Title Page
        Title Page 1
        Title Page 2
    Abstract
        Page 1
    Gamma-ray profile...
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
    References
        Page 16
        Copyright
            Main

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

by

Richard A. Johnson


Florida Geological Survey
Tallahassee, Florida
1991












19-o '~


SCIENCE
LIBRARY







A GAMMA-RAY PROFILE INVESTIGATION OF THE UPPER
PLEISTOCENE MIAMI LIMESTONE OF SOUTH FLORIDA

by
Richard A. Johnson*, P.G. No. 60








ABSTRACT

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.















NOTE

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.
1


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

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

intensity.

Figure 3 gives an index of lithologic patterns used in the

3





















T
North

0 10 mi


SGammuua ray profile
Figure 1. Locations of relatively thick gaama zy profiles obtained
from the Miami Linestone.
4



















COMPOSITE GAMMA RAY PROFILE OF THE MIAMI LIMESTONE


Zone
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


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



C heilotow bryosoan caloindit*
(oolltio)



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


maslive quart. sandstone
(oolitio)



ooaoldlo eozrytallised




hbuzwed quas sandstone
(oolitio)



massive ooid calcarenite


massive unfosslifezous, very finely
recrystallised limestone (nonoolltio)


Figaz 3. Index of lithologio pattern used In the figure of this paper.


~il4 ;


1.
. 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

location.

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

7











GAMMA RAY PROFILE 1
Coral Gables Canal


c/s


60 70 80


4


ri



.~~~~~ 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'


2


1.

Zone


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


I
t.7jr


Zom


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.

10















GAMMA RAY PROFILE 3
Dania Cutoff Canal


60 70 80


90 100 c/8
gone


I
12


I


4

3I
3


* 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 -- --- -- -


~F~i~ii:









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-

12













GAMMA RAY PROFILE 6


Hillsboro Canal



50 60 70 80 oe/
SZone


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


10'

2


)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

14










SOrUT


NORTH


I>

MSL


Figure .
Gama-& profile
orzzet.loa,


2

1
7 FT


bonep


SCALES



JI L'
*>
i


feet


meters









Miami gamma-ray zone 4 are consistently recorded as lower inten-

sity.



REFERENCES
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.










FLRD GEOLOSk ( IC SUfRiW


COPYRIGHT NOTICE
[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.