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.