FROM SHOCKED BASEMENT TO FALLOUT SPHERULES: THE CORING RECORD AT THE CHESAPEAKE BAY CRATER

The USGS and its collaborators have completed seven completely cored boreholes to document the stratigraphic and structural attributes of the Chesapeake Bay impact crater. With 2,921 m of core to calibrate >2,000 km of seismic reflection profiles, we have considerably improved the conceptual model of this 85-km-diameter impact crater. Seismic profiles indicate that the surface of crystalline basement is highly fractured and faulted. Cores also reveal that basement rocks (metagranodiorite) are intensely fractured. Basement cores from the outer part of the annular trough display no shock metamorphism. Basement clasts derived from the center of the crater, however, display shock fractures, planar deformation features, melt rock, and glass spherules indicative of shock pressures of up to 60 GPa. Seismic profiles show that kilometer-sized, rotated, stratified megablocks overlie the basement surface in the annular trough. Cores from the megablocks reveal mainly unconsolidated sands with horizontal to moderately inclined (15-degrees) bedding. Above the megablocks seismic profiles indicate a thick interval (>1 km) of chaotic reflections attributable to a breccia unit containing large clasts. Cores reveal that this interval consists of sediment-clast-dominated, upward fining, crater-fill breccia, some clasts of which are 20 m thick. Microfossils show that the breccia clasts represent all pre-Oligocene sedimentary units of the Virginia Coastal Plain (Lower Cretaceous to upper Eocene strata). Capping the crater-fill breccia is a thin (0.6 m) of clayey silt, devoid of indigenous fossils and too thin to discern on seismic profiles. This barren interval contains evidence of impact fallout debris. The evidence consists of millimeter-sized, lattices of framboidal pyrite, the pores are of which are uniform in shape (spherical) and size (~1 mm in diameter). The pyrite appears to have formed an embracing framework around a layer of stacked glass spherules, which subsequently altered to clay. We inadvertently washed the clay out of the pyrite lattice during preparation for micropaleontological analysis. This succession of lithofacies cored in the Chesapeake Bay impact crater is compelling substantiation that a giant bolide struck the Atlantic continental shelf of North American during the late Eocene, ~35.8 Ma.