SHALLOW SEISMIC REFLECTION IMAGING ACROSS THE CHESAPEAKE BAY IMPACT INNER CRATER RIM

Preliminary results from an ongoing high-resolution seismic reflection project across the inner crater rim of the Chesapeake Bay Impact Crater will be presented. Since its discovery in 1993, much of the detailed work on the Chesapeake Bay Impact Crater has focused on the effects of the impact on the geologic, structural and hydrogeologic framework of the outer rim of the crater. Less is known about the inner crater, which is the region that was most deeply excavated by the impact and that, like the Chicxulub structure, is likely to have the most complex structure and stratigraphy. The primary goal of this project is to use images of the transition from the inner crater to the outer crater to study the effects of impact processes on both pre-existing shallow marine stratigraphy, and post-impact stratigraphy of the continental shelf. A 600 m seismic line across the projected inner crater rim was collected in September 2002 as part of the USGS Chesapeake Bay Impact Crater Project. Pre-processing (editing noise traces, automatic gain control, and band-pass filter for display) of the top 2 s of the seismic profile shows three strong reflectors at 0.2 s, 0.3 s, and 0.5 s, which can indicate post impact sediments, and weaker deeper events. Interactive spectral analysis of the raw data shows a frequency content for the reflectors between 100 Hz and 200 Hz. These data were collected using two 60-channel seismographs, 120 geophones, and a Betsy seisgun. All shots (at 10 m spacing) were recorded by all geophones (at 5 m) with a sampling interval of 0.5 ms. An additional 2-3 km of seismic data will be collected by the University of Delaware group in the summer of 2003 along roads that lie on both sides of the inner rim as it has been mapped in the existing literature (Powars and Bruce, 1999).