Anthony F. Randazzo

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.

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

Abstract 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 Two processes of dolomitization are suggested: (a) homogeneous dolomitization resulting in a single-stage development of microtextured (groundmass crystals

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

Abstract 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. 14 C activity implies some relatively young dolomite exists, probably as rims on older crystals.

Sedimentology and paleoecology of middle and upper eocene carbonate shoreline sequences, Crystal River, Florida, U.S.A.

Abstract Petrographic analyses of six cores penetrating the Inglis and Avon Park Formations, from Crystal River, Florida, U.S.A., reveal a general regressive—transgressive cycle, aerial exposure and diagenesis. Three distinct lithofacies are represented: (I) biomicrite—poorly washed biosparite; (II) dolomitized biopelmicrite—biosparite and algal dololithite; and (III) pellet- and intraclast-bearing biosparite. These lithofacies were deposited under shallow-marine conditions, at times even above the strand line. Lithofacies I was deposited in a shallow basin partly covered by marine grasses and dasyclad algae. Lithofacies II was deposited in the supratidal and intertidal zones and closely resembles the sedimentary characteristics of Holocene mud-flat deposits. Lithofacies III is the shallow-marine counterpart of the supratidal dolostone of lithofacies II and can be compared with a Holocene bank environment. This study illustrates the importance of Holocene analogues in determining and interpreting environments and demonstrates the need to consider sedimentary processes, both physical and biological, which operate in these zones.

Post-Miocene subtropical karst evolution, lower Suwannee River basin, Florida

Morphometric characteristics of ∼25 000 karstic depressions suggest that the last phase of the post-Miocene karstic evolution within the lower Suwannee River basin of Florida has been controlled by the lower sea-level stands of the Pleistocene and the formation of the Suwannee River. During the Pleistocene, as interglacial seas retreated, marine terraces formed by sequential sea-level lowstands and the time period of subaerial exposure diminished toward the sea. Consequently, geomorphologically younger karst landforms formed as the elevation of marine terraces decreased. The evolutionary geomorphological development of this heavily karstified region produced more frequent and/or larger and more complex depressions at higher elevations. A geographic information system analysis of morphometric and spatial distribution parameters of the karstic depressions within the lower Suwannee River basin reveals that the Florida karst is represented by broad, shallow depressions with an average density of 6.07/km 2 and an average pitting index of 14.5. Morphometric and spatial distribution parameters of karstic depressions show a great variation within the lower Suwannee River area and thus preclude a simple morphoclimatic classification of karst landforms. The Tertiary carbonate rocks of the subtropical Florida karst have relatively less joint frequency and significant primary porosity, and do not produce the extreme karst landforms observed in the massive limestones of the tropics.

Mineralogical changes along the freshwater/saltwater interface of a modern aquifer

Abstract Occurrence of dolomite is related to the distribution of geological and hydrochemical parameters in cores and wells penetrating the Floridan aquifer along portions of the west coast of peninsular Florida. Trace element analyses of the cores indicate correlations between strontium and sodium concentrations and the particular carbonate phase. Chloride, sulfate, and conductivity values indicate the position and extent of the freshwater/saltwater interface. Thick sequences of carbonate rocks in western peninsular Florida have been dolomitized in the freshwater/saltwater mixing zone of the coastal aquifer. Correlations between the occurrence of dolomite and specific stratigraphic formations suggest that dolomitization was rock-selective. Evidence of continuous formation of dolomite indicates that the process is actively occurring.

Authigenic fluorite in dolomitic rocks of the Floridan aquifer

Fluorite in Eocene carbonate mudstone of Florida is reported for the first time. Fluorite occurs within primary gypsum nodules at a depth of 235–240 m below the surface. Water chemistry indicates undersaturation for gypsum and fluorite, and their survival is attributed to a lack of groundwater circulation. Fluorite probably formed penecontemporaneously with gypsum in a sabkha environment from seawater-concentrated brine. An additional source of fluoride may have been fluvial transport of salts weathered from Appalachian volcanic rocks.

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

Abstract 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.

Selected geochemical factors influencing diagenesis of eocene carbonate rocks, peninsular Florida, U.S.A.

Abstract The geochemical significance of three selected ions (Mg2+, Na+, and Sr2+) supports a model of dolomitization by brackish groundwater. This groundwater zone contains sufficient quantities of Mg2+ to facilitate dolomitization ( Mg Ca ratios 1 ). Rising and falling of sea level and fluctuations of the phreatic zone related to climatic variations account for the thickness of the dolomite layers and the chemical distributions within these layers. Sodium concentrations in the calcite are 70–185 ppm, indicating formation in brackish water. Dolomite has sodium concentrations between 50–1400 ppm, suggesting formation in waters of similar salinity. Strontium in calcite ranges from 320–600 ppm, suggesting diagenesis in slightly saline waters in an open system. Dolomite contains 241 ppm Sr2+ on the average and calcite has 418 ppm Sr2+. The Sr2+ concentrations of the dolomite are characteristic of diagenesis in water less saline than sea water. Average strontium concentrations in the dolomite occur in two distinct groups, 260 ppm for dolomite with 39–43 mole-% MgCo3 and 195 ppm for the dolomite with 44–50 mole-% MgCO3. The difference in the Sr2+ concentrations of the two dolomite groups indicates the higher mole-% MgCO3 dolomite recrystallized in a less saline environment than the lower mole-% MgCO3 dolomite. These different environments are attributed to a relatively more saline coastal environment and a less saline inland environment. The more nearly stoichiometric dolomite (44–50 mole-% MgCO3) has less scatter when mole-% MgCO3 is plotted against Sr2+ and Na+. This suggests a greater approach to equilibrium with the dolomitizing fluid than the lower mole-% MgCO3 (39–43) dolomite. The more saline environment has higher Mg/Ca ratios and promotes more calcium-rich dolomite during diagenesis because of the inhibition from competing foreign ions and because it is thermodynamically a more favorable environment which causes more rapid crystallization. The less saline waters allow recrystallization to proceed more slowly, producing better ordering in the dolomites, textural preservation and development of subhedral to euhedral rhombic crystals.

Lithoplaision ocalae: A new trace fossil from the Ocala Limestone (Eocene), Florida

The upper Eocene Ocala Limestone of Florida contains an unique and abundant plug‐shaped trace fossil, the origin and paleoenvironmental significance of which remain enigmatic. High‐angle to near‐vertical in‐situ orientations and other field and laboratory data clearly favor formation by biogenic processes over nonbiogenic (physical) processes. The fossils are elongated, subcylindrical to conical structures possessing an internal central tube surrounded by a thick wall. Skeletal debris, cemented by sparry calcite, composes both the tube and wall components. The burrows, interpreted to have been formed by an organism maintaining a vertical position extending downward from the sediment/water interface, have been given the new ichnogenus and ichnospecies names Lithoplaision ocalae.