INSIGHTS INTO THE STRUCTURALLY COMPLEX INNER CRATER OF THE CHESAPEAKE BAY IMPACT STRUCTURE FROM GEOPHYSICS AND DEEP TEST HOLES

Preliminary analyses of marine and land-based seismic-refraction, seismic-reflection, gravity, and magnetotelluric (MT) data, as well as data from an 823-m-deep partially cored USGS test hole and a 1.76-km-deep, continuously cored ICDP-USGS test hole provide insights into the configuration of the inner crater of the buried, late Eocene Chesapeake Bay impact structure. The main features of the 35- to 38-km-diameter inner crater are a broad central uplift surrounded by an elliptical, structurally collapsed moat. The inner crater is bounded by a complex, variably-collapsed margin with a raised rim. The raised rim in part coincides with locally stacked, top-of-basement seismic-reflection signatures that may represent partially preserved crystalline-flap remnants. Gravity, MT, seismic-velocity, and seismic-reflection images define a NW-SE-elongate, ~10-km-wide (short axis) central uplift that rises about 1 km above the floor of the surrounding moat. Seismic velocity images suggest that a major decrease in the velocity gradient at the 5.6 km/s contour may represent the top of coherent basement rocks that underlie the crater floor. A strong set of continuous reflectors descends from the central uplift at a depth of 2.0 km down to ~3.2 km depth over a 5 km horizontal distance and may represent the flank of the central uplift where slumped megablocks sharply overlie autochthonous, fractured crystalline-basement rocks. The USGS test hole on the central uplift penetrated sediment-clast breccia below 355-m depth and polymict suevitic breccia and monomict brecciated gneiss below 655-m depth. The velocity and seismic-reflection data suggest impactites may be ~1.4 km thick over the central uplift. The moat is ~15 km wide and corresponds to a gravity minimum, comparatively low electrical resistivities, and relatively continuous, subhorizontal, inward-dipping seismic reflections from synimpact and postimpact sedimentary sections. Inward-dipping reflections and relatively steep gradients of gravity (19.5 to 25.0 mGal), resistivity, and seismic-velocity mark the inner-crater margin. The ICDP-USGS deep corehole was drilled in the moat and recovered 1,322 m of impactites below 444 m of postimpact sediments, where seismic-reflection data suggests that impactites may reach ~2.7 km in thickness.