PETROGRAPHIC CHARACTERISTICS OF THE UPDIP MIDDLE EOCENE CARBONATE-CLASTIC MICROFACIES – SAVANNAH RIVER SITE, SOUTH CAROLINA

Five mixed carbonate-terrigenous lithofacies were recognized in the middle Eocene calcareous strata from sedimentological studies of cores in the Savannah River Site area of the Atlantic Coastal Plain of South Carolina; quartz sand, calcareous quartz sand, sandy carbonate, muddy carbonate, and a transitional lithofacies that ranges from a sandy, muddy carbonate to a calcareous sandy mud. These lithofacies were subdivided into microfacies based on petrographic examination; quartz sand; lime mud, terrigenous mud, microsparite, siliceous mudstone, skeletal quartz sand; quartz-rich skeletal calcareous mud; skeletal wackestone and quartz-rich skeletal wackestone; skeletal packstone and quartz-rich skeletal packstone; quartz-rich, glauconitic skeletal wackestone and packstone; and sandy skeletal grainstone. Carbonate-rich facies are more prevalent downdip, while terrigenous rich facies dominant updip.

The transition from terrigenous-rich to carbonate-rich occurs updip in a narrow zone less than 5 km wide. The primary control on areal facies distribution is depositional environment, controlled by sea level eustacy, and amount, rate and locus of terrigenous influx. Petrographic characteristics of the updip microfacies will be presented.

Diagenetic pathways vary with facies type, but generally include: (1) marine phreatic — grain micritization, and radially fibrous cementation; (2) freshwater phreatic — inversion of high-Mg calcite to low-Mg calcite, dissolution of aragonitic allochems, formation of moldic porosity, precipitation of syntaxial calcite overgrowths on echinoderms, formation of calcite spar and isopachous calcite cement, neomorphism of micrite to microspar and pseudospar, precipitation of opal-CT lepispheres, replacement of mollusk shells by chalcedony, and precipitation of zeolites within secondary moldic pores. Quartz-rich facies have high interparticle porosity and excellent permeability. Mud-rich facies have low to moderate porosities and permeabilities owing to the isolation of moldic and vug pores. Lithologic and geophysical data for more than 90 boreholes were used in this study. Methods included thin section examination, insoluble residue analysis, and SEM.