2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 6
Presentation Time: 2:55 PM

Asymmetric (Dipping) Layered Target Constrained Infilling of the Chesapeake Bay Impact Structure

POWARS, David S.1, EDWARDS, Lucy E.1, CATCHINGS, Rufus D.2, GOHN, Gregory S.1 and HORTON, J. Wright1, (1)U.S. Geological Survey, 926A National Center, Reston, VA 20192, (2)Earthquake Science Center, U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025, dspowars@usgs.gov

The Chesapeake Bay impact structure formed ~35.5 Ma when a ~3-km-diameter impactor crashed into the Atlantic Ocean ~160 km offshore from Virginia's late Eocene paleo-coastline. Recent framework compilation reveals new details of the complex, asymmetric, multi-layered, wet-target that consisted of an eastward-dipping crystalline basement overlain by an eastward-thickening wedge of unconsolidated sediments (~450 to 1,220 m) and an eastward-deepening ocean (90 to >300 m). The eastern part of the target had significantly deeper basement rocks, deeper and thicker sediment cover, and deeper water than the western part. This asymmetry significantly affected the complex late-stage infilling of the structure's central crater (collapsed transient cavity).

The top of the crystalline basement is deepest at the eastern margin of the central crater (~1.4 km). This level is about the same as the top of the suevite and base of deepest sediment-dominated breccia and blocks in the “moat” of the central crater in the ICDP-USGS Eyreville core, which suggests the possibility that spillover constrained the ultimate crater geometry. Elevations of the tops of sediment-dominated breccias in the central crater and preimpact sediments outside the central crater's eastern rim are approximately the same, suggesting equilibrium between erosion and deposition with possible spillover. The variable thickness of the sediment layer and differences in strength between the sediments and the underlying crystalline rocks possibly contributed to a terraced, asymmetrical excavation flow out to various distances from the present rim of the central crater.

High and low velocity intervals that extend down to 3.5-km-depth on a USGS regional seismic survey may record effects of the transient-cavity collapse. The upper crater fill was deposited and (or) reworked by ocean resurge and multi-directional oscillation waves. This fill contains shocked rocks and melt particles that represent late-stage ejecta fallback.