Avon Park Formation Research Articles

Characterization of a multi-layer karst aquifer through analysis of tidal fluctuation

Interaction between ocean tides and coastal aquifers is an important process influencing the quality and quantity of groundwater resources. In this study, high frequency groundwater fluctuation along two transects from the Gulf of Mexico to the Upper Floridan Aquifer (UFA) were analyzed to characterize the hydrogeologic structure of the multi-layer karst aquifer. Tidal efficiency and time lag of harmonic constants were calculated from groundwater and seawater level and aquifer diffusivity were estimated using the Jacob-Ferris equation. Measurable tidal signals were observed at a maximum inland distance of approximately 7–8 km due to the high conductivity of the unconfined karst aquifer. The largest tidal fluctuation was observed in the deeper unit of the aquifer (146–155 m below land surface), which indicate the hydraulic contact between seawater and the high permeability zone of Avon Park Formation. The estimated values of hydraulic diffusivities agreed well with the regional groundwater flow model despite the highly heterogeneous nature of karst aquifer. Our analysis suggested that tidal springs are important sea water/groundwater interfaces and the conduit networks of tidal creeks, caves, and fractures are preferential pathways of tidal propagation.

Paleohydrological modeling of penesaline reflux dolomitization: Avon Park Formation (Middle Eocene), East Central Florida

Previous geochemical, petrological, and sedimentological investigations of the Middle Eocene Avon Park Formation (Middle Eocene) of Florida suggest that dolomitization occurred in the Eocene from normal to hypersaline seawater. Reflux of penesaline water (i.e., seawater evaporatively concentrated below evaporite mineral precipitation) is the most likely mechanism to supply the required magnesium. Density-dependent solute-transport modeling was performed to evaluate whether penesaline reflux under known or inferred Middle Eocene paleohydrological conditions could have provided enough magnesium to account for the mass of dolomite present in three cores of the upper part of the Avon Park Formation from the City of Daytona Beach. Rather than a simple down and out flow pattern, the modeling results indicate that an initial rapid vertical convection-dominated flow regime first occurs in which native groundwater is displaced by refluxed brine, and followed by a longer-duration regime dominated by steady-state lateral flow. Dolomitization from a brine with a 89.6 ppt TDS concentration would require an average minimum reflux (at 100% dolomitization efficiency) on the Florida Platform of about 8130 m3 per m2 to provide the required magnesium. Modeling results indicate that such reflux could occur over 0.7–3 million years, depending upon the aquifer hydraulic parameter values. Longer time periods would be required for the reflux of lower salinity brines and lower dolomitization efficiencies. Penesaline reflux is thus hydrologically plausible for the dolomites of the upper Avon Park Formation.

Insights Into the Dolomitization Process and Porosity Modification in Sucrosic Dolostones, Avon Park Formation (Middle Eocene), East-Central Florida, U.S.A.

Abstract The Avon Park Formation (middle Eocene) in central Florida, U.S.A., contains shallow-water carbonates that have been replaced by dolomite to varying degrees, ranging from partially replaced limestones, to highly porous sucrosic dolostones, to, less commonly, low-porosity dense dolostones. The relationships between dolomitization and porosity and permeability were studied focusing on three 305-m-long cores taken in the City of Daytona Beach. Stable-isotope data from pure dolostones (mean δ18O = +3.91‰ V-PDB) indicate dolomite precipitation in Eocene penesaline pore waters, which would be expected to have been at or above saturation with respect to calcite. Nuclear magnetic log-derived porosity and permeability data indicate that dolomitization did not materially change total porosity values at the bed and formation scale, but did result in a general increase in pore size and an associated substantial increase in permeability compared to limestone precursors. Dolomitization differentially affects the porosity and permeability of carbonate strata on the scale of individual crystals, beds, and formations. At the crystal scale, dolomitization occurs in a volume-for-volume manner in which the space occupied by the former porous calcium carbonate is replaced by a solid dolomite crystal with an associated reduction in porosity. Dolomite crystal precipitation was principally responsible for calcite dissolution both at the actual site of dolomite crystal growth and in the adjoining rock mass. Carbonate is passively scavenged from the formation, which results in no significant porosity change at the formation scale. Moldic pores after allochems formed mainly in beds that experienced high degrees of dolomitization, which demonstrates the intimate association of the dolomitization process with carbonate dissolution. The model of force of crystallization-controlled replacement provides a plausible explanation for key observations concerning the dolomitization process in the Avon Park Formation and elsewhere: (1) volume-for-volume replacement at a crystal scale, (2) coupled growth of dolomite crystals and dissolution of host calcium carbonate matrix, and (3) automorphic replacement by euhedral dolomite crystals. The force-of-crystallization model also does not require an influx of externally derived water that is undersaturated with respect to calcite to dissolve calcite, a fact that could simplify diagenetic models of porosity generation in dolostones. The later addition of external carbonate can result in a substantial reduction in porosity by the precipitation of dolomite cement, which could convert a high porosity sucrosic dolostone into a dense “Paleozoic type” dolostone.

Determination of arsenic concentration and distribution in the Floridan Aquifer System

Here we report on the different sampling strategies for almost seven years of sampling rocks/sediments for the determination of As within the Intermediate Aquifer System (IAS) and upper Floridan Aquifer System (FAS), a very large and productive limestone aquifer spanning from Georgia into Florida. In the FAS, As contamination has become a recurring problem during aquifer storage and recovery (ASR), particularly in central and south Florida. To investigate these phenomena, samples from solid drill cores and rock cuttings were collected from the Hawthorn Group, Suwannee Limestone, Ocala Limestone and Avon Park Formation. Samples were taken along drill cores and rock cuttings (referred to as ‘interval’ samples) or from particular drill core sections and rock cuttings (referred to as ‘targeted’ samples) likely to contain elevated concentrations of As as indicated by the presence of pyrite, hydrous ferric oxide, organic matter, clay minerals, fracture surfaces, and high permeable (moldic) zones. Arsenic was present in all of the stratigraphic units at low concentrations, close to the global average for As in limestone of 2.6 mg/kg. The highest As concentration was 69 mg/kg. In all units, however, the average bulk As concentration in the targeted samples was substantially higher than that in the interval samples. Based on direct spot measurements by electron microprobe and indirect calculations, pyrite was identified as the main source of As in the FAS. Concentrations in pyrite ranged from less than 100 mg/kg to more than 11,000 mg/kg. Because pyrite is heterogeneously distributed, both vertically and horizontally in the sampled stratigraphic units, the same was observed for the distribution of As. However, As concentrations generally decreased with depth, i.e., highest As values in the Hawthorn Group and lowest As values in the Ocala Limestone and Avon Park Formation. Compared to pyrite, other trace minerals contained much less As. The average As concentrations of the two types of sample media (solid cores and rock cuttings) were quite similar. These results indicate that if simply the average bulk rock As concentration of a geologic unit is the desired outcome of an investigation, either interval or targeted sampling of rock cuttings, seems to be sufficient. This is particularly important when time and money are a factor. This approach could work equally well for any other trace element. Structural sedimentary information, such as fractures, etc., is likely lost, however, when sampling rock cuttings. Thus, if this information is required, solid core samples need to be collected by hollow core diamond drilling.

Diagenesis and porosity preservation in Eocene microporous limestones, South Florida, USA

Microporous limestones may contain immobile, capillary-bound (irreducible) water that is only in diffusional contact with mobile pore waters or in some reservoirs may contain producible hydrocarbons. The preservation and distribution of microporosity impact both subsurface fluid flow and solute transport. The diagenesis of microporous limestones has received relatively little attention because their very fine grains and cements are not amenable to standard analytical methods. The Ocala Limestone (Upper Eocene) and upper Avon Park Formation (Middle Eocene) in South Florida contain microporous micritic limestones (mudstones to packstones) that are at an intermediate stage of diagenesis. The limestones have been exposed to the active near-surface environment, but have not yet reached a burial depth sufficient for intense chemical compaction and associated porosity reduction. Nuclear magnetic resonance (NMR) logging allowed for the quantification of total porosity, pore-size distribution, and permeability. The Ocala Limestone and Avon Park Formation have different predominant microfacies and porosity size distributions, but yet both retain total porosities predominantly between 35% and 37%. Estimated microporosities range mostly between 12% and 45%. The mudstones and wackestones of the Ocala Limestone have significantly lower permeabilities (mostly 3 to 12 md) than the wackestones to grainstones of the Avon Park Formation (commonly in the 100 to 3000 md range), which have more mixed and overall coarser pore sizes. Computer modeling using carbon and oxygen stable data indicates that the studied microporous limestones underwent only a low degree of chemical diagenetic alteration, despite likely experiencing episodes of freshwater flushing associated with post-depositional sea-level lowstands. The Ocala Limestone and Avon Park Formation limestones illustrate the general concept that total porosity is often largely preserved through early diagenesis (although may undergo intra-formational redistribution) and that confined aquifers are diagenetic quiescent environments.

Retention of High Permeability During Shallow Burial (300 to 500 M) of Carbonate Grainstones

Abstract A progressive decrease in matrix permeability occurs with burial in the Paleogene limestones of west-central Florida, yet grainstones with permeabilities > 400 md persist below burial depths of 300 m. Why some grainstones retain high permeability during passage through the shallow-burial realm was investigated using 85 dominantly foraminifera and micritized-grain grainstones from the Eocene Avon Park Formation. Twenty-seven variables representing diagenesis, stratigraphic packaging, depositional energy, grain types, and pore-network attributes were quantified. Permeabilities range from 23 to 7,160 md and thin-section porosities range from 14% to 36%. Grain size (lower coarse to upper fine sand) covaries positively with sorting (poor to very good). The samples are from three different systems tracts within the Avon Park, and sampled grainstone bodies ranged in thickness from 0.5 to 3.5 m. Evidence of diagenesis include mechanical and chemical burial compaction, pre-compaction cements, and dissolution. Original interparticle porosity loss due to compaction (COPL) and cementation (CEPL) range from 0 to 35.3% and 1.6 to 26.2% respectively. Most original intraparticle porosity is occluded, but most moldic porosity, which ranges from 1.2% to 12%, remains open. Multivariate regression reveals that 61% of the variance in permeability is attributed to grain size, CEPL, COPL, and systems tract (a proxy for relative abundance of compactable foraminifera versus robust micritized grains). Large grain size means a large initial permeability; all other regression components reflect diagenetic reductions in permeability. Observed ranges in CEPL and COPL values mean they can reduce permeability by more than three orders of magnitude whereas the other regression parameters reduce it by only one order of magnitude. Retention of high (> 400 md) permeability can occur by (1) cementation of ~ 40% of the original interparticle pore space, which inhibits subsequent compaction yet preserves high permeability, or (2) a combination of cementation and compaction that destroys less than two thirds of the original interparticle pore space. These relations, derived from originally calcite-rich sediments, provide a quantitative reference by which to evaluate permeability preservation during shallow burial in numerical simulation of diagenesis and reservoir development. Capillary-pressure data also support the hypothesis that pore geometries, and hence permeability, respond differently to compaction versus cementation. Cement-dominated samples exhibit a dominance of porosity behind intermediate-sized pores at higher permeabilities than compaction-dominated samples. Compaction also does not yield a large change in the amount of porosity behind pore throats smaller than 1 μ at any permeability above 20 md, whereas cementation-dominated diagenesis generates significant amounts of pore space behind those small throats once permeability goes below 100 md. These differences arise because cementation converts the original interparticle pores to a system of interparticle polyhedral and sheet-like pores connected by progressively smaller pore throats between cement crystals, whereas compaction merely constricts the original interparticle porosity behind smaller intergranular pore throats.

Middle Eocene terrestrial palynomorphs from the Dolime Minerals and Gulf Hammock quarries, Florida, U.S.A.

A small but diverse assemblage of pollen and spores recovered from marine sediments of the Avon Park Formation (middle Eocene) in west central Florida, represents the oldest land flora described from the state. Twenty‐one pollen taxa from terrestrial plants, are identified from the Dolime Minerals and Gulf Hammock quarries. The presence of mangrove pollen associates such as Acrostichum, Nypa, and Pelliciera associated with seagrass macrofossils, combined with evidence from invertebrate fossils and sedimentary structures, suggest that the sediments of the Avon Park Formation, at these quarry sites, were deposited in a shallow marine lagoon, perhaps near the mouth of a river or stream in a protected area that had some tidal influence. This is the earliest report of terrestrial vegetation and near marine vegetation of Florida and documents the presence of a Florida landmass during middle Eocene time.

MIDDLE EOCENE TERRESTRIAL PALYNOMORPHS FROM THE DOLIME MINERALS AND GULF HAMMOCK QUARRIES, FLORIDA, U.S.A.

A small but diverse assemblage of pollen and spores recovered from marine sediments of the Avon Park Formation (middle Eocene) in west central Florida, represents the oldest land flora described from the state. Twenty-one pollen taxa from terrestrial plants, are identified from the Dolime Minerals and Gulf Hammock quarries. The presence of mangrove pollen associates such as Acrostichum, Nypa, and Pelliciera associated with seagrass macrofossils, combined with evidence from invertebrate fossils and sedimentary structures, suggest that the sediments of the Avon Park Formation, at these quarry sites, were deposited in a shallow marine lagoon, perhaps near the mouth of a river or stream in a protected area that had some tidal influence. This is the earliest report of terrestrial vegetation and near marine vegetation of Florida and documents the presence of a Florida landmass during middle Eocene time.

Water-rock interactions in a modern coastal mixing zone

The chemical evolution of ground water and the diagenetic history of the rocks in a modern saline–fresh-water mixing zone were investigated to test the hypothesis that this zone is a chemically active environment in which carbonate mineral diagenesis occurs. The field site was in the unconfined section of the upper Floridan aquifer system in west-central Florida, a regionally extensive hydrostratigraphic unit composed of Eocene–Oligocene limestone and dolomite. The mixing zone, marked by a steep concentration gradient, is >90 m thick at the study site in New Port Richey, Pasco County, Florida. Interpretation of aqueous geochemical data indicates that the coastal zone is most likely a location where three waters of distinct composition mix. A fresh Ca-HCO3 water that derives its composition from calcite dissolution and CO2 influx in the up-gradient recharge area, a Na-Cl water that results from salt-water intrusion, and a Ca-SO4–rich water that upwells subsequent to contact with underlying gypsiferous beds of the Avon Park Formation are likely all converging in this coastal setting. Poor core recovery and an enlarged core hole provide evidence of dissolution. The ground water, however, is oversaturated with respect to calcite; thus, the large-scale dissolution features observed are unrelated to modern-day mixing. The mixing of Ca-SO4–rich ground water in the coastal region led to oversaturation with respect to calcite. At the petrographic scale, there is little evidence of diagenesis related to the modern mixing zone. Calcite crystals from the lower section of the cored interval have isotopic signatures that are different than those from the upper section, suggesting that the latter has undergone more water-rock interactions. Calcium-rich dolomite, which occurred relatively late in the paragenetic sequence, has thin cathodoluminescent zonations but shows no evidence of subsequent alteration. The paragenetic sequence includes marine micritization and cementation; meteoric mineralogic stabilization; dissolution enhancement of porosity; phreatic, equant calcite cements; and echinoderm overgrowths, all typical of the region and not unique to the mixing zone.

An Example of Mixing-zone Dolomite, Middle Eocene Avon Park Formation, Floridan Aquifer System

ABSTRACT A late-formed dolomite cement in a core of the Middle Eocene Avon Park Formation, peninsular Florida, provides an example of dolomite cement from a mixing zone and illustrates how dolomite textural alteration and stabilization can occur at earth-surface conditions. The Avon Park Formation is a pervasively dolomitized peritidal platform carbonate 400 m thick in the Floridan aquifer system. Typical Avon Park dolomite is inclusion-rich, fine-grained (< 40 mm), noncathodoluminescent, highly porous (average, 20%), and formed during the Eocene by normal to hypersaline seawater (18O = + 3.7 PDB; 13C = + 2.0; 87Sr/86Sr = 0.70778; Sr = 167 ppm). In a 20 m interval in a core from southwest Florida, inclusion-free, cathodoluminescent dolomite overgrows the early-formed noncathodoluminescent marine dolomite. The cathodoluminescent dolomite cement profoundly alters the texture of Avon Park dolomite from typical Cenozoic-like porous, poorly crystalline dolomite to hard, dense, low-porosity, highly crystalline Paleozoic-like dolomite. The dolomite cement is not a replacement of limestone but an overgrowth of early-formed marine dolomite and pore-occluding cement. From analyses of samples containing both the early marine dolomite and the late dolomite cement, quantitative estimates of pure cathodoluminescent dolomite cement are: 13C = -0.5 and 18O = +1.7; 87Sr/86Sr = 0.7085; Sr = 225 ppm. The stable-isotope compositions are intermediate between Avon Park marine dolomite and dolomite predicted to be in equilibrium with Floridan aquifer freshwater (estimated 18O = 0 to + 0.5, P B), indicating the dolomite precipitated from waters intermediate in composition between seawater and freshwater. Due to the rise in seawater 87Sr/86Sr since the Middle Eocene, the 87Sr/86Sr composition of the cathodoluminescent dolomite cement demonstrates that it must have formed after the Middle Miocene (incorporating radiogenic Sr from seawater or rocks of Middle Miocene age or younger). A water sample was taken from this same core from an interval in the Avon Park Formation in the dilute part of the modern mixing zone, approximately 60 m above the first occurrence of altered dolomite. Fluids from this interval have 3400 ppm TDS and are a mixture of 94% Floridan aquifer freshwater and 6% normal seawater. These pore fluids are Sr-rich (24 ppm) and SO4-rich (755 ppm) due to extensive Avon Park gypsum dissolution and have a 87Sr/86Sr composition (0.7078) in equilibrium with Avon Park host rock. The salinity of the fluid that precipitated the cathodoluminescent dolomite cement is quantitatively calculated with a fluid-fluid mixing model using (1) the Sr concentration (24 ppm) and 87Sr/86Sr composition (0.7078) of the von Park pore fluid sample from the core, (2) the Sr concentration (8 ppm) and 87Sr/86Sr compositions of Modern (0.7092) and Late Miocene (0.7089) seawater, and (3) the 87Sr/86Sr composition (0.7085) of the cathodoluminescent dolomite cement. These calculations indicate that the dolomite cement precipitated from a mixed marine-meteoric fluid with salinity approximately 75% seawater. This same fluid would be both calcite supersaturated and contain about 12 ppm Sr. Such a fluid could precipitate dolomite cement with 225 ppm Sr using a reasonable Kd = 0.05; this accounts for the higher Sr concentrations in the mixing-zone dolomite cement (225 ppm) relative to Avon Park marne dolomite (167 ppm). This study demonstrates that: (1) dolomite precipitated from a 75% seawater mixing-zone fluid that was both calcite saturated and sulfate-rich, and (2) dramatic textural maturation and stabilization in dolomite can occur in the near surface environment, without elevated temperature and burial conditions.

Late-Stage Diagenesis in the Floridan Aquifer, Middle Eocene Avon Park Formation

Late-Stage Diagenesis in the Floridan Aquifer, Middle Eocene Avon Park Formation

Animal-Plant Relationships and Paleobiogeography of an Eocene Seagrass Community from Florida

An excellent example of a preserved seagrass community occurs in the late Middle Eocene Avon Park Formation of west-central, peninsular Florida. This fossil assemblage provides some of the first detailed information about the evolutionary development of animal/seagrass interactions, and in addition, documents the pre-Miocene existence of seagrasses in Florida. The most common seagrasses present are species of Thalassodendron and Cymodocea. A strong Tethyan paleobiogeographic connection, previously noted among the Eocene molluscs of Florida, is also supported by the seagrasses and dugongs. We hypothesize that this assemblage of seagrasses was much more widespread in the Neotropics during the Paleogene and into the Neogene. Their absence now can be explained by the changing circulation patterns and cooling initiated by the closing of the Panamanian Isthmus and/or the onset of Plio-Pleistocene glaciation

Paleoecology of shallow-marine carbonate environments, middle Eocene of Peninsular Florida

The middle Eocene Avon Park Formation of west central Florida contains a carbonate sequence that accumulated under supratidal to shallow subtidal conditions. Vertically repetitive sedimentary sequences document transgressive regressive, open marine to shoreline cyclicity. Newly discovered cross-bedding and abundant fossils support these paleoenvironmental interpretations. The rocks are dolomitic and bioclastic packstones and grainstones. Allochems are chiefly benthic foraminifers, bivalves, echinoids, and algae. Planar cross-bedding occurs within megaripple sets. Bed thicknesses, lateral extent of bed types, and paleocurrent directions are nonuniform, indicating migrating or shifting hydrodynamic regimes. Trace fossils, including Ophiomorpha, are well preserved and suggest maximum water depths of three to four meters in a shoreline transition zone. Invertebrate, vertebrate, trace, and plant fossils represent more than 70 species. Abundant irregular echinoids (clypeasteroids, spatangoids, cassiduloids, and a holectypoid) also suggest sandy shoreline conditions. Erosional truncations of burrows indicate periodic storm action along the shoreface. Modern environmental and ecological analogues to this study area include the south Florida shelf and the Bahamian Platform. Mud-free, carbonate sands displaying megarippled cross-beds and extensive burrowing occur adjacent to sea grass beds with finer-grained sediments and greater faunal diversity/density. Shifting conditions often result in redistribution of facies.

Water/Rock Interaction Efficiency and Seawater Dolomitization in the Eocene Avon Park Formation, Floridan Aquifer: ABSTRACT

The Floridan aquifer has often been proposed as a system of extensive meteoric carbonate diagenesis and mixing zone dolomitization. However, the dominance of marine isotope (C, O, {sup 87}Sr/{sup 86}Sr) and trace element (Sr, Fe, Mn) compositions in dolomites and limestones in the Eocene Avon Park Formation, Floridan aquifer, suggests that the very active low temperature meteoric groundwater system has, over the past 40 m.y., been an inefficient mechanism of diagenesis. {delta}{sup 18}O values of all but two replacement dolomites sampled range from +2.0 to +5.1 (PDB) with high Sr concentrations (90-325 ppm), indicating dolomitization by near-normal marine water involving no significant interaction with meteoric groundwater. The two {delta}{sup 18}O-depleted (0.0 {plus minus} 1) dolomites have low Sr concentrations ({approximately}100 ppm) suggesting limited recrystallization in meteoric water. Several dolomite samples have radiogenic {sup 87}Sr/{sup 86}Sr compositions (0.70810-0.70883 {plus minus} 2), but have heavy oxygen isotope compositions (> +2.0) and high Sr concentrations (<200 ppm) suggesting precipitation from cold Miocene age or younger seawater that circulated through the Florida platform. Most limestone stable isotope compositions cluster around marine values (({delta}{sup 18}O = {minus}1 to +1, PDB) {delta}{sup 13}C = +0.5 to +2.5) and have Eocene seawater {sup 87}Sr/{sup 86}Sr compositions (0.70775more » {plus minus} 2 to 0.70779 {plus minus} 2) with 400 to 500 ppm Sr. Isotopic compositions of limestones from the east coast of Florida are all within these ranges. Only some limestones from central Florida and the west coast contain depleted stable isotopic compositions and low Sr concentrations. The sample with the most depleted stable isotope values has a radiogenic {sup 87}Sr/{sup 86}Sr composition (0.70870 {plus minus} 2), suggesting that diagenetic meteoric water migrated through post-Miocene strata.« less

Characterization of dolomitic rocks from the coastal mixing zone of the Floridan aquifer, Florida, U.S.A.

Petrographic, XRD, SEM, cathodoluminescence, and microprobe analyses were conducted to characterize dolomitic strata from a core penetrating the coastal mixing zone of the Floridan aquifer. Lithologic, geochemical, and water chemistry data were used to identify diagenetic trends, including modern water/rock interaction. The depositional environment of these middle Eocene carbonate sediments of the Avon Park Formation was intertidal to supratidal. Sabkha conditions caused penecontemporaneous deposition of evaporites and accessory minerals. Present water chemistry indicates undersaturation for gypsum and its survival is attributed to sluggish groundwater circulation. Dolomite crystals were commonly found to be compositionally zoned with crystal centers generally calcium-rich and less stable than the more stoichiometric crystal rims. Three stages of dissolution were recognized from SEM studies of these crystals: (1) initial development of dissolution pits; (2) partial dissolution of the unstable crystal center; and (3) complete dissolution of the unstable crystal center leaving hollow dolomite. Present-day mixing zone water is saturated with respect to stoichiometric dolomite. Near-stoichiometric dolomite cement may be filling pores in this zone and non-stoichiometric dolomite may be dissolving. The instability of calcian dolomite in the mixing zone may be related to compositionally modulated crystal lattice defects. Pervasive dissolved crystal centers in the saline zone are believed to have been highly disordered penecontemporaneous dolomite. Stoichiometric dolomite does not appear to be precipitating in this zone because of the presence of competing ions. Stable carbon and oxygen isotopic values for dolomite are comparable to those reported for Plio-Pleistocene dolomites. 14C activity implies some relatively young dolomite exists, probably as rims on older crystals.

The occurrence of fossil seagrasses in the Avon Park formation (late middle eocene), Levy County, Florida (U.S.A.)

Within the Avon Park Formation of west-central Florida, U.S.A., fossil seagrasses are preserved as carbonized imprints within the bedding planes of a micritic dolomitized limestone. Six species could be recognized, but only for three of them was the material sufficiently complete to allow a formal description. These three species are Thalassodendron auricula - leporis den Hartog n.sp., Cymodocea floridana den Hartog n.sp. and Thalassia testudinum Banks ex König. The genera Thalassodendron and Cymodocea have not been recorded for America before.

Classification and description of dolomitic fabrics of rocks from the Floridan aquifer, U.S.A.

Biozones of the Lake City Formation, Avon Park Formation and Ocala Limestone are characterized by interbedded, massive, fossiliferous carbonate rocks (wackestones to grainstones) and thinly bedded, peloidal and carbonaceous rocks (mudstones and wackestones). Several horizons have been partly or completely dolomitized. A wide range of early to late stages of dolomitic fabrics are recognized. The fabrics are classified descriptively as equigranular (unimodal) or inequigranular (multimodal). Fabrics composed of crystals < 0.002 mm 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: (a) homogeneous dolomitization resulting in a single-stage development of microtextured (groundmass crystals < 0.016 mm in diameter) aphanotopic, peloidal and mosaic fabrics; and (b) heterogeneous dolomitization resulting in multistage development of porphyrotopic and some mosaic fabrics. It is possible to deduce the original depositional fabric and lithofacies from the dolomitic fabric except where extensive neomorphism has occurred.