PETROLOGY AND DIAGENESIS OF AVON PARK FORMATION AND OCALA LIMESTONE, SOUTHEAST GEORGIA COASTAL PLAIN

Middle and late Eocene carbonate strata were sampled from the C.D. Hopkins No. 1 well, located 3.5 mi east of Gardi, Wayne Co., Georgia. Fifty-eight blue dyed, epoxy-impregnated thin sections were point counted to delineate petrographic microfacies and assess diagenetic effects on pore system evolution.

The 645-ft thick Avon Park is a quartz-rich, glauconitic wackestone-packstone that grades upward into skeletal packstone, grainstone, and crystalline dolomite. Nodular chert, wispy argillaceous seams, and thin lignitic layers are common. The overlying Ocala Limestone is a 309-ft thick skeletal grainstone and packstone with minor wackestone and crystalline dolomite. Dominant allochems are larger Foraminifera (Asterocyclina sp., Lepidocyclina sp.), bryozoans, mollusks, echinoderms, and calcareous algae; glauconite and terrigenous quartz are rare. Both units were deposited on a shallow, warm-water carbonate bank.

Diagenetic effects in limestones include grain micritization, leaching of aragonitic skeletal grains, neomorphism of micrite to microspar, precipitation of isopachous calcite cement and coarse spar in grainstone, and syntaxial calcite overgrowths on echinoderm fragments. The abundance of allochem ghosts, molds formed by dissolution of former fossils, and preservation of limestone diagenetic fabrics indicate that most of the crystalline dolomite formed by replacement of lithified, shallow-water limestones. The dolomite formed by meteoric water-sea water mixing in the subsurface shoreline environment. Evidence supporting this includes dolomite coarseness, preservation of original limestone fabrics (indicating slow crystallization), and presence of limpid dolomite rhombs and syntaxial rims. Grainstone and crystalline dolomite show excellent porosity and permeability because they have high percentages of well-connected intercrystal, vug, and moldic pores.