T269 Terrestrial Impact Structures: Origin, Structure, and Evolution

Tuesday, 7 October 2008
1:30 PM - 5:30 PM

Impact to Immediate Post-Impact Effects of the Chesapeake Bay Event

Paper number: T269-11
Program: Technical Sessions
Time: 4:10 PM – 4:30 PM

Authors

Abstract

The shallow marine, late Eocene, Chesapeake Bay impact event formed an 85-90 km wide structure in which a 35-40 km wide central basement crater is surrounded by a shallower annular trough. The ICDP-USGS Eyreville A and B cores, drilled in 2005, indicate that the deepest part of the central crater is filled with a varied succession of impactites and sediments. The suevitic and lithic impact breccias consist of two parts: a lower, melt-poor section of polymict impact breccias and cataclastic gneiss blocks, and an upper, melt-rich section of suevites and clast-rich impact melt rocks. Above these impactites is a succession of slumped clastic sediments, debris-flow deposits, turbidites and olistostromes, and a 275-m megablock of basement-derived granite. During the modification stage, these slumps, slides, debris flows, turbidity currents, and avalanches were partly triggered by the post-impact crater collapse as well as ocean resurge into the cavity, and associated tsunamis. The upper part of the impact-generated succession is largely a diamicton consisting of muddy quartz-glauconite sand with mixed-age sediment clasts, less abundant crystalline-rock clasts, and small amounts of shocked minerals and impact-melt particles, with numerous fining-upward sequences in the upper part. The persistence in the mixture of various age components in the matrix of the diamicton indicates homogenization by ocean resurge and oscillation resurge prior to any early post-impact redeposition. The uppermost part is capped by stratified deposits, including turbidites. Finely laminated mudstones (Chickahominy Formation) represent the return to normal marine, continental-shelf, depositional conditions. Similar successions of impact to post-impact sedimentary deposits are recognized in other marine impact structures.

In addition to these early impact/post-impact successions, the target area and surroundings may have experienced related major events such as wildfires, dust storms, and local climatic changes. An important goal of succeeding analyses is to test for possible traces of these events.