Sedimentology and depositional environments of middle Eocene terrigenous-carbonate strata, southeastern atlantic coastal plain, USA

Abstract

Basin-margin sediments of middle Eocene age in the Savannah River Site area consist of five terrigenous-carbonate lithofacies: quartz sand; calcareous quartz sand; sandy carbonate; muddy carbonate; and a transitional lithofacies that ranges from sandy, muddy carbonate to calcareous mud. The middle Eocene sediment package, which includes calcareous parts of the upper Congaree, Warley Hill, and Santee Formations, dips southeast at 4.7 m/km and thickens from 2 m at its updip edge to 95 m downdip. The presence of glauconite and a diverse faunal assemblage in all lithofacies suggests deposition in clear, well-oxygenated, open-marine waters of normal salinity on the inner to middle shelf with periods of marginal marine, nearshore, and deltaic influence. Coarse-grained terrigenous sand and calcareous sand, deposited in higher-energy, nearshore environments, occur near the updip limit. Fine-grained terrigenous mud, calcareous mud, and sandy and muddy carbonate are located downdip and accumulated in quieter water conditions on the inner and middle shelf. The transition from terrigenous to carbonate sediment occurs near the updip limit in a narrow zone less than 5 km wide. Three depositional sequences, which contain transgressive and highstand system tracts, are recognized within the middle Eocene calcareous interval. One is assigned to the upper Congaree Formation ( TA 3.5 3.6 cycles ). The main control on areal distribution Hill Formation (TA3.4 cycle), and one to the Santee Formation ( TA 3.5 3.6 cycles ). The main control on areal distribution of facies was depositional environment, which was controlled primarily by sea-level eustasy and the amount, rate and locus of terrigenous influx. In updip areas, however, sediment distribution and thickness were also influenced by middle Eocene growth faulting. Diagenetic pathways vary with facies type, but generally include: (1) marine phreatic — boring of skeletal fragments by algae and fungi, grain micritization, and radially fibrous cementation; and (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 porosity and permeability owing to the isolation of moldic and vug pores.