Guidebook to the correlation of geophysical well logs within the St. Johns River Water Management District

(r

METRIC CONVERSION FACTORS

To eliminate duplication of parenthetical conversion of units in the text of reports, the Florida
Geological Survey has adopted the practice of inserting a tabular listing of conversion factors.
For readers who prefer metric units to the customary U.S. units used in this report, the following
conversion factors are provided.

MULTIPLY BY TO OBTAIN

inches 25.4 millimeters
feet 0.3048 meters
miles 1.609 kilometers

ABBREVIATIONS USED IN THIS REPORT

FGS Florida Geological Survey

SJRWMD St. Johns River Water Management District

USGS U.S. Geological Survey

MSL Mean Sea Level: Sea Level refers to the National Geodetic Vertical Datum of
1929 a geodetic datum derived from a general adjustment of the first-order
level nets of both the United States and Canada, formerly called "Sea Level
Datum of 1929." Mean sea level provides a consistent and corellatable datum for
referencing elevations of geologic strata.

BLS Below Land Surface, a depth expressed in feet in this report. Land surface, (or
some point slightly above land surface) is the typical datum upon which well logs
are based. Since land surface elevations vary considerably with topography, the
depth below land surface at which a geologic marker lies does not provide a cor-
relatable datum for constructing cross sections. Mean sea level provides the only
constant statewide datum for such correlations.

To convert depths below land surface (BLS) to depths relative to mean sea level
(MSL), subtract the depth BLS from the land surface elevation. For example: a
very high gamma peak representing the top of the Hawthorn Group occurs on a
log at 100 feet BLS. The land surface elevation at the well is 120 feet MSL. To
find the elevation of the gamma peak relative to mean sea level, subtract 100 feet
from 120 feet, which equals 20 feet, or 20 feet above mean sea level. If the depth
to the top of the Hawthorn Group is greater than the depth to mean sea level, the
resulting MSL value is negative, or below mean sea level.

Cover illustration compiled by Frank Rupert from U.S. Geological Survey false color satellite image (1989) and gamma log cross sections developed
during this study. It is provided for illustrative purposes only, and no geospatial accuracy is implied or intended.

STATE OF FLORIDA

DEPARTMENT OF ENVIRONMENTAL PROTECTION
David B. Struhs, Secretary

DIVISION OF RESOURCE ASSESSMENT AND MANAGEMENT
Edwin J. Conklin, Director

FLORIDA GEOLOGICAL SURVEY
Walter Schmidt, State Geologist and Chief

SPECIAL PUBLICATION NO. 50

GUIDEBOOK TO THE CORRELATION OF GEOPHYSICAL WELL LOGS WITHIN THE
ST. JOHNS RIVER WATER MANAGEMENT DISTRICT

by

Jeff Davis, Richard Johnson, Don Boniol and Frank Rupert

Published by the

FLORIDA GEOLOGICAL SURVEY
Tallahassee, Florida
in cooperation with the
ST. JOHNS RIVER WATER MANAGEMENT DISTRICT
Palatka, Florida
2001

Printed for the
Florida Geological Survey

Tallahassee
2001

ISSN 0085-0640

II

LETTER OF TRANSMITTAL

FLORIDA GEOLOGICAL SURVEY

Tallahassee, Florida
2001

Governor Jeb Bush
Tallahassee, Florida 32301

Dear Governor Bush:

The Florida Geological Survey, Division of Resource Assessment and Management, Department
of Environmental Protection, is publishing as Special Publication No. 50, Guidebook to the
Correlation of Geophysical Well Logs Within the St. Johns River Water Management District,
prepared by Jeff Davis and Don Boniol of the St. Johns River Water Management District and
Survey staff geologists Richard Johnson and Frank Rupert. The publication describes a correla-
tion between geophysical well logs and geology within the St. Johns River Water Management
District. This information will be useful for citizens such as water well drillers and environmental-
ly conscious persons as well as municipal, county, state and federal agencies in interpreting the
natural geological and hydrological environments of northeastern Florida.

Respectfully,

Walter Schmidt, Ph.D.
State Geologist and Chief
Florida Geological Survey

This work is dedicated to Richard Alan Johnson, 12/09/1949 5/27/2000,
coauthor, colleague, and friend.

CONTENTS

page

IN T R O D U C T IO N ................. ......... .............................. .1
M ETHO DS .................................. ................. ....... .2
GEOPHYSICAL WELL LOGS ........... ..................................7
G am m a Log .............. ..................... .......... ....... .... 7
Electric Log .......... ................................ ....... ..7
STRATIGRAPHY ................................... .......... ......... .8
Paleocene Series Cedar Keys Formation ............................... .. .8
Lithostratigraphy ................................. ................ .10
G am m a Logs ................................................. ..... 11
E electric Logs ....................................................... 11
Lower Eocene Series Oldsmar Formation .................................... 11
Litho stratig ra phy . . . . . . . . . . . . . . . . . . . . . . . . .. 11
Gamma Logs .............. ................... ................... 13
Electric Logs .......... ...... ................ .................... 13
Middle Eocene Series Avon Park Formation .................................. .13
Lithostratigraphy ............. ..................... ......... .. . 14
G am m a Logs ................... ................................... 14
Electric Logs ............................... ....... ................. 17
Upper Eocene Series Ocala Limestone ....................................... 20
Lithostratigraphy ................................. ................ .20
G am m a Logs ......... ........ .................. ......... ........ 22
Electric Logs ..................................................... 25
Oligocene Series Suwannee Limestone .................................... .29
Lithostratigraphy .................................. ................ 29
G am m a Logs ................... ................................... 29
Electric Logs ....................................................... 30
Oligocene to Pliocene Series Hawthorn Group ................................. 30
Lithostratigraphy .................................. ................ 31
G am m a Logs ................... ................................... 31
Electric Logs ......................................................... 32
Upper Pliocene Series Tamiami Formation .................................. .35
Lithostratigraphy .................................. ................ 35
G am m a Logs ................... ................................... 35
Electric Logs ................. ..................................... .36
Upper Pliocene Series Cypresshead Formation .............................. .36
Lithostratigraphy .................................. ................ 36
G am m a Logs ................... ................................... 37
Electric Logs ........................................................ .37
Upper Pliocene to Pleistocene Series Nashua Formation and Okeechobee formation .... 37
Lithostratigraphy ................................. ................ .39
G am m a and Electric Logs ............................................. 39
Pleistocene Series Anastasia Formation ................................... .. 41
Lithostratigraphy ................................. ................ .41
G am m a Logs ................... ................................... 41
E electric Logs ....................................................... .42

V

Pleistocene to Holocene Series Undifferentiated Sand, Clay, and Shell ............... .42
Lithostratigraphy .................................. ............ ..... 42
Gamma and Electric Logs ................ ............................43
SUBSURFACE FEATURES AFFECTING THE STRATIGRAPHY AND LOG
CORRELATIONS IN THE SJRWMD ....................................... .43
Paleosinks .................................................. ..... . .43
Structure ................ ................... ............ ....... ... 44
GAMMA LOG SIGNATURES AND CROSS SECTIONS ........................... .47
CO NC LUSIO NS .................................................. ........ 49
REFERENC ES .............................................. ........ 50

ILLUSTRATIONS

FIGURE
1. Location of W ells Used in Figures 2 19 .................................... .3
2. Gamma log of well BR1217, Brevard County ................................ 5
3. Gamma and Electric logs of L-0729, Lake County ............................ 6
4. Electric and Focused Guard log of well N-0222, Nassau County ...................9
5. Gamma and Electric logs of well D-0349, Duval County ........................ 12
6. Gamma log of well P-0619, Putnam County ................................. 15
7. Gamma log of well IR0748, Indian River County ............................. .16
8. Gamma log of well L-0005, Lake County ................................... 18
9. Gamma and Electric logs of well OR0465, Orange County ................... .. .19
10. Gamma and Electric logs of well P-0172, Putnam County ...................... .21
11. Gamma log of well D-0176, Duval County ................................. 23
12. Gamma and Electric logs of well L-0094, Lake County ...................... .. 24
13. Gamma log of well F-0162, Flagler County ................................ 26
14. Gamma and Electric logs of well IR0338, Indian River County .................... 27
15. Gamma and Electric logs of well SJ0148, St. Johns County ................... .. 28
16. Gamma log of well SJ0177, St. Johns County ............................ .. .33
17. Gamma log of well D-0520, Duval County ............................... .34
18. Gamma log of well M-0410, Marion County .............................. .38
19. Gamma log of well F-0019 Flagler County .............................. .. .40
20. Subsurface structures in the SJRWMD ................................... .45

APPENDIX A. Cross Sections through the SJRWMD ............................. 53
Location of Gamma Log Cross Sections, Northern SJRWMD West to East Sections ...... .54
Gamma Log Cross Section A-A' ......................................... 55
Gamma Log Cross Section B-B' ......................................... 56
Gamma Log Cross Section C-C' ......................................... 57
Gamma Log Cross Section D-D' ......................................... 58
Gamma Log Cross Section E-E' ......................................... 59
Gamma Log Cross Section F-F' ........................... ....... ...... 60
Gamma Log Cross Section G-G' ......................................... 61

Location of Gamma Log Cross Sections, Southern SJRWMD West to East Sections ..... .62
Gamma Log Cross Section H-H' ......................................... 63

Gamma Log Cross Section I-I' .......................................... 64
Gamma Log Cross Section J-J' ........................................... 65
Gamma Log Cross Section K-K' .................. ..................... 66
Gamma Log Cross Section L-L' ................ ................ ....... 67
Gamma Log Cross Section M-M' ............................... ........ 68
Gamma Log Cross Section N-N' ................. ................ ...... 69
Gamma Log Cross Section 0-0' ................. ................ ..... 70
Gamma Log Cross Section P-P' .................. ..................... 71
Gamma Log Cross Section Q-Q' ................. ................ ..... 72
Gamma Log Cross Section R-R' ................. ................ ..... 73

Location Of Gamma Log Cross Sections, North to South Sections ................... 74
Gamma Log Cross Section S-S' ........................................75
Gamma Log Cross Section T-T'............................... ......... 76
Gamma Log Cross Section U-U' ......... ............. .... .... 77
Gamma Log Cross Section V-V' .................. ..................... 78
Gamma Log Cross Section W-W ................. ................ .... 79
Gamma Log Cross Section X-X' ................. ..................... 80
Gamma Log Cross Section Y-Y' .................. ..................... 81
Gamma Log Cross Section Z-Z'............................... ......... 82
Gamma Log Cross Section AA-AA' ................. ........... ........... 83
Gamma Log Cross Section BB-BB' ......................................84
Gamma Log Cross Section CC-CC' ...................................... 85
Gamma Log Cross Section DD-DD' ...................................... 86
Gamma Log Cross Section EE-EE' ......................................87
Gamma Log Cross Section FF-FF' ..................................... 88
Gamma Log Cross Section GG-GG' ..................................... 89
Gamma Log Cross Section HH-HH' ...................................... 90
Gamma Log Cross Section 1-11' ......................................... 91

Location of Gamma Log Cross Sections, Regional North to South Sections .............92
Gamma Log Cross Section JJ-JJ' ...................................... 93
Gamma Log Cross Section KK-KK' .................. ................... 94
Gamma Log Cross Section LL-LL' ................................ ..... 95

APPENDIX B. Table of Reference Logs used in this study ....................... 97

APPENDIX C. Annotated Bibliography of Published Geophysical Well Logs
within (or very near) the SJRWMD ............. ................... .103

viii

FLORIDA GEOLOGICAL SURVEY

GUIDEBOOK TO THE CORRELATION OF GEOPHYSICAL WELL LOGS WITHIN
THE ST. JOHNS RIVER WATER MANAGEMENT DISTRICT

by
Jeff Davis, PG No. 844, Richard Johnson, Don Boniol and Frank Rupert

INTRODUCTION

The St. Johns River Water Management District (SJRWMD) maintains a database of over
2,500 wells that have geophysical logs in digital format. The Florida Geological Survey (FGS)
also maintains a database of lithologic descriptions of wells throughout the State of Florida. Many
of the lithologic logs have geologic contacts identified. Prior to this study, few of the SJRWMD
geophysical logs had been correlated to the corresponding lithologic logs or to neighboring wells.
It was apparent to geological staff at both agencies that such correlations, along with identifica-
tion of distinct and recognizable log signatures for the different lithologic units, would serve as an
extremely useful tool in subsurface hydrogeological investigations within the SJRWMD.

This guidebook identifies the correlation of geophysical well logs (natural gamma and elec-
tric logs) within the SJRWMD. The correlations were documented through a comprehensive
review of existing well log data and literature. Typical natural gamma log signatures for geolog-
ic units in the SJRWMD have been recognized by Johnson (1984), Miller (1986), Scott (1988a),
Duncan et al. (1994) and Green et al. (1995). Geophysical logs are presented in cross sections
and individual figures to serve as reference logs for correlation purposes. These reference logs
exhibit a characteristic log response that can be identified in other logs. Additionally there is suf-
ficient lithologic data available to identify specific geologic units.

This study includes the geophysical log characterization and correlation for the entire SJR-
WMD and encompasses all the geological units commonly penetrated by water wells. The major
geologic units considered in this report include the following Cenozoic strata: Paleocene Cedar
Keys Formation; the Eocene Oldsmar Formation, Avon Park Formation, and Ocala Limestone;
the Oligocene Suwannee Limestone; the Miocene Hawthorn Group; and the various Pliocene,
Pleistocene, and Holocene formations. These units are discussed in detail in the Stratigraphy
section.

Reference logs are identified to establish an objective standard for geophysical correlations
of spatially separated well logs, much as a type section is used as a geologic formation refer-
ence. A reference log well has lithologies that exhibit characteristic geophysical log responses.
Additionally, there is sufficient information to identify a number of formations in the well. Ideally,
a reference log would have cores or cuttings described by a geologist and have a basic geo-
physical log suite consisting of natural gamma, normal electric and caliper logs.

Other wells may not have a lithologic description but do have a geophysical log which can
be correlated to a reference log. Such a well log is designated as a correlated log. Since there
is limited lithologic data, fewer geologic units may be identified in a correlated log. A database of
correlated logs is currently being developed based on the reference logs identified in this report.
Primarily, reference logs were used in the construction of a series of geological cross sections

SPECIAL PUBLICATION NO. 50

(Appendix A). These cross sections provide a reference framework for correlation of logs from
other sites throughout the SJRWMD. Appendix B presents a table with attributes of the reference
logs that identify which lithologic log was used for geologic unit identification, geologic unit
boundaries, location, and other pertinent information. The cross sections and tables do not
include geologic contacts for the Pliocene, Pleistocene, and Holocene sediments. The log
response to individual units within these post-Miocene sediments is too variable to identify con-
sistently recognizable log signatures.

The guidebook is intended to be used as a field tool during drilling and logging operations,
as well as to establish a documented basis (metadata), for the geologic units in the SJRWMD
Geographic Information System data sets. It will also provide citizens and professionals with
interpretations of geophysical log response (primarily natural gamma and electric normal resis-
tivity) correlated with stratigraphy and lithology of the subsurface formations that can be applied
to both well site planning and technical hydrological and geological research.

METHODS

A review of geophysical and lithologic data on file at SJRWMD and FGS identified 180 ref-
erence logs. Identification of wells with sufficient log data to be a reference log involved a review
of geophysical and lithologic data on file at the SJRWMD and FGS data repositories. These data
are accessed and displayed using GeoSys software (Arrington & Lindquist, 1987) and are avail-
able on the FGS website. The digital files for the geophysical data were obtained by real time
acquisition using SJRWMD logging equipment or by digitization of existing analog files, logs in
published literature (Appendix C), and logs that were provided by private well logging companies
and other agents. The lithologic logs used for this report were chosen for completeness and reli-
ability of description. Geophysical logs were used to determine the elevation of the geologic
boundary. In some cases, both a lithologic log and geophysical log were not available for a well.
In these cases a nearby well with a reliable lithologic log was used to confirm the geologic units.

Geophysical logs from eighteen reference wells were chosen to demonstrate typical log sig-
natures throughout the SJRWMD. The location of these wells is shown in Figure 1. Variations in
the absolute value of a natural gamma log response to a particular rock type will occur depend-
ing on borehole conditions, scaling units (counts per second [cps], American Petroleum Instistute
[API]), and probe design. To establish as much consistency as possible, the logs used to show
typical signatures were first normalized by dividing each value by the maximum value in the log
and multiplying by 100. In this way all logs plot on a 0 to 100, unitless scale. The gamma logs
for these wells were then color coded to delineate relative gamma ray intensity based on a four
interval system to provide consistency in descriptions. This system is similar to oilfield tech-
niques of calculating a gamma ray index to identify a baseline value to end member lithologies
(Dresser Atlas, 1975). While oilfield applications work best using the end members of sand to
shale, a pure limestone to clay and phosphate range is more applicable to the SJRWMD.

Delineation of intensity zones is used to standardize and simplify descriptions of gamma ray
response to various lithologies. The gamma intensity produced from pure limestone is herein
described as low intensity. The gamma intensity produced from the clays and phosphates is
described as high intensity. Between the low intensity and high intensity zones, low moderate

FLORIDA GEOLOGICAL SURVEY

Figure 1. Locations of wells used in Figures 2 19.

Location of Well Logs
Used in Figures 2 19

Explanation

0 10 20 Miles

1:1911948
Figure Wells
, County Boundaries 1M Florida
SSJRWMD Boundary

,
::
r

SPECIAL PUBLICATION NO. 50

intensity and high moderate intensity terms are used. The differentiation between low moderate
intensity and high moderate intensity can help distinguish clean sands and dolostones from
lithologies such as organic (peat, lignite) and dolostones that contain accessory minerals with
higher radioactivity. Units containing anhydrite, glauconite, or chert may also be represented in
these moderate intensity zones. Though the boundaries between geologic units often are
marked by a recognizable change in gamma ray intensity, other factors should be considered
when evaluating geologic unit boundaries.

Eocene carbonates show the lowest intensity gamma peaks of the Tertiary formations in
Florida. A detailed description of these units is presented in the Stratigraphy section of this
report. The low intensity baseline is defined by the maximum value of the lowest intensity zone
within the Ocala Limestone. For example, the Ocala Limestone occurs in well BR1217 from a
depth of 116 to 244 feet BLS (Figure 2). The gamma intensity is less than 7 for the entire Ocala
Limestone section. In this case the low intensity baseline is drawn at 7 and all values less than
seven are colored light blue. The high baseline is drawn on the mean value recorded in the
Miocene Hawthorn Group, which occurs at a value of 30 in Figure 2. In this example, everything
above a value of 30 is considered as high intensity and colored orange. The low moderate inten-
sity zone is determined from the median value for the Eocene carbonate section. In Figure 2, the
low moderate intensity zone extends from the low intensity baseline up to a line positioned at the
median value of the underlying Eocene carbonate peaks (at a value of approximately 12). The
high moderate intensity zone extends from this line up to the high intensity baseline and includes
the highest peaks within the Eocene carbonate section.

In certain cases, the units within the Ocala Limestone range from low intensity to high mod-
erate intensity. For the well L-0729 (Figure 3), only the zone from about 100 to 140 feet BLS is
used to define the low intensity baseline. This log also contains peaks present in the Eocene sec-
tion that are near the same magnitude as the peaks in the Miocene section. In this log the mean
value is 26 for the Miocene section, and is used to define the high moderate to high intensity
boundary. These intensity designations are used as a consistent method to describe gamma log
intensity and assist with correlation between logs based on relative gamma ray intensity. These
descriptive terms are used in this report for the gamma log cross sections that are presented in
Appendix A but are not color coded.

Additional descriptive terms are used to describe a characteristic gamma response to litho-
logic changes within a specific interval. For strata where thin interbeds produce a series of high
and low peaks within a short depth interval, the term "uneven" is used. An example of this can
be seen in Figure 2, well BR1217 in the interval from 2,050 to 2,600 feet BLS and in Figure 13,
well F-0162 in the interval from 300 to 400 feet BLS. A section of massive, pure carbonate lime-
stone that produces an interval with a relatively flat profile such as seen in Figure 2 (well BR1217
from 1,500 to 1,550 feet BLS) and Figure 11 (well D-0176 from 510 to 650 BLS) is defined as
"even" intensity. The logs shown in the following figures demonstrate this qualitative classifica-
tion of gamma ray intensity from wells in different counties in the SJRWMD.

FLORIDA GEOLOGICAL SURVEY

Ly Undifferentiated Sand Clay Snell

n Hawthorn Group
Ocala Limestone

- HM H Avon Park Formation

0 -

-100

-200

-300

-400

-500

-600

-700

-800

-900

-1000

-1100

-1200

-1300

-1400

-1500

-1600

-1700

-1800

-1900

-2000

-2100

-2200

-2300

-2400

-2500

-2600

-2700
0

Oldsmar Formation

Cedar Keys
Formation
(unconfirmed)

Legend

Low Intensity (L)
Low Moderate
Intensity- (LM)
High Moderate
Intensity (HM)
High Intensity (H)

10 20 30 40 50 60 70 80
Gamma Intensity (unitless, normalized)
Figure 2. Gamma log of well BR1217, Brevard County.

90 100

* Dashed lines are boundaries for relative intensity

W- I-

c
I -

slc---~

r--- I

c-

SPECIAL PUBLICATION NO. 50

Gamma Log

Electric Log

0

-100 awhorn Group
Ocala Limestone
-200

-300

-400
Avon Park Formation
-500

-600

-700

-800

-900

-1000
Legend

-1100 Low Intensity (L)

-1200 M Low Moderate
Intensity (LM)
-1300 M High Moderate
Inensty HMI
1400 High Intensity (H)

-1500

-1600

-1700

-1800
-1800

-1900 Oldsmar
Formation
-2000

-2100

-2200

-2300 Cedar Keys
-2400 Formation
-2400
0 10 20 30 40 50 60 70 80 90 100

Gamma Intensity (unitless, normalized)

Guard Log for
S thissa ine

0 200 400 600 800 1000

Resistivity (ohm-m)

Figure 3. Gamma and Electric logs of well L-0729, Lake County.

FLORIDA GEOLOGICAL SURVEY

GEOPHYSICAL WELL LOGS

Gamma Log

The gamma log (natural gamma log) records the naturally occurring gamma photon radioac-
tive intensity in the sediment or rock composing the borehole wall. It is the most widely used
nuclear log for groundwater applications (Keys, 1988). In peninsular Florida, this radioactivity
predominantly results from inclusions of highly radioactive phosphate grains, from moderately
radioactive clay-minerals, and from radioactive organic material or peat. Kwader (1982) dis-
cussed the effect on gamma log response from clay minerals and phosphates that are typical of
the Hawthorn Group sediments found in Florida.

A gamma log can be run through both metal and plastic casing provided the borehole diam-
eter is not excessive and a minimum thickness of cement grout is in place between the casing
and the borehole wall. Additional "strings" of casing also reduce the sensitivity of the gamma log.

Electric Log

In this report the term "electric log" refers to any of the geophysical probes that measure
potential differences due to the flow of electric current in and adjacent to a borehole. The pre-
dominant type of electric logs available that are most useful for log correlation within the SJR-
WMD are Single Point Resistance, Long (64") and Short (16") Normal electric logs. These logs
are especially useful for identifying lithologic changes in carbonates where the rocks do not con-
tain enough radioactive material to cause changes in the gamma log response. The electric logs
can also be used to derive porosity within the sediment or rock surrounding the borehole.
Penetration distance into the surrounding lithologic material varies with diameter of the borehole
and the type of electric logging tool in use. The penetration is generally the same as the elec-
trode spacing so that 16-inch normal resistivity probes penetrate approximately 16 inches into
the borehole wall material.

Porous rocks provide electrical flow pathways through the ground water contained in their
interconnected pores, and register as low resistivity on electric logs. Conversely, nonporous
(massive) rocks resist electrical current flow, and register as high resistivity. Generally, in penin-
sular Florida, nonporous massive evaporites are recorded as high resistivity, and massive (non-
porous) limestone and dolostone are recorded as moderate to high resistivity. Porous limestone
and dolostone are recorded as low resistivity. Clay as well as peat are recorded as low resistiv-
ity, and pure quartz sand is recorded as moderate resistivity.

Most electric logs available for use in water wells in peninsular Florida can only be run in the
uncased or openhole portion of boreholes. In general, small diameter (2-4 inch) wells yield the
best and most accurate electric logs. This is because the logging probe samples a larger vol-
ume of rock or sediment in the borehole wall, rather than the fluid (usually water or drilling mud)
filling the borehole. Neutron logs have been used in the cased portions of wells to obtain simi-
lar information as electric logs. These can be recorded in plastic or metal casing. However, since
the probes use a nuclear source their usage is limited and logs are not readily available in the
SJRWMD.

SPECIAL PUBLICATION NO. 50

Other electric logs such as Focused Guard, Dual Induction and Fluid Resistivity are avail-
able for only a few wells and, therefore, have limited value for correlation purposes. Since the
normal electric logs are greatly affected by the salinity of the fluid within both the borehole and
the formation, the normal electric log responses discussed herein represent the formation resis-
tivity as unaffected by high salinity fluids. The Focused Guard log can be used when the salini-
ty is high.

The electric logs for wells L-0729 (Figure 3) and N-0222 (Figure 4) demonstrate several fea-
tures that may be encountered in wells where saline water is encountered. Often, the salinity of
the formation fluids increases dramatically in this environment and may cause a normal electric
log to be attenuated or even flatten. This attenuation can be seen in the normal electric log for
N-0222 at 1,300 feet BLS (Figure 4). The highly saline water has flattened the electric log so that
no bedding can be distinguished below that point. The electric log would therefore be useless in
identifying any formation boundaries below 1,300 feet. The focused guard log, however, shows
many zones of high resistivity that could be used to identify the boundaries. In well L-0729
(Figure 3) elevated chlorides were encountered below 2,000 feet. The electric log shows a gen-
eral decrease in resisitivity but there is still some bed resolution. A section of the guard log that
was run on L-0729 shows more thin bed resolution and higher resistivity but no new high resis-
itivity beds are identified. This adds confidence to the formation picks that are determined from
the electric logs.

STRATIGRAPHY

The Cenozoic stratigraphic column of the SJRWMD has been described in good detail by
Miller (1986). In the northern portion of the District, the Cenozoic strata can be subdivided into
two broad portions: a lower carbonate section, composed almost exclusively of limestone (calci-
um carbonate) and dolostone (calcium-magnesium carbonate), and an upper predominantly sili-
ciclastic (quartz silt/sand/gravel and clay-mineral clay) section. The lower carbonate section also
contains variable amounts of the evaporite minerals gypsum (hydrated calcium sulfate) and
anhydrite (anhydrous calcium sulfate) toward its base. In the southern part of the SJRWMD car-
bonates comprise a significant portion of the Paleocene through Miocene strata, with siliciclas-
tics comprising the Pliocene and younger part of the section.

Paleocene Series
Cedar Keys Formation

Cole (1944) proposed the name Cedar Keys Formation for cream to tan colored, carbonates
underlying peninsular Florida. The Cedar Keys Formation is the oldest unit commonly penetrat-
ed by wells in the SJRWMD. It is composed of lower anhydrite and upper dolostone litholgic
zones (modified from Chen, 1965). The top of this unit generally lies at elevations below -1500
feet MSL in the SJRWMD. The Cedar Keys Formation ranges from about 400 thick under the
northern portion of the District, to 1200 feet or more under the southern portion (Chen, 1965;
Miller, 1986). Water wells and monitor wells typically do not penetrate the entire Cedar Keys sec-
tion.

FLORIDA GEOLOGICAL SURVEY

-400
1

-600 -

-800 -

-1600 -

S-1200
7. Fresh v
a)

Saline v

-1400

-1600

-1800

-2000
0 100 200 300

ohm-r
16" Normal Electric Log
Focused Guard Log

Figure 4. Electric and Focused Guard logs of well N-0222, Nassau County.

9

SPECIAL PUBLICATION NO. 50

Lithostratigraphy

The lower anhydrite lithozone of the Cedar Keys Formation consists of interbedded gray,
brown, or clear, massive anhydrite and gray to tan dolostone. The lower lithozone typically com-
prises up to two-thirds of the Cedar Keys thickness (Miller, 1986). Lower lithozone anhydrite
characteristically occurs as sand-to-pebble-sized blebs surrounded by thin walls of dolostone; as
discrete beds; as bands or laminae; as intergranular and foraminiferal moldic porosity fill in dolo-
stone; and as rare discrete sand sized crystals in dolostone. White to clear gypsum may com-
pose thin beds, bands, blebs, veins and porosity infillings in dolostone. Thin interbeds of gray to
tan recrystallized dolostone also occur in the lower anhydrite lithozone.

The upper dolostine lithozone of the Cedar Keys Formation characteristically consists of
gray to tan, relatively porous, finely recrystallized dolostone. Gypsiferous dolostone, containing
white to clear gypsum, also occurs toward the base of the upper dolostone lithozone.

Gamma Logs

On gamma logs, the lower anhydrite lithozone is recorded as even, high moderate intensi-
ty dolostone peaks interspersed with low to low moderate intensity valleys representing anhy-
drite beds. Figure 2 illustrates the typical signature on a gamma log for the upper portion of the
lower anhydrite lithozone of the Cedar Keys Formation. These logs are from an injection test well
(BR1217, located in east-central Brevard County) that partially penetrates the Cedar Keys
Formation. The top of the lower anhydrite lithozone can be identified by the presence of a dis-
crete anhydrite bed centered at approximately 2,680 feet BLS that is recorded as a low intensi-
ty valley on the gamma log. Locally, the bedded anhydrite contains traces of very finely particu-
late peat that is radioactive. Therefore, slightly peaty anhydrite beds may not be as well-defined
on gamma logs as pure anhydrite beds. However, the lower anhydrite lithozone of the Cedar
Keys Formation is generally not easily identified on the gamma log; it is characteristically best
defined on the electric log. The indistinct contact between the Cedar Keys Formation and the
overlying Oldsmar Formation can be seen in Figure 2 at approximately 2,400 to 2,500 feet BLS.

A SJRWMD monitoring well (L-0729) was recently drilled in southern Lake County near Lake
Louisa. This well penetrated the top of the Cedar Keys Formation at approximately 2,090 feet
BLS. The gamma log shown in Figure 3 shows the low moderate intensity below 2,250 feet typ-
ical of the gypsum-rich dolostone that is found in the Cedar Keys Formation. The high intensity
zone seen between 2,105 and 2,250 feet BLS is unusual, but may be due to the presence of clay
and silt.

Because both the upper lithozone Cedar Keys Formation and the lower lithozone Oldsmar
Formation consist of dolostone, their traces, as recorded on gamma logs, are quite similar. The
uneven low moderate to high moderate intensity recorded at the contact between the Cedar
Keys Formation and the Oldsmar Formation generally cannot be distinguished using gamma
logs alone.

FLORIDA GEOLOGICAL SURVEY

Electric Logs

The lower anhydrite lithozone of the Cedar Keys Formation is typically recorded on electric
logs as a distinct series of thick, high resistivity (very low porosity) peaks representing discrete
anhydrite beds, alternating with lower resistivity (higher porosity) dolostone intervals.

The upper dolostone lithozone of the Cedar Keys Formation is easily identified on most elec-
tric logs; the porous dolostone is characteristically recorded as a relatively flat, low resistivity
(high porosity) line (Chen, 1965). This trace pattern contrasts sharply with that recorded from
the overlying base of the Lower Eocene Oldsmar Formation, which consists of very hard recrys-
tallized low porosity (high resistivity) dolostone. Figure 5 illustrates this trace pattern from an
electric log obtained from a U.S. Geological Survey deep monitor well D-0349 located in west-
ern Duval County. The contact between low resistivity uppermost Cedar Keys Formation and
high resistivity basal Oldsmar Formation occurs at about 1,975 feet BLS.

For well L-0729, a Focused Guard log was also run in the interval for the Cedar Keys
Formation and is included in Figure 3 for comparison. Water quality samples from this zone indi-
cated an increase in conductivity. The electric log is smoother and has somewhat lower values
than the Focused guard log. The Focused guard log shows higher bed resolution but no new
high resistivity zones are identified that would indicate the pore fluid was masking the response.
Thus the low resistivity recorded for this zone is primarily caused by the formation materials.
These two figures emphasize the advantages of electric logs over gamma logs for identifying the
Cedar Keys Formation, however water quality should always be considered when evaluating the
electric logs for this unit.

Lower Eocene Series
Oldsmar Formation

All Lower Eocene carbonate rocks underlying Florida are included in the Oldsmar Formation
of Applin and Applin (1944). The Oldsmar Formation is subdivided into lower and upper litho-
zones (modified from Chen, 1965). The top of this unit typically occurs at elevations of -965 to
-2,332 feet MSL in the SJRWMD. Within the SJRWMD, the thickness of the Oldsmar Formation
generally ranges between 400 and 1,100 feet thick.

Lithostratigraphy

The lower lithozone of the Oldsmar Formation consists of very dark brown to dark gray, very
hard and massive dolostone. Traces of glauconite, pyrite, peat and phosphate occur throughout
the lower dolostone lithozone. The upper lithozone is composed of dolomitic, recrystallized, cal-
carenitic limestone and brown recrystallized dolostone. Near the top of the formation an impure
carbonate section of highly variable thickness contains chert, peat, glauconite, pyrite, phosphate,
clay, and granule to pebble sized quartz crystal masses. This section represents the "glauconitic
zone" of Duncan et al. (1994) and the silicicc zone" of Johnson (1984). In Brevard County,
Duncan et al. (1994) picked the upper contact of the Oldsmar Formation at the top of this impure
carbonate section. Marking the top of the Oldsmar Formation, a relatively thin (0-60 feet) and
somewhat discontinuous bed of white to light tan, pure, porous, foraminiferal calcarenitic lime-

FLORIDA GEOLOGICAL SURVEY

Oldsmar Formation

I I I I I I I I I
0 10 20 30 40 50 60 70 8(
Gamma Intensity (unitless, normalized)

Figure 6. Gamma log of well P-0619, Putnam County.

0

-100

-200

-300

-400

-500

-600

-700

-800

-900

-1000

-1100

-1200

-1300

_IAnn

Legend

Low Intensity (L)
Low Moderate
Intensity- (LM)
High Moderate
Intensity (HM)
SHigh Intensity (H)

Hawt~horn Group

Ocala Limestone

Avon Park Formation

.. . .. . .

-I '^

FLORIDA GEOLOGICAL SURVEY

stone occurs above the impure carbonate interval and directly below the brown, massive, crys-
talline dolostone occurring at the base of the overlying Avon Park Formation.

Gamma Logs

Because the lower lithozone of the Oldsmar Formation consists almost exclusively of dolo-
stone, uneven high moderate intensity, with interspersed minor low moderate intensity peaks, is
characteristically recorded on gamma logs. Therefore the lower lithozone Oldsmar cannot be dis-
tinguished from upper lithozone Cedar Keys Formation on the sole basis of the gamma log. The
example shown in Figure 5 demonstrates the similarity in gamma response at the contact of the
Oldsmar Formation and the underlying Cedar Keys Formation at 1,975 feet BLS. This gamma
log was obtained from a deep test/observation well (D-0349) located in western Duval County.

The interval from 1,530 to 1,975 feet BLS in Figure 5 illustrates a typical gamma log
response from the Oldsmar Formation. The interval shows a section of uneven low moderate to
high moderate intensity from approximately 1,800 to 1,975 feet BLS, representing dolostone in
the the lower lithozone.

The upper lithozone is indicated by a series of predominantly uneven low to low moderate
intensity limestone and dolostone peaks lying between 1,545 and 1,800 feet BLS. Interspersed
high moderate to high intensity peaks likely reflect the moderately radioactive phosphate, clay,
glauconite and peat content in the upper lithozone. The overall lower intensity of the upper litho-
zone contrasts with the higher intensity recorded below in the lower dolostone lithozone of the
Oldsmar Formation.

Electric Logs

On electric logs, the lower dolostone lithozone of the Oldsmar Formation is characterized by
a thick series of high resistivity peaks interspersed with very thin low resistivity valleys. This
trace pattern contrasts greatly with the low, even resistivity typical of the subjacent upper dolo-
stone lithozone Cedar Keys Formation. Figure 5 shows an electric log for well D-0349. It depicts
the lower dolostone lithozone as a characteristically distinct series of high resistivity peaks
between approximately 1,800 to 1,975 feet BLS.

The upper lithozone of the Oldsmar Formation (about 1,530 to 1,800 feet BLS on Figure 5)
is recorded on electric logs as alternating higher resistivity peaks and lower resistivity valleys typ-
ical of alternating lower and higher porosity carbonate beds.

Middle Eocene Series
Avon Park Formation

Miller (1986) grouped the lithologically similar Avon Park Limestone and Lake City
Limestone of Applin and Applin (1944) into a single unit, the Avon Park Formation. The Avon
Park Formation comprises the Middle Eocene carbonates occurring under the SJRWMD. Within
the SJRWMD the top of this unit typically occurs at elevations of-92 to -850 feet MSL. The thick-
ness of the Avon Park Formation varies between 600 and 1550 feet.

SPECIAL PUBLICATION NO. 50

Lithostratigraphy

The Avon Park Formation characteristically consists of dark brown to dark tan to dark gray,
variably peaty recrystallized dolostone interbedded with white to tan, recrystallized foraminiferal
limestone. Beds of tan to brown to gray, dolomitic limestone and dolostone also are common.
Three dolostone lithozones are commonly present in this formation. A relatively continuous and
massive dolostone, commonly occurs at the base of the Avon Park Formation. An upper dolo-
stone lithozone, comprised of recrystallized dolostone with interbedded limestone, typically
occurs within 50 to 200 feet of the upper contact. A less continuous middle dolostone lithozone
may also be present, separated from the more continuous lower and upper dolostone lithozones
by sections of limestone and dolomitic limestone.

The Avon Park Formation typically contains variable amounts of black to dark brown, finely
particulate to fibrous, partially decomposed organic material or peat. The peat occurs very fine-
ly disseminated, or as sand to pebble sized blebs, as easily identifiable leaf or seagrass plant
fossils, as laminations or stringers, and as discrete beds. Within the upper portion of the middle
dolostone lithozone (if present) and at the base of the upper dolostone lithozone, two 5 to 15 feet
thick discrete beds of peat (Chen, 1965) occur relatively continuously in the northern two-thirds
of the SJRWMD (north of Brevard, eastern Osceola, and northeastern Okeechobee Counties).
In the southern one-third of the district, the thick peat beds are locally replaced by intervals of
peaty dolostone and recrystallized limestone. The lower dolostone lithozone of the Avon Park
Formation may contain yellow to orange pyrite and green glauconite grains; the middle dolostone
lithozone also locally contains glauconite, but commonly lacks the pyrite content. This typical
lithology of the base of the lower dolostone lithozone of the Avon Park Formation (dark brown to
dark gray, peaty, pyritiferous, glauconitic dolostone) differs markedly from the tan to white, pure,
calcarenitic limestone bed occurring at the top of the underlying upper lithozone Oldsmar
Formation.

At the top of the Avon Park Formation, the uppermost upper lithozone characteristically con-
sists of brown to orange recrystallized dolostone interbedded with light to dark tan limestone.
These lithologies are easily differentiated from the calcarenitic limestone typical of the overlying
basal lower lithozone Ocala Limestone.

Gamma Logs

Due to the characteristic content of moderately radioactive dolostone and highly radioactive
peat in the Avon Park Formation, the interval is typically recorded on gamma logs as uneven low
moderate to high moderate intensity. Moderately radioactive glauconite increases gamma
intensity recorded in the lower lithozone of the Avon Park Formation. Two discrete peat beds,
one in the middle lithozone and the other at the base of the upper lithozone, are typically record-
ed as high intensity peaks or as a series of high moderate intensity peaks, where present. Figure
6 illustrates the gamma log obtained from a well (P-0619) in north-central Putnam County which
clearly displays these two high intensity peaks. The stratigraphically lower peak is centered at
approximately 660 feet BLS and the upper peak at about 540 feet BLS.

Figure 7 illustrates a typical gamma log of the upper portion of the Avon Park Formation,
obtained from a livestock supply well (IR0748) located in southeastern Indian River County. In

SPECIAL PUBLICATION NO. 50

undifferentiated, sand,
clay, and shell

Hawlhorn Group

0

-100

-200

-300

-400

-500

-600

-700

-800

-900

-1000

-1100

-1200

-1300

-1400

-1500

-1600 -

-1700

-1800
0

Fi

Avon Park Formation

Ocala Limestone

Legend

Low Intensity (L)
Low Moderate
Intensity- (LM)
High Moderate
Intensity (HM)
S High Intensity (H)

50
Gamma Intensity (unitless, normalized)
gure 7. Gamma log of well IR0748, Indian River County.

SPECIAL PUBLICATION NO. 50

80 90 100

Figure 8. Gamma log of well L-0005, Lake County.

0

-20

-40

-60

-80

-100

-120

-140

-160

-180

-200

-220

-240

I -260

_ -280
CL
o -300

-320

-340

-360

-380

-400

-420

-440

-460

-480

-500

-520

-540

0 10 20 30 40 50 60 70
Gamma Intensity (unitless, normalized)

FLORIDA GEOLOGICAL SURVEY

IR0748, the Avon Park Formation-Ocala Limestone contact occurs at approximately 590 feet
BLS. The low intensity of basal Ocala Limestone contrasts markedly with the uneven low mod-
erate to high moderate intensity characteristic of the Avon Park Formation below. The Avon Park
Formation also contains beds that produce high intensity units such as those seen from 800 to
860 feet BLS in IR0748. These high intensity units may not be laterally extensive; however, they
are helpful in identifying the presence of the Avon Park Formation. This contrasts with the even
low intensity characteristic of the underlying calcarenite bed marking the top of the underlying
Oldsmar Formation.

The top of the Avon Park Formation is characteristically recorded on gamma logs as an
interval of low moderate or high moderate intensity peaks. This contrasts markedly with the lower
intensity typically recorded in the overlying Ocala Limestone. This contrast between the top of
the Avon Park Formation and the Ocala Limestone is demonstrated in Figure 6 where the con-
tact in the gamma log for P-0619 is identified at a depth of 255 feet BLS. Figure 2 depicts the
gamma log obtained from well BR1217 located in east central Brevard County. Between approx-
imately 225 to 300 feet BLS, the peaty uppermost upper lithozone Avon Park Formation is char-
acteristically recorded as a series of very closely spaced low moderate intensity peaks. Above
approximately 225 feet BLS, the Ocala Limestone is typically recorded as even low intensity.

In Alachua, Marion, and Lake Counties, the top of the Avon Park Formation locally contains
clay in addition to peat; this results in an exceptionally distinct interval of high moderate to high
intensity marking the formational top on the gamma log. Characteristic gamma log response for
the Avon Park Formation-Ocala Limestone contact in these counties can be seen in the cross
sections in Appendix A, in particular, well M-0060 in section T-T', wells A-0375 and M-0139 in
section S-S', and wells L-0121 and L-0122 in section I-I'.

Figure 8 illustrates the gamma trace obtained from a public supply well (L-0005) in south-
central Lake County in which the top of the Avon Park Formation contains clay. Below the even,
low and low moderate intensity characteristically recorded in the Ocala Limestone (about 190 to
260 feet BLS), the uppermost upper zone Avon Park Formation is recorded as a series of high
intensity peaks (about 260 to 325 feet BLS).

Electric Logs

On electric logs, the Avon Park Formation is characteristically recorded as alternating low to
very low resistivity valleys (corresponding to moderate to high porosity limestone or dolostone,
peaty carbonate, and/or discrete peat beds) and high to very high resistivity peaks (correspon-
ding to hard and massive low porosity dolostone). The three (lower, middle and upper) dolostone
lithozones are typically recorded as thick intervals containing abundant, closely spaced, moder-
ate to high resistivity (low porosity) peaks separated by thin, sharp, low to very low resistivity val-
leys (which may represent fractures/joints or intercalations of peaty carbonate or discrete peat
beds). This characteristic trace pattern is shown on Figure 9, an electric log from deep observa-
tion well OR0465 at Lake Ivanhoe in Orlando, Orange County. The lower dolostone lithozone
extends from approximately 1,445 feet BLS to about 1,800 feet BLS, the middle dolostone litho-
zone includes from 1,000 feet BLS to about 1,300 feet BLS, and the upper dolostone lithozone
extends from about 400 feet BLS to about 500 feet BLS. Since the upper lithozone of the Avon

FLORIDA GEOLOGICAL SURVEY

Gamma Log

0

-100 -

-200

-300

-400

-500

-600

-700

-800

-900

-1000

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-1200

-1300

-1400

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-1600 -

-1700

-1800 -

-1900

-2000

-2100

-2200

Electric Log

0 10 20 30 40 50 60 70 80 90 100 0 250 500 750 1000 1250 1

Gamma Intensity (unitless, normalized) Resistivity (ohm-m

Figure 9. Gamma and Electric logs of well OR0465, Orange County.

500 1750 2000

)

' ' '

FLORIDA GEOLOGICAL SURVEY

Electric Log

undifferentiated sand,
201 clay, and shell

Hawthorn Group

-140

-160
-18 Ocala Limestone

-200 -

-220

-240 -

-260 .
SAvon Park
S-280 Formalion
-300 -.(_______

-400

-420 -

-440 --
-460 -

-480

-500
-o460 -

-520 -

-540
100 0 10 20 30 40 50 60 70 80 90 100
Resistivity (ohm-m)

Figure 10. Gamma and Electric logs of well P-0172, Putnam County.

Gamma Log

Legend

Low Intensity (L)

Low Moderate
Inlensity (LM)
* High Moderate
Intensity (HM)
High Intensity (H)

0 10 20 30 40 50 60 70 80 90
Gamma Intensity (unitless, normalized)

__

-I

e

SPECIAL PUBLICATION NO. 50

Park Formation contains peat and clay, it is characteristically recorded on electric logs as an
interval of even, low resistivity or a series of thin, low resistivity peaks and valleys.

Figure 10 depicts the log response for the top of the Avon Park Formation as recorded on
an electric log obtained from a SJRWMD observation well (P-0172) east-central Putnam County.
The peaty uppermost Avon Park Formation is recorded as an even, low resistivity valley from
approximately 245 to 265 feet BLS, with the basal Ocala Limestone high resistivity peak record-
ed just above. At 310 feet BLS the high resistivity peaks of the upper dolostone lihtozone begins.

Variations in the resistivity response at the Avon Park Ocala Limestone contact may make
correlations using resistivity logs alone difficult. A combination of electric and gamma log may be
the only way to recognize the contact. The upper dolostone lithozone is most easily recognized
by the high resistivity peaks. Once this upper dolostone lithozone is identified, the gamma log for
the interval above this zone can be reviewed for a decrease in gamma intensity.

Upper Eocene Series
Ocala Limestone

Dall and Harris (1892) first used the name Ocala Limestone for marine carbonate rocks
exposed in quarries near Ocala, Marion County. It is found throughout most of Florida. The
Ocala is subdivided into upper and lower units (after Applin and Applin, 1944; Scott, 1993). The
top of the unit occurs at elevations between 80 feet MSL in western Alachua County and -660
feet MSL in St. Lucie County. It ranges in thickness from 0 feet, where it is absent on structural
highs, to about 400 feet in Duval County.

Lithostratigraphy

The Ocala Limestone typically consists of white or tan, homogeneous, porous and perme-
able, thickly bedded, foraminiferal limestone containing abundant granule to pebble sized
foraminifera, echinoids, mollusks, corals, and bryozoans. The Ocala Limestone characteristical-
ly consists of upper and lower lithozones (modified from Applin and Applin, 1944) which differ
only slightly in average grain size and minor dolomite content.

The lower lithozone characteristically consists of white, tan, or light yellow, foraminiferal cal-
carenite and calcilutite, commonly with sparry calcite cement. Thick relatively soft intervals are
interbedded with thinner, hard to very hard, finely recrystallized limestone with varying degrees
of molluscan moldic porosity. Recrystallized dolomitic limestone beds occur discontinuously in
the lower lithozone, as well as a basal section composed of very hard, molluscan to echinoid cal-
ciruditic limestone. The pure calcarenitic to calciruditic limestone lithologies typical of the lower
lithozone Ocala Limestone contrast markedly with the clayey and/or peaty dolostone and lime-
stone characteristic of the top of the underlying Avon Park Formation.

The upper lithozone of the Ocala Limestone is characteristically composed of white to light
tan, thickly bedded, extremely fossiliferous, foraminiferal calciruditic limestone interbedded with
fossiliferous, foraminiferal calcarenitic limestone. Both types of limestone typically contain vari-
able amounts of calcilutite cement; however, moderate to high intergranular porosity is never-

SPECIAL PUBLICATION NO. 50

Gamma Log

Legend

Lu Inlnsy (L)
I,. LOWMiderate

High Moderate
Intenstr (HM)
SHuHigr Inltirily (H)

Hawthorn Group

-~ AfV .1

0

-100-

-200

-300

-400

-800
-600

-700

-600

-900

- Y -

-700

-600

-900

-1000

-1100

-1200

-1300

-1400

-1500

-1600

-1700

-1600

-1900

I -

0 10 20 30 40 50 60 70 80 90 100

Gamma Intensity (unitless, normalized)

Avon Park
Formation

Oldsmar Formation

Cedar Keys
Formation

500 1000 1

Resistivity (ohm-m)

Figure 5. Gamma and Electric logs of well D-0349, Duval County.

12

Electric Log

Ocala Limestone

-1000

-1100

-1200

-1300 -

-1400

-100 -

-1600

-1700

-1800

-1900-

-2000

-200 -

-2200

~ ~ ~ ~ ~ ~ ~

L- ...II..--

-2200

3
-c;

SPECIAL PUBLICATION NO. 50

theless common. As in the lower lithozone, relatively thin (0.5-2 feet), very hard beds of mollus-
can, moldic, finely recrystallized limestone occur interbedded with the calcirudite and calcaren-
ite.

The top of the pure foraminiferal calcirudite or calcarenite of the Ocala Limestone differs
greatly from the phosphatic, predominantly siliciclastic lithology of the overlying Hawthorn Group.
In eastern Indian River and southeastern Brevard Counties, Suwannee Limestone occurs above
the Ocala Limestone. Again, the somewhat phosphatic, slightly peaty, variably dolomitic cal-
carenite of the Suwannee Limestone is easily differentiated from the pure calcirudite to cal-
carenite characteristic of upper lithozone Ocala Limestone.

Gamma Logs

The Ocala Limestone is easily identified on both gamma and electric logs. Because the
Ocala Limestone is predominantly composed of very pure limestone, the interval is typically
recorded on gamma logs as low intensity. The Ocala Limestone characteristically produces the
lowest intensity recorded in the carbonate section of the Cenozoic stratigraphic column and is
therefore used as a low baseline for the relative gamma intensity scale used in this report. The
top of the Ocala Limestone is easily identified on most gamma logs. Over much of the SJRWMD,
the base of the overlying Hawthorn Group is characteristically recorded as a high intensity peak
just above the low intensity typical of the uppermost Ocala Limestone. This may be observed in
Figure 10, well P-0172, at about 125 feet BLS. In many gamma logs, the entire Ocala Limestone
section produces only low intensity. This is an indication that the upper lithozone and the lower
lithozone may only differ slightly. Examples of Ocala Limestone gamma response that are pri-
marily low intensity throughout can be seen in wells BR1217 (Figure 2), D-0349 (Figure 5),
IR0748 (Figure 6), and IR0338 (Figurel4). In some logs, a low moderate intensity zone is record-
ed on the top of the Ocala Limestone because the paleokarst has allowed clay and phosphate
from the overlying formations to migrate downward and accumulate.

Other Ocala Limestone sections show both an upper and lower lithozone gamma log
response. The gamma log for D-0176 (Figure 11) shows the characteristic upper lithozone
gamma log response from 510 to 650 feet BLS. From 650 to 740 feet BLS the even low moder-
ate and/or high moderate intensity typical of the lower lithozone in the interval can be seen.
Other examples where both lithozones can be distinguished are shown in wells L-0005 (Figure
8), F-0162 (Figure 13), SJ0148 (Figure 15), D-0520 (Figure 17), and F-0019 (Figure 19). In most
areas of the SJRWMD, the lower lithozone of the Ocala Limestone is recorded as slightly high-
er intensity due to the presence of dolomitic limestone beds. However, the gamma intensity
recorded in this zone is generally lower than that recorded in the underlying upper lithozone Avon
Park Formation.

The gamma log from well L-0094 (Figure 12), a water supply well located in Astatula, cen-
tral Lake County, is an example of a log from the ridge areas where sand is mined. Note that the
undifferentiated sand, clay and shell sediments at depths above 100 feet BLS have the lowest
gamma intensity of the entire section. This is related to the very clean sands that occur above
the Hawthorn Group. Also in this log, the low intensity of the Ocala Limestone is higher than in
wells in other counties (Figures 7-11).

FLORIDA GEOLOGICAL SURVEY

undifferentiated sand clay, and shell

-150 A -

iIo -- .

-200 Hay

-250

-300

-350

-400 J,______-- -__--
-400 -

-450 -

-500

-550 -

-600o Oc

-650 -

-700

-750

-800

-850

-900 Avc

-950

-1000

-1050

-1100

-1150

-1200

-1250

-1300

thorn Group

ala Limestone

)n Park Formation

Legend

Low Intensity (L)
Low Moderate
Intensity- (LM)
High Moderate
Intensity (HM)
High Intensity (H)

80 90 100

Figure 11. Gamma log of well D-0176, Duval County.

0 10 20 30 40 50 60 70
Gamma Intensity (unitless, normalized)

I

SPECIAL PUBLICATION NO. 50

Gamma Log

Electric Log

Legend

Low Intensity (L)
SLow Moderate
Intensity- (LM)
SHigh Moderate
Intensily- (HM)
m High Intensity (H)

0

-50

-100

-150

undifferentiated sand
clay and shell

Hawthorn Group

Ocala Limestone

Avon Park
Formation

1 I-350 I-I-I I
10 20 30 40 50 60 70 80 90 100 0 200 400 600 800 1000

Gamma Intensity (unitiess, normalized) Resistivity (ohm-m)
Figure 12. Gamma and Electric logs of well L-0094, Lake County.

U

-200 -

-250 -

-300

-350

I_
~
I _

FLORIDA GEOLOGICAL SURVEY

Figure 13 shows the gamma log obtained from well F-0162 located in northeastern Flagler
County. The top of the Ocala Limestone consists of characteristic even low intensity below the
basal Hawthorn Group high intensity peak at approximately 145 feet BLS. Notice the transition
zone of low moderate intensity from about 145 to 152 feet BLS, followed by low intensity to 200
feet BLS. This demonstrates the gamma response where clay and other minerals from the over-
lying Hawthorn Group have either filled lows in the paleokarst of the Ocala or have been deposit-
ed in the pore space thereby increasing the gamma intensity. Other examples of this effect can
be seen in the gamma cross sections which are discussed later in this report.

In eastern Indian River and southeastern Brevard Counties, the Suwannee Limestone
occurs above the Ocala Limestone. The base of the phosphatic, silty, and peaty Suwannee is
typically recorded as high moderate gamma intensity, which contrasts with the low intensity
recorded in the uppermost Ocala Limestone. Figure 14 illustrates the typical Suwannee
Limestone/Ocala Limestone contact as recorded in a gamma log obtained from well IR0338
located in south-central Indian River County. The contact between the top of the Ocala
Limestone at 380 feet BLS is characteristically recorded as relatively uneven low intensity lying
directly below the high moderate intensity typical of the Suwannee Limestone in this area.

Electric Logs

On electric logs, the Ocala Limestone is highly variable but is most often recorded as a
series of relatively thin, moderate to high resistivity peaks (corresponding to interbeds of hard,
lower porosity limestone or dolomitic limestone) between broad, low resistivity valleys (repre-
senting porous limestone or moldic recrystallized limestone). Figure 12 depicts the electric log
obtained from well L-0094. In this log, the Ocala Limestone recorded as uneven, higher resistiv-
ity peaks centered at 210 and 250 feet BLS. The Ocala is generally sandwiched between the low
resistivity typical of the subjacent peaty to locally argillaceous Avon Park Formation (contact at
approximately 255 feet BLS), and the very low resistivity typical of the overlying partially silici-
clastic Hawthorn Group (contact at about 185 feet BLS). The base of the Ocala Limestone is
often recorded as a relatively thick (10 to 15 feet) high resistivity, single or double peak marking
the presence of the basal, very hard and recrystallized, low porosity, molluscan or echinoid lime-
stone bed between approximately 245 to 255 feet BLS. This peak strongly contrasts with the
broad low resistivity valleys) characteristically marking the peaty top of the underlying Avon Park
Formation below about 255 feet BLS.

The top of the Ocala Limestone is also typically recorded as a moderate to high resistivity
peak on electric logs. This trace pattern may sharply contrast with the pattern recorded in the
base of the Hawthorn Group, which, in portions of the SJRWMD, is recorded as a low resistivity
valley (Figure 15, above approximately 185 feet BLS). The base of the Hawthorn Group here is
locally composed of phosphatic, quartz sandy clay. In other areas (e.g., eastern Putnam and
southern St. Johns Counties), basal Hawthorn Group (Penney Farms Formation) is composed
of brown, very hard, very low porosity, crystalline dolostone, recorded as a very high resistivity
peak, as illustrated in Figure 15. This electric log was obtained from an agricultural supply well
SJ0148 located in southwestern St. Johns County. The log records a very high peak at the base
of the undifferentiated Hawthorn centered at about 175 feet BLS, representing the basal

SPECIAL PUBLICATION NO. 50

Ocala Limestone

Avon Park Formation

Legend

Low Intensity (L)
Low Moderate
Intensity (LM)
High Moderate
Intensity (HM)
High Intensity (H)

90 100

0

-50

-100

-150

-200

-250

-300

-350

-400

0 10 20 30 40 50 60 70 80
Gamma Intensity (unitless, normalized)
Figure 13. Gamma log of well F-0162, Flagler County.

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Gamma Log

Electric Log

300 -300

-350 Suwannee
Limestone

-400 -400 -
Legend
SLownternaiy- () Ocala
SLoModere Limestone
Low Moee
-450 H hly (Ir M
SInlrHily (HM)
I ligh Intensity (1)

-500 0 i -500 i
0 10 20 30 40 50 60 70 80 90 100 -20 0 20 40 60 80 100 120 140 160
Gamma Intensity (unitless. normalized) Resistivily (ohm-m)

Figure 14. Gamma and Electric logs of well IR0338, Indian River County.

SPECIAL PUBLICATION NO. 50

Gamma Log

0

-50

-100

-150

-200

-250

-300

-350

-400

-450

-500

Electric Log

0 10 20 30 40 50 60 70 80 90 100
Gamma Intensity (unitless, normalized)

0 50 100 150
Resistivity (ohm-m)

Figure 15. Gamma and Electric logs of well SJ0148, St. Johns County.

Legend

FLORIDA GEOLOGICAL SURVEY

Hawthorn dolostone bed. The top of the Ocala Limestone is recorded as a relatively atypical
decrease in resistivity. This log does, however, highlight the variability in electric log response
since the entire section is an even pattern indicative of a massive carbonate with no differing
interbedded lithology. For cases like this, and in general, it is necessary to use the gamma log in
conjunction with the electric log in determining the top of the Ocala.

Where the Suwannee Limestone occurs above the Ocala Limestone (southeastern Brevard
and eastern Indian River Counties), the contact, as recorded on electric logs, is typically not well-
defined. Because both the Suwannee Limestone and the Ocala Limestone are generally com-
posed of porous limestone, these formations are recorded similarly on electric logs. Locally in
southeastern Indian River County, the basal Suwannee Limestone is significantly more porous
than the thick beds of massive recrystallized very hard limestone at the top of uppermost Ocala
Limestone. This produces a low resistivity zone on the log directly above the very high resistivi-
ty peak recorded at the top of the Ocala.

Oligocene Series
Suwannee Limestone

Cooke and Mansfield (1936) proposed the name Suwannee Limestone for limestone
exposed along the Suwannee River, between the towns of Ellaville and White Springs,
Suwannee and Hamilton Counties. Most older literature assigned the Oligocene carbonates in
the SJRWMD, which locally are restricted to the southeastern portion of the District, to the
Suwannee Limestone. Recent work by Brewster-Wingard et al. (1997) recognized that a large
portion of these peninsular Florida Oligocene carbonates are actually Arcadia Formation, of the
basal Hawthorn Group. For the purposes of this report the older convention of considering these
sediments as Suwannee Limestone is used. In the SJRWMD the top of the unit typically occurs
at elevations between -300 and -425 feet MSL.

Lithostratigraphy

The Suwannee Limestone consists of tan to brown, moderately to very porous, variably
dolomitic, microfossiliferous calcarenitic limestone containing variable concentrations of silt sized
phosphate grains and rare peat blebs. The interval is 60 feet or less in thickness over most of its
extent and it thins to pinch out inland to the west. However, in extreme southeastern Indian River
County, located approximately one mile south of Vero Beach, an anomalous maximum thickness
of 288 feet occurs (Appendix A, gamma cross section HH-HH', well IR0930). The thickness of
the formation below the southern one-half of the barrier island in Indian River County is also
anomalous (150-200 feet).

Gamma Logs

On gamma logs from wells in this area, the Suwannee Limestone is characteristically
recorded as uneven low to high intensity. The low intensity zones correlate with relatively pure
nonphosphatic, nonpeaty intervals and high moderate intensity represents more dolomitic, phos-
phatic and peaty beds of carbonate. A typical Suwannee Limestone gamma trace is illustrated
in Figure 14, obtained from a water supply well (IR0338) located in Vero Beach, southeastern

SPECIAL PUBLICATION NO. 50

Indian River County. The uneven low moderate to high moderate intensity from approximately
343 to 380 feet BLS contrasts with the low intensity, relatively even trace characteristically pro-
duced in the upper lithozone Ocala Limestone below approximately 380 feet BLS. The charac-
teristic thick, high intensity, basal Hawthorn Group peak is centered at about 315 feet BLS direct-
ly above the lesser intensity typically associated with the uppermost Suwannee Limestone
(below approximately 343 feet BLS). The basal Hawthorn Group invariably contains substantial-
ly higher concentrations of phosphate than the uppermost Suwannee Limestone. This produces
an easily identified peak above the top of the Suwannee Limestone on gamma logs.

Electric Logs

The Suwannee Limestone is recorded on electric logs as a series of broad, low resistivity
valleys interspersed with low, somewhat higher resistivity peaks. In general, the Suwannee
Limestone cannot be differentiated from either the underlying Ocala Limestone or the lower dolo-
stone lithozone of the overlying Hawthorn Group using only the electric log. At certain sites the
top of the Ocala Limestone is easily identified on electric logs by a relatively thick (10 to 15 feet)
extremely high resistivity peak, therefore the Suwannee Limestone can be identified by a
decrease in resisitivity. Where basal Hawthorn Group consists of quartz sandy clay, a very low
resistivity valley overlies the significantly higher resistivity recorded in the uppermost Suwannee
Limestone.

Oligocene to Pliocene Series
Hawthorn Group

Dall and Harris (1892) first used the name Hawthorne beds for phosphatic sediments
exposed near the town of Hawthorne, Alachua County. The unit has undergone considerable
nomenclatural evolution through the years. It was first designated as a formation by Matson and
Clapp (1909). Scott (1988a) raised the Hawthorn to group status, and recognized five forma-
tions of the group within the SJRWMD. The Coosawhatchie, Marks Head, and Penney Farms
Formations occur in the northern portion of the SJRWMD. These units extend southward to the
Lake County area, where the formations become indistinguishable in cores and are generally
referred to as Hawthorn Group undifferentiated. In the southern portion of the SJRWMD, from
the Polk-Osceola-Brevard County area southward, the Peace River and Arcadia Formations
comprise the Hawthorn Group. Delineation of the individual formations is generally possible in
cores. However, most of the well data is from cuttings in which it is generally not possible to dif-
ferentiate formations within the Hawthorn Group. Additionally, identification of individual forma-
tions using gamma logs alone is difficult or not possible throughout most of the SJRWMD.
Therefore, in this report, the unit is referred to as Hawthorn Group even if the individual forma-
tions can be distinguished in some wells.

Within the SJRWMD, the elevation of the top of the Hawthorn Group ranges from 150 feet
MSL in central Alachua County, to approximately -175 feet MSL in south-central Duval County.
The unit dips and thickens from the west-central part of the SJRWMD to the east-northeast into
the trough of the Jacksonville Basin, and southward into the Okeechobee Basin. Thickness of
the Hawthorn Group ranges from 0 feet in central Volusia County, where it is absent over the
crest of the Sanford High, to approximately 500 feet in deeper subsuface basins.

FLORIDA GEOLOGICAL SURVEY

Lithostratigraphy

The Hawthorn Group (Scott, 1988a) is an extremely heterogeneous mixture of both silici-
clastic and carbonate lithofacies, divisible into lower and upper lithozones. Carbonate lithofacies
predominate in the lower lithozone (Penny Farms and Arcadia Formations). However, relatively
thin interbeds of siliciclastic material commonly occur in the lower lithozone. Lithologies char-
acteristic of the lower lithozone of the Hawthorn Group include tan, brown, gray, and white,
sandy, phosphatic dolostone and (relatively rare) limestone. Gray to brown chert locally occurs
in the lower lithozone of the Hawthorn Group. The chert may be associated with white to light
brown, slightly quartz sandy, variably phosphatic, recrystallized dolostone (representing the
Arcadia Formation of Scott, 1988a) in the southern portions of the SJRWMD (Indian River,
southern Brevard, southeastern Osceola and northeastern Okeechobee Counties). Quartz sand
and phosphatic dolostone breccias and conglomerates also commonly occur within the lower
lithozone of the Hawthorn Group.

Siliciclastic lithofacies predominate in the upper lithozone (Marks Head, Coosawhatchie,
and Peace River Formations), although interbeds of carbonate also commonly occur in the upper
siliciclastic lithozone. The upper lithozone contains olive-green, blue, and/or brown, phosphatic
clay, quartz sand and dolosilt. The carbonate beds may have increased porosity due to mollusk
molds. There are few macrofossils present in any of the units.

The predominant unifying lithologic character of the carbonate and siliciclastic lithofacies
composing the Hawthorn Group is the presence of black, brown to amber, very fine sand to peb-
ble sized phosphate grains in sufficient quantities to greatly affect gamma ray intensity. An
exception to this is the Charlton Member of the Coosawhatchie Formation (Scott, 1988a) in
northern SJRWMD (Duval County and portions of Baker, Clay and Nassau Counties). In this
area, the Charlton Member marks the top of the Hawthorn Group, and consists predominantly of
brown to dark gray, nonphosphatic to only sparsely phosphatic, molluscan to ostracod to
foraminiferal moldic dolostone as well as green to blue clay. In the remainder of the SJRWMD,
the top of the Hawthorn Group is characteristically composed of relatively phosphatic lithologies
which are normally easily distinguishable from the nonphosphatic to sparsely phosphatic litholo-
gies typical of overlying formations.

Gamma Logs

Because the Hawthorn Group characteristically contains variable, but relatively high,
amounts of radioactive phosphate sand and gravel, the interval is typically recorded on gamma
logs as a series of sharp to very broad, high moderate and high intensity units correlating with
lower and higher concentrations of phosphate and/or clay. The gamma log is especially useful in
picking the base of the Hawthorn Group. Within the SJRWMD, the basal Hawthorn Group typi-
cally displays a distinct high intensity peak on gamma logs. Where the Ocala Limestone occurs
below the Hawthorn Group (most of the SJRWMD excluding southeastern Brevard and eastern
Indian River Counties), the top of the Ocala is characteristically recorded as low intensity in sharp
contrast to the high intensity typical of the basal Hawthorn. In eastern Indian River and south-
eastern Brevard Counties, the Suwannee Limestone occurs below the Hawthorn Group. Since

SPECIAL PUBLICATION NO. 50

the upper Suwannee Limestone may be recorded as a series of thin high moderate intensity
peaks it may appear somewhat similar to the much thicker peak series recorded within the
Hawthorn Group. Despite the similarities, gamma intensity characteristic of the Suwannee is
invariably lower than the high moderate or high intensity typical of basal Hawthorn.

The gamma log pattern typical of the Hawthorn Group is illustrated on a gamma log (Figure
16) obtained from a FGS corehole (W-13751; Scott #2; SJ0177) located in northern St. Johns
County. The pattern for the complete Hawthorn Group section occurs between 105 feet and 325
feet BLS in the log. The upper contact (105 feet BSL) is identified by an increase from low and
low moderate intensity in the overlying surficial sediments to high intensity in the upper Hawthorn
Group sediments. At the lower contact (325 feet BSL) a sharp contrast occurs where the high
intensity of the basal Hawthorn Group overlies the low intensity units of the Ocala Limestone.

There are two lithologies that typically occur in the upper siliciclastic lithozone (Peace River
Formation) of the Hawthorn Group in southern Brevard, southeastern Osceola, northeastern
Okeechobee, and Indian River Counties. Thick sections of clay or homogeneous dolosilt are
present and are recorded as predominately high intensity interbedded with high moderate inten-
sity peaks. Figure 14 displays the gamma log obtained from a flowing well (IR0338) located in
southeastern Indian River County which illustrates this typical trace pattern. The top of the Peace
River Formation is characteristically marked by high intensity at approximately 128 feet BLS
(contrasting with the high moderate intensity of the locally overlying Tamiami Formation). The
remainder of the Peace River Formation is characteristically recorded as high intensity with
interbedded high moderate intensity units downward to approximately 311 feet BLS, where the
top of the Arcadia Formation of Scott (1988a) occurs. The Arcadia Formation (or lower dolostone
lithozone) is recorded as a series of high intensity peaks. The base of the Arcadia Formation is
represented by the basal Hawthorn Group high intensity peak centered at about 315 feet BLS,
below which locally occurs the lower intensity typical of the Suwannee Limestone.

In Duval, Nassau, Clay and Baker Counties in the northern portion of the SJRWMD, where
the nonphosphatic to sparsely phosphatic Charlton Member (of the Coosawhatchie Formation of
Scott, 1988a) occurs at the top of the Hawthorn Group, the upper contact of the Hawthorn is not
clearly defined by a high intensity phosphate peak on gamma logs. The Charlton Member may
be recorded on gamma logs as low moderate to high moderate intensity depending upon the
lithologic variations. A minimum of an electric log or (preferably) reliable well samples in some
form are required for confirmation of the presence of the member. Figure 17 depicts the gamma
log obtained from a public supply well (D-0520) located in northwestern Duval County. The base
of the Hawthorn Group remains characteristically well-defined, represented by a high intensity
peak centered at about 415 feet BLS; however, the uppermost portion, locally represented by the
Charlton Member, is recorded as a thin interval of high moderate intensity between approxi-
mately 95-125 feet BLS.

Electric Logs

The Hawthorn Group is recorded as an extremely variable trace in a variety of different pat-
terns on electric logs due to its heterogeneous lithologic nature. In many wells, no electric logs
have been recorded since the well casing has been set into the underlying Ocala Limestone and

FLORIDA GEOLOGICAL SURVEY

undifferentiated sand,
clay, and shell

-250

-300

Ocala Limestone

0 10 20 30 40 50 60 70 80 90 1
Gamma Intensity (unitless, normalized)
Figure 16. Gamma log of well SJ0177, St. Johns County.

-50

-100

-150

Jr

SPECIAL PUBLICATION NO. 50

undifferentiated sand,
clay, and shell

0

-50

-100

-150

-200

-250

-300

-350

-400

-450

-500

-550

-600

-650

-700

-750

-800

-850

-900

-950

-1000

-1050

-1100

Ocala Limestone

t-

Avon Park Formation

Legend

Low Intensity (L)
Low Moderate
Intensity- (LM)
SHigh Moderate
Intensity (HM)
SHigh Intensity (H)

0 10 20 30 40 50 60 70 80

Gamma Intensity (unitless, normalized)
Figure 17. Gamma log of well D-0520, Duval County.

90 100

Hawthorn Group
Hawthorn Group

--- --- ---

RUN-

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the log cannot record through the casing. Since the Hawthorn Group is composed of both silici-
clastic beds, which are typically recorded as low resistivity, and carbonate beds, which are typi-
cally recorded as higher than the siliciclastic beds, electric logs can be used to differentiate these
units. The volume of phosphate encountered in these sediments is insufficient to affect electric
logs. Considering the limitations imposed by the presence of casing and the high variability of
the sediments, the gamma log remains the best indicator for the presence or absence of the
Hawthorn Group.

Upper Pliocene Series
Tamiami Formation

Mansfield (1939) proposed the name Tamiami limestone for rock exposed in shallow ditch-
es along the Tamiami Trail (U.S. Highway 41) in Collier and Monroe Counties, Florida. Hunter
(1968) modified the name to Tamiami Formation. The Tamiami Formation occurs within the SJR-
WMD in eastern Indian River and southeastern Brevard Counties, where the interval is less than
40 feet thick (Johnson, 1993). The interval also can be traced in the subsurface northward to
the vicinity of St. Augustine, east-central St. Johns County, where it is discontinuous, and thins
to between 0 and 10 feet thick. Depth to the top of the unit varies between approximately 100
and 150 feet BLS.

Lithostratigraphy

Within the SJRWMD, the Tamiami Formation typically consists of gray to tan to white, mod-
erately to well-indurated, slightly phosphatic, quartz sandy, variably recrystallized calcarenitic
limestone, to very hard, molluscan moldic, recrystallized micritic limestone (Johnson, 1993). The
Tamiami Formation is most recrystallized and thickest in the immediate vicinity of the Atlantic
coast (beneath the barrier islands), becoming less recrystallized (more shelly and less moldic)
and pinching out inland toward the west. The Tamiami Formation directly overlies the top of the
Hawthorn Group. It underlies the Pliocene to Pleistocene Okeechobee formation of Scott (1994)
to the south, or the Nashua Formation (Huddlestun, 1988) to the north. These latter two forma-
tions can be differentiated from the Tamiami Formation by their content of unrecrystallized shell
material, whereas the Tamiami is predominantly recrystallized and moldic.

Gamma Logs

On gamma logs, the slightly phosphatic Tamiami Formation is commonly not easily recog-
nizable, since the formations below and above may locally incorporate phosphate grains.
However, the concentrations of phosphate within the Tamiami Formation are characteristically
less than those typical of the underlying Hawthorn Group; thus, the Tamiami may be recorded as
uneven low moderate or high moderate intensity peaks and valleys. The Tamiami can be identi-
fied by the marked change in intensity from the underlying high intensity at the top of the
Hawthorn Group. Moreover, because the overlying Okeechobee formation or Nashua Formation
typically contain moderately radioactive clay, higher gamma intensity is locally recorded above
the Tamiami Formation. An example of the gamma response to the Tamiami Formation can be
seen in Figure 14, well IR0338, in the interval from 123 to 138 feet BLS. In general, however, the
presence or absence of the Tamiami Formation in any given well is not determinable from the

SPECIAL PUBLICATION NO. 50

gamma log alone.

Electric Logs

In eastern Brevard and Indian River Counties, the Tamiami Formation is characteristically
recorded on electric logs as a moderate resistivity peak or series of very closely spaced peaks
(Johnson, 1993) between the markedly lower resistivity characteristically recorded in the upper-
most Hawthorn Group, and basal (predominantly siliciclastic) Okeechobee formation. This trace
pattern is depicted in Figure 14, an FGS corehole (IR0338) located in east-central Indian River
County. In this well the Tamiami Formation is typically recorded as a moderate resistivity peak
centered at approximately 123 feet BLS, with lower resistivity siliciclastic beds below (Hawthorn
Group at 138 feet BLS) and above (Okeechobee formation above about 120 feet BLS). To the
north of central coastal Brevard County, the Tamiami Formation thins and is commonly more dif-
ficult to recognize on electric logs.

Because the lithologies of the remaining post-Hawthorn Group formations are extremely
variable over relatively short horizontal distances, geophysical log response is also very local
and highly variable. Furthermore, these formations are relatively discontinuous and may be local-
ly very thin or absent; reliable cores or well cuttings are required for confirmation of the presence
of these intervals at any specific well location.

Upper Pliocene Series
Cypresshead Formation

Huddlestun (1988) applied the name Cypresshead Formation to Late Pliocene, clayey, grav-
elly quartz sands in southeastern Georgia. Scott (1988b) extended the unit into Florida. The
Cypresshead Formation includes the Citronelle Formation sediments of Pirkle, et al. (1963) in
peninsular Florida. The formation occurs only beneath the higher elevation ridges near the cen-
tral north-northwest/south-southeast axis of peninsular Florida (e.g., the Mt. Dora Ridge). It typ-
ically varies between about 30 and 80 feet thick in the SJRWMD. Depth to the top of the unit is
generally less than 20 feet BLS, and it commonly forms the land surface on the higher ridges in
the central Florida peninsula.

Lithostratigraphy

The Cypresshead Formation is typically composed of unfossiliferous, variably argillaceous
quartz sand, silt and gravel, that can be separated into three zones based on lithology (modified
from Pirkle et al., 1963). One zone is a relatively thick basal lithozone characteristically consist-
ing of white or lavender, thickly bedded, sparsely argillaceous, very fine to very coarse quartz
sand and granule to pebble sized gravel with variable amounts of quartz silt. The middle litho-
zone is characteristically red, orange, white or lavender in color. It may be banded, laminated,
cross-bedded quartz sand and silt which contains higher percentages of clay matrix than the
basal lithozone. Quartz gravel and discrete clay beds also occur locally within the middle litho-
zone. The middle lithozone is typically both thinner and more thinly bedded when compared to
the basal white lithozone. The upper argillaceous lithozone is characteristically comprised of dark
orange to dark red, argillaceous, homogeneous, very fine to very coarse quartz sand with gran-

FLORIDA GEOLOGICAL SURVEY

ule to pebble sized quartz or quartz sandstone grains scattered homogeneously throughout. This
lithozone typically contains up to 10 to 20 percent clay matrix.

The Cypresshead Formation is overlain by undifferentiated sand, clay, and shell (UDSCS)
or forms the land surface, and is underlain by the Hawthorn Group. Because the Cypresshead
Formation lacks all traces of phosphate, the interval is easily distinguished from the phosphatic
Hawthorn Group below.

Gamma Logs

On gamma logs, the Cypresshead Formation is locally recorded as a relatively thick low to
low moderate intensity interval (correlating with the basal white lithozone), a middle somewhat
higher intensity interval (correlating with the middle somewhat more argillaceous lithozone), and
an upper thinner section comprised of one to three, low to low moderate intensity peaks (repre-
senting the upper argillaceous lithozone). An example of the gamma response from the
Cypresshead Formation can be seen in Figure 18, well M-0410 in the interval from 33 to 45 feet
BLS. However, the presence of this trace pattern on a gamma log obtained from a well in the
correct geographical area is not conclusive proof of the existence of the Cypresshead Formation
in any given well. In the immediate vicinity of the inland ridges, peaty quartz sand overlying clean
quartz sand and gravel (UDSCS) also locally produce a similar gamma trace pattern. Good qual-
ity well samples are always required to confirm the presence of the Cypresshead Formation.

Electric Logs

The Cypresshead Formation cannot be distinguished from UDSC on electric logs due to
similar local compositions (i.e., quartz sand) and because the Cypresshead, where present, is
stratigraphically located at or near the top of the column at or near land surface (like UDSCS)
and is typically cased or screened off.

Upper Pliocene to Pleistocene Series
Nashua Formation and Okeechobee formation

Matson and Clapp (1909) proposed the name Nashua marl for molluscan fossiliferous sands
exposed near the town of Nashua, on the St. Johns River, in St. Johns County. Huddlestun
(1988) elevated the unit to a formation, and included within it the Pliocene and Pleistocene shelly
sands in northeastern Florida and southeastern Georgia.

Scott (1993; 1994) applied the name Okeechobee formation (informal) to similar age mol-
luscan fossiliferous units in the southern peninsula. The Okeechobee formation encompasses
all or parts of several units originally named on biostratigraphic criteria, including the
Caloosahatchee, Bermont, and Ft. Thompson Formations. The Nashua Formation grades
southward into the Okeechobee formation. Nashua Formation Okeechobee formation
sediments typically vary from about 50 to 115 feet thick, with a maximum thickness of 135 feet
observed in one well in Volusia County. Depth to the top of the units ranges from land surface
to 90 feet BLS.

SPECIAL PUBLICATION NO. 50

undifferentiated sand,
clay, and shell

L Cypresshead Formation

Hawthorn Group

-90

-100

-110

-120

-130

-140 -

Ocala Limestone

Legend
Low Intensity (L)
Low Moderate
Intensity- (LM)
High Moderate
Intensity (HM)
I High Intensity (H)

-150

0 10 20 30 40 50 60 70 80
Gamma Intensity (unitless, normalized)
Figure 18. Gamma log of well M-0410, Marion County.

I

FLORIDA GEOLOGICAL SURVEY

Lithostratigraphy

The Nashua Formation and Okeechobee formation consist of gray to tan to brown to green-
gray to black, variably fossiliferous and variably phosphatic, argillaceous quartz sand; quartz
sandy clay; quartz sandy, molluscan limestone; and variably argillaceous quartz sandy shell
beds. Each specific lithology occurs discontinuously, grading into other lithologies or pinching out
over horizontal distances of a few feet to a few miles. The Nashua Formation occurs from Volusia
and Seminole Counties north to Nassau County, while the Okeechobee formation occurs in the
same stratigraphic position from Indian River County to northern Brevard, southern Orange, and
eastern Osceola Counties. The Nashua Formation grades to the west and north into the
Cypresshead Formation (Huddlestun, 1988) in Clay, Baker, Duval and Nassau Counties by
becoming mostly unfossiliferous, completely siliciclastic, and very fine grained.

The predominant defining characteristic of the Nashua Formation and Okeechobee forma-
tion is the presence of unaltered macrofossil material, in highly variable concentrations. Typical
macrofossils present in these formations include mollusks pelecypodss, gastropods,
scaphopods), corals, bryozoans, barnacles, crabs, echinoids and echinoid spines.
Characteristically, this fossil material is unworn and unabraded, frequently whole, and pelecy-
pods locally remain articulated and in life position. This occurs because these formations were
deposited in low energy paleoenvironments (e.g., lagoonal, landward of a barrier island). The
pelecypod Chione cancellata is common throughout both intervals, but may be locally rare to
absent. The upper portion of the Okeechobee formation is typically less fossiliferous than the
lower portion, and locally contains beds of unfossiliferous, peaty, quartz sand.

The Nashua Formation and Okeechobee formation are underlain by either the Tamiami
Formation (along the Atlantic coast north to the vicinity of St. Augustine, east central St. Johns
County) or the upper siliciclastic lithozone of the Hawthorn Group (in the remainder of the SJR-
WMD). The Nashua Formation and Okeechobee formation differ from the Tamiami Formation in
that the latter is composed almost exclusively of fully recrystallized molluscan moldic limestone,
whereas the Nashua and Okeechobee are predominantly siliciclastic. Locally, where the basal
Nashua Formation or Okeechobee formation contains beds of limestone, this lithology is char-
acteristically less recrystallized, with most of the contained shell material unaltered. Where these
two formations are underlain by the Hawthorn Group, the substantially higher phosphate con-
centrations typically occurring in the upper siliciclastic Hawthorn lithozone serve to distinguish
the interval from the Nashua Formation and Okeechobee formation. Moreover, the Hawthorn
Group characteristically contains substantially lesser concentrations of macrofossils such as
mollusks when compared to the overlying Nashua Formation or Okeechobee formation.
Additionally, the Nashua Formation becomes discontinuous in the northern portion of the SJR-
WMD (Clay, Baker, Duval, northern St. Johns, and Nassau Counties).

Gamma and Electric Logs

Due to the variable lithologies and variable amounts of phosphate found within these two
formations, geophysical log response is also quite variable. Figure 19 illustrates the gamma log
obtained from an FGS corehole (W-15282; Washington Oaks State Gardens #1; F-0019) locat-
ed in northeastern Flagler County on the barrier island. The Nashua is recorded in this well

SPECIAL PUBLICATION NO. 50

Anastasia Formation

Ocala Limestone

-Avon Park Formation (need to confirm)

Legend

Low Intensity (L)
Low Moderate
Intensity- (LM)
SHigh Moderate
Intensity (HM)
i High Intensity (H)

10 20 30 40 50 60 70 80
Gamma Intensity (unitless, normalized)

Figure 19. Gamma log of well F-0019, Flagler County.

40

0

-50

-100

-150

-200

-250

-300

-350

-400

-450

-500 -
0

FLORIDA GEOLOGICAL SURVEY

between 40 and 92 feet BLS, and consists of a series of low moderate to high moderate inten-
sity peaks, culminating in a high intensity peak representing a moldic limestone at the base of
the unit. In general, argillaceous and somewhat phosphatic lithologies are recorded as very
uneven, low moderate to moderate intensity on gamma logs; clay content is recorded as very
low to low resistivity on electric logs; and nonphosphatic, quartz sandy limestone beds or shell
beds are recorded as low intensity on gamma logs and as low moderate to moderate resistivity
peaks on electric logs. Again, some form of reliable well sample is necessary to accurately deter-
mine presence or absence of these formations in any specific well.

Pleistocene Series
Anastasia Formation

Sellards (1912) applied the name Anastasia Formation to shelly sands and coquina rock
exposed along the east coast of the Florida peninsula. The relatively discontinuous Anastasia
Formation occurs within the SJRWMD only along the Atlantic coast from Indian River County
north to southeastern St. Johns County (vicinity of St. Augustine). It forms the core of the Atlantic
Coastal Ridge along much of its length. In the SJRWMD, maximum thickness is about 70 feet.
The top of the Anastasia Formation varies from land surface to about 30 feet BLS.

Lithostratigraphy

The Anastasia Formation is characteristically comprised of nonphosphatic, orange to tan to
white, worn and abraded shell (predominantly mollusk) beds, molluscan limestone, and variably
shelly unconsolidated quartz sand to moderately consolidated quartz sandstone (Johnson,
1994). The shell beds vary locally and may contain traces of black to dark brown very finely par-
ticulate peat as stringers and laminae. The Anastasia Formation represents high-energy beach,
intertidal or offshore bar paleoenvironments of deposition. Shell material is characteristically
worn, abraded and predominantly fragmental (Johnson, 1994). Additionally, the common pres-
ence of Donax variabilis, a small pelecypod, underscores the depositional higher energy nature
of the Anastasia Formation (Johnson, 1994).

The Anastasia Formation occurs beneath the Atlantic barrier islands and extends no more
than 15 miles inland on the mainland to the west, grading into the upper portion of the
Okeechobee formation in Brevard and Indian River Counties by change in environment of dep-
osition from high to low energy. The lower portion of the Okeechobee formation or the Nashua
Formation occurs stratigraphically below the Anastasia Formation in the southern and northern
portions, respectively, of the SJRWMD. Either Holocene UDSCS (typically black, unconsolidat-
ed, peaty quartz sand) occurs above the Anastasia Formation, or the interval forms local land
surface.

Gamma Logs

On gamma logs, the Anastasia Formation is recorded as either even low intensity, repre-
senting nonphosphatic, nonargillaceous, nonpeaty shell beds, limestone or quartz sand/sand-
stone, or as uneven low moderate intensity where peat or other (nonphosphatic) locally occur-
ring heavy mineral grains are present within these lithologies. Figure 19 illustrates the gamma

SPECIAL PUBLICATION NO. 50

log obtained from a corehole (W-15282; F-0019) located in northeastern Flagler County on the
barrier island. The Anastasia Formation is recorded at this specific location as uneven low to low
moderate intensity from approximately 40 feet BLS (top of the high moderate intensity peak rep-
resenting the top of the Nashua Formation) to very near land surface. However, where peat
and/or heavy minerals occur, where the borehole is larger in diameter, or in other areas to the
south away from the type area (Anastasia Island), the gamma trace may be poorly defined and
not recognizable. Where reliable well samples are available and lithologies typical of the
Anastasia Formation are confirmed present, its basal contact with the underlying Nashua
Formation (north) or lower Okeechobee formation (south) is typically distinguishable on gamma
logs. The uppermost portions of these underlying formations typically contain both phosphate
grains and clay, recorded as a sharp and significant increase in intensity with respect to basal
nonphosphatic and nonargillaceous Anastasia Formation. This gamma trace pattern is also illus-
trated on Figure 19; the top of the phosphatic argillaceous Nashua Formation is recorded as a
distinct moderate intensity peak centered at approximately 45 feet BLS, just below the much
lower intensity of basal Anastasia Formation.

Electric Logs

On electric logs, interbeds of dense, relatively nonporous limestone and well-consolidated,
nonporous quartz sandstone within the Anastasia Formation are recorded as relatively broad,
low moderate resistivity peaks alternating with low resistivity valleys representing porous inter-
vals (such as unconsolidated shell beds or clean quartz sand). This even to uneven, low peak
and valley pattern on both gamma and electric geophysical logs is not exclusive to the Anastasia
Formation; thus, the formation generally cannot be distinguished reliably on the basis of geo-
physical logs alone. Again, reliable core or well cutting samples must be utilized to detect the
presence of the Anastasia Formation in any particular well within its area of occurrence.

Pleistocene to Holocene Series
Undifferentiated Sand, Clay and Shell

Undifferentiated sand, clay and shell (UDSCS) occurs discontinuously throughout the SJR-
WMD, varying from zero to over 200 feet in thickness. In this report, the UDSCS has been used
to label the post-Miocene sediments on most of the figures and cross sections. The exception to
this are Figure 18 (M-0410), showing an example of the gamma response in the Cypresshead
Formation, and Figure 19 (F-0019), showing an example of the Nashua and Anastasia
Formations. Since the post-Miocene units are difficult, if not impossible, to correlate using
gamma logs alone, this seemed to be the most practical solution.

Lithostratigraphy

This interval (which does not constitute a formal formation) is extremely lithologically vari-
able: quartz silt/sand/gravel to clay to shell material to limestone, and all combinations of these
lithological continue end points. Moreover, lithologies commonly change over extremely short
horizontal distances, on the order of inches to feet. However, the most common lithology encoun-
tered in the SJRWMD is tan to gray, very poorly consolidated to unconsolidated, unfossiliferous,
pure to peaty quartz sand which contains very low percentages of sand sized, undifferentiated

SPECIAL PUBLICATION NO. 50

heavy mineral grains. In addition, brown to dark gray, unfossiliferous, variably argillaceous quartz
sand is also relatively common throughout the SJRWMD.

Gamma and Electric Logs

Due to the pronounced lithological variability and discontinuity of the UDSCS stratigraphic
interval, no reliable and correlatable patterns occur on geophysical logs; good quality well sam-
ples must be available in order to identify the interval in any specific well. Generally, pure quartz
sand is recorded on gamma logs as even low to low moderate intensity, whereas peaty quartz
sand is recorded as uneven low to low moderate intensity, and argillaceous quartz sand or quartz
sandy clay as low moderate intensity peaks. Furthermore, because the UDSCS interval is strati-
graphically located at land surface at the top of the Cenozoic column, is not everywhere water
saturated, and is typically cased or screened in most water supply wells, neither electric nor neu-
tron logs can be used for identification or correlation.

SUBSURFACE FEATURES AFFECTING THE STRATIGRAPHY AND LOG
CORRELATIONS IN THE SJRWMD

The geologic strata discussed above were deposited in a relatively flat-lying sequence, with
progressively younger sediments overlying older units. The aerial extent, dip, and thickness of
these geologic units have been influenced by a number of local and regional factors, including
pre-existing structural features, paleo-erosion events, post-depositional subsidence and karst
activity. Data are largely lacking on the local extent of paleo-erosion and subsidence. However,
two better-documented types of features which significantly affect the configuration of the strata
underlying the SJRWMD are buried paleosinks and regional subsurface geologic structural fea-
tures.

Paleosinks

The term paleosink (paleokarst) is generally used to describe a buried karst feature that was
formed under different conditions than the current geologic setting (Ford and Williams, 1992).
The karst features include cover collapse sinkholes, solution sinkholes, cover subsidence sinks
and solution pipes. The feature may or may not have visible signs at land surface. Paleosinks
have been blamed for anomalous results in drilling projects such as unusually thick or missing
sections and can even be mistaken as evidence of faults. One of the best ways to understand
what a buried paleosink looks like is to see results from surface geophysical techniques such as
high resolution seismic reflection profiling (HRSP). HRSP has been used extensively to map
paleokarst beneath lakes (Kindinger et al., 1994, 1999, 2000; Locker et al., 1988; Sacks et al.,
1991) in northeast Florida.

To identify paleosinks using borehole geophysical techniques it generally requires logs from
several closely spaced wells. An excellent example of using gamma logs to identify a pale-
osinkhole was done at the University of Florida motor pool site (Edelstein, 1993). During this
study, fifteen wells were drilled in and around an area containing a leaking underground fuel stor-
age tank. The high intensity of the Hawthorn Group could be seen only in wells around the
perimeter of the paleosink whereas only low moderate or high moderate intensity units could be

SPECIAL PUBLICATION NO. 50

seen in the area disturbed by the sinkhole subsidence. In other cases, the high intensity units of
the Hawthorn Group may show marked changes in elevation over short distances with an
accompanying thickening of the overlying sands and clays. This is a strong indication that the
wells were drilled along the slope of a buried paleosinkhole. Other effects of paleokarst on
gamma logs occur where clays and phosphates from the Hawthorn Group have been transport-
ed downward into voids in the underlying limestones. The gamma counts may be higher than
would be expected from the pure limestone. Gamma logs used in this report were chosen more
to reflect the regional trends rather than the localized effects that paleokarst would cause. When
correlating gamma logs, the effects of paleosinks should be considered when anomalies are
identified.

Structure

A series of subsurface geologic structures significantly influence the distribution and config-
uration of the Middle Eocene and younger geologic units underlying the SJRWMD. Early litera-
ture on these features generally attributed their formation to structural events, such as uplift,
faulting, or structural downwarping. Due to a paucity of data on the features, their actual modes
of origin are uncertain. Therefore, modern nomenclature for the features attempts to avoid a
deformational connotation (Scott, 1988). In general, positive (high) features bring Eocene car-
bonate units close to the surface. This has resulted in either non-deposition of younger units, or
erosion of younger units that once covered the carbonate bedrock comprising the feature.
Negative (low) features are basins, with the top of Eocene carbonates lying deeper than adja-
cent areas. These basins typically accumulated increased thicknesses of post-Eocene siliciclas-
tic sediments. Figure 20 illustrates the locations of the major subsurface structural features
affecting the SJRWMD. As detailed below, the influence of the features on the geologic strata
may be observed on cross sections in Appendix A.

Lying just west of the SJRWMD is one of the most significant subsurface structures in
Florida: a broad, northwest-southeast trending positive feature named the Ocala Platform
(Hopkins, 1920; Vernon, 1951; Scott, 1988a). The Ocala Platform crests under Levy County and
forms an extensive karst plain, comprised of Middle Eocene to Oligocene carbonates under the
central Big Bend and north-central peninsular areas. The carbonates dip in all directions away
from the crest of the Ocala Platform. Dips are generally around 0.1 degree, or about 10 feet per
mile (Tom Scott, 2001, personal communication). The top of the Eocene Ocala Limestone typi-
cally deepens from approximately 90 feet above MSL in northern Alachua County (Well A-0438,
cross section E-E') to over -500 feet MSL in northeastern Nassau County (Well N-0277, cross
section A-A') in the trough of the adjacent Jacksonville Basin. Younger geologic units pinch out
against the flanks of the Ocala Platform. Cross section JJ-JJ' runs approximately parallel to the
strike of this feature, along its eastern flank. This section shows the generally shallow and gen-
tly-dipping structural surfaces of the Eocene Avon Park Formation and Ocala Limestone in the
western part of the SJRWMD. The Miocene Hawthorn Group is absent over the crest of the
Platform, west of the SJRWMD. It dips and thickens to the east-northeast off the eastern flank
of the platform (section E-E').

The Jacksonville Basin (Goodell and Yon, 1960) underlies Duval and eastern Nassau
Counties. It is the most prominent subsurface low in the northern Florida peninsula. In the

SPECIAL PUBLICATION NO. 50

GEORGIA

Southeast
Georgia
Embayment

Jacksonville
Basin
\St. Johns
Platform

St. Johns River Water
Management District

Osceola
SLow

bee

0 50 100 150 Miles
II
0 80 160 240 Kilometers
SCALE

Figure 20. Subsurface structures in the SJRWMD (modified from Scott, 1988a).

SPECIAL PUBLICATION NO. 50

trough of the basin, Hawthorn Group sediments attain thicknesses in excess of 450 feet (Well D-
1118, cross section AA-AA'). The Jacksonville Basin is a sub-basin of the much larger Southeast
Georgia Embayment, and is separated from the latter by a positive feature named the Nassau
Nose (Scott, 1983). The Nassau Nose is situated under north-central Nassau County, where its
influence causes a slight rise of the top of Ocala Limestone (Well N-0221, cross sections U-U'
and KK-KK').

The Sanford High (Vernon, 1951) is a positive subsurface feature located under Seminole
and Volusia Counties. Cross section I-I' illustrates the influence of this feature on the local stra-
ta. The structural surfaces of the Avon Park Formation and Ocala Limestone rise at the crest of
the high at wells L-0122 and V-0254. Middle Eocene Avon Park Formation carbonates form the
core of the feature, and the Ocala Limestone and Hawthorn Group may be missing from some
areas (well V-0254) over the crest of the Sanford High. In these areas Avon Park Formation car-
bonates lie immediately below post-Hawthorn sediments.

North and south of the Sanford High two low, broad structural platforms are evident on the
erosional surface of the Ocala Limestone. The St. Johns Platform (Riggs, 1979a, b) extends
northward under St. Johns County, plunging gently into the Jacksonville Basin. Well F-0251
(cross section AA-AA) is drilled near the crest of the St. Johns Platform. West-east cross sec-
tion D-D' illustrates the Hawthorn Group sediments deepening off the Ocala Platform on the
west, then climbing onto the St. Johns Platform at well SJ0164.

To the south, the Brevard Platform (Riggs, 1979a, b) underlies Brevard County, and plunges
gently to the south-southeast towards the Okeechobee Basin of southern Florida. Section I1-11'
runs nearly parallel to the strike of the Brevard Platform and illustrates the gently dipping nature
(three feet per mile) of the Avon Park Formation and Ocala Limestone along the feature. At the
southern end of the platform the dip of the Eocene strata increases (to about 20 feet per mile)
southward into the basin. Cross section HH-HH' illustrates the southward-dipping surfaces of the
Eocene and Oligocene units off the Brevard Platform into the Okeechobee Basin.

Situated between the southern ends of the Ocala and Brevard Platforms are two significant
subsurface features named the Kissimmee Faulted Flexure and the Osceola Low (Vernon,
1951). The Kissimmee Faulted Flexure, originally considered by Vernon to be a fault-bounded
block, is a high on the Middle Eocene Avon Park Formation (Scott, 1988a). Well P00013 in
cross section P-P' represents the crest of the feature. Although not shown on the present sec-
tions, Ocala Limestone and Hawthorn Group sediments may be absent over a portion of the fea-
ture due to erosion. The Osceola Low is a north-south trending low, or trough, on the erosional
surface of the Ocala Limestone. Sediments of the Hawthorn Group are thicker within the low
than in immediately adjacent areas. The middle portion of cross section P-P' and cross section
Z-Z' (wells OS00005A and OS0068) illustrate this thickening. Here Hawthorn sediments attain
a maximum thickness of about 200 feet. Although Vernon (1951) noted up to 350 feet of
Miocene sediments within the Osceola Low, this anomalous data was apparently derived from a
well drilled in a paleosinkhole located in the trough of the low (Tom Scott, 1999, personal com-
munication).

The stratigraphy of the southernmost portion of the SJRWMD is influenced by a large neg-

FLORIDA GEOLOGICAL SURVEY

ative structure named the Okeechobee Basin (Riggs, 1979a, b). This feature underlies much of
southern Florida. Eocene and Oligocene carbonates and the overlying Hawthorn Group
sediments dip and thicken into the basin towards the south and southeast. The southern por-
tions of cross sections Z-Z, 'DD-DD', HH-HH' and spanning southern Brevard, St. Lucie, Indian
River, and Okeechobee Counties, illustrate the accentuated dip of the strata into the
Okeechobee Basin.

GAMMA LOG SIGNATURES AND CROSS SECTIONS

The gamma log cross sections (Appendix A) not only show the subsurface structural fea-
tures but also illustrate the similarities and variations in gamma log signature from one region to
the next. Gamma log signature can be described as a characteristic pattern of peaks and valleys
in a log that can also be recognized in other gamma logs. The idea of signature is more obvious
when viewed in a cross section since the pattern of peaks and valleys for individual gamma logs
can be recognized in the other logs of the cross section.

The gamma log for well D-0176 (Figure 11) demonstrates a typical signature for a com-
plete stratigraphic sequence from land surface down into the Avon Park Formation. The log has
four characteristic zones. One is an upper zone with low and low moderate intensity peaks which
correspond to the post Hawthorn Group sediments (0 to 48 feet BLS). It is underlain by zone two
which is predominately high and high moderate intensity peaks but also contains low and low
moderate intensity peaks which correspond to the Hawthorn Group sediments (48 to 505 feet
BLS). Below that is zone three which consists of low intensity peaks underlain by low moderate
intensity peaks which corresponds to the upper and lower lithozones of the Ocala Limestone
(505 to 730 feet BLS). The lowest zone is predominately high moderate and low moderate inten-
sity peaks but may be interbedded with low and high intensity peaks (730 to 1275 feet BLS). The
actual thickness of the different zones will vary greatly throughout the SJRWMD, however, the
general pattern (or parts thereof) can be recognized over most of the region. Many of the cross
sections demonstrate this recognized signature that can be traced laterally for many miles.
Sections A-A', B-B', C-C', F-F', J-J', N-N', R-R', T-T', U-U', V-V', W-W', DD-DD', FF-FF', and JJ-
JJ' are good examples of typical log signature patterns.

An anomaly to this simplistic pattern can be seen in gamma log cross section KK-KK'
(Appendix A). This section runs through the center of the SJRWMD from the northern boundary
in Nassau County almost to the southern boundary in Indian River County. This covers a dis-
tance of approximately 230 miles. The signature discussed above is best illustrated in the north-
ern wells (N-0221, C-0142, and C-0123) and in the southern wells (OR0015, OS00005, and
IR0314). The central part of the cross section at well V-0254 highlights the most variability of a
gamma log signature. The only similarities in V-0254 to a complete stratigraphic sequence occur
in the Avon Park Formation sediments which show the low moderate and high moderate inten-
sity sections. Even the undifferentiated sand, clay, and shell contains a 20' thick high intensity
unit instead of the normally low and low moderate intensity that is generally seen.

The east-central region of the SJRWMD illustrates how the signature changes over the
structural highs where complete sections have been eroded or never deposited. Scott (1988a)
constructed an isopach of the Hawthorn Group sediments that shows the areas in this region

SPECIAL PUBLICATION NO. 50

where the Hawthorn Group is missing. In cross section BB-BB', for example, Hawthorn Group
and Ocala Limestone sediments are missing from all wells south of F-0294 and F-0251, respec-
tively. The extreme variation of the Hawthorn Group sediments in thickness of the entire group,
thickness of individual units, and lateral continuity or discontinuity of individual high intensity units
can be seen by trying to trace a particular unit from one well to the next.

The change in gamma intensity between the Avon Park Formation and the overlying
Ocala Limestone can be traced laterally for many miles as demonstrated in the gamma cross
sections. A good example of how the contact can be traced laterally is demonstrated in gamma
cross section W-W which runs from northern Nassau County for 70 miles into southern Putnam
County. The top of the Avon Park Formation can easily be identified in all of the logs where the
formation is present. Other sections such as B-B', K-K', S-S', Y-Y', Z-Z', and DD-DD' demon-
strate the general character of the contact. Since the change is generally from either low inten-
sity to low moderate intensity or low moderate to high moderate intensity, borehole conditions
such as cavities or a large diameter bore can attenuate the gamma response and greatly limit
the ability to distinguish the contact.

The top of the Hawthorn Group in many sections (e.g. B-B', C-C', and D-D') can often be
identified as the first high intensity peaks. However, there are many logs where the top is locat-
ed on either low moderate or high moderate intensity peaks (e.g. OR0614 in section Y-Y',
OS00005a in section Z-Z', SJ0163 in section AA-AA, SJ0025 in section FF-FF', and N-0117 in
section FF-FF' ). Wells OR0015 and P-0418 in section KK-KK' illustrate the problems associat-
ed with identifying the top of the Hawthorn Group from the gamma logs alone. The boundary for
OR0015 required lithologic data because the change in gamma intensity was too slight to use
for identification. In P-0418, the Hawthorn Group is overlain by sediments with high intensity
units that were identified as younger sediments. Gamma cross section EE-EE' demonstrates the
extremes seen in the east-central region of the district. In EE-EE' there is no Hawthorn Group in
any of the logs, the Ocala Limestone pinches out to the west, and the gamma peaks in the undif-
ferentiated sand, clay, and shell range from a high intensity signature that could be confused with
the Hawthorn Group (wells V-0254, V-0267, and V-0304) to a low intensity (well V-0819).

The gamma log cross sections for areas north of G-G', south of N-N', and west of V-V'
show fairly typical gamma signatures except for variations in thickness. The gamma signature
for the region bordered by G-G', N-N', V-V' and the Atlantic Ocean either have units missing, or
units that are very thin. Correlations between logs in this region are further complicated because
units near the surface may be comprised of reworked Hawthorn Group sediments that contain
sufficient clay and phosphate to produce low moderate to high intensity peaks that can be con-
fused with original Hawthorn sediments. In the regions over the structural highs, it is important
to have other supporting data when identifying geologic unit boundaries.

FLORIDA GEOLOGICAL SURVEY

CONCLUSIONS

The SJRWMD and the FGS reviewed the databases of geophysical and lithologic well logs
to identify reference logs for correlation of geologic units throughout the SJRWMD. This cooper-
ative effort has resulted in 38 gamma log cross sections and descriptions of key gamma log sig-
natures for the geologic units within the Cenozoic Era.

Typical gamma log signatures for most geologic units may be recognized in a newly logged
well by the following procedure. First, the location of the well should be identified relative to the
nearest cross section to determine approximate depths the units are to be expected and if they
are to be present at all (e.g. units are missing in Volusia and northeast Seminole Counties).
Second, the relative gamma log intensity for particular zones should be determined based on a
qualitative visual estimation or a quantitative determination of intensity zones. The quantitative
method described herein requires a general idea of where the Ocala Limestone and Hawthorn
Group sediments occur in the log. For a log that penetrates a complete Cenozoic section, zones
of low intensity, low moderate intensity, high moderate intensity and high intensity can be iden-
tified. The examples presented were normalized first to minimize the differences due to equip-
ment, units of measurement (cps, API), and borehole effects. The examples presented also uti-
lized a standard color scheme to help in correlating units from one log to another. Third, the log
can be compared to the nearest cross section or reference log to correlate signatures.

A compilation of reference logs was developed (Appendix B) as documentation of the log
data that were used to establish contacts of geologic units. The reference logs are from wells
that have detailed lithologic descriptions either from that well or from one or more nearby wells.
In most cases, the lithologic logs were used to identify the geologic unit and the geophysical logs
were used to define the elevation of contact.

The gamma log cross sections in Appendix A were developed to demonstrate how gamma
log signatures are consistent over large areas and to identify the areas with the highest variabil-
ity. There is sufficient cross section coverage such that any new logs should have a cross sec-
tion close enough for correlation purposes. The reference logs can be used to correlate addi-
tional gamma logs so that more detailed cross sections can be constructed.

The majority of wells within SJRWMD either have incomplete or no lithologic data available
to help identify geologic contacts in geophysical logs. With this foundation of reference logs, a
large data base of correlated geophysical logs can now be developed with sufficient coverage to
provide input for ground water models, create maps of geologic surfaces, and provide a frame-
work for predictive geologic assessments for drilling and water supply investigations.

SPECIAL PUBLICATION NO. 50

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FLORIDA GEOLOGICAL SURVEY

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Petersburg, Florida, Final Report to St. Johns River Water Management District, 39 p.

Mansfield, W. C., 1939, Notes on the upper Tertiary and Pleistocene mollusks of peninsular
Florida: Florida Geological Survey Bulletin 18, 75 p.

Matson, G. G., and Clapp, F. G., 1909, A preliminary report on the geology of Florida with spe-
cial reference to the stratigraphy: Florida Geological Survey 2nd Annual Report, p. 25-173.

Miller, J. A., 1986, Hydrogeologic framework of the Floridan aquifer system in Florida and parts
of Georgia, Alabama, and South Carolina: U. S. Geological Survey Professional Paper
1403-B, 91 p.

SPECIAL PUBLICATION NO. 50

Pirkle, E. C., Yoho, W H., Allen, A. T., and Edgar, A. C., 1963, Citronelle sediments of peninsu-
lar Florida: Quarterly Journal of the Florida Academy of Sciences, v. 26, p.. 105-149.

Riggs, S. R., 1979a, Petrology of the Tertiary phosphorite system of Florida: Economic Geology,
v. 74, p. 195-220.

1979b, Phosphorite sedimentation in Florida a model phosphogenic system:
Economic Geology, v. 74, p. 285-314.

Sacks, L. A, Lee, T. M. and Tihansky, A. B., 1991, Hydrogeologic setting and preliminary data
analysis for the hydrologic budget assessment of Lake Barco, an acidic seepage lake in
Putnam county, Florida: U.S. Geological Survey Water Resources Investigation Report no.
91-4180. 28 p..

Scott, T. M., 1983, The Hawthorn Formation of northeast Florida: Part 1 The geology of the
Hawthorn Formation of northeast Florida: Florida Bureau of Geology Report of Investigation
91, p. 1-32.

1988a, The lithostratigraphy of the Hawthorn Group (Miocene) in Florida: Florida
Geological Survey Bulletin 59, 148 p.

.___ 1988b, The Cypresshead Formation in northern peninsular Florida: In:
Southeastern Geological Society Fieldtrip Guidebook, 1988, pp. 70-72.

1993, Geologic map of Okeechobee County, Florida: Florida Geological Survey
Open File Map Series 54.

1994, The Okeechobee formation: a preliminary reassessment of the latest
Pliocene to late Pleistocene lithostratigraphy of southern Florida [abstract]: Florida Academy
of Sciences, Florida Scientist, v. 57, supplement 1, p. 41.

Sellards, E. H., 1912, The soils and other surface residual materials of Florida: Florida
Geological Survey 4th Annual Report, p. 1-79.

Vernon, R. O., 1951, Geology of Citrus and Levy Counties, Florida: Florida Geological Survey
Bulletin 33, 256 p.

FLORIDA GEOLOGICAL SURVEY

APPENDIX A: Cross Sections through the SJRWMD

SPECIAL PUBLICATION NO. 50

Location of gamma log cross sections, northern SJRWMD
West to East Sections

LOCATION OF GAMMA
LOG CROSS SECTIONS

Northern SJRWMD West to East Sections

Explanation
Well
| County Boundary
St\SJRWMD Boundary

0 7 14 Miles

1:908177

~ji~iri~L~

Gamma Log Cross Section A-A'

N-0220

S-UDSCS

HTRN

0 CS 240
CPS

OCAL..

0 320
CPS AVPK

Miles

N-0277

N-0131
411

-1200

Legend

UDSCS Undifferentiated sand, clay, & shell
HTRN HawthornGroup
OCAL Ocala Limestone
AVPK Avon Park Formation

0
CPS

Scale = 1/278066

Gamma Log Cross Section B-B'

N-0237 D-0349

BA0009

-500

0" -1000
0 1)

LU

-1500

-2000

-500
O0

0-

i-
-o
C

-1000 C
0

0
z
z

01
-1500

-2000

Scale= 1/520666

Gamma Log Cross Section C-C'

D-0153

D-0165

D-0070

0

-200

Ss -400

LU

-600

-800

UDSCS

SiHTRN

A- --- -
0 22D

OCAL
0 150
1 0 240
0\ 220 CPS
CPS

Legend

UDSCS Undifferentiated sand, clay, & shell
HTRN -Hawthorn Group
OCAL Ocala Limestone
AVPK Avon Park Formation

0 10

20 30

Scale = 1/579911

BA0023

D-4560

SJ0178

i 1;

AVPK

0 220
CPS

Gamma Log Cross Section D-D'
B-0006

100 oo C-0138 100
C-0137

5 SJ0164 SJ0163
C-0473
UDSCS
0 -----
-o

S -100 -100 c

0.
c- 0
7I -H X^ HTRN -3-

O
-200 i 0 -: -200
z
o 220 0
C _______

20 PS Legend
-300 -300

Scale= 1/481294

Gamma Log Cross Section E-E'

C-0490

E'
150

100

50

aU

(S
Cu
a)
w

0 10

20 30

40
Miles

80
Scale = 1/731024

A-0006B

A-0071

F

0

-200

o0 .6 -400
0 ,-
o

-600

-800

-8oo

Gamma Log Cross Section F-F'

10 20 30 40 50 60 70

Scale = 1/720837

Legend

UDSCS Undifferentiated sand, clay, & shell
HTRN Hawthorn Group
OCAL Ocala Limestone
AVPK Avon Park Formation

Gamma Log Cross Section G-G'

Scale = 1/570957

M-0139

G
100

0

-100

-200

-300

s ^
LU
C)
-.
0

Miles

SPECIAL PUBLICATION NO. 50

Location of Gamma Log Cross Sections, southern West to East sections.

LOCATION OF GAMMA
LOG CROSS SECTIONS

Southern SJRWMD West to East Sections

Explanation
Well
SCounty Boundary
, SJRWMD Boundary
0 8 16 Miles

1:1068693

E ms

Gamma Log Cross Section H-H'

H M-0122 M-0044 M-0115 V-0225 v-0831 H'
a, VT TV-0187 V-0200

UDSCS
122 22D I--------- OCAL CPS 110

220 AVPK
-200 C -200
S220 -n
CPS 22
0

-400 -400 0G
Sm
-0
0r

-600 -600 r-
Co

Legend
m
UDSCS Undifferentiated sand, clay, & shell
-0 HTRN Hawthorn Group -- co
OCAL Ocala Limestone
AVPK Avon Park Formation U CPs -

-1000 T -1000

10 20 30 40 50 60 70

Miles

Scale = 1/669016

Gamma Log Cross Section I-I'

M-0310

Scale = 1/570211

Im
100

0

-100

-200

ci)
ILl

Gamma Log Cross Section J-J'

L-0467

L-0078 L-0106 UDSCS52 S j25

0 HT0 I
OCAL----

AVPK -n
-200 -200 0

Tm
180

-HTRN Hawthorn Group
O

0 160
-600 -00 C

HTRN Hawthom Group
OCAL Ocala Limestone
AVPK Avon Park Formation

0160
CPS
5 10 15 20 25 30
Miles
Scale = 1/292944

Gamma Log Cross Section K-K'
S-1225
K S-0083 S-0552 0801 V-0238 V-0235 -0570 K'

SUDSCS
HTRN --- UDSCSDA

-100 -.100 A

0

-'o
AVPKAVPK

0 90c

UDSCS Undifferentiated sand, clay, & shell
AVPK Avon Park Formation

Ii CPS
S5 10 15 20 25 35-
00 Legend 00

30 0 5 10 15 2 25 30 35
-40 0 - - - -------------- ; - - ---------- -4 0 0

0 5 10 15 20 25 30 35

Scale= 1/309154

Gamma Log Cross Section L-L'

L-0443

-100
-n

m
0
r-
-300 0
0
I-

r-
-400 C)
C

m
ac<
-500

-600

3 45
Scale= 1/413854

(D
,o

a,
LU

UDSCS Undifferentiated sand, clay, & shell
HTRN Hawthorn Group
OCAL Ocala Limestone
AVPK Avon Park Formation

Gamma Log Cross Section M-M"

Scale = 1/522888

M OR0551
M-F

0C
EL

Gamma Log Cross Section N-N'

N OR0314 OR0304
SOR0305 OR0618 N

HTRN
;OR110 BR0617

---- HTRN ---
-200 ----- ----- OCAL -200
a 220 AVPK r
CPSCPS
__ O
-400 0 220 Ps -400 G
m
S0 260 0
0cPs r--

1 0
il -600 -600 >

I-
CD
C

-800 -800
Legend
UDSCS Undifferentiated sand, clay, & shell
SHTRN Hawthorn Group
-1000 OCAL Ocala Limestone -1000

Scale = 1/434509

Gamma Log Cross Section 0-0'

OS00006

Scale = 1/570309

0
100

0

-100

g -200

LL

-300

-400

UDSCS Undifferentiated sand, clay & shell
HTRN Hawthorn Group
OCAL Ocala Limestone

Gamma Log Cross Section P-P'

P

100

0

-100

-200

y -300
LU

-400

-500

-600

-700

P00004

P00013

70
Scale = 1/654672

0 10 20 30 40 50 60

Gamma Log Cross Section Q-Q'

OS00019

0-I

CPS
15 20 25 30 35 40

50
Scale = 1/472376

OS0002

iu

Gamma Log Cross Section R-R'

IR0314 IR0954

IR0748

-400 -n
0

o

0
-800
G)
m
0
I--

r-
-800
C)
0

c

-1400

-1600

Scale = 1/313941

OK0003

R

0

-200

?

-S
.0
(U
w

0 5 10 15 20 25 30 35

SPECIAL PUBLICATION NO. 50

Location of Gamma Log Cross Sections, North-South Sections

LOCATION OF GAMMA
LOG CROSS SECTIONS

North to South Sections

Explanation
Well
County Boundary
.; SJRWMD Boundary

0 10 20 Miles

1:1895350

.--..;
I

Gamma Log Cross Section S-S'

200

0

-200

-J
Cn C
-400
LLU

-600

-800

Scale = 1/1197915

Gamma Log Cross Section T-T'

T C-0496 D.nnnL., .. C-0134

Scale = 1/826356

-1
0) -
Cu
Cu)

-1000

UDSCS Undifferentiated sand, clay, & shell
HTRN Hawthorn Group
OCAL Ocala Limestone
AVPK Avon Park Formation

Gamma Log Cross Section U-U'

U N-0221 N-0131 N-0237 D-4560 C-0142 C-0193 U'
100 0P-051 0
SPI -0619 UDSCS M-0115
4-P-0473

_L --- -- =
------ -- HTRN
-100 II I I I -- so -- -100

-200 -- ----_, --__ ---300

- 00------ 00
0

- CPS .o o r- C
r
400 -400
24 220 Vt
SAVP K An Pk

,m
A-500 -500

Legend

-6o -000 UDSCS Undifferentiated sand, clay, & shell -600oo
SHTRN Hawthorn Group
---- OCAL Ocala Limestone
~AVPK Avon Park Formation
-700

Scale = 1/1083203

0 50 100

Gamma Log Cross Section V-V'
L-0467
V M-0068 i OR0551 00007 V'
M-0115 --- UDSCS OS00021 OS00019
0 0VP

400ll ------------------o }II I AP LIIIII IIIII I T"- ------.--I- --III I
-200 -200
CP 0C

S-6 o00 -600-
o
-800 -00
CCPS

c600 -

-1oo HTRN Hawthorn Group -IOO
z
z

-800 __ -800
Legend -

UDSCS Undifferenfiated sand, clay, & shell CPS
-1000 HTRN Hawthorn Group I -1000

Scale = 1/977023

Gamma Log Cross Section W-W'

S N-0220

-200

-400

-800

-1000

-1200

10 20 30 40

Scale = 1/701328

P-0474

W'

-200

-200

Legend

UDSCS Undifferentiated sand, clay, & shell
HTRN Hawthorn Group
OCAL Ocala Limestone
AVPK Avon Park Formation

Gamma Log Cross Section X-X'

X P-0474 D-na 7 ino i f,) X'

i
0 -i
I
C)
c
rn
t

60

Scale = 1/532969

0 10 20 30 40 50
Miles

Gamma Log Cross Section Y-Y'

Y -2S-1351 -1216 S-1402 OR0305 OR0614 OS0041 Y'

UDSCS

a 0
7- 0-

-200 -200

-o0 >- UDSCS Undifferentiated sand, clay, & shell -
-30 HTRN Ha -300w n

C

-cs OCAL Ocala Limestone

AVPK Avon Park Formation
i- -- ---, HTRN Hawthorn Group
600 AVPK -Avon Park Formation
^00~~~~0 --- --- a -- L ---------------------

Scale = 1/439377

Gamma Log Cross Section Z-Z'

Z

0

-200

-400
_(D
00 0

u.
-600

-800

-1000

OR0614 OS00005

0S0068

OK0003

0 10

20 30 40 50
Miles

60
Scale= 1/594782

OS0002

Gamma Log Section AA-AA'

AA D-1118 .... SJ0177

Scale = 1/807228

A'
0

CO

Ld
c
00 0
C^

Gamma Log Cross Section BB-BB'

V-0273

V-0307 V -0254 V-0780 S-0080

UDSCS
F-0294 F-0251
V-0375

UDSCS

0 280

Scp8
. . .. .P.... . ....I 1..I I I I I I I l.l ll0 C 10 0 A V P K

CPS

Legend CPS E

UDSCS Undifferentiated sand, clay, & shell
HTRN Hawthorn Group 130
OCAL Ocala Limestone cPs cps
AVPK Avon Park Formation

CI S

10 20 30 40
Miles

50

60
Scale = 1/529433

BB
1n

HTRN

BB'
I

-al
00

~n~b~n r ~LC-rrrTTnFI

lIUU

vI0

7

--OCAL

Gamma Log Cross Section CC-CC'

CC

0 -

-100

Miles

cc

LU

Scale = 11533441

Gamma Log Cross Section DD-DD'

OK0003

DD'
0

-100

Scale = 1/462921

DD OS0220

a,
C
03
oI
w

25
Miles

Gamma Log Cross Section EE-EE'

V-0031

V-0819

0

-200

-400

0 0
a

L.
00 0

0 -600

-800

-1000

AVPK CAI

a so
775
CPS
0 130 0 110
PS 80 CPS CPS

CPS

Legend

UDSCS Undifferentiated sand, clay, & shell
OCAL Ocala Limestone
AVPK Avon Park Formation

0
CPS

0

-200
-n
r
0

m
400>

0
O

-600 0
I--

r
)C
C

m
-800 -<

20
Scale = 1/189014

EE V-0254

V-0267

V-0183

EE'

V-0304

UDSCS

Gamma Log Cross Section FF-FF'

SJ0025

SJ0798

SJ0128 F-0162 F-0312

FF'

-00

-1000

10 20 30 40

Scale = 1/843415

FF N-0117C

D-0403

ID

0 L

Gamma Log Cross Section GG-GG'

GG F-0312 V-0842 V-0817A v-0119 V-0570 BR1572 GG'

Scale = 1/588770

0

HTRN
-100

-200

-300

0 .

MJ

Miles

Gamma Log Cross Section HH-HH'

HH'

HH
0

-200

-400

a)
CO
CD
Ia)
;
LU

0 5 10 15 20 25 30 35 40 45 50
Miles
Scale = 1/448273

	Front Cover
	Title Page
	Front Cover
	Table of Contents
	Main
	Reference
	Appendix

MILLISECOND	CLASS.METHOD	MESSAGE
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57	html_echo_mainwriter.add_text_to_page	Finished reading and writing the file