Influence of paleochannels on submarine groundwater flow from CHIRP seismic surveys and sediment cores on the shelf offshore Charleston, South Carolina

Abstract:

The continental shelf off Charleston, South Carolina is the site of an on-going project to understand fluid flow and geochemical budgets across the seafloor over a broad area of typical passive margin. The continental shelf of the east coast of the United States is commonly incised with paleochannels, sub. The paleochannel fill consists of a lithology and permeability distinct from the surrounding material, changing the rate of submarine groundwater discharge across these features. We have visually described 13 cores from 0.5 to 6.5 meters in length and sampled and tested for permeability and grain size using a laser diffraction particle size analyzer and a permeameter. Each core is at the intersection of two perpendicular 0.5-10 kHz chirp lines covering about 220 sq km to about 14 kilometers offshore. The chirp lines are spaced about every 5.5 kilometers perpendicular to shore and about 4.5 kilometers parallel to shore. The chirp data define the entire extent of the channels and the volume of the infill.

Comparison of a sediment core in a paleochannel with a core directly adjacent suggests much higher permeability in the channel. Both cores have shelly gravel on the seafloor, but layer outside the channel is 1.3 m thick while in the channel there is a thin veneer of gravel. The channel fill has a 3 meter thick layer of coarse arenite sand. The core outside the channel has a 1.7 meter thick layer of silty sand interbedded with thin silt layers above a 0.8 meter thick layer of muddy shell hash. A layer of massive brown mud at the bottom of the core that in the channel is 3.2 meters thick and outside of the channel is 1.9 meters thick acts as a basal confining unit.

Future work will provide an estimate of the influence of paleochannels on the flow across the seafloor on a passive continental shelf by using these data to constrain numerical groundwater flow models.