DEPOSITIONAL ENVIRONMENTS DURING SEA-LEVEL RISE BASED ON PALEOCHANNEL FILL OFFSHORE CHARLESTON, SC

An extensive CHIRP survey covering about 220 sq km and 1-14 kilometers offshore of Charleston, South Carolina with 13 collocated sediment cores provides a rare glimpse of the shallow stratigraphy of a shelf paleochannel system in great detail. This set of paleochannels was incised into continental shelf of the east coast of the United States during the last glacial lowstand, and are relatively common features on U.S. coastal passive margins. 13 cores from 0.5 to 6.5 meters in length are visually described and 5 selected for analysis of grain size using a laser diffraction particle size analyzer, porosity using water displacement, and permeability with a permeameter. Each core is at the intersection of two perpendicular 0.5-10 kHz chirp lines that are spaced at 5.5 kilometers shore-perpendicular to shore and 4.5 kilometers shore-parallel. A stratigraphic framework created in Petrel to integrate all data spatially is used to interpret the size and extent of the paleochannel system. We found that paleochannel fill was distinct from surrounding sediment in terms of facies and permeability. Paleochannels were characterized by a >0.5m thick layer of mud at their base that generally coarsened upward in a transgressive sequence. In contrast, surrounding sediment was characterized by thinner layers and often skipped facies associated with tidal inlets, beach sands, and dunes. Our results are one of few to combine seismic and sediment cores at paleochannel sites on the U.S. southeast margin to interpret the paleo-environment during sea-level transgression. Our results suggest that paleochannels capture a relatively complete stratigraphy of a transgression. Paleochannels also catch large organic sediment deposits during transgressions, and may hinder rather than serve as conduits for submarine groundwater flow on the continental shelf.