HR: 1340h
AN: S23B-0263    [Abstracts]
TI: Seismic Profiles Across The Chesapeake Bay Impact Crater
AU: * Goldman, M R
EM: goldman@usgs.gov
AF: U.S. Geological Survey, MS 977 345 Middlefield Rd, Menlo Park, CA 94114 United States
AU: Catchings, R D
EM: catching@usgs.gov
AF: U.S. Geological Survey, MS 977 345 Middlefield Rd, Menlo Park, CA 94114 United States
AU: Powars, D S
EM: dpowars@usgs.gov
AF: U.S. Geological Survey, MS 926A National Center, Reston, VA 20192 United States
AU: Gohn, G S
EM: ggohn@usgs.gov
AF: U.S. Geological Survey, MS 926A National Center, Reston, VA 20192 United States
AB: In September and October 2004, the U.S. Geological Survey and Virginia Tech acquired a series of seismic reflection and refraction profiles along the southern Delmarva Peninsula, Virginia. Our objective was to determine the overall geometry and structure of the 35-million-year-old Chesapeake Bay Impact Crater (CBIC). One of the profiles, an approximately 7-km-long east-west-trending seismic refraction and low-fold reflection profile was acquired about 6 km north of the town of Cape Charles. Seismic sources (5 to 45 kg) for the main east-west profile consisted of explosions in approximately 20-m-deep boreholes, spaced at about 1 km intervals; data were recorded with 146 PASSCAL "Texan" seismographs, spaced at 50-m intervals across land surfaces and water bodies. We also acquired an approximately 2-km-long, high-resolution (5 m shot and geophone spacing) seismic profile along a section of the main east-west profile using a combination of small (0.5 kg) explosive and seisgun sources. These profiles were located across part of the crater moat, north of the central uplift. We developed tomographic velocity images and stacked CDP images for both the main east-west profile and the high-resolution profile. Tomographic velocity images suggest that coherent basement rocks (greater than 5.5 km/s) are relatively shallow (about 1.6 km) beneath the eastern end of the main east-west seismic profile, but these higher velocities deepen (to about 2.1 km) near the western end of the profile. The overlying rocks and sediments also dip to the west along the profile. Post-impact sediments (less than 2.5 km/s) are approximately 0.7 km thick and are underlain by 0.7 km of brecciated rocks (about 2.5 to 4.0 km/s) and the top of fractured basement (about 4.0 to 5.5 km/s) at 1.4 km depth. The reflection and tomographic images suggest similar subsurface structures, including westward dips of higher-velocity layers near the western end of the profile. High-resolution CDP images suggest relatively planar layering near the central part of the main east-west profile, with strong reflections at 100 and 400 m depth. The east-west profiles, combined with two north-south-trending profiles, suggest that the CBIC is similar to many other impact structures, in which central basement uplift is surrounded by a relatively deeper moat.
DE: 7270 Tomography (6982, 8180)
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: Fall Meeting 2005