Small Mud and Sand Filled Paleochannels Offshore Charleston, SC.

摘要

The spatial variations of Quaternary sediments on the continental shelf record the progression of depositional environments during the latest sea-level rise, and are fundamental controls on submarine groundwater discharge (SGD) flow. SGD on continental shelves is an important but poorly constrained process. Spatial variations of lithologies on continental shelves, such as sand-filled paleochannels, are assumed to play a fundamental role in controlling where SGD occurs but coincident seismic mapping and sedimentological data are rare. Here we combine seismic mapping of paleochannel locations with sediment cores to examine the sedimentary characteristics of paleochannel fill and interfluvial deposits. To establish paleochannel locations and abundance and the nature of sedimentary deposits in these features, we collected high-frequency seismic reflection (CHIRP) surveys along with 13 vibracores in a 150 km2 area north of Charleston, South Carolina. Based on the core data, we defined eight lithofacies correlated to specific depositional environments based on grain size, sorting, rounding, sedimentary structures, and stacking patterns. Our results indicate that paleochannels were inundated by rising sea level and became a series of tidal creeks and estuarine channels. Paleochannels smaller than 400 meters wide were filled with structureless marsh mud, while paleochannels larger than 400 meters wide contain a thick layer of structureless mud interbedded with silt overlain by well-sorted quartzose sand. The greater accommodation associated with the larger channels likely played a role in preserving the sandy sediments. Due to more than 1.3 meters of low permeability mud at the base of paleochannels, paleochannels in this area are almost certainly not preferential paths for SGD. A widespread layer of interfluvial mud was identified in 50% of the CHIRP track lines. This mud layer was likely deposited in a mudflat or similar back-barrier environment and is likely a leaky confining layer with respect to SGD. Neither the paleochannels nor the mud layer were found more than 11 km off the present shore, indicating the paleoshoreline was approximately this far offshore during incision of these paleochannels. Mud and silt, followed finally by sand, filled the channels and interfluvial areas as estuarine mudflat and barrier island complexes migrated landward as sea level rose.