay pseudomorphs, which replace aggregates of bladed zeolite. The basalt is now broken into pieces by the expansion of this clay by as much as 15 percent. Below the upper 3 m, a pink laumontite is dominant in amygdules. Minor chalcedony occurs with the zeolite. Groundmass plagioclase and pyroxene are partially replaced by zeolite. Residual glassy patches are opaque black to red. As depth increases, the basalt appears much fresher; plagioclase and pyroxene are unaltered, and chalcedony, quartz, and smectite dominate the secondary mineral assemblage. Sulfides are rare and are found only in the lower portion of the flow.

FLOW 2
The top of flow 2 is directly beneath flow 1 at 785 m. The total thickness is unknown because drilling was terminated within this flow at 792 m. The vesicular top extends down to 790 m depth and is significantly less altered than is the top of flow 1. The basalt is aphyric throughout. The upper 0.5 m and the deepest 1 m drilled are streaked red and green. This lowest zone is highly fractured and probably quite permeable. Textures range from hyalophitic to intersertal. The plagioclase and


A3







STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


pyroxene are partially altered to zeolite near the top but are quite fresh throughout the remainder of the unit. Residual glassy patches are altered to smectite. Amygdules are commonly filled by zeolite, but calcite, chalcedony, and smectite also occur. The fractured zone in the lowermost 1 m contains many thin veins of smectite.

CC#2 Basalt
At a depth of 776 m, basalt was encountered beneath a conglomerate in a sequence similar to the sequence in CC#1. A total thickness of 131 m of basalt was recovered before drilling was terminated. Three flows are distinguished in this hole.

FLOW 1
The top is at 776 m and the base at 782 m. The upper 3 m are highly vesicular and moderately altered in appearance. Vesicles and amygdules constitute as much as 25 percent of the upper portion of the flow. Amygdules are commonly filled by coarse aggregates of a pink, bladed zeolite and amorphous, green smectite. Many amygdules are very complex with several different generations of zeolite and (or) smectite. This flow is aphyric throughout. The upper part is altered and greenish but grades to a gray-green color in the lower, fresher part of the flow. The groundmass texture grades from hyalophitic to intersertal. Common segregation veins have a coarser texture and more vesicles than the surrounding basalt and consequently have more associated zeolite, chalcedony, quartz, and smectite. Sulfides are also preferentially associated with the segregations. Some segregation veins are brecciated and filled by a matrix of chalcedony. Pipe vesicles occur in the base of the flow. In the upper part of the flow, plagioclase is partially altered to zeolite.

FLOW 2
A 4-cm layer of green smectite, zeolite, and calcite separates flow 1 from flow 2. Flow 2 is 89 m thick between 782 m and 871 m depth. The upper 8 m are highly vesicular with 10-20 percent vesicles, which are found in sheeted horizontal layers that were produced as a result of flow. At about 6 m below the top, bands of finegrained and coarser grained basalt occur, which are commonly oriented as much as 30 from the horizontal plane. The coarser grained basalt is always more vesicular. The vesicular zone has a greenish cast and appears somewhat more altered than material below. No phenocrysts are observed in the upper 8 m of the flow. The flow is massive from 790 to 820 m. This basalt is gray, appears fresh, and has less than 2 percent vesicles. At 820 m, another horizontal vesicle sheet about 1 m


thick occurs within the flow. A few plagioclase phenocrysts are observed at 825 m. Between 830 and 850 m, the basalt is highly fractured; these fractures are often associated with thin smectite veins. Recovery of core in this interval was locally poor, although there was no indication of any flow boundary. At 860 m, a few plagioclase phenocrysts and a few pseudomorphs of smectite after olivine phenocrysts are found. Glomerocrysts of plagioclase, pyroxene, and olivine (replaced by smectite pseudomorphs) may also be found. The phenocrysts are generally less than 5 mm in diameter. The base of the flow has about 5 percent amygdules, most of which are filled by zeolite. The groundmass texture generally varies from hyalophitic to subophitic, but several zones within the flow display unusual textures related to flow segregation during crystallization. A mottled texture at 790 m is produced by swirls of oriented plagioclase microlites. Amygdules are commonly filled by basalts having one of two types of texture. One filling displays coarse, flowalined plagioclase microlites. The other is a very finegrained, dark, quench-textured material. In the upper 12 m of the flow, zeolite and smectite are the dominant secondary minerals in vesicles. Between 794 and 867 m, calcite, chalcedony, and smectite are dominant. Below 867 m, zeolite again becomes an important secondary mineral. In the massive basalt, sulfide is commonly found in fine veins, often with smectite, and as disseminated grains. Groundmass minerals are mostly fresh throughout the flow; areas of former residual glass are altered to smectite.

FLOW 3
A 5-cm layer of calcite and zeolite occurs between flows 2 and 3. The top of flow 3 is at 871 m; however, the total thickness is unknown because drilling was terminated at 907 m. The upper 24 m is highly amygdaloidal, although the amygdules are irregularly distributed. The amygdules tend to be alined in horizontal sheets, and most are filled with zeolite or smectite. At 877 m, the basalt is coarsely amygdaloidal; these amygdules are mostly filled by zeolite. As depth increases, the number of vesicles increase, and amygdules are filled by light green, fibrous prehnite, or, more commonly, by pink, bladed zeolite; together, vesicles and amygdules constitute up to 20 percent of the rock. At 895 m and below the basalt is massive with few vesicles and has a much fresher overall appearance. The upper portion of the flow is aphyric, but, at 902 m, phenocrysts of plagioclase and pyroxene occur along with pseudomorphs of smectite after olivine phenocrysts. Some horizontal sheets of vesicles occur near the bottom of the core at 904 m. Phenocrysts do not occur within these vesicle sheets. Clots with the same three types of


A4







GEOCHEMISTRY OF SUBSURFACE BASALTS NEAR CHARLESTON


phenocrysts also occur in this zone. The groundmass textures range from hyalophitic to intergranular. At the top of flow 3 the groundmass has been largely altered to a highly birefringent, fine-grained aggregate that may be prehnite. At greater depths the groundmass minerals are fresh and the patches of former residual glass are replaced by smectite.

CC#3 Basalt
Coring began in basalt at 775 m, immediately after cuttings had indicated a change from the overlying sediments. The basalt extends down to 1,031 m, although much of the interval between was not cored. A minimum thickness of 256 m is therefore recorded for the Clubhouse Crossroads basalt. A total of seven basalt flows are recognized, though it is likely some were missed in the uncored intervals.

FLOWS 1, 2, AND 3
These are very thin, highly vesicular flows that are similar to each other in most respects. All are aphyric, and they may represent flow lobes of a single eruption. Flow 1 is 2.7 m thick; its top is at 775 m and its base at 777.7 m. Flow 2 is beneath a 5-cm layer of smectite and zeolite that contains angular fragments of basalt. The base of flow 2 is at 782.5 m. Flow 3 is also beneath a 5-cm layer of smectite and zeolite that contains angular fragments of basalt. Coring was terminated at 784.6 m, within this flow. Vesicles and amygdules compose as much as 25 percent of these flows, commonly are flattened and in horizontal sheets, and commonly are filled by zeolite and smectite. The texture of the groundmass is hyalophitic throughout. Alteration is moderate to intense; in the upper flow, most plagioclase has been altered to zeolite, though some secondary chalcedony and potassium feldspar are also found. Calcite occurs in minor veins throughout the flows.

FLOW 4
Coring was resumed at 921 m. Because of the long uncored interval, the basalt encountered at 921 m was assigned to a new unit. The base of this unit is at 924 m. The basalt of flow 4 is medium grained, massive, gray, and relatively fresh. The base of the flow is very finegrained and black and contains considerable smectite. In appearance, it is quite similar to the sandstone that underlies it. Rare microphenocrysts of olivine are replaced by smectite pseudomorphs. The rock is essentially free of vesicles. The textures range from hyalophitic to subophitic. Zeolite is rare in this flow. Smectite replaces the residual glassy material. Sulfides occur as fine disseminated grains and in veins with smectite.


SEDIMENTARY BED
A thin sedimentary unit between flows 4 and 5 grades from a fine-grained black sandstone at its top (924 m) to a coarse-grained red sandstone at its base (926 m). Angular fragments of phyric olivine basalt occur in the upper black sediments, and the lower meter of red sandstone contains rounded, irregular pieces of phyric olivine basalt. Quartz and potassium feldspar occur as common detrital grains in the sedimentary bed.

FLOW 5
Coring of flow 5 was stopped at 930 m and thus only 4 m were recovered. The top of flow 5 is sparsely vesicular (less than 5 percent); its very-fine-grained quench texture suggests that the basalt may have intruded into unconsolidated sediment as a sill or invasive flow. The flow contains 10-20 percent olivine phenocrysts that were unevenly distributed in sheets apparently by segregation during flow. All olivine is replaced by smectite or rarely by calcite. Below the quenched top, the basalt is visibly coarser grained. One meter below the top, the groundmass is ophitic. The upper part of the flow is red, but the flow grades with depth to a dark green. Amygdules contain smectite and calcite.

FLOW 6
Coring was resumed at 984 m. Because the recovered basalt was aphyric, it was assigned to a new unit whose base is at 1,021 m. The basalt is fine grained with few vesicles or amygdules. The texture is mostly hyalophitic to intergranular, and the rock appears to be quite fresh. Several small segregation veins of coarser plagioclase and pyroxene, which occur between 995 and 1,000 m depth, commonly have vugs filled with chalcedony and smectite. A few veins have calcite, chalcedony, and smectite. Zeolite is present in the few amygdules which occur in the base of the flow between 1,015 and 1,020 m. The formerly glassy base of this flow is altered to smectite.

FLOW 7
A layer of calcite, smectite, and angular basalt fragments occurs between flows 6 and 7. The top of flow 7 is at 1,021 m. The base of the flow overlies an arkosic sandstone at 1,031 m. The top of the flow is an altered selvedge having amygdules filled by zeolite and calcite. The basalt is aphyric and dark gray to black. Very large vugs and amygdules occur near the top. The amygdules are filled by very coarse, bladed aggregates of zeolite or by quartz crystals, whereas the vugs remain empty. Pipe vesicles are found at the base of the flow. The texture varies from hyalophitic to intersertal. Sulfides and native copper occur but are rare.


A5









STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


CC#2

Major-Element Chemistry

Chemical analyses were made on eight core samples selected from CC#2 to represent the different flow units encountered in that hole. The major oxide and normative compositions of these basalts are presented in table 1. The data indicate that all the samples have undergone hydration; total H20 contents range from 2 to 5 percent. Contents of the mobile element K20 show a six-fold variation, whereas the contents of the relatively stable minor elements TiO2 (1.0 percent) and P205 (0.15 percent) are essentially the same in all the rocks. A plot of the normative compositions of the samples in the nor-


mative diopside-hypersthene-olivine-nepheline-quartz diagram is shown in figure 2. Normative calculations were made on a water-free basis and an assumed Fe203/(FeO + Fe203) ratio of 0.15 to standardize the effects of hydration and oxidation.
Six of the eight samples cluster in the quartznormative tholeiite field and have essentially the same composition as do the basalts from CC#1. Sample 786, which has the highest H20 and Na20 contents, is slightly nepheline normative, and sample 878, which has the highest K20 and high H20 contents, plots in the olivinenormative tholeiite field. The departure of these two samples from the cluster of samples in the quartznormative field reflects the mobility of the alkalies,


TABLE 1. -Major-oxide and normative mineral composition, in weight percent, of basalts from CC#2, near Charleston, S. C.
[Analyses by Z. A. Hamlin and F. W. Brown]

Depth below surface
(meters) _ _ _ _ 780 786 819 836 842 869 878 908
Major-oxide composition
Sio2 ----------------------- 53.1 49.4 53.2 53.8 53.6 53.3 51.2 53.0
A120 ------------------- 14.3 14.0 14.3 14.2 14.4 14.4 13.4 14.5
Fe20 ------------------- 2.3 3.1 3.1 2.4 2.8 1.8 4.3 2.2
FeO -------------------- 8.6 7.8 7.8 8.5 8.6 8.7 6.5 8.6
MgO -------------------- 6.0 6.3 6.2 5.9 5.9 6.1 6.4 6.3
CaO -------------------- 9.5 7.4 9.6 9.5 9.7 8.9 7.2 10.2
Na2O -------------------- 2.5 4.7 2.3 2.2 2.3 2.3 3.4 2.3
K20 --------------------- .25 .87 .57 .35 .33 .81 1.5 .23
H20+ - ------------------- 2.3 4.6 1.2 1.5 1.2 1.9 3.0 1.2
H20-------------------- __1.1 1.0 1.2 .95 1.0 .90 1.7 .64
TiO2 ------------------- 1.0 .97 1.0 1.0 .99 .99 .99 1.1
P205 -------------------- .15 .14 .16 .15 .15 .15 .13 .14
MnO -------------------- .15 .17 .16 .16 .16 .17 .17 .17
CO2 --------------------- .02 .02 .02 .04 .13 .01 .01 .01
Total --------------- 101 100 101 99 101 100 100 101
Major-oxide composition recalculated volatile-freel
Si02 --------------------- 54.3 52.2 54.2 54.9 54.3 54.6 53.9 53.7
A120 --------------------.. 14.6 14.8 14.6 14.5 14.6 14.8 14.1 14.7
Fe203 - --------- 1.8 1.9 1.8 1.8 1.9 1.8 1.8 1.8
FeO -------------------- 9.3 9.5 9.2 9.2 9.6 9.0 9.3 9.1
MgO -------------------- 6.1 6.7 6.3 6.0 6.0 6.3 6.7 6.4
CaO -------------------- 9.7 7.8 9.7 9.6 9.7 9.1 7.6 10.3
Na2O -------------------- 2.6 5.0 2.3 2.2 2.3 2.4 3.6 2.3
K20 --------------------- .26 .92 .58 .36 .33 .83 1.6 .23
TiO2 -------------------- 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.1
P20 -------------------- .15 .15 .16 .15 .15 .15 .14 .14
MnO -------------------- .15 .18 .16 .16 .16 .17 .18 .17
Total --------------- 100 100 100 100 100 100 100 100
Normative mineral composition'
Q ----------------------- 6.73 __ 6.40 8.83 7.63 6.75 _- 6.37
Or ---------------------- 1.51 5.42 3.42 2.11 1.97 4.90 9.33 1.37
Ab ---------------------- 21.63 36.87 19.81 18.98 19.72 19.94 30.31 19.72
An ---------------------- 27.67 15.34 27.49 28.39 28.37 27.22 17.77 28.94
N e _ _ _ - _ _ _ _ _ - - -- 2.77 ___ ___ _ __Wo --------------------- 8.09 9.31 8.25 7.68 7.72 7.07 7.88 8.91
En ---------------------- 15.28 4.82 15.72 14.98 14.89 15.56 14.35 15.90
Fs ---------------------- 14.12 4.23 13.96 14.09 14.68 13.69 12.10 13.73
Fo ----------- -- 8.23 -_ ___ __ __- 1.70 -F a _ _- ... _ _ _ _ _ _ _ _ 7.97 _._ _ _ ___ --- 1.58
Mt ---------------------- 2.63 2.70 2.60 2.62 2.72 2.55 2.64 2.59
Il ---------------------- 1.94 1.94 1.93 1.93 1.90 1.92 1.98 2.11
Ap ---------------------- .36 .35 .38 .36 .36 .36 .32 .33
Total --------------- 100 100 100 100 100 100 100 100
'Based on analyses recalculated to 100 percent volatile-free oxides; Fe2Oz/(FeO+Fe2,O) ratio assumed to be 0.15.


A6







GEOCHEMISTRY OF SUBSURFACE BASALTS NEAR CHARLESTON


Di





Ne - Qz



- ALKALI OLIVINE BASALTS OLIVINE QUARTZ THOLEIITES
THOLEIITES 7860 780
908 0836
81950842
8690
0878










01 Hy

FIGURE 2.-Normative mineralogy of basalts from CC#2 plotted on diopside (Di)-hypersthene (Hy)-olivine (01)-nepheline (Ne)-quartz (Qz) diagram. Data are from table 1.


caused by alteration processes, rather than original magma composition.

Trace-Element Chemistry

Trace-element contents and selected interelement ratios in samples from CC#2 are given in table 2. The presentation of the data is similar to that proposed by Taylor (1965) whereby elements are grouped mainly on the basis of ionic radii and charges and hence similar geochemical behavior. Twenty-nine trace elements were determined for the eight samples selected for majorelement chemistry. Selected trace elements were determined for 16 additional samples in order to detect possible variations in composition among individual eruptions and to assess on a more refined scale the extent and degree of alteration.
The elements of large ionic radii (Rb, Ba, and K), which are highly susceptible to remobilization by postcrystallization processes, show the widest ranges in


abundance. In our study of basalts from CC#1 (Gottfried and others, 1977), Rb was found to be the most sensitive indicator of alteration. The same appears to be true for the suite of samples from CC#2. The range of variation of Rb (more than fiftyfold: 1.1-59 ppm) is greater than that found for Ba and Sr. The only K-related element that appears relatively stable is Pb, which varies for the most part within a factor of two. In contrast to the wide range of variation in largecation abundances, the high-valence cations (Th, U, Zr, Hf, Nb, Ta) show striking uniformity in abundance throughout the suite. The limited scatter in the abundance data may be due primarily to analytical uncertainties rather than to alteration processes. Th and U contents range from about 1.5 to 2.3 ppm and 0.5 to 0.8 ppm, respectively. With one exception, the Th/U ratios fall in the narrow range of 3.0 to 3.8. The uniformity in U contents is somewhat surprising; studies of other rocks that have undergone hydration and oxidation have indicated that U is extremely mobile.


A7









A8


STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 TABLE 2.-Trace-element abundance, in parts per million, and selected interelement


Depth below surface
(meters) _--_____ 776.5 780 783.3 786 790 798 803.4 808.9 814 819 829

Large cations
Rb _ ----- 10 3.4 1.6 34 6.0 10 8.4 37 59 10 17
Ba --------------- 270 80 14 110 45 72 57 89 370 140 140
K* _____- _____ __ 2,200 __ 7,600 ---- --- --- --- 4,800
Sr ---------- 380 180 64 170 61 210 160 170 160 160 180
Cal . .... _ _. 69,000 __ 56,000 __. __ ___ __ 70,000 _Pb"* ------- 7.4 5.1 7.7 5.5 9.1 4.8 5.4 5.6 7.6 4.9 5.3
K/Rb _ ___ _ __ 650 -_- 220 -- _- ___ __ _ 480
Ba/Rb ------------- 27 24 8.8 3.2 7.5 7.2 6.8 2.4 6.3 14 8.2
K/Ba _-- _ _ _ _ _ _ 28 __ 69 __ __ _ _ .__ 34

High-valence cations
Th' _ _ _ _____ __ 2.0 __ 1.9 __ 2.1
U _ -_ _ _ _ .6 _ .5 -_- ___ .7
Zr" ____-_- __ 78 90 83 78 100 77 87 77 78 90 90
H fp -___ _ _ -__ -__ 2.1 _ 2.1 __ _ - _- 2.1
Nb'* _ _ _ ____ 4.9 6.1 -_ 5.9 5.3 __ 5.6 5.2 6.2 6.5 4.9
Ta' _ __ .34 .45 __ __ _- __ _ .53
Th/U - _ _ _ ___ __ 3.3 __ 3.8 - - 3.0
Zr/Hf _ _ _ _ _ __ __ 43 __ 37 -- 43
(Nb x 100)/Ti _____ __ .10 __ .10 - .11
Nb/Ta ___ _ _ _.- . __ 18 _ 13 -_ __ 12

Ferromagnesian elements
Cc2. --______ 36 50 44 49 50 39 44 34 37 48 45
Cu. _ _ 16 19 18 20 22 17 17 18 24 20 20
Li* __-_______ 3.9 7.4 2.2 5.2 8.2 19 12 20 20 10 4.8
Ni ----_-___ 19 21 22 23 25 20 23 20 22 21 25
Zn+ __- _____- 88 93 89 76 93 90 85 93 87 94 89
Cr, ____-___ 50 68 56 68 66 50 54 50 51 63 66
Gal* __________ 12 15 20 17 15 15 17 16 15 15 17
sc" __ _____ 39 39 42 37 50 40 46 40 40 41 46
vl _ 230 270 280 300 370 270 270 280 260 250 290
Ni/Co -------------- .53 .42 .50 .47 .50 .51 .52 .59 .59 .44 .56

Rare-earth elements
La _ _ _ _ __ 10 9 9
Ce __ __ -______ __- 20 20 21
Nd ___ ___- _ _ _ _ 14 __ 12 14
SM _ _ _ 3.2 3.1 3.2
Eu _ _ _ ___ _ _ _ .93 __ .94 - -99
Gd ------------ 3.0 2.8 3 4
Tb ------------ .77 __ .71 .84
H o _ _ _ _ __-_ _.5 .7 .6
Tm ----------- .33 -- .32 .36
Yb ------------ 2.7 2.6 3.1
Lu ------------ .42 __ .44 .44
- - -29 _ 25 28


Zr and Hf contents range from 78 to 100 ppm and 1.9 to 2.1 ppm, respectively; these ranges are similar to those in CC#1 basalts. Nb contents (4.9-6.8 ppm) are somewhat lower than those found in CC#1 (6.8-7.7 ppm). The Nb/Ta ratios in CC#2 samples range from 12 to 18 and average 14; these are significantly lower than the average ratio of 25 found in CC#1.
Except for Li, the elements of ferromagnesian affinity (Co, Ni, Zn, Cr, Sc, and V) are quite uniform in abundance. The low Cu contents, generally in the range 16-20 ppm, are an unusual feature of the test-hole basalts, as are the rather low Ni/Co ratios of 0.42-0.63 (Gottfried and others, 1977). The uniformity in abundance of the REE and Y provide further confirmation of the generally recognized stability of these elements during alteration (Gottfried


and others, 1977). The chondrite-normalized REE pattern, based on analysis of eight samples of basalt, shows enrichment of the light REE relative to the heavy REE (fig. 3). Omitted from the pattern are the Tb and Ho data, which have rather large analytical uncertainties.


CC#3

Major-Element Chemistry

Studies of the volcanic and underlying lower Mesozoic(?) sedimentary rocks encountered in CC#3 permit a more complete and detailed interpretation of the volcanic-tectonic evolution and pre-Cretaceous geologic history of the Charleston area than do analyses of the basalt sections in CC#1 and CC#2. The sedimentary beds and their implications for tectonism are discussed










GEOCHEMISTRY OF SUBSURFACE BASALTS NEAR CHARLESTON


A9


ratios in basalt from Clubhouse Crossroads test hole #2, near Charleston, South Carolina


836 842 853.7 858.5 864 869 872 878 888 889 899 905 908
Large cations -Continued

19 24 8.6 17 14 21 24 54 2.5 1.1 1.6 18 17
190 140 160 150 170 150 180 200 83 7 91 150 130
3,000 2,700 ___ __ __ 6,900 -__ 12,000 __ __ __ 1,900
170 170 240 220 260 170 210 210 250 95 200 190 180
69,000 70,000 -_ -_ __ 65,000 -__ 54,000 - ___ -_ 74,000
6.2 4.5 7.8 5.4 5.4 4.5 7.1 4.1 4.7 5.1 5.5 4.5 3.8
160 110 __ _ 330 __ 220 __ -- __ - 110
10 5.8 19 8.8 12 7.1 7.5 3.7 33 6.4 57 8.3 7.6
16 19 ___ __ 46 __ 60 - __ __ 15

High-valence cations -Continued

2.0 2.1 __- ___ __ 2.3 __ 1.7 ___ __ 1.5
.6 --- --- -- .7 --.8 --- --.5
91 93 91 85 82 85 86 82 85 84 89 90 89
2.1 2.0 -__ __ __ 2.0 __ 1.9 _ __ -- 2.0
6.5 5.9 6.4 ___ 6.3 6.8 6.0 6.0 6.0 6.7 6.1 6.3 5.9
49 .49 ___ .43 .42 __ - - .39
3.3 ___ -- --- _- 3.3 _ 2.1 __ _ _ 3.0
44 47 __ __ _- 43 __ 43 _- ___ 45
.11 .099 __ __ __ .11 __ .10 _ __ _ .090
13 12 __ __ __ 16 __ 14 _ _ _ 15

Ferromagnesian elements -Continued

48 47 46 47 43 44 53 43 42 45 49 49 54
20 21 21 20 19 19 22 30 23 22 24 38 43
5.8 6.2 15 17 25 9.8 16 15 17 8.1 6.6 2.7 1.8
22 23 24 24 21 21 24 25 20 25 24 31 34
97 85 93 90 88 89 100 75 87 79 89 93 82
69 76 74 59 58 72 62 97 56 62 71 130 170
16 17 18 18 15 16 15 12 17 18 18 18 18
44 45 50 49 46 40 38 37 40 39 41 42 43
270 280 290 290 270 290 280 300 - 290 300 300 300 300
.46 .49 .52 .49 .49 .48 .45 .58 .48 .56 .49 .63 .63

Rare-earth elements -Continued

10 9 ___ __ _ 10 __ 9 -_ - -- 9
20 20 ___ ___ ___ 20 _. 18 __--- _ 17
12 12 ___ ___ ___ 12 ___ 11 _- - --- 9
3.1 3.2 __- _- _- 3.2 ___ 3.0 __ - _ - __ 3.1
.93 .93 __ __ .95 __ .87 --- _-- --- .94
3.4 3.1 __ _ _ _ _ 3.1 _ _ 2.9 ___ ___ ___ _ _ 3.2
.71 .77 __ _ __ .76 -_ .73 __- __ __ ___ .86
.7 .6 ___ __ __ .5 __ _ _ -- - - - -- - .7
.31 .31 _- - _ __ .33 _ .32 ___ __ __ _ .34
2.6 2.5 __ ___ __ 2.8 __ 2.5 __ __ _ __ 2.8
.41 .41 _- - -- .42 _. .39 __- - - .44
29 30 __ 26 25 __- _ __ 29


elsewhere in this volume (Gohn and others, 1983). In addition to the underlying sedimentary rocks, CC#3 contains a thin sandstone bed at about 925 m between an underlying strongly altered olivine-rich flow and an overlying aphyric flow (fig. 1); these rocks document a brief cessation of volcanism and a distinct change in magma chemistry.
The major-element chemistry and normative composition of 14 samples from CC#3 are presented in table 3. Evidence of alteration of the cored basalt from the top of the section (777-785 m) is clearly shown by the high and variable contents of H20 and CO2. Uniformity in TiO2 and P205 contents in these samples suggests that the original major-element composition of these samples was the same. The threefold range in K20 is thus


ascribed to its mobility during alteration. Major-oxide analyses of basalts from the next cored interval (921-930 m) indicate that the sample at 921 m is similar to the overlying samples. However, these analyses also confirm petrographic observations of basalt of a significantly different magma type in the interval from 926 to 930 m. Three samples (from 926.3, 928, and 930 m) representing different textural variants of this olivinerich flow have been analyzed. Rather intense alteration of these samples is indicated by high H20, CO2, and Fe203/FeO ratios. The high MgO contents (11.5-16.6 percent) are consistent with high modal olivine contents, although some of the variation in MgO could be due to alteration. The nearly constant and relatively low amounts of TiO2 (0.52 to 0.56 percent) and P205 (0.08 to







STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


w 20 I
0

0
cc


40[ 30


101-


5


La Ce Pr Nd Pm Sm Eu Gd Tb RARE EARTH ELEMENTS


DY Ho Er Tm Yb Lu


FIGURE 3.-Average abundances of REE in basalts from CC#2. Data are from table 2. Each vertical bar indicates the range of normalized values of eight samples.


0.10 percent) indicate that the samples were originally of rather similar compositions and that this flow unit represents a relatively primitive magma. The six samples of basalt from 984 m to the base of the basalt section at 1,031 m, though showing variable degrees of alteration, are virtually indistinguishable, on the basis of their stable minor elements (P205 and TiO2), from the basalts from the top of the section. A plot of the normative compositions of the 14 samples from CC#3 is shown in figure 4. As expected, the normative data show a rather wide scatter of points extending from the quartz-tholeiite field to the olivinetholeiite and alkali-olivine fields. In contrast, the immobile minor elements indicate the presence of two distinct magma types, each of which shows little if any recognizable range in composition. Comparison of the major-oxide chemistry, especially the stable TiO2 contents, of the CC#3 samples to the chemistry of exposed eastern North American, lower Mesozoic tholeiites (Weigand and Ragland, 1970) and quartz-normative tholeiites in CC#1 (Gottfried and others, 1977) suggests that only high-Ti, quartz-normative tholeiites occur between 777 and 921 m and between 984 and 1,030 m. Ap-


parent olivine-normative compositions for rocks in those intervals are ascribed to chemical alteration of the samples. Olivine-normative compositions that reflect original magma chemistry and not alteration effects are found only in samples from 926.3 to 930 m.

Trace-Element Chemistry

Although petrographic and stable minor-element data indicate the presence of two magma types, additional data are required to effectively distinguish the magmatic affinities of the two suites. The immobile trace elements listed in table 4 make possible the recognition of diagnostic geochemical features on a more refined scale and also have implications for the tectonic setting.
In the CC#3 samples, extensive redistribution of Rb resulted in overlapping of Rb contents, at low concentration levels, between the olivine-rich unit and the more evolved tholeiitic rocks in the section. Contents of highvalence cations are virtually the same, within analytical error, in samples from the top of the section (777 to 921 m) and those from the basal flows (984 to 1,030 m). They


I
I
I










I--~ ~ I I I I I I I I I I


A10










GEOCHEMISTRY OF SUBSURFACE BASALTS NEAR CHARLESTON TABLE 3.-Major-oxide and normative mineral compositions, in weight percent, of basalts from CC#9, near Charleston, S. C.
[Analyses by Z. A. Hanlin and F. W. Brown]


Depth below surface


All


are also the same as those in samples from CC#2 and CC#1. The concentrations of Th, Zr, and Hf in the three olivine-rich samples are about half those found in the quartz-normative tholeiitic flows. Nb contents, although slightly lower than those in the tholeiites, appear to be relatively enriched in the olivine-rich samples as compared to the relative abundances of the other highvalence cations. The relatively high (Nb x 100)/Ti ratio (0.18) and Nb/Ta ratio (32) in one of the olivine-rich samples (from 926.3 m) may be due mainly to analytical error in the Nb determination rather than to low Ti and Ta contents. Previous studies of suites of tholeiitic basalts and diabases from widely different regions have shown that Nb/Ta ratios are quite similar: 13.5 in the Dillsburg, Pa., sill; 14.4 in diabases of Tasmania; and 16


in upper Paleozoic diabase dikes from Scotland (Gottfried and others, 1968; Macdonald and others, 1981).
The Th/U ratios in the CC#3 samples are quite uniform, and are mostly in the range from 3 to 4. These ratios, like the results for CC#2, suggest that the original U contents have not been seriously modified. Similar Th and U contents and similar Th/U ratios were found in samples of Triassic-Jurassic tholeiitic diabases from Fairfax County, Va. (Larsen and Gottfried, 1960) and the Palisades sill, New Jersey (Heier and Rogers, 1963).
The chondrite-normalized . REE patterns of the olivine- and quartz-normative basalts are shown on figure 5. The light REE pattern for the quartznormative basalts of CC#3 is similar to that for the


(meters) 777 779 782 784.4 921 926.3 928 930 984 1002.2 1014 1020.4 1024 1030
Major-oxide composition
siO -------------------- 47.8 47.8 46.2 55.1 52.1 42.9 44.3 43.2 52.9 52.2 53.7 52.8 52.5 47.8
Al.0 _-_------ _ 14.2 14.0 14.8 13.3 14.3 14.6 13.2 13.4 14.1 13.8 14.1 14.3 14.1 14.6
FeO ------------------- 5.6 3.9 4.7 4.5 3.4 6.6 4.9 3.8 3.8 2.4 3.7 2.9 3.7 3.9
FeO -------------------- 6.4 7.0 7.0 5.3 7.8 3.8 5.4 6.9 8.0 9.7 8.3 8.4 7.9 8.0
MgO -------------------- 5.9 6.4 6.5 6.0 6.7 10.6 12.8 15.3 5.6 5.9 5.3 5.7 6.3 8.3
CaO -------------------- 5.0 6.5 5.8 5.8 10.6 11.4 8.5 7.4 8.7 9.7 9.3 9.2 5.7 6.9
NaO ------- ---- 3.6 4.5 5.0 4.2 2.3 1.6 1.2 1.2 2.8 2.5 2.6 3.3 5.5 3.3
K20 --------- --- 1.3 .44 .93 .74 .16 .05 .27 .15 .64 .64 .74 .34 .05 .05
H20+ - ----------- 5.9 5.8 5.5 3.4 1.1 2.7 4.9 5.4 1.2 1.3 .52 1.4 2.4 2.9
H20--------------------- 2.3 2.2 1.7 1.5 1.5 3.3 4.5 2.8 .41 .34 .53 .99 .92 2.0
TiO ----------- --- 1.1 .92 1.1 .89 1.1 .52 .47 .49 1.1 .96 1.1 1.1 .97 .99
P20, -------------- .16 .15 .16 .15 .14 .09 .07 .08 .18 .16 .18 .18 .16 .16
MnO ------------ .30 .21 .24 .19 .17 .18 .14 .22 .24 .22 .21 .21 .21 .22
C02 -------------------- .24 .09 .08 .36 .04 .90 .82 .10 .02 .24 .14 .02 .13 .02
Total ___---_- 100 100 100 101 101 99 101 100 100 100 101 101 100 99

Major-oxide composition recalculated volatile-free
SiO--------- -_____ - 52.2 52.0 50.0 57.3 52.8 46.5 48.6 47.0 53.9 53.2 54.1 53.6 54.1 50.8
AlO ______._____ 15.5 15.3 16.0 13.8 14.5 15.8 14.5 14.5 14.4 14.1 14.2 14.5 14.5 15.5
Fe20, -.____----- 6.1 4.2 5.1 4.7 3.4 7.1 5.4 4.1 3.9 2.4 3.7 3.0 3.8 4.1
FeO --------- --- 7.0 7.6 7.5 5.5 7.9 4.1 6.0 7.5 8.2 9.9 8.4 8.5 8.1 8.5
MgO ------------------- 6.4 7.0 7.1 6.2 6.8 11.5 14.0 16.6 5.7 6.0 5.4 5.8 6.5 8.8
CaO __ _ 5.4 7.1 6.2 6.0 10.7 12.4 9.3 8.0 8.9 9.8 9.4 9.4 5.8 7.3
Nao ------------------- 4.0 4.9 5.4 4.3 2.3 1.7 1.3 1.3 2.9 2.6 2.6 3.4 5.7 3.5
K20 _ _ _ 1.4 .48 1.0 .77 .16 .05 .30 .16 .65 .65 .74 .35 .05 .05
Tio, -------------------- 1.2 1.0 1.2 .93 1.1 .56 .52 .53 1.1 .98 1.1 1.1 1.0 1.1
P201 -------------------- .17 .16 .17 .16 .14 .10 .08 .09 .18 .16 .18 .18 .17 .17
MnO ------------------- .33 .23 .26 .20 .17 .20 .15 .24 .24 .22 .21 .21 .22 .23
Total _ --- _ 100 100 100 100 100 100 100 100 100 100 100 100 100 100

Normative mineral composition'
Q ___- _ _ -_ _ 4.81 4.64 __ _ _ 4.92 4.25 6.21 2.78
Or ---------- --- 9.75 2.84 5.97 4.58 .95 .32 1.77 .96 3.86 3.86 4.42 2.04 .30 .31
Ab ------------- 33.52 41.71 33.98 37.26 19.74 14.94 11.31 11.06 24.21 21.63 22.25 28.40 48.11 29.70
An _-__------- 19.98 18.29 16.50 15.99 28.63 35.88 33.23 33.48 24.54 25.09 24.90 23.60 14.09 26.46
Ne-------------- --- 6.49 --- --- -- --- - --- -- ---
Wo 1.80 6.37 5.43 4.39 9.81 7.92 2.87 2.16 7.60 8.90 8.18 8.97 5.47 3.62
En ------------- 9.53 3.52 2.78 15.66 16.93 12.54 26.67 18.94 14.25 15.02 13.35 14.44 7.30 14.30
Fs ---------- --- 9.75 3.05 2.51 12.78 14.16 6.34 11.25 7.10 15.21 16.05 15.30 14.49 6.87 10.48
Fo ---------- --- 4.65 9.76 10.37 _- __ 11.63 6.19 15.81 _ _ __ 6.25 5.38
Fa .-_--------__ 5.24 9.33 10.32 ___ _- 6.47 2.87 6.53 __ -_ __ 6.48 4.35
Mt ----------- --- 3.04 2.78 2.94 2.36 2.66 2.59 2.64 2.71 2.82 2.93 2.84 2.70 2.80 2.95
Il _ _------------ 2.29 1.91 2.27 1.77 2.12 1.09 .99 1.01 2.13 1.86 2.11 2.12 1.90 2.00
Ap ________ --____- __.41 .38 .41 .37 .33 .23 .18 .20 .43 .38 .43 .43 .39 .40
Total __- ___ 100 100 100 100 100 100 100 100 100 100 100 100 100 100
Based on analyses recalculated to 100 percent volatile-free oxides; Fe20,/(FeO+Fe20s) ratio assumed to be 0.15.








STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


Di
A\ /\ /\ /\ /\ /\ \A\\ /\ / /\ /\ /\ /\ /\ /\ /\ /\




Ne - -\ Qz



- ALKALI OLIVINE BASALTS OLIVINE QUARTZ THOLEIITES
THOLEIITES 9210 01014 1020.40. 0782 1002 0984

779
926.3 %1024 0784.4





000 1030.80

0777 0928
0930

01 Hy

FIGURE 4.-Normative mineralogy of basalts from CC#3 plotted on diopside (Di)-hypersthene (Hy)-olivine (Ol)-nepheline (Ne)-quartz (Qz) diagram. Data are from table 4.


quartz-normative basalts from CC#2 and CC#1 (Gottfried and others, 1977). The patterns for olivine- and quartz-normative basalts in CC#3 are subparallel, which stresses their close chemical relationship even though the abundances of the REE in the olivine-normative type are one-half of those of the quartz-normative type. Variations in the abundance of some elements in the ferromagnesian group have important petrographic implications and also provide a means of "fingerprinting" units which are otherwise indistinguishable. As noted above, flows from the top of the section (777 to 921 m) have virtually the same contents of stable minor elements (P and Ti) and trace elements (Th, U, Nb, Zr, Hf) as do those from the base of the section (984 to 1,030 m). However, the lower flows contain 3 to 4 times as much Cu and nearly twice as much Ni and have higher Ni/Co ratios than do the upper flows. The average Cu content in the upper flows (20 ppm) is similar to that in flows from CC#2 (22 ppm) and CC#1 (25 ppm) but contrasts with the average of 75 ppm in the lower flows in CC#3. The low and relatively uniform Cu contents in


most of the samples of the upper flows suggest that low Cu is for the most part an intrinsic feature of these evolved basalts. Low Cu contents have also been noted by Weigand and Ragland (1970) in some Mesozoic highTi, quartz-normative diabases. The low Cu contents and low Ni/Co ratios in CC#1 basalts were previously ascribed to the preeruption separation of an immiscible sulfide melt into which Cu and Ni had been strongly partitioned (Gottfried and others, 1977). Recent experimental studies on the partitioning of Ni, Co, and Cu and the solubility of sulfur in silicate melts provide valuable insight into the nature of the processes that control the observed abundance variations of these elements in the Clubhouse Crossroads basalts. The solubility of sulfur in silicate melts at high temperature and pressure has been shown by Mysen and Popp (1980) to be strongly pressure dependent; that is, solubility decreases with decreasing pressure. They found that a melt at 10 kbar has 50 percent less sulfur than does the same melt at 20 kbar. Thus a decrease in pressure would cause excess sulfur to separate as an immiscible sulfide melt. A strong


A12








TABLE 4.- Trace-element abundances, in parts per million, and selected interelement ratios in basalt from Clubhouse Crossroads test hole #8, near Charleston, S. C.
[Y determined by emission spectrography; La, Ce, Sm, Eu, Tb, and Ho determined by epithermal neutron activation analysis (NAA); other elements, including all elements of samples from 926.3 rm and 930 m, determined by thermal NAA. -, not determined]

Depth below surface
(meters) 777 779 782 784.4 921 924 926.3 928 930 984 1002.2 1014 1020.4 1024 1030
Large cations
Rb ------- -- 44 8.5 36 9.8 1.5 270 1.5 4.1 2.3 17 19 27 7.5 2.0 2.1
Ba2 _____ 550 240 180 74 110 250 70 69 64 170 180 210 200 54 61
K_ 12,000 4,000 8,300 6,400 1,300 49,000 400 2,500 1,300 5,400 5,400 6,100 2,900 400 400
Sr2_ 410 360 260 150 150 76 110 83 60 230 240 220 310 260 220 M
Ca2 - ____ 39,000 51,000 44,000 43,000 77,000 -_ 89,000 66,000 57,000 64,000 70,000 67,000 67,000 41,000 52,000 0
Pb -- -- __ _ 5.5 6.1 6.0 4.8 4.0 16 3.0 2.1 2.2 5.5 7.0 6.6 5.1 4.2 2.5 m
K/Rb- -- __ _ 270 470 230 650 870 180 270 610 570 320 280 230 390 200 190 x
Ba/Rb ---- 13 28 5.0 7.6 73 .93 47 17 28 10 9.5 7.8 27 27 29 $
K/Ba --------- 22 17 46 86 12 200 5.7 35 20 32 30 29 15 7.4 6.6
High-valence cations
Th --------- 2.6 2.1 2.4 2.1 1.7 __ 1.0 1.0 1.0 2.4 2.2 2.2 2.1 1.8 1.9 0
U _ .7 .6 .6 .6 .5 __- .3 ___ .7 .7 .7 .7 .5 .6 I
Zr4 ._____ 87 77 87 82 85 110 53 46 45 100 110 94 94 84 77 M
Hf -_________ 2.5 2.0 2.2 2.1 2.2 __ 1.1 1.1 1.2 2.6 2.4 2.3 2.4 2.2 2.0
Nb5 ------_ 6.8 5.9 6.9 5.8 5.9 _- 5.5 4.5 4.1 6.2 5.6 6.4 5.9 5.7 6.4 W
Ta ---------- .42 .27 .47 .26 .28 __ .17 .21 - .41 .36 .37 .39 .28 .33
ThU --------- 3.7 3.5 4.0 3.5 3.4 __ 3.3 ___ 3.4 3.1 3.1 3.0 3.6 3.2 M
Zr/Hf -------- 35 39 40 39 39 -_- 48 42 38 38 46 41 39 38 39
(Nbx100)/Ti_ .10 .11 .10 .11 .09 ___ .18 .16 .14 .09 .10 .10 .09 .10 .11 M
Nb/Ta -------- 16 22 15 22 21 __ 32 21 -_ 15 16 17 15 20 19 m
Ferromagnesian elements
Co -____- 67 44 43 38 49 33 69 70 68 43 42 42 48 46 51 %
Cu- _ _--------- 20 18 22 19 48 37 150 120 98 66 73 64 82 79 85 O
Li ------------ 7.5 6.0 8.9 27 2.0 5.3 11 7.8 33 8.4 19 13 12 22 26 z
Ni2 ----___--- 26 22 20 20 33 200 530 530 530 33 27 35 37 42 35
Zn_---------- 50 62 68 69 81 63 56 62 74 92 94 110 91 80 82 g
Cr...____.. 57 52 50 48 190 370 790 800 650 32 25 36 27 35 28
Ga - --------- 14 16 14 13 17 12 13 11 12 17 17 16 15 14 14 W
SC3-___-- ------44 44 41 41 43 19 43 33 39 41 39 43 42 40 37
---__-____ 290 280 290 250 300 82 230 180 230 320 330 300 290 290 240 e
Ni/Co -____ .39 .50 .47 .53 .67 6.1 7.7 7.6 7.8 .77 .64 .83 .77 .91 .69 L

Rare-earth elements z
La----------- 11 10 10 10 8 ___ 5.5 5 5 12 11 11 11 9 10
Ce ----------- 20 23 20 20 20 ___ 14 12 12 22 24 26 32 27 23
Nd ------- 14 10 13 12 12 __ 9 __ 6.5 15 13 15 13 14 13
Sm ---------- 3.4 3.0 3.4 3.0 3.2 _-- 1.8 1.6 1.7 3.8 3.5 3.6 3.5 3.3 3.4
Eu ---------- 1.07 .94 1.09 1.10 1.10 __ .62 .57 .51 1.32 1.18 1.06 1.20 .90 1.20
Tb ----------- .76 .78 .75 .67 .81 ___ .41 .83 .79 .69 .81 .74 .69
Ho------- .8 .7 .8 .7 .8 __ .7 .6 .6 .8 .8 .9 .8 .8 .8
Yb ------- 2.9 2.5 2.6 2.6 2.7 __ 2.5 2.0 2.3 3.0 3.0 2.9 3.0 2.8 2.9
Lu _______ .44 .35 .38 .35 .43 _- .36 .34 .34 .43 .43 .43 .43 .43 .41
Y -------- 24 23 25 25 29 20 23 17 20 28 28 29 29 26 25







A14


FE


La Ce Pr Nd Pm Sm Eu Gd Tb RARE EARTH ELEMENTS


Dy Ho Er Tm Yb Lu


FIGURE 5.-Average abundances of REE in basalts from CC#3. Data are from table 4. Each vertical bar indicates the range of normalized values of 11 quartz-normative samples or 1-3 olivine-normative samples.


preference of Ni, Cu, and Co for the sulfide liquid is clearly indicated by the large partition coefficients that were experimentally determined by Rajamani and Naldrett (1978) between sulfide liquid and a basalt liquid with 8.3 percent MgO. Their values for the partition coefficient D [D = (weight percent metal in sulfide liquid)/(weight percent metal in basalt liquid)] at 1235'C for Ni, Cu, and Co are 274, 245, and 80 respectively. Although the process of separation of a sulfide melt could drastically deplete these elements in residual liquids, the effect on the major-element composition would be negligible if, as suggested by Mysen and Popp (1980), the immiscible sulfide melt amounts to only 0.5 weight percent. The threefold difference in Cu contents among the CC#3 basalts, in which other elements are mainly isochemical, is thus probably the result of differences in physical-chemical conditions during separation of the immiscible sulfide melts. As noted above, the olivine-rich basalts contain about half the abundances of the stable incompatible minor and trace elements (P, Ti, Zr, Hf, Th, U, and Ta) of the quartz-normative varieties. Olivine-rich basalts are also marked by high Ni and Cr contents and high Ni/Co ratios. The chemistry of the olivine basalts is consistent


with their being parental to the quartz-normative type to which they evolved by a crystal-fractionation process. The two-fold enrichment of the incompatible elements indicates that at least 50 percent crystallization and separation of olivine pyroxene from parental melts is necessary to produce quartz-normative tholeiites. In addition, the Cu and Ni/Co data point to some sulfide fractionation. Experimental studies suggest that quartznormative tholeiites evolve from olivine-tholeiite magma by crystal fractionation at crustal pressures of less than 8 to 10 kbar (Yoder, 1976).
The olivine-normative basalts are stratigraphically intercalated between quartz-normative flows. This precludes the sampled olivine-normative rocks from being directly parental to the underlying quartznormative flows. They may, however, have the composition of such parents. The high MgO (11.5-16.6 percent), Ni (530 ppm), and Cr (650-800 ppm) contents and high Ni/Co ratios (7.7) may be particularly significant; Mysen (1978) and Mysen and Popp (1980) have suggested that partial melting of peridotite at about 20 kbar would result in a tholeiitic liquid that contains 600-700 ppm Ni. The olivine basalts may represent near-primary magmas that have been little modified by crystal fractionation


STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 EXPLANATION Quartz-normative (11 samples)

- .. Olivine-normative (1 to 3 samples)







-


30 -


20


0

0
c c







GEOCHEMISTRY OF SUBSURFACE BASALTS NEAR CHARLESTON


during ascent from the mantle. The low Ni/Co ratios of the quartz-normative basalts preclude the possibility that they were in equilibrium with mantle peridotite. Thus a polybaric evolutionary history involving silicate and sulfide fractionation must be envisioned for these flows. The geochemical data are consistent with the possibility that the parental melt for both the olivinenormative and the quartz-normative basalts was generated within a relatively homogeneous peridotite source.
The trace-element chemistry of the sedimentary bed at 924-926 m (table 4, sample 924) reflects the mineralogic composition of this detrital unit. Very high K (49,000 ppm) and Rb (270 ppm) contents reflect the presence of potassium feldspar and illite that almost certainly were not derived from the enclosing basalt flows. High Ni (200 ppm) and Cr (370 ppm) contents agree with the observation of olivine-basalt clasts in the sedimentary bed.

TECTONIC SETTING AND MAGMATIC HISTORY
A particularly valuable result of the geochemical study of the test-hole basalts is the use of their geochemical features to characterize their tectonic setting. Gottfried and others (1977) attempted such a characterization of the CC#1 basalts by using the Ti-Zr, Ti-Zr-Y, and Ti-ZrSr discriminant diagrams of Pearce and Cann (1973). The results obtained were mutually contradictory; the basalts were described on various plots as calc-alkalic basalts of the island-arc series, as ocean-floor basalts, and as island-are tholeiitic basalts. Gottfried and others (1977) concluded that the available plots were inappropriate for tectonic characterization of CC#1 basalts.
More recently, Wood and others (1979) and Wood (1980) have proposed the use of a Th-Hf-Ta triangular diagram for tectonomagmatic classification. A plot of several Clubhouse Crossroads basalts and, for comparison, a typical ocean-ridge basalt and two Paleozoic continental tholeiitic diabases from Scotland is shown in figure 6. The Clubhouse Crossroads basalts plot in the field of basalts from destructive plate margins. However, the test-hole basalts are closely analogous chemically to basalts that occur in Atlantic-type passive continental margins and that represent the early cycles of magmatism that precede or accompany rifting and continental separation. Thus, either the diagram is inappropriately classifying the Clubhouse Crossroads basalts or the comparisons with basalts of rifted continental margins made in our previous study (Gottfried and others, 1977) are spurious.
We suggest that the geochemical characteristics of the subsurface basalts of the Charleston area produce the inappropriate classification on the Th-Hf-Ta plot.


The Clubhouse Crossroads basalts are characterized by low abundances of the relatively small ions Ti, Zr, Hf, and Ta. Macdonald and others (1981, their table 10) have shown that the Triassic-Jurassic passive-margin basalts of eastern North America, and presumably the test-hole basalts, belong to a type of intraplate tholeiite that is relatively enriched in Th. Thus, on the Th-Hf-Ta diagram (fig. 6), the test-hole basalts are displaced towards the Th apex and plot in the destructive-platemargin field. The data base used by Wood (1980) for delineation of his field did not adequately account for tholeiitic basalts of passive continental margins that are chemically similar to those of eastern North America. Although published discriminant diagrams do not appear appropriate for interpretation of the studied rocks, the results of this extensive geochemical study (tables 5, 6) of the basalts from CC#2 and CC#3 confirm our previous interpretation (Gottfried and others, 1977) regarding the magmatic affinities and tectonic setting of the basalt rocks. Recent radiometric ages determined by refined techniques (Lanphere, 1983) substantiate the correlation by stable minor and trace elements of the Clubhouse Crossroads quartz-normative basalts with the lower Mesozoic high-Ti, quartz-normative diabases of eastern North America. The olivine-normative basalt intercalated with the quartz-normative basalts in CC#3 shows close' geochemical similarities to the common olivine-normative diabases from the eastern North American province. Thus two of the three main chemical types of the eastern North American province have been recognized in the Clubhouse Crossroads basalts. In addition, the quartz-normative basalts from Clubhouse Crossroads are virtually indistinguishable from the quartz-normative tholeiitic rocks from eastern North America on the basis of their REE patterns and REE absolute abundances (fig. 7). The REE pattern for the olivine basalt from CC#3 "fingerprints" this rock as belonging to the olivine-normative type of eastern North America (Ragland and others, 1971). The wide ranges in contents of most of the major elements in the quartznormative rocks (table 5) reflect postmagmatic chemical changes. The average TiO2 and P201, contents in all of the quartz-normative basalts are virtually identical, as is the case for Th and Hf and some other stable trace elements. However, Cu and, to a lesser extent, Ni contents in the lower quartz-normative basalts from CC#3 are significantly greater than those encountered in the quartz-normative rocks at higher levels in the test holes. These differences can reasonably be attributed to differences in preeruptional history; namely, during magmatic ascent, the separation of an immiscible sulfide liquid into which Ni, Cu, and Co are strongly partitioned. In addition to providing important clues to the petrogenetic history of these basalts, the abundance


A15








A16 STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886

Hf
EXPLANATION
FIELDS
A Normal-type mid-ocean-ridge basalt
B Enriched-type mid-ocean-ridge basalt
and tholeiitic within-plate basalts
C Alkaline within-plate basalts
D Destructive-plate-margin basalts
TEST-HOLE BASALTS AND OTHER
THOLEIITIC BASALTS AND DIABASES
Test-hole basalts A
791. CC#1 836o CC#2 928 @ CC#3
DR-10 y Typical ocean-ridge basalt
1 m Continental diabases from
Midland Valley, Scotland 21




DR-101


B








@2 908 2M
-791-928--;


C







Th Ta

FIGURE 6.-Th-Hf-Ta data for samples of Clubhouse Crossroads basalt, ocean-ridge basalt, and continental tholeiltic diabase plotted on tectonomagmatic discrimination diagram of Wood (1980). Data sources: CC#1 (Gottfried and others, 1977); CC#2 and CC#3 (tables 2 and 4, this paper); ocean-ridge basalt from Juan de Fuca Ridge (David Gottfried, unpub. data, 1979); continental tholefitic diabase from Scotland (Macdonald and others, 1981).

relations of these elements have potential economic ap- central Alabama yielded ages of 161-168 m.y. for plication to further our understanding of the formation quartz-normative diabase and 184-193 m.y. for olivineof Ni and Ni-Cu sulfide deposits that are associated with normative diabases (Deininger and others, 1975). Field similar mafic volcanic rocks. studies of a swarm of olivine-normative diabase dikes
Previously, studies have proposed that the tholeiitic (Smith and others, 1975) in Pennsylvania showed the olivine diabases and flows of the eastern North presence of fracture cleavage in many of the olivineAmerican province represented the earliest stage of tholeiitic dikes, which was not observed in quartzvolcanism in that province. Whole-rock K-Ar normative dikes. This suggested that the olivinemeasurements of undeformed diabase dikes in east- normative magma was the earliest erupted type.









GEOCHEMISTRY OF SUBSURFACE BASALTS NEAR CHARLESTON A17

TABLE 5.-Average major-oxide composition, in weight percent, of subsurface basalts from Clubhouse Crossroads test holes CC#1, CC#2, and CC#8, near Charleston, S. C.
[Recalculated volatile-free. Range is given in parentheses after average. Averages derived as follows: CC#1, averages are for 5 least altered samples, from 771, 774, 779, 782, and 791 m below
surface; ranges are for all samples. CC#2, averages are for 8 samples, from depths of 780, 786, 879, 836, 842, 869, 878, and 908 m below surface. CC#3 upper quartz-normative, averages are for 4 samples, from depths of 777, 779, 782, and 784.4 m below surface. CC#3 lower quartz-normative, averages are for 6 samples, from depths of 984, 1002.2, 1014, 1020.4, 1024, and
1030 m below surface. CC#3 olivine-normative, averages are for 3 samples, from depths of 926.3, 928, and 930 m below surface.]
Sample location ---------------- CC#1 CC#2 CC#3 CC#3
Upper quartz- Lower quartz- Olivine
normative normative normative
SiO, ------------------ 54.0 (53.1-68.5) 53.9 (52.1-54.8) 52.9 (50.0-57.3) 53.3 (50.8-54.1) 47.4 (46.5-48.6)
A1,0 ------------------ 14.0 (7.3-16.7) 14.6 (14.1-14.8) 15.2 (13.8-16.0) 14.5 (14.1-15.5) 14.9 (14.5-15.8)
FeO, ---------------- 3.1 (2.5-9.5) 2.9 (1.8-4.6) 5.0 (4.2-6.1) 3.5 (2.4-4.1) 5.5 (4.1-7.1)
FeO ------------------ 8.6 (.79-9.0) 8.4 (6.8-8.9) 6.9 (5.5-7.6) 8.5 (8.1-9.9) 5.9 (4.1-7.5)
MgO ----------------- 6.0 (2.6-6.9) 6.3 (6.0-6.7) 6.7 (6.2-7.1) 6.4 (5.4-8.8) 14.0 (11.5-16.6)
CaO ------------------ 9.0 (7.1-9.6) 9.2 (7.6-10.3) 6.2 (5.4-7.1) 8.4 (5.8-9.8) 9.9 (8.0-12.4)
Na0 ----------------- 2.8 (.51-3.6) 2.8 (2.2-5.0) 4.7 (4.0-5.4) 3.5 (2.6-5.7) 1.4 (1.3-1.7)
K,0 ------------------ .54 (.02-1.4) .63 (.23-1.6) .99 (.48-1.4) .42 (.05-.74) .17 (.05-.30)
TiO, ------------------- .97 (.82-1.1) 1.0 (1.0-1.1) 1.1 (.93-1.2) 1.1 (.98-1.1) .54 (.52-.56)
P,0, ----------------- .14 (.12-.15) .15 (.14-.16) .17 (.16-.17) .17 (.16-.18) .08 (.08-.10)
MnO2 ----------------- - .23 (.14-.27) .17 (.16-.18) .26 (.20-.33) .22 (.21-.24) .20 (.15-.24)


154 EXPLANATION
al. 1 Eastern North American high-Ti, quartz-normative diabase
- - - 2 Eastern North American olivine-normative diabase
----3 Olivine-normative basalts from CC#3 e....... e 4 Quartz-normative basalts from CC#1
- ------5 Quartz-normative basalts from CC#3
- - - 6 Quartz-normative basalts from CC#2

3




5
6
-3

-- --- ---..- - .- - --- - -- - - 2


I I I I I I I I I I


La Ce Pr Nd Pm Sm Eu Gd Tb RARE EARTH ELEMENTS


Dy Ho Er Tm Yb Lu


FIGURE 7.-- Comparison of average abundances of REE in tholeiitic basalts of eastern North America and subsurface basalts from Clubhouse
Crossroads test holes. Data sources: eastern North American high-Ti, quartz-normative diabase and eastern North American olivinenormative diabase (Ragland and others, 1971); CC#1 (Gottfried and others, 1977); CC#2 and CC#3 (tables 2 and 4, this paper).


301-


20



z



1o
0 -


r I









A18


STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886

TABLE 6.-Average trace-element abundances in subsurface basalts from Clubhouse Crossroads test holes CC#1, CC#2, and CC#S, near Charleston, S. C.
[Range given in parentheses after average. n.d., not determined]


Sample location ___ CC#1 CC#2 CC#3 CC#3 CC#3
Upper quartz- Lower quartz- Olivine
normative normative normative
Large cations
Rb* _____-_-_ 16 (10-19) 17 (1.1-59) 25 (9-44) 12 (2-27) 2.7 (1.5-4.1)
Ba2. ________ 130 (100-140) 130 (7-370) 260 (74-550) 150 (54-210) 68 (64-70)
K - -____- 4,500 (4,100- 5,200 (1,900- 8,200 (3,700- 3,500 (400- 1,400 (6005,300) 12,000) 11,000) 6,100) 2,200)
Sr __ ----------- 190 (160-220) 190 (61-380) 300 (50-410) 250 (220-310) 84 (60-110)
Ca2' ___ 64,000 (62,000- 66,000 (51,000- 44,000 (36,000- 60,000 (41,000- 71,000 (53,00066,000) 73,000) 50,000) 69,000) 82,000)
Pb2* __--_ 4.5 (3.9-5) 5.7 (3.8-9.1) 5.6 (4.8-6.1) 5.2 (2.5-7) 2.4 (2.1-3.0)
K/Rb _________ 300 (220-410) 290 (110-650) 410 (230-650) 270 (190-390) 480 (270-610)
Ba/Rb ___-_ 8.1 (5.8-14) 9.4 (3.2-24) 10 (5-28) 18 (7.8-29) 31 (17-47)
K/Ba _____-___- 35 (29-39) 36 (15-69) 43 (17-86) 20 (5.7-35) 20 (6.6-32)
High-valence cations
Th -* _ 2.1 (2.0-2.2) 2.0 (1.5-2.1) 2.3 (1.7-2.6) 2.1 (1.8-2.4) 1.0 1.0
U* ------ 0.6 (.5-.8) 0.6 (.5-.7) 0.65 (.5-.7) .3
Zr_ _ 76 (69-82) 86 (78-100) 83 (77-87) 93 (77-100) 48 (45-53)
Hf__ 2.1 (2.0-2.1) 2.0 (1.9-2.1) 2.2 (2.0-2.5) 2.3 (2.0-2.6) 1.1 (1.1-1.2)
Nb' 7.3 (6.8-7.7) 6.0 (4.9-6.8) 6.4 (5.8-6.9) 6.0 (5.6-6.4) 4.7 (4.1-5.5)
Ta'' __________ 0.29 (.26-.32) 0.44 (.34-.53) 0.36 (.26-.47) .36 (.28-.41) 1.19 (.17-.21)
Th/U -------- 3.1 (2.1-3.8) 3.7 (3.4-4.0) 3.2 (3.0-3.6) 3.3
Zr/Hf ____- 36 (35-39) 43 (37-47) 38 (35-40) 40 (38-46) 44 (38-48)
(Nbx 100)/Ti __ 0.13 (.12-.14) 0.10 (.09-.11) 0.11 (.10-.11) .098 (.09-.11) .16 (.14-.18)
Nb/Ta _-___ 25 (23-26) 14 (12-18) 19 (15-22) 17 (15-20) 127 (21-32)
Ferromagnesian elements
Co2 ____ 46 (40-52) 45 (34-54) 48 (38-67) 45 (42-51) 69 (68-70)
Cu2' _ _ .. - 25. (16-34) 22 (16-43) 20 (18-22) 75 (64-85) 120 (98-150)
Li+ 9.4 (5-14) 11 (1.8-25) 12 (6-27) 17 (8.4-26) 17 (7.8-33)
Ni2+ 17 (14-20) 23 (19-34) 22 (20-26) 35 (27-42) 530 (530)
Zn2 ----------- 88 (82-96) 89 (75-100) 62 (50-69) 92 (80-110) 64 (56-74)
Crl* _ ----- 33 (28-36) 71 (50-170) 52 (48-57) 31 (25-36) 750 (650-800)
Ca -_______ 16 (15-18) 16 (12-20) 14 (13-16) 16 (14-17) 12 (11-13)
Sc3' 51 (46-56) 42 (37-50) 43 (41-44) 40 (37-43) 38 (33-43)
V3+ 320 (250-400) 280 (230-370) 280 (250-290) 300 (240-330) 210 (180-230)
Ni/Co __-__ .38 (.31-.48) .52 (.42-.63) .47 (.39-.53) .77 (.64-.91) 7.7 (7.6-7.8)
Rare-earth elements
La 210 (9-11) 49.4 (9-10) 10 6(10-11) 11 6(9-12) 5.2 7(5-5.5)
Ce ------- 19 (18-19) 20 (17-21) 21 (20-23) 26 (22-32) 13 (12-14)
Nd ------ -10 12 (9-14) 12 (10-14) 14 (13-15) 7.8 (6.5-9)
Sm ----- 3.0 (3.0-3.1) 3.1 (3.0-3.2) 3.2 (3.0-3.4) 3.5 (3.3-3.8) 1.7 (1.6-1.8)
Eu --_____ .99 (.95-1.02) .94 (.87-.99) 1.05 (.94-1.10) 1.14 (.90-1.32) .57 (.51-.62)
Gd _____- - n.d. n.d. 3.1 (2.8-3.4) n.d. n.d. n.d. n.d. n.d. n.d.
Th ----------- .70 (.68-.73) .77 (.71-.86) .74 (.67-.78) .76 (.69-.83) .41 n.d.
Ho ______- n.d. n.d. .6 (.5-.7) .8 (.7-.8) .8 (.8-.9) .6 (.6-.7)
Tm n.d. n.d. .33 (.31-.36) n.d. n.d. n.d. n.d. n.d. n.d.
Yb __ _ 2.2 (2.1-2.3) 2.7 (2.5-3.1) 2.7 (2.5-2.9) 2.9 (2.8-3.0) 2.3 (2.0-2.5)
Lu -________ -.45 (.44-.47) .42 (.39-.44) .38 .43 .36 n.d.
Y -___ 32 (28-34) 28 (25-30) 24 (23-25) 28 (25-29) 20 (17-23)
I Derived from two samples, from 926.3 and 928 m below surface.
2 REE averages derived from three samples, from 771, 779, and 791 m below surface.
'Derived from one sample, from 779 m below surface.
'REE derived from eight samples, from 780, 786, 819, 836, 842, 869, 878, and 908 m below surface.
'Derived from four samples, from 777, 779, 782, and 784.4 m below surface.
'Derived from six samples, from 984, 1002.2, 1014, 1030.4, 1024, and 1030 m below surface.
'Derived from three samples, from 926.3, 928, and 930 m below surface.


However, the extensive geochronologic study of exposed lower Mesozoic dikes in Georgia by Dooley and Wampler (1983) indicates that excess 40Ar in most of these dikes resulted in variable and highly discordant K-Ar ages. Further, several of their samples that contained little excess 40Ar included both olivine- and quartz-normative tholeiites having apparent ages of


190-195 m.y. The new data from CC#3 unequivocally show the presence of an olivine-tholeiite flow intercalated within a sequence of quartz-normative tholeiites. It thus appears that there were repetitive cycles and possibly penecontemporaneous eruptions of olivinenormative and quartz-normative magmas. Weigand and Ragland (1970) discussed the geographic distribution of








GEOCHEMISTRY OF SUBSURFACE BASALTS NEAR CHARLESTON


the main chemical magma types in the eastern North American province. They proposed that differences in tectonic environment could account for the absence of the olivine-normative magma type in the northeast part of the province. The close association in space and time of these contrasting magmas at Clubhouse Crossroads suggests that the olivine-tholeiitic magma may represent merely a greater degree of melting, hence differences in the thermal regime in the mantle, rather than any significant change in tectonic environment.


REFERENCES CITED

Baedecker, P. A., Rowe, J. J., and Steinnes, Eiliv, 1977, Application of
epithermal neutron activation in multi-element analysis of silicate rocks employing both coaxial Ge(Li) and low energy photon detector systems: Journal of Radioanalytical Chemistry, v. 40, p.
115-146.
Deininger, R. W., Dallmeyer, R. D., and Neathery, T. L., 1975,
Chemical variations and K-Ar ages of diabase dikes in east-central Alabama [abs.]: Geological Society of America Abstracts with Programs, v. 7, no. 4, p. 482.
Dooley, R. E., and Wampler, J. M., 1983, Potassium-argon relations
in diabase dikes of Georgia: the influence of excess 40Ar on the geochronology of early Mesozoic igneous and tectonic events, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity: U.S. Geological
Survey Professional Paper 1313, p. M1-M24.
Gohn, G. S., Gottfried, David, Lanphere, M. A., and Higgins, B. B.,
1978, Regional implications of Triassic or Jurassic age for basalt and sedimentary red beds in the South Carolina Coastal Plain:
Science, v. 202, no. 4370, p. 887-890.
Gohn, G. S., Houser, B. B., and Schneider, R. R., 1983, Geology of
the lower Mesozoic(?) sedimentary rocks in Clubhouse Crossroads test hole #3, Dorchester County, South Carolina, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886- Tectonics and seismicity: U.S. Geological Survey Professional Paper 1313, p. D1-D17.
Gottfried, David, Annell, C. S., and Schwarz, L. J., 1977, Geochemistry of subsurface basalt from the deep corehole (Clubhouse Crossroads corehole 1) near Charleston, South Carolina-Magma type and tectonic implications, in Rankin, D. W., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-A preliminary report: U.S. Geological Survey Professional Paper
1028, p. 91-113.
Gottfried, David, Greenland, L. P., and Campbell, E. Y., 1968,
Variation of Nb-Ta, Zr-Hf, Th-U and K-Cs in two diabasegranophyre suites: Geochimica et Cosmochimica Acta, v. 32, no. 9,
p. 925-947.
Heier, K. S., and Rogers, J. J. W., 1963, Radiometric determination of
thorium, uranium and potassium in basalts and in two magmatic


differentiation series: Geochimica et Cosmochimica Acta, v. 27, no.
2, p. 137-154.
Lanphere, M. A., 1983, 40Ar/9Ar ages of basalt from Clubhouse Crossroads test hole #2, near Charleston, South Carolina, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity: U.S. Geological Survey Professional Paper 1313, p. B1-B8.
Larsen, E. S., III, and Gottfried, David, 1960, Uranium and thorium
in selected suites of igneous rocks: American Journal of Science, v.
258-A, p. 151-169.
Macdonald, R., Gottfried, David, Farrington, M. J., Brown, F. W.,
and Skinner, N. G., 1981, Geochemistry of a continental tholeiite suite: late Paleozoic quartz dolerite dykes of Scotland: Royal Society of Edinburgh Transactions, Earth Sciences, v. 72, p. 57-74. Mysen, B. 0., 1978, Experimental determination of nickel partition
coefficients between liquid, pargasite, and garnet peridotite minerals and concentration limits of behavior according to Henry's law at high pressure and temperature: American Journal of
Science, v. 278, no. 2, p. 217-243.
Mysen, B. 0., and Popp, R. K., 1980, Solubility of sulfur in CaMgSi206
and NaAlSi3O8 melts at high pressure and temperature with controlled f02 and fS2: American Journal of Science, v. 280, no. 1, p.
78-92.
Pearce, J. A., and Cann, J. R., 1973, Tectonic setting of basic volcanic
rocks determined using trace element analyses: Earth and
Planetary Science Letters, v. 19, no. 2, p. 290-300.
Ragland, P. C., Brunfelt, A. 0., and Weigand, P. W., 1971, Rareearth abundances in Mesozoic dolerite dikes from eastern United States, in Brunfelt, A. 0., and Steinnes, Eiliv, eds., Activation analysis in geochemistry and cosmochemistry: Oslo, Universitetsforlaget, p. 227-235.
Rajamani, V., and Naldrett, A. J., 1978, Partitioning of Fe, Co, Ni, and
Cu between sulfide liquid and basaltic melts and the composition of Ni-Cu sulfide deposits: Economic Geology, v. 73, no. 1, p. 82-93. Smith, R. C., II, Rose, A. W., and Lanning, R. M., 1975, Geology and
geochemistry of Triassic diabase in Pennsylvania: Geological
Society of America Bulletin, v. 86, no. 7, p. 943-955.
Taylor, S. R., 1965, Geochemical analysis by spark source mass
spectrometry: Geochimica et Cosmochimica Acta, v. 29, no. 12, p.
1243-1261.
Weigand, P. W., and Ragland, P. C., 1970, Geochemistry of Mesozoic
dolerite dikes from eastern North America: Contributions to
Mineralogy and Petrology, v. 29, no. 3, p. 195-214.
Wood, D. A., 1980, The application of a Th-Hf-Ta diagram to problems
of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary volcanic province: Earth and Planetary Science Letters, v. 50, no.
1, p. 11-30.
Wood, D. A., Joron, J. L., and Treuil, M., 1979, A reappraisal of the
use of trace elements to classify and discriminate between magma series erupted in different tectonic settings: Earth and Planetary
Science Letters, v. 45, no. 2, p. 326-336.
Yoder, H. S., Jr., 1976, Generation of basaltic magma: Washington,
D.C., National Academy of Sciences, 265 p.


A19



























































































































4

















40Ar/39Ar Ages of Basalt From Clubhouse Crossroads Test Hole #2, Near Charleston, South Carolina


By MARVIN A. LANPHERE

STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-TECTONICS AND SEISMICITY


GEOLOGICAL SURVEY PROFESSIONAL PAPER 1313-B


UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:1983




















CONTENTS


Abstract --------------------------------------In tro d u ctio n _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - _ - _ _ - _ Geologic setting - _Techniques -----------------------------------Results of age determinations on basalt samples --------------------BBH#1 analyses _________-- -
B B H #3 analyses _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ___- _
BBH#5 analyses ---------------------------------Discussion --------------------------------------R eferences cited _ _ _ _ _ _-- __________- ____ _ _-___ _


Page
----- BI
- 1
_ _ _ 1
- 2 - 2 _ _ 4 _ _ 4 -- 5
-- 5
-_ 7


ILLUSTRATIONS


FIGURE 1. Age-spectrum diagram for 40Ar/39Ar incremental heating experiment on BBH#1 basalt -__________________2. Isochron diagram for 40Ar/39Ar incremental heating experiment on BBH#1 basalt _-_______- _- _________3. Age-spectrum diagram for 40Ar/39Ar incremental heating experiment on BBH#3 basalt -_______- __________4. Isochron diagram for 40Ar/39Ar incremental heating experiment on BBH#3 basalt _____________________5. Age-spectrum diagram for 40Ar/39Ar incremental heating experiment on BBH#5 basalt ____-- ______________6. Isochron diagram for 40Ar/39Ar incremental heating experiment on BBH#5 basalt ____-_- ____________-


Page B4
4 5 5 6 6


TABLES


TABLE 1. Analytical data for 40Ar/39Ar total-fusion and incremental heating experiments on three basalt samples from CC#2 near
C harleston , S . C -__ _ _ _ _ _ _ _- _ _ _ _ _ _ _ _ _ - _ - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
2. Summary of age-spectrum and isochron data from 40Ar/39Ar incremental heating experiments on three basalt samples
from CC#2 near Charleston, S. C. - ------------------------------------------------------------


Page


--- B3

4


III


























s






























































































a














STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886TECTONICS AND SEISMICITY


40Ar/39Ar AGES OF BASALT FROM CLUBHOUSE CROSSROADS TEST HOLE #2, NEAR CHARLESTON, SOUTH CAROLINA By MARVIN A. LANPHERE


ABSTRACT
40Ar/39Ar total-fusion ages of three samples of basalt from Clubhouse Crossroads test hole #2, near Charleston, S.C., range from 182 to 236 m.y. (million years); only one of the total-fusion ages agrees within analytical uncertainty with conventional K-Ar ages of the same samples. Data from 40Ar/39Ar incremental heating experiments indicate that only one sample meets the criteria for a reliable crystallization age. The 40Ar/36Ar versus "9Ar/36Ar isochron age for this basalt is 184 3.3 m.y. This age is in good agreement with reliable ages of tectonically related lower Mesozoic diabase intrusions in eastern North America and Liberia. The ages of all these intrusions are consistent with their emplacement shortly after initiation of central Atlantic rifting about 190 m.y. ago.

INTRODUCTION

Many workers believe that the separation of the African and North American plates and the formation of the Atlantic Ocean were the initial events in the breakup of Pangea (Dietz and Holden, 1970; Pitman and Talwani, 1972; Phillips and Forsyth, 1972; Larson and Pitman, 1972; Smith and others, 1973). Basaltic igneous activity of Triassic to Jurassic age in northeastern North America and northwestern Africa is probably related to the extensional tectonics that produced rifting of the continent (Dietz and Holden, 1970; Dalrymple and others, 1975). Triassic or Jurassic basalt flows in deep test wells near Charleston, S. C., indicate that the early Mesozoic tectonic framework of the Southeastern United States is the same as that of northeastern North America (Gohn and others, 1978). The current study is an extension of isotopic dating work on the basalts near Charleston.
Acknowledgments.-I thank S. E. Sims and J. C. Von Essen of the U.S. Geological Survey (USGS) for the argon measurements and D. H. Rusling and the staff of the USGS reactor facility for fast-neutron irradiation of samples. Deep drilling and related investigations by the USGS in the Charleston, S. C., area are supported by the


U.S. Nuclear Regulatory Commission, Office of Nuclear Research, under Agreement No. AT(49-25)-1000.

GEOLOGIC SETTING
Fault-bounded basins filled primarily with clastic sedimentary rocks of Triassic and Jurassic age are a characteristic geologic feature of the eastern seaboard of North A erica. Some basins contain interbedded basalt flows and diabase dikes and sills. The dikes typically extend outside individual basins. Such basins are exposed in the Piedmont from North Carolina on the southeast to Nova Scotia on the northeast. Sediments of the Atlantic Coastal Plain conceal subsurface lower Mesozoic rocks in South Carolina, Georgia, Florida, and Alabama, which have been documented from deep wells or inferred from geophysical surveys.
Three deep test wells drilled by the USGS at Clubhouse Crossroads, Dorchester County, S. C., penetrated a pre-Upper Cretaceous sequence of subaerial, tholeiitic basalt flows underlain by red sandstone, mudstone, and conglomerate (Gohn and others, 1978). Two basalt samples from Clubhouse Crossroads test hole 1 (CC#1) yielded K-Ar ages of 97.0+ 4.2 and 111+ 4 m.y. (million years). These ages have been recalculated from those originally reported by Gottfried and others (1977) by using the isotopic abundance and decay constants for 40K recommended by Steiger and Jager (1977). These ages must be considered minimum, however, because the basalts have been somewhat altered. The basalts unconformably underlie strata of Cretaceous (Cenomanian) age (Hazel and others, 1977) in the test hole; no direct stratigraphic evidence for a maximum age for the basalts is available.
Petrographic and major-element chemical data ob- tained from samples of basalt from CC#1 (Gottfried and others, 1977) indicate that the rocks have undergone slight to extreme oxidation, hydration, and hydrotherB1








STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


mal alteration; H20 and CO2 contents are relatively high and variable. Secondary minerals resulting from hydrothermal alteration include laumontite, calcite, chlorite, and stilbite. I examined thin sections of the two samples from CC#1 on which K-Ar ages had been measured, and I concluded that these samples were too altered to yield reliable ages.
Conventional K-Ar ages were subsequently measured on three additional samples of basalt from Clubhouse Crossroads test hole 2 (CC#2) (Gohn and others, 1978). Location of the test hole is shown in Gohn and others (1983, fig. 1). The 132-m-thick section of basalt in CC#2 was divided into 3 flow units by Gottfried and others (1983); Phillips (1983) further divided these 3 units into 10 subflows on the basis of paleomagnetic measurements. The three basalt samples used for K-Ar measurements-BBH#1 (818.7 m), BBH#3 (842.3 m), and BBH#5 (907.4 m) - are from flows designated by Phillips as 2-2b, 2-2d, and 2-3d, respectively. The samples are slightly altered but, on the basis of thin-section examination, were considered suitable for dating.
Duplicate analyses of the three basalt samples from CC#2 yielded ages of 204 4.1, 162 3.2, and 186 +3.7 m.y. for BBH#1, BBH#3, and BBH#5, respectively. These data confirmed that the basalts, and presumably the underlying sedimentary red beds, are Triassic or Jurassic in age and are equivalent to rocks in the exposed early Mesozoic basins along the Atlantic seaboard; however, the poor agreement of these ages (Gohn and others, 1978) was disturbing. The ages are not in correct stratigraphic order, and it seems unlikely that basalt was erupted over a period exceeding 40 m.y. Therefore, 40Ar/39Ar analyses were made on the same three samples of basalt in an attempt to refine the age determinations.

TECHNIQUES

Samples for 40Ar/39Ar analysis were small cores (6 mm in diameter and 1 cm long) taken from the same slices of core from CC#2 as were the samples for conventional K-Ar analysis. The samples for 40Ar/39Ar dating were sealed in air in flat-bottomed fused silica vials and were irradiated at 1 megawatt for 25 to 30 hours in the USGS TRIGA' reactor, where they received a neutron dose of approximately 2.5-3 x 1018 nvt (neutron density-velocity-time). Further details of the reactor flux characteristics, the flux monitor mineral, and the corrections for interfering K- and Ca-derived Ar isotopes have been given by Dalrymple and Lanphere (1971) and by Dalrymple and others (1981).


'The use of trade names in this publication is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.


Irradiated samples were fused by induction heating, and the Ar was purified in a standard extraction line. Temperatures for incremental heating experiments were measured by means of a platinum/platinumrhodium thermocouple inserted into a small hole in the bottom of a machined molybdenum crucible and are accurate within about + 20'C (Lanphere and Dalrymple, 1971). Samples were maintained for 30 minutes at each temperature during incremental heating experiments. Ar analyses were made by means of a Nier-type, 15.24-cm radius, 60'-sector magnet, single-collector mass spectrometer, and analog data acquisition.


RESULTS OF AGE DETERMINATIONS ON BASALT SAMPLES

Data for total-fusion and incremental heating 40Ar/39Ar experiments are presented in table 1. Errors given for ages of individual samples or gas increments are estimates of one standard deviation of analytical precision. These errors were calculated by using formulas derived by differentiation of the 40Ar/39Ar and age equations (Dalrymple and Lanphere, 1971). The agreement between total-fusion 40Ar/39Ar and conventional K-Ar age determinations for the three basalt samples is not particularly good. For only one sample, BBH#5, do the two ages agree within analytical uncertainty. The ages for BBH#3 differ by slightly more than analytical uncertainty, and the total-fusion and conventional ages of BBH#1 differ by nearly 15 percent. The discordance in ages probably reflects the effect of alteration of the K-Ar system. A considerable body of data on the effects of alteration on K-Ar age determinations has been obtained from studies of Hawaiian-type basalts altered in the submarine environment. No comparable data exist for continental tholeiites such as those in the Clubhouse Crossroads drill holes. The general pattern of the Hawaiian data may be similar, however. Clague and others (1975) and Dalrymple and Clague (1976) found that the total-fusion 40Ar/39Ar ages of altered Hawaiian basalts generally were greater than conventional K-Ar ages for the same rocks. They proposed that this pattern was due to proportional loss of radiogenic 40Ar and K-derived 39Ar from K-bearing clays. Dalrymple and others (1980) devised a parameter to portray this discordance in age. This parameter, 6, is the total-fusion age less the conventional age and is expressed as a percentage of the total-fusion age. Dalrymple and others (1980) have suggested that for samples having 6 values that are less than +5, the total-fusion 40Ar/39Ar and conventional K-Ar ages probably are quite close to crystallization ages. Only BBH#5, which has a 6 value of
-2.4, fits this criterion for crystallization age, though


B2








40Ar/39Ar AGES OF BASALT FROM CLUBHOUSE CROSSROADS TEST HOLE #2


TABLE 1.-Analytical data for 4"Ar/39Ar total fusion and incremental heating experiments on three basalt samples from CC#2, near Charleston, S. C.
[Subscripts in columns 9-12 indicate radiogenic (R), calcium-derived (Ca), and potassium-derived (K) argon]
Depth be- Tempera- "Ar Calculated
low surface ture (percent of "Are. "Are. "Ar. "Ar, age'
Sample no. (meters) (*C) J' "Ar/"Ar "Ar/"Ar' "Ar/"Ar total Ar) (percent) (percent) (percent) (percent) (m.y.)
Total fusion experiments
BBH#1 __________----_ - 818.7 ___- 0.005600 29.10 4.466 0.01550 ____ 7.8 0.3 85.5 <0.1 236 2.9'
BBH#3 ------------------- 842.3 ____ .005600 52.65 12.85 .1216 _ 2.9 .8 33.7 <.1 172 4.5'
BBH#5 ------------------- 907.4 ____ .006821 24.54 18.28 .003608 13.8 1.2 62.5 <.1 182 2.8'
Incremental heating experiments
BBH#1 ------------------ 818.7 500 0.005600 34.02 0.8163 0.006458 43.1 3.4 <0.1 94.6 <0.1 299 3.5
600 21.53 4.755 .005877 17.5 22.0 .3 93.7 <.1 194 2.4
700 23.07 6.759 .01539 8.7 11.9 .4 82.6 <.1 184 3.1
800 24.31 2.550 .01499 10.3 4.6 .2 82.6 <.1 193 2.7
900 19.94 1.531 .005252 15.2 7.9 .1 92.8 <.1 178 2.3
975 23.14 6.960 .04734 1.8 4.0 .4 41.9 <.1 95.8 7.2
1050 38.78 20.86 .1213 .8 4.7 1.3 11.9 <.1 46.6 18.5
FUSE 34.68 58.47 .09643 2.6 16.5 3.7 31.4 <.1 111 9.0

BBH#3 ______- - _________ 842.3 500 0.005600 54.04 1.828 0.1321 14.3 0.4 0.1 28.0 <01 147 6.3
600 45.80 3.831 .09457 14.1 1.1 .2 39.6 <.1 175 6.3
700 29.84 6.360 .04573 25.7 3.8 .4 56.4 <.1 163 4.3
800 28.78 8.853 .04266 17.9 5.6 .6 58.6 <.1 164 4.3
900 35.18 9.406 .06867 7.0 3.7 .6 44.4 <.1 152 10.3
975 26.29 11.78 .03694 13.8 8.7 .7 62.0 <.1 159 3.8
1050 34.14 6.987 .07551 3.6 25.2 4.4 51.1 <.1 176 10.9
FUSE 41.82 7.325 .1135 3.6 17.5 4.6 33.8 <.1 144 16.3
00.0
BBH#5-_-__________ 907.4 500 0.006821 57.99 22.05 0.1409 6.4 0.4 0.1 28.5 <0.1 193 107
600 26.98 3.260 .03939 10.0 2.3 .2 57.8 <.1 183 7.1
700 27.99 6.113 .04059 16.8 4.1 .4 58.9 <.1 193 5.3
800 22.56 8.063 .02457 27.2 8.9 .5 70.7 <.1 187 3.5
900 23.04 11.18 .02814 10.2 10.8 .7 67.8 <.1 184 5.3
975 26.72 12.10 .04445 5.5 7.4 .8 54.5 <.1 172 8.0
1050 21.14 17.80 .02853 12.4 17.0 1.1 66.9 <.1 168 5.3
FUSE 19.53 79.93 .03358 11.5 64.7 5.1 82.1 <.1 197 6.0
A function of the age of the monitor mineral and of the integrated fast-neutron flux.
Corrected for "Ar decay; half-life= 35.1 days.
'X =0.581 x1 0 yr, , =y h4.962 x 10 yr ' The figures are estimated standard deviation of analytical precision at the 68 percent level of confidence.
'Conventional K-Ar ages of BBH#1, B #3, and B #5 are 204 4.1, 162 3.2, and 186 3.7, respectively (Gohn and others, 1978).


BBH#3, which has a 6 value of + 5.8, is close. BBH#1 has a 6 value of + 13.6.
40Ar/39Ar incremental heating experiments were done on all three samples of basalt, and the analytical data are presented in table 1. The data are treated in two different ways: (1) as an age-spectrum diagram in which the apparent age of each gas increment is plotted as a function of the cumulative percentage of 39Ar released and (2) as an 40Ar/36Ar versus 39Ar/36Ar correlation diagram in which the slope of the line (isochron) fitted to the data points indicates the age of the sample and the intercept on the ordinate indicates the composition of nonradiogenic Ar in the sample. The age of each gas increment (table 1) is calculated by assuming that the nonradiogenic Ar has the composition of atmospheric Ar (40Ar/36Ar = 295.5). The interpretation of age spectra is based on shape; in the ideal case of an undisturbed sample, all gas increments will have the same apparent age (Turner, 1968; Dalrymple and Lanphere, 1974). The interpretation of age spectra for disturbed or altered samples is not straightforward, but workers generally agree that, for igneous rocks, a crystallization age may be represented by a plateau, which is a plotted set of adjacent gas increments having the same apparent age and representing a significant proportion of the 39Ar released from the sample.
No assumption about the isotopic composition of nonradiogenic Ar in a sample is made for an isochron


diagram. For each gas increment, the 40Ar/36Ar ratio is plotted versus the 39Ar/36Ar ratio after the ratios have been corrected for interfering Ar isotopes produced during irradiation. In an unaltered and undisturbed sample, all gas increments will form a straight line (isochron) having an 40Ar/36Ar intercept of 295.5 and a slope that is proportional to the age of the sample. For disturbed samples, the interpretation may be more complicated because decisions have to be made on which points to include in fitting an isochron. Isochrons are fitted by using the York 2 least-squares cubic fit and correlated errors (York, 1969), and by using the formula recommended by York (quoted in Ozima and others, 1977) for the linearregression correlation coefficient, r.
To make the interpretation of age spectra and isochrons as objective as possible, Lanphere and Dalrymple (1978) suggested a set of conservative criteria for an acceptable crystallization age. These criteria are:
1. A well-defined plateau in an age-spectrum diagram
formed by three or more contiguous gas increments representing at least 50 percent of the
39Ar released.
2. A well-defined isochron for the plateau points having
a York 2 fit index [SUMS/(N -2)]2 of less than 2.5

'SUMS/(N -2) is a goodness-of-fit parameter of a straight line to a set of data; SUMS is the weighted sum of the squares of the residuals, and N is the number of data.


B3








STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


TABLE 2. -Summary of age-spectrum and isochron datafrom *ArI3Ar incremental heating experiments on three basalt samples from CC#2, near Charleston, S. C.
Age spectrum Isochron
Depth Temperature Weighted
Sample below surface increments mean age'Ar Age "Ar/"Ar
no. (meters) used (00) (my. (percent) (my.) intercept SUMS/(N-2)
BBH#1 --------------- 818.7 600-900 187 1.3 51.7 192 15.4 -4 330 1445
BBH#3 ___ _ 842.3 700-975 161 3.1 64.4 167 2.0 281 28 1.7
BBH#5 -------------- 907.4 500-900 187 3.1 70.5 184 3.3 302 6 1.6
500-1050 183 2.6 88.5 175 1.0 309 14 8.8
See footnote, p. B3.


(Brooks and others, 1972; Dalrymple and Lanphere, 1974).
3. Concordant isochron and plateau ages.
4. An 40Ar/36Ar intercept for the isochron that is not
significantly different from the atmospheric
value of 295.5.
Samples that do not meet these criteria are considered too disturbed or altered to yield reliable crystallization ages. The age-spectrum and isochron data (table 2) show that only one sample from this study, BBH#5, meets the criteria for a reliable crystallization age. The results for each sample are discussed below.

BBH#1 Analyses
The most disturbed of the three samples is BBH#1 (figs. 1, 2). The initial gas increment removed at 500'C has an apparent age of 299 m.y., more than 50 percent older than the succeeding four gas increments. The 600'-900'C gas increments contain about 52 percent of the 39Ar released and define a reasonably good plateau that has a weighted mean age of 187 +1.3 m.y.; each datum was weighted by the inverse of its estimated variance. The first 95 percent of the 39Ar released produces an age spectrum (fig. 1) that resembles those for igneous rocks known to contain excess 40Ar (Lanphere and Dalrymple, 1971; Brereton, 1972; Ozima and Saito,


400


Z 300


< 200
1
< 100 M_


o
0


10 20 30 40 'Ar RELEASED, IN


50 60 70 80 90 CUMULATIVE PERCENT


100


FIGURE 1.-Age-spectrum diagram for 40Ar/"5Ar incremental heating experiment on BBH#1 basalt. t, is the weighted mean plateau age.


5500


3500 F-


1500


-500
0


+ 500'C


EXPLANATION


0U


Fuse
-1050-c


50 100 1
"Ar/aeAr


sed in fit 6000 0
ot used in fit 900"C o-70000
8000 ',
Age 192 15.4 m.y.
-' -"ArPAr intercept= - 4 330


VC


SUMS/(N -2) = 1445


50 200 250


FIGURE 2. -Isochron diagram for 40Ar/9Ar incremental heating experiment on BBH#1 basalt.

1973; Kaneoka, 1974; Dalrymple and others, 1975; Lanphere and Dalrymple, 1976). The high-temperature gas increments from BBH#1, however, do not show an increase in age to produce the typical "saddle-shaped" spectrum. Dallmeyer (1975) did not believe that a saddle-shaped spectrum indicates the presence of excess 40Ar; he suggested that for samples containing excess 40Ar, anomalously older apparent ages are found only in the low-temperature fractions.
The isochron diagram for BBH#1 (fig. 2) shows the scatter of data typical of isochron diagrams for samples known to contain excess 40Ar (Brereton, 1972; Dalrymple and others, 1975; Lanphere and Dalrymple, 1976). This scatter is shown very clearly by the large uncertainty in the 40Ar/36Ar intercept and the extremely large value of SUMS/(N-2) (table 2). The 192 15.4 m.y. age indicated by an isochron drawn through the points that define the age-spectrum plateau agrees with the plateau age, though the agreement may be fortuitous because the isochron age has a large uncertainty.

BBH#3 Analyses
The 700'-975'C gas increments for BBH#3 contain about 64 percent of the 39Ar released, and the agespectrum diagram shows a well-defined plateau that has an age of 161 3.1 m.y. (fig. 3). The initial fraction


-500*C
============ = 187.t.3 0

- ~ ~ 0' _8001 ooC 90 700oC---


B4








40Ar/39Ar AGES OF BASALT FROM CLUBHOUSE CROSSROADS TEST HOLE #2


250 225 200 < 175 w 150 , 125

100


0 10 20 30 40 50 60 70 80 90
"Ar RELEASED, IN CUMULATIVE PERCENT


100


FIGURE 3.- Age-spectrum diagram for 40Ar/I9Ar incremental heating experiment on BBH#3 basalt. t, is the weighted mean plateau age.


released at 500*C has an apparent age that is significantly younger than the plateau age. An age spectrum having this shape is characteristic of postcrystallization argon loss (Turner, 1968; Lanphere and Dalrymple, 1971). The isochron age of 167 2.0 m.y. for BBH#3 (fig. 4) differs from the plateau age by slightly more than the analytical uncertainties. The intercept of the isochron (table 2) agrees with the atmospheric 40Ar/36Ar ratio but has a large uncertainty. The isochron statistics (table 2) indicate that the fit does not have excessive scatter.

BBH#5 Analyses
The most straightforward age spectrum is that of BBH#5 (fig. 5). The 500'-900'C gas increments contain about 70 percent of the 39Ar released and define a good plateau that has an apparent age of 187 + 3.1 m.y. If the 500'-1050'C gas increments, which contain about 88 percent of the 39Ar released, are used, the plateau age is 183 + 2.6 m.y. The isochron (fig. 6) and plateau ages are concordant for the 5000 -900'C gas increments, but the two ages are discordant for the 500' -1050'C gas increments (table 2). The fit of the isochron and the precision of the intercept are better for the 5000 -900'C gas increments.

DISCUSSION

The objective of this study was to refine the age of basalt in CC#2, and I believe that this objective has been achieved. The incremental heating experiments show that two of the basalt samples are markedly disturbed; the age spectra give evidence for excess 40Ar in one sample and for loss of 40Ar in the other. The 40-m.y. spread in the conventional K-Ar ages, therefore, is not real. The only sample that meets all the given criteria


800 600 400


20


01


0


10


20
39Ar/36Ar


30


40


FIGURE 4.- Isochron diagram for 40Ar/39Ar incremental heating experiment on BBH#3 basalt.

for a reliable crystallization age is BBH#5. The agespectrum and isochron ages for the gas increments released between 500*C and 900'C are concordant for this sample. If the 9750C and 10500C increments are included, however, the age-spectrum and isochron ages are discordant. Thus, the 500'-900*C increments yield the best available information on the age of BBH#5. The isochron age of 184 +3.3 m.y. is preferred because no assumption needs to be made about the isotopic composition of nonradiogenic argon in the sample. The conventional age of 186 3.7 m.y. and the total-fusion 40Ar/39Ar age of 182 + 2.8 m.y. are concordant with the incremental heating data. Taken collectively, these data indicate that 184 m.y. is a good estimate for the crystallization age of sample BBH#5.
The 40Ar/39Ar incremental heating data for sample BBH#3 show that the sample is slightly but definitely disturbed; it meets all the criteria for a reliable crystallization age except concordance of age-spectrum and isochron ages. However, the shape of the age spectrum is typical of postcrystallization argon loss. The isochron age of 167 m.y. for BBH#3 is about 9 percent younger than the preferred age for BBH#5. BBH#3 overlies BBH#5 by 65 m, but there is no way to estimate the time interval between eruption of the two flow units.
Sample BBH#1 clearly is the most disturbed of the three samples of basalt. In the age-spectrum diagram (fig. 1), the low-temperature (5000C) gas increment is significantly older than succeeding fractions. In the high-temperature increments, the age spectrum does not show older apparent ages typical of samples known to contain excess 40Ar. An alternative interpretation for the high apparent age of the 5000C fraction is that it reflects recoil of 39Ar out of the basalt following the 39K (n,p)39Ar reaction. Turner and Cadogan (1974) calculated that the recoil energy of 39Ar typically will be


- |: tp =161 3.1 - |
600*c
700c 800c 900 c975C
500'C------------------~ ~
50~ ~~0_


- I . I I


- EXPLANATION 975'C
0 Used in fit 800'C
- + Not used in fit 700C

- 1050'C +

600*C+ 9001C
- Fuse
- 500'C Age =167 -2 m.y.
-'Ar/P6Ar intercept = 281 - 28 SUMS/(N - 2) = 1.7


B5


6








STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


225
z
200 < 175 w 150 a- 125


FIGURE 5.--Age-spectrum diagram for 40Ar/39Ar incremental heating experiment on BBH#5 basalt. tp is the weighted mean plateau age.


approximately 200 kiloelectron volts (keV), which would cause a recoil distance of 0.08 ym on the average. This recoil could easily cause loss of K-derived 39Ar from finegrained alteration products in the basalt. Loss of 39Ar would result in an anomalously high 40Ar/39Ar age. Loss of 39Ar by recoil, however, is not a factor in the conventional age determination of BBH#1, 204+4.1 m.y., which is more than 10 percent higher than the preferred age of BBH#5. The latter sample, on the basis of stratigraphy, must be the oldest of the three. Therefore, BBH#1 must contain some excess 40Ar that was loosely bound and removed in the 500 C gas increment (table 1, fig. 5). The 6000 -900'C gas increments of BBH#1 have age-spectrum and isochron ages that agree with those of BBH#5. However, in the absence of the independent data provided by sample BBH#5, a reliable crystallization age cannot be inferred from the analytical data for BBH#1.
The most reliable age for the Clubhouse Crossroads basalt is the preferred age of 184 + 3.3 m.y. for sample BBH#5. Phillips (1983) estimated an age for the basalt unit as a whole from the paleoinclination of samples from all three drill holes at Clubhouse Crossroads. He compared the paleoinclination with a paleoinclinationversus-time curve produced from the pole positions of Irving (1977). From his paleoinclination data, Phillips suggested that the most probable age of the basalt is 170 m.y; probability that the age is between 110 and 196 m.y. is 95 percent. Given the uncertainty in the paleoinclination-versus-time curve (Phillips, 1983), the agreement in the age inferred from the paleomagnetic data and the K-Ar age measured on BBH#5 is good.
Most workers believe that the basaltic igneous activity in eastern North America and northwestern Africa is related to rifting and separation of the African and North American plates (Dietz and Holden, 1970; Dalrymple and others, 1975). The consensus is that this


1800 1400 1000 600


0


20 40
39Ar/36Ar


60 80


FIGURE 6.-Isochron diagram for 40Ar/39Ar incremental heating experiment on BBH#5 basalt.


separation began 180 to 200 m.y. ago. The principal evidence for this conclusion includes the age of the basaltic rocks (Dalrymple and others, 1975; Sutter and Smith, 1979), analysis of the magnetic-anomaly pattern in the North Atlantic Ocean (Pitman and Talwani, 1972; Phillips and Forsyth, 1972; Larson and Pitman, 1972), and extrapolation of the sea-floor spreading rate in the northwestern Atlantic Ocean to the Continental Rise (Geotimes, 1970).
The coincidence of paleomagnetic poles derived from mafic volcanic rocks from eastern North America (Pennsylvania to Nova Scotia) and northwestern Africa (Liberia, Sierra Leone, and Morocco) indicates that the two continents could not have been separated very far before latest Triassic to earliest Jurassic time (Larson and LaFountain, 1970; Dalrymple and others, 1975). Reliable K-Ar ages for diabase dikes in Liberia range from 177 to 196 m.y. (Dalrymple and others, 1975). K-Ar ages of basalt flows and diabase intrusive rocks in eastern North America from Massachusetts to Maryland show considerable scatter, which Armstrong and Besancon (1970) have attributed to potassium inhomogeneity and (or) loss of radiogenic 40Ar during superimposed low-grade metamorphism. For the basalt in the Clubhouse Crossroads drill holes, the alteration appears to be deuteric; no independent evidence exists for a later metamorphic event in the region.
Sutter and Smith (1979) subsequently made 40Ar/39Ar incremental heating experiments on diabase intrusive rocks from Connecticut and Maryland. Four of six samples from diabase intrusive rocks in Connecticut and two of four samples from intrusive rocks in Maryland yielded data that meet the criteria outlined for a reliable crystallization age. The four samples from Connecticut have 40Ar/36Ar versus 39Ar/36Ar isochron ages ranging


2501


I I I , I . I I , 1
50 60 70 80 90 100 CUMULATIVE PERCENT


vu
0 10 20 30 40
39Ar RELEASED. IN


500C t p=187 3.1 Fuse
___0._ __ __C 9751C
- 0 --- 0 _G


H


i I . I


EXPLANATION Fuse +
o Used in fit + Not used in fit


800'C
9001C +1050*C
700'C
600*C Age=184 3.3 m.y.
-975'C "ArI36Ar intercept= 302 t6 SUMS/(N - 2) = 1.6 500'C


B6









40Ar/39Ar AGES OF BASALT FROM CLUBHOUSE CROSSROADS TEST HOLE #2


from 182 to 192 m.y., and the two samples from Maryland have isochron ages ranging from 186 to 189 m.y.
In conclusion, the most reliable age for the Clubhouse Crossroads basalt is 184 3.3 m.y. This age is in good agreement with the reliable ages of diabase intrusive rocks farther north in eastern North America and in Liberia. All these data are consistent with the initiation of central Atlantic rifting about 190 m.y. ago.


REFERENCES CITED

Armstrong, R. L., and Besancon, James, 1970, A Triassic time scale
dilemma-K-Ar dating of Upper Triassic mafic igneous rocks, eastern U.S.A. and Canada and post-Upper Triassic plutons, western Idaho, U.S.A.: Eclogae Geologicae Helvetiae, v. 63, no. 1,
p. 15-28.
Brereton, N. R., 1972, A reappraisal of the 40Ar/39Ar stepwise degassing technique: Royal Astronomical Society Geophysical Journal, v. 27, no. 5, p. 449-478.
Brooks, C., Hart, S. R., and Wendt, I., 1972, Realistic use of twoerror regression treatments as applied to rubidium-strontium data: Reviews of Geophysics and Space Physics, v. 10, no. 2, p. 551-577. Clague, D. A., Dalrymple, G. B., and Moberly, Ralph, 1975, Petrography and K-Ar ages of dredged volcanic rocks from the western Hawaiian Ridge and the southern Emperor Seamount Chain:
Geological Society of America Bulletin, v. 86, no. 7, p. 991-998. Dallmeyer, R. D., 1975, 40Ar/39Ar release spectra of biotite and
hornblende from the Cortlandt and Rosetown plutons, New York, and their regional implications: Journal of Geology, v. 83, no. 5, p.
629-643.
Dalrymple, G. B., Alexander, E. C., Jr., Lanphere, M. A., and Kraker,
G. P., 1981, Irradiation of samples for 40Ar/39Ar dating using the Geological Survey TRIGA reactor: U.S. Geological Survey Professional Paper 1176, 55 p.
Dalrymple, G. B., and Clague, D. A., 1976, Age of the HawaiianEmperor Bend: Earth and Planetary Science Letters, v. 31, no. 3,
p. 313-329.
Dalrymple, G. B., Gromm6, C. S., and White, R. W., 1975, Potassiumargon age and paleomagnetism of diabase dikes in Liberia-Initiation of central Atlantic rifting: Geological Society of America
Bulletin, v. 86, no. 3, p. 399-411.
Dalrymple, G. B., and Lanphere, M. A., 1971, 40Ar/39Ar technique of
K-Ar dating: A comparison with the conventional technique: Earth
and Planetary Science Letters, v. 12, no. 3, p. 300-308.
1974, 40Ar/39Ar age spectra of some undisturbed terrestrial
samples: Geochimica et Cosmochimica Acta, v. 38, no. 5, p.
715-738.
Dalrymple, G. B., Lanphere, M. A., and Clague, D. A., 1980, Conventional and 40Ar/39Ar K-Ar ages of volcanic rocks from Ojin (Site 430), Nintoku (Site 432), and Suiko (Site 433) seamounts and the chronology of volcanic propagation along the Hawaiian-Emperor Chain, in California University, Scripps Institution of Oceanography, La Jolla, Initial reports of the Deep Sea Drilling Project, Volume 55: Washington, D. C., National Science Foundation, p. 659-693.
Dietz, R. S., and Holden, J. C., 1970, Reconstruction of PangaeaBreakup and dispersion of continents, Permian to present: Journal
of Geophysical Research, v. 75, no. 26, p. 4939-4956.
Geotimes, 1970, Deep Sea Drilling Project-Leg 12: v. 15, no. 9,
p. 14-16.
Gohn, G. S., Gottfried, David, Lanphere, M. A., and Higgins, B. B.,
1978, Regional implications of Triassic or Jurassic age for basalt


and sedimentary red beds in the South Carolina coastal plain:
Science, v. 202, no. 4370, p. 887-890.
Gohn, G. S., Houser, B. B., and Schneider, R. R., 1983, Geology of the
lower Mesozoic(?) sedimentary rocks in Clubhouse Crossroads test hole #3, near Charleston, South Carolina, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity: U.S. Geological Survey Professional Paper 1313, p. D1-D17.
Gottfried, David, Annell, C. S., and Byerly, G. R., 1983, Geochemistry
and tectonic significance of subsurface basalts from Charleston, South Carolina--Clubhouse Crossroads test holes #2 and #3, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity: U.S. Geological
Survey Professional Paper 1313, p. Al-A19.
Gottfried, David, Annell, C. S., and Schwarz, L. J., 1977, Geochemistry of subsurface basalt from the deep corehole (Clubhouse Crossroads corehole 1) near Charleston, South Carolina-Magma type and tectonic implications, in Rankin, D. W., ed., Studies related to the Charleston, South Carolina, earthquake of 1886 -A preliminary report: U.S. Geological Survey Professional Paper
1028, p. 91-113.
Hazel, J. E., Bybell, L. M., Christopher, R. A., Frederiksen, N. 0.,
May, F. E., McLean, D. M., Poore, R. Z., Smith, C. C., Sohl, N. F., Valentine, P. C., and Witmer, R. J., 1977, Biostratigraphy of the deep corehole (Clubhouse Crossroads corehole 1) near Charleston, South Carolina, in Rankin, D. W., ed., Studies related to the Charleston, South Carolina, earthquake of 1886 -A preliminary report: U.S. Geological Survey Professional Paper 1028, p. 71-89. Irving, Edward, 1977, Drift of the major continental blocks since
the Devonian: Nature, v. 270, no. 5635, p. 304-309.
Kaneoka, Ichiro, .1974, Investigation of excess argon in ultramafic
rocks from the Kola Peninsula by the 40Ar/39Ar method: Earth and
Planetary Science Letters, v. 22, no. 2, p. 145-156.
Lanphere, M. A., and Dalrymple, G. B., 1971, A test of the 40Ar/39Ar
age spectrum technique on some terrestrial materials: Earth and
Planetary Science Letters, v. 12, no. 4, p. 359-372.
1976, Identification of excess 40Ar by the 40Ar/39Ar age
spectrum technique: Earth and Planetary Science Letters, v. 32,
no. 2, p. 141-148.
1978, The use of 40Ar/39Ar data in evaluation of disturbed K-Ar
systems, in Zartman, R. E., ed., Short papers of the Fourth International Conference, Geochronology, Cosmochronology, and Isotope Geology: U.S. Geological Survey Open-File Report 78-701,
p. 241-243.
Larson, E. E., and LaFountain, L., 1970, Timing of the breakup of the
continents around the Atlantic as determined by paleomagnetism:
Earth and Planetary Science Letters, v. 8, no. 5, p. 341-351.
Larson, R. L., and Pitman, W. C., III, 1972, World-wide correlation of
Mesozoic magnetic anomalies, and its implications: Geological
Society of America Bulletin, v. 83, no. 12, p. 3645-3662.
Ozima, Minoru, Honda, Masahiko, and Saito, Kazuo, 1977, 40Ar/39Ar
ages of guyots in the western Pacific and discussion of their evolution: Royal Astronomical Society Geophysical Journal, v. 51, no. 2,
p. 475-485.
Ozima, Minoru, and Saito, Kazuo, 1973, 40Ar/39Ar stepwise degassing
experiments on some submarine rocks: Earth and Planetary
Science Letters, v. 20, no. 1, p. 77-87.
Phillips, J. D., 1983, Paleomagnetic investigations of the Clubhouse
Crossroads basalt, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886- Tectonics and seismicity: U.S. Geological Survey Professional Paper 1313, p.
Ci-C18.
Phillips, J. D., and Forsyth, D., 1972, Plate tectonics, paleomagnetism,
and the opening of the Atlantic: Geological Society of America
Bulletin, v. 83, no. 6, p. 1579-1600.


B7









STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


Pitman, W. C., III, and Talwani, Manik, 1972, Sea-floor spreading in
the North Atlantic: Geological Society of America Bulletin, v. 83,
no. 3, p. 619-646.
Smith, A. G., Briden, J. C., and Drewry, G. E., 1973, Phanerozoic
world maps: Special Papers in Paleontology, no. 12, p. 1-42.
Steiger, R. H., and Jager, E., 1977, Subcommission of geochronology -Convention on the use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, no.
3, p. 359-362.
Sutter, J. F., and Smith, T. E., 1979, 40Ar/39Ar ages of diabase
intrusions from Newark trend basins in Connecticut and


Maryland-Initiation of central Atlantic rifting: American Journal
of Science, v. 279, no. 7, p. 808-831.
Turner, G., 1968, The distribution of potassium and argon in chondrites, in Ahrens, L. H., ed., Origin and distribution of the
elements: New York, Pergamon Press, p. 387-398.
Turner, G., and Cadogan, P. H., 1974, Possible effects of 39Ar recoil
in 40Ar/39Ar dating, in Proceeding of the Fifth Lunar Science Conference: Geochimica et Cosmochimica Acta Supplement 5, v. 2, p.
1601-1615.
York, Derek, 1969, Least squares fitting of a straight line with correlated errors: Earth and Planetary Science Letters, v. 5, p.
320-324.


B8

















Paleomagnetic Investigations of the Clubhouse Crossroads Basalt


By JEFFREY D. PHILLIPS

STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-TECTONICS AND SEISMICITY GEOLOGICAL SURVEY PROFESSIONAL PAPER 1313-C

tT 0p


UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:1983




























CONTENTS



Page
Abstract -------------------------- --------------- - -- ---- C1
Introduction ----------____________________----------- - ---- 1
Technique -------------------------___ ____---------__-__---- 3
Thermomagnetic analysis -__-_-__---------------------- ----- 3
Measurements ___________________________________----------_---_ ----- 5
Results --------------_-_-----------------_____________________________ 10
Location of flow boundaries ----------------------------- ----- 10
Correlation of flows ---------------------------------__ _ -13
Age of the basalt ------------------------------ - -------- 14
Conclusions ____ ---------- ---_ _ _ ------- 17
References cited ---------__---_________ ---------------- 17


ILLUSTRATIONS


Page
FIGURE 1. Index map showing location of the Clubhouse Crossroads test holes ----------------------------------- C2
2. Thermomagnetic curves for four basalt samples from CC#2 and CC#3 -------------------------------- ---- 4
3. Graph showing paleoinclination and age of the basalt -------------------___---------- -------- 15
4. Time line showing age comparison with other eastern North American Mesozoic igneous rocks ------------------------- 16



TABLES



Page
TABLE 1. Paleomagnetic measurements ---------------------------------------------------------------------- C6
2. Measured physical properties -------------------------------------------------------- ------ 10
3. Means of inclination and intensity as a function of depth --- ___-__-------------------------------------- 11
4. Evidence for flow boundaries - ---- -- ------------------------ 13
5. Means of inclination and intensity as a function of flow ----------------------------------------------------- 14
III




















































































































































































































































































































s
s













STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886TECTONICS AND SEISMICITY PALEOMAGNETIC INVESTIGATIONS OF THE CLUBHOUSE CROSSROADS BASALT



By JEFFREY D. PHILLIPS


ABSTRACT
Paleomagnetic investigations have been undertaken on partially oriented basalt samples recovered from three USGS (U.S. Geological Survey) deep test holes at Clubhouse Crossroads near Charleston, S. C. The basalt unit lies at the base of the Coastal Plain sedimentary section; it is 256 m thick in test hole #3. It overlies and is partially interbedded with a sedimentary red-bed unit of probable Triassic or Early Jurassic age. On the basis of the paleomagnetic evidence and the geologic descriptions of the cores, 23 flows can be identified. Six of the flows have negative magnetic inclinations, which are interpreted as indicating periods of reversed polarity; one test hole contains a definite sequence of five reversed-polarity intervals separated by four normalpolarity intervals. The mean thermal remanent-magnetization (TRM) inclination for the 23 flows after magnetic cleaning is 35.4 3.2*. Comparison of this value with a paleoinclination curve for the Charleston area reveals that the age of the basalt has a 95-percent chance of being in the range 110-196 m.y. Comparison of the Clubhouse Crossroads basalt with other eastern North American basalts and diabases suggests that the true age is more likely to be in the older part of this range.

INTRODUCTION

Between 1975 and 1977, the U.S. Geological Survey drilled three deep test holes (CC#1, CC#2, and CC#3) near the hamlet of Clubhouse Crossroads, Dorchester County, S. C. (fig. 1) as part of a study of the epicentral area of the 1886 Charleston earthquake. All three penetrated the Coastal Plain sedimentary section, and two of them, CC#1 and CC#2, bottomed in a thick basalt unit underlying the Coastal Plain section. The third test hole, CC#3, penetrated 256 m of basalt and bottomed in a 121-m-thick sedimentary red-bed section of probable Triassic or Early Jurassic age (Gohn and others, 1978).
Paleomagnetic investigations of the basalt were undertaken with two goals in mind. First, flow boundaries within each core were to be detected paleomagnetically, and flows were to be correlated among the three holes. Such a correlation might indicate faults or other tectonic features in the area. Second, the mean inclination of the paleomagnetic samples was to be com-


pared to the paleoinclination curve for the site, as predicted from other North American paleomagnetic studies, in order to estimate the age of the basalts. This would serve as a check on the early Mesozoic radiometric ages obtained for the basalts and aid in the geochronological interpretation of the cores.
Earlier paleomagnetic studies of eastern North American Mesozoic basalts and diabases as summarized by de Boer (1968), Beck (1972), and Smith (1976) have established the following:
1. The igneous rocks associated with eastern North
American early Mesozoic basins are all normally magnetized, which suggests that they formed during the Late Triassic-Early Jurassic interval
of predominantly normal polarity.
2. Most radiometric ages for eastern North American
Mesozoic basalts and diabases fall within the range 180-200 million years (m.y.), which supports a Late Triassic-Early Jurassic age.
3. The position of the magnetic pole relative to North
America changed rapidly during the Triassic and Early Jurassic. This permits paleomagnetic studies to be used to determine relative ages of lower Mesozoic units and to correlate these units
over large areas.
4. The basalts and diabases associated with eastern
North American early Mesozoic basins are of an age similar to that of the associated sedimentary
basin-fill materials.
Acknowledgments. -W. E. Huff, Jr., provided laboratory assistance, and S. H. Perlman and D. Gottfried provided geologic descriptions of test holes 2 and 3. Reviewers were D. Watson, R. Reynolds, and R. Simpson. Deep drilling and related investigations by the U.S. Geological Survey in the Charleston, S. C., area are supported by the U.S. Nuclear Regulatory Commission, Office of Nuclear Research, under Agreement No. AT(49-25)-1000.
C1









STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


810*


Line Fall





Fayetteville

0


17A


163


CC#2

137 CC#3
Clubhouse Crossroads

CC#1







0 DORCHESTER COUNTY
CHARLESTON COUNTY 37

-1 38
0 0 1 2 KILOMETERS
0


mberton


Florence
0


32'
50'
Myrtle Beach

Qb 0
Lake
Marion
Orangeburg
0 0
AugustaB
Bowman Moultrie Georgetown




River Summerville

0ns0
Area of t-inset0 o 7 Charleston
7 07


Beaufort

0 50 100 KILOMETERS


- Savannah
Fs FIGURE 1. -Index map showing the location of the Clubhouse Crossroads test holes CC#1, CC#2, and CC#3, near Charleston, S. C.


C2


82*


35'-


32" 55'


Lu
0


340


32'


80"


79'


-


33*







PALEOMAGNETIC INVESTIGATIONS OF THE CLUBHOUSE CROSSROADS BASALT


TECHNIQUE
A total of 240 m of basalt core was available for study. In CC#1, the basalt was cored continuously from its top at 750 m depth to the bottom of the hole at 792 m, a total of 42 m. In CC#2, the basalt was cored continuously from its top at 776 m to the bottom of the hole at 907 m, a total of 131 m. In CC#3, the basalt was cored in three discontinuous sections. These extended from the top of the basalt at 775 m to 785 m (10 m), from 921 m to 930 m (9 m), and from 983 m to the base of the basalt unit at 1,031 m (48 m). The basalt core is 8 cm in diameter and is fractured into segments 3-50 cm long. Each piece of the core is oriented with respect to the vertical, but there is no azimuthal control.
Samples of the cores were gathered at depths where field descriptions of the cores indicated the possibility of a new flow, and paleomagnetic specimens were prepared. Each sample of core was drilled down its axis with a 2.5-cm-diameter core drill. The resulting 2.5-cmdiameter core was cut into two to four 2.3-cm-long cylindrical specimens. Each specimen was oriented with respect to the vertical, and usually all specimens from a single 2.5-cm-diameter core were assigned the same arbitrary azimuth.
Paleomagnetic directions and intensities of all specimens were measured on high-speed spinner magnetometers (Doell and Cox, 1965), and two magneticcleaning techniques were tried (Tarling, 1971, p. 66-69). In a preliminary study, alternating field (af) demagnetization in a two-axis tumbler with peak fields of 30, 60, and 80 mT (millitesla) was attempted on samples from CC#1 and 2. Although samples from CC#1 retained stable magnetization directions throughout the af cleaning process, all samples from CC#2 became unstable as a result of the cleaning. An alternative cleaning method, thermal demagnetization, was therefore tried. CC#1 and 2 were resampled, and the new specimens were cleaned in a magnetically shielded furnace. Thermal demagnetization at a range of temperatures from 1000 C to 5500 C resulted in greater stability for samples from CC#2 than did af demagnetization. The natural remanent magnetization (NRM) measurements presented here are for samples from both the preliminary and the subsequent studies. However, only the thermally cleaned samples are considered in defining the thermal remanent magnetization (TRM) acquired by the basalts during their initial cooling. Magnetic stability was assessed on the basis of clustering of paleomagnetic inclinations (Tarling, 1971, p. 85-86). For example, specimen NRM inclinations of a particular sample might cluster loosely. Lowtemperature demagnetization might cause the specimen inclinations to diverge somewhat, but at higher temperatures the demagnetization process should bring


the inclinations together again. The mean inclination at the demagnetization step that produces the greatest clustering of the specimen inclinations is taken as a measure of the TRM inclination of the sample. An unstable sample is one for which the specimen inclinations fail to converge during the demagnetization process.

THERMOMAGNETIC ANALYSIS

Prior to an extensive program of thermal cleaning and remanence measurement, thermomagnetic curves for several specimens were produced on a Curie balance (Doell and Cox, 1967). These curves helped in designing the thermal cleaning process and in identifying the magnetic minerals present in the samples through their Curie temperatures.
Figure 2 shows thermomagnetic curves (saturation magnetization against temperature) for four different basalt specimens. Curie points and transition temperatures are indicated by inflections in the curves. The heating and cooling were done in air at a slow rate (typically just under 1 hour for an entire heating and cooling cycle) to insure that the sample was always close to thermal equilibrium. A strong magnetic field (- 180 mT) was applied to the sample during the entire cycle. The cooling curves do not reproduce the heating curves, which indicates that some of the magnetic minerals were altered during laboratory heating in air. Although the magnetic minerals have not been examined in polished section, we can make inferences about the magnetic mineralogy from the thermomagnetic curves.
The dominant magnetic mineral in fresh basalts is titanomagnetite, a solid-solution-series mineral having a composition intermediate between that of ulv6spinel (FesTiO4) and that of magnetite (Fe3O4). The Curie temperature of titanomagnetite ranges from -1530 C for ulv6spinel to 5800 C for magnetite (Akimoto and others, 1957). Initial titanomagnetite compositions in basalts must contain more than 50 mole percent ulv6spinel (Carmichael and Nicholls, 1967; Merrill, 1975). However, oxidation can reduce the proportion of ulv6spinel. At high temperatures (250-700' C), such as might exist during the initial cooling of the basalt, titanomagnetite can undergo oxidation to form magnetite and ilmenite (FeTiO3) lamellae. The product will have a Curie temperature between 5000 C and 5800 C and a strong TRM recording the local direction of the earth's magnetic field at the time of cooling (Gromm6 and others, 1969).
The specimens used to produce the curves in figures 2B, C, and D have Curie points at around 5800 C, suggesting that the magnetic remanence in these specimens is carried by nearly pure magnetite. This is most likely


C3








STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


CC#3 779 m










Tc=610*C
0 100 200 300 400 500 600
A




CC#3
922 m


= 310*C








T =585*C
0 100 200 300 400 500 600
C


CC#2
859.5 m










Tc=582*C
| - I


0 100 200 300 400 500 600
B


CC
1 00d











= 580'C


0 100 200 300 400 500 600
D


TEMPERATURE. IN DEGREES CELSIUS (*C)
FIGURE 2. -Thermomagnetic curves for four basalt specimens from CC#2 (859.5 m depth) and CC#3 (779, 922,
and 1,004 m depth). Curie points (T,) estimated from the intersections of tangent lines with one another are
indicated. Arrows on curves indicate which is the heating curve and which is the cooling curve.


the primary magnetic mineral produced during the initial cooling of the Clubhouse Crossroads basalt. It should be a reliable recorder of the paleomagnetic field directions at the time of cooling.
Magnetite can also form at times other than during the initial cooling of a basalt. For example, it can form through high-temperature oxidation of titanomagnetite during a reheating event, such as the emplacement of a nearby intrusion. In this case the basalt would tend to be uniformly remagnetized in the direction of the earth's


field at the time of reheating. As will be seen later, the TRM directions of the Clubhouse Crossroads basalts are far from uniform. Thus a reheating event is an unlikely explanation for the 5800 C Curie temperatures.
At low temperatures (50-135* C) and under oxidizing conditions, an original titanomagnetite in a basalt will oxidize to the metastable iron oxide titanomaghemite. At slightly higher temperatures (1350 C to the Curie point of the titanomagnetite), this oxidation is accompanied by unmixing of the titanomaghemite into mag-


z
0

w zj CC


#3
4m


C4







PALEOMAGNETIC INVESTIGATIONS OF THE CLUBHOUSE CROSSROADS BASALT


netite, hematite-ilmenite, and rutile (Johnson and Merrill, 1973). Magnetite formed in this way will contain a strong chemical remanent magnetization (CRM), which could easily be mistaken for a stable TRM. Although it is difficult to rule out CRM entirely as a source of the stable remanence in the Clubhouse Crossroads basalts, two factors suggest that most of the stable remanence is TRM. First, the present temperature near the top of the basalt is about 40* C (Sass and Ziagos, 1977). Barring much deeper burial, much higher radiogenic heating, or a local reheating event, it is unlikely that the Clubhouse Crossroads basalt has experienced temperatures in excess of 135* C since its initial cooling. Second, there is no evidence in the thermomagnetic curves for the presence of residual titanomagnetite or titanomaghemite. If titanomaghemite were present in the samples, the cooling curves would probably lie above the heating curves (Ozima and Larson, 1970), whereas just the opposite behavior is observed in figure 2.
The curves of figures 2A and C contain inflections near 3100 C. These inflections probably result from the decomposition of maghemite into hematite. Maghemite (yFe203) is a low-temperature (50-250' C) oxidation product of magnetite (Johnson and Merrill, 1972; Stacey and Banerjee, 1974, p. 30). Depending upon the grain size of the magnetite, the chemical remanence acquired during maghemitization can be parallel either to the original TRM or to the ambient field at the time of oxidation (Johnson and Merrill, 1972, 1974). Fortunately, in most cases, thermal cleaning in a field-free space at temperatures above 3500 C can be used to reduce the effect of the CRM and to recover the original TRM directions. Thus the presence of maghemite partially explains the need for thermal cleaning of the samples. The presence of a third magnetic mineral is indicated in the thermomagnetic curve of figure 2A. The Curie point of 6100 C is too high for magnetite; most likely it is due to an ilmeno-hematite of composition Fe.92T.osO3 (Stacey and Banerjee, 1974, p. 37). The presence of titanium suggests that the ilmeno-hematite, like the magnetite, is a primary magnetic mineral. [If the titanium were absent, the resulting hematite could be explained as a byproduct of maghemitization (Johnson and Merrill, 1972).] At this composition, the ilmeno-hematite should be weakly magnetic and may be a carrier of primary remanence.
It should be noted that minerals of the hematiteilmenite solid-solution series are the best documented self-reversing minerals (Stacey and Banerjee, 1974, p. 166). Thus the presence of ilmeno-hematite in one sample of the Clubhouse Crossroads basalt raises the remote possibility of stable self-reversals. However, at this particular composition, ilmeno-hematite is not known to


self-reverse, and in general self-reversals are rarely observed in subaerial basalts (Merrill, 1975). Thermomagnetic analysis has indicated that three magnetic minerals are present in the Clubhouse Crossroads basalts. Magnetite is probably the carrier of primary remanence. Maghemite is present as a lowtemperature oxidation product of the magnetite. Ilmeno-hematite, perhaps rarely present, is another probable carrier of primary remanence. The magnetic mineralogy is consistent with the hypothesis of a subaerial basalt that cooled in a highly oxidizing environment and has not been reheated above 1350 C since its initial cooling.
The thermomagnetic curves were considered in the design of the thermal demagnetization experiment. Owing to the apparent range of magnetic minerals present in the samples, cleaning at a single temperature or within a small range of temperatures was deemed inappropriate. Thorough cleaning in small temperature steps, although desirable, was impractical because of the large number of samples. Therefore a procedure was chosen in which large temperature steps were used in cleaning all samples. In addition to the NRM measurements, measurements were taken after heating in air and in zero magnetic field to 100 or 1500 C, 3000 C, 5000, and 5500 C. At low temperatures the samples were heated rapidly and allowed to equilibrate at the desired temperature for 10 minutes prior to cooling and measurement. At higher temperatures, the heating rate was slowed to prevent the samples from exploding from expansion of trapped moisture. Despite these precautions, some samples were destroyed during heating.

MEASUREMENTS
The complete set of paleomagnetic measurements is presented in table 1. Because the azimuth of each specimen is arbitrary, the declinations in this table are unrelated to geomagnetic field directions. Only the changes in declination seen during demagnetization may have some significance. Although every precaution was taken to ensure that the specimens were correctly oriented with respect to the vertical, inconsistencies in the table suggest that one specimen at 858.3 m depth in CC#2 was inverted. In the following analysis the signs of the inclinations have been changed for this specimen. In addition to measurements made on basalt specimens, table 1 contains measurements made on red-bed specimens from two depths near the bottom of CC#3. During the course of the study, the density and magnetic susceptibility of 60 randomly selected specimens were measured. These results (table 2) are useful for estimating the contribution of the basalt to the gravity and magnetic fields at the ground surface (Daniels and others, 1983).


C5











C6 STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886

TABLE 1.-Paleomagnetic measurements-natural remanent magnetization (NRM) and remanent magnetization after thermal cleaning at various temperatures
[Inc, inclination; Dec, declination; J, intensity (in amperelmeter). Each sample contained several specimens] Depth of sample
below surface NRM 150*C 3000C 500*C 550*C
(meters) Inc Dec J Inc Dee J Inc Dec J Inc Dee J Inc Dec J

CC#1

756.5 54.2 318.6 3.19x 10-'

760.5 __- ________-_- 41.1 23.7 1.36 42.2 16.0 1.05

762.9 ___.____-__-__- 34.9 355.0 1.82 39.3 359.0 1.57 36.3 358.8 1.49 36.7 356.8 1.35 36.0 356.1 9.85 x 10-'
50.2 94.3 6.50x10-' 39.4 92.3 6.97x10-1 38.4 90.4 7.03x10-'

766.9 ________- 48.7 188.9 6.50 x 10-'
49.1 124.2 2.31 x 10- 49.1 118.3 1.79 x 10- 48.0 115.7 1.78 x 10- 47.8 118.4 1.63x 10'
37.7 110.8 1.75x10- 37.0 122.3 1.55x10- 35.8 123.8 1.70x10-' 38.8 124.7 1.59x10770.2 39.9 156.3 1.02
42.3 144.9 9.22x10- 38.3 147.0 8.71x10- 36.2 147.8 8.70x10772.4 ______-_-41.8 339.7 4.24
40.6 344.4 4.07 36.3 340.7 3.99 35.7 345.1 3.51 36.3 342.5 2.58

780.0 ______________- 37.2 268.5 8.33 37.8 304.8 7.69 35.7 305.2 7.23 35.5 302.6 7.22 34.4 310.5 5.07
68.5 105.9 1.78 x 10- 67.1 86.0 1.16 x 10- 71.2 74.5 1.29 x 10-' 49.3 82.4 1.06 x 10787.3 _______- __- 49.8 327.1 6.80 x 10' 60.1 18.4 1.13 x 10' 34.6 9.5 5.97 x 10-2 31.0 350.2 4.42 x 10- 29.7 331.7 3.52 x 10789.4 36.1 146.3 2.82
36.8 149.5 3.02 35.0 149.2 2.94 33.7 144.4 2.85 33.4 145.0 1.87

790.7 ___- __________- 58.5 102.8 1.02 56.5 91.7 1.33 48.1 90.2 9.29x10' 40.7 85.0 8.39x10' 38.4 89.3 5.09x10CC#2

776.0 -__________-32.4 300.6 9.64x10-1 -42.4 296.0 9.00x10-' -45.8 292.3 6.37x10' -48.7 291.1 3.33x10' -47.1 300.8 2.58x10-


-15.0 281.4 1.38 -18.4 284.1 1.10

_______ 28.6 288.1 1.60 x 10-'
30.9 321.3 2.29 x 10-'
-19.7 324.8 2.41 x 10-'
2.1 305.6 3.14 x 10' 77.6 159.4 2.01 x 10'


778.2 _------ 15.2 185.2 1.05 x 10-'
56.2 185.7 1.61 x 10-' 70.3 189.5 8.06 x 10'2

779.1 _______- 5.9 275.9 2.62 x 10-'
3.5 253.4 2.19 x 10-' 49.1 344.2 4.25 x 10-'

779.7 _______- 21.3 27.3 7.36 x 1020.0 42.8 7.96 x 10-' 20.9 71.0 7.85x 10782.1 19.6 168.6 7.66x 10'
44.5 174.5 8.03x1044.1 204.7 9.51x10783.3 -13.4 113.3 6.95x10-3
9.0 99.1 4.58x10-3 16.2 89.1 6.23x10-3

785.1 _____- _-50.2 94.4 2.73x10-3
-80.8 124.4 1.83x10'
-54.9 82.0 5.57x10-'

785.5 __- ___- _-___- 5.7 71.3 4.70x1066.8 249.5 4.70 x 10' 40.8 265.4 6.60x10-


55.8 117.0 4.37 x 10-' 57.2 115.6 2.28 x 10'3 44.0 114.6 3.28x10'1


- 3.7 131.7 2.14x10-2 17.1 116.1 1.07x10' 34.3 131.7 5.23x 10'2 15.8 114.0 2.62x10' 15.8 117.7 1.97 x 1047.6 117.3 3.98x 10' 67.7 171.9 4.62 x 1071.6 200.7 6.04 x10' 62.3 168.0 7.61x10'

-21.1 85.6 1.63
-20.4 106.2 8.74x 10'
-13.4 122.8 7.02x10' 37.5 82.3 5.44 x 10'


-22.4 284.2 1.28 -27.3 284.4 6.42x10-'
6.0 242.5 5.17 x 10' -13.2 266.5 2.55 x 10- - - 23.3 280.8 1.12x10- - - 26.5 291.3 1.13 x 10-1
- - - -15.7 321.0 1.72 x 10-1
- - - -14.8 322.3 1.76 x 10-'


-10.4 166.9 1.68x10- -21.5 160.9 1.52x 10-' 23.3 192.4 1.02 x 10- 1.6 198.4 7.25 x 10-' 44.2 180.0 4.48 x 10 25.6 184.2 2.18 x 10-'

-11.6 284.7 1.38x10- 11.9 307.0 5.67x 10-'
- 6.0 290.3 8.14 x 10-2 37.2 315.6 4.92 x 10-' 24.2 9.3 2.37 x 10- 31.0 15.3 1.19 x 10-'


-31.3 288.6 2.56x10- -23.5 292.4 1.83x10-16.1 274.2 8.95 x 10- -38.0 285.1 6.36 x 1046.7 308.0 3.58x102 31.3 316.0 3.07x10-2 51.5 300.5 3.97 x 102 23.7 300.4 3.25 x 10-16.9 333.0 5.39 x 10- -31.2 319.8 2.82 x 10-18.2 338.4 6.95 x 10- - 9.0 343.1 3.46 x 10-26.3 158.9 7.51 x 10-'
-13.0 203.7 3.56 x 10-2 24.7 182.1 8.70 x 10-'

14.1 320.4 2.76x 10-2 36.6 324.7 2.49 x 10-' 28.2 18.6 5.20 x 10-'


-30.4 115.7 6.09x10' -46.9 116.2 5.04x10' -56.6 114.5 3.03x10-' -53.9 127.7 3.25x103
-11.4 104.8 3.39x10' -25.4 108.6 2.38x10-' -42.3 108.7 1.06x10-3 -38.5 103.7 1.23x103
- 9.9 94.4 4.14x10-' -25.3 96.5 2.97x10' -61.6 92.3 1.69x10- -53.8 110.5 2.58x10'

5.8 127.7 1.76x10- -18.3 133.0 1.41x10' -46.5 143.7 1.25x10-30.0 141.1 4.11x10' -37.4 145.3 4.10xl10 -48.4 144.1 3.58x10-3
-29.3 93.1 3.74x10-3 -43.6 97.0 3.64x10- -56.4 100.2 2.20x103


49.4 103.5 1.76 x 10' 23.3 46.8 4.40 x 104 -43.4 67.7 3.89 x 104 45.3 108.4 1.00 x 10-3 36.5 99.5 4.05 x 10-' -38.0 88.4 2.39 x 104
24.6 100.1 1.40x10-' 12.7 96.3 7.13x10-' -44.4 42.8 6.09x10'

-29.6 128.6 1.76x10' -44.5 129.7 1.64x10-1 -48.8 128.6 1.27x103.0 123.3 5.93x10' -26.4 133.5 5.06 x10-2 -42.7 129.4 3.81Xl10
1.6 138.8 4.2lxl102 -15.5 136.5 3.14x10- -27.4 131.9 2.08x10-


777.8


786.3


789.4 _ _ _ _ _ _ _790.7 794.3











PALEOMAGNETIC INVESTIGATIONS OF THE CLUBHOUSE CROSSROADS BASALT


C7


TABLE 1.-Paleomagnetic measurements-natural remanent magnetization (NRM) and remanent magnetization after thermal cleaning at various temperatures -Continued
[Inc, inclination; Dec, declination; J, intensity (in ampere/meter). Each sample contained several specimens] Depth of sample
below surface NRM 150*C 300*C 500C 550'C
(meters) Inc Dec J Inc Dec J Inc Dec J Inc Dec J Inc Dec J

CC#2-Continued


795.5 _-- _- _- __ -32.9 213.8
-41.2 10.0
-30.7 210.8


1.69 2.69 1.68


-45.0 297.0 2.03
-46.9 294.9 2.56
-42.7 27.5 2.39


-43.5 209.3 1.81
-47.3 6.3 2.70
-42.6 208.3 1.74

-48.3 300.2 2.42
-49.4 296.8 2.63
-47.0 30.2 2.50


-47.5 207.3 1.52
-49.9 4.0 2.19
-46.7 205.9 1.36


-46.1 207.6 1.03
-51.8 5.2 1.54
-44.8 206.6 9.22x10'


-51.3 301.1 2.27 -51.2 302.4 1.52
-49.9 34.0 2.25 -50.0 33.8 1.65


801.0 ----------------21.0 96.2
- 1.8 114.5 7.1 122.1 24.7 126.2


2.04 1.22 1.04
9.73 x 10'


801.6 -------------- 37.7 68.4 9.88x 10' 15.8 56.3 7.69 x10' 3.8 52.9 6.75x 10' 4.2 53.2 4.22x 10'
14.3 75.6 1.30 - 0.5 65.3 1.10 - 4.9 61.9 8.59x10' - 4.4 59.5 5.59x10'
26.1 51.9 1.10 3.5 48.1 9.42x10-' - 8.9 56.3 6.55.10-1 - 8.9 55.1 4.17x10-


802.5 _ _---- -40.9 35.7
-53.5 24.4
-37.5 113.0 30.6 293.6
-16.9 323.2

802.8 ----------------22.7 63.3
-28.5 64.1
-43.3 138.6 804.6- ___ ____ ____- 6.7 318.5
- 6.1 144.0
- 5.3 301.2


1.83
8.87 x 10' 8.85 x 10' 5.58 x 10' 9.83 x 10'

1.53
1.57
1.94
9.84 x 10' 8.77x 10'1
1.50


-32.2 60.6
-36.4 65.0
-47.7 137.1
-26.5 317.6
-26.8 149.6
-24.5 308.4


1.62 1.65
2.01
9.54 x 10' 8.23x 10'
1.14


-41.6 64.4
-49.9 137.1
-38.8 317.6
-38.0 155.7
-35.3 310.1


1.52
1.97
8.16 x10' 6.98x 10' 8.44x 10'


-41.8 299.5
-39.7 61.1
-51.0 137.1
-42.2 318.4
-39.9 156.2
-36.7 310.5


6.56 x 10' 8.46x101.16
4.68x 10' 4.06 x 10' 5.54x 10'


4.6 111.8 1.15
- 6.3 122.1 1.66
- 4.0 36.5 1.58


-43.9 154.5
-46.1 165.1
-38.4 161.9
-38.1 156.0


814.4 ___-- _-29.6 196.8
49.7 263.1 21.3 210.7 36.5 203.4

819.0 9.3 139.8
21.9 171.7 32.1 193.5 39.0 192.3 48.7 48.9 42.1 283.6


2.29 2.98 2.18 2.55

3.72x 10' 2.95 x 10' 6.73x 10'
1.36

1.09 1.06
6.83x 10'
1.11
6.64x 10' 6.60 x 10-1


- 9.4 96.0 6.67x 10'
-15.2 114.1 7.88x 10'
-18.0 22.6 8.96 x 10'


-43.0 153.4
-51.9 158.7
-36.6 160.5
-39.9 153.2


3.24 3.05 2.23 2.61


-23.2 98.1 4.09x 10' -26.7 85.1 2.34x 10'
-26.6 10.0 5.22x10' -30.1 1.4 3.18x10'


-49.3 160.2
-51.9 168.1
-47.7 166.3
-46.7 157.9


2.29 2.33 1.27 1.81


-48.1 163.5
-52.5 168.7
-49.7 168.4
-43.4 161.2


1.15 -49.1 163.5 9.73 x10'
1.38 -53.0 169.1 1.17
7.00x10' -50.8 168.6 5.98x10' 9.25 x10' -44.1 160.2 7.79 x10'


32.6 18.5 1.53 40.2 1.4 1.18 34.9 23.0 1.76


._ _ _ _ _ _- 45.5 73.6
22.5 222.4 27.4 230.5
- 4.1 84.7 - 3.4 83.1 - 2.7 98.2


'825.7 ____


- 9.8 242.1
-11.1 239.0
-12.2 284.4
-10.8 280.0


828.8 -.__ _ _ - 1.7 235.1
2.4 243.8
- 3.9 146.3


2.03 2.15 2.69 2.56
3.64 3.68

6.00
4.81 5.54 3.59

4.90 4.62 4.61


32.9 26.6 1.29 43.1 16.7 1.00 33.2 21.6 1.44

38.9 284.0 2.09 17.9 234.8 2.13 18.7 235.5 2.67


-10.3 245.1
- 8.9 245.1
-14.7 280.2
-13.1 282.7


5.41 5.19 5.59
5.45


17.8 30.2 6.66 x10' 22.8 26.5 5.95 x 10' 20.7 27.6 7.54 x 10'

36.1 286.5 1.64 14.7 226.0 1.74 18.0 226.8 1.87


-13.5 244.9
-11.2 244.2
-17.3 281.8
-15.4 282.2


4.52 4.46
4.88
4.64


19.4 27.4 2.75 x 10' 39.7 14.0 1.30 x 10'


35.1 289.2 3.90x 1019.2 231.9 2.44 x 10' 16.8 234.9 2.13x 10'


31.9 240.8 75.9 130.8 30.2 54.6 8.9 66.7


1.67x 10' 1.03 x 10' 1.43 x 10' 1.79 x 10'


- 8.3 132.4 4.64x 10'
- 3.4 72.0 8.92 x10-2
-10.2 256.4 7.47x 10' 24.1 241.7 9.13x10'


835.8 ____


- 6.1 139.9 6.25
- 9.8 138.8 5.23
2.1 298.7 5.47


838.8 -39.1 103.2
-41.0 108.8
-42.7 326.4 840.9 0.2 170.7
- 2.5 170.1
-10.2 172.9
-19.8 177.9

842.8 .___ _ _ _ _ 25.1 280.6 30.6 277.1 28.2 276.2


3.97 3.92
4.32 4.90 4.70 4.68 4.46

4.49 4.14 4.10


- 7.0 141.9 5.94
-10.1 140.0 4.96
- 2.9 296.6 5.27

-41.2 106.1 4.07
-43.1 111.4 4.02
-44.6 327.4 4.42


- 6.2 140.2 4.71
- 9.1 141.1 4.23
- 3.8 296.3 4.51

-42.3 107.1 3.78
-43.7 111.8 3.70
-46.0 324.7 3.86


2.1 144.8 3.31x 10'
-23.0 188.7 3.60x 10-2
-12.7 221.3 4.31x10-65.5 94.9 2.40x10-68.6 92.4 1.60x10'
-66.3 346.3 1.36 x10'


798.0 _ _ _-


808.9 '813.8


1819.3 823.0








STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


TABLE .-Paleomagnetic measurements-natural remanent magnetization (NRM) and remanent magnetization after thermal cleaning at various temperatures - Continued
[Inc, inclination; Dec, declination; J, intensity (in ampere/meter). Each sample contained several specimens] Depth of sample
below surface NRM 150*C 300*C 500*C 550*C
(meters) Inc Dec J Inc Dec J Inc Dec J Inc Dec J Inc Dec J

CC#2-Continued


846.7 ___ 39.0 44.7
45.5 23.2 45.3 304.1


853.7


_ 34.9 9.0
35.4 1.2 32.8 343.2 49.9 308.9 67.1 323.2 57.8 263.3


2.51 2.40 1.78

1.76
1.94 1.62 2.15 1.53 1.95


32.8 41.4 1.85 38.4 27.2 1.70 38.3 315.2 1.53




41.0 311.2 1.51 50.6 320.4 1.10 45.5 264.6 1.42


27.8 36.3 9.83x10- 23.2 36.7 2.94x10' 19.8 44.9 4.10xl10 33.1 26.6 1.07 28.3 31.6 2.08x10- 19.7 40.9 2.25xl1032.2 319.3 1.10 23.8 336.2 1.17xl10 19.1 284.7 9.30x 10-'




31.7 315.3 1.01 27.3 320.7 2.74x101 - 3.8 320.4 2.13xl10
36.1 317.6 7.65xl10 30.3 320.9 2.00x10' -74.7 250.6 1.17x10 29.6 262.5 7.72x10' 25.8 263.5 2.41x10-' -73.6 339.3 1.46x10-'


858.3 11.5 298.0
20.2 267.8 18.5 245.2 '-15.4 23.6

'859.5 55.0 116.3
49.4 108.1 39.7 352.0 44.2 352.8


863.8


79.2 91.1 63.6 31.4 62.0 4.8 68.2 304.9
-56.7 217.1
-60.5 178.2
-76.0 104.5 72.6 150.6

71.6 42.8 51.8 10.6 48.9 8.0


869.0


'869.3 - 59.9 15.0
58.9 359.7 38.0 357.9 36.3 3.0


1.28
1.69
2.58
1.87

2.10
2.29
2.16
2.52

9.05 x 10-' 9.90 x 10' 8.69 x 10'
1.03
1.46 x10' 1.42 x 10'
1.03
7.12 x 105.71 x10' 6.47x 10' 5.30 x 107.03 x 10' 7.19 x 10' 7.74 x 10' 7.87x 10'


-20.4 27.1 1.81 -26.7 26.8 1.44 -28.3 26.3 4.71x10-


48.5 123.5 43.6 115.4 43.2 350.5 42.2 355.6





-43.4 188.1
- 0.7 210.8 81.1 223.0 72.7 177.9


64.4 359.8 60.3 358.8 45.3 3.9


2.10 2.16 1.95 1.89





2.59 x 107.23 x 10-1 8.07 x 10-' 8.03x10'a


6.44 x 10'a 6.39 x 10-' 5.81 x 10'


42.8 146.8 31.9 122.8 29.3 352.6 29.9 355.4





-17.0 242.3
4.1 220.2 67.7 166.5 57.7 150.4


38.3 345.0 34.9 344.7 37.9 21.5 32.2 19.6


1.08 1.33 1.10
1.21





2.70 x 10-a 8.13x10-a 4.34x 10-a 5.07 x 10'


2.79 x 10' 2.93 x 10' 3.38 x 103.39 x 10'


25.9 132.5 25.4 125.2 23.0 356.2 20.2 352.3





50.4 93.8 56.4 176.6 53.3 144.9 49.5 148.6


17.5 339.4 11.9 347.7 23.0 29.6 25.6 35.3


2.97 x 10a 3.42 x 10a 2.60x10-a 1.82x10-a


7.62 x 10-a 5.85 x 10-' 2.26x 10 2.83 x 10'


1.08 x 10' 1.26 x 10'a 1.57 x 10-a 1.14x 10-'


-59.9 337.6 2.48x1038.1 282.8 4.2lxl10 48.9 8.0 5.30 x 10-


'873.3 50.3 235.6
36.5 245.2


878.1


1.55x10' 29.3 238.7 2.00x 10' 17.2 238.9 1.13xl0 9.0 238.0 2.86x10' 12.7 232.2 2.12x102.59x10- - - - 10.6 249.1 1.25xl10 14.1 241.7 3.17x10- -18.5 242.0 2.9x10'
- 22.4 241.4 2.16xl10- 7.6 239.9 1.33x10-2 -15.7 245.5 2.79x10-3 -25.9 83.6 1.03x103


-20.9 19.8 3.22x10' 42.2 344.9 3.46 x 10' 53.7 46.9 4.31x10-


883.9 -24.2 127.3
-24.9 102.5
-19.4 118.0


1.20 1.27 1.53


-37.8 144.1 1.04
-38.9 117.7 1.05
-32.6 132.8 1.24


-45.2 146.5 6.29x10' -48.4 147.2 3.55x10-46.3 121.7 7.74x10' -49.2 122.4 4.84x10-39.1 135.2 8.61x101 -43.7 137.6 4.65x10-


__ -40.4 55.2 2.96
-38.7 47.8 2.78 -34.3 52.3 2.58


887.6 -44.7 20.4
-47.6 12.0
-48.4 71.9

888.8 - 8.9 215.8
51.4 247.3 52.1 249.8


2.58 2.39
2.04

1.36 x 10-' 3.21 x 10' 2.48 x 10-


__-_ 46.2 205.6 1.14x10' -17.3 326.0 9.88x10- -28.1 334.0 6.19x10-2 -38.3 331.3 3.68x1030.7 267.7 2.38 x 10' -31.1 .3 1.88x 10' -42.2 11.5 1.61x 10-' -53.6 15.8 8.75x 10-2

18.5 116.8 1.82x10-' 24.1 119.9 7.05x10-a 29.0 121.4 2.98x10-2 38.4 116.9 1.39x10'
24.5 120.6 1.78x10' 30.8 121.7 7.22x10-2 36.3 122.9 3.08x10-2 40.1 123.7 1.39x10-


896.1 -17.1 10.9
-13.8 86.9
-18.3 76.1

897.3 -34.7 355.6
-42.7 358.5 7.1 39.2 82.3 124.9 71.4 80.5

902.8 36.0 51.6
77.3 140.6 75.8 115.0


6.36x 10-a 5.07x 10'
6.94

6.75 x 10'
1.11
3.37 x 10' 2.62 x 10' 1.78x10'

1.29
7.21 x 10' 7.71 x 10'


-57.9 286.6 3.77x10- -55.1 253.3 3.17x10- -53.8 244.8 2.2xl10'
-68.7 337.9 2.64x10-' -55.9 305.5 2.19x10-a -54.4 297.0 1.61x10-65.4 354.6 3.4xl10-' -58.8 310.6 2.39x10-' -58.3 300.3 1.55x10'


C8


50.0 128.7 27.2 99.9 16.1 46.6 41.5 48.0


7.06 x 10-a 9.37 x 107.33 x 103.44 x 10-2


870.5


17.8 322.1 9.5 344.4 23.7 31.8 10.5 29.4


6.08 x 10-a 6.93 x 10' 1.13 x 108.48 x 10-a


885.4


890.0


894.0









PALEOMAGNETIC INVESTIGATIONS OF THE CLUBHOUSE CROSSROADS BASALT


C9


TABLE 1.-Paleomagnetic measurements -natural remanent magnetization (NRM) and remanent magnetization after thermal cleaning at various temperatures -Continued [Inc, inclination; Dec, declination; J, intensity (in anperemeter). Each sample contained several specimens] Depth of sample
below surface NRM 150*C 300*C 500*C 550'C
(meters) Inc Dec J Inc Dec J Inc Dec J Inc Dec J Ine Dec J

CC#2-Continued

904.3 -_----- 5.5 20.6 2.26
10.0 10.9 3.06
5.6 0.1 3.04
34.8 93.7 2.01 17.7 92.5 1.53 1.4 96.2 1.16 -23.1 102.1 1.64x1014.5 15.4 2.04 3.3 13.2 1.78 -13.5 11.8 1.46 -49.7 9.3 1.89x1'10.9 131.0 2.20 6.8 133.1 2.04 - 7.6 137.4 1.49 -15.9 152.0 1.47x10'

905.3 -------------- 2.5 145.5 4.19
51.5 62.3 1.58

CC#3


777 --- 61.2
84.3 51.1 61.8

779 -----------_-_ - - 11.9
-17.3
-32.5

'781 __- _ _ _ _ _ _ 67.6 68.5
64.0

782 ---------------- 24.5
33.5 31.2

784 _ _ _ _ _____- 21.9
53.2

921 ---------------- 65.8
71.3

922 60.8
46.0 70.1 21.6
2.1 '60.2


134.8 151.9 59.5 88.8

159.8
140.0 146.7

312.2
341.9 338.3

73.0
82.4 92.8

91.1 89.1

51.1
54.8

48.9 57.9 .3
225.5 252.7
353.4


'927 -__-___ _--_ 49.2 284.4 52.9 277.9 48.2 282.2

928 .__ _ _ _ _ _ _ 46.9 226.6 52.5 229.1

984 __.__ --_ _ _ _ 19.9 275.7 15.7 281.1

'987 ____- _ _ _ _ 67.5 292.0 59.0 306.4 59.9 304.3

'989 64.7 20.9
69.9 356.0 54.0 18.8

'994 __________ 38.1 227.0
67.4 213.4 62.2 220.9 51.6 225.6

'1004 _- _ _ _ _ __ 47.1 324.3 47.9 349.2 63.4 92.3 60.7 108.0

'1012 __.___ _ _ _ _ 78.6 292.9 79.1 257.6 84.8 183.6

1014 75.3 203.4
66.5 235.9 65.5 218.2

1024 .__ -_ _ -__ 14.3 270.3 30.4 272.3 45.2 56.7 52.6 57.1

1031 ____ _ _ _ _ _ 38.1 152.1 35.9 152.2 22.4 178.9

31047 _ 17.2 157.0
14.8 163.7

'1146 ___- __ 77.7 3.4 75.4 93.3 71.3 13.6


6.53x 10-' 9.53 x 10'
1.41
1.02

8.59x 101.19
2.72

1.06
7.73x 10-' 7.13x 10'

1.33
1.67
1.35

2.64x 10' 1.41xl10-'

5.33x 10' 5.85x 10'

6.16x 10' 3.25 x 10' 4.39 x 10' 1.93 x 10'
2.85x10' 4.94 x 10'

3.06 x 10'1 3.58x10'1 2.33x 10'

2.34 x 10' 2.89 x 10'

5.24
6.58

1.03
8.99 x 10'1 9.31 x 10'

9.13x 10' 7.11x101.10

7.07 x 10'1 8.76 x 10'1 8.20 x 10'
1.03

3.80 x 10'1 5.18 x 10'1
1.13
9.01 x 10'

8.99 x 10'1 9.05 x 10' 9.81x10-'

1.08
1.03
7.56x 10'

2.92 x 10' 3.55 x 10' 5.89 x 10' 6.15 x 10'

4.25x 10'1 4.68 x 10' 4.82 x 10'

2.72 x 10' 1.68x10'

3.95 x 10-3 3.13 x 10-' 3.25 x 10-'


51.4 104.5 3.21xl10 38.9 105.8 50.3 110.6 6.00 x 10-' 41.2 110.0 44.2 64.0 8.46 x 10-' 34.5 60.9


-17.3 152.6
-24.4 143.2
-35.7 148.4

66.3 306.4 65.9 334.9 61.5 333.7

20.3 72.5 20.1 81.8 17.5 90.9

13.2 277.3 25.9 279.0




55.0 57.8 36.4 69.9



77.5 100.1

50.0 283.5 51.3 277.3 46.9 283.0

45.6 228.4 48.8 227.6

12.2 276.8 10.5 277.9

66.7 271.3 64.7 283.0 66.6 265.6

72.6 11.1 76.0 265.7 63.3 3.0

35.6 224.0 58.5 216.9 50.1 213.7 51.5 208.7

79.3 314.4

59.0 105.4 57.0 111.8

70.8 230.7 68.2 229.0 65.5 201.0

65.5 212.2 63.4 228.4 57.6 223.0



29.9 56.2 35.3 57.3

27.1 168.7 23.4 157.6 20.0 186.5

9.8 159.6 8.6 165.0

71.3 32.2 69.1 50.2 59.2 356.8


9.56 x 10'
1.27
2.75

9.24 x 10' 6.68x10'1 6.26 x 10'

1.21
1.41
1.08

1.47 x 10'1 1.13x 10'1




3.63x 10' 2.32 x 10'



5.19x 10'1

2.73x 10' 3.29x 10' 2.15x 10'

2.19x 10' 2.78 x 10'

5.55
6.85

9.89 x 10-' 8.33x 10'
1.04

8.28x 10' 7.37x 10' 9.06 x 10'

6.04 x 10' 7.64 x 10' 8.96 x 10'
1.01

6.15x10'

8.95 x 10' 7.78 x 10'

8.03 x 10' 8.00 x 10' 8.51 x 10'1

7.56x 10' 7.48x 10' 7.33 x 10'



3.91 x 10'1 3.52x 10'

3.91 x 10' 3.63 x 10' 3.68x 10'

2.72 x 10'1 1.83x 10'

2.70 x 10-3 2.33x 10'3 2.76 x 10'1


-31.6 152.0
-29.2 143.9
-38.9 149.5

54.0 248.8 52.2 251.5 52.7 255.7

16.3 73.7 12.9 80.2 9.0 88.2

14.6 281.0 18.9 284.5




36.4 61.7 12.7 75.2 53.9 246.3


73.9 253.1

52.5 283.1 53.4 277.8 50.3 283.7

45.6 228.5 48.4 227.4

9.7 277.9 8.4 277.9

58.1 264.7 55.1 267.5 56.6 251.1



59.9 346.3

19.2 221.5 34.0 213.9 31.4 215.6 32.7 214.9


72.7 181.2 41.3 122.3 34.9 125.6

60.6 233.5 58.0 231.4 60.3 218.7

57.2 216.4 55.5 231.5 53.0 227.9



21.4 55.0 25.4 59.0

24.9 168.6 23.0 165.0 17.6 179.6

5.2 159.9 9.6 163.4

84.4 86.4 63.5 16.1 46.9 357.7


1.9Sxl10 24.9 112.3 8.12xl10' 3.88x10-' 31.0 114.9 1.32 x 10' 5.05x10' 28.7 74.8 1.55 x 10'


8.14 x 10'1
1.13
2.44

5.64 x 10' 3.28 x 10' 2.98 x 10'

8.75 x 10'1
1.00
6.76x 10'

8.42 x 100 7.05x 10'


-39.1 153.8
-34.5 142.9
-42.6 150.4

45.1 237.2 38.8 237.4 38.0 237.7

15.0 73.8 11.2 79.8 8.5 86.4

12.1 283.9 8.0 286.4


1.70 X 10- 34.0 64.3 1.35 x 10' 6.6 69.8 1.18x10-' 55.6 257.2
- 15.7 266.7
-17.7 266.4 1.63x10' 56.6 246.4

2.53x10' 51.1 283.5 3.08x10' 49.8 279.3 2.09x10' 49.1 283.5

2.06x10' 44.7 232.1 2.62x 10 50.3 221.6

5.42 8.1 277.8
6.65 7.2 278.8

5.87 x 10' 45.9 266.2 5.87 x 10' 42.0 268.9 6.29 x 10' 47.4 258.6

- 50.1 323.0
- 41.3 245.7
4.35 x 10' 51.6 329.8

6.45x10' 16.3 221.9 6.24 x 10' 21.7 216.2 7.08 x 10 18.1 217.1 7.37 x 10' 19.8 217.1


3.99 x 10 6.04 x 10' 5.47 x 10'

4.71 x 10' 4.62x 10' 4.28x 10'

5.09 x 10' 5.13x 10' 5.34 x 10'



3.01 x 10' 2.45 x 10'

2.85x 10' 2.88 x 10' 3.34x 10'

2.74 x 10'1 1.83x 10'

1.88x 10'3 1.80x 10' 2.44 X 10-3


5.74 x 101 5.80 x 10' 8.28 x 10'

3.18x10' 1.65 x 10' 1.67x 10'

3.53 x 103.93 x 10' 3.07 x 10'

3.63 x 10' 2.71 x 10'




6.27 x 104.49 x 10' 4.82 x 10-2 4.49 x 10-2 5.75 x 105.04 x 10-2

1.68x10' 1.87 x 10' 1.30 x 10'

6.81 x 10-2 6.18 x 10-2

4.17
5.12

2.32x 10-' 2.10x 10' 2.77x 10'

1.69 x 102.05x10' 1.96 x 10'

4.06 x 10' 3.67 x 10' 4.59 x 10' 4.33x 10'


47.9 225.0 2.61 x 10' 56.8 230.7 7.28x10-2 57.7 239.4 8.58xl10


42.4 244.2 47.3 194.6
-13.3 180.4 58.9 123.2

50.7 282.9 48.8 277.5 50.2 283.2






41.4 264.4 32.6 265.4 46.7 262.5

43.3 343.3 34.7 243.3 27.6 348.9

9.3 220.7 12.2 219.3 11.9 217.0 14.3 216.8


2.57 x 10' 1.68 x 10-2 1.08x 10-' 2.25x 10-'

1.20 x 10' 1.55 x 10' 9.36 x 101.34x 10' 1.02 x 10' 1.48x 107.42 x 10' 1.09 x 10' 9.90 X 10-2

2.33x 10' 2.19 x 10' 3.17 x 10' 2.74 x 10'


43.0 161.8 1.68 x 10' 33.0 168.2 1.22 x 10' 21.6 125.5 3.06x10 18.8 127.7 2.02x10' 15.2 134.9 3.0xlO 15.8 129.3 1.99 x 1048.6 227.1 2.67x1W0 42.9 238.9 2.44x10' 43.8 233.7 2.68 x 10'



17.0 56.4 2.14x10' 21.6 58.1 1.62x10-


4.9 159.3 8.7 164.0

73.2 168.4 31.3 26.9 31.1 2.5


2.57 x 10-' 1.75x 10'

1.40 x 10' 1.81 x 103 2.58 x 10'


'Samples cleaned at 1000C rather than 150*C. Sample probably inverted. ' Red-bed sample.








STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


TABLE 2.-Measured physical properties
[Density not available for samples from CC#1]
Depth Nubro Remanent Magei Denst
below surface samber maettion su cetibty D
(meters) sampes m suse g"")
cc#1
756.5 1 3.19 x10-1 3.41 X10-3
766.9 1 6.50xl1-1 1.02x10-2
772.4 1 4.24 2.84 x 10-2
780.0 1 8.33 3.14 x 10-2
787.3 1 6.80 x 10 1.75 x 10-2
789.4 1 2.82 2.98 x 10-2
790.7 1 1.02 3.83x 10-2
CC#2
777.8 4 2.29 x 10-1 3.43 x 10- 2.788
813.8 4 2.48 1.43x 10-2 2.669
819.3 3 1.47 6.40x 10-2 2.892
825.7 4 4.89 1.73 x 10-2 2.852
859.5 4 2.26 4.64x 10-2 2.901
869.3 4 7.45 x 10-1 2.94 x 10-2 2.872
873.3 3 2.00 x10-2 1.22 x10-3 2.598
cc#3
781 3 8.36 x10-1 1.57 x10-2 2.585
922 4 3.30 x 10-1 3.38 x 10-2 2.895
927 3 2.94 x 10-1 2.92 x 10-2 2.658
987 3 9.52 x 10-1 4.64 x 10-2 2.886
989 3 8.94x 101 3.97x 102 2.884
994 4 8.69 x 10-1 3.38 x 10-2 2.851
1004 4 6.69 x 10-1 3.09 x 10-2 2.885
1012 3 9.28 x10-1 4.07x10-2 2.917



In the usual statistical analysis of paleomagnetic data, remanent directions are assumed to have a Fisherian distribution (Tarling, 1971, p. 75). Briden and Ward (1966) and Kono (1980) have developed a method for determining Fisherian statistics when only inclination measurements are available. Use of their technique was attempted in this study, but unfortunately it often failed to converge to a solution, apparently owing to the small number of specimens at each sample depth. Accordingly, in this study statistical analyses assume a normal distribution for the inclination measurements. When comparison could be made with the more exact method, the results were found to be in good agreement. This study has also adopted the standard assumption that the magnetization intensities have a log-normal distribution (Tarling, 1971, p. 87).

RESULTS

Location of Flow Boundaries

As a first step in the interpretation of the paleomagnetic measurements, TRM parameters as a function of depth were estimated. From the information in table 1, the mean inclination and its standard deviation were calculated as a function of depth and demagnetization temperature. At each depth, the mean inclination at the demagnetization temperature that minimized the stan-


dard deviation was chosen as the best estimate of the TRM inclination. The resulting parameters are given in table 3.
Flow boundaries for the three test holes were then chosen on the basis of the existing geologic descriptions and the TRM inclinations. (Further details are provided in the following section.) For CC#1, the paleomagnetic results do not disagree with the presence of a single flow boundary as described by Gohn and others (1977). Although sample TRM inclinations were not determined for the lower flow (flow 1-2), specimen inclinations appear indistinguishable across the flow boundary; therefore it is likely that the two flows are close in age. For CC#2, the locations of the two major flow boundaries described by Gottfried and others (1983) are in agreement with the NRM intensities, which show a marked decrease at the top of each major flow. This decrease in NRM intensity may be due to a combination of flow surface weathering immediately following emplacement and hydrothermal alteration during burial. However, more than two flow boundaries are required to explain the paleomagnetic measurements. For this reason, each of the three major flows in CC#2 has been divided into subflows at depths where the sign of the TRM inclination changes. Where lithologic evidence was lacking, NRM inclination changes were sometimes used in an attempt to pin down the flow boundary more precisely than was possible by using the relatively sparse TRM measurements. Most of the required additional flow boundaries are located near lithologic changes identified by Gottfried and others (1983) and S. H. Perlman (unpubl. data, 1977). A similar procedure was used on the data from CC#3. The complete basis for choosing flow boundaries is given in table 4. There will always be some uncertainty in the locations of flow boundaries. For example, many thin flows may have been missed by the paleomagnetic sampling. This is particularly true near the bottom of CC#2, where the sampling is sparse and the lithologic description indicates many possible disruptions. On the other hand, some of the chosen flow boundaries may be superfluous. For example, the flows labeled 2-3a and 2-3b originally may have been the top and bottom parts of a single flow, but when flow 2-2d was emplaced above it, the top part (2-3a) was reheated and remagnetized so that its polarity is now different from that of the lower part (2-3b). Despite these uncertainties, the flow boundaries given in table 4 should provide a good basis for geologic correlation and paleomagnetic dating.
The next step in the interpretation was to determine the mean TRM inclination for each flow. This was done by averaging sample mean TRM inclinations. The results (table 5) can be used to correlate flows among the three test holes and to estimate the age of the basalt.


C10









PALEOMAGNETIC INVESTIGATIONS OF THE CLUBHOUSE CROSSROADS BASALT


TABLE 3.-Means of inclination and intensity as afunction of depth [Flows are categorized by the number of the test hole, number of the major flow within the test hole, and letter of the subflow within the major flow. Queries (?) indicate anomalous samples,
which are not included in flow averages. N, number of specimens; Inc, inclination; S.D., standard deviation; J, intensity ( in ampere/meter).]
Depth Natural remanent magnetization Thermal remanent magnetization
Flow below surface N Inc S.D. J N Inc S.D. J T(*)
Flow metersa)
CC#1
1-1 756.5 1 54.2 - 3.19 x 10-1
760.5 2 41.7 0.8 1.19
762.9 3 41.5 7.9 1.23 2 37.6 1.2 9.74 x 10-1 300
766.9 3 45.2 6.5 2.97 x 10-1 2 43.3 6.4 1.61 x 101 500
770.2 2 41.1 1.7 9.70 x 10772.4 2 41.2 .9 4.15
780.0 3 47.8 17.9 2.25 2 41.9 10.5 7.40x10-1 500

1-2 787.3 2 55.0 7.3 2.77 x 101
789.4 2 36.5 .5 2.92
790.7 2 57.5 1.4 1.16
CC#2


776.0 777.8 778.2

779.1 779.7


2-1b


2-2a 782.1
783.3 785.1 785.5 786.3 789.4 790.7 794.3 795.5

798.0 801.0 ? 801.6
802.5 802.8
804.6 808.9 813.8

2-2b 814.4
819.0 819.3 823.0


2-2c 2-2d


2-3b


825.7 828.8 835.8 838.8
840.9

842.8 846.7 853.7 858.3 859.5 863.8 869.0 869.3 870.5

873.3 878.1

883.9 885.4 887.6 888.8 890.0


3
5
3


-21.9 9.2
23.9 36.5 47.2 28.6


3 19.5 25.7
3 20.7 .7

3 36.1 14.3
3 3.9 15.4
3 -28.5 69.4
3 37.8 30.7
3 52.3 7.3
5 15.9 13.5
4 62.3 10.5
4 -4.4 28.1
3 -34.9 5.5


3
4
3
5
3
3
3
4

4
6
3
6


-44.9 2.1
2.3 19.0 26.0 11.7
-23.6 33.0
-31.5 10.6
-6.0 .7
-1.9 5.8
-41.6 4.0

34.3 12.0 32.2 14.5 35.9 3.9 14.2 20.8


4 -11.0 1.0
3 -1.1 3.2
3 -4.5 6.0
3 -40.9 1.8
4 -8.1 9.0

3 28.0 2.8
3 43.3 3.7
6 46.3 14.2
4 16.4 3.8
4 47.1 6.6
8 19.1 69.5
3 57.4 12.4
4 48.3 12.9
3 9.0 59.9

2 43.4 9.8
3 38.9 16.6

3 -22.8 3.0
3 -37.8 3.2
3 -46.9 2.0
3 31.5 35.0
2 38.5 11.0


1.14
2.23 x 10-1 1.11 x 10-1

2.90 x 10-1 7.72 x 10-'

8.36 x 10-' 5.83 x 10-3 3.03 x 10-1 5.26 x 103.20 x 10-3 1.44 x 10-2 5.39 x 108.59 x 10-1
1.97

2.32 1.26
1.12
9.53 x 10-1
1.67 1.09
1.44 2.48

5.63 x 10-1 8.53 x 10-'
1.47 2.72

4.89 4.71 5.63
4.07 4.68

4.24 2.21 1.81 1.80 2.26
5.76 x 10-1 5.81 x 107.45 x 10-' 3.81 x 10-'

2.00 x 10-2 3.63 x 101

1.33 2.77 2.33
2.21 x 10-1 1.65 x 10-1


3
4
3


-36.2
4.8 1.9


11.9 23.2 23.6


3 26.3 11.4


3
3

3
3


-48.7
-50.4

-41.9
-39.6


8.9 5.3

3.4 11.0


3 . -48.0 1.7


2

3

2

2
4


-50.6

-3.3

-45.8

-28.4
-48.9


.9

6.5 5.9

2.4 2.3


3 20.4 2.5
3 23.7 10.0


4

3
3



3
3

4
4

4


-14.4

-6.4
-66.8



19.5 27.8 23.6
52.4 35.8


2.6 2.7 1.6



.4
2.3 2.6 3.1 2.9


1.44 x 10-' 550 1.40 x 10-1 300 6.22 x 10-2 300

3.29 x 10- 500



2.18 x 10-3 550 2.14 x 10-3 500

3.84 x 10-4 500 2.16 x 10- 500


1.65


300


1.58 500

7.24 x 10-' 300 1.73 300

2.73 x 10-1 500 1.87 300



6.69 x 101 300 2.73 x 10-' 500 4.62 x 102 300 4.48 300
1.73 x 101 500



2.05 x 10-2 550 2.36 x 10-1 500 2.63 x 10- 500 1.30 x 101 500 3.11x10- 300


2 25.9 4.9 2.08 x 102 100


3 -47.1 3.0 4.31x10-1 500



2 -24.2 9.8 1.36 x 101 150


C11







STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


TABLE 3.-Means of inclination and intensity as afunction of depth-Continued [Flows are categorized by the number of the test hole, number of the major flow within the test hole, and letter of the subflow within the major flow. Queries (?) indicate anomalous samples,
which are not included in flow averages. N, number of specimens; Inc, inclination; S.D., standard deviation; J, intensity ( in ampere/meter).]
Depth Natural remanent magnetization Thermal remanent magnetization
Flow bewsurace N Inc S.D. J N Inc S.D. J T(*C)
CC#2-Continued
2-3c 894.0 2 21.5 4.2 1.80 x 10-1 2 39.3 1.2 1.39 x 10-2 500
2-3d 896.1 3 -16.4 2.3 1.31 3 -56.6 2.0 2.55 x 10-1 300
897.3 5 16.7 58.2 4.11x 10-1
902.8 3 63.0 23.4 8.95 x 10? 904.3 6 13.6 11.0 2.40 3 -6.6 7.5 1.36 300
905.3 2 27.0 34.7 2.57
CC#3
3-1 777 4 64.6 14.0 9.73 x 10-1 3 28.2 3.1 1.18 x 10-1 500
3-2a 779 3 -12.6 22.6 6.53 x 10-1 3 -38.7 4.1 3.02 x 10-1 500
3-2b 781 3 66.7 2.4 8.36 x 10-1 3 53.0 .9 3.81 x 10-1 300
3-2c 782 3 29.7 4.7 1.44 3 19.3 1.6 1.23 150
3-3 784 2 37.6 22.1 1.93 x 10-1 2 10.1 2.9 3.14 x 10-2 500
3-4 921 2 68.6 3.9 5.58 x 10-1
922 6 43.5 26.4 3.66 x 10-1 3 56.3 20.6 3.52 x 10-1 150
3-5 927 3 50.1 2.5 2.94 x 10-1 3 49.9 1.0 1.20 x 10-1 550
928 2 49.7 4.0 2.60 x 10-1 2 47.0 2.0 2.32 x 10- 300
? 984 2 17.8 3.0 5.87 2 7.7 .6 4.62 500
3-6a 987 3 62.1 4.7 9.52 x 10- 3 66.0 1.1 9.50 x 10-1 100
989 3 62.9 8.1 8.94x10-1 3 47.7 5.6 1.89x10-1 500
3-6b 994 4 54.8 12.9 8.69 x 10-1 4 11.9 2.1 2.58 x 10-1 550
1004 4 44.0 27.0 6.69 x 10-1 3 22.5 9.2 1.70 x 10-1 550
3-6c 1012 3 80.8 3.4 9.28 x 10-1 3 59.6 1.4 4.53 x 10-1 300
1014 3 69.1 5.4 9.44 x 10-1 3 55.2 2.1 5.19 x 10-1 300
3-7 1024 4 35.6 17.0 4.40 x 101 2 23.4 2.8 2.72 x 10-1 300
1031 3 32.1 8.5 4.58 x 10-1 3 23.5 3.6 3.74 x 10-1 150
sed 1047 2 16.0 1.7 2.14 x 10- 2 9.2 .8 2.23 x 10-2 150
sed 1146 3 74.8 3.2 3.44 x 10-'
'Thermomagnetic cleaning temperature.


Details on flow determinations.- To aid in the determination of flow boundaries, a statistical test was used to compare pairs of sample mean TRM inclinations. The test statistic is given by

U= , (1)


where mk is the mean TRM inclination at depth k, and Amk is the standard error in the mean

Amk = . (2)


Here ak is the standard deviation of the TRM inclinations at depth k, and nk is the number of specimens used to estimate mk and o-k. The test statistic U is used to test the hypothesis that the mean TRM inclinations at the two depths are identical. If the value of U is greater than 2.0, then the hypothesis is rejected at the 95-percent confidence level.
For each test hole, the test statistic was computed for every possible pairing of samples. Pairs for which the


hypothesis of equivalent mean inclinations could not be rejected at the 95-percent confidence level were found to be linked together into groups of samples. Note that paired adjacent samples need not have satisfied the statistical test in order to be classified in the same group. Rather they were only required to satisfy the test when each was paired with a third sample elsewhere in the hole. Due to the large scatter in the inclinations, the more rigorous approach would have yielded nearly as many groupings (flows) as there are samples in test holes 2 and 3.
GC#1 was found to have a single group of samples having a mean TRM inclination of 40.9'. Results for CC#2 were equivocal until the two samples with standard errors in the mean greater than 100 were rejected from the analysis. Three groupings were then found having mean TRM inclinations of 26.9', -41.2', and -5.4*. The latter grouping contained three isolated samples that were rejected from further analysis as being anomalous. Two samples found to belong to no grouping were also rejected from further analysis. Rejected samples are indicated by question marks in the flow col-


C12








PALEOMAGNETIC INVESTIGATIONS OF THE CLUBHOUSE CROSSROADS BASALT


TABLE 4.-Evidence for flow boundaries


Flow


Depth below surface (m) to flow top feature


Paleomagnetic evidence


Lithologic evidence


CC#1
1-1 750.0 Top of basalt
1-2 784.7 Sharp contact
792.0 Base of core
CC#2
2-la 775.3 Top of basalt
776.0 Top of core
2-lb 777.8 Mixed TRM inclinations
778.2 Mixed TRM inclinations

2-2a 781.6 TRM inclination changes from + at 779.1 to - at 783.3 Sharp contact
793.5 Gradational contact
801.6 Shallow TRM inclination
804.6 Unstable sample
2-2b 814.0 TRM inclination changes from - at 813.8 to + at 819.3
NRM inclination changes from - at 813.8 to + at 814.4 2-2c 824.0 TRM inclination changes from + at 823.0 to - at 825.7
835.8 Shallow TRM inclination
838.8 Steep TRM inclination Sheet of vesicles
2-2d 842.0 TRM inclination changes from - at 838.8 to + at 846.7
NRM inclination changes from - at 840.9 to + at 842.8

2-3a 871.3 Breccia zone
2-3b 883.0 TRM inclination changes from + at 873.3 to - at 883.9 Gradational zone
NRM inclination changes from + at 878.1 to - at 883.9 888.0 Gradational contact
2-3c 892.0 TRM inclination changes from - at 890.0 to + at 894.0
2-3d 895.0 TRM inclination changes from + at 894.0 to - at 896.1 Gradational contact
899.3 Gradational contact
901.3 Sharp contact
903.0 Gradational contact
904.3 Shallow TRM inclination
907.0 Base of core
CC#3
3-1 775.0 Top of core
3-2a 777.7 TRM inclination changes from + at 777 to - at 779 Sharp contact
3-2b 780.0 TRM inclination changes from - at 779 to + at 781
3-2c 781.5 TRM inclination shallows by 33.7* between 781 and 782

3-3 782.6 Sharp contact
785.0 Base of core
3-4 921.0 Top of core
3-5 925.5 Sedimentary wedge
930.0 Base of core
3-6a 983.0 Top of core
3-6b 992.0 TRM inclination steepens by 35.8* between 989 and 994
3-6c 1008.0 TRM inclination steepens by 37.10 between 1004 and 1012
3-7 1021.3 TRM inclination shallows by 39.0* between 1014 and 1024 Sharp contact
1031.0 Base of basalt


umn of table 3. CC#3 also produced three groupings, having mean TRM inclinations of 54.30, 18.30, and
-38.70.
Flow boundaries were located on the basis of the following criteria, listed in order of their importance:
1. Geological description of the core provided unequivocal evidence for a flow boundary;
2. Sample TRM inclinations changed groupings across a
probable flow boundary mentioned in the
geologic description;
3. Sample TRM inclinations changed groupings without
geologic evidence for a flow boundary.


Flows determined using criterion 1 are designated by numbers. Criteria 2 and 3 are used to subdivide the numbered flows into alphabetized subflows.

Correlation of Flows

Correlation of flows among the three test holes is hindered by the limited amount of vertical overlap among the cored sections of the holes and by the horizontal distance between the holes. However, CC#2 and 3 are only about 760 meters apart, and it appears likely either that flow 2-1b (TRM inclination 26.30) and flow 3-1 (TRM inclination 28.20) represent the same


C13







STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


TABLE 5.-Means ofinclination and intensity as afunction offlow [ND, number of depths from which samples for analysis were taken; Inc, inclination; S.D.,
standard deviation; J, intensity (in amperelmeter)]
Natural remanent magnetization Thermal remanent magnetization Flow ND Inc S.D. J ND Inc S.D. J
cc#1
1-1 7 44.7 4.9 1.03 3 40.9 3.0 4.88x 10-'
1-2 3 49.7 11.5 9.79x 10-' CC#2
2-la 1 -21.9 - 1.14 1 -36.2 - 1.44 x 10-'
2-1b 2 20.1 .8 4.73x 10- 1 26.3 - 3.29x 10-'
2-2a 16 -0.4 33.7 3.06 x 10-' 9 -44.7 7.2 7.64 x 102
2-2b 4 29.2 10.1 1.18 2 22.1 2.3 4.27 x 10-'
2-2c 3 -5.4 5.1 4.76 1 -14.4 - 4.62x10-2
2-2d 8 36.6 17.0 1.36 4 26.7 7.0 1.41 x 10-'
2-3a 2 41.2 3.2 8.52x 10-' 1 25.9 - 2.08x 102-3b 5 -7.5 39.8 7.93x10 2 -35.7 16.2 2.42 102-3c 1 21.5 - 1.80 X10-1 1 39.3 - 1.39 x10-1
2-3d 4 22.6 32.7 1.05 1 -56.6 - 2.55x 10-'
CC#3
3-1 1 64.6 - 4.97x10-' 1 28.2 - 1.18x10
3-2a 1 -12.6 - 6.53x 10-: 1 -38.7 - 3.02x 103-2b 1 66.7 - 8.36 x 10- 1 53.0 - 3.81 x 10-'
3-2c 1 29.7 - 1.44 1 19.3 - 1.23
3-3 1 37.6 - 1.93 X 10-' 1 10.1 - 3.14 x 10-2
3-4 2 56.1 17.8 4.52 x10-1 1 56.3 - 3.52 x10-1
3-5 2 49.9 .3 2.76 x10-1 2 48.5 2.1 1.67 x10-1
3-6a 2 62.5 .6 9.23 x 10-' 2 56.9 12.9 4.24 x 10-'
3-6b 2 49.4 7.6 7.62x10- 2 17.2 7.4 2.09x10-1
3-6c 2 75.0 8.3 9.34x10' 2 57.4 3.1 4.85x10
3-7 2 33.9 2.5 4.49 x10-1 2 23.5 .1 3.19 x10'



flow or that flow 2-la (TRM inclination -36.2*) and flow 3-2a (TRM inclination of -38.7") represent the same flow. In the former case, the major flow boundary at 777.7 m in CC#3 corresponds to the major flow boundary at 781.6 m in CC#2, and flows 3-2b, 3-2c, and 3-3 are absent in CC#2. In the latter case, the major flow boundary at 777.7 m in CC#3 corresponds to the top of the basalt in CC#2, and the major flow boundary at 782.6 m in CC#3 corresponds to the major flow boundary at 781.6 m in CC#2. In this interpretation, flows 3-2b, 3-2c and 2-lb become independent thin flows, and flow 3-3 could be the remagnetized top of flow 2-2a, the remagnetization having occurred at the time of emplacement of flow 3-2c. In either case, the flow boundaries at the top of CC#2 and 3 have a near-horizontal apparent dip, which indicates little tilting since extrusion.
Because CC#1 extends to only 792 m, it is possible that the basalt flows sampled in CC#1 lie entirely above the section sampled in CC#2 and 3. Alternatively, the top flow in CC#3 could correspond to the lower flow in CC#1. Thus the paleomagnetic measurements in the three test holes can be reconciled without requiring the presence of faults or other tectonic elements.

Age of the Basalt

After the basalt unit has been divided into flows and the mean TRM inclination has been estimated for each flow (except flow 1-2), the mean inclination of the basalt unit as a whole can be estimated. This number can be compared to a graph of paleoinclination against time for


the Charleston area in order to estimate the age of the basalt. The unit mean inclination is determined by averaging the absolute values of the flow means. Possible correlations of flows among the three holes were not considered in the averaging process; therefore some flows may have been sampled twice. The mean inclination computed from the 22 flow means is 35.4 +3.2'. Ignoring possible errors in the flow averages and assuming a normal distribution for the sample means, the unit mean inclination has a 95-percent chance of falling in the range 35.4 +6.40.
In figure 3 this mean inclination and its 95-percent confidence band have been superimposed on a paleoinclination curve that shows the mean inclination at Charleston (lat 33' N., long 80' W.) as a function of time for the past 300 million years. The paleoinclination curve was generated by using the pole positions of Irving (1977). The dashed lines in the figure show the 95-percent confidence region about the mean paleoinclination curve.
On the basis of the present study, the most probable age of the basalts corresponds to the time at which the paleoinclination curve crosses 35.4 degrees. This turns out to be 170 m.y. ago. The actual intersection of the two curves, however, has a 95-percent chance of falling anywhere within the ruled area. Thus we can be 95-percent certain that the true age of the basalts lies between 110 m.y. and 196 m.y. This time range runs from the end of the Triassic, through the Jurassic, and into the Early Cretaceous. For comparison with radiometric age dates (fig. 4), we also have computed the onestandard-error range (68-percent confidence interval) on the paleomagnetic age to be 120 m.y. to 188 m.y. The large uncertainty in the age of the basalts is a result of both the uncertainty in the paleoinclination curve and the uncertainty in the mean magnetic inclination of the Clubhouse Crossroads basalt unit. It is likely that a more precise age could have been determined if completely oriented cores had been available to provide azimuthal information.
Figure 4 is a time line comparing paleomagnetic and radiometric ages of the Clubhouse Crossroads basalt with the geomagnetic reversal history and with radiometric ages for other eastern North American Mesozoic igneous rocks. The figure shows that neither the paleomagnetic data nor the radiometric age measurements provide sufficient constraints on the age of the Clubhouse Crossroads basalt to permit comparison with the other Mesozoic units. However, the presence of normal and reversely magnetized flows in the basalt unit suggests either that the age of the basalt may differ from the age of other early Mesozoic basalts and diabases or that certain periods of lower Mesozoic magnetic reversals are inadequately documented within the paleomagnetic record.


C14








PALEOMAGNETIC INVESTIGATIONS OF THE CLUBHOUSE CROSSROADS BASALT


, S
t~S / S


LU

0
Z

0
zj fl 9
z
zL
w


S S
S ~
S / 5/

S , S


100 200
TIME, IN MILLIONS OF YEARS BEFORE PRESENT


FIGURE 3.-Paleoinclination and age of the basalt. The solid curve shows the mean inclination at Charleston (lat 33* N., long 80*W.) as a function of past time. The dashed curves represent 95-percent confidence limits about the mean. These curves were computed from pole positions given by Irving (1977). The horizontal line at 35.4* represents the mean TRM inclination of the Clubhouse Crossroads


From the marine magnetic record, frequent magnetic reversals are known to have occurred during the Late Jurassic and Early Cretaceous (Larson and Hilde, 1975). From paleomagnetic measurements made on land, another period of frequent reversals is known to have occurred in the Early Triassic (McElhinny, 1971). These two periods of frequent reversals are separated by a long period of normal polarity, containing few reversedpolarity intervals. It is during this long period of predominantly normal polarity that most of the Mesozoic igneous rocks of eastern North America apparently were formed.
Regional tectonics suggest that the Clubhouse Crossroads basalts probably are not old enough to have formed during the Early Triassic interval of frequent reversals. On the other hand, the initial Cretaceous radiometric ages for the CC#1 basalt (Gottfried and others, 1977) suggest that they may have formed during the Late Jurassic-Early Cretaceous reversal sequence. In that case, the Clubhouse Crossroads basalts would be much younger than the other eastern North American basalts and diabases associated with early Mesozoic basins. Instead their age would be similar to the youngest intrusive rocks of the White Mountain Plutonic-Volcanic Suite of New England (Foland and


basalt, as computed from 22 flow means. The intersection of this line with the solid curve gives the most probable age of the basalt as 170 m.y. The ruled area is the 95-percent confidence region about this intersection. Thus the actual age of the basalt has a 95-percent chance of falling in the range 110-196 m.y.



others, 1971; Armstrong and Stump, 1971). There are three problems with this interpretation. First, like the other eastern North American Mesozoic basalts and diabases, the Clubhouse Crossroads basalts are found close to a probable Triassic-Jurassic basin. Secondly, the presence of a thin sedimentary red-bed unit between flows 3-4 and 3-5 of CC#3 (Gottfried and others, 1983) indicates that the Clubhouse Crossroads basalts, like the other eastern North American basalts and diabases of Mesozoic age, are closely associated in time with the sedimentary rocks they intrude or overlie. Thirdly, geochemical analyses indicate that the Clubhouse Crossroads basalt is of the same chemical type as other eastern North American Triassic-Jurassic basalts (Gottfried and others, 1977, 1983). Thus it is likely that the Clubhouse Crossroads basalts are similar in age to the igneous rocks of other eastern North American early Mesozoic basins. That is, they are of Late Triassic and Early Jurassic age. As a corollary, the reversely magnetized flows of the Clubhouse Crossroads basalt are evidence for a period of frequent reversals during the Late Triassic or Early Jurassic.
Opdyke and McElhinny (1965) and Brock (1968) present evidence for short reversed-polarity intervals within the Late Triassic-Early Jurassic normal-polarity


bUl I I I I


40 \-


5/~


20 H-


(1


0


300


........... ........... .


-


C15


. I I I I


- -------


35.4


0









STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


CRETACEOUS JURASSIC TRIASSIC PERMIAN
I I I I I





2-+0


31








5
2 -*1*AF3-+1 p








T I
I a
I H






7 [.+-+I -in.--@-I I---+-- -* SO-++ - I-+4---80 100 120 140 160 180 200 220 24
TIME. IN MILLIONS OF YEARS BEFORE PRESENT


0


FIGURE 4.-A time line comparing the paleomagnetic and radiometric ages for the Clubhouse Crossroads basalt with geomagnetic field behavior and with radiometric ages for other eastern North American Mesozoic igneous rocks. The following features are shown: 1. Geomagnetic field behavior. Dark bands represent normal polarity. Light bands represent reversed polarity. Shaded bands represent mixed polarity (Larson and Hilde, 1975; McElhinny, 1971; Opdyke and McElhinny, 1965; Brock, 1968;
Reeve and Helsley, 1972; Steiner and Helsley, 1974).
2. Biostratigraphic age of the oldest Cretaceous sediment in CC#1 (Hazel and others, 1977).
3. Paleomagnetic age of the Clubhouse Crossroads basalt (this study).


interval. Apparently two such reversed intervals can be identified from paleomagnetic studies of South African igneous rocks, one at 195 m.y. near the Triassic-Jurassic boundary and the other around 170 m.y. The reversed flows seen in the Clubhouse Crossroads basalt could date from either of these times, although the younger date corresponds well to the most probable age of the basalt as determined by this study and might therefore


4. Radiometric ages of the Clubhouse Crossroads basalt (Gohn and others, 1978; Lanphere, 1983).
5. Radiometric ages of other southeastern North American Mesozoic basalt and diabase (Gohn .and others, 1978; Watts
and Noltimier, 1974).
6. Radiometric ages of northeastern North American Mesozoic basalt and diabase (de Boer, 1968; Dallmeyer, 1975; Sutter
and Smith, 1979).
7. Radiometric ages of the rocks of the White Mountain PlutonicVolcanic Suite (Foland and others, 1971; Armstrong and
Stump, 1971: Hurley and others, 1960).




be preferred. The presence of five distinct reversed intervals in CC#2 indicates that, if the correlation with the 170-m.y. reversed event is correct, this event is actually a sequence of reversals, similar in form to the short Late Triassic reversal sequences seen by Reeve and Helsley (1972) and Steiner and Helsley (1974) in the Chinle (New Mexico) and Kayenta (Africa) formations.


C16








PALEOMAGNETIC INVESTIGATIONS OF THE CLUBHOUSE CROSSROADS BASALT


It should be mentioned that Opdyke and Wensink (1966) found reversely magnetized samples at three sites in the White Mountains; samples from two of the sites were thought to be of Early Jurassic age, whereas samples from the third site were radiometrically dated (K-Ar) as Early Cretaceous. However, K-Ar dating by Foland and others (1971) has since established that one set of the "Jurassic" samples is indeed Early Cretaceous. The other site is undated, but similar paleomagnetic directions in all samples suggest a similar Cretaceous age for all three.
CONCLUSIONS
Although hampered by the lack of completely oriented cores and by the relatively small amount of overlap in the depth ranges of the cored sections of the three test holes, paleomagnetic investigations of the Clubhouse Crossroads basalt have yielded worthwhile results. The age of the basalt has been confirmed as Mesozoic and probably Early Jurassic. No faults are required to correlate flows among the three holes. Reversed polarities and a definite polarity sequence have been found for the first time in eastern North American igneous rocks associated with early Mesozoic basins. In addition, the description of the basalt unit itself has been expanded.
Thermomagnetic analysis indicates that the main carrier of the remanence in the basalt is nearly pure magnetite. In some samples having high Curie points, the remanence may be carried by primary ilmenohematite. Low Curie points are also seen, providing evidence for maghemite. The inference is that all the original unoxidized titanomagnetite in the basalt underwent high-temperature oxidation during the initial cooling to form nearly pure magnetite. Some of the magnetite has undergone low-temperature oxidation since cooling, which has resulted in the production of maghemite. Both the magnetite and the ilmenohematite can be expected to provide reliable paleomagnetic remanent directions.
On the basis of the paleomagnetic measurements and the geologic descriptions of the cores, 23 flows have been identified. These are divided as follows: 2 flows in CC#1, 10 flows in CC#2, and 11 flows in CC#3. One of the top two flows in CC#2 may correspond to one of the top two flows in CC#3. The flow boundaries are essentially horizontal.
The mean TRM inclination of the flows is 35.4' 3.20. Comparing this figure with a paleoinclination curve for the Charleston area indicates that the most probable age of the basalt is 170 m.y. and that the actual age has a 95-percent chance of falling in the range 110-196 m.y. Similarities between the Clubhouse Crossroads basalt and other lower Mesozoic eastern North American basalts and diabases suggests that the true age is more likely to fall in the older end of this range.


The flow inclinations in CC#2 exhibit a definite polarity sequence. Five reversed-polarity intervals are separated by four normal-polarity intervals. Reversed polarities are not seen in other eastern North American Mesozoic basalts and diabases. However, reversed polarities and polarity sequences are known from other studies of Upper Triassic and Lower Jurassic rocks in the Southwestern United States and in southern Africa.

REFERENCES CITED
Akimoto, Syun-iti, Katsura, Tukashi, and Yoshida, Minoru, 1957,
Magnetic properties of TiFe204-FeO4 system and their change with oxidation: Journal of Geomagnetism and Geoelectricity, v. 9,
no. 4, p. 165-178.
Armstrong, R. L., and Stump, Edmund, 1971, Additional K-Ar dates,
White Mountain magma series, New England: American Journal
of Science, v. 270, no. 5, p. 331-333.
Beck, M. E., Jr., 1972, Paleomagnetism of Upper Triassic diabase
from southeastern Pennsylvania: further results: Journal of
Geophysical Research, v. 77, no. 29, p. 5673-5687.
Briden, J. C., and Ward, M. A., 1966, Analysis of magnetic inclination
in borecores: Pure and Applied Geophysics, v. 63, no. 1, p.
133-152.
Brock, A., 1968, Paleomagnetism of the Nuanetsi igneous province
and its bearing upon the sequence of Karroo igneous activity in southern Africa: Journal of Geophysical Research, v. 73, no. 4, p.
1389-1397.
Carmichael, I.S.E., and Nicholls, J., 1967, Iron-titanium oxides and
oxygen fugacities in volcanic rocks: Journal of Geophysical
Research, v. 72, no. 18, p. 4665-4687.
Dallmeyer, R. D., 1975, The Palisades sill: a Jurassic intrusion?
Evidence from 40Ar/39Ar incremental release ages: Geology, v. 3,
no. 5, p. 243-245.
Daniels, D. L., Zietz, Isidore, and Popenoe, Peter, 1983, Distribution
of subsurface lower Mesozoic rocks in the Southeastern United States as interpreted from regional aeromagnetic and gravity maps, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity: U.S.
Geological Survey Professional Paper 1313, p. K1-K24.
de Boer, Jelle, 1968, Paleomagnetic differentiation and correlation of
the Late Triassic volcanic rocks in the central Appalachians (with special reference to the Connecticut Valley): Geological Society of
America Bulletin, v. 79, no. 5, p. 609-626.
Doell, R. R., and Cox, Allan, 1965, Measurement of the remanent
magnetization of igneous rocks: U.S. Geological Survey Bulletin
1203-A, p. A1-A32.
C1967, Recording magnetic balance, in Collinson, D. W., Creer, K. M., and Runcorn, S. K., eds., Methods in
paleomagnetism: New York, Elsevier, p. 440-444.
Foland, K. A., Quinn, A. W., and Giletti, B. J., 1971, K-Ar and Rb-Sr
Jurassic and Cretaceous ages for intrusives of the White Mountain magma series, northern New England: American Journal of
Science, v. 270, no. 5, p. 321-330.
Gehn, G. S., Gottfried, David, Lanphere, M. A., and Higgins, B. B.,
1978, Regional implications of Triassic or Jurassic age for basalt and sedimentary red beds in the South Carolina Coastal Plain:
Science, v. 202, no. 4370, p. 887-890.
Gohn, G. S., Higgins, B. B., Smith, C. C., and Owens, J. P., 1977,
Lithostratigraphy of the deep corehole (Clubhouse Crossroads corehole 1) near Charleston, South Carolina, in Rankin, D. W., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-A preliminary report: U.S. Geological Survey Professional
Paper 1028, p. 58-70.


C17








STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


Gottfried, David, Annell, C. S., and Byerly, G. R., 1983, Geochemistry and tectonic significance of subsurface basalts near Charleston, South Carolina; Clubhouse Crossroads test holes #2 and #3, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity:
U.S. Geological Survey Professional Paper 1313, p. A1-A19.
Gottfried, David, Annell, C. S., and Schwarz, L. J., 1977,
Geochemistry of subsurface basalt from the deep corehole (Clubhouse Crossroads corehole 1) near Charleston, South Carolina-Magma type and tectonic implications, in Rankin, D.
W., ed., Studies related to the Charleston, South Carolina, earthquake of 1886 -A preliminary report: U.S. Geological Survey Professional Paper 1028, p. 91-113.
Grommd, C. S., Wright, T. L., and Peck, D. L., 1969, Magnetic properties and oxidation of iron-titanium oxide minerals in Alae and Makaopuhi lava lakes, Hawaii: Journal of Geophysical Research, v.
74, no. 22, p. 5277-5293.
Hazel, J. E., Bybell, L. M., Christopher, R. A., Frederiksen, N. 0.,
May, F. E., McLean, D. M., Poore, R. Z., Smith, C. C., Sohl, N. F., Valentine, P. C., and Witmer, R. J., 1977, Biostratigraphy of the deep corehole (Clubhouse Crossroads corehole 1) near Charleston, South Carolina, in Rankin, D. W., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-A preliminary report: U.S. Geological Survey Professional Paper 1028, p. 71-89. Hurley, P. M., Fairbairn, H. W., Pinson, W. H., and Faure, G., 1960,
K-Ar and Rb-Sr minimum ages for the Pennsylvanian section in the Narragansett Basin: Geochimica et Cosmochimica Acta, v. 18,
nos. 3/4, p. 247-258.
Irving, E., 1977, Drift of the major continental blocks since the Devonian: Nature, v. 270, no. 5635, p. 304-309.
Johnson, H. P., and Merrill, R. T., 1972, Magnetic and mineralogical
changes associated with low-temperature oxidation of magnetite:
Journal of Geophysical Research, v. 77, no. 2, p. 334-341.
1973, Low temperature oxidation of a titanomagnetite and the
implications for paleomagnetism: Journal of Geophysical
Research, v. 78, no. 23, p. 4938-4949.
1974, Low-temperature oxidation of a single-domain
magnetite: Journal of Geophysical Research, v. 79, no. 35, p.
5533-5534.
Kono, M., 1980, Statistics of paleomagnetic inclination data: Journal
of Geophysical Research, v. 85, no. B7, p. 3878-3882.
Lanphere, M. A., 1983, 40Ar/39Ar ages of basalt from Clubhouse
Crossroads test hole #2, near Charleston, South Carolina, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity: U.S. Geological Survey
Professional Paper 1313, p. B1-B8.


Larson, R. L., and Hilde, T. W. C., 1975, A revised time scale of
magnetic reversals for the Early Cretaceous and Late Jurassic:
Journal of Geophysical Research, v. 80, no. 17, p. 2586-2594.
McElhinny, M. W., 1971, Geomagnetic reversals during the
Phanerozoic: Science, v. 172, no. 3979, p. 157-159.
Merrill, R. T., 1975, Magnetic effects associated with chemical changes
in igneous rocks: Geophysical Surveys, v. 2, no. 3, p. 277-311. Opdyke, [N.] D., and McElhinny, M. W., 1965, The reversal at the
Triassic Jurassic boundary and its bearing on the correlation of Karroo igneous activity in Southern Africa [abs.]: EOS, American
Geophysical Union Transactions, v. 46, no. 1, p. 65.
Opdyke, N. D., and Wensink, H., 1966, Paleomagnetism of rocks from
the White Mountain plutonic-volcanic series in New Hampshire and Vermont: Journal of Geophysical Research, v. 71, no. 12, p.
3045-3051.
Ozima, Mituko, and Larson, E. E., 1970, Low- and high-temperature
oxidation of titanomagnetite in relation to irreversible changes in the magnetic properties of submarine basalts: Journal of
Geophysical Research, v. 75, no. 5, p. 1003-1017.
Reeve, S. C., and Helsley, C. E., 1972, Magnetic reversal sequence in
the upper portion of the Chinle Formation, Montoya, New Mexico: Geological Society of America Bulletin, v.83, no. 12, p. 3795-3811. Sass, J. H., and Ziagos, J. P., 1977, Heat flow from a corehole near
Charleston, South Carolina, in Rankin, D. W., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-A preliminary report: U.S. Geological Survey Professional Paper
1028, p. 115-118.
Smith, T. E., 1976, Paleomagnetic study of lower Mesozoic diabase
dikes and sills of Connecticut and Maryland: Canadian Journal of
Earth Sciences, v. 13, no. 4, p. 597-609.
Stacey, F. D., and Banerjee, S. K., 1974, The physical principles of
rock magnetism: Amsterdam, Elsevier, 195 p.
Steiner, M. B., and Helsley, C. E., 1974, Magnetic polarity sequence
of the Upper Triassic Kayenta Formation: Geology, v. 2, no. 4, p.
191-194.
Sutter, J. F., and Smith, T. E., 1979, 40Ar/9Ar ages of diabase intrusions from Newark trend basins in Connecticut and Maryland; initiation of central Atlantic rifting: American Journal of Science, v.
279, no. 7, p. 808-831.
Tarling, D. H., 1971, Principles and applications of paleomagnetism:
London, Chapman and Hall, 164 p.
Watts, Doyle, and Noltimier, H. C., 1974, Paleomagnetic study of
diabase dikes in the inner Piedmont of N. Carolina and Georgia [abs.]: EOS, American Geophysical Union Transactions, v. 55, no.
7, p. 675.


C18

















Geology of the Lower Mesozoic(?) Sedimentary Rocks in Clubhouse Crossroads Test Hole #3, Near Charleston, South Carolina


By GREGORY S. GOHN, BRENDA B. HOUSER, and RAY R. SCHNEIDER STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-TECTONICS AND SEISMICITY GEOLOGICAL SURVEY PROFESSIONAL PAPER 1313-D


T


UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:1983




















CONTENTS


Abstract
Introduction _ _ _- ____ _ _ ____ _ _ ___ _ ____ _ ____ _ __Stratigraphy - _ _ ____ _ _ ___- _ _ ____ _ ____ _ ____ _ __Coastal Plain section __ _____ ______ ____- _______Basalt
Red-bed section
Description of the red-bed section _______ _____- ______Fine-grained facies _ _______ __________ _____Lithology and primary sedimentary structures __-__Diagenetic features _____________ _______ __-


Page D1
1 1 1 3 3
4 4 .4
6


Description of the red-bed section--Continued
Fine-grained facies- Continued
Mineralogy and petrology -___-___- _____- __-Coarse-grained facies _-_________ _____- _______Lithology and sedimentary structures _____-___-Mineralogy and petrology ___________- _______Depositional environments ___- _______________ ___Fine-rained faces
Coarse-grained facies-- - - - - - - - - - - - - -
Tectonic implications
References cited -----


ILLUSTRATIONS


Page


FIGURE 1. Index map showing the location of the Clubhouse Crossroads test holes ---------2. Generalized stratigraphic cross section through the Clubhouse Crossroads test holes __.
3. Stratigraphic column for the lower Mesozoic(?) red-bed sequence in CC#3 --------4. Graphic log for the fine-grained faces ------------------------5. Photographs of core segments showing sedimentary features of the fine-grained facies.
6. Graphic log for the coarse-grained facies ________ ___-------------7. Photographs showing sedimentary features of the coarse-grained facies ----------


D2
3
4
5
8 11
12


TABLES


Page


TABLE 1. Mineralogy of the clay-sized fraction - -----------------------2. Mineralogy of the light-mineral fraction -_____-- -- _____ --------------------------- -- --------3. Modal analyses of sandstone ---------------------------------------------4. Percentages of heavy minerals -------------------.------.---_----------III


D7
7
10 11


Page


D6
7 7 7 10
14 15 15 15


-- _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - - - - - -
------------------------------------------------_ _ _ _ _ _ _ _ __-- - - - _ _ _ _ _ _ _ _ -
--------------------------
























































































































































4













STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886TECTONICS AND SEISMICITY


GEOLOGY OF THE LOWER MESOZOIC(?) SEDIMENTARY ROCKS IN
CLUBHOUSE CROSSROADS TEST HOLE #3, NEAR CHARLESTON, SOUTH CAROLINA


By GREGORY S. GOHN, BRENDA B. HOUSER, and RAY R. SCHNEIDER


ABSTRACT
In Clubhouse Crossroads drill hole #3, near Charleston, S. C., a minimum of 121 m of well-consolidated sedimentary red beds underlies 256 m of subaerial basalt flows. The basalt flows are of Early Jurassic age and underlie 775 m of Cretaceous and Cenozoic Coastal Plain deposits. The red beds are of probable Late Triassic and (or) Early Jurassic age, on the basis of the age of the basalt and the lithologic similarity of the red beds to lower Mesozoic red beds exposed elsewhere in the Eastern United States. The red-bed section is divided into an upper fine-grained facies (39 m) consisting of mudstones, siltstones, and argillaceous sandstones and a lower coarse-grained faces (82 m) consisting of mudstones and conglomeratic sandstones. Sandstones in the fine-grained facies show several types of low-amplitude cross-stratification, whereas conglomeratic beds in the coarse-grained facies are inversely and normally graded. The predominant sandstones in both facies are arkosic wackes that contain abundant quartz-feldspar lithic fragments as well as abundant feldspar. Detrital lithic fragments and heavy minerals indicate that the red beds' provenance consisted primarily of granitic rocks. Microbrecciated granitic rock, basalt, and mylonite are additional types of detrital rock fragments found in the deposit. Comparison of the red-bed section with exposed Triassic-Jurassic red beds and modern continental sediments suggests that the Clubhouse Crossroads rocks were deposited in fluvial and alluvial-fan environments.

INTRODUCTION
Between January 1975 and May 1977, the U.S. Geological Survey drilled three deep stratigraphic test holes near Clubhouse Crossroads, Dorchester County, S. C. (fig. 1). Clubhouse Crossroads #1 and #2 penetrated a poorly consolidated Coastal Plain section and bottomed in a sequence of basalt flows. Clubhouse Crossroads #3 (CC#3) penetrated the basalt and bottomed in a sequence of well-indurated sedimentary red beds that is the subject of this report.
CC#3 is located at lat 32*54.18' N., long 80 18.95' W., in the south-central part of the Clubhouse Crossroads 7.5-minute quadrangle, 0.8 km northeast of Clubhouse Crossroads and about 40 km west-northwest of Charleston, S. C. CC#3 was drilled to a total depth of


1,152 m below a surface elevation of 6.7 m. Field descriptions of cuttings collected from the Coastal Plain section and of cuttings and cores recovered from the basalt and red-bed sections have been published by Schneider and others (1979). Geophysical logs were run only in the Coastal Plain section because the hole was blocked at the top of the basalt. Acknowledgments. -Deep drilling and other phases of the investigations conducted by the U.S. Geological Survey near Charleston, S. C., were supported by the U.S. Nuclear Regulatory Commission, Office of Nuclear Research, under Agreement No. AT(49-25)-1000. Melodie Hess performed the X-ray diffraction analyses of the red-bed samples; R. Wayne Burrow and Sharon L. King assisted in other laboratory analyses of the samples. The Clubhouse Crossroads test holes were drilled by a U.S. Army Corps of Engineers team from the Mobile, Ala., district. We express our appreciation to the Westvaco Company for use of their land for drilling sites.

STRATIGRAPHY
The Clubhouse Crossroads test holes encountered three major subsurface stratigraphic sequences: Coastal Plain sediments of Late Cretaceous and Cenozoic age, subaerial basalt flows of Early Jurassic age, and sedimentary red beds of presumed Triassic and (or) Jurassic age (fig. 2). Stratigraphic data from studies of the basalt and the younger sediments are reviewed in this section as ancillary data for a discussion of the stratigraphy of the red-bed section.

Coastal Plain Section
The Upper Cretaceous and Cenozoic section in the Clubhouse Crossroads test holes ranges in thickness from 750 m in CC#1 to 776 and 775 m in CC#2 and #3, D1









D2 STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886

820 810 80 79
80'20'11




17A



32- 841 6
55'7

c CC#2 A
Fayetteville
35* - CC#3 0
Clubhouse Crossroads CC#1--Lumberton





DORCHESTER COUNTY '165
00
j~jiqFlorence
rn 0
34* 2 KILOMETERS
340'ec 32'
50'
Myrtle
Beach

Lake
FALL cOrangeburg Mro

AugustaBowman Lake 0
B Moultrie Georgetown




RiVer Summerville


\) -. Area of
rn inset
7 Charleston





Beaufort

0 50 100 KILOMETERS



320 - Savannaho


FIGURE 1.-Index map showing the location of the Clubhouse Crossroads stratigraphic test holes near Charleston, S. C. Cross section A-A' is shown in figure 2. Route numbers of roads are shown in inset.








LOWER MESOZOIC(?) SEDIMENTARY ROCKS IN CLUBHOUSE CROSSROADS TEST HOLE #3


respectively. The Coastal Plain section was continuously cored in CC#1, and preliminary reports on the lithostratigraphy (Gohn and others, 1977) and biostratigraphy (Hazel and others, 1977) of this section have been published. Early Late Cretaceous (late Cenomanian) fossils (Hazel and others, 1977; Hattner and Wise, 1980) in sediments 18 m above the top of the basalt provide a minimum age for the underlying rocks (fig. 2).

Basalt
The basalt unit contains numerous individual subaerial flows of massive to amygdaloidal, moderately to strongly altered basalt (Gottfried and others, 1983; Phillips, 1983). Both the top (775 m) and bottom (1,031 m) contacts of the 256-m-thick basalt unit were penetrated in CC#3 (fig. 2).
Whole-rock K-Ar ages for heavily altered samples of basalt from CC#1 (Gohn and others, 1977; Gottfried and others, 1977) initially suggested a middle Cretaceous age for the basalt flows. Subsequent whole-rock K-Ar and 40Ar/39Ar ages and paleomagnetic data for less altered samples from the other holes indicate an older and presently accepted Early Jurassic age (Gohn and others, 1978; Lanphere, 1983; Phillips, 1983). An initial geochemical study of the basalt by Gottfried and others (1977) provided insight into the age, origin, and inferred tectonic affinities of the basalt that have been supported by the additional radiometric ages and by additional geochemical and petrologic studies (Gottfried and


A


CC#1


L1 7


200


cn




LI
0


400 600 800 1000 -


1200


others, 1983). The principal conclusions of the basalt studies to date are that the basalt in the Clubhouse Crossroads test holes has geochemical and inferred tectonic affinities with mafic igneous rocks from rifted passive continental margins in general and the eastern North American, lower Mesozoic tholeiitic diabases and basalts in particular.
A 2-m-thick bed of argillaceous red sandstone within the basalt sequence was cored at a depth of about 925 m, and other thin sedimentary beds may exist undetected in uncored sections of the basalt. The sandstone at 925 m contains abundant basalt fragments and quartz and minor microcline and perthite. The quartz and potassium feldspar suggest that the source of the sediment was not limited to the enclosing basalt. The presence of red sediments within the basalt section also suggests that the basalt has some temporal affinity with the underlying red-bed section.

Red-bed Section
In CC#3, 121 m of well-indurated sedimentary rocks ,were penetrated between depths of 1,031 and 1,152 m (fig. 3) before drilling was terminated because of mechanical problems. Of the 121 m of red beds, the top 23 m and the bottom 7.8 m were cored with moderate recovery, whereas only cuttings were obtained from the remaining 90.2 m.
From the top of the red-bed sequence at 1,031 m to a depth of about 1,070 m, the section consists of redcolored mudstone and smaller amounts of siltstone and


CC#3


CC#2


A'


Cenozoic sediments


239 m -


240 m


244 m


Upper Cretaceous sediments


750 m late Cenomanian fossils 792 m.L Basalt


907 m


Basalt


0 0.5 1 KILOMETER
I I I


Red


775m [776 m _


1031 m beds 1152 m


FIGURE 2.-Generalized stratigraphic cross section through the Clubhouse Crossroads test holes. Location of section shown in figure 1. Approximate altitudes of the tops of the drill holes are: 6 m (#1), 6 m (#2), and 7 m (#3).


1A


D3








STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


DEPTH, IN METERS
1025



10401060 1080




1100




1120 1140


1152-


LITHOLOGY


DESCRIPTION


x x x x x x Basalt X X X X X X Bsl

Red beds, fine-grained faces: massive redand green-mottled mudstone interbed- - -- - ded with red- and green-mottled, current- _- - - bedded siltstone and fine- to mediumgrained sandstone





- -,

Red beds, coarse-grained facies: red- and
- green-mottled, arkosic, coarse-grained to
conglomeratic sandstone interbedded with massive, red- and green-mottled
mudstone


* z


- I



- ~









EXPLANATION
Basalt

Sandstone, coarse-grained to conglomeratic, argillaceous
Sandstone, very-fine- to fine-grained,
argillaceous
Siltstone and mudstone

Core loss


FIGURE 3.-- Stratigraphic column for the lower Mesozoic(?) red-bed sequence in CC#3.


fine-grained sandstone. Cuttings from the interval between 1,066.8 and 1,075.9 m indicate that the lithology changes to coarse-grained or conglomeratic sandstone and minor amounts of red mudstone. Therefore, an upper fine-grained facies and a lower coarse-grained facies are defined, and the boundary is placed, somewhat arbitrarily, at 1,070 m.
Although plant debris was observed, no biostratigraphically useful macro- or microfossils were found in the red-bed cores. Nine samples processed for palynomorphs were barren (R. A. Christopher, oral commun., 1977). Age estimates for the red-bed sequence can be based, therefore, only upon the limits imposed by the age of the overlying unit and the uncertain regional lithostratigraphic correlation of this sequence with other similar rock units. The Early Jurassic age for the basalt imposes a minimum age limit of Early Jurassic for the red beds; an additional constraining age of early Late Cretaceous is supplied by the fossils in the basal Coastal Plain sediments. Gohn and others (1978) noted the lithologic similarity of the Clubhouse Crossroads red beds to the exposed fossiliferous Triassic and Jurassic Newark Group of the Eastern United States (Cornet and others, 1973; Cornet and Traverse, 1975) and the association of both exposed and buried rock sequences with radiometrically dated lower Mesozoic basaltic rocks. Although no age-specific data can be cited, these similarities provide some basis for provisionally assigning a Late Triassic and (or) Early Jurassic age to the red-bed sequence below the basalt in CC#3.
Lower Mesozoic red-bed sequences also are associated with basaltic rocks in fault-bounded basins below parts of the Georgia and Florida Coastal Plain and parts of the South Carolina Coastal Plain other than those discussed here (Marine and Siple, 1974; Popenoe and Zietz, 1977; Chowns and Williams, 1983; Daniels and others, 1983). Gohn and others (1978) interpreted the presence of lower Mesozoic basalt and lower Mesozoic(?) red beds in the Clubhouse Crossroads section to represent an extension of this lower Mesozoic(?) sub-Coastal Plain sequence into the Charleston, S. C., area.

DESCRIPTION OF THE RED-BED SECTION
Fine-grained Facies
LITHOLOGY AND PRIMARY SEDIMENTARY STRUCTURES The fine-grained facies of the red-bed section consists of about 39 m of color-mottled mudstones, siltstones, and argillaceous very-fine- to medium-grained sandstones. Sandstones and siltstones are current-bedded; these beds are grouped in 1- to 2-m-thick sections concentrated near the top of the section (fig. 4). Mudstones typically occur in 1- to 4-m-thick sections. Carbonaceous plant debris is common as finely comminuted grains


D4









LOWER MESOZOIC(?) SEDIMENTARY ROCKS IN CLUBHOUSE CROSSROADS TEST HOLE #3


CORING RUNS 1030.8


DEPTH,


DEPTH, IN METERS

1031 -


1035 -


1038.44CORE 9
CORE 10


1040 -


CORE 10 1044.90 CORE 11


1046.2 CORE 11
CORE 12


1045 -


1050 -


1054.4 1055 _J


LITHOLOGY CONTACTS I TEXTURE STRUCTURES


BASALT


CORE LOSS

















CORE LOSS





















CORE LOSS


CUTTINGS


4ciV
2 cm 1 cm














5 cm


123456789


I Fill H


1 cm ??


2 cm 1 cm


L _________ -I- __________


ED


dU


FIGURE 4.-Graphic log for the fine-grained facies of the red-bed section showing rock types, contact relationships, textures (determined from core with hand lens), and sedimentary structures.


D5


I


EXPLANATION
LITHOLOGY
[7-. Fine- to medium-grained
sandstone
Very fine- to fine-grained
sandstone
Siltstone to very fine-grained
sandstone Mudstone

CONTACTS
Sharp, erosional, amount of
relief shown
-,s ,-- As above with erosional
intraclasts
--- Sharp, irregular, amount of
relief shown
Gradational, thickness of gradational zone shown
TEXTURE
1 Clay <41A
2 Silt <63g
Sand 63A-2mm
3 VF <125, 4 F <250Ap 5 M <500A 6 C <1 mm 7 VC <2 mm 8 Granule <4 mm 9 Pebble <64 mm

STRUCTURES
Crossbeds, flaser, set height
<1 cm
vw- Crossbeds, planar, set height
5 cm to 1 m
Horizontal planar beds
: Crossbeds, planar, flat base,
set height 1 to 2 cm
C Crossbeds, trough, erosional
base, set height 1 to 2 cm
Calcite nodules
Feature abundant or well developed
Feature common or obvious

Feature sparse or poorly developed


. . .







STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


along bedding and crossbedding in sandstones and as larger fragments throughout mudstones. The sandstones and siltstones show a variety of smallscale (low-amplitude) primary sedimentary structures (figs. 4, 5). Costs of trough crossbeds having individual set heights of less than 2 cm are typical of the siltstone; similarly small sets of planar cross lamination and flaser bedding, in addition to horizontal bedding, characterize the sandstones. A 1-m-thick set of planar crossbeds occurs in a fine-grained sandstone bed near the top of the section. The lower contacts of the sandstone beds typically are erosional and show several centimeters of relief. Mudstone and siltstone intraclasts and quartz granules overlie the erosional surfaces. Near the top of the core, at about 1,032.0 to 1,032.8 m, there is a broad and irregular contact zone between an upper poorly sorted medium-grained sandstone and a lower sequence of fine-grained sandstone and mudstone that contains abundant calcite nodules (fig. 5A). Within the contact zone as seen in the core, the medium-grained sandstone occurs in irregular patches that are not vertically continuous with the main body of this rock type at the top of the section (figs. 4, 5A). These structures are interpreted to be soft-sediment deformation features, possibly ball-and-pillow structure, produced by sediment loading on a water-saturated, unstable substrate. As additional evidence of sediment loading, a possible flame structure occurs at the principal contact between the two units at 1032.4 m.
The mudstones in the fine-grained facies are typically massive, nonfissile, fine-grained rocks lacking primary stratification. However, numerous secondary features and zones of disrupted fabrics are common.

DIAGENETIC FEATURES
Calcite occurs in irregularly shaped nodules as large as 6 cm and irregular blebs as small as 1 mm. The nodules consist of microspar in submicroscopic to 100-P grains. In thin sections, the nodule interiors show a wide range of organization; some have fine-grained rims and coarser grained interiors, whereas others have a translucent clotted fabric. Most nodules contain at least a few relict detrital grains or small patches of matrix. A continuum in the replacement of detrital material can be seen in the thin sections, from areas devoid of calcite to areas where detrital matrix and framework grains are partially replaced to areas showing well-formed nodules. The nodules occur in all lithologies, but the large nodules tend to be concentrated at specific horizons, particularly the calcareous interval near the top of the section (figs. 4, 5A).
The color-mottling of the red-bed section is a secondary feature that cross-cuts primary stratification. Most of the rocks of the fine-grained facies are grayish red


(5R4/2, 10R4/2) or pale reddish brown (10R5/4) (Goddard and others, 1948). Non-red parts of the core are grayish green (10GY5/2, 5G5/2, 5G6/1) or grayish yellow green (5GY7/2). The occurrence of colors other than red is related to depositional and diagenetic features of the core. Green is found in halos around carbonized plant material in fine-grained beds (fig. 5F), in halos around calcite nodules, in sandstones containing fine-grained carbonized material (fig. 5C), and in mudstones that occur just below carbonaceous sandstones or as intraclasts within those sandstones (figs. 5B, C, D). Curving, irregular slickensided surfaces pervade the mudstones and occur at all orientations, including horizontal (fig. 5G). The thicker mudstone sections of the core typically are broken and partly disaggregated due to breakage along the slickensided surfaces. The absence of a preferred orientation of the slickensides may have resulted from random particle orientation in the massive mudstone. These surfaces indicate that considerable mechanical deformation of the mudstone has occurred. This deformation may have been caused by lithostatic loading, by volumetric mineralogic changes during diagenesis, or by tectonic stress.

MINERALOGY AND PETROLOGY
Clay mineralogy. - Two distinct clay-mineral suites occur in the fine-grained facies (table 1). Samples above 1,035 m contain mostly interlayered illite-smectite, which was found to consist of 100 percent expandable layers and is, therefore, true montmorillonite. Only minor amounts of kaolinite-group minerals or chlorite and moderate amounts of illite are present. In samples from 1,039.24 m to 1,066.8 m, the dominant mineral is discrete illite with lesser amounts of illite-smectite and minor kaolinite and chlorite. The illite-smectite is of relatively low expandability (40 to 62 percent). The two clay-mineral suites correlate with specific rock types in the core. The upper suite, dominated by mixed-layer clay, is contained within sandstones, whereas the lower, illite-dominated suite is contained within mudstones and siltstones.
Sand mineralogy - light-mineral fraction. - The lightmineral fraction of sand samples (63-125 p) isolated from sandstones of the fine-grained facies were studied by means of refractive index oils in grain mounts (table 2). In all studied samples, feldspar predominated over quartz; plagioclase was more abundant than potassium feldspar. Albite and oligoclase, in subequal amounts, were the only plagioclase minerals recognized. Sandstone petrology.- Sandstones in the fine-grained facies are compositionally and texturally immature rocks characterized by high matrix contents and abundant unstable detrital grains, including polycrystalline quartz, feldspar, and quartz-feldspar lithic fragments


D6










LOWER MESOZOIC(?) SEDIMENTARY ROCKS


TABLE 1. -Mineralogy of the <2-p, clay-sized fraction of 23 samples from the red-bed section
[Determined by X-ray diffraction. Samples were sieved and centrifuged after removal of the carbonate fraction by means of acetic acid. Semiquantitative determinations of the types and amount of the minerals present follow the methods of Reynolds and Hower (1970) and Perry and Hower (1970). Tr, trace.]


Depth of sample below surface (in)
1,032.74. .......
1,033.03--------_1,033.80-------1,034.23.........-1,039.23.........-1,040.03 ........
1,040.45.........-1,042.00-,.4..----1,043.25. .4 ___.. 1,047.02.--,-.4-__1,050.94 1,051.78_ -......
1,052.03---------::
1,052.20 ........
1,060.7-1,066.8
1,094.2-1,104.0--1,122.3-1,131.4--1,122.3-1,131.4--1,145.34.........-1,145.67 -- -- --
1,146.67 -- -- --
1,146.95.........-1,147.40 -- - - -
*Also contains chins


Percent
Constituent minerals (percent) expandable Mixed-layer layers in Kaolinite illite- Iite
group Illite smectite smectite
0 50 50 100
14 36 50 100
0 10 90 100
8 19 73 100
18* 76 6 45
20* 72 8 40
15 77 5 49
8* 35 57 40
14* 46 40 49
19* 70 11 62
14* 76 10 45
16* 71 13 49

20* 61 19 50
13* 60 27 45
14* 43 43 40
7- 47 46 45


62 38 Tr
8* 77 15


Tr 9.
10*
Tr 10*


24 20 40 11 37


76 71 50 89 53


40

64 90 82 93 62


ite.


Lithology sandstone sandstone sandstone sandstone mudstone mudstone
siltetone mudstone mudstone siltstone mudstone mudstone
mudstone mudstone mudatone cuttings mudatone and conglomerate cuttings granitic rock fragment mudstone cuttings
conglomerate muld tone conglomerate conglomerate conglomerate


(table 3). The framework grains are only moderately rounded. These rocks typically contain at least 10 percent argillaceous matrix, which consists of detrital mica and chlorite, fine-grained quartz and feldspar, and clay minerals, all of which are iron stained. Locally the detrital matrix is partly to completely replaced by secondary calcite.
Using the classification of Williams and others (1954, p. 292-293), we classified most of the studied sandstones as arkosic or feldspathic wackes. Sandstones containing abundant locally derived siltstone and mudstone intraclasts are lithic wackes (sample from 1,032.0 m, table 3).


Coarse-grained Facies.

LITHOLOGY AND SEDIMENTARY STRUCTURES

The coarse-grained facies consists of 82 m of very poorly sorted, argillaceous, coarse-grained to conglomeratic sandstones and interbedded mudstones (figs. 6, 7). The sandstones in the cored interval are in 0.5- to 1.0-m-thick beds that alternate with 0.25- to 0.5-m.-thick mudstones (fig. 6). The upper contacts of sandstone beds typically are sharp and slightly undulating; as much as 1 cm of relief is visible in the core. Basal contacts of sandstone beds are also sharp. However, recognizing exact basal contacts is difficult because the lower part of each sandstone bed has a high matrix content and resembles the underlying mudstone bed. Each basal sandstone contact is best delineated by the abundance of large calcite


IN CLUBHOUSE CROSSROADS TEST HOLE #3 D7

TABLE 2.-Mineralogy of the light-mineral fraction of sand-sized grains (68-125 p) from sandstones
[Determined by means of refractive index oils and the central-focal-masking technique. 100 grains were counted per sample. Analyses are given as percentages]
Depth of ample Potassium
below surface (m) Quartz feldspar Albite Oligoclase 1,032.7 ------- 45 15 29 11
1,033.0 ------- 23 4 26 47
1,033.8-------- 25 4 17 54
1,034.2 ----------24 6 45 25
1,047.0 ------- 14 2 32 52



nodules in the upper part of the underlying mudstone (figs. 6, 7D). Sedimentary structures in the sandstone beds consist of inverse or inverse-to-normal size grading (fig. 6). Pebbles and granules occur in the center or top of each bed, whereas sand-sized grains and fine-grained matrix are distributed throughout each bed ("coarse-tail grading"). Other forms of stratification and ordered fabrics are difficult to discern but are probably present. For example, in figure 7A, the top half of the core shows faint inclined bedding (upper left to lower right), and there is a tendency toward subhorizontal alinement of the larger, flat clasts.
Mudstones in the coarse-grained facies are similar to those in the fine-grained facies and typically are massive, red- and green-mottled rocks. No primary sedimentary structures were observed in the mudstones; their most obvious features are common calcite nodules and veinlets (figs. 7C, 7D).
Rocks of the coarse-grained facies are dominantly grayish red (5R4/2 or 10R4/2). Nonred color mottling is typically yellowish gray (5Y7/2) or pale greenish yellow (10Y8/2) and occurs within and around carbonate nodules and veinlets and plant debris.


MINERALOGY AND PETROLOGY

Clay mineralogy. -The clay-mineral suite found in samples between depths of 1,094.2 m and the bottom of the section differs from both clay suites found in the fine-grained facies (table 1). In the coarse-grained facies, mixed-layer illite-smectite of moderate expandability (64 to 93 percent) is the dominant mineral. Illite is common but in relatively smaller percentages than above; chlorite and a kaolinite-group mineral are present in amounts from trace to 10 percent. A kaolinitegroup mineral (62 percent) and illite (38 percent) constitute the clay-sized fraction of a chip of granitic rock from the interval between 1,122.3 and 1,131.4 in. In that sample, these two minerals are interpreted to represent the principal alteration minerals of the feldspars in the granitic rock.

Sandstone petrology. -Argillaceous conglomeratic sandstones in the coarse-grained facies are extremely immature rocks characterized by high matrix content and large amounts of unstable polycrystalline grains, particularly quartz-feldspar lithic fragments (table 3;








D8


L -~ ~14










-i-il.. ...


B


I


'1





/' -ii

-1


A


FIGURE 5.-Core segments showing sedimentary features of the finegrained facies. A, Medium-grained sandstone overlies (upper highlighted contact) and is interlayered with (at base of ruler) argillaceous fine-grained sandstone. Lower irregular lens of the coarser lithology and a possible flame structure at the upper contact (arrow) may be due to sediment loading. Calcite nodules (dark) occur in both lithologies. Length of ruler is 15 cm. Upper contact is from a depth of about 1,032.1 m. B, Fine- to medium-grained sandstone (upper third of sample) contains numerous grayish-green (light) and grayish-red (dark) siltstone clasts. Sandstone overlies (highlighted contact) the eroded top of red- and green-mottled siltstone containing numerous 1- to 5-mm calcite nodules. Mottled siltstone grades


downward into dominantly red siltstone that lacks nodules. Scale in inches; bar equals one cm. Base of sandstone is from depth of 1,035.30 m. C, Medium-grained, cross-laminated sandstone overlies the eroded top of grayish-green mudstone at lower highlighted contact (11-cm mark). Quartz granules and small mudstone clasts occur in base of sandstone, and carbonaceous debris is concentrated along cross laminations. Medium-grained sandstone grades upward into fine-grained sandstone with planar, horizontal laminations. Horizontal laminations are truncated at top along moderately dipping erosional surface that displays several hemispherical pits (burrows?). Dipping erosional surface is overlain by mudstone. Base of sample is from 1,047.7 m depth. Scale in centimeters.


I














U)J,









0


1 2 13 14 1 5
F


Figure 5.-Continued. -D, Contact (highlighted) between an upper color-mottled fine-grained sandstone and a lower color-mottled mudstone. The mudstone is mottled grayish red (dark) and grayish green (light). The sandstone is mottled grayish red and light olive gray and displays several small sets of planar to gently curving cross laminations. The bases of the sets and the sandstone-mudstone contact are erosional; the latter contact has about one centimeter of relief. Cross lamination is disrupted by the presence of thin mudstone clasts at about the 6-cm mark (foreset slumping?). The cross-laminated sandstone grades upward (above the 8-cm mark)


If)


(Y)


into wavy laminated (not visible) red siltstone. Scale in centimeters. Base of sample is from a depth of 1,046.0 m. E, Dominantly red siltstone displaying faint sets of trough cross laminations. Scale in centimeters. Sample is from a depth of about 1,046.6 m. F, Grayishgreen halo around carbonized plant fragment in dominantly grayishred mudstone. Scale in centimeters. Sample from a depth of 1,050.8 m. G, Slickensided horizontal surface in mudstone from a depth of about 1,047.9 m (wet for photograph). Green mottles and carbonaceous material are also visible. Diameter of core is about 6.7 cm.


Cr








STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


TABLE 3.-Modal analyses (in percent) and petrologic classifications of sandstones from the red-bed section
[250 grains were counted per slide. The sandstone classification is from Williams and others (1954, p. 292-293)]
Lithic fragments
Depth of Quartz Feldspar g Fine- Other
saml Qurt Fedsa fedpr rmed and
ge Mono- Poly- Potas- so-plutni sedirnen- unknown
crystane cryst line Total i cs Total rock tary rock Total Matrix Calcite minerals Sandstone classification
Fine-grained foci es:
1,032.0 - - -- 22.4 6.0 28.4 4.8 3.6 8.4 2.0 14.8 16.8 36.4 8.4 1.6 lithic wacke
1,035.3-------------24.0 15.6 39.6 12.0 5.6 17.6 12.4 1.2 13.6 13.6 14.8 .8 arkose or arkosic wacke
1,046.0 -------------32.4 6.8 39.2 2.4 2.8 5.2 2.4 -- 2.4 24.0 27.6 1.6 feldspathic wacke
Coarse-grained foci es:
1,144.7 ------------ 3.2 4.8 8.0 2.4 8.0 10.4 65.6 __ 65.6 10.4 5.2 .4 arkose
1146.7 8.0 7.4 15.4 9.2 7.8 17.0 25.6 25.6 39.6 1.2 12 arkosic wake
1,146.9 ------- 2.8 11.2 14.0 12.4 5.6 18.0 34.0 -- 34.0 29.2 4.8 -- arkosic wacke
3'Includes perthite.
Includes myrmekite.


fig. 7). The conglomeratic sandstones are classified as arkoses or arkosic wackes (Williams and others, 1954, p. 292-293).
The quartz-feldspar lithic fragments represent granitic rocks composed of plagioclase, perthite, myrmekite, quartz, chlorite, epidote, opaque minerals, and sphene. Feldspar and quartz also compose most of the single-crystal sand-sized grains in the unit and display the same microstructures and types of alteration both as single grains and in granitic clasts (Gohn, 1983). Less common types of lithic fragments include altered basaltic rocks, mylonite, and microbreccia. Sediment provenance. - Gohn (1983) discusses in detail the mineralogy and petrology of the rock fragments in the conglomeratic sandstones and concludes that the sediment source area consisted primarily of granitic plutonic rocks. Because the granitic clasts are contained within beds interpreted to have been deposited close to their source area (see following section on depositional environments), at least part of the pre-Mesozoic basement in the greater Charleston area must consist of these granitic rocks (Gohn, 1983). In addition, the microbreccia and mylonite clasts indicate that two styles (and times?) of deformation have affected the local basement. The basalt clasts indicate that a period of basaltic magmatism older than the one represented by the basalt above the red-bed section in CC#3 occurred in the area.
Nonopaque, heavy-mineral suites present in both the fine-grained and the coarse-grained units (table 4) are characterized by low diversity of mineral species and a dominance of epidote. Heavy-mineral fractions separated from samples at the top of the section (samples at 1,034.3 m and above) and at the bottom of the section (samples at 1,145.7 m and below) consist of more than 85 percent epidote. Zircon, tourmaline, apatite, biotite(?), chlorite, and garnet are the only other minerals present in significant amounts. The samples at 1,042 m and 1,047 m contain more diverse suites but are dominated by zircon (1,042 m) or phyllosilicates (1,047 m). Samples consisting of rotary cuttings between depths of 1,061 and 1,131 m contain more diverse suites than do the cored samples, although epidote and biotite


(or stained chlorite) remain the most common minerals. Because the cuttings unavoidably contain cavings from higher stratigraphic levels, the relatively high mineralogic diversity of these samples was attributed to contamination. Epidote, zircon, tourmaline, white mica, and iron-stained chlorite were also seen as detrital grains in thin sections, as was detrital sphene. Chlorite flakes as large as 1.0 mm and white mica flakes as large as 0.2 mm were observed.
The low diversity of the heavy-mineral suites in the core samples is interpreted to indicate that the sediments were derived from a limited variety of sourcerock types. In addition, the lack of dilution of this limited suite by other minerals from other sources suggests that the source rocks were close to the site of deposition. The abundance of epidote suggests that selective diagenetic removal of mineral species by intrastratal solution is not the cause of the low diversity. In general, the composition of the heavy-mineral suite supports the interpretation made from the rock-fragment data that the source area was dominantly granitic. Epidote is a common to abundant secondary mineral within all types of rock fragments in the deposit, including the granitic clasts (Gohn, 1983), and its abundance as individual detrital grains in the heavy-mineral suite, therefore, is to be expected. The remainder of the observed heavy minerals that are common in the samples (zircon, tourmaline, apatite, mica, sphene) are all typical accessories in granitoid rocks. Chlorite was observed to be common in the detrital granodiorite, microbreccia, and basalt clasts in the deposit (Gohn, 1983).

DEPOSITIONAL ENVIRONMENTS

The depositional history and environments represented by the red-bed section are interpreted herein through comparison with sediments found in exposed early Mesozoic basins of the Eastern United States and through comparison with modern deposits in Israel and the Southwestern United States. The basis for comparison with the Mesozoic rocks is the general lithologic


D10









LOWER MESOZOIC(?) SEDIMENTARY ROCKS IN CLUBHOUSE CROSSROADS TEST HOLE #3


D11


TABLE 4.-Percentages of heavy minerals in red-bed samples (very fine sand fraction) [The heavy minerals were studied in refractive index oils; 200-500 grains were counted per slide. Tr, noted but not counted or less than 1 percent. , cuttings]
Depth of sample 1061- 1094- 1122below surface (m) 1032.8 1033.0 1034.0 1034.3 1042.0 1047.0 1067* 1104 1131* 1145.7 1146.7 1147.0
Zircon ------------- 5 Tr 1 Tr 78 Tr 4 2 6 7 5 4
Tourmaline --------- 5 0 Tr Tr 1 Tr 14 5 3 Tr 0 Tr
Rutile ------------- Tr 0 0 0 1 0 0 0 Tr Tr 0 0
Apatite ----------- 1 1 3 Tr 4 2 19 10 7 0 0 0
Muscovite ---------- Tr 0 0 2 Tr 36 7 5 5 Tr 0 0
Biotite ------------- 1 0 0 0 Tr 47 28 38 53 1 2 2

Epidote ------------ 86 98 95 97 5 11 14 31 16 90 92 94
Chlorite ------------ Tr 1 Tr Tr 1 3 0 2 5 Tr 0 0
Garnet ------------- 1 0 0 0 10 0 10 4 5 0 0 0
Sillimanite ---------- 0 0 0 0 0 Tr 1 0 0 0 0 0
Amphibole ---------- 0 0 0 0 0 0 2 3 0 Tr Tr 0
Pyroxene ----------- 0 0 0 0 0 0 0 Tr Tr 0 0 0


LITHOLOGY


CUTTINGS


.,..000 *O
*,O,0*00..
00 00 0*0. .00.0.:
0~ 0
0....~0~** 4.
0.0. *0 *.~*0~ *0*0
* *0050.....0. 0.00
000o.0O '.*~ 00,0~*
000000 .*o *'. 00.4
0 .'....0 0 o
0 *0 0 00 000000 000 0 0 00..00....000

*0o0~ .00 00~. oOO .o~0. ~ *.~
o ~.: *0.:....~ .0.0.0: *o 00'.~0 00. .0*. 0.
*o0* 0~*o o0*0.0..00:::o,:.00.%o. O 5..0 ~ *. 0000 ..0..s'~0*..
.00...*0,..., 0
*0* W"~
00 0 0.o*.*0~00 .00
*00.
.00~~0 .~ ~ ~~ooo* ,s,
0.00. 00. 0 0,00
*5'040
.0 0*,0~ ~ 000 .'.00. 0.000.







CORE LOSS








(0.ORF LOSS


BED
CONTACTS


TEXTURE
1 2 3 4 5 6 7 8 9 I I I I I I


STRUCTURES













D . -1


EXPLANATION


DEPTH, IN METERS
1144




1145 1146 1147 1148 1149 1150



1152-


TEXTURE
1 Clay 2 Silt Sand 3 VF
4 F 5 M 6 C
7 VC


<41.
<63 63t-2 mm <125g
<250/c <500sA <1 mm
<2 mm


8 Granule <4 mm
9 Pebble <64 mm


STRUCTURE
* .-. Normal grading ._- Inverse grading
Calcite nodules
Abundant or well
developed
Common or obvious
Sparse or poorly
developed


FIGuRE 6.-- Graphic log for the coarse-grained faces of the red-bed section showing rock types, contact relationships, textures (determined from core with hand lens), and sedimentary structures.


similarity and probable correlation of the Clubhouse Crossroads red-bed section with the exposed Triassic and Jurassic red beds. However, inherent limitations exist in studying a discontinuously cored section and comparing it with exposed sections. These limitations include the lack of information on the large-scale


geometries of beds and the limited information on the vertical sequence of units and structures in the cores.
The exposed Triassic-Jurassic red beds and associated igneous rocks, and their tectonic setting, have been extensively researched. The general conclusion of most authors is that exposed Triassic-Jurassic sedimentary


CONTACTS
Flat, sharp
1cm Irregular,amount
of relief shown

LITHOLOGY

Argillaceous,
conglomeratic
sandstone

Mudstone


'









D12


SB


A


FIGURE 7.-Sedimentary features of the coarse-grained facies. A, Typical conglomeratic sandstone showing dominance of granitic (light-colored) debris in the sand- and granule-sized fraction. Large (dark) rock fragments are basalt. Note the poor sorting, local variations in matrix content, and possible inclined stratification (in upper third of sample). Sample from a depth of 1,144.7 m. Scale = 15 cm. B, Rock fragments in conglomeratic bed. Largest clast is granite. Dark fine-grained clast at upper right is basalt. Smaller light-colored clast near center is epidote-rich cataclastic rock. Note the poor sort-


ing and lack of an ordered fabric. Core segment illustrates local inverse size grading of clasts. Sample from a depth of 1,144.6 m. Scale in cm. C, Red mudstone containing green-rimmed calcite nodules and veinlets. Sample from a depth of 1,145.5 m. Scale in cm. D, Slabbed section showing contact (highlighted) between sandstone bed and underlying mudstone containing large calcite nodules. Smaller white grains are granitic material in sandstone and calcite in mudstone. Sample from a depth of 1,146.8 m. Inverted scale in cm.


STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 I" ~U4J4








Ln














'Uav








LOWER MESOZOIC(?) SEDIMENTARY ROCKS IN CLUBHOUSE CROSSROADS TEST HOLE #3 D13 I,1





































D
D-


FIGURE 7.-Continued.


C







STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


rocks represent deposition in a variety of continental environments within closed rift basins (grabens). Interpreted environments typically include alluvial fans, braided and meandering streams and associated floodplains, and permanent and intermittent lakes (for example, Van Houten, 1969, 1980; Hubert and others, 1978; Manspeizer, 1980).

Fine-grained Facies
Generally, active sedimentary processes during deposition of the fine-grained facies were relatively low energy. This characteristic is suggested by the fine grain size of the facies as a whole, the moderate sorting of the sandstones, and the small amplitude of the cross stratification. However, several of the sandstones shown in figure 5 contain mudstone intraclasts, which illustrates that energy levels were occasionally high enough to erode partially consolidated, stable substrates. Rapid deposition on unstable, water-saturated substrates also occurred, as suggested by the deformed and intermixed lithologies shown in figure 5A. The flaser bedding (fig. 4) indicates that occasionally traction movement of sand alternated with settling of fine sediments from slack water. The absence of body fossils and the rare occurrence of possible biogenic structures (fig. 5C) suggest deposition in an environment lacking an abundant fauna.
The lithologies, sedimentary structures, and inferred depositional processes of the fine-grained facies in CC#3 are generally similar to those documented in sections of grayish-red mudstones and associated thin sandstones and siltstones in exposed sequences of Triassic-Jurassic rocks. These exposed rocks are generally interpreted to represent vertically accreting floodplain (overbank) deposits associated with fluvial systems or the distal ends of alluvial fans (Hubert and others, 1978, p. 104, figs. 18, 35; Manspeizer, 1980, p. 331, 334, 343).
However, some sedimentary features considered to be diagnostic of typical fluvial floodplain deposits (Reineck and Singh, 1975, p. 244-253) are absent in the finegrained facies of the Clubhouse Crossroads section. Specifically, climbing-ripple lamination in sandstones and horizontal lamination in mudstones were not found. In addition, channel-sand deposits, a necessary element in fluvial systems, are not represented in the Clubhouse Crossroads core unless the rocks between 1,035.5 m and the top of the red-bed section (fig. 4) represent one or more truncated, relatively fine-grained, channel-bar deposits. Plant root casts, mud cracks, and raindrop impressions also were not observed in the mudstones. The absence of these features may indicate that the finegrained facies was deposited in an environment somewhat different from the generally recognized fluvial floodplain environment associated with perennial streams.


Because of the massive character of the dominant mudstone and the paucity of structures and vertical sedimentary sequences typical of fluvial channel deposits, the fine-grained facies bears a close resemblance to sedimentary sequences in the semi-arid southern Basin and Range province, which have been deposited on the floors of dry closed basins (B. Houser, unpub. data, 1981). A dry-basin floor is defined topographically as the relatively flat, generally elongate axial part of a sediment-filled closed basin, flanked by alluvial fans or pediments and exclusive of the parts of the basin occupied by lakes or playas. A key element of the dry-basin floor environment is the high ratio of the volume of transported sediment to the volume of water available to transport this material. Gerson (1977) has described sediment transport by ephemeral streams in an analogous environment near the Dead Sea in Israel.
Dry-basin floors are environments that contain finegrained alluvium derived from the distal parts of adjacent alluvial fans and also reworked from the basin floor itself. The sediment is transported episodically by overloaded streams during floods, chiefly in the form of slurries and less commonly as mudflows. Coarse-grained sediment (sand and gravel) is deposited in shallow channels on distal fan surfaces during waning flow or on the largely unchanneled surface of the basin floor in response either to waning flow or to the abrupt decrease in topographic gradient at the alluvial fan-basin floor boundary. After deposition of the coarser grained material, high concentrations of suspended fine-grained sediments remain in the transporting fluid. These sediments ultimately are deposited in beds with characteristics that are either typical of mudflow deposits or intermediate between those of mudflow deposits and water-lain sediments (Bull, 1963; Gerson, 1977).
Hence, the characteristic features of the sedimentary sequences of dry-basin floors are interpreted to result from deposition from slurries and differ from the more commonly noted sedimentary features of floodplains of perennial streams where water budgets are larger and sediment concentrations in channels and floodwaters are relatively lower. In particular, bimodal sorting into massive mudstone beds (lacking horizontal lamination) and thinner lenses of sand or gravel is diagnostic of drybasin-floor sediments.
The similarity of the sedimentary features seen in the fine-grained facies of the core to those outlined above for deposits of dry-basin floors suggests that this may have been the depositional environment of the finegrained facies. In addition, the calcite nodules (fig. 5) in the core resemble features in upper Paleozoic and Mesozoic continental rocks that are typically interpreted to represent pedogenic carbonate deposits (caliche) (Van Houten, 1973; Marine and Siple, 1974; Steel, 1974;


D14







LOWER MESOZOIC(?) SEDIMENTARY ROCKS IN CLUBHOUSE CROSSROADS TEST HOLE #3


Jensen, 1975; Hubert, 1977). Paleosol horizons marked by caliche zones are also common in dry-basin floors of the semi-arid Southwestern United States and are taken as evidence of significantly long periods of subaerial exposure between depositional events, during which time little erosion occurred. Caliche in the Clubhouse Crossroads red beds is incipiently developed (mostly type 1 of Steel, 1974; stage 1 of Hubert, 1977) and consists only of scattered nodules and calcite cement in sandstones. Laterally continuous nodular limestone layers and laminar, brecciated, or pisolitic limestone are absent.
Also compatible with the interpretation of a continental (fluvial-alluvial) environment is the dominantly red color of the fine-grained and coarse-grained facies, which suggests oxidation of these rocks in a subaerial setting except where local reducing conditions (producing colors other than red) were maintained by the presence of plant material (fig. 5C, F) or were produced by the calcification process.

Coarse-grained Facies
Beds of the coarse-grained facies represent deposition by debris flow (sediment-gravity flow) rather than by water-gravity flow. As discussed by Middleton and Hampton (1973), debris flows are cohesive, high-density mixtures of water, mud, and coarser grained sediment that possess a finite yield strength and buoyancy and that move sluggishly downslope under the influence of gravity. Characteristics of the conglomeratic sandstones of the coarse-grained facies that suggest debris flow include inverse and inverse-to-normal size grading of clasts within beds, matrix-supported textures, limited alinement of large and perhaps small clasts in a finegrained matrix (indicating shearing of a cohesive sediment-water mixture), and very poor sorting (figs. 6, 7). Although mudstones in the coarse-grained facies show few primary features, they may represent mud flows that moved in a manner similar to the debris flows and hence may be genetically related to the mudstones of the fine-grained facies.
The identification of debris-flow deposits in a sequence of continental rocks is evidence for the interpretation of an alluvial-fan environment. Rust (1979) concluded, in a summary discussion of alluvial fans (p. 12), "that with few exceptions debris flow deposits characterize alluvial fans and can be used (together with other criteria) to recognize ancient fan deposits * * " although other depositional processes do occur on fans. As examples, exposed sequences of Triassic and Jurassic conglomerates and associated rocks displaying many of the same structures and textures as the coarse-grained facies of the Clubhouse Crossroads section have been interpreted as alluvial fan deposits (Randazzo and others,


1970; Cloos and Pettijohn, 1973; Lindholm and others, 1979; Hubert and others, 1978). Additionally, in modern continental settings, the natural conditions observed to promote debris flow are typical of alluvial fans, particularly those in semi-arid climates (Bull, 1972). Our interpretation is, therefore, that the coarse-grained facies of the Clubhouse Crossroads red beds represents deposition on an ancient alluvial fan and probably, on the basis of the lack of boulders and the proportion of fine-grained beds in the deposit, on the medial to distal part of the fan.
Tectonic Implications
Several corollaries follow from the interpretation of alluvial-fan and related sedimentary environments for the Clubhouse Crossroads red-bed section. Firstly, debris flow (and mud flows) do not transport sediment for long distances. An extremely long transport distance of over 20 km for a 1941 Wrightwood, Calif., flow reported by Sharp and Nobles (1953) can probably be considered an upper limit for movement by a single debris flow. Accordingly, as discussed above, detrital lithic fragments and other sediments in the con'glomeratic sandstones can be considered representative of at least some of the pre-Mesozoic rocks underlying the Charleston-Summerville area.
Secondly, late Cenozoic alluvial fans in the Western United States (for example, Hooke, 1967, figs. 2, 3; Bull, 1972, p. 77-81), inferred lower Mesozoic alluvial-fan deposits in the Eastern United States (for example, Hubert and others, 1978, p. 9; Lindholm and others, 1979; Ratcliffe, 1980, p. 288-292; Manspeizer, 1980), and other examples of alluvial-fan deposits (for example, Rust, 1979) are spatially associated with faulted basin margins. In these cases, near-surface faulting produced the topographic relief necessary for accumulation of the alluvial-fan deposits. Therefore, by analogy, the coarsegrained facies of the Clubhouse Crossroads red beds probably records deposition in an area of formerly great topographic relief, which was perhaps produced by faulting. This tentative conclusion, based on sedimentfacies analysis, is consistent with interpretations, from seismic surveys, that locally thick (up to 1 km) or thin sequences of red beds occur in a faulted horst-andgraben terrane below Jurassic basalt and (or) Cretaceous sediments in the Charleston-Summerville area (Talwani, 1977; Ackermann, 1983; Hamilton and others, 1983; Schilt and others, 1983).

REFERENCES CITED
Ackermann, H. D., 1983, Seismic-refraction study in the area of the
Charleston, South Carolina, 1886 earthquake, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity: U.S. Geological Survey Professional Paper 1313, p. F1-F20.


D15








STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


Bull, W. B., 1963, Alluvial-fan deposits in western Fresno County,
California: Journal of Geology, v. 71, no. 2, p. 243-251.
1972, Recognition of alluvial-fan deposits in the stratigraphic
record, in Rigby, J. K., and Hamblin, W. K., eds., Recognition of ancient sedimentary environments: Society of Economic Paleontologists and Mineralogists Special Publication no. 16, p. 63-83. Chowns, T. M., and Williams, C. T., 1983, Pre-Cretaceous rocks beneath the Georgia Coastal Plain- Regional implications, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity: U.S. Geological Survey
Professional Paper 1313, p. L1-L42.
Cloos, Ernst, and Pettijohn, F. J., 1973, Southern border of the
Triassic basin, west of York, Pennsylvania: Fault or overlap?:
Geological Society of America Bulletin, v. 84, no. 2, p. 523-535. Cornet, Bruce, and Traverse, Alfred, 1975, Palynological contributions to the chronology and stratigraphy of the Hartford basin in Connecticut and Massachusetts: Geoscience and Man, v. 11, p.
1-33.
Cornet, Bruce, Traverse, Alfred, and McDonald, N. G., 1973, Fossil
spores, pollen, and fishes from Connecticut indicate Early Jurassic age for part of the Newark Group: Science, v. 182, no. 4118, p.
1243-1247.
Daniels, D. L., Zietz, Isidore, and Popenoe, Peter, 1983, Distribution of subsurface lower Mesozoic rocks in the Southeastern United States as interpreted from regional aeromagnetic and gravity maps, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886 -Tectonics and seismicity: U.S. Geological Survey Professional Paper 1313, p.
K1-K24.
Gerson, Ran, 1977, Sediment transport for desert watersheds in
erodible materials: Earth Surface Processes, v. 2, no. 4, p.
343-361.
Goddard, E. N., and others, 1948, Rock-color chart: Washington,
D.C., National Research Council, 6 p. (republished by Geological
Society of America, 1951; reprinted 1975).
Gohn, G. S., 1983, Geology of the basement rocks near Charleston,
South Carolina-Data from detrital rock fragments in lower Mesozoic(?) rocks in Clubhouse Crossroads test hole #3, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity: U.S. Geological Survey
Professional Paper 1313, p. El-E22.
Gohn, G. S., Gottfried, David, Lanphere, M. A., and Higgins, B. B.,
1978, Regional implications of Triassic or Jurassic age for basalt and sedimentary red beds in the South Carolina Coastal Plain:
Science, v. 202, no. 4370, p. 887-890.
Gohn, G. S., Higgins, B. B., Smith, C. C., and Owens, J. P., 1977,
Lithostratigraphy of the deep corehole (Clubhouse Crossroads corehole 1) near Charleston, South Carolina, in Rankin, D. W., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-A preliminary report: U.S. Geological Survey Professional
Paper 1028, p. 59-70.
Gottfried, David, Annell, C. S., and Byerly, G. R., 1983, Geochemistry
and tectonic significance of subsurface basalts near Charleston, South Carolina; Clubhouse Crossroads test holes #2 and #3, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity: U.S. Geological
Survey Professional Paper 1313, p. A1-A19.
Gottfried, David, Annell, C. S., and Schwarz, L. J., 1977, Geochemistry of subsurface basalt from the deep corehole (Clubhouse Crossroads corehole 1) near Charleston, South Carolina-Magma type and tectonic implications, in Rankin, D. W., ed., Studies related to the Charleston, South Carolina, earthquake of 1886- A preliminary report: U.S. Geological Survey Professional Paper
1028, p. 91-113.


Hamilton, R. M., Behrendt, J. C., and Ackermann, H. D., 1983, Land
multichannel seismic-reflection evidence for tectonic features near Charleston, South Carolina, in Gohn, G. S. ed., Studies related to the Charleston, South Carolina, earthquake of 1886- Tectonics and seismicity: U.S. Geological Survey Professional Paper 1313, p.
11-118.
Hattner, J. G., and Wise, S. W., Jr., 1980, Upper Cretaceous calcareous nannofossil biostratigraphy of South Carolina: South Carolina
Geology, v. 24, no. 2, p. 41-117.
Hazel, J. E., Bybell, L. M., Christopher, R. A., Frederiksen, N. 0.,
May, F. E., McLean, D. M., Poore, R. Z., Smith, C. C., Sohl, N. F., Valentine, P. C., and Witmer, R. J., 1977, Biostratigraphy of the deep corehole (Clubhouse Crossroads corehole 1) near Charleston, South Carolina, in Rankin, D. W., ed., Studies related to the Charleston, South Carolina, earthquake of 1886- A preliminary report: U.S. Geological Survey Professional Paper 1028, p. 71-89. Hooke, R. L., 1967, Processes on arid-region alluvial fans: Journal
of Geology, v. 75, no. 4, p. 438-460.
Hubert, J. F., 1977, Paleosol caliche in the New Haven Arkose, Connecticut; Record of semiaridity in Late Triassic-Early Jurassic
time: Geology, v. 5, no. 5, p. 302-304.
Hubert, J. F., Reed, A. A., Dowdall, W. L., and Gilchrist, J. M., 1978,
Guide to the redbeds of central Connecticut: 1978 field trip, eastern section of Society of Economic Paleontologists and
Mineralogists, Amherst, Massachusetts, 129 p.
Jensen, L. R., 1975, Late Triassic redbeds, Kingsport area: Maritime
Sediments, v. 11, no. 2, p. 77-81.
Lanphere, M. A., 1983, 40Ar/9Ar ages of basalt from Clubhouse
Crossroads test hole #2, near Charleston, South Carolina, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity: U.S. Geological Survey
Professional Paper 1313, p. B1-B8.
Lindholm, R. C., Hazlett, J. M., and Fagin, S. W., 1979, Petrology of
Triassic-Jurassic conglomerates in the Culpeper basin, Virginia:
Journal of Sedimentary Petrology, v. 49, no. 4, p. 1245-1261.
Manspeizer, Warren, 1980, Rift tectonics inferred from volcanic and
elastic structures, in Manspeizer, Warren, ed., Field studies of New Jersey geology and guide to field trips: 52nd Annual Meeting of the New York State Geological Association, Rutgers University,
Newark, 1980, p. 314-350.
Marine, I. W., and Siple, G. E., 1974, Buried Triassic basin in the central Savannah River area, South Carolina and Georgia: Geological
Society of America Bulletin, v. 85, no. 2, p. 311-320.
Middleton, G. V., and Hampton, M. A., 1973, Sediment gravity flows:
Mechanics of flow and deposition, in Middleton, G. V., and Bouma, A. H., co-chairmen, Turbidites and deep-water sedimentation: Lecture notes for Short Course, Pacific section, Society of Economic
Paleontologists and Mineralogists, Anaheim, 1973, p. 1-38.
Perry, Ed, and Hower, John, 1970, Burial diagenesis of Gulf Coast
pelitic sediments: Clays and Clay Minerals, v. 18, no. 3, p. 165-177. Phillips, J. D., 1983, Paleomagnetic investigations of the Clubhouse
Crossroads basalt, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886- Tectonics and seismicity: U.S. Geological Survey Professional Paper 1313, p.
C1-C18.
Popenoe, Peter, and Zietz, Isidore, 1977, The nature of the geophysical
basement beneath the Coastal Plain of South Carolina and northeastern Georgia, in Rankin, D. W., ed., Studies related to the Charleston, South Carolina, earthquake of 1886- A preliminary report: U.S. Geological Survey Professional Paper 1028, p.
119-137.
Randazzo, A. F., Swe, Win, and Wheeler, W. H., 1970, A study of
tectonic influence on Triassic sedimentation-the Wadesboro basin, central Piedmont: Journal of Sedimentary Petrology, v. 40,
no. 3, p. 998-1006.


D16









LOWER MESOZOIC(?) SEDIMENTARY ROCKS IN CLUBHOUSE CROSSROADS TEST HOLE #3


Ratcliffe, N. M., 1980, Brittle faults (Ramapo fault) and phyllonitic
ductile shear zones in the basement rocks of the Ramapo seismic zones, New York and New Jersey, and their relationship to current seismicity, in Manspeizer, Warren, ed., Field studies of New Jersey geology and guide to field trips: 52nd Annual Meeting of the New York State Geological Association, Rutgers University,
Newark, New Jersey, 1980, p. 278-311.
Reineck, H.-E., and Singh, I. B., 1975, Depositional sedimentary environments, with reference to terrigenous plastics: New York,
Springer-Verlag, 439 p.
Reynolds, R. C., and Hower, John, 1970, The nature of interlayering
in mixed-layer illite-montmorillonites: Clays and Clay Minerals, v.
18, no. 1, p. 25-36.
Rust, B. R., 1979, Coarse alluvial deposits, in Walker, R. G., ed.,
Facies models: Geoscience Canada Reprint Series 1, Geological
Association of Canada, p. 9-21.
Schilt, F. S., Brown, L. D., Oliver, J. E., and Kaufman, Sidney, 1983,
Subsurface structure near Charleston, South Carolina: Results of COCORP reflection profiling in the Atlantic Coastal Plain, in Gohn, G. S., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-Tectonics and seismicity: U.S. Geological
Survey Professional Paper 1313, p. H1-H19.
Schneider, R. R., Gohn, G. S., Force, L. M., and King, S. L., 1979,
Lithologic log for a deep stratigraphic test hole, Clubhouse Crossroads No. 3, Dorchester County, South Carolina: U.S.
Geological Survey Open-File Report 79-449, 23 p.
Sharp, R. P., and Nobles, L. H., 1953, Mudflow of 1941 at Wright-


wood, southern California: Geological Society of America Bulletin,
v. 64, no. 5, p. 547-560.
Steel, R. J., 1974, Cornstone (fossil caliche)- Its origin, stratigraphic
and sedimentological importance in the New Red Sandstone,
western Scotland: Journal of Geology, v. 82, no. 3, p. 351-369. Talwani, Pradeep, 1977, A preliminary shallow crustal model between
Columbia and Charleston, South Carolina, determined from quarry blast monitoring and other geophysical data, in Rankin, D. W., ed., Studies related to the Charleston, South Carolina, earthquake of 1886-A preliminary report: U.S. Geological Survey Professional
Paper 1028, p. 177-187.
Van Houten, F. B., 1969, Late Triassic Newark Group, north-central
New Jersey and adjacent Pennsylvania and New York, in Subitzky, Seymour, ed., Geology of selected areas in New Jersey and eastern Pennsylvania and guidebook of excursions: New
Brunswick, New Jersey, Rutgers University Press, p. 314-347.
1973, Origin of red beds, a review- 1961-1972, in Donath,
F. A., Stehli, F. G., and Wetherill, G. W., eds., Annual Review of Earth and Planetary Sciences, v. 1: Palo Alto, Annual Reviews,
Inc., p. 39-61.
1980, Late Triassic part of Newark Supergroup, Delaware
River section, west central New Jersey, in Manspeizer, Warren, ed., Field studies of New Jersey geology and guide to field trips: 52nd Annual Meeting of the New York State Geological Association, Rutgers University, Newark, New Jersey, 1980, p. 264-276. Williams, Howel, Turner, F. J., and Gilbert, C. M., 1954, Petrography
-An introduction to the study of rocks in thin sections: San Francisco, W. H. Freeman and Company, 406 p.


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x
















Geology of the Basement Rocks Near Charleston, South Carolina-Data from Detrital Rock Fragments in Lower Mesozoic(?) Rocks in Clubhouse Crossroads Test Hole #3

By GREGORY S. GOHN

STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-TECTONICS AND SEISMICITY



GEOLOGICAL SURVEY PROFESSIONAL PAPER 1313-E


UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:1983



























































































































































































































































1


















CONTENTS


Abstract ____... ___...________...._...______..
Introduction ----------------------------------Red-bed section --------------------------------Lithofacies ---------------------------------Evidence for local sediment source ____-_____-_Data from core _ __- _ _ _ _ _______ ___ _Data from geophysical surveys _________-__Petrology of detrital rock fragments -_____-______-Granodiorite --------------------------------Mineralogy ------------------------------Petrologic composition ---------------------Texture --------------------------------Alteration ------------------------------Mylonite ----- _----------------------------Microbreccia ---------------------------------


Mineralogy


Page

El
1 3 3
4 4 5 6 7 8 9 10 11 11 11
12


Petrology of detrital rock fragments-Continued
Microbreccia- Continued
Texture - -------------------------------Epidosite -------------------------------Basalt ------------------------------------Mineralogy -----------------------------Texture ----------- --------------------Alteration ------------------------------Petrologic classification ___ _ ___ _ _ _ _ _ _ _Discussion -----------------------------------Regional basement-rock provinces ______-_____Geologic events inferred from petrographic data ____-Granodiorite plutonism --Basalt magmatism ------------------------Mafic plutonism --------------------------Deformation events -----------------------,References cited --------------------------------


ILLUSTRATIONS


FIGURE 1. Location map and generalized geologic column for CC#3 ----------------------------------------------2. Stratigraphic column showing sedimentary facies of the red-bed sequence in CC#3 -----------------------------3. Graphic log of depositional units recognized in the bottom core from the red-bed sequence ------------------------4. Photomicrographs showing textures of conglomeratic beds ---------------------------------------------5. Photographs of cores of conglomeratic beds -----------------------------------6-10. Photomicrographs of:
6. Plagioclase and myrmekite in granodioritic clasts -----------------------------------------------7. Potassium feldspar, quartz, and accessory minerals in granodioritic clasts ___---------------------------------8. Mylonite clasts - - -- --------------------------------------9. Microbreccia clasts --------------------------------------------10. Basalt clasts _-_---_----------------------------------------11. Generalized geologic map of the Southeastern United States showing subsurface basement-rock provinces -_-____- ____-


Page

E2
3
4 6 7

8 10
12 13 15 16


TABLES


Page


TABLE 1. Modal analyses of representative cong
2. Modal analyses of granodioritic clasts.


omeratic rocks - -------------------------------------------------___ ---------------------------------HI


Page


E14 14 14 14 15 15 16 16 16 17 17 17 18 18
20


E5 11


__ ________--- __ _ __--- --------



















STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886TECTONICS AND SEISMICITY



GEOLOGY OF THE BASEMENT ROCKS NEAR CHARLESTON,
SOUTH CAROLINA-DATA FROM DETRITAL ROCK FRAGMENTS IN
LOWER MESOZOIC(?) ROCKS IN CLUBHOUSE CROSSROADS TEST HOLE #3


By GREGORY S. GOHN


ABSTRACT
Detrital rock fragments in conglomeratic lower Mesozoic(?) sedimentary rocks in Clubhouse Crossroads drill hole #3 are the only material available for direct geologic study of the pre-Mesozoic crystalline basement in the Charleston, S. C., area. No drill holes in the area have penetrated the pre-Mesozoic section. Many lines of evidence from the core (textures, mineralogy, sedimentary structures) and from local geophysical surveys (basement-surface configuration) suggest that the source area for these compositionally immature, poorly sorted conglomeratic sediments was proximal to their site of deposition. Four types of basement rocks occur as detrital clasts; in decreasing order of abundance, these types are granodiorite and similar plutonic rocks, microbreccia, basalt, and mylonite. Plutonic igneous-rock fragments grouped as granodiorite have apparent compositions ranging from tonalite or granodiorite to granite. Alteration of the granodiorite has occurred, and the present mineralogy is typically saussuritized plagioclase, perthite, quartz, chlorite, opaque minerals, epidote, and sphene. Microbreccia clasts have a protolith that closely resembles the granodiorite clasts, but the microbreccia shows considerable comminution of grains, fracturing, and secondary epidote, chlorite, quartz, and zeolite(?) mineralization in veinlets. Basalt clasts in the deposit have also been mineralogically altered to saussuritized plagioclase, chlorite, epidote, and other secondary minerals, but they do not show obvious effects of penetrative deformation. Mylonite clasts have a mineralogy similar to that of the granodiorite clasts, but the mylonite clasts have been strongly deformed, and their texture is dominated by fluxion structure and porphyroclasts. Inferences about the geologic history represented by these detrital rock fragments can be made from the petrologic data. However, the lack of data on the larger geometries of the rock units involved and on their relative and absolute ages limits these interpretations. At least one episode of granodiorite intrusion into an unknown sequence of host rocks has been documented, as has an episode of basaltic volcanism or shallow plutonism. The basalt cannot be temporally related to the younger lower Mesozoic basalt encountered higher in the drill hole. The mylonite and microbreccia may represent relatively ductile and relatively brittle deformation, respectively, within a single fault zone, or they may represent contrasting styles of deformation in different fault zones of different age. Deformation textures are restricted to the interiors of the detrital mylonite and microbreccia clasts and do not extend into the enclosing sedimentary matrix. Therefore, on the basis of the probable age of the sedimentary section, the minimum age of the faulting is established as early Mesozoic. The mylonite and microbreccia are direct evidence of ancient faulting in basement rocks of the Charleston area.


INTRODUCTION

In recent years, studies of the historic and modern seismicity in the Charleston, S. C., area have included efforts to understand the geology of the pre-Cretaceous 'rocks below the South Carolina Coastal Plain and the tectonic history these rocks encode. Important results of these efforts include the recognition of widespread subsurface lower Mesozoic volcanic rocks and lower Mesozoic(?) sedimentary rocks in the Charleston area and the recognition of their stratigraphic equivalence to similar rocks throughout the subsurface of the Southeastern United States and adjacent offshore areas (for example, Gohn and others, 1978). To date, studies of the older, pre-Mesozoic crystalline basement in the Charleston area have been restricted to geophysical investigations including gravity and aeromagnetic studies (Long and Champion, 1977; Phillips, 1977; Popenoe and Zietz, 1977; Daniels and others, 1983), seismicrefraction surveys (Ackermann, 1977, 1983; Talwani, 1977), electrical resistivity surveys (Campbell, 1977), and seismic-reflection surveys (Hamilton and others, 1983; Schilt and others, 1983). Geologic investigations of pre-Mesozoic rocks in the Charleston area were precluded by the absence of boreholes deep enough to penetrate that section. In other parts of the Southeast, however, geologic data from deep boreholes have been used, in conjunction with regional potential field data, to produce maps of the basement rocks (Popenoe and Zietz, 1977; Chowns and Williams, 1983; Daniels and others, 1983).
The deepest borehole in the Charleston area is Clubhouse Crossroads #3 (CC#3) in southwestern Dorchester County, about 40 km west-northwest of Charleston (fig. 1). A generalized geologic column for CC#3 is given in figure 1, and discussions of the preCretaceous rocks in this section are included in papers by Gottfried and others (1983) and Gohn and others El









E2


FIGuRE 1.- Location map and generalized geologic column for CC#3.


STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886



810 80*
83* 1 81* 79* 770 TENN)
340- Columbia T NORTH CAROLINA
350CAROLINA


330 GEORGIA Area of figure











0 Orangeburg
Lake Marion SURFACE UNITS GEOLOGIC DEPTH,
ELEVATION COLUMN IN METERS
CC#3
+6.7 meters ... . 0- Lake
-- -.-.Moultrie



Coastal
Plain
sediments .


o Summerville 330

775 Clubhouse
Basalt Crossroads Middleton
flows + Place

Sedimentary 1031
red beds .Charleston
-e beds 1152-Co




Beaufort






0 25 50 KILOMETERS
[2 _____________







GEOLOGY OF THE BASEMENT ROCKS NEAR CHARLESTON


(1978, 1983). In this stratigraphic test hole, below 775 m of Cretaceous and Cenozoic Coastal Plain sediments, 256 m of subaerial basalt flows overlie a minimum of 121 m of sedimentary red beds. On the basis of radiometric ages for the basalt and regional lithostratigraphic relationships of the basalt and red beds, Gohn and others (1978, 1983) assigned a Late Triassic to Early Jurassic age to the combined basalt and red-bed section. Due to mechanical failures, CC#3 did not penetrate the crystalline basement thought to underlie the red-bed sequence. Ackermann (1983) suggests, on the basis of seismic-refraction data, that CC#3 may have stopped only a few tens of meters above that basement. However, a 3.1-m core recovered from the bottom (1,144-1,147 m) of CC#3 does contribute to our understanding of the basement geology in the Charleston area. Below a depth of about 1,070 m in the hole, the red-bed section consists of interbedded conglomeratic sandstones and mudstones, most of which are represented only by rotary cuttings. The bottom core, however, consists predominantly of conglomeratic sandstone that contains detrital fragments, as large as 4 cm, of crystalline rocks. Numerous lines of evidence suggest that these rock fragments were derived from areas proximal to their site of deposition and that they represent, therefore, the only material available for direct geologic investigation of the crystalline basement near Charleston. For this report, the 2.0- to 40-mm-sized polymineralic fragments were studied petrographically to determine the types of rock present and to provide data from which inferences about the geology of the crystalline basement could be made. Comparisons of the petrologic data with the geophysical interpretations of the basement are also presented. Acknowledgments. -This work was supported by the U.S. Nuclear Regulatory Commission, Office of Nuclear Research, under Agreement No. AT(49-25)-1000. I thank my colleagues Peter Lyttle, Norman Hatch, Louis Pavlides, David Gottfried, Gilbert Espenshade, Douglas Rankin, Brian Leavy, and especially Nicholas Ratcliffe and J. Wright Horton for their many helpful discussions of igneous and mylonitic rocks.

RED-BED SECTION
Lithofacies
The general geology of the red-bed section in CC#3 is discussed by Gohn and others (1983), and a generalized description of the red-bed lithofacies is given in figure 2. From the base of the overlying basalt, at a depth of 1,030.8 m, to a depth of 1,070 m, the section consists of red and green mudstones, siltstones, and fine-grained sandstones. Due to the fine grain size, these rocks do not lend themselves as readily to studies of sediment


DEPTH, LITHOLOGY DESCRIPTION
IN METERS
- 1ma _


1040 1060




108011001120




1140 1152-


X X X X X

- -
- -









-

-


- 0


- ~









-


Basalt


Red beds, fine-grained facides: massive redand green-mottled mudstone interbedded with red- and green-mottled, currentbedded siltstone and fine- to mediumgrained sandstone







Red beds, coarse-grained facies: red- and green-mottled, arkosic, coarse-g rained to conglomeratic sandstone interbedded with massive, red- and green-mottled mudstone


EXPLANATION

V Basalt
Sandstone, coarse-grained to conglomeratic, argillaceous
E] Sandstone, very-fine- to fine-grained,
argillaceous
Siltstone and mudstone

El Core loss


FIGURE 2.-Stratigraphic column showing sedimentary faces of the
lower Mesozoic(?) red-bed sequence in CC#3.


E3







STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886


provenance and basement geology as do underlying coarser-grained deposits. Beginning with the rotary cuttings from the interval between 1,067 and 1,076 m, drilling chips of conglomeratic sandstone and chips consisting entirely of individual crystalline-rock fragments appear. The depth of 1,070 m has been selected arbitrarily as the top of this conglomeratic unit. This lithology was penetrated to a depth of 1,144 m where a
3.1-m core was recovered.
The conglomeratic beds in the bottom core are divided into seven depositional units (fig. 3). Four inversegraded or inverse-to-normal-graded beds occur in the core. Starting at the base, each of these beds consists of a matrix-supported conglomeratic mudstone that grades upward into framework-grain-supported argillaceous conglomerate or argillaceous conglomeratic sandstone. Thin mudstone layers intervene between the graded conglomeratic beds. Color-mottling, carbonate nodules, clay mineralogy, and other sedimentary aspects of this core are discussed by Gohn and others (1983). The locations in the core of thin sections examined during the present study are shown by arrows on figure 3.

Evidence for Local Sediment Source

The detrital rock fragments in the conglomerates from the bottom core are relevant to the basement geology of the Charleston area only if their source area was proximal to their site of deposition at Clubhouse Crossroads. Several lines of evidence suggest that such proximity was the case.

DATA FROM CORE

Compositional immaturity. -The conglomeratic beds in the bottom core are poorly sorted deposits containing abundant mineralogically and texturally unstable detrital rock fragments. Although the maturity of a sedimentary deposit may depend on several factors, including intensity of source-rock weathering before transportation and diagenetic alteration after deposition, the interpretation is made here that the time and distance of sediment transport was insufficient to produce appreciable breakdown of polycrystalline grains into texturally stable grains or appreciable chemical alteration of unstable minerals to secondary products. Similarly, the abundance of unstable grains indicates that the pretransportation and postdepositional processes also contributed little to the maturation of the deposit.
The relative maturity of a sediment or sedimentary rock may be represented by a ratio of the stable grains (monocrystalline quartz, chert, orthoquartzite lithic fragments) to the unstable grains (feldspar, polycrystalline quartz, other lithic fragments) in the deposit.


DEPTH, IN METERS
1144












114511461147-


LITHOLOGY 1 GRADED THIN
BEDDING SECTIONS


CUTTINGS


'0 0 0 *0 0 00 .a0 00 0 0 00 0 00 0 ,
.. .. . "0 0
00. *0 .00
'0-0000 -000 0000

1*00 00 0000: ,*00 0 *00000*00 0 0 * 00 0o 0 0 *0 0* 0
0 0 0 0
00 000 00 000 00 00( 00 0 0 0 00 0 00 0 0 0(
.00 0'.%o0000 000, 000 00 0 0 00 00 0000 00_ C


Normal


Inverse


0 00000, 0 00 -0
o' . 0' 00 0
0 *.* 0* *
000 0 00 .* 0 0000 0 0 0* 0 00 0 0-0 0 00. 0 00% 00 0 00 0 0 0 C 0 0 0 0 0v r s 00 0 0 0 0
- .. 0 0* -0*.,
000 00 00 Invers
0 ? 000s S0 * o0 0 o 0 0




0 0 00 0 00 0 0 Normal


1.0 :..* Inverse
00'o . 0 _00000' .-0 . 0. 0 0. -0000 0" 0* - *
- - - -___ _0 0000


0 oo 00 00 00 00.000**.0
0 -0000.0,0
*o'o o.* o*,
*0 0 0 0
000000
_-00 0 0.0000 .00-00 o: -0..00 000 000--


Inverse


4


No recovery
EXPLANATION
[* * Conglomeratic sandstone

N Conglomeratic mudstone
- Mudstone

FIGURE 3. -Depositional units recognized in the bottom core from the red-bed sequence. Normally and inversely graded parts of the conglomeratic beds are labeled. Arrows mark locations in the core of samples from which sets of thin sections were cut.


E4




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