PALEONTOLOGY OF CRATER-FILL DEPOSITS, CHESAPEAKE BAY IMPACT STRUCTURE, VIRGINIA

A variety of sediments were deposited during and after the meteorite impact that created the Chesapeake Bay impact structure. The only fossils recovered from crater unit A, which consists of angled parautochthonous megablocks of Cretaceous sediments, are from a spore-pollen sample near the top of the unit, which is middle Albian to early Cenomanian in age (Potomac Group). Crater unit B is a clast-supported breccia. Spore-pollen samples from this unit were derived from the Cretaceous Potomac Group. At least toward the end of crater unit B deposition, the fine sediment settling out of the water column in the crater was composed of a mixture of materials derived from both the Potomac Group and from various Lower Tertiary formations in the region. The Exmore beds, a matrix-supported breccia, contain clasts that vary in age from Early Cretaceous to late Eocene. Most clasts derived from the Potomac Group come from the upper part of the Group (Cenomanian). Lower Tertiary clasts are predominantly of late Paleocene and early Eocene age. Samples from matrix materials that make up the bulk of the Exmore beds are of mixed Late Cretaceous, Paleocene, and Eocene ages. Many specimens of dinoflagellate cysts are fragmented, curled up, or otherwise degraded; specimens of one species of nannofossil also tend to be broken. This degradation of the fossils was probably caused by some combination of heat, shock, and abrasion resulting from the oceanic impact.

Post-impact sedimentation includes the rapidly deposited Chickahominy Formation, several Oligocene and early Miocene units that are only known in and around the impact structure, and a middle Miocene record that includes several unconformities. A second episode of rapid deposition is preserved as the late Miocene St. Marys and Eastover Formations. Reworked microfossils are conspicuous in many of the samples from the upper Eocene and younger units, and some of the reworked microfossils show evidence of damage caused by the impact. Erosion of impact debris deposited outside the crater, as well as continued movement along faults and fractures in and around the outer margin of the crater, are likely causes.