Donald J. Colquhoun

A Fossil Assemblage from the Wicomico Formation in Berkeley County, South Carolina

Both megafossils and fossil Foraminifera have been obtained from unconsolidated sediments of the Wicomico Formation, which underlies the Penholoway terrace, a coastal feature that formed when the shoreline of the Atlantic Ocean stood about 70 feet higher than at present. Some of the fossils are Tertiary species and are assumed to have been derived from rocks of that age; the others are species commonly found in near-shore deposits of Quaternary age as well as in present-day south-Atlantic coastal waters. The Penholoway terrace in this area is shown to have been formed during a pause in marine regression from the Wicomico stand, when the sea stood near 100 feet. The relation of the Wicomico and Penholoway terraces to other coastal terraces suggests a Yarmouth age for the fossil-yielding beds.

Stratigraphic correlation of oligocene marginal marine and fluvial deposits across the middle and lower coastal plain, South Carolina

Abstract The age and stratigraphic relationship of the Upland Unit, which crops out at the Savannah River Site near Barnwell, South Carolina has been the focus of many recent investigations. The geological interpretation of the Upland Unit is particularly significant since it serves as the upper confining unit used in the storage of lowlevel radioactive wastes at the Savannah River Site. The age and regional extent of the unit is also important in providing an accurate geological map of the upper coastal plain. The age of the Upland Unit has been in dispute because it lacks datable material. Extensive coring and seismic studies have been conducted to investigate the occurrence and regional distribution of this stratigraphic unit. Lithologically, the Upland Unit consists of poorly sorted, clayey to silty, medium- to coarse-grained sands and gravels of fluvial origin. Similar quartz gravel deposits have been reported from the Chandler Bridge Formation near Charleston, South Carolina. These ‘Upland-like’ gravels are the oldest gravels of Tertiary age reported in the lower coastal plain. The Chandler Bridge Formation is interpreted as a downdip marginal marine facies of an extensive fluvial drainage system which once extended from the upper to the lower coastal plain of South Carolina. Where present, the Chandler Bridge Formation is overlain by nodular phosphate deposits of the Edisto Formation (late Oligocene to early Miocene), and underlain by the Ashley Formation of late Oligocene age. Pollen and dinoflagellate analyses conducted on sediment samples also confirm a late Chattian age for the Chandler Bridge Formation. Consequently, if quartz clasts in the Chandler Bridge Formation represent fluvial transport of Upland Unit gravels from the upper coastal plain which seems likely, then the age of the Upland Unit can be no younger than late Oligocene. Lithologic and stratigraphic analyses suggest that the Upland Unit and the Chandler Bridge Formation are correlative and represent a sequence of fluvial and marginal marine deposits of late Oligocene age. The depositional history of these formational units provides additional information concerning the geological evolution of the middle and lower coastal plain of South Carolina.

A review of Cenozoic evolution of the southeastern United States Atlantic coast north of the Georgia trough

Abstract From late Eocene or Oligocene to the present the pattern of cyclic sediment deposition on the Atlantic coastal plain of the southeastern United States is the product of: (1) a relatively stable trailing edge continental margin with gentle downwarping; (2) deposition of siliciclastics by major Appalachian Mountain and Piedmont draining rivers and shelf generated carbonates; (3) a humid temperate to subtropical climate; (4) a mesotidal coast with moderate wave energy and periodic tropical storms; (5) dominant longshore drift towards the southwest; (6) a history of sea level change that was moderately submergent from late Cretaceous through late Eocene or Oligocene and mildly emergent during the Neogene. Intermittent fluctuations in sea level (up to 100 m in the Neogene) subdivide the stratigraphy as unconformities during the late Oligocene through Holocene, but are less pronounced in the Paleogene. Each of these six factors has influenced the geomorphology, sedimentation, stratigraphy, petrology and geometry of the Cenozoic sedimentary units and the diversity of surficial Quaternary coastal deposits. During the Quaternary sea level change would have been produced by glacial-interglacial cycles. The late Eocene-Oligocene to early Miocene and the late Miocene transition experienced a major sea level decline influenced by ice cap formation in addition to climatic cycle induced sea level fluctuations.

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

Abstract Surficial and near-surface soils of the South Carolina Coastal Plain reflect a variety of lithologies and depositional environments that are difficult to differentiate because of intense leaching and abrupt or laterally inconsistent facies changes. Binocular microscopic examination, scanning electron microscopic/energy dispersive X-ray (SEM/EDX) observations, and X-ray diffraction (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 the units results in alternation of mineralogic signatures within a low-gradient fluvial/transitional/marine depositional system. Late Eocene and Early Oligocene offshore sediments were deposited in a mixed carbonate–siliciclastic, middle- to outer-shelf environment that was subjected to periods of erosion or non-deposition during transgressive events. Detrital and diagenetic 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-energy fluvial deposits of the Late Eocene Tobacco Road Sand is based on distinctive hydroxy-interlayered vermiculite (HIV) signatures. Intervals of HIV-enrichment are coincident with accumulations of carbonaceous material and identified as paleosols; these “soils” are used to infer offshore transgressive periods. Onshore sediments of the Late Eocene Dry Branch Formation contain high concentrations of smectite and flocculated, relatively poorly crystallized kaolinite flakes reflective of marine depositional conditions. At the base of this unit, authigenic Ca-minerals (Ca-zeolites and calcite) and quartz lepispheres (opal-CT) form coatings on and between sand grains. Late Eocene siliceous microfossils that contribute to opal-CT formation are identified in southwestern North Atlantic continental margin shelf, slope and rise deposits and are a valuable correlation tool for onshore sediments.

Scouring of Buried Pleistocene Barrier Complexes as a Source of Channel Sand in Tidal Creeks, North Island Quadrangle, South Carolina

Scouring of buried Pleistocene barrier complexes has been documented through the use of aerial photographs, seismic data, and mineralogic, sedimentary and faunal analyses. This scouring is responsible for large bodies of sand occurring at meander bends and for isolated bodies of clean sand in mud sequences.