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WATER IiRESOURCES researc center Publication No. 46 GEOHYQROLOGIC MODEL OF THE FLORIDAN AQUIFER IN THE SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT By Anthony F. Randazzo (Principal Investigator) Department of Geology University of Florida Gainesville UNIVERSITY OF FLORIDA
Transcript
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WATER IiRESOURCES researc center

Publication No. 46

GEOHYQROLOGIC MODEL OF THE FLORIDAN AQUIFER IN THE SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

By

Anthony F. Randazzo (Principal Investigator)

Department of Geology University of Florida

Gainesville

UNIVERSITY OF FLORIDA

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GEOHYDROIOGIC IDDEL OF THE FLORIDAN AQUIFER IN THE SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

By

Anthony F. Randazzo

Publication No. 46

FLORIDA WATER RESOURCES RESEARCH CENTER

RESEARCH PROJECT TECHNICAL CCMPLEl'ION REPORT

OWRT Project Number B-032-FLA

Matching Grant Agreerrent Number

14-34-0001-7148

Report Submitted: January 7, 1980

The work upon which this report is based was supported in part by funds provided by the United States Depa.rt:rrent of the

Interior, Office of Water Research and Technology as authorized under the Water Resources

Research Act of 1964 as a:rrended

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TABLE OF CONTENTS

Abs tract .............................................. i v

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1

Methods of Study................................. 1

Previous Work.................................... 3

Manatee Springs and Homosassa Springs ................. 4

Stratigraphy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4

Sedimentology-Lithofacies. . . . . . . . . . . • . . . . . . . . . . .. 4

Cyclic Sedimentation ........................ lO Depositional Environments ................... lO

Diagenesis ....................................... 12

Crystallization Texture and Fabric Classification ...................•.......... 12 Porosity .................................... 16 Fabric Selectivity of Dolomitization ........ 24 Summ.ary ......... 1 •••••••••••••••••••••••••••• 32

Romp Cores .........•.................................. 33

Avon Park Formation .................•............ 33

Cyclicity ................................... 33 Recognition of Depositional Environments in Avon Park Strata .•....................... 37 General Lithology .............•............. 38 Depositional Facies ...............•......... 43

Ocala Limestone .................................. 44

Lower Ocala Limestone ..............•.....•.. 44 Upper Ocala Limestone .....•....•..•....•.... 45

Ballast Point, Brandon, and Duette Cores .•........•... 47

Stratigraphy ..................................... 48

Stratigraphic Interpretation ........•....... 48

Diagenesis ....................................... 52

Geochemistry .......................................... 58

Importance of Sodium .............•..••........... 60

Sodium in Calcite ..........•................ 62 Sodium in Dolomite .......................... 62

ii

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Importance of Strontium ........................... 66

strontium Strontium

in in

Calcite .• Dolomite.

Strontium and Sodium in Relation to

.. 66 . . . . . . .67

Mole-Percent-MgC03. . . . . . . . . . . . . . • .•........... 67

Calcite .. Dolomite .•

.68 . .. 69

Summary ................................................ 70

References Cited ....................................... 74

iii

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ABSTRACT

Nineteen cores from the northern portion of the South-west Florida Water Management District and three counties adjacent to it were analyzed by megascopic, microscopic, x-ray diffraction, and atomic absorption spectrometry techniques for the determination of lithofacies, paleontology, mineralogy, and geochemistry. These wells penetrate biozones representative of the "Tampa", Suwannee, Ocala, Avon Park, and Lake City Formations. The Avon Park and Lake City Formations are characterized by interbedded massive, fossiliferous carbonate (limestone and dolomite) rocks (wackestone to grainstone) and thinly bedded peloidal and carbonaceous rocks (mudstone and wackestone). These represent subtidal (open marine and lagoonal), intertidal, and supratidal deposition. The Ocala Limestone is characterized by thickly bedded, fossiliferous limestone (mostly packstone and grainstone), representing subtidal and some intertidal depositional environments. The Suwannee Limestone consists of interbedded fossiliferous, partly dolomitized carbonate rocks (mudstone to grainstone) and an algal boundstone facies. These Suwannee deposits represent subtidal, intertidal and supratidal sedimentation. The "Tampa" Formation represents deposition in shallower marine waters and contains substantial quantities of clay and quartz and phosphatic sand, interbedded with carbonate rocks (mudstone to packstone). Numerous depositional cycles were recognized by cyclic occurrences of rock types and depositional environments.

A wide range of diagenetic fabrics from early to late stages of development occur. Fabrics are classified descriptively as equigranular (uni-modal) or inequigranular (multi-modal). Fabrics composed of crystals <O.002mm in diameter (unresolvable) are termed aphanotopic. Equigranular fabrics include sutured mosaic and sieve mosaic fabrics, and a somewhat problematic peloidal fabric. Inequigranular fabrics include porphyrotopic, poikilotopic, fogged mosaic and spotted mosaic fabrics. Two processes of dolomitization are suggested: homogeneous dolo­mitization resulting in single-stage development of micro­textured (groundmass crystals <O.016mm in diameter) aphanotopic, peloidal and mosaic fabrics; and heterogeneous dolomitization resulting in multi-stage development of porphyrotopic and some mosaic fabrics. The pattern of crystallization fabric distri­bution appears to be related to sedimentologically defined depositional cycles.

Undolomitized rocks generally have high visible porosity, consisting mostly of interparticle and intraparticle pores. Dolomites have variable amounts of visible porosity, consisting mostly of moldic and vug pores, and the type of porosity can be related to the type of crystallization fabric. Greatest total porosity is found in cavernous zones associated with formational contacts.

iv

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The distribution of Na+ and Sr2+ ions and mole-percent­MgC03 in carbonate rocks of the Floridan Aquifer supports a mixing zone model of diagenetic dolomitization. The inland phreatic zone consists of brackish solutions in the Floridan Aquifer. A coastal mixing zone exists at the salt/fresh water interface, but the flow patterns of the Floridan Aquifer can modify the three dimensional shape of this interface. Areas of locally high discharge, such as artesian springs, move the interface seaward. Variations in sea level and fluctuations of the phreatic zone related to climatic and tectonic changes could cause the dolomitizing solutions to contact large volumes of rock through time, resulting in the thick sequences of dolomite in the Floridan Aquifer.

The Na+ content of carbonate rocks is an approximate indicator of the salinity of the latest diagenetic solution. Sodium concentrations of the calcites are 37-970 ppm, indicating a slightly saline diagenetic fluid. Dolomite has 281-1963 ppm Na+ and was formed in a slightly more saline solution.

Strontium concentrations of carbonate rocks reflect the salinity of the latest diagenetic fluid and the diagenetic mineralogy. The Sr2+ concentration range of the calcites is 89-968 ppm, demonstrating diagenesis in a slightly saline solution, probably in an open system. The average Sr2+ content of the dolomites is approximately 59% of that in the calcites, indicating a more saline diagenetic environment for dolomite than calcite.

The more nearly stoichiometric dolomite generally has a narrower range of Sr2+ concentrations than non-stoichiometric dolomite indicating a more saline diagenetic environment for the non-stoichiometric dolomite where a greater number of competing ions inhibit dolomite ordering.

The formation of dolomite and its influence on rock texture and porosity are directly related to past and present hydrologic regimes interacting with aquifer limestones.

v

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INTRODUCTION

The Floridan Aquifer represents one of the world's finest aquifer systems and is relied upon as Florida's principal supply of fresh water. Detailed geologic knowledge of the Tertiary limestones comprising the aquifer is only now beginning to be compiled. Identification of the type and distribution of minerals, the arrangement, character and quantity of lithologic constituents,and the correlation of stratigraphic horizons would aid greatly our understanding of groundwater flow systems. The inaccuracies and uncertainties of our geologic knowledge of the Floridan Aquifer could be resolved by establishing a petrogrphic and geohydrologic model. Such a model would demonstrate the lithologic evolution of the aquifer and would be of fundamental importance in water management practices.

This research effort was concentrated in the northern portion of the Southwest Florida Water Management District where drill-cored materials were most available for study (Figure 1). The principal stratigraphic formations studied

were the Lake City, Avon Park, Ocala, Suwannee, and "Tampa" units. Rocks from this area were compared with those studied in an earlier investigation (Randazzo, 1976b).

The petrologic characteristics of the Floridan Aquifer in the current study area have been described. The original depositional environments of the rocks were deduced and the effects of diagenesis have been recognized. Results have been applied to the clarification of stratigraphic problems. Geochemical analyses of the rocks have revealed the history of water/rock interactions. A better understanding of how porosity evolved has been achieved. Petrologic and geochemical parameters which control the functioning of hydrologic systems have been evaluated and may be useful in the prediction or deduction of future aquifer changes.

Methods of Study

In this study nineteen cored sections were examined from the northern portion of the Southwest Florida Water Management District and three counties adjacent to it (Figure 1). Numerous samples were also taken from quarries in the area. More than 1,000 thin sections were prepared and analyzed to determine the constituents of the rocks and the diagenetic changes that have occurred. The composition of the rocks was determined by point counting with approximately 250-300 point counts made on each thin section.

Mineralogy was determined by X-ray diffraction analysis. Where calcite and dolomite were present in the same sample, a thin section of that sample was stained (Friedman, 1959) to determine which constituents of that rock were of calcite

1

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I ]I

GH· REPRESENTS 8 CLOSELY SPACED CORES

o

LEVY

25

SCALE

MARION

.CP

HERNANDO

PASCO

HILLSBOROUGH

eaR

-0 MANATEE

50 km

Figure 1. Index map of the study area showing core locations (B = Bell, MS= Manatee Springs, GH = Gulf Hammock, LE = Levy County-ROMP #124, RS = Rainbow Springs, CP = Cotton Plant, HS= Homosassa Springs, H = Hernando County-ROMP #107, LA = Lake CountY-ROMP #101, BP = Ballast Point, BR = Brandon, 0 = Duette).

2

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and which were dolomite. A1azarin red-S in a solution of NaOH was used to distinguish dolomite from calcite. The scanning electron microscope was also employed for minute textural studies and the determination of Mg/Ca ratios of selected a11ochemica1 constituents. Atomic absorption spectrometry was used for strontium and sodium detection and quantification. Mole-percent MgC03 of calcite and dolomite were calculated from data obtained by X-ray diffraction and atomic absorption spectrometry. The rock classification used herein is that proposed by Folk (1962). Grain-supported and matrix-supported distinctions have been made and the classification scheme of Dunham (1962) is used where appro­priate.

Previous Work

The petrology and depositional environments of the carbonate units comprising the Floridan Aquifer have only recently been investigated. The broadly defined works of Vernon (1951) and Chen (1965) have been utilized in an attempt to relate modern carbonate shoreline processes to the sedimentologic environment represented (Randazzo and Saroop, 1976; Randazzo et a1., 1977). Several studies have been concerned with the nature and diagenetic alteration of carbonate rocks (Bricker, 1971; Purser, 1973; Folk, 1974; Folk and Land, 1975; Veizer et a1., 1977). The geochemical history and characteristics of these rocks and their re­lationship to hydrologic conditions have been addressed by Randazzo (1976b) and Randazzo and Hickey (1978).

The oldest exposed rocks in Florida are Late Middle Eocene (Avon Park Formation). These rocks crop out on the crest of the Ocala Arch and are surrounded by rocks of Late Eocene age (Ocala Limestone) which occur on the flanks of the arch. These formations are the principal units composing the Floridan Aquifer. The Suwannee Limestone of Oligocene age, often exposed at the surface, is an important part of the aquifer in certain areas of Florida. The Lake City Formation of Middle Eocene age occurs only in the subsurface but is also a significant component of the Floridan Aquifer.

This investigation involved a number of graduate students who made noteworthy contributions to the total research effort. The reader is directed to the works of Saroop (1974), Stone (1975), Hickey (1976), Liu (1978), Zachos (1978), and Fenk (1979) for elaborate details on the lithologic and paleontologic charac­teristics of the various lithofacies recognized. Hickey (1976), Sarver (1978), Zachos (1978), and Metrin (1979) discussed the diagenetic and geochemical aspects of the important carbonate rock-forming minerals, calcite and dolomite. Our combined efforts have resulted in the verification of a model of dolomiti­zation by groundwater in the peninsula of Florida as postulated by Hanshaw et a1., (1971). Some of the results of our studies are summarized in a number of recent pUblications (Randazzo and Saroop, 1976; Randazzo, 1976a; and Randazzo and Hickey, 1978). References to other important contributions regarding strati­graphy, petrology, geochemistry and hydrology are presented in later sections of this report.

3

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MANATEE SPRINGS AND HOMOSASSA SPRINGS

Stratigraphy

Rock cores were drilled at Manatee Springs (MS) and Homosassa Springs (HS) by the Florida Bureau of Geology. The oldest of the three formations penetrated in these wells is the Lake City Formation which is recognized by the following combination of features: presence of highly carbonaceous beds; clay beds; common quartz; glauconite and gypsum; and by poiki1otopic fabric seen in thin section. Fabu1aria mat1eyi, Archaias co1umbiensis, and Dictyoconus americanus occur together 14.68m above the top of the Lake City in the MS core, and F. mat1eyi and A. co1umbiensis occur together 22.56m below the top in the HS core. Dictyoconus americanus has not been positively identified in the HS well. Two distinct depositional cycles are recognized in the Lake City Formation in the study material.

The Avon Park Formation overlies the Lake City For­mation in both wells. The top of the Avon Park is deter­mined by the first occurrence downward of a mudrock litho­facies which corresponds with the first occurrence of dolomite. The top is also marked by cavernous porosity; increased amounts of quartz, gypsum, and metallic sulfides; by poiki1otopic fabric seen in thin section; and by the first occurrence of Dictyoconus cookei in both cores. Five distinct depositional cycles are differentiated in the Avon Park represented by the study material.

The Avon Park Formation is overlain by the Ocala Lime­stone. The Ocala Limestone has been biostratigraphica11y zoned by many authors using many different taxonomic groups and types of zones. A comparision of various zonations is shown in Table 1.

Both wells enter the Ocala Limestone at an erosional unconformity with overlying Pleistocene or Holocene sands and do not represent the entire formation. Problems concerning recognition of the top of the entire unit will not be considered here (see Hunter, 1976). The Ocala Limestone is represented by one depositional cycle.

Sedimen to1ogy - Li tho facies

Rocks of the MS and HS cores can be divided into four major lithofacies (Table 2). Mudrocks [represented by mudstone (Dunham, 1962)J have grain contents of less than 10%, and for the most part contain less than 5% grains. Rocks with 10% or more grains [represented by wackestone, packstone and grainstone (Dunham, 1962)J are divided here on the basis of grain type. Skeletal grains (Leighton and Pendexter, 1962) and peloids (McKee and Gutschick, 1969) make up the most significant grains volumetrically (Figure 2) and the variation in their ratios can be used to differentiate most of the rocks. A few samples contain

4

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Table 1. Ocala biofacies.

Pur; (1957) Cheetham (1963) McCullough (1969) Nicol & Shaak (1973) Zachos & Shaak (1978) Williams et al. (1977)

Lepidocyclina chaperi Spondylus dumosus faunizone faun; zone -

Asterocyclina-Spirulaea Spi rul aed. vernoni 01·iJ1.Q.P19US wetherbyi vernoni faunizone Floridina antigua biozone biozone

I-.. ----~--~ ... . Nummulites vanderstoki faunizone Amusium ocalanu~ ---(C~~clJ-;rent range zone {=Camerina willcoxi}- biolone------Hemic~there faunizone

Lepidocyclina-Pseudophragmina' faunizone Exputens ocalensis i

Ul biozone

Spiroloculina newberryensis Oligopygus haldeman; faunizone Tubucellaria nodifera biozone

fauni zone Opercul;noides mood)branchensis (=Camerina willcoxi

faunizone

Operculinoides jacksonensis (=Camerina willcoxi)'

faunizone Concurrent range zone

Periarchus lyel'i floridanus- Periarchus lyelli Plectofrondicularia? faunizone Oligopygus phelan;

inglisiana faunizone bi ozone

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Table 2. Lithofacies terminology.

Dunham Mudstone Wackestone, Packstone, Grainstone (1962)

<10% Grains ~lO% Grains I

<: 0

:;::; ~75% Other Grains >75% Other Grains .,... '" 0 0-E 0 Skeletal/Peloidal Grain Ratio '-'

<1 ~1

II thofaci es Mudrod Peloidal Rock Tenn

Skeletal Rock Characterized By Major Constituent

6

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-....J

0/0 Other Groins

0" 100

• •

Peloidal , Rocks

Skeletal Rocks

• •

% Pe~oido' .. te~. !laa I • .,

Grains

Figure 2. Distribution of grain types in the MS and HS cores.

'10 Sketetal Graihs

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LEGEND

Peloidal Rock ~.

Skeletal Rock D Mud rock

Clay a Quartz Sand [Z] ..... ... . .... . .... .. ..

Figure 3. Skeletal/Peloidal grain ratios and lithofacies distrubution iri the MS and HS cores. .

8

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MS

Skeletal/Peloidal Rctio

0

-.~ 3: 30

"t-0,

a. ~ 3: o Q)

m CII) '-

0

~. r ~ r-

~o~ .r= -Q,. Q)

o

150

co

~ .(0

·u 0 ~

0 .r::. -..J

en CD ·0 c -.... 0

.t::. -..J

Cycle VIII

Cycle Vii

Cycle VI

Cycle IV

CycJe II

Cycle I

9

HS

Sit.lctal/Peloidal Ratio 0 I C)

0

.c. -Co CD C

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greater than 75% grains other than skeletal Or peloidal, and these are referred to minor lithofacies named according to the major constituent. Skeletal grains include all recognizable remains of hard parts secreted by organisms. They can generally be divided into fragmental and non-fragmental gnains, but the distinction is difficult to determine from thin section. Peloids are allochems composed of non-structured, micro-or crypto­crystalline material. They may represent wholly micritized skeletal grains, mud aggregates, pellets, and, in diagenetically altered rocks, the problematic clotted fabric or structured grumeleuse of Cayeaux (1935) (see especially Bathurst, 1975, p. 511-513). The term is very useful in that no genetic origin is assumed in its usage. Peloidal rocks are characterized by skeletal/peloidal grain ratios less than 1; skeletal rocks by ratios of 1 or greater. The distribution of lithofacies in the two cores is shown in Figure 3.

Cyclic Sedimentation

The variation in the skeletal/peloidal grain ratios are shown diagrammatically in Figure 3. Plotted values are restricted to samples containing 10% or more grains. Values were measured from point-counts of thin sections, and sample only certain portions of the cores (i.e., the diagrammed variation is only a sample of the real variation in the cores). The similarity in trends is, nevertheless, close. Corresponding variations in the sense if not the actual magnitude of the ratios is used as a basis of correlation. The mudrock lithofacies mayor may not be correlatable, which is in part caused by the diagenesis of otherwise recognizable skeletal or peloidal rocks which results in fabrics lacking original grains. Correlation of the study cores on this basis makes possible the recognition of repeated cycles of generally peloidal to skeletal sedimentation. Closer examination of these grossly defined cycles and consideration for finer grain-type distinctions, assessory minerals, and sedi­mentary structures suggests that these may in face represent actual cycles or carbonate sedimentation, with more than local significance.

Depositional Environments

Four major depositional environments can be recognized by a combination of criteria, including grain size and type, miner­alogy, and sedimentary structures.

Open Marine: This is the normal marine environment, repre­sented by waters of average salinity, temperature, and composition, and with a bottom environment characterized by slightly oxidizing conditions. Sediments deposited in this environment are charac­terized by a lack of thin stratification or lamination, visible organic material, gypsum, or significant amounts of clay minerals. Large echinoids, mobile pelecypods, bryozoans, and abundant benthic foraminifera are diagnostic. The extensive dolomitization of these rocks makes it difficult to distinguish low and high energy deposits

10

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(i.e., deep and shallow subtidal), but corals and coralline algae, when present, indicate fairly shallow, agitated waters (Purdy, 1963). Skeletal packstones and grainstones are dominant, with some wackestones.

Lagoonal: This environment is characterized by salinities, temperatures, and bottom Eh different from the open marine environment, from which it is isolated by actual barriers or by restriction of circulation by shallowness. It is analogous to the shelf lagoon environment described by Purdy (1963) and includes the adjacent subtidal environment discussed by Shinn et ale (1969). Salinities generally are higher than open marine, temperatures may rise infrequently as high as 40°C (Glynn, 1968), and bottom conditions may be slightly oxidizing or reducing. Sediments consist mainly of pellets or mud aggregates (Shinn et al., 1969; Purdy, 1963), combined here under the term peloids. Water is quiet and non-agitated, since, as Purdy (1963) notes: " ... the preservation as well as the formation of pellets is dependent upon minimal bottom agitation" (p. 484). Visible organic matter may be present, and its decomposition may have induced strong reducing conditions in the sediment, and thus restriction of infauna. Lamination and stratification are not common, but may be present in deeper and quieter parts of the shelf or lagoon. Generally the faunas will show less diversity; mudstone, wackestone, and pellet grainstone predominate, and the rocks are mostly peloidal.

Intertidal: This environment is characterized by an abundance of preserved sedimentary structures. Evans (1965) in a detailed study of a tidal flat deposit, delineated six major zones parallel to coastal strike. In a vertical sequence, coarsest beds are at the bottom, well-sorted sands are in the middle, and fine silts and muds at the top. The deposits are characterized by laminated sands, silts, and muds. Well-developed burrows may be present and some portions of the deposits may be completely bioturbated (Shinn et al., 1969). Park (1976) in studies along the Persian Gulf, stated that " ... optimum conditions for stromatolite development in the Trucial Coast are restricted to the mid and upper intertidal areas" (p. 382). Thin beds of organic debris may be present. The lowest portion of the tidal flat may be characterized by abundant rock and metazoan fragments (Evans, 1965). Tidal creeks cross the flats and have distinctive sedimentary characteristics analogous to those described for fluvial (point bar) deposits, though on a much smaller scale. According to Evans (1965): "The meandering of the creeks gives rise to cross-stratification on a scale not seen in the other sub-environments (of the tidal flat). Erosion of the concave outer bank is accompanied by the deposition of inclined strata with dips up to 20° on the convex inner bank. The meandering creek produces a planar surface of erosion, commonly covered by a layer of shells and mud pellets, derived from the banks, which is buried

11

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beneath the cross-stratified deposit of the prograding bank" (p. 226). Shinn et ala (1969) include the tidal creeks in the subtidal zone, but they are obviously characteristic of the tidal flat environment, and their deposits are intimately associated with intertidal deposits.

Supratidal: This environment may include salt marshes, beaches and beach ridges, and sabkhas. Salt marsh deposits are typically well-laminated, fine-grained, and very carbonaceous (Evans, 1965; Shinn et al., 1969). They may be marked by root casts, fenestral porosity (Shinn, 1968a; Shinn et al., 1969), algal stromatolites, and laminate crusts (Multer and Hoffmeister, 1968). Supratidal ponds may result in deposits of fine clays and evaporite minerals. Creeks in the supratidal marsh, according to Evans (1965), may differ from creeks in the intertidal zone, with the base of the cross-stratification " ... marked by a bed of jumbled angular blocks of marsh sedi­ments, produced by undercutting of the creek walls. The overlying set of strata is again wedge-shaped and individual laminae may show dips of up to 80 0 " (p. 226). Beach ridges are generally characterized by graded laminae and cross-beds, and the deposits are very well-sorted, though ranging from fine to coarse in grain size. Sabkha-type deposits are evaporitic in character, and contain gypsum and halite or crystal molds, and are generally laminated by algal stromatolites.

Diagenesis

Crystallization Texture and Fabric Classification

The basic terminology described and defined by Friedman (1965) is used here. His terms for crystallization textures are retained. The definitions of euhedral, subhedral, and anhedral are those of general usage by North American geologists and do not require explanation. The terms are defined by Friedman (1965), following Cross et ala (1906). There are three major groups of crystallization fabrics (Table 3): equi­granular and inequigranular, distinguished by uni-modal and multi-modal crystal size distributions, respectively; and aphanotopic, composed of crystals smaller than 0.002mm in diameter (unresolvable). The first two groups are further divided into idiotopic (mostly euhedral textures), hypidio­topic(mostly subhedral textures), and xenotopic (mostly anhedral textures) .

The Friedman classification of fabrics is expanded here by finer distinction of types, particularly inequigranular types (Table 3). Equigranular fabrics are divided into mosaic and peloidal fabrics, inequigranular into porphyrotopic, poikilotopic, and mosaic fabrics. Aphanotopic fabrics can not be further subdivided by use of the optical microscope. Porphyrotopic fabrics are further divided into floating-rhomb (Figure 4a) and contact-rhomb (Figure 4b). Floating-rhomb and contact-rhomb fabrics are characterized by isolated or loosely aggregated euhedral or subhedral crystals, respectively,

12

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I-' W

Table 3. Crystallization fabric terminology (modified after Friedman, 1965).

Peloidal

Peloidal

Equigranular Inequigranular

Mosaic Mosaic Porphyrotopi c

Sutured Sieve S~otted Fogged . Contact- Floating-~1osaic Mosaic r40saic r10saic

~ ---

Size classes O.256mm - O.016mm diameter O.016mm - O.002mm diameter <O.002mm diameter

Rhomb Rhomb ~--

Term

Mi cro-crys ta 1 s Aphanotopic crystals

Poikilotopic

PoikilotoQic

Aphanotopic

Aphanotopic

AQhanotoQic

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Figure 4. a)

b)

c)

d)

e)

f}

HS 62, 28.65mbt (meters below top). Plane light. Peloidal lithofacies, partially dolomitized. Inequigranular, idiotopic floating-rhomb porphyrotopic. Small, euhedral dolomite rhombohedra in aphanotopic calcite groundmass. Large foraminifer is Dictyoconus cookei.

MS 299, l45.85mbt. Plane light. Skeletal lithofacies, partially dolomitized. Inequigranular, hypidiotopic contact-rhomb porphyrotopic. Pockets of aphanotopic calcite surrounded by coarse, subhedral dolomite crystals.

MS 106, 52.65mbt. Plane light. Mudrock lithofacies, dolomitized. Inequigranular, xenotopic fogged mosaic.

MS 60, 35.66mbt. Plane light. Skeletal lithofacies, dolomitized. Inequigranular, xenotopic spotted mosaic. Spots are micritized foraminifera.

MS 266, l30.00mbt. Crossed polars. Mudrock lithofacies, partially dolomitized. In­equigranular, ~enotopic poikilotopic. Small, anhedral to subhedral dolomite crystals embedded in coarse calcite groundmass.

MS 266, l30.00rnbt. Crossed polars. Same as above, slide rotated to extinction of calcite.

14

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c. d.

e. f.-

15

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contained in a fine-grained matrix. Mosaic fabrics are divided into fogged (Figure 4c) and spotted (Figure 4d) mosaic fabrics. Fogged mosaic fabric is characterized by irregular or diffuse areas of very fine crystals contained in a coarse mosaic groundmass. Isolated and well-defined peloids ("spots" or "blebs") of fine to very fine crystals in a coarse mosaic groundmass are characteristic of spotted mosaic fabric. Poikilo­topic fabric (Friedman, 1965, p. 651) is relatively rare in the study material, but is distinctive when it occurs. Wholerock poikilotopic fabric is characterized by fine dolomite crystals contained in large sparry calcite 6rystals (Figures 4e, fl. The term can also be used to describe the small-scale fabric of calcitic micrite contained in sparry calcite overgrowths on skeletal grains (Figure Sa).

Equigranular mosaic fabrics can be further divided into sutured (Figure 5b) and sieve (Figure 5c) mosaic fabrics. Tightly packed anhedral crystals, generally with little or no inter­crystal porosity, are characteristic of sutured mosaic fabric. Sieve mosaic fabric, on the other hand, is characterized by loosely packed anhedral to euhedral crystals and high moldic and intercrystal porosity. It is in part analogous to the sucrosic or "sugary" texture of many authors. Peloidal fabric is distinctive but problematic, and characterized by distinct to indistinct "clotting" of crystals of essentially uni-modal size distribution (Figure 5d).

A combined texture and fabric nomenclature is used to describe any crystallized rock sample, e.g., idiotopic floating­rhomb fabric; or xenotopic sutured mosaic fabric.

The size scales recommended by Friedman (1965, p. 653) are arbitrary and do not conform to natural size breaks in the study material. Crystals in this material fall into three major size classes (according to length of major diameter) : (l) O.256mm to O.Ol6mm, (2) O.Ol6mm to O.002mm, and (3) less than O.002~~. No term is used to describe the first size classi the second size class is indicated by the prefix micro- added to the fabric term, and crystals in the third size class are termed aphanotopic. For example, the term microxenotopic fogged mosaic indicates that the crystals in the groundmass are anhedral and fall in the size range O.016mm to O.002mm. Crystals in the aphanotopic size range can not be resolved well enough for textural classification.

Porosity

Classification

The Choquette and Pray (l970) classification of porosity is followed, with the following exceptions. No distinction is made between intraparticle porosity and growth-framework porosity of solitary corals or bryozoa or shelter porosity inside echinoids. No distinction is made between channel or cavern porosity. Root moldic porosity is considered to be simple moldic.

16

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The process of grinding thin sections alters the amount of intercrystal porosity, the actual amount varying according to the original grain size, texture, and fabric of the rock. A significant amount of intercrystal porosity may also be submicroscopic, particularly in rocks with a large percentage of aphanotopic grains. Errors in estimation are not known, but may range over 100%; for this reason only visible porosity is reported here.

Distribution

Excluding cavernous porosity, total visible porosity in the MS and HS cores appears to vary randomly (Figure 6), although the amount of porosity is generally low at depositional cycle boundaries. Cavernous porosity, on the other hand, is clearly associated with the formational contacts as selected in this study. Difference in the dissolution characteristics of calcite and dolomite probably accounts for caverns at the Avon Park-Ocala formational contact (see Goodell and Garman, ~69), and suggests its formation during the telogenetic stage (Choquette and Pray, 1970) of diagenesis. Enhanced solution beneath the clay beds marking the top of the Lake City Formation in the HS core may account for cavernous porosity here.

Porosity is dominantly fabric selective (Table 4), a conclusion arrived at also by Textoris et ale (1972) and Randazzo et ale (1977).Interparticle porosity is greatest in rocks in which the original depositional fabric is preserved, whether dolomitized or not, but intraparticle porosity is greatest in those not dolomitized. Moldic porosity varies greatly, since it is dependent on original skeletal content, but is found predominantly in crystallized fabrics and is greatest in equigranular sieve mosaic fabric. Fenestral porosity is rare in the study material, and is restricted to equigranular fabrics only. Non-fabric selective vug porosity is restricted to crystallized fabrics, but is distributed among them and shows no apparent fabric selectivity other than occurrence in dolomite. The fabric selectivity of the porosity suggests that interpretable distribution of porosity can be seen in the cores.

Manatee Springs Core

Distribution of porosity types in the ~1S core (Figure 7) correlates closely with fabric distribution and also with the depositional cycles described. Cycles I and VIII, both represented by significant amounts of calcite, are characterized by interparticle porosity [primary (Choquette and Pray, ~970)], somewhat modified by pore filling and cementation [mesogenetic (Choquette and Pray,)1970)]. Crystallized fabrics are for the most part characterized by moldic porosity [eogenetic or mesogenetic (Choquette and Pray, 1970)], although vug development may be important. Moldic porosity is generally greatest in the upper portions of the depositional cycles, directly related to the distribution of the skeletal rock lithofacies (see Figure 3). Sieve mosaic fabrics often have a significant amount of moldic porosity, and, coupled with their inherent high intercrystal porosity, are often soft and may form zones of high transmissivity. Moldic porosity in inequigranular mosaic fabrics may not lead to

17

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Figure 5. a)

b)

c)

d)

e)

f)

HS 35, 20.73mbt. Crossed polars. Skeletal lithofacies, undolomitized. Aphanotopic calcite embedded in sparry calcite overgrowth on echinoid grain (between arrows).

MS 284, 140.21mbt. Plane light. Mudrock lithofacies, dolomitized. Equigranular, hypidiotopic sutured mosaic.

MS 263, 128.40mbt. Plane light. Hudrock lithofacies, dolomitized. Equigranular, xenotopic sieve mosaic. Dolomite crystals, some of which are hollow (arrows), contain dark nuclei.

MS 262, 127.86mbt. Plane light. Peloidal lithofacies, dolomitized. Equigranular, microxenotopic peloidal.

MS 279, 137.46 mbt. Plane light. Mudrock lithofacies, dolomitized. Equigranular microxenotopic sutured mosaic.

MS 151-B, 71.63mbt. Plane light. Skeletal lithofacies, dolomitized. Inequigranular, hypidiotopic spotted mosaic. Spot is transverse section of micritized foraminifer.

18

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a. b.

c. -DI~ d.

e. f.

19

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Table 4. Fabric selectivity of porosity.

Q.J Q.J Q.J ..- ..- .->, u U ..Q +oJ ..... . .... ..... .....

+oJ +oJ .- VI VI ~ ~ 1'0 .... 0 1'0 1'0 ~ >~ c.. c.. U +oJ 0 ~ 1'0 ..... VI .-~ Q.J ~ "C Q.J 1'0

+oJ +oJ ..- :: 0', +oJ :: :: 0 Q.J :;, 0 - ..... :E: ~ ::> ~

Peloidal 0.06 a 5.5 0~2 1.3 7.1

Sutured t·1osa i c 0 0 4.8 O. 1 0.9 5.8

Sieve Mosaic a 0.03 13.3 0.3 1.2 14.8

Spotted Mosaic 0 0 5.0 0 3.0 8.0

Fogged Mosaic 0 0 7.3 0 2.2 9.5

Contact-Rhomb 0 0.6 3.3 a 1.5 5.4 Porphyrotopi c

Floating-Rhomb 3.4 2.2 0.4 a 0.6 6.7 Porphyrotopi c

Uncrysta 11 i zed 9.4 1.9 3.0 a a 14.4 Fabri cs

All values in area percent of total rock.

20

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MS HS • % Porosity % Porosity 1) .! "" 20 40 60 80 100 " <.JO "" 0 I I I I I , <.JO 20 40 60 10

I

Caverns~

Caverns)

caverns)

Figure 6. Distribution of total visible. porosity in the ~'lS and HS cores. 21

100 I

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Iype of Porosity.

Interparticle Intrapartict e

. Moldie Fenestra.!

.. Vuq

LEGEND

.£!ysta lliz otion Fabric

............. Equigranular _ . .--.----. Sutured Mosaic Sieve Mosaic

.-. ... --.-.-,. Peloidal -~ ........... Ineq,uigranuicr

Ftoating-Rhomb· Porphyrotopic Contact-Rhomb Porphyrotop i c

Fogged Mosaic . Spotted Mosaic Poikilotopi c

Aphanotopic. Original Fabric

Preserved

IXXI 1000 1 I·~· -I

rz Z ZI

WmLa ~\~ ~. [I i I [I -[ ]

Figure 7. Distribution of porosity types (excluding cavernous) in the Manatee Springs core and comparison with distribution of crystallization fabrics.

22

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o

---o Q..

~. :c­o cg

CD CI) l.. Q;) -cg

::E.

c o

o

~ .. ' ... .

10

.. ... . . . . . '

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

Porosity

20

~ .... , :... ........ 4 .. _ .......... .

;":00 ..• _ ....... . ... I.

23

30

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better permeability since the pores are often not interconnected. Vug porosity is probably necessary for high permeability in these fabrics.

Highest permeabilities are suggested at the base of Cycle I (vug development in mosaic fabric) and in the center of Cycle I (interparticle porosity in the calcitic fabrics) i in the upper part of Cycle IV and Cycle V (moldic and vug porosity in sieve mosaic fabric); and in the parts of Cycle VIII with high inter­particle porosity (Figure 7).

Homosassa Core

Distribution of porosity types in the HS core (Figure 8) also correlates closely with the depositional cycles, although it differs from the distribution found in the MS core (Figure 7). Cycle VIII, represented entirely by calcite, is characterized by interparticle porosity. Crystallized fabrics are for the most part characterized by moldic porosity, but vug development is more significant in this core than in the MS core.

Highest permeabilities are suggested in Cycle III, the upper part of Cycle IV, and in Cycle V (moldic porosity in sieve mosaic and aphanotopic fabrics); the lower part of Cycle IV and lower part of Cycle VI (vug porosity in peloidal and inequigranular mosaic fabrics); and in Cycle VIII (interparticle porosity) (Figure 8) •

Fabric Selectivity of Dolomitization

The sequence and association of crystallization fabrics and porosity evidenced in the HS and MS cores reveal much about their mode of origin and their genetic relationships.

Heterogeneous Dolomitization

A continuous spectrum of dolomite fabrics ranging from early stage floating-rhomb porphyrotopic (Figure 4a) to later stage contact-rhomb porphyrotopic (Figure 4b) fabrics is clearly revealed in the study rocks. These fabrics often grade into mosaic fabrics (Figures 4c,d), which appear to represent the completion stage of dolomitization in the rock (Figure 9). This process of dolo­mitization is multistage, and a full range of fabrics is present in the study rocks. Dolomite porphyrotopes are almost invariably restricted to aphanotopic (mud) calcite matrix, and therefore the appearance of the final fabric is dependent upon the occurrence and distribution of mud matrix in the original fabric (Figure 10). Because of this restriction, the process is here termed heterogeneous dolomitization. This process is initiated by nucleation of isolated dolomite crystals in an aphanotopic matrix consisting of calcite (or possibly aragonite). The preservation of a full range of fabrics indicates the process takes place during a geologically significant period of time (probably on the order of a thousand years or more). Heterogeneous dolomitization of mudstone leads to the formation of sutured mosaic fabric (Figure 10). Heterogenous

24

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dolomitization of wackestone leads to spotted mosaic fabric if the allochems are also dolomitized, or to sutured or sieve mosaic fabrics if they are dissolved (Figure 10). The large number of allochems present in a packstone causes the dolomite porphyrotopes to penetrate the grains and destroy original outlines, resulting in diffuse patches of finer-grained crystals and fogged mosaic fabric, or to sutured or sieve mosaic fabrics if the allochems are dissolved (Figure 10). No true grain-stones were observed that had more than 1% porphyrotopic dolomite; presumably grains tones can not be dolomitized by heterogeneous dolomitization since they contain no mud matrix.

Homogeneous Dolomitization

The process of heterogeneous dolomitization can not explain the origin of aphanotopic or peloidal fabric, or of dolomitized grainstones. The excellent preservation of original grainstone fabric in some dolomite samples is not possible if dolomitization proceeded by porphyrotopic growth, and heterogeneous dolomiti­zation of a mudstone or peloidal grainstone would lead to sutured mosaic fabric. These particular fabrics are always composed of fine or very fine dolomite crystals, generally with mean diameters less than 0.016mm, and they are here referred to as micro-textured fabrics. Some mosaic fabrics are also composed of crystals in this size range, and these are also referred to as micro-textured. Dolomitization of micro-textured fabrics is not dependent upon the occurrence of mud matrix and the process is termed homo­geneous dolomitization. Micro-textured fabrics are always composed of 99+% dolomite and no full range of fabrics is preserved as for heterogeneous fabrics. The process is, therefore, single-stage and nucleation of dolomite crystals occurs at a large number of sites throughout the rock and growth is completed in a geologically insignificant period of time. Destruction of allochems is probably minimized, and original fabrics often preserved. Fogged and spotted micro-textured mosaics are uncommon. Homogeneous dolo­mitization of mudstone leads to micro-textured sutured mosaic fabric, or, if the rate of nucleation is very high (essentially spontaneous throughout the rock), it can lead to aphanotopic fabric (Figure 11). Wackestone and packstone are generally dolo­mitized to micro-textured sieve mosaic fabric, although fogged or spotted mosaic fabrics might be developed (Figure 11). Homo­geneous dolomitization preserves the original fabric of peloidal grainstone to a large degree (Figure 11).

Poikilotopic Fabric

The origin of poikilotopic fabric is problematic. This type of fabric is rare in the study rocks and its isolated occur­rences provide little evidence of mode of origin. The character of the mixed mineralogy and the association with unconformities or zones of weathering suggest that it might originate by de­dolomitization of crystallized fabrics (see Friedman, 1965, p. 651).

25

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lype of Porosity.

Interparticle Intraparti cl e Maidie Fenestral Vug

LEGEND

~ystallizQtjon Fabric

E qui granular Sutured Mosaic Sieve Mosaic Peloidal

Inequigranular Floatin 9 -Rhom b Porph yroto pic Contoct-Rhom b Porphyroto pic

Fogged Mosaic Spotted Mosaic Poikilotopic

Aphonotopic Original Fabric

Preserved

LXXI [0001 1···1

VI21

Wi/@· ~\\'1 lS"1 [II I tl -( :

Figure 8. Distribution of porosity types (excluding cavernous) in the HS core and comparison with distribution of crystallization fabrics.

26

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o

.... 0 ~

~ 3 0 -CD III en "-CD -CD :E c -.c > -0. -CD 0

----

c .9 -o N ::0 - '-O..Q -0 ~u. U 0

000 00

000 00

••• •• ••• • • .~ .. 000 .-. ••••

••

10

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

.. -. .. ...... ~~::~~~.-

--.-... ~ -------

27

Porosity

20 30 40

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LEGEND

Lithofac ie s Peloidal Rock VI/ZIl Skeletdl Rock I I Mudrock _ Clay & Qu ortz

Sand D

% Dolomite ::

Dolomite Dolomite -I-Calcite x 100

~y.stallizotion Fabric Equigranufor

Sutured Mosaic Sieve Mosaic Peloidal

Inequi;ranuJ a r Floating - Rhomb

Porphyrotopic Confact <a Rhomb

P orphyrotopic Fogged Mosaic Spotted Mosaic PoikHotopic

Aphanotopic Original Fabric

Preserved

~ 10 001 I •• a I

VI/1

Willa t\\Wi tSS4 fllIlI -

Figure 9. Distribution of crystallization fabrics in the ~1S and and HS cores and comparison with lithofacies distribution and variation in carbonate mineralogy.

28

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N V)-

g Depth g In Meters Below Top Of g Well 0

I. I I fJ 10 ~

(") \! '< n CD

o o o 2. ~ -'\ I , ..J 100 CD (f)

~"11 ~ ~~ I 11

~! ~ J

\~ \ (")

'< '< n 0 CD <D

< <

Depth 8 In

Lithofacies

Crystallization Fabric

Crys tallization Fabric

Lithofacies

o ~ o o 0-3 ::;: CD

100

I (J)

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Co)

Q.. 0 -0 ~

~ .c Q.. ~

0 a.

(J .-o tn. o :E

Original Fabric

Floatin;-Rhomb Stage

Contact·Rhomb Stage

Completion Stave

Wackestone Mudstone Packstone

! 1 !

Spotted Moaalc Fog gad Mosaic

Sieve Mosaic

Figure 10. Genesis of crystallization fabrics by heterogeneous dolomitization.

30

c 0 .--0

.!:::! :: E 0 -0 e

04-0

CD CD ~

0' CD e es c en 0 CD ~ (J c

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w I-'

Mudstone

Original Fabric .. .. .... . . ( . ..' '. ". J a .. .... ..

Nucleation ~ u ~. _ . • -7 ~ •. -.~.'.Q' ... 3 (;' ~.·Q.~:;c(.:~ (;j ~ 'q • .: o. tIi 'd to

Packlton., Waok •• ton.

("~'i··,·"'J .. '... . . ....

~ ;: .... P·-z~,v . 'a.~CI ~ II • • 41. 0' '4 ir D. 6, 0.'''' e .. __ .=a ...

.. E

o .c .- U

Peloidal Grainstone

~~-II c ---~ ~ c .!:! ....

J E 0 0 0

~o _. ~ :;

r.-Ot Q)

0

Ot 1 T ... 0 c Completion ( • '. . ... -.' .- J ~L ~

::I _ .-. . . 0- U) • • ••• a·!II_ (1)4 0

A phonotoplc ~Mm\

fA Icro - textured Sutured MOlOle

Mlcro- textured FoggedMoaolc

~~

Micro -textured Sieve MOlaic

Figure 11. Genesis of crystallization fabrics derived by homogeneous dolomitization.

Q) .... 0

• .s

1 Peloidal

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Summary

There is a descriptive basis for the classification of crystallization fabrics (primarily dolomites). Categori-zation of the rock fabrics found in the two study cores reveals patterns similar to those of the depositional cycles deduced from sedimentologic considerations. Type and distri­bution of porosity also occur in patterns that can be associated with the depositional cycles. The various crystallization fabrics caused by dolomitization can be placed into a genetic framework and there is a fabric dependency upon both the process of dolomitization and the original fabric. Unless the rock has undergone repeated neomorphism of dolomitized fabrics, it is generally possible to deduce the original depositional fabric (and thus the lithofacies) from even highly crystallized fabrics. Only in the case of sutured mosaic fabric is the original fabric in great doubt, primarily because repeated neomorphism will lead to this fabric.

32

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ROMP CORES

Three cores were extracted by the Southwest Florida Water Management District from locations in north central and northwestern peninsular Florida as part of its Regional Observation and Monitor-Well Program (ROMP). The location of these cores (#101, #107, #124) are shown in Figure 1. At the writing of this report some ten additional ROMP cores are in various stages of investigation. The major stratigraphic units encountered are the Avon Park and Ocala Formations (F igure 12).

Avon Park Formation

The Avon Park Formation exhibits many features which are strikingly similar to carbonate sediments produced in modern tidal-flat environments. These features include: flat, millimeter-thick laminations; undulating stromato­lites; vertical burrows; desiccation cracks; "birdseye" structures; root casts and molds, flat pebbles; sediment mottling; evaporite mineral molds; and thin, micritic beds. These features have been described as being products of tidal-flat accretion over adjacent shallow marine deposits.

Petrographic examination has revealed the presence of eleven commonly occurring subfacies representative of the supratidal, intertidal and subtidal zones. The repetitive vertical arrangement of these rock types has enabled the recognition of cyclic sedimentation patterns in Avon Park strata.

Cyclicity

The vertical sequence of interbedded lithologies found in the Avon Park Formation depict a complex model of sediment accumulation in which the recognition of cyclic depositional patterns proves to be, " ... the single most illuminating factor in making sense of the seemingly bewildering vertical parade of subfacies" (Reinhardt and Hardie, 1976). The vertical organization of lithologies represents a repetition of sedimentation cycles in which the basic Avon Park depositional cycle is represented by an offlapping or progradational facies sequence (Figure 13). Subtidal facies represents the lowest unit in most depositional cycles. This is often in sharp contact with a bed which represents the top of the underlying cycle. Periods of submergence to subtidal levels were followed by shoaling and progradation of intertidal and supratidal deposits over subtidal sediments. Interruptions in these progradational trends are indicated by numerous in­complete cycles marked by minor unconformable contacts found along cyclic boundaries.

33

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LEGEND THINLY BEDDED AND LAMINATED MUDSTONE

FLATLY LAMINATED, PELLETED MUDSTONE TO PACKSTONE

ALGAL BIOLITHITE

ROOTED AND MOTTLED FORAMINIFERAL WACKESTONE

PELLETED WACKESTONE TO PACKSTONE

ROOTED AND BURROWED MUDSTONE

BIOCLASTIC WACKESTONE TO MUDSTONE

FORAMINIFERAL-ECHINOID WACl<ESTONE TO PACKSTONE

FORAMINIFERAL PACl<STONE TO GR AINSTONE

DICTYOCONID WACKESTONE TO PACKSTONE

MILIOLID PACKSTONE

PELOID-MILlOLlD PACKSTONE

COMPOSITE GRAIN-MIXED SKELETAL GRAINSTONE TO PACKSTONE

FRAGMENTAL GRAINSTONE TO PACKSTONE

FRAGMENTAL PACKSTONE TO WACKESTONE

LARGE FORAMINIFERAL WACKESTONE TO PACKSTONE

NUMMULITID WACKESTONE TO PACKSTONE

LIMESTONE [ rr I : 11 DOLOMITIC LIMESTONE

DOLOMITE I ~ ~ I CALClTlC DOLOMITE

LITHOLOGIC BOUNDARY

I I E:;~~?a fi~~~.:-~ ~~~:f~~!

~I

lI!:tm.EI ~~

1::=::3 I: :: 5 ;]

FAUNIZCNE BOUNDARY - - - ____ _ DEPTH IN METERS BELOW TOP OF WELL

Figure 12. Columnar sections showing the relationship of assemblage zones to lithologic units in the ROMP cores, corrected to datum at the top of the Avon Park Formation.

34

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<t ...J <t U o a: l.IJ

~

cc I­Z ::l

Nummulites 'Mllcati Zone

Levy, County ...::::.-,-=: ? Well 12.4

~?­

Spircloculi/1f1 sBmlnoiensis­

Amphi$iegina pinarel1$is ~

Zone-

i..Jiur:Jnda fIDrid:Jntl,

D/cryOClJlXJ$ , f/crit:itJl1U$.

fl&tJoItel Zone'

-

35

. -.:::.::: I::I::::;:~ Spirclocullna

seminolensis-1-.--'-1 Amphist8gllltl

1-4---<;~'

iCII.jJ1izone , aOllterated ;­bv. ddo- , !'l1itizo-;' llon-;

"

Uluonlll/o !/addona,

OlctyoccntJS !/oriaonus,

o.&::JoKei Zooe

Lake County Well 101 r:""' __ -~ __

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--I OJ c c::

1J...

CORE

....,.~.

SEDIMENTARY FEATURES

./ EROSIONAL CONTACT

LITHOLOGY: Algal biolithites; thinly bedded and laminated mudscones to wackestones. STRUCTURES: Thin algal and sedimentary laminations, des­

~_._--~~~ iccation cracks, fenestral F-'-=:"-"'~':::--="'" cavities, rare burrows.

- - .. _o---!!:,

--:.. - -- - FOSSILS: Sparse to absent

,-=--J. ~~-, ~- LITHOLOGY: Skeletal and pel­~ if ~ letal mudstones, wackestones

- II \ ~ 0 1- and packs tones.

DEPOSIT10NAL ENVIRONMENT

Shallow

Lagoon

Supratidal

Mudflat

;- [ __ 0 • STRUCTURES: Vertical burrows,

J. <t (' root casts and molds, sediment /I' .. AlII :-.. 'J mottling, thin micritic beds.

o () 0 .. ' ~ 0- 120 FOSSILS: Low faunal diversi ty

Inte(tlJoi

Mudflat " ~ 0- (small forams, ostracods and

-. .," I ~ t) 0 I f ewer gas tropoc.s) . ~ .• ~.'O°,a~ o 0 0 - LITHOLOGY: Skeletal wacke-

.. e. " II ~ 0 0 f ~ stones to grains t'Jnes. '0 • 0 0 0

... 0 .. STRUCTURES: Micrites show a o 00"." 0 I) 0° a \) Q. 0°,0 • (I ~ IlQ ( general lack of sedimentary .)' 0 00 QQ 0 f b' b f- • . .0.. 0 12 ~ " o~ 12 I) II a rJ.c ecause 0 J.nt ens J.ve (I. c). 0 0 I) 0 0 .0· reworking by burrowinO'o organ-

o· 00'''' Q I) • -o~0ci0o ooo .. o.~ .. 1) isms; sparites may show faint

0'1) 0'0 00 rJr) current-produced laminations. 0" :I 0" 12.." 0 0#

I) GOCoCl~ ·0 ~o &' ,0 <:I -;, FOSSILS: Moderate faunal di-o QC) 0 .. 0 0" c~ 0' versi ty containing a varie ty 00°.000'0 <!t. 0,0' O.400.~1) 0 :;oo"~ of species of foraminifera, ~ I) 0 t. 0 0 4 • 0 ~o dasyclad algae, echinoids, ~ .OOO!) .~·OQ04

Q', I:) Q ~CI 00 Q 0 0 $' bryozoans, solitary corals

ShalJow

Lagoon

.0000 CI 0.0 1:1°" and molluscs. h (1'-';' <) .~OJ.. _________________ --'-------

-_.~ 1r'7'" --~ ~ V \. Incomplete cycles are marked by minor ~ __ 1 unconformitiss found along cyclic

- ~ 0 U .... boundries ..

Figure 13. Schematic drawing showing the essential aspects of an Avon Park depositional cycle. The vertical sequence of sedimentary textures and structures is produced by the progradation of supratidal and intertidal mudflat deposits over shallow lagoonal sediments.

36

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Recognition of Depositional Environments in Avon Park Strata

Although each of the ROMP cores used in this study displayed its own unique set of interpretive problems, similarities were found to exist which enabled the recogni­tion of the supratidal, intertidal and subtidal zones within the Avon Park strata.

The most characteristic feature of supratidal deposits in the Avon Park is thin, algal and sedimentary laminations. Crinkled and flat laminar morphologies similar to those described by Hardie and Ginsburg (1977) are abundantly represented in supratidal facies. Many beds consist of laminated fine and medium crystalline dolomites which pre­sumably have replaced original, finely bedded or laminated, muddy sediments. Supratidal rocks are largely mud-supported. Invertebrate skeletal remains are sparse and pellets usually represent the dominant allochemical constituent. Burrow structures are rare and, where they do occur, they stand out in vivid contrast against the well-preserved, laminated matrix of the rock. Desiccation structures are found in supratidal beds and include laminoid fenestral cavities ("birdseye" vugs), algal mat deformation structures, flat pebbles and vertically oriented cracks disrupting algal laminations and thin micritic beds. No evidence of evaporite minerals were found in the ROMP cores although small amounts of gypsum and the presence of evaporite mineral molds have been previously reported from supratidal facies within the Avon Park (Saroop, 1974; Hickey, 1976).

The boundary between rocks representing the supratidal and the underlying intertidal zones is typically gradational (Figure 13). Changes in the nature of preserved sedimentary structures and a relatively greater abundance and diversity of fossils serve to distinguish these tidal deposits. Intertidal rocks are characterized by a predominance of micrite, indicative of the low tidal and wave energies which prevailed in the depositional environment. Fossils consist of a sparse number of individuals and relatively few species (mostly foraminifera and ostracods with fewer gastropods) . These beds are further distinguished by the presence of preserved sedimentary structures including vertical burrows, open root tubules and a churned-to-wispy-sediment mottling. These features are of debatable value as primary indicators of a particular subenvironment within a shallow nearshore regime of sediments. They do, however, provide clues to the role of organisms in the paleoenvironment which may have exerted some control over sediment disrupting processes and allowing for the preservation of these structures.

Recognizable transition in fossil communities, as seen in vertical sequence, was the most useful criteria enabling the differentiation between subtidal and intertidal facies.

37

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The fauna of subtidal rocks is represented by a larger number of individuals and wider diversity of fossils than associated intertidal and supratidal deposits. A general lack of stratification in subtidal rocks may be an indi­cation of early bioturbative processes; the activities of burrowing and sediment-ingesting organisms result in the destruction of primary sedimentary structures and in the homogenization of the ambient sediment body (Shinn, 1968b; Heckel, 1972). Locally, wave and current energy was strong enough to winnow interstitial muds and reorganize the sedi­ments into flat to low angle, current laminations.

General Lithology

The general distribution of Avon Park lithologies in the ROMP cores is shown in Figures 14, l5 and l6. The lower part of the Avon Park in these cores (unit A) shows a dominance of supratidal-intertidal ("tidal-flat") sedimentation over accompanying subtidal phases. Vernon (l95l, p. 96) had described this lithology as a " ... tan to brown, thin-bedded and laminated very finely crystalline dolomite •... " Well­preserved, thinly laminated deposits, interpreted as algal stromatolites, are abundantly represented in this lowermost unit. These distinctly laminar rocks were originally described by Vernon as varve-like features with the implication that they represented slowly~settled organic and plant residues in a relatively deep body of water. Dolomitization is more extensive in this unit than in overlying strata, often to a degree such that the original microfabric of the rock is largely obscured and sometimes obliterated. Desiccation features in algal bedding structures are generally more abundant than in overlying lithologies.

The central portion (unit B ) of the formation is similar in many aspects (particularly in the ROMP core #l24 to the Avon Park unit designated by Randazzo and Saroop (l976) as Lithofacies I. Vernon's (l95l, p. 96) description of this lithology is repeated below:

Cream to brown, pasty and fragmental, peat flecked and seamed, very fossiliferous marine limestone. This bed is extremely rich in well-preserved bryozoa, foraminifers and ostracods, and the fauna is concentrated and somewhat deformed along thin beds that are interbedded with peat and more barren pasty limestone seems to give the rock a laminated and mottled appearance, to which the term 'molasses and butter' has been applied by some geologists.

The interbedding of rock types observed by Vernon may be expressed in terms of cyclic depositional patterns. Sub-tidal beds, characterized by their abundant and distinct microfauna, are interlayered with sparsely fossiliferous carbonate rocks of tidal-flat origin. The lack of a conspicuous

38

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LEGEND THINLY BEDDED AND LAMINATED MUDSTONE I~ -=---=--==1 FLATLY LAMINATED, PELLETED MUDSTONE TO PACKSTONE f I ALGAL BIOLITHITE H ROOTED AND MOTTLED FORAMINIFERAL WACKESTONE

PELLETED WACKESTONE TO PACKSTONE

ROOTED AND BURROWED MUDSTONE

BIOCLASTIC WACKESTONE TO MUDSTONE

FORAMINIFERAL-ECHINOID WACKESTONE TO PACKSTONE

FORAMINIFERAL PACKSTONE TO GRAINSTONE

D1CTYOCONID WACKESTONE TO PACKSTONE

MILlOLID i'ACKSTONE

PELOID-MILlOLlD PACKSTONE

COMPOSITE GRAIN·-MiXED SKELETAL GRAINSTONE TO PACKSTONE

FRAGMENTAL GRAINSTONE TO PACKSTONE

FRAGMENTAL PACKSTONE TO WACKESTONE

LARGE FORAMINIFERAL WACKESTONE TO PACKSTONE

NUMMULlT1D WACKESTONE TO PACKSTONE

LIMESTONE :0 DOLOMITE

.. " .. CHERT

.. • .. ... AI. ...

... ... .' .It. ... ...

Symbolic patterns for Figures 14, 15, and 16.

39

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25 -

4: ~

30- 4: U 0 a:

35- ~

W a. a.

Z ::::l

40- f2 (Jj

4e- W I--

~ -1 4: ~-<r <i -l u <r 0

55 U a:

~ r.n 0 3:. a:: 9 !.Usa r0-W :.E ~ sa-a:: 0 0

~ ro.- u

z ~

Z l-a. 75- ::::l W 0

60- Z -Q

85 ~ 2 0:::

90- p u.. III

~ ro-95 - Z CC ::::l

100-~

0 illJ 105 ~

MINERALOGY LITHOLOGY TIDAL ZONES SUI) inlenub

FAUNIZONES (from Hunter, 1976)

Nummulites willcox; Zone

- ----

SpirolocullnQ' s8l11i(lolensis:.-

Amphistsgino

pinarensis Ct:Jsdeni Zone-

-----

LltuoneJla ffontiana,

Di ctyOCOI1lJS f/oridanus,

D. caokei Zone

- ----Figure 14. ROMP well #107 (H). Mineralogy, lithology, tidal zones

and biozonations. 40

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35

40

45

50

(J)

~ 55 I-IJJ

:E 60 IJJ. a:: o <oJ 65

~

:r 70 I-

fu 0 75

80

85

90

95 < I­%

100 :::)

MINERALOGY LITHOLOGY TIDAL ZONES sup inter sub

~NIZONES (from Hunter, 1976)

Spir%QJI/no slJI11inolensis­

Ampl7isteglna plnarensis CC$denl

Zone

Lituonellc f/on'tiana,

Oictyoconus florltianus,

O. cookel Zone

110 ____ _

Figure 15. ROMP well #101 (LA). Mineralogy, lithology, tidal zones, and biozonations.

41

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5 -

10 -

15 -

20-

2.5 -

30-

65

701

75~ I

. (J) ...J

<t ...J <l: U o

<C

It: z :::>

MINERALOGY LllMOlOGY TIDAL ZONES

sup inter sub

~, FAUNIZONES

(from Hunter) 1976)

Spirolocl.llina seminolensis­

Ampnistegfna pinorensis cosdeni

Zone

Lijl.lonel/(l flon'dana,

Oicfyoconus floridanus,

O. cooke;' Zone

Figure 16. ROMP well #124 (LE). Mineralogy, lithology, tidal zones, biozonations.

42

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fauna imparts a "barren" appearance to these rocks. Por­tions of this unit have been partially or completely dolo­mitized. Dolomitization is more extensive in ROMP core #124. This portion of the Avon Park records a gradual net change in sea level. Subtidal facies predominate in the lower portion of the unit. Cycles become shorter and less regular in the upper portions of the lithology where tidal-flat carbonates dominate.

Vernon (1951, p. 96) describes the uppermost lithology of the Avon Park as a " ... cream to brown, highly fossil­iferous, miliolid rich, marine, fragmental to pasty lime­stone .... " He remarks that this unit sometimes appears as a coquina of "cones" (Le., dictyoconid foraminifera) with locally abundant specimens of the small echinoid, Neolaganum (=Peronella) dalli. The unit (unit C) is characterized by

wide vertical and lateral variations in character. This lithology, as it is represented in the ROMP core #124, bears a close resemblance to the carbonate rocks designated by Randazzo and Saroop (1976) as Lithofacies II. The unit has been extensively dolomitized in this locality. Lenses of peat and carbonaceous plant remains are abundant in the upper beds of this core. Dolomite represents only a small portion of this unit in ROMP core #107 and is absent in ROMP core #101. The preservation of fossils is typically poor in these beds and often molds or impressions are all that remain of the original tests. Subtidal deposits in the lower portion of this unit are characterized by a low diversity fauna dominated by species of dictyconid and large miliolid foraminifera. These beds become highly fossiliferous towards the top of the unit and support a diverse array of fossils.

Depositional Facies

Eleven commonly occurring subfacies representing the supratidal, intertidal and subtidal zones have been re­cognized in these cores of the Avon Park Formation. These include:

I. Supratidal rocks a. thinly bedded and laminated mudstone b. flatly laminated, pelleted mudstone-wackstone c. algal biolithite

II. Intertidal rocks a. rooted and mottled, foraminiferal wackestone b. pelleted wackestone to packstone c. rooted and burrowed mudstone d. microlaminated, bioclastic wackestone to mudstone

III. Subtidal rocks a. foraminiferal-echinoid wackestone to packstone b. foraminiferal packstone-grainstone c. dictyoconid wackestone to packstone d. miliolid packstone

43

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All lithologies have been partially to completely replaced by dolomite. The distribution of facies in the three cores is shown in Figures 14, 15, and 16. The de­positional environments and possible water depths of sub­facies in the Avon Park Formation are described in Table 5. Figure 17 illustrates the generalized distribution of depositional environments.

Ocala Limestone

In contrast to the complex cyclic sequences expressed by the Avon Park strata, the rocks of the Ocala Limestone are interpreted as products of a more constant and stable depositional environment. The presence of an abundant and diverse marine fauna signifies a transition to less-restricted, more normal, open marine conditions. In general, the change from the tidal-flat and restricted subtidal deposits of the Avon Park to the offshore, high and low energy marine deposits of the Ocala Limestone is indicative of an overall trans­gressive sequence of sedimentation.

Chen (1965, p. 76) has concluded that the sediments of the Ocala had been deposited under warm and shallow water marine conditions on a relatively flat and broad carbonate shelf or platform similar to the present day Great Bahama Banks.

The ROMP cores reveal that the Ocala Limestone is composed of two major lithologies, separating the strata into distinct upper and lower lithologic units (Figures 3 and 7). Deposits representing shallow-water, moderately high energy conditions occur in the lower unit where deposition is interpreted to have taken place relatively close to shore. These grade into slightly deeper water, lower energy deposits which characterize the upper unit. Data from the ROMP cores are most consistent with the interpretation of Applin and Applin (1944), dividing the Ocala Limestone into an upper and a lower member.

Lower Ocala Limestone

The Lower Ocala unit occupies the same stratigraphic interval recognized by the U.S. Geological Survey as the Lower Ocala, and would include the Williston and (at least part of) the Inglis designations of Puri (1957). This portion of the Ocala has been generally described as a light cream to tan colored miliolid limestone, porous, friable, microcoquinoid in appearance, often chalky and generally harder than strata characterizing the upper unit. Petrographic examination reveals that this lower unit consists predominately of cleanly washed skeletal packs tones to grains tones in marked contrast to the overlying muddier deposits of the Upper Ocala. The contact between rocks of the Upper and Lower Ocala is gradational.

44

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Depositional Facies

Three commonly occurring depositional subfacies have been identified in the rocks of the Lower Ocala, on the basis of lithology, textures, constituent grain composition and grain attributes. These are:

Peloidal/miliolid packstone (subtidal sand flat facies) .

Composite grain/mixed skeletal grainstone-packstone (detrital shoal facies).

Fragmental grainstone-packstone (intertidal shoal facies) .

These subfacies delineate distinctive depositional subdivisions of the open shelf environment. The vertical arrangement of lithologies in Lower Ocala strata is shown in Flgures 14, 15 and 16. The depositional environments and possible water depths of subfacies in the Lower Ocala Limestone are described in Table 5. Figure 17 illustrates the generalized distribution of depositional environments.

Upper Ocala Limestone

The carbonate deposits of the Upper Ocala Limestone represent a transition into progressively deeper water environments of sedimentation. A continuation of the general transgressive sequence is suggested by the presence of a deeper-water faunal assemblage and the deposition of wackestones and muddy packestones over the cleanly washed deposits of the lower unit. Megascopically, the unit is very pale orange in color. Beds are uniformly textured and range from sparsely granular to coquinoid in appearance. The rock is friable and generally softer than the lower unit.

The Upper Ocala is characterized by the presence of abundant whole and fragmented tests of large foraminifera. Specimens which have been identified from the ROMP cores include:

Nummulites willcoxi Lepidocycl~n~ ocalana Heterostegina ocalana Spiroloculina newberryensis Sphaerogypsina globula Textularia sp.

Whole tests of small, thin-shelled foraminifera, ostracods, echinoid plates and spines, small gastropids and fragments of pelecypids and bryozoans are also present. Grain sizes range from coarse silt-sized (O.04mm) bioclasts to coarse pebble-sized grains measuring from several millimeters to

45

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·:::';":':;:(;::;;\~::'·::l:\>,.,::" " , ' .. ' ~ _ \ .. , ' ',. ::~~'.:,""': ':~·i:· :':. :?III '(}t/~\~~ _ ,.' ,

INTERTlDAL 1 •• ' ··;·;·:ri:. ,:~ : .. - - '- :.. :,- -- :, :=.--_ ..c, :';'::":-, -.' AND 1 RESTRICTE0 1 ' , • '1--- =.::= -::.--::~-:-.~.>_

SUPRATlDAL 1 LAGOON I SHALLOW ~ : . -::'_'-":.y.:'.,; .. ,::., ":.

MUDFUTS 1 . ISUBTlDA.1.. ,0 1 SHALLOW SU8T1DALI . ·~~~I (INTERTIDAL I SANOF1-ATS I . ":''):'''::Y;": , I SHOALS 1 (OFFSHORE, OPEN

CYCLIC SEDIMENTATION: NEARSHORE SHELF : SHELF

Figure 17. Generalized relationship among depositional environments interpreted for the Avon Park Formation and the Ocala Limestone.

Table 5. Depositional environments and probable water depths of subfacies in the ROMP cores.

FACIES

a. ntiHl.'f BEDOED AND LAMINATa) MUDSiONE I b. F1..ATLY U."INATE'O,r~..uTED MUDSTONE

TO ~CXS'TONE Co AI.GA.L. BIOLiTHITE

a. ROOTED AND MO~D FORAMINIFERAL WACKESTONE

IT b. FEl.l.tTED WACXESTONE TO PACKSTONE C. ROOTED AND BURROWED MUDSTONE d. 310CLASTIC WACXESTOHE TO hlUDS'TOHE

a. FORAhlINIFE.:tAL. -!0IIN010 WACXESTONE TO P*.CXSTONE

rrr b. FORAhllMIFERAL PACXSTONE TO GR4INS'TONE C. OICT'1OCONID WACXESiONE TO PI\CXSTONE Ii IiIIUOl..1D PACXS"':"CNE

a. PE1.0ID-..aI..IOl..ID ~CXSTDNE

ITo. 'COMPOUND GRAIN-MIXED SKEI.£TAI. GRAINSTONE TO ~CXS1ONE

C~ FRAGMENTAL GRAINSTONE TO P*.CXS'TCNE

O. FRAGhlENTAL. ?!lCXSTOHE TO WACXES'TCNE

Y b. L.ARGE, PER~RATE FORAMINIFERAL WACXES"':"CNE TO ~CKSTONE

c. HU .. hlULlT10 WACXESTOHE TO PACXSTONE

46

ENVIRONMENTS OF OE?OSIT10N AND ?~OBABL:: WAnF! OE?T'HS

r INTERTIDAL MUDFLAT

.~ PRCT£CTEO, SHAU.O""WAToR L.AGOON. WATER DEPTH L~SS THAN 10 METERS.

}- SHALl.OW SUBTIDAL SANOF1...ATS. WATER OEPTH L£SS THAN to METERS

~ !'H ALLOW SUBTI DAl.. TO INTERT1DAI. SHOAL.S. W4Ta! OEPT'''' O-iO METtRS.

~ OFnHORE, OPEN SHE1.F. WAitH OEPTH GREATtF! THAN ~ METtRS.

\I. ...J LI.I --en LI.I c::: 0 :::: en c:::

'" LI.I ..J z

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a few centimeters or more along their long axis. Some of the larger tests have been broken and extensively frag­mented; however, they show little signs of mechanical abra­sion in that grain boundaries tend to be ragged or angular in shape. The evidence suggests that most fragments were derived through biologically controlled processes. Some large burrow structures are encountered which are probably of crustacean origin. These are preserved as open tubes which show a characteristic dense micritic lining. Similar burrows have been attributed to crustaceans by Shinn (1968b).

The upper unit is characterized by a largely mud­supported fabric. The matrix is composed of micrite and very finely crystalline microspar often cluttered by silt-sized bioclasts. It sometimes displays a clotted fabric although discrete pellets are rarely observed.

Depositional Facies

The limestone of the Upper Ocala can be subdivided into its component subfacies. Three primary depositional sub­facies are recognized. These are:

Bioclastic packstone to wackestone Large perforate foraminiferal wackestone to packstone Nummulitid wackestone to packstone

Subdivisions were based primarily upon paleontologic criteria including the relative abundance and identity of the various faunal constituents. The vertical arrangement of lithologies in Upper Ocala strata is shown in Figure 14. The depositional environments and possible water depths of subfacies in the Upper Ocala Limestone are described in Table 5. Figure 17 illustrates the inferred distribution of depositional environments.

BALLAST POINT, BRANDON, AND DUETTE CORES

Drill cores, located at Ballast Point and Brandon in Hillsborough County, and at Duette in Manatee County, were obtained from the Florida Bureau of Geology. These cores penetrate the "Tampa Formation" and Suwannee Limestone (Figure 18).

The Suwannee Limestone is of Oligocene age, and defined as a biostratigraphic unit rather than a lithostratigraphic unit. In the area of study, it is lithologically conformable with the underlying Late Eocene Ocala Limestone. An un­conformity is present toward the north in Hernando County (Yon and Hendry, 1972, p. 30) and Citrus County (Vernon, 1951, p. 177). The pelecypod ~~usium ocalanum, which is apparently present at and below the depth of about 116m from sea level within the Ballast Point core, is utilized herein as a biostratigraphic marker of the Upper Ocala (McCullough, 1969, p. 1) .

47

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Because the Suwannee Limestone and Tampa Formation have been defined on the basis of biostratigraphic evidence, confident recognition and correlation have been difficult at best. These traditional formational names are invalid on the basis of the Code of Stratigraphic Nomenclature (Randazzo, 1976a). King and Wright (1979, p. 1605) have defined the Tampa Formation from biostratigraphic and geochronologic criteria. The Tampa Formation is lithologically similar in places to other "post-Suwannee" units. The Tampa Formation described in the cores studied may actually be the Hawthorn Formation. Because of the uncertainity of its identity, references made to the Tampa Formation in the remaining part of this report will be offset in quotation marks.

Stratigraphy

The regional stratigraphy of the "Tampa Formation" and Suwannee Limestone is summarized by Applin and Applin (1944) and Puri and Vernon (1964).

The Ballast Point, Brandon and Duette drill cores were used in making a northwest-southeast correlation of the "Tampa Formation" and Suwannee Limestone (Figure 18). The correlation was done on the basis of petrographic simi­larities between cores. As an aid to correlation, vertical variation diagrams of orthochemical, allochemical, and terrigenous constituents were constructed and compared with one another. Although neomorphism and silicification are locally intensive within the cores, orthochemical components (micrite, microspar, pseudospar and sparite) and ghosts of allochemical grains are generally recognizable. Among these diagrams, the similarities in vertical variation of contents of fossils, pellets, and micrite (and microspar) among cores proved most useful in correlation.

From the correlation, a composite vertical sequence (Figure 19) was constructed to show the changes in lithology and stratigraphic relationships of the various lithologic types. There are four lithofacies distinguished from one another by macroscopic appearance. These lithofacies are further divided into 19 subfacies on the basis of microscopic and macroscopic attributes. The distinctions among these subfacies reflect differences in the energy levels represented (Figure 19, Table 6).

Stratigraphic Interpretation

Petrographically derived subfacies classifications are listed in Figure 19 with their inferred depositional environments (i.e., supratidal, intertidal, shallow subtidal and deep subtidal) .

The sequence is subdivided into four distinct litho­facies representing different environments of deposition.

Lithofacies I together with lithofacies II makes up the major sequence of the Suwannee Limestone which has a distinct

48

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~ \.0

SE A LEVEL BALLAST PT. o

If)

0: W t- t- 30 w ~

60

~-.L~...I;: -- .]:-- - _ T-

IX

r -=r

.~

BRANDON

" "

KEY

'-'­

'-.......

o 6 12 L I I

KILOME TERS

'-'-

....... , " .......

'-

IHf1tllimestone of "Tampa Formation"

~ Suwannee limestone

~~jJ Crystal River Formation

D Part of core not studied

fV\N'J\ Disconformities

DUETTE

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

'-....... ~\.;

....... ~.L-~J.;.l

Figure 18. Stratigraphic sections and correlations of the "Tampa Formation" and Suwannee Limestone.

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I § '.~ .. I - g ,~ '0 . - g, E 0 -

- I "" ~J "&151~o G~ 5 I ~ I :;r;1 ~. ~~- ~

I

I ~, ~

I : ,~

I ...... i w

I ,-:'

~ r~· ~ I-

'" w 1m it ;j -l I ~ u I w w I ~ z z iP ~

I :J tr)

~ ~ H~

II I

Deposltionot Environment s

I-I --'

..J --'I g

~ ~ 1<" ;::

!- ~ 0.0 ~ i ~ ~~ <l-

"" ::) ... :::::::l "" '" ~ lIlCl1 C

I :: ... I

I I I T

l.

I r

1:

·1

I ~ T

..L

- I T"

i I 1:

I f 1

,

KEY

IlIOSPA""TE TO POORLY WAS>t£C SlOSl'O.RI TE

, .

SANOY INTlI .. ""OI. TE

INTRAMiC/IUl)'TE

00l.0""TI2EO :»No. "IC~IT(

A~GAI. 8101.ITHlTE

015(;ONFOII'" T T

M.CRO . OISCO"FO"M' TV

~

~ I~~v'v'l v'v' o []

I I

A~o.'U"""TE 1 ... tf.:JHJCSS 1:1 r-;:--l _____________________ ,_£_C1_�0_~_._,_._5_~_l_!_ER_5 _______ e~_._5_T&_~ __ ~_IV_ .. _R __ .O_~_M_~_T_'O_N_ ~-.J I

Figure 19. mation" Ba 11 ast

Generalized columnar section of the "Tampa For­and Suwannee Limestone, based on data from the Point, Brandon, and Duette cores.

50

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Table 6. Characteristics ·of depositional environments of the "Tampa Formation ll and Suwannee Limestone.

ENVIRONN£NTS

CHARACTERI STI CS SUPRATIDAL INTERTIDAL SHALLm~

SUBTIDAL DEEP

SUBTIDAL

Foss i 1 fragments )t •••••••••••••• ~ - - ---+----~~-----i

Intraclasts '-----.....;..----....:i----- -~ ~ ......... . Pellets ~ ..... ••••••• 41. ~ ~- ----+-----..01-- _ - .. ~-

~ Terrigenous quartzl-____ ..... _____ -!~- ____ ~ ••••••••••••

5 ~&~P~OO~s~p~h~a~t~e~s~a~n=ds~----------+_--------_+----------+_----------j §E Terri genous !:: cl avs J... ---- ---i---.... ~ -- ........... ~----~ .....J

)-.' ----....:---- -_ ... ~ -_ ....... ' ......... ~----~ Micrite

Sparite f-." •• • • • • • • • - ... - .-. --..;-----~- -.-- ••• "1 Do 1 omi te '-----~- - - • • • • • I

~~------------~--------~---------+--------~~--------. ~~cl~a~ib~~dse'lesl I - - - ••••••.•••• ~ ........... ,

~. Hor;z~~~~~ws ............ ; ................ ~ _____ ( ~

~ SWl r 1 eo and ver-:-____ ~- _____ ................ 1 ••••••••••• 5 ti ca 1 burrO\,iS 1 c::::: I ~ Micrite-Vl envelopes

Peat seams

Arti c. cora 1-1 i nes & codiaceans Dasyclad ca 1 ci spheres l:.ncrustose Coral lines

>-C.!l Mo 11 us cs o

I j ...... ~.--. -----~--. - - 1" ........... . I

~ ............ III r- - - ~ .... - --,....-----! I - - -I-------! .- - .-.. ------ ..

•• III •••• 4· ••••••••• f------..., I

.. ' ... ·+0-----...,1· ..... ................ - ~ ........ - ~ -+------~

5 ~S~o~l~i~t-a-ry-------4-----------+-------~----+-----------+L--------~ ~ 1--__ ~c;;.or;..;a~l:..;;s~ __ _+_-------_i_I----'-·-··-· .,-"-"--+-" -. -" -" -" -" -" ---··-"+r--------------I1' Uoi Cl 'l I j

~ ~o--0_n~~~~r~a~i~s~ __ .41---------+----------+r----·---------------+1_._ .. _._._. ____ ~~ Bryozoans ! .. · · · +- - - - _. _I "'1

j t I !

jl Algal ~--~---. II I L-~ __ ~st~r~u~c~t~u~r~es~~ ___________ ~ _________ ~ ___________________ ~

---- Abundant --- Common ........ • Rare

5l

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Late Oligocene fauna. This section can be subdivided into nine subfacies. Lithofacies III is the top unit of the Suwannee Limestone and is subdivided into three subfacies. This lithofacies is interpreted as a shoreline sequence with oscillations in sea level which cause changes in the type of deposition from time to time. In all, the Suwannee Limestone consists of lithofacies I-III which recorded a general transgressive phase with minor episodes of regression, and lithofacies III recorded a general regression with a later transgressive phase.

The boundary between the Suwannee Limestone and the overlying "Tampa Formation" is represented by a discon­formity, indicating sub-aerial exposure and erosion. This may be related to the Ocala Uplift which Vernon (1951) dated as post-Oligocene in age.

Upon an erosional surface was deposited lithofacies IV of the "Tampa Formation" which is typical of the Lower Miocene Series and characterized by a lithology and fossil assemblage different from that of the underlying beds. Lithofacies IV, making up the limestone portion of the "Tampa Formation" studied, is subdivided into seven subfacies and represents two carbonate cycles of sedimentation.

Diagenesis

The most important processes of diagenesis within the varied facies of the Ballast Point, Brandon and Duette are those of calcitization, dissolution, micrite-envelope formation, micritization, compaction, cementation, neomorphism and replacement (dolomitization and silicification). All of the microfacies found within the cores reflect the effects of several of these processes, acting either simultaneously or sequentially (Liu, 1978). Table 7 lists the prominent diagenetic features developed in the Suwannee Limestone and the "Tampa Formation."

There are only four common porosity types present in the Ballast Point, Brandon and Duette cored sections, although nearly all porosity types described by Choquette and Pray (1970) were encountered. These four basic types are inter-particle, intraparticle, moldic and non-fabric-selective vugs. Usually a combination of different pore types is found in a subfacies (Figure 20). The visible porosity in different microfacies varies mostly between 3 to 22% (yigure 21). Sizes of pores generally vary from micropores (less than 0.06mm) in interparticle to intraparticle types to small megapores (less than 32mm) in vugs, molds and inter­particle types.

Textoris et al. (1972) felt that dissolution of the skeletal allochems, especially in sediments with interparticle pores, would provide for the reprecipitation of low-Mg calcite nearby as cement and at times invert location of

52

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Table 7. Prominent diagenetic features displayed within the subfacies of the SUvJannee Limestone and !tTampa Forma ti on. II

Subfacies Diagenetic Features

IVg ExtensivE recrystallization of micrite to microspar. Dissolution of fossils. Two generations of cementation.

IVf Dissolution of fossils and infilling of the resultant molds with two generations of sparry calcite cementation.

Fossi1s commonly coated by micrite envelopes.

rVe In Ballast Point and Duette cores: completely do10-mitized with preservation of original texture; oartial coalescive neomorphism of dolomite with ~riginal texture oblitera~ed or destroyed.

In Brandon core: partly dolomitized by coalescive neomorphism without preservation of original texture.

IVd In the lower portions; extensive recr'ystall;zation of micrite to microspar and pseudospar; partial replace­ment of detrita1 q~artz grains by sparry taleite.

IVe Mieritized allochems coated by micrite envelopes. Dissolution of fossils. Partia1 replacement of sparry calcite cement by silica

(cl1ert) .

IVb Dissolution of fossils and infilling of the resultant molds Ivith sparry calcite cement.

Fossils coated by micrite envelopes.

IVa Allochemical grains coate<! by micrite envelopes. Two generations of cementation.

II Ie Allochems coated by micrite envelopes.

rIIb

Two generations of cementation. In Ballast Point core, partial replacement of echinoid

fragments by silica (chert).

Dissolution of fossils. In upper part of subfacies: In middle part of subfacies:

of micrite to microspar. In lower part of s ubfaci as:

crysta 11.1 zati on of rni cri te

53

high degree of silicification. extensive recrystallization

partial silicification; re­to microspar.

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Table 7. (continued)

Subfacies Diagenetic Features

IlIa A high degree of micritization completely destroying the original algal cellular structure (Banner and Wood, 1964).

Cementation of fenestral pores by sparry calcite.

lIe Recrystallized fOS$11 grains, especially dasyclad algal segments.

Dissolution of fossils and infilling of the resultant molds with sparry calcite cement.

In Brandon core, high degree of silicification.

lId Micritized allochems. High degree of cementation developed in intergranular

pores. Compacti on and mecnani ca 1 breaJ<do\'Jn of allochems.

IIc Dissolution of fossils and infilling of the resultant mo 1 ds \vi th spa rry ca 1 ci te cement.

Micritized allochems.

lIb High degree of micritization of allochems. Dissolution af fossils. High degree of cementation developed in intergranular pores.

IIa In upper section: micritized allochems coated by micrite envelopes; high degree of cementation; partly silicified echinoid fragments; al1och~ms rim-encrusted by calcite crystals in Ballast Point core.

In lower section: micritized allochems coated by micrite en­velopes; partly silicified echinoid fragments.

Id Recrystallized fossil grains. Slight pyritization in the micrite matrix.

Ic Micritized fossil grains. High degree of cementation developed in inter- and intra­

granular pores. Slight replacementcf allochems by silica (chert).

54

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Table 7. (continued)

Subfacies Diagenetic Features

Ib Micritized allochems.

1a

Very high degree of cementation developed in inter­and intra-granular pores.

Micritized a1lochems. High degree af cementation. Partial replacement af echinoid fragments by silica

(chert).

55

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>­I--V1 o c::: o a.. -l c:t: I­o I-

w.... o I..LJ <..!:l c:t: I­z I..LJ U c::: I..LJ a..

Suwannee Suwannee Bi ospari te Subfacies Biomicrite Subfacies (Subfacies Ic) (Subfacies II Ic)

60

6°1 34.9

45 45 II

37.5 I I 11

~ I I 11

30 30 I II!

19.6 I I I

15,7 1 I

13 J5 12.5 I III - - 9.8 I /1 - - - . I I

~- 1 I Q....J rn § Q 0 0

0 m ~

o

Suwannee Biomicrite Subfacies (Subfacies Ire)

48.0

20.0

"Tampa" Sandy Clayey Micro­sparite Subfacies (Subfacies IVd)

53.9

15.4

3C.e t I II I I II m W 9.0

~ ~

::1 30~ 15 m--= = -- -oj ==-=

I I ill I II

LLL.G Suwannee Biosparite (Subfaci es

24.5 --

Subfaci es Ira)

50.8

40

16.9 20

-

7.7

o Inter- Intra- Moldic Vug and

Particles Channel TYPES OF PORES

"Tampa ll Pelecypod Biomicrudite Subfacies (Subfacies IVb)~ 1.3

6.3 5!l

Inter- Intra­Particles

\ \1

\1

12.S [lTiI] ~

~·101d;c Vug and Channel

Figure 20. Major types of pores and relative percentages in selected subfacies of the "Tampa Formation" and Suwannee Limestone.

56

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SEA BALLAsr POINT LEV E L 0 r---"'r-r--,....--,....--r-r--1"--'

CORE BRANDON DUErrE

(Jj

a:: I.J.J

f-w ~

Z -I f-a... w a

30

\ 50 ~

~ 60

110

:20

130

140 1 I I o :0

I

I [

( ! ! ! I ! I 1 I ! , ! , ! I

20 30 40 0 10 20 30 40 0 PERCENT POROSITY

I I 1 I

20 30 40

Figure 21. Variations in visible porosity in the "Tampa Formation" and Suwannee Limestone.

57

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original pore spaces. The moldic porosity type (maximum up to 17% of a rock unit) is completely controlled by the abundance of original skeletal allochems which have been dissolved. This pore type is common in both sparites and micrites (Figure 20 ). Skeletons may be completely dissolved and the resultant voids subsequently solution-enlarged, or filled with low-Mg sparry calcite to varying degrees.

Interparticle pores are common in sparites. The pore spaces may reach as high as 21% in volume. Original porosity was higher and dependent on packing and shapes of allochems. Interparticle porosity still exists commonly in mollusc­bearing beds (Figure 20) because of the differences in the shapes of fossils and packing. The intraparticle pore type may be found in any skeletal grains which originally had open chambers (foraminifera and bryozoans). It is common in the various rock types of both sparites and micrites, and may be as much as 5% of the rock volume.

The two rather common, non-fabric-selective vug and channel pore types are grouped together due to their normally common genesis. Most vugs are probably solution-enlarged molds, and some channels are often simply connections between them. Vug porosity may reach as much as 24% in rock volume. Many channels are solution-enlarged cracks of various origins, including fractures. Vugs and channels may be in various stages of infilling and are more common in the micritic subfacies.

GEOCHEMISTRY

The Na+ and Sr 2+ concentrations and mole-percent-MgC03 of selected pure calcite or dolomite samples from 16 of the Eocene carbonate rock cores studies (Figure 1) were measured and the results were used to evaluate a model of diagenetic dolomitization. In the process of evaluating this model of dolomitization, an attempt was made to delineate new, and/or confirm reported relationships between Sr 2+ and Na+ and mole-percent-MgC03 and the latest diagenetic solution affecting the rocks. The geographic position of each core and its Na+ and Sr 2+ content were considered in determing the affect, if any, of groundwater flow patterns of the Floridan Aquifer on the Na+ and Sr 2+ content of diagenetic carbonate minerals formed within the aquifer. The distribution of Sr 2+ was evaluated to determine if the original mineralogy of the sediments has any control over the present diagenetic products.

The geochemical significance of Sr 2+ and Na+ in calcite and dolomite stems from their ability to substitute in the cation layers of the crystal structures of these minerals. The calcite crystal structure consists of alternating layers of Ca 2+ ions and C03 2- radicals. The dolomite crystal structure has alternating layers of Ca 2+ and Mg 2+ ions with layers of CO 32- radicals in between them. High-Mg calcite

58

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has more than 4 mole-percent-MgC03 and low-Mg calcite has less than 4 mole-percent-MgC03. Dolomite in carbonate rocks of the Floridan Aquifer has been classified as more nearly stoichiometric if it contains 44-50 mole-percent-MgCo3 and non-stoichiometric if it has 39-43 mole-percent-MgC03 .

The geochemical significance of ions such as Sr2+, Na+, and Mg2+ in the formation and diagenesis of carbonate rocks has been discussed by a number of authors (Odum, 1957; Kinsman, 1969; Beherns and Land, 1972; Land and Hoops, 1973; Viezer and Demovic, 1974; Folk and Land, 1975; Randazzo and Hickey, 1978; Sarver, 1978, Metrin, 1979).

Odum (1957) reported Sr2T/ca2+ ratios in ancient rocks much lower than materials of modern sediments, suggesting that the sediments had been replaced. Kinsman (1969, p. 487) stated that "The Sr2+/ Ca2+ ratio of a precipitating solution plays a dominant role in determining the Sr2+ concentration of precipitated carbonate minerals."

Kinsman (1969, p. 501) also found that calcites pre­cipitated from a single solution in the down flow areas had higher Sr2+values than those formed in the up flow areas. Beherns and Land (1972, p. 159) proposed " .•. that if the Ca-planes in dolomite behave like calcite and the Mg-planes exclude the larger Sr2+ion nearly completely, dolomite should contain approximately half the amount of strontium as would a calcite co-existing at equilibrium ."

Land and Hoops (1973, p. 613) indicated that, "The bulk sodium content of carbonate rocks is a crude but useful indicator of the salinity of genetic and diagenetic solutions," and that Na+ substitutes with equal facility into Ca2+ and Mg2+ lattice positions in dolomite. Viezer and Demovic (1974) suggested that the Sr2+ content of carbonate rocks is facies controlled with the high Sr2+ concentrations inherited from predominately aragonitic sediments. Folk and Land (1975) found that lower salinities enabled more stoichiometric dolomite to form because of the lower concentrations of other ions competing with Mg2+ and Ca2+ for sites in the dolomite crystal structure.

In this study, a model of diagenetic dolomitization involving the mixing of fresh and salt water to produce a dolomitizing solution as addressed by Hanshaw et ale (1971), Badiozamani (1973), and Randazzo et ale (1977) was evaluated. This zone of mixing can occur at the interface between fresh and saline subsurface waters in a coastal regime and farther inland in a brackish zone (Hanshaw et al., 1971; Land, 1973). This model has been proposed for a " ... later and continuing stage of dolomitization ... " for the Eocene Ocala and Avon Park rocks of the Floridan Aquifer (Randazzo et al.,1977).

59

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The sixteen cores analyzed include the Bell (B-­Gilchrist County), Rainbow Springs (RS--Marion County), Cotton Plant (CP--Marion County), and eight from Gulf Ham­mock (GH--Levy County) as well as ROMP #101 (LA--Lake County), #107 (H--Hernando County), #124 (LE--Levy County) and Homosassa Springs (HS--Citrus County) and Manatee Springs (MS--Levy County). Detailed geology for these cores is contained in Stone (1975--GH cores) ,Hickey (1976--B, RS, CP cores), Zachos (1978--HS, MS cores), and Fenk (1979--LA, H, LE cores). Pure calcite and dolomite samples from these cores were analyzed by X-ray diffraction and atomic ab­sorption spectrometry.

Samples were ground .to a powder, dissolved and diluted to appropriate volumes. Analysis was done on a Perkin-Elmer 403 Model Atomic Absorption Spectrophotometer. An Amdahl-470 V62 model computer was used to calculate parts per million (ppm) of each element analyzed. Magnesium data were calculated in terms of mole-percent-MgC03' The data were plotted to determine significant trends. Methods for quanti­tative analysis of Sr2+, Na+, Mg2+ and Ca2+ are presented in Sarver (1978) and Metrin (1979).

Importance of Sodium

The most abundant cation in sea water is Na+ (Land et al., 1975). The partitioning coefficients of Na+/Mg2+ and Na+/Ca2+ for natural carbonate has not been defined suffi­ciently to be employed confidently in the determination of the paleosalinity at the time of deposition. However, the overall Na+ content may be a rough indicator of the salinity of the latest diagenetic fluid (Land and Hoops, 1973; Viezer et al., 1978). In the model of diagenetic dolomitization evaluated in this study, dolomitization occurs in the mixing zone of fresh and saline subsurface waters. In Florida, the coastal mixing zone is formed at the interface of the fresh water lense and sea water at the depth based on the Ghyben­Herzberg relation (Walton, 1970) (Figure 22). Sea water in the vicinity of Miami Beach has 10,970 ppm of Na+ and fresh ground water from the interior of the Florida peninsula has combined Na+ and K+ concentrations ranging from a few parts per million to about 40 ppm (Stringfield, 1966, p. 155). In the zone of mixing, the Na+ concentration will be directly proportional to the degree of mixing (Badiozamani, 1973). Land and Hoops (1973) pointed out that the trapped and absorbed Na+ could affect the determination of the Na+ in the crystal structure of calcite and dolomite. However flushing of the Floridan Aquifer with fresh ground water would remove most of the Na+ ions released into solution during the dissolution and replacement of the original sediments.

60

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I

- ---

, I:'~

.. 0. ~. .

. . , ,

.'

/. J. . ,

.. . ... . ,_ .... - ..... . . . . .

.. .. .. ' ... . '.' . . .... : .... ;,'

~ ~ .. .. ... .. .. , ..... ~ . , :,' . , . .. - ~ ... . , .. -ft""'·· .. - ~ .. - ... ('.. '. ..-.. ... ..

• 0 •

o· .. .. .... - ... - •• f' ..

"

" .--. : ..... - .. . . . .

Figure 22. The coastal zone of mixing environment of peninsular Florida.

61

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Sodium in Calcite

Removal of aragonite and high-Mg calcite sediments from the marine environment where they are stable, can bring about their replacement by more stable low-Mg calcite and/or dolomite. Whether the change is to low-Mg calcite or dolomite is dependent upon the mineralogy of the original sediments, early-formed diagenetic products and the chemistry of the diagenetic environment. Pure calcite samples from the cores in this study have a Na+ content range of 37-970 ppm and an average of 196 ppm (Table 8). Land and Hoops (1973) reported Na+ concentrations of 1,010 ppm or more for Holocene marine carbonate sediments (Table 10). Data from these two studies show a lower Na+ content for the Eocene carbonates, indicating that the formation of low-Mg calcite took place in an environment less saline than sea water. This environment may have been the inland brackish zone of the Floridan Aquifer or the coastal salt/fresh water interface.

Sodium in Dolomite

Land and Hoops (1973) suggested that Na+ is able to substitute into the Ca2+ and Mg2+ positions of the dolomite structure with equal facility. Therefore, the Na+ concen­trations of calcite and dolomite may be compared directly. The replacement of aragonite and high-Mg calcite by dolomite reflects the environment of diagenesis. TheNa+ concentration of the dolomite studied ranged from 50-1,963 ppm, with an average of 887 ppm (Table 9). Comparison of this average with the 196 ppm average Na+ content of the calcites, indicates that the dolomites were precipitated in a more saline environ­ment than the calcites. When the Na+ concentrations of the Eocene dolomites in this study are compared to modern marine dolomites with Na+ concentrations of 2,000 ppm and more (Land, 1973; Land and Hoops, 1973), a diagenetic environment considerably less saline than sea water is indicated.

These low Na+ values for the calcites and dolomites in this study indicate that even slight mixing of hypersaline and fresh water can cause dolomitization. A modern dolomite with 2,000-5,000 ppm Na+ can form by reaction with hypersaline brines, but, if 5-30% of the brine is mixed with fresh water to form a more dilute dolomitizing solution, the resulting dolomite will have only 100-1,500 ppm Na+. Badiozamani (1973, p. 769) stated that 5-30% sea water is enough to cause undersaturation of CaC03 and oversaturation of CaMg(C03)2. Land (1973) found that as little as 3-4% sea water would also cause oversaturation of CaMg(C03)2, resulting in dolo­mitization.

Vertical variation diagrams of Na+ concentrations are presented by Sarver (1978) and Metrin (1979). They show higher Na+ concentrations in the lower portion of the LA, H, HS, B, CP, RS and several of the GH cores. LE and MS cores show higher Na+ values in the center region of the core. The MS core also has higher Na+ values in its lowest portion. These areas of higher Na+ values could be the result of diagenesis by solutions more saline than that which affected other portions of the core at other times.

62

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Table 8. Sodium and Sr2+ contents of calcite for all cores

Calcite

Nos of Strontium ppm Sodium EEm Cores Samples Range Mean Range Mean

LA 31 279-968 505 140-432 238

H 23 89-520 285 37-296 143

LE 5 264-507 388 150-970 389

HS 8 274-546 359 138-735 295

MS 9 259-408 322 125-286 189

GH 21 244-639 486 71-843 202

RS 6 297-444 366 85-176 136

CP 11 297-639 377 85-152 113

B' 7 279-368 322 77-127 96

63

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Table 9. Sodium and Sr2+ contents of stoichiometric and non-stoichiometric dolomite for all cores.

Dolomite

Strontium ppm

39-43 mo1e-percent-MgC03 44-50 mo1e-percent-MgC03 Group GrouE

Nos of Nos of Cores Samples Range Mean Samples Range Mean

LA 11 215-319 256 0

H 6 215-314 246 1 254

LE 14 201-370 246 6 150-211 174

HS 13 192-334 246 17 141-183 152

MS 20 201-289 234 9 132-173 149

GH 50 210-403 282 8 258-354 314

RS 12 170-262 236 17 132-157 140

CP 11 210-279 240 2 162-183 173

B 12 201-275 229 10 119-266 159

Sodium ppm

39-43 mo1e-percent-MgC03 44-50 mo1e-percent-MgC03 Group Group

Nos of Nos of Cores Samples Range Mean Samples Range Mean

LA 11 363-1,075 690 0

H 6 548-1,285 754 1 984

LE 14 557-1,088 810 6 344-862 581

HS 13 897-1,767 1,143 17 223-1,963 1,134

MS 20 290-1,527 907 9 281-599 377

GH 50 566-1,520 975 8 857-1,323 1,037

RS 12 504-1,329 1,061 17 205 ... 889 335

CP 11 517-867 701 2 50-693 372

B 12 766-1,426 1,029 10 233-1,575 556

64

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Table 10. Sodium values in carbonate rocks reported in previous works.

Description and Reference

Modern marine reef calcite and aragonite sediments (Land and Hoops, 1973)

Holocene dolomites (Land and Hoops, 1973)

Pleistocene dolomite of Jamaica Pleistocene calcite of Jamaica (Land, 1973)

Eocene dolomite of Egypt (Land et al., 1975)

Sodi urn . (ppm)

1,140-2,520

l,010-3,050

400

less than 200

213-475

Table 11. Strontium values in carbonate rocks reported in previous works.

Description and Reference

Modern marine reef calcite and aragonite sediments (Land, 1973)

Modern dolomite from the Persian Gulf (Land and Hoops, 1973)

Pleistocene dolomite of Jamaica Pleistocene calcite of Jamaica (Land, 1973)

Carboniferous dolomite of Northumber­land, England Carboniferous calcite of Northumber­land, England (Al-Hashimi, 1976)

Eocene dolomite of Egypt (Land et al., 1975)

Platteville Formation, Ordovi.cian, dolomite Platteville Formation, Ordovician, calcite (Badiozamani, 1973)

65

Strontium (ppm)

1,200-4,140

640

220

440

132

618

90

37

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Importance of Strontium

Kinsman (1969) has concluded that the Sr2+ concen­t:ation of precipitated carb~nate minerals,i~ ma~nly dete:­mlned by the Sr2+/ Ca2+ ratlo of the preclpltatlng Solutlon. The crystal structure of individual carbonate minerals also affects the Sr2+ concentration. The Sr2+ content of sea water is higher than that of fresh ground water. Odum (1951a, b) reported an average Sr2+ concentration of 8.1 ppm for sea water and less than 1 ppm for fresh ground water of Florida. Because of this difference in Sr2+ content between sea water and fresh water, carbonate minerals may reflect the relative degree of mixing of fresh and saline subsurface waters in the environment that produced them. Kinsman (1969+ p. 488) found sea water to have a fairly constant Sr2+/ ca2 ratio of (0.86±0.4) X 10-2 except where influenced by continental waters nearshore. The Sr2+ content of sea water can be concentrated in the supratidal environment to a Sr2+/ Ca2+ range of (0.8 to 1.2) X 10-2 over a range of solutions from normal sea water to nine times concentrated sea brines (Kinsman, 1969, p. 490). Kinsman (1969, p. 490) reported a

median Sr2+/ Ca2+ ratio of 3.2 X 10-2 for surface continen­tal waters. Subsurface continental waters have a wide range of Sr2+/ Ca2+ values depending upon the mineralogy of the rocks through which they flow, but are generally less than sea water (Kinsman, 1969, p. 490).

The Sr2+ in carbonate rocks is also reflected in the mineralogy present. The aragonite crystal structure can accommodate high amounts of Sr2+ (Bathurst, 1975, p. 241). Modern marine sediments are composed chiefly of aragonite and high-Mg calcite and have average Sr2+ concentrations of more than 3,000 ppm (Land, 1973). The carbonate minerals in ancient carbonate rocks are mostly low-Mg calcite and dolomite, which do not incorporate as much Sr2+ as aragonite (Bathurst, 1975, p. 241). Table 11 presents Sr2+ values for some modern and ancient marine carbonates. The ancient carbonate rocks have much less Sr2+ than the modern carbonate sediments, indicating later and continuing diagenesis in water less saline than sea water. The lower Sr2+ content of ancient marine carbonates also reflects the lesser ability of calcite and dolomite to incorporate Sr2+ into their crystal structures than aragonite.

Strontium in Calcite

The partitioning coefficients for precipitated aragonite and calcite are determined by the Sr2+/ Ca2+ ratio of the minerals formed. The partitioning coefficient for calcite is KSr=0.14±0.02 at 25 COand is dependent upon faunal mineralogy as we 1 as temperature (Kinsman, 1969, p. 488). Based on the Sr2+/ca2+ ratio of sea water, and the calcite partitioning coefficient, calcite precipitated from sea water should have about

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1,200 ppm Sr2+. Calcites with less Sr 2+ would have been precipitated from water less saline than sea water. This would occur in sediments in an open system with solutions less 2saline than sea water. The calcites in this study have a Sr + concentration range of 89-968 ppm with an average of 403 ppm (Table 8). This is comparable with other values reported for ancient marine limestones (Table 11).

Strontium in Dolomite

Synthesis of dolomite at low temperatures has never been accomplished because of very slow kinetics involved in its formation (Berner, 1971). Therefore, the partitioning co~fficient is undefined and cannot be used to predict the Sr + content of dolomite precipitating from sea water as was done with calcite. However, as stated earlier, the dolomite crystal stru~ture should contain approximately half the amount of Sr + as would calcite precipitated from the same solution (~ehrens and Land, 1972). This would be approximately 600 ppm Sr +, a value supported by Land and Hoops (1973, Table 4).

The Sr~+ values of the dolomites in this study range from 119-403 ppm with an average of 239 ppm (Table 9). This suggests that the latest diagenetic solution in which the do~~mite formed was less.sali~e than sea water. The average Sr content of the calc1tes 1S 403 ppm. The Sr 2+ content of the dolomites is approximately 59% of that in the calcites. This is some 9% more than the amount predicted by Behrens and Land (1972) based on the incorporation of Sr~+ into the dolomite crystal strQcture. Land (1973) stated that this difference in the Sr l + content was caused by an outside source and suggested sea water as that source. This would indicate a greater relative amount of salt water present in the dolomitizing zone and is compatible with the Na+ data, indiciating a more saline diagenetic environment for dolomite than calcite. Calcites show considerably higher Sr2+ concentrations than the dolomite~ because of the greater ability of calcite to incorporate Sr + into its crystal structure.

Strontium and Sodium in Relation to Mole-Percent-MgC03

The relationship between the Sr2+ and Na+ concentrations and the mole-percent-MgC03 of dolomite was a means by which the diagenetic model of dolomitization was evaluated. The same data for calcite was used to interpret the chemistry of the diagenetic environment. In the Floridan Aquifer, there is a brackish zone between fresh and saline phreatic waters (both saturated with calcite). This brackish zone is under­saturated with calcite and supersaturated with dolomite (Hanshaw et al., 1971). Dissolution of calcite can occur in the undersaturated zone and physical mixing of the brackish waters can cause CO2 to degas, enabling dolomite to precipitate.

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Badiozamani (1973) found that when under saturation of calcite and degassing of C02 occur, a solution with as little as 3-30% sea water can cause replacement by dolomiti­zation. Folk and Land (1975) proposed that with a Mg2+/Ca2+ ratio of at least 1, dolomitization can occur. Also they found that lower salinities produced more ordered dolomites with higher mole-percent-MgC03 because there are fewer ions competing for positions in the dolomite structure. This suggests that hi1h mole-percent-MgC03 dolomites should have lower Na+ and Sr + concentrations and would indicate the relative salinity of the latest diagenetic solution.

Sections of the cores in this study contained both calcite and dolomite. These sections of mixed mineralogies could have resulted from the depletion of Mg2+ ions in the solution by precipitation of dolomite and the subsequent formation of low Na+ and Sr2+ calcite as more fresh water flushed through the system (Land, 1973). The Na+ and Sr2+ concentrations associated with various mole-percent-MgC03 contents of carbonate rocks are an indication of the relative salinity of the environment during neomorphism.

The mixing zone model of dolomitization would be able to produce the thick dolomite sequences in Florida because " ... sea level changes, climatic changes, and/or the occasional uplift or downwarp of the Florida platform ... " (Hanshawet al., 1971, p. 722) could cause the brackish zone to move within the system and contact large volumes of rock inland and bring about substantial lateral migration of the sea water/fresh water coastal zone of mixing.

Calcite

Nearly all of the calcite in this study is low-Mg calcite with less than 4 mole-percent-MgC03' It has much lower Sr2+ and Na+ concentrations than modern marine carbonate sediments (Tables 8 and 10). This may be the result of diagenesis in an open system with fresh or slightly saline water because calcite formation is inhibited by Mg2+ ions (Berner, 1966). The slight changes of mole-percent-MgC03 that did occur in the calcites had no corresponding changes in Na+ and Sr2+ concentrations. However, the ranges of Na+ and Sr2+ values in the calcites could be caused by the original mineralogy of the sediments (Veizer and Demovic, 1974) or by the position of the core in relation to the flow patterns of the aquifer (Kinsman, 1969). If the original sediments were aragonite, a higher content of Sr2+ should be inherited by the precipitated calcites (Veizer and Demovic, 1974). If the calcites were in the down flow direction or discharge area of the aquifer, they should have higher Sr2+ concentrations (Kinsman, 1969). These two factors, along with the salinity of the diagenetic solutions, could cause the wide variation of Sr2+ in the calcites.

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Dolomite

As stated earlier, the dolomites in this study fall into two groups, more nearly stoichiometric and non-sto­ichiometric dolomite. Both groups have distinct Sr2+ and Na+ ranges (Table 9).

The more nearly stoichiometric dolomite (44-50 mole­percent-MgC03) occurs mainly in the lower portions of the B, RS, CP, LE, MS and HS cores.

The higher mole-percent-MgC03 dolomites generally have a narrower range of Sr2+ concentrations than the non­stoichiometric dolomites (Table 9). This may suggest a longer resident time for the diagenetic fluids and possibly a greater approach to equilibrium between crystals and solution. The Sr2+ values for the 44-50 mole-percent-MgC03 group are generally less than the 39-43 mole-percent-MgC03 dolomites. Folk and Land (1975) attributed this type of relationship to the formation of higher mole-percent-MgC03 dolomite in less saline water allowing slower, more precise ordering to occur with less inhibition by competing ions.

The data in this study show a positive correlation between Na+ and Sr2+ in the dolomites of the B, RS, CP, GH, MS and LE cores. These cores are in an area of n ••• large discharge of artesian water ... n (Stringfield, 1966, p. 130). This large discharge could flush most of the trapped and absorbed Na+ from the rocks. The LA, Hand HS cores do not show this positive correlation between the Sr2+ and Na+ values (Metrin, 1979). The Na+ concentrations vary over a wide range, while the Sr2+ values are fairly constant. The LA core is in an area of local recharge (Stringfield, 1966, p. 126). The Hand HS cores are in an area of local recharge where the aquifer is at the surface. However,water also discharges as large springs along the coast and on the floor of the gulf (Stringfield, 1966, p. 130). This high discharge (5.376m3/sec for Homosassa Springs; United States Geological Survey, 1974) in the area of these cores could cause fluctuations in the ground water geochemistry, as the large amounts of fresh water discharge lower the salinity of sea water. This could cause the salt/fresh water interface to move seaward. These variations in the salinity of the diagenetic fluids could cause the wide ranges of Na+ values in these cores. Also, Na+ could be contributed by trace amounts of clay pre-sent in the original sample.

The changes in salinity that produced the distinct ranges of Na+ and Sr2+ concentrations of the two dolomite groups may have been caused by movement of the dolomitizing fluids through the system. In response to climatic and/or tectonic changes, inland brackish fluids would pass through the aquifer as the phreatic zone fluctuated vertically. The coastal salt/fresh water interface would migrate laterally as changes in sea level occurred. This could cause changes

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in the Na+ and Sr2+ concentrations of the dolomites of a core as the dolomitizing ~olutions flowed through the aquifer producing later and continuing dolomitization of the Floridan Aquifer (Randazzo et al., 1977, p. 501).

The variations between Sr2+ and Na+ ranges in coastal and inland cores are the result of the mineralogy of the original sediments and/or the high discharge of fresh water in the area of the coastal cores in this study, causing modification of the three dimensional shape of the salt/fresh water interface and consequent fluctuations of the ground water geochemistry.

The results of this geochemical study of the distribution of Na+ and Sr2+ within Eocene carbonate rocks of the Floridan Aquifer support a mixing zone model of dolomitization, which dolomitizes this sequence of rocks, with solutions less saline than sea water. These dilute saline solutions can be formed at the coastal salt/fresh water interface, modified by fresh water discharge, or in an inland regime in the brackish zone of the phreatic system.

SUMMARY

The carbonate units comprising the Floridan Aquifer in the northern portion of the Southwest Florida Water Manage­ment District were deposited in a shallow marine environment and represent a complex history of deposition and diagenesis. A number of lithofacies have been recognized and examination of the vertical distribution of lithologies among cores has enabled the recognition of cyclic sedimentation patterns in the strata.

The Lake City Formation is represented by two distinct depositional cycles. An open marine environment changed to intertidal and supratidal environments. Shallow water is indicated by the presence of algal boundstones and wacke stones and slightly deeper water by foraminiferal wackestones. The upper portion of the last cycle is represented by lagoonal deposition of mudstones, gypsum-bearing clay beds and a significant amount of carbonaceous matter.

The carbonate rocks of the Avon Park Formation provide evidence of deposition in an environmental complex consisting of subaerially exposed tidal mudflats and intervening, protected, shallow-water lagoons. A number of carbonate facies have been recognized and examination of the vertical distribution of lithologies within cored intervals has enabled the recognition of cyclic sedimentation patterns in Avon Park strata.

Subtidal facies show a general lack of sedimentary structures and are represented by a greater abundance and wider diversity of fossils than associated intertidal and supratidal deposits. The lithologic and paleontologic charac­teristics of these subtidal rocks suggest that they were

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deposited in environments ranging from a broad carbonate shelf or platform to a low energy lagoonal setting. Inter­tidal rocks are characterized by a predominance of micrite, indicative of the low tidal and wave energies which pre­vailed in the depositional environment. Fossils are sparse and the low faunal diversity of intertidal rocks may be a reflection of unstable ecologic conditions which had limited the presence of organisms in this tidal zone. Sedimentary structures preserved in intertidal rocks include: vertical burrows; root casts and molds; sediment mottling; and thin, micritic beds. The most characteristic feature of the supratidal rocks is thi~, algal and Sedimentary laminations. Crinkled and flat laminar morphologies similar to those found in the modern carbonate deposits of Andros Island, Bahamas are abundantly represented in supratidal facies. A scarcity of desiccation structures and evaporite minerals in supratidal beds may suggest that deposition had taken place under general humid paleoclimatic conditions.

The sediments of the Ocala Limestone were deposited under warm and open marine conditions on a relatively flat and broad carbonate shelf or platform similar to the present day Great Bahama Bank. The Ocala Limestone is composed of two major lithologies, separating the strata into distinct upper and lower lithologic units. ,Deposits representing shallow subtidal to intertidal, comparatively high energy, open or slightly restricted marine conditions occur in the lower unit where deposition is interpreted to have taken place relatively close to the shore. As a lithology, the Lower Ocala is characterized by thickly bedded fossiliferous limestone consisting predominantly of cleanly washed skeletal packstones to grainstones.

The carbonate deposits of the Upper Ocala represent a transition into progressively deeper water, lower energy environments of sedimentation. The unit is characterized by the presence of a deeper-water faunal assemblage, including abundant whole and fragmented tests of large foraminifera, and the deposition of wackestones and muddy packstones over the cleanly washed deposits of the lower unit.

The change from the tidal-flat and restricted subtidal deposits of the Avon Park to the offshore, high and low energy marine deposits of the Ocala Limestone is indication of an overall transgressive sequence of sedimentation. The vertical succession and cyclic alternations of lithologies is interpreted to be the result of periodic sea level fluctuations characterized by local and repeated seaward shifts in the shoreline environment.

The Suwannee Limestone and "Tampa Formation," as they occur in the subsurface of Hillsborough and Manatee Counties, south Florida, were deposited on a shallow marine carbonate bank. The Suwannee Limestone is a white, whitish-gray to yellowish-gray, allochemical limestone characterized by its abundance of fossils and relative importance of pore-filling sparite. The Suwannee Limestone consists of three lithofacies and represents a series of rock types deposited mostly in an agitated water, shallow subtidal zone above wave base.

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The microcrystalline limestone and autochthonous algal limestone of the Suwannee are only present near the top of the sequence. These rock types can be attributed to their deposition in the supratidal environment.

The disconformable erosional surface between the Suwannee Limestone and the "Tampa Formation" represents a depositional hiatus. Furthermore, a few micro-disconformities are found within both of these formations, each representing local diastems.

The "Tampa Formation" is a gray or dark gray, sandy, clayey, allochemical limestone characterized by its abundance of micrite, extra-basinal materials and local concentrations of fossils. Microcrystalline limestone, dolomite and partly dolomitized limestones are found only in the middle part of the sequence. The sequence was deposited mostly in the supratidal and intertidal environments.

In all, the Suwannee Limestone in the area records a general transgressive-regressive sequence which ended with subaerial exposure and erosion. The erosional surface is overlain by a basal limestone conglomerate which was followed by another phase of sedimentation during which the "Tampa Formation" was deposited.

Diagenetic (crystallization) fabrics can be classified on the basis of constituent crystal size distributions (uni­or multi-modal) and combination of textural types. Basic categories are:

(a) Equigranular (i) peloidal (ii) sutured mosaic (iii) sieve mosaic

(b) Inequigranular (i) spotted mosaic (ii) fogged mosaic (iii) contact-rhomb porphyrotopic (iv) floating-rhomb porphyrotopic (v) poikilotopic

(c) Aphanotopic

Consideration of natural associations and genetic relationships of crystallization fabrics makes it possible to deduce the original uncrystallized fabrics in many cases. Type of porosity development is directly related to type of crystallization fabric, except in the case of vug or cavernous porosity (nonfabric selective). Vug porosity occurs sporad­ically in dolomitized fabrics. Cavernous porosity is associated with formational contacts and may be caused by gross mineralogic changes in the rocks (e.g., the change from dominantly calcitic to dominantly dolomitic rocks).

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The close agreement of depositional cycles, patterns of crystallization fabric distribution, and patterns of porosity development in these cores suggests that predictive models of mineralogy, fabric, and porosity development can be constructed. The viability of such models will depend upon reproducibility and recognition of these patterns in other wells.

Atomic absorption spectrometry measurements of Sr2+, Na+ and mole-percent-MgC03 have allowed the recognition of distribution patterns related to depth, geographic position, and diagenetic mineralogy.

Sodium and Sr2+ concentrations provide evidence in support of a mixing zone model of diagenetic dolomitization as presented by Hanshaw et al. (1971) and Land (1973). This mixing zone can occur on the coast at the salt/fresh water interface or in the brackish zone of the phreatic system farther inland. Diagenesis occurred within a dynamic open system in the Floridan Aquifer resulting in dolomitization when sufficient Mg2+ ions were available and calcitization when the supply of Mg2+ ions was lacking.

Higher concentrations of Sr2+ and Na+ in dolomite than in calcite indicate a more saline diagenetic environment during the formation of dolomite. The Sr2+ and Na+ con­centrations and mole-percent-MgC03 in calcite indicate diagenesis in a uniform environment less saline than sea water. Comparison of the Sr2+ and Na+ concentrations of the Eocene dolomites with modern marine dolomites indicates diagenesis in an environment less saline than sea water. Sr2+ and Na+ concentrations in dolomite are generally lower in the higher mole-percent-MgC03 dolomites, indicating a less saline environment of formation with fewer ions competing for sites in the dolomite structure. The Sr2+ values in calcite could be inherited from original aragonitic sediments or could be the result of formation in the discharge area of the aquifer.

Some of the coastal cores of this study have lower Na+ and Sr2+ concentrations than the inland cores, suggesting a less saline diagenetic environment for the coastal cores. The large springs with high fresh water discharge near the coastal cores of this study could account for this difference by lowering the salinity of sea water and causing a seaward shift of the salt/fresh water interface. The Sr2+ values for the LA, Hand LE cores reflect the presence of calcite or dolomite. The LA and H cores have higher Na+ values in their lower portions. The LE core has higher Na+ values in its center region. This suggests fluctuations in the chemistry of the diagenetic environment as the dolomitizing solutions moved through the aquifer in response to climatic, tectonic, and/or sea level changes.

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