HR: 09:30h
AN: U21E-07    [Abstracts]
TI: Multi-directional Block and Breccia Emplacement in the Moat of the Chesapeake Bay Impact Structure Revealed by High-Resolution Seismic Reflection Surveys near the 1.76-km-deep, ICDP-USGS Eyreville Corehole
AU: * Powars, D S
EM: dspowars@usgs.gov
AF: U.S. Geological Survey, 926A National Center, Reston, VA 20192, United States
AU: Catchings, R D
EM: catching@usgs.gov
AF: U.S. Geological Survey, 345 Middlefiled Rd., Menlo park, CA 94857, United States
AU: Gohn, G S
EM: ggohn@usgs.gov
AF: U.S. Geological Survey, 926A National Center, Reston, VA 20192, United States
AU: Horton, J W
EM: whorton@usgs.gov
AF: U.S. Geological Survey, 926A National Center, Reston, VA 20192, United States
AU: Edwards, L E
EM: leedward@usgs.gov
AF: U.S. Geological Survey, 926A National Center, Reston, VA 20192, United States
AU: Rymer, M J
EM: mrymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefiled Rd., Menlo park, CA 94857, United States
AU: Goldman, M R
EM: goldman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefiled Rd., Menlo park, CA 94857, United States
AU: Gandhok, G
EM: ggandhok@hotmail.com
AF: U.S. Geological Survey, 345 Middlefiled Rd., Menlo park, CA 94857, United States
AB: In 2004–2006, the USGS acquired four 1- to 2-km-long, high-resolution seismic reflection and refraction profiles (5-m source and geophone spacing) across the northern half of the central crater of the Chesapeake Bay impact structure (CBIS). The resulting seismic images provide details of the upper 1.0 km of the moat that surrounds the central uplift within the central crater and serve locally as a complement to 38 km of deep seismic data acquired across the CBIS by the USGS and Virginia Tech in 2004. Two profiles that cross at the 1.76-km-deep, ICDP- USGS Eyreville corehole (EC) provide semi-3D views of the stratigraphy and structures. Preliminary data processing and analysis indicate excellent correlation of the images with the EC, including individual block and impact-debris patterns in the upper 300 to 400 m of the CBIS. Variations in reflection patterns correspond to variations in conditions and processes within the central crater during impact. The seismic images show strong, mostly continuous, subhorizontal reflections typical of marine strata for the post-impact sediment units identified in the EC above 444 m depth. These strata dip toward, and thicken above, the center of the moat with relatively minor disruption; underlying synimpact units are highly disrupted. The upper 50-75 m of synimpact deposits have subhorizontal, overlapping-to-shingled reflections on all profiles that dip and pinch-out in various directions and correlate with multiple fining-upwards sequences in matrix-supported breccia of the EC (527-444 m depth). These shingled units are interpreted as deposits formed by multiple tsunami wave reverberations. Seismic signatures vary with location in the underlying 200- to 300-m-thick section of the moat. In the central part, discontinuous, chaotic reflections that bound relatively continuous lenses correlate with resurge breccias in the EC (618-527 m depth). These units are underlain by relatively continuous reflections that pinch-out in various directions as well as chaotic, discontinuous reflections (some imbricated); this interval correlates with clast- supported, sediment-clast breccias assigned to the lower resurge section in the EC (867-618 m depth). Seismic images of the inner part of the moat show more continuous seismic reflections, suggesting more uniform, uni- directional movement of breccias from the central uplift. Collectively, the seismic images suggest that the upper impactites in the central crater are highly variable laterally, with similar units at different depths deposited from multiple directions at multiple times.
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 6022 Impact phenomena (5420, 8136)
DE: 8136 Impact phenomena (5420, 6022)
SC: Union [U]
MN: 2007 Fall Meeting