THE CORED RECORD OF IMPACT-DERIVED LITHOFACIES IN THE CHESAPEAKE BAY IMPACT CRATER: SERENDIPITY VERSUS STRATEGY

The Chesapeake Bay impact structure has been drilled in two stages, a serendipitous stage followed by a strategic stage. In the serendipitous stage (1986 -1992), four coreholes were drilled before the impact structure had been identified. These early cores established the presence of a subsurface impact breccia 10-55 m thick. Preliminary breccia analyses revealed stratigraphically mixed clasts and sparse evidence of shock metamorphism (planar deformation features in quartz grains) comparable to the Bunte Breccia of the Ries crater (southern Germany). Subsequent acquisition of extensive seismic reflection surveys showed that two of the cores had inadvertently penetrated an 85-km-wide complex impact crater. The seismic profiles indicated that lithofacies recognized in the coreholes were widespread within the crater, and that additional uncored lithofacies were present. This led to the strategic stage of drilling (2000-2001), in which three more coreholes were drilled to document additional lithofacies. The combination of serendipity and strategic planning has allowed recognition of five impact-derived lithofacies (from deepest to shallowest): (1) Slumpback facies – kilometer-scale megaslides, megaslumps, and collapse structures derived from the sedimentary outer walls of the crater; activated by acoustic fluidization; (2) Surgeback facies – meter- to decameter-scale clasts scoured from the seafloor, crater walls, and crater floor by the collapsing 300-m-thick marine water column; (3) Washback facies – boulder- to sand-sized clasts scoured from the seafloor and coastal plain by impact generated tsunamis and then washed back to sea; (4) Hypercane(?) facies – silt- and clay-sized debris stirred from the seafloor by impact-generated hypercanes and moved back into the crater by turbidity currents and debris flows; and (5) Fallout facies – sand- to silt-sized debris containing porous pyrite microlattices, which originally enclosed glass microspherules produced by the impact. At least one additional inferred lithofacies, a fallback facies, presumably of mainly crystalline clasts, remains to be documented in the inner basin of the crater, which has not yet been drilled.