STRATIGRAPHIC SEQUENCES IN UPPER CRETACEOUS COASTAL PLAIN SEDIMENTS, DORCHESTER AND BERKELEY COUNTIES, SOUTH CAROLINA

The vertical stacking pattern of terrestrial and marginal-marine facies distinguishes four Upper Cretaceous depositional sequences in cores taken from Dorchester and Berkeley Counties, SC. These sequences form part of a stratigraphic framework that can be traced into the subsurface of North Carolina and correlated to sequences observed in the Elizabethtown and Kure Beach cores in the Cape Fear River basin. Cenomanian to Santonian sediments from USGS cores DOR-037 and BRK-644 were evaluated using insoluble residue and particle size analyses, constrained by biostratigraphic data, and show a similar progression of systems tracts. Sediments from a highstand systems tract (HST), characterized by rooted and oxidized paleosols, overlie either Cenomanian marine (DOR-037) or non-marine (BRK-644) sediments. BRK-644 also contains interbeds of sand. A sequence boundary is marked by an influx of coarse clastic sediment. The overlying lowstand systems tract (LST) contains a thick succession of amalgamated braided channel deposits with common orthoclase and quartz pebbles. Four parasequences, bounded by thin paleosols, are identified in the LST. The ensuing increase in accommodation space and back stepping of facies produced a shift from a braided to meandering stream pattern characterized by channel sands encased in floodplain muds, typical of deposition during a transgressive systems tract (TST). Marine influence is suggested in the overlying muddy sands by a strong spike in carbonate content and reworking of the sediments, suggestive of a transgressive ravinement surface merged with a second sequence boundary. The overlying sequence is believed to be correlative to the Pleasant Creek I sequence observed in NC. Sediments of the TST are dominated by organic-rich, estuarine to marginal-marine sediments of Coniacian/Santonian age. Organic mud characterizes the TST in DOR-037, while sand deposition is more prevalent in BRK-644. A mud-rich, highly calcareous bed in this core may represent the maximum flooding surface which separates the TST from coarsening-upward HST sands. A major floral gap marks the third sequence boundary. This sequence, thought to correspond to the Santonian Pleasant Creek II sequence, is characterized by a thin, muddy TST, shoaling upward to glauconitic-rich sands of the HST in DOR-037.