Following the River: Quaternary Valleys of the Santee River as Integral Components of a Regional Stratigraphic Framework

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. The Quaternary valleys and offshore depositional elements of the Santee have, however, been widely understudied.

The Santee River has occupied its current valley for at least the last 80 kyr; however, integrated offshore and onshore mapping has confirmed the location of an older paleo-Santee valley. This paleovalley intersects the coast beneath Bulls Bay, 35 km to the south of the modern valley. Both valleys incise into structurally deformed Paleocene, Eocene, and Oligocene siliciclastic and carbonate rocks and extend offshore for at least 25 km. On the basis of geometric relationships with onshore geomorphic features, the more southerly paleo-Santee valley was active during the mid-Pleistocene. It is probable that fault-related accommodation played a key role in the avulsion of the Santee River from its more southerly trending paleovalley into its modern valley.

Offshore, the inner continental shelf is covered by a patchwork of Quaternary and pre-Quaternary deposits. Adjacent offshore paleovalleys and paleochannels preserve mostly mud-rich, aggradational transgressive deposits, while extra-channel deposits record at least two, distinct episodes of coastal progradation. Older progradational units are of Pleistocene age and extend basinward to the modern shelf edge. Younger progradational units are coincident with the seafloor from Bulls Bay to just north of the modern Santee Delta. These younger, sand-rich, progradational units are of mid-Holocene age and are likely the result of an increased sediment supply to the coast during this time. Defining the spatial and temporal distributions of paleovalleys, paleochannels, bounding surfaces, and depositional elements and the relationships between these features is key to understanding the geologic evolution of the region.