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