|UFDC Home||myUFDC Home | Help ||
CITATION SEARCH THUMBNAILS PAGE IMAGE ZOOMABLE
STANDARD VIEW MARC VIEW
|Table of Contents|
Title Page 1
Title Page 2
State of Florida
Department of Natural Resources
Tom Gardner, Executive Director
Division of Resource Management
Jeremy Craft, Director
Florida Geological Survey
Walter Schmidt, State Geologist and Chief
Open File Report 42
A Gamma-Ray Profile Investigation of the Upper Pleistocene
Miami Limestone of South Florida
Richard A. Johnson
Florida Geological Survey
A GAMMA-RAY PROFILE INVESTIGATION OF THE UPPER
PLEISTOCENE MIAMI LIMESTONE OF SOUTH FLORIDA
Richard A. Johnson*, P.G. No. 60
The Miami Limestone of southeastern peninsular Florida consists
of white to yellow to brown-yellow, oolitic or pelletal lime-
stone. Nineteen hand-held scintillometer gamma-ray profiles
of the Miami are obtained from outcrops in Dade, Broward, Palm
Beach, and Monroe Counties, and a continuous core from Dade
County. Four well-defined, alternating higher and lower intensity
gamma-ray zones are detected, with a higher intensity zone at
the base of the Miami directly overlying the low intensity of
the Fort Thompson Formation (limestone). The uppermost Miami
consists of a low intensity zone at or near land surface.
This Open File Report is adapted from a paper presented at the
Florida Academy of Sciences 55th Annual Meeting, May 10, 1991,
at Saint Leo College, Saint Leo, Florida.
*Independent professional geologist (State of Florida License
No. 60), P.O. Box 3560, Tallahassee 32315-3560.
UNIVERSITY OF FLORIDA LIFAARIES
The upper Pleistocene Miami Limestone consists of white to
yellow to brown-yellow, very slightly to abundantly sandy
(quartz), oolitic or pelletal limestone and minor oolitic or
pelletal sandstone which occurs at or near land surface in
southeastern and south-Central peninsular Florida, and in the
Lower Keys. The Miami is a relatively thin stratigraphic unit,
attaining a maximum thickness of only 35 feet (Hoffmeister et
al., 1967). Over most of its extent, it averages less than 15
Because the Miami Limestone occurs at or near land surface
and is very thin, much of the formation is both above water
level and masked behind casing in most water wells of the area.
As a result, very little information currently exists with regard
to the Miami's borehole geophysical signature, that is, gamma-
ray, neutron, or electric-resistivity patterns characteristic
of the formation.
In order to study the geophysical signature of the Miami
Limestone, a technique was applied which utilizes surface expo-
sures: outcrop gamma-ray profiling (Johnson, 1989). Since the
Miami occurs at or very near land surface, and since a relatively
large number of exposures are known to exist, the technique
appeared ideal for this purpose.
Outcrop gamma-ray profiling utilizes a very sensitive, hand-
held scintillometer to determine gamma photon intensity vertical-
ly across a clean surface exposure (or continuous core). The
individual measurements, in standardized counts-per-second (cps),
are then plotted against vertical distance along the exposure.
The result is a profile which shows the gamma-ray intensity
patterns characteristic of the formation in the same manner
that a borehole gamma-ray log would.
For this study, gamma-ray profiles were run on a total of
18 surface exposures of the Miami Limestone in Dade, Monroe,
Palm Beach, and Broward Counties. Of these 18 outcrop profiles,
6 profiles recorded a significant fraction of the total thickness
of the Miami at each location (Figure 1). In addition, a gamma-
ray profile was constructed using the same instrumentation on
a Florida Geological Survey continuous core (W-16395, Everglades
#1) obtained from southwestern Dade County (profile 2 on Figure
1). The core penetrated the underlying Fort Thompson Formation,
and thus, the gamma-ray signatures of the basal contact and
lower portion of the Miami could be conveniently studied.
Four gamma-ray zones were consistently detected in these
profiles of the Miami Limestone (Figure 2). In this paper, these
are termed gamma-ray zones 1, 2, 3 and 4, in ascending strati-
graphic order. Gamma-ray zone 1 represents the basal portion
of the Miami and is characterized by higher intensity. Gamma-
ray zone 2, the lower-middle-Miami zone, is recorded as very
low intensity. Gamma-ray zone 3, the upper-middle-Miami zone,
repeats the basal zones's higher intensity. Gamma-ray zone 4,
representing uppermost-Miami, is again characterized by lower
Figure 3 gives an index of lithologic patterns used in the
0 10 mi
SGammuua ray profile
Figure 1. Locations of relatively thick gaama zy profiles obtained
from the Miami Linestone.
COMPOSITE GAMMA RAY PROFILE OF THE MIAMI LIMESTONE
3 Upper Middle
2 Lower Middle
Figure 2. Composite gamma ray profile of the Miami Limestone.
azgllaoeous, lightly o 0ani
quarts sand (noaolitio)
ft aolluakan oaloieudite
S(oolitio, Do sp. or Chioon
C heilotow bryosoan caloindit*
S foraminifezL (Archaas ap.) calci-
maslive quart. sandstone
hbuzwed quas sandstone
massive ooid calcarenite
massive unfosslifezous, very finely
recrystallised limestone (nonoolltio)
Figaz 3. Index of lithologio pattern used In the figure of this paper.
. l I% I
I I A I
* I I J-'
figures of this report. See also Johnson (1991) for a complete
list and general areas of occurence of the 5 lithofacies found
to be characteristic of the Miami Limestone.
The thickest exposure of the Miami Limestone (Figure 4) is
located just north of the traffic circle at the intersection
of LeJeune Road and Sunset Drive in the city of Coral Gables,
Dade County (Puri and Vernon, 1964), where the Coral Gables
Waterway transects the Atlantic Coastal Ridge (Township 54 south,
Range 41 east, Section 29, southeast quarter of southwest quar-
ter). The 19 feet of limestone exposed here consist predominantly
of cross-bedded ooid calcarenite faces (Johnson, 1991), with
three very thin beds of Donax sp. molluskan calcirudite subfacies
(Johnson, 1991). This exposure represents slightly more than
one-half of the total thickness of the Miami Limestone at this
The gamma-ray profile of this exposure (Figure 4) shows: a
very small portion of the basal-Miami, higher intensity, zone
1; a very well-defined, lower-middle-Miami, low intensity, zone
2; a relatively thick and well-defined, upper-middle-Miami,
higher intensity, zone 3; and a very thin, uppermost-Miami,
lower intensity, zone 4. This profile contains portions of all
four gamma-ray zones characteristic of the Miami.
The gamma-ray profile obtained from the Florida Geological
Survey core (W-16395) in southwestern Dade County (Township
58 south, Range 371 east, Section 1, northwest quarter) is shown
in Figure 5. Approximately 15.5 feet of slightly to highly
GAMMA RAY PROFILE 1
Coral Gables Canal
60 70 80
.~~~~~ I I L- R
l~~~ ~ ~ |- 1- i'i1 1 *i'i'i T^1 1-1.-
9' E" ,
'~~~~~~~ im 'i.'i 1 1 ,i T n
12 '1'I 'I 'I'I 'I 'I' I'
Figue 4i. Section diLsgxm and gamma-ry profile obtained from the thickest exposure of Miami Limestone, hde
County. See Figure 3 for index of lithologio patterns.
GAMMA RAY PROFILE 2
Florida Geological Survey Core W-16 395
50 60 70 80 o/r
ITS FPort Thompeon PoFuton
Figure 5. Section diagram and gaam-zay profile obtained from the Florida Geological Survey core in southwestern
., O?-..-*-.. ..* ,. *a r*> 4.* -* -* 9 4* r* --4* '^I.-...
burrowed, bryozoan and foraminiferal calcirudite subfacies
(Johnson, 1991) of the Miami overlie the nonoolitic, very finely
recrystallized, very hard and massive limestone of the Fort
Thompson Formation. On this profile, the basal-Miami gamma-ray
zone 1 is clearly shown as a very well-defined higher intensity
interval above the low intensity of Fort Thompson limestone.
A very thin, slightly argillaceous, undifferentiated quartz
sand bed overlies the Miami Limestone at land surface, and is
recorded on the profile as slightly higher intensity. The low
intensity of gamma-ray zone 2 occurs immediately above the
basal-Miami, higher intensity, zone 1. Again, the low intensity
of zone 2 is very well-defined on this profile. Generally, zone
2 represents the best-defined and most-correlatable gamma-ray
zone in the Miami, and can be easily identified on all seven
of the relatively complete profiles.
In the thickest exposure of Miami Limestone in Broward County
(Figure 6), approximately 12 feet of Miami are exhibited in
the walls of the Dania Cutoff Canal at the Atlantic Coastal
Ridge in the city of Dania (Township 50 south, Range 42 east,
Section 34, northeast quarter of northwest quarter), representing
approximately one-third of the formation at this location. The
Miami consists predominantly of hard, microsparry to coarsely
sparry, Donax sp. molluskan calcirudite subfacies, with uppermost
and basal beds of ooid calcarenite faces.
The gamma-ray profile obtained from this exposure (Figure
6) shows gamma-ray zones 2, 3 and 4 of the Miami Limestone.
GAMMA RAY PROFILE 3
Dania Cutoff Canal
60 70 80
90 100 c/8
* IO l
t ,* I
I ~ ~ 101-5 i- i'l l ess '
Figure 6. Section diagram and gaa-ray profile obtained from the thickest exposure of Miui Limestone in Bzoward
County. See Figure 3 for index of lithologio patterns.
----- 1 -- --- -- -
The lower intensity of the top of zone 2 at the base of the
exposure generally correlates with non-shelly, very burrowed,
ooid calcarenite faces. The top of the Miami (also non-shelly
ooid calcarenite) is recorded on the profile as the low intensity
of zone 4.
The thickest exposure of the Miami in Palm Beach County (Figure
7) occurs in the banks of the Hillsboro Canal (Township 47 south,
Range 41 east, Section 26, southwest quarter) where a maximum
thickness of 10 feet is exposed at extreme low water level.
This thickness represents more than three-quarters of the forma-
tion at this location. The Miami consists of slightly sandy
(quartz), massive, Chione cancellata molluskan calcirudite
subfacies; abundantly sandy (quartz), massive unfossiliferous
calcilutite to massive, unfossiliferous, oolitic quartz sandstone
facies (Johnson, 1991); very burrowed oomoldic-recrystallized
faces (Johnson, 1991) with common C. cancellata; and somewhat
burrowed, C. cancellata-rich, oolitic quartz sandstone facies
at the base.
The gamma-ray profile of the exposure (Figure 7) exhibits
zones 2, 3 and 4 of the Miami Limestone. Zones 3 and 4, the
upper two zones, are very well-defined and relatively thick,
however, only the extreme uppermost portion of zone 2, the
lower-middle-Miami, lower intensity zone, is recorded. This
profile exhibits the same shape as the previously-considered
profile from the Dania Cutoff Canal in Broward County (Figure
6), since the higher intensity of zone 3 is recorded as a rela-
GAMMA RAY PROFILE 6
50 60 70 80 oe/
Figure 7. Section diagram and gama-ray profile obtained from the thickest exposure of Miani Limestone in Pala
Beach County. See Figure 3 for index of lithologio patterns.
)m. *.l ~ ~l'l .lI'
r* zz,.:. L:I:X( Gz!1I. I
rZ7-- -" 7% 3 C-
tively thin peak at the top of the zone, which slowly descends
into the lower intensity of zone 2 below.
Figure 8 shows a geographic correlation, from north (south-
eastern Palm Beach County) to south (southwestern Dade County),
of the 7 most-complete gamma ray profiles obtained from the
Miami Limestone. Mean sea level (MSL) is represented by the
horizontal dashed line. Profile 2, most of which is below MSL,
represents the Florida Geological Survey core in southwestern
Dade County. Profile 6 represents the Hillsboro Canal, Palm
Beach County, exposure near the northernmost extent of the Miami.
Gamma-ray zones 2, 3 and 4 can be identified on all of the
outcrop profiles, and the general curve shape remains consistent
across the entire area (however, no correlation between lithology
and gamma-ray intensity was identified in the Miami Limestone).
Because no relatively thick and complete exposures of the
Miami Limestone occur in the Lower Keys of southern Monroe
County, no gamma-ray profiles of the Miami obtained from that
area are illustrated in this paper. However, zones 3 and 4,
the upper 2 zones, are recognizable on all profiles of the Miami
obtained from the Keys.
In summary, the upper Pleistocene Miami Limestone of south
Florida can be geophysically divided into four well-defined
gamma-ray zones of alternating higher and lower intensity. The
basal Miami gamma-ray zone 1 and the upper-middle Miami gamma-ray
zone 3 are consistently recorded as higher intensity, and the
lower-middle Miami gamma-ray zone 2 as well as the uppermost
Miami gamma-ray zone 4 are consistently recorded as lower inten-
Hoffmeister, J.E., Stockman, K.W., and Multer, H.G., 1967, Miami
Limestone of Florida and its Recent Bahamian counterpart:
Geological Society of America Bulletin, v. 78, pp. 175-190.
Johnson, R.A., 1989, Stratigraphic correlation of outcrop gamma
ray profiles in Florida: Florida Geological Survey Open
File Report 26, 27 p.
Johnson, R.A., 1991, Lithofacies of the upper Pleistocene Miami
Limestone of south Florida [abstract]: Florida Scientist,
v. 54, Supplement 1, p. 28.
Puri, H.S. and Vernon, R.O., 1964, Summary of the geology of
Florida and a guidebook to the classic exposures (revised):
Florida Geological Survey Special Publication 5 (revised),
FLRD GEOLOSk ( IC SUfRiW
[year of publication as printed] Florida Geological Survey [source text]
The Florida Geological Survey holds all rights to the source text of
this electronic resource on behalf of the State of Florida. The
Florida Geological Survey shall be considered the copyright holder
for the text of this publication.
Under the Statutes of the State of Florida (FS 257.05; 257.105, and
377.075), the Florida Geologic Survey (Tallahassee, FL), publisher of
the Florida Geologic Survey, as a division of state government,
makes its documents public (i.e., published) and extends to the
state's official agencies and libraries, including the University of
Florida's Smathers Libraries, rights of reproduction.
The Florida Geological Survey has made its publications available to
the University of Florida, on behalf of the State University System of
Florida, for the purpose of digitization and Internet distribution.
The Florida Geological Survey reserves all rights to its publications.
All uses, excluding those made under "fair use" provisions of U.S.
copyright legislation (U.S. Code, Title 17, Section 107), are
restricted. Contact the Florida Geological Survey for additional
information and permissions.