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 Gamma-ray profile...

A gamma-ray profile investigation of the Upper Pleistocene Miami limestone of south Florida ( FGS: Open file report 42 )
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Permanent Link: http://ufdc.ufl.edu/UF00001041/00001
 Material Information
Title: A gamma-ray profile investigation of the Upper Pleistocene Miami limestone of south Florida ( FGS: Open file report 42 )
Series Title: ( FGS: Open file report 42 )
Physical Description: 16 p. : ill., maps ; 28 cm.
Language: English
Creator: Johnson, Richard A ( Richard Alan ), 1949-
Florida Geological Survey
Publisher: Florida Geological Survey
Place of Publication: Tallahassee
Publication Date: 1991
Subjects / Keywords: Geology, Stratigraphic -- Pleistocene   ( lcsh )
Geology -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
Statement of Responsibility: by Richard A. Johnson.
Bibliography: Includes bibliographical references (p. 16)
General Note: Cover title.
Funding: Digitized as a collaborative project with the Florida Geological Survey, Florida Department of Environmental Protection.
 Record Information
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management:
The author dedicated the work to the public domain by waiving all of his or her rights to the work worldwide under copyright law and all related or neighboring legal rights he or she had in the work, to the extent allowable by law.
Resource Identifier: aleph - 001754183
oclc - 25642049
notis - AJG7172
System ID: UF00001041:00001

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Full Text

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
Tallahassee, Florida

19-o '~



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.


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

feet thick.

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.


4 Uppermost

3 Upper Middle

2 Lower Middle

4 Basal

increasing intensity-

Figure 2. Composite gamma ray profile of the Miami Limestone.

azgllaoeous, lightly o 0ani
quarts sand (noaolitio)

ooid oaloaxsonit

ft aolluakan oaloieudite
S(oolitio, Do sp. or Chioon

C heilotow bryosoan caloindit*

S foraminifezL (Archaas ap.) calci-
rudite (oelitic)

maslive quart. sandstone

ooaoldlo eozrytallised

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.

~il4 ;

. l I% 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


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.


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.


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-



Hillsboro Canal

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.

T 0



)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






Figure .
Gama-& profile


7 FT






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),
312 p.


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