STRUCTURE AND MORPHOLOGY OF THE CHESAPEAKE BAY SUBMARINE IMPACT CRATER

The structure and morphology of the Chesapeake Bay submarine impact structure are interpreted from a large database of seismic reflection (>2000 line-km) and gravity data, calibrated with seven continuously cored boreholes drilled inside and near the crater. The inferred structure is that of a complex crater, with an annular trough (21 km wide), a peak-ring (9 km wide; 126-m relief), an inner basin (28 km wide; 1-2-km relief), and a central peak (5 km wide; 620-m relief), all constructed from Neoproterozoic crystalline rocks (~600 my old) that form the basement under the Virginia Coastal Plain. In contrast, the outer rim (85-km average diameter) is constructed of poorly consolidated, mainly Lower Cretaceous, nonmarine sediments. The rim forms a steep (300-1200-m relief), irregularly circular, fault scarp. Seismic profiles show that hundreds of normal faults (25-50-m throw) and a few reverse faults offset the surface of the crystalline basement within the crater. Groups of concentric normal faults slice the preimpact sedimentary section outside the crater rim (generally no farther than ~10 km away from the rim). Residual gravity anomaly patterns over the impact structure are in good agreement with the principal seismically-identified features in the center of the structure (peak ring, inner basin, central peak), and a gravity model with elevated basement ridges simulates the peak-ring morphology. The gravity data, however, give no indication of the presence of an outer rim. One of the most unusual features associated with this submarine structure is a field of 23 inferred secondary craters (0.4-4.2-km apparent diameters; 50-700-m apparent depths) imaged on seismic profiles northwest of the primary crater. Each secondary crater displays rim faults and chaotic crater-fill reflections typical of small simple craters, and several display raised rims. The structure and morphology of the Chesapeake Bay impact structure are similar to those of other complex craters of our planetary system. In age, size, and morphology, for example, the Chesapeake Bay structure is nearly a twin to the Popigai structure of Northern Siberia. King crater, on the Moon's farside, also is a good analog.