A QUATERNARY STRATIGRAPHIC FRAMEWORK FOR THE SANTEE DELTA REGION OF THE ATLANTIC COASTAL PLAIN AND INNER CONTINENTAL SHELF, SOUTH CAROLINA, U.S.A

Modern fluvial valleys, paleovalleys and paleochannels, regional stratigraphic bounding surfaces, as well as aggradational and progradational lithosomes are key components of an integrated stratigraphic framework for the coastal plain and inner shelf of the Santee Delta region along the SE Atlantic margin of the United States. The Piedmont-draining Santee River of South Carolina forms the only river-fed delta along the US east coast, and as such, has been a major sediment source for the region throughout recent geologic history. In addition to the modern river valley, offshore and onshore mapping incorporating various geophysical and geological datasets confirm the location of a separate precursor Santee paleovalley. Both valleys incise into pre-Quaternary siliciclastic and carbonate rocks. On the basis of previous age assignments of mapped onshore stratigraphy and geometric relationships with onshore geomorphic features, the more southerly paleo-Santee valley was active as recently as the mid-Pleistocene. The avulsion from this older valley may be attributable to the creation of fault-related accommodation. Offshore, a complex network of paleochannels can be projected onshore where they align with these ancient river valleys.

In addition to the readily identifiable fluvial paleoincisions, the pre-Quaternary sedimentary units of the inner continental shelf are covered by a patchwork of Pleistocene and Holocene marine and paralic deposits. Many of these units appear massive, lacking internal stratal geometries that would yield clues as to their origin. However, evidence of several regression phases are evidenced by distinct stratigraphic units containing internal geometries that indicate significant progradation. The oldest of these progradational units are of Pleistocene age, extend from the nearshore to the outer shelf, and exhibit internal geometries and stacking patterns consistent with deposition within a deltaic shoreface. The younger progradational units are of Holocene age, coincident with the seafloor, and have internal and external geometries similar to sand ridges documented elsewhere on the US Atlantic inner shelf.