2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 9
Presentation Time: 1:30 PM-5:30 PM


POWARS, David S.1, EDWARDS, Lucy E.1, BRUCE, T. Scott2 and JOHNSON, Gerald H.3, (1)U.S. Geol Survey, National Center MS 926A, Reston, VA 20192, (2)Virginia Dept. of Environmental Quality, Richmond, VA 23240, (3)Geology Department, College of William and Mary, Williamsburg, VA 23187, dspowars@usgs.gov

Seismic and corehole data indicate that, like other oceanic impacts, the late Eocene Chesapeake Bay impact crater is wider and contains more syn-impact debris than otherwise equivalent “dry-target” counterparts. The outer 10 to15 km of the crater’s annular trough was filled rapidly during late-stage crater modification by outer-margin wall collapse, resurge processes, and tsunami washback that interacted with severely shaken and locally fluidized, water-saturated, pre-impact sediments. Therefore, the outer annular trough is a variable pile of highly fractured and fault-bounded parautochthonous mega-slump blocks overlain and injected by the catastrophic wet jumble of sediments. The crater holds 100 to 900 m of impact modified or generated deposits that subsequently were buried by mostly fine-grained, post-impact marine deposits. Various geomorphic expressions, high rates of sea-level rise, and historical earthquakes argue for ongoing subsidence. The buried crater has affected the depositional history, sedimentary patterns, and stratigraphic relations of the post-impact units that cover it. The post-impact stratigraphic units consist of 140-550 m of upper Eocene to Holocene deposits represented by fairly continuous, planar seismic reflections that are faulted, but to a much lesser degree than the underlying syn-impact deposits. Numerous angled fractures with slickensides and a gouge-filled fault within cores of the post-impact section provide ground-truth for the structures seen on the seismic images. These post-impact fractures and faults may be related to continued compaction and megablock movement. The seismic images also show that most post-impact deposits dip slightly towards the inner basin. Except for Pleistocene fluvial-estuarine deposits, all of the post-impact deposits are marine clays, silts, and very fine to very coarse sands that locally contain diatomaceous, glauconitic, shelly, and calcite-cemented intervals. The creation of a persistent bathymetric low due to post-impact loading and compaction has resulted in the preservation of several post-impact stratigraphic units (upper Eocene Chickahominy Fm, Oligocene Delmarva and Drummonds Corner beds, and lower Miocene Newport News beds) that are present only within or near the Chesapeake Bay impact structure.