Petrology of Lower and Middle Eocene Carbonate Rocks, Floridan Aquifer, Central Florida: ABSTRACT

Abstract

ABSTRACT Study of cores from a U.S. Geological Survey test well near Polk City indicates that the Avon Park-Lake City (Claibornian) and Oldsmar (Sabinian) Formations, which comprise most of the Floridan aquifer in central Florida, can be divided into six microfacies: foraminiferal mudstone; foraminiferal wackestone-packstone; foraminiferal grainstone; nodular anhydrite; laminated dolomicrite; and replacement dolomite. Dolomite containing variable amounts of nodular anhydrite forms more than 90 percent of the Avon Park-Lake City interval, whereas the Oldsmar is chiefly limestone. The depositional model inferred for these units is a broad, shallow-water marine platform with environments ranging from supratidal-sabkha to shallow-water shelf. Diagenetic pathways vary with rock type, but generally include: (1) marine phreatic -- grain micritization, and radially fibrous cementation within foraminiferal tests; (2) meteoric vadose -- minor leaching of aragonitic grains; and (3) meteroic phreatic -- neomorphism of unstable grains, dissolution of aragonitic allochems, formation of isopachous equant calcite cement and coarse spar in grainstones, and syntaxial calcite overgrowths on echinoderms. Several episodes of dolomite formation are recognized. Laminated dolomicrite formed syngenetically in a supratidalsabkha environment. Crystalline dolomite with nodular anhydrite formed early by replacement of limestone through reflux of dense, Mg-rich brines. Replacement dolomite not associated with evaporites and containing limpid crystals probably formed later by a mixed-water process in the subsurface environment. Late diagenetic processes affecting crystalline dolomites include hydration of anyhdrite to gypsum, partial dissolution of gypsum, alteration of gypsum to calcite (minor), and dissolution of calcian dolomite cores in stoichiometric crystals. Crystalline dolomite and grainstone are the only rock types that have high enough porosities and permeabilities to provide significant yields of water. Medium and finely crystalline dolomites show best values of porosity and permeability because they have high percentages of intercrystal and moldic pores that are well connected. Filling of pores by anhydrite or gypsum can significantly reduce porosity and permeability.