2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 8
Presentation Time: 3:35 PM


POWARS, David S.1, PIERCE, Herbert A.2, DANIELS, David L.2, GOHN, Gregory S.1, CATCHINGS, Rufus D.3, HORTON Jr, J. Wright1 and EDWARDS, Lucy E.1, (1)U.S. Geol Survey, 926A National Center, Reston, VA 20192, (2)U.S. Geol Survey, 954 National Center, Reston, VA 20192, (3)U.S. Geol Survey, 345 Middlefield Rd, MS-977, Menlo Park, CA 94025, dpowars@visuallink.com

New gravity, magnetotelluric (MT), and high-resolution seismic-reflection surveys, plus samples and geophysical logs from a new 823-m-deep, partially cored test hole, were combined with previous geophysical and corehole data to produce new insights into the structure and stratigraphy of the buried, late Eocene Chesapeake Bay impact structure. A gravity maximum, comparatively high resistivities at moderate depths, and shallow basement reflections coincide in the middle of the central crater, thereby indicating a central uplift. The test hole encountered sediment-clast breccias below 355-m depth and crystalline-clast breccias and brecciated gneiss below 655-m depth on the uplift’s eastern flank. Coinciding gravity minima, comparatively low resistivities at moderate depths, and inward-dipping reflections from synimpact and postimpact sedimentary sections indicate that a complex structural moat surrounds the central uplift within the central crater. Gravity maps show small, low-amplitude maxima, and the MT survey suggests local structural uplift, within the moat. Steep gravity and resistivity gradients mark the central crater’s outer margin in the same areas where seismic reflections from preimpact materials are truncated. MT data suggest fracturing of crystalline rocks to depths of at least 6 km in the central crater. Most seismic profiles that cross the impact structure’s annular trough display one or more extensional collapse structures that range in width from 0.5 to 3.9 km. These features consist of abundant short, small-displacement faults rather than large through-going faults. Most of the collapse structures disrupt the entire section from crystalline basement rocks upward through preimpact and synimpact sediments. The collapse structures locally coincide with compressional structures in the basement and with the locations of the greatest postimpact subsidence in the annular trough. Where seismic profiles cross, the collapse structures appear concentric to the central crater. Correlation of the collapse structures around the annular trough defines four main concentric structural rings that locally merge or bifurcate.