SEISMIC-REFLECTION IDENTIFICATION OF SUSQUEHANNA RIVER PALEOCHANNELS ON THE MID-ATLANTIC COASTAL-PLAIN

Land-based, high-resolution seismic-reflection methods were used to im age Quaternary paleochannels of the Susquehanna River system. Using a portable, 12-channel signal-enhancing seismograph, 12 accelerometers a s receivers, and a 4.54-kg sledge hammer struck against an aluminum pl ate as a source, a sixfold, multichannel seismic profile 2.5 km long w as acquired at Taylors Island, Maryland. On the processed seismic prof ile, pronounced high-amplitude seismic reflections delineate the uncon formity between Quaternary and underlying Tertiary sediments and the d isconformable contact separating Miocene and Eocene deposits. Subsurfa ce-seismic stratigraphic relationships that clearly indicate the prese nce of two paleochannels were observed, one believed to be the Exmore paleochannel, projected to underlie northern Taylors Island based on m arine seismic data. An overlapping sequence of fill sediments was obse rved on the eastern margin of the Exmore paleochannel. The second pale ochannel may be a tributary of the Exmore or possibly the western edge of the younger Eastville paleochannel. Results from this study indica te that land-based, shallow, high-resolution seismic-reflection data c an be used to delineate subsurface geomorphology successfully in coast al plain environments. This technique of defining erosional surfaces a nd depositional units beneath present land areas, when integrated with chronostratigraphic data, is a powerful tool for developing a better understanding of the Quaternary record. (C) 1994 University of Washing ton.