Synimpact-postimpact transition inside Chesapeake Bay crater

Geology
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Abstract

The transition from synimpact to postimpact sedimentation inside Chesapeake Bay impact crater began with accumulation of fallout debris, the final synimpact deposit. Evi dence of a synimpact fallout layer at this site comes from the presence of unusual, millimeter- scale, pyrite microstructures at the top of the Exmore crater-fill breccia. The porous geometry of the pyrite microstructures indicates that they originally were part of a more extensive pyrite lattice that encompassed a layer of millimeter-scale glass microspherules—fallout melt particles produced by the bolide impact. Above this microspherule layer is the initial postimpact deposit, a laminated clay-silt-sand unit, 19 cm thick. This laminated unit is a dead zone, which contains abundant stratigraphically mixed and diagenetically altered or impact-altered microfossils (foraminifera, calcareous nannofossils, dinoflagellates, ostracodes), but no evidence of indigenous biota. By extrapolation of sediment- accumulation rates, I estimate that conditions unfavorable to microbiota persisted for as little as <1 k.y. to 10 k.y. after the bolide impact. Subsequently, an abrupt improvement of the late Eocene paleoenvironment allowed species-rich assemblages of foraminifera, ostracodes, dinoflagellates, radiolarians, and calcareous nannoplankton to quickly reoccupy the crater basin, as documented in the first sample of the Chickahominy Formation above the dead zone.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Synimpact-postimpact transition inside Chesapeake Bay crater
Series title Geology
DOI 10.1130/0091-7613(2002)030<0995:SPTICB>2.0.CO;2
Volume 30
Issue 11
Year Published 2002
Language English
Publisher Geological Society of America
Contributing office(s) Woods Hole Coastal and Marine Science Center
Description 4 p.
First page 995
Last page 998
Country United States
Other Geospatial Chesapeake Bay