Depositional and diagenetic signatures of Late Eocene-Oligocene sediments,South Carolina

Surficial and near-surface soils of the South Carolina Coastal Plain reflec t a variety of lithologies and depositional environments that are difficult to differentiate because of intense leaching and abrupt or laterally incon sistent facies changes. Binocular microscopic examination, scanning electro n microscopic/energy dispersive X-ray (SEM/EDX) observations, and X-ray dif fraction (XRD) analyses indicate that onshore Late Eocene to Late Oligocene Barnwell Group sediments are transitional facies ranging from high-energy fluvial deposits to offshore siliciclastic shelf sands. Interfingering of t he units results in alternation of mineralogic signatures within a low-grad ient fluvial/transitional/marine depositional system. Late Eocene and Early Oligocene offshore sediments were deposited in a mixed carbonate-silicicla stic, middle- to outer-shelf environment that was subjected to periods of e rosion or non-deposition during transgressive events. Detrital and diagenet ic characteristics of the onshore kaolinite-enriched, Late Oligocene Upland Unit sediments reflect deposition in a high- to low-energy fluvial system. Differentiation between these uppermost sediments and the underlying low-e nergy fluvial deposits of the Late Eocene Tobacco Road Sand is based on dis tinctive hydroxy-interlayered vermiculite (HIV) signatures. Intervals of HI V-enrichment are coincident with accumulations of carbonaceous material and identified as paleosols; these "soils" are used to infer offshore transgre ssive periods. Onshore sediments of the Late Eocene Dry Branch Formation co ntain high concentrations of smectite and flocculated, relatively poorly cr ystallized kaolinite flakes reflective of marine depositional conditions. A t the base of this unit, authigenic Ca-minerals (Ca-zeolites and calcite) a nd quartz lepispheres (opal-CT) form coatings on and between sand grains. L ate Eocene siliceous microfossils that contribute to opal-CT formation are identified in southwestern North Atlantic continental margin shelf slope an d rise deposits and are a valuable correlation tool for onshore sediments. (C) 2000 Elsevier Science B.V. All rights reserved.