Distribution and Origin of Impact-Generated Debris: Western Annular Trough, Chesapeake Bay Impact Crater
Abstract
The buried 35-million-old Chesapeake Bay impact structure is up to 160 km wide and possibly excavated 3.3 km deep. Within the CBIS is a 90-km-wide Chesapeake Bay impact crater has a 38-km-wide inner basin with a central uplift surrounded by a bowl-shaped zone of basement rocks faulted down to about 11 km depth. A 21- to 31-km-wide annular trough outside the inner basin and peak ring has a slumped terrace zone at its outer margin and is bounded by an outer rim consisting of 300- to 500-m-high gullied escarpment. An outer fracture zone up to 35 km in width surrounds the crater. The USGS and affiliated institutions recently drilled three deep coreholes in the western part of the structure's annular trough and completed high-resolution seismic reflection and audio-magnetotelluric (AMT) surveys across its southwestern margin. The coreholes are located 22.7, 19.7, and 8 km outside the inner basin. Both marine- and land-based seismic data reveal numerous faults that displace the top of basement and overlying Coastal Plain sediments in the annular trough and the outer fracture zone. Due to contrasting resistivity across the crater's margin the AMT data are useful locating the outer rim. The seismic profile from the NASA Langley Research Center, Va., and the deep Langley core (19.7 km outside the inner basin) indicate that impact-generated debris near the outer rim can be characterized by a three-part physical stratigraphy. (1) Above granitic basement rock, unit A consists of 183.8 m of parautochthonous Cretaceous sediments of the Potomac Formation, tensionally fractured in situ and disrupted during late-stage gravitational collapse of the crater. The deepest occurrence of impact-induced sediment fluidization is probably in the upper beds of unit A. Seismic data indicate unit A is pervasively disrupted by short, subvertical to low-angle faults. (2) Middle unit B consists of a clast-supported diamicton (173 m thick) containing zones of moderately deformed Potomac megaclasts (1-25 m) separated by disruption zones consisting of strongly deformed and disaggregated clasts (4 mm to 1 m) of Potomac sediments and dissociated glauconitic Tertiary sediments, and sparse crystalline-rock clasts. These features suggest that unit B has experienced extensive fluidization, mixing, and local resedimentation. Seismically unit B is represented by discontinous reflections that fill excavated areas. (3) The Exmore beds consist of 33.8 m matrix-supported, muddy, quartz-glauconite sand with clasts of pre-impact Tertiary and Cretaceous sediments (granules to meters), and sparse shocked quartz and brecciated crystalline-rock clasts. The seismic survey shows multiple depositional packages within the Exmore beds that truncate underlying ones and have wavy upper surfaces. The core closest to the inner basin contains a different sequence of impact-generated debris; a basal 53.6 m of rubble is overlain in turn by 285 m of unit B deposits and 91.5 m of the Exmore beds. Apparently up to 8 km outside the inner basin all the pre-impact sediments were excavated by the impact. The presence of Tertiary dinocyst fragments at the base of unit B indicates deposition at nondestructive temperatures (<2000C) as backsurge or tsunami deposits or sediment dikes. Apparently the outer annular trough is a variable pile of mega-slump blocks, slump blocks, multiple debris flows, and tsunami-backwash deposits.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2002
- Bibcode:
- 2002AGUSM.T21A..02P
- Keywords:
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- 4564 Tsunamis and storm surges;
- 5420 Impact phenomena (includes cratering);
- 5717 Impact phenomena;
- 6022 Impact phenomena