THE CHESAPEAKE BAY BOLIDE IMPACT - A CONVULSIVE EVENT IN ATLANTIC COASTAL-PLAIN EVOLUTION

Until recently, Cenozoic evolution of the Atlantic Coastal Plain has b een viewed as a subcyclical continuum of deposition and erosion. Marin e transgressions alternated with regressions on a slowly subsiding pas sive continental margin, their orderly succession modified mainly by i sostatic adjustments, occasional Appalachian tectonism, and paleoclima tic change. This passive scenario was dramatically transformed in the late Eocene, however, by a bolide impact on the inner continental shel f. The resultant crater is now buried 400-500 m beneath lower Chesapea ke Bay, its surrounding peninsulas, and the continental shelf east of Delmarva Peninsula. This convulsive event, and the giant tsunami it en gendered, fundamentally changed the regional geological framework and depositional regime of the Virginia Coastal Plain, and produced the fo llowing principal consequences. (1) The impact excavated a roughly cir cular crater, twice the size of Rhode Island (similar to 6400 km(2)) a nd nearly as deep as the Grand Canyon (similar to 1.3 km deep). (2) Th e excavation truncated all existing ground-water aquifers in the targe t area by gouging similar to 4300 km(3) of rock from the upper lithosp here, including Proterozoic and Paleozoic crystalline basement rocks a nd Middle Jurassic to upper Eocene sedimentary rocks. (3) Synimpact de positional processes, including ejecta fallback, massive crater-wall f ailure, water-column collapse, and tsunami backwash, filled the crater with a porous breccia lens, 600-1200 m thick, at a phenomenal rate of similar to 1200 m/hr. The breccia lens replaced the truncated ground- water aquifers with a single 4300 km(3) reservoir, characterized by gr ound water similar to 1.5 times saltier than normal sea water (chlorin ities as high as 25,700 mg/l). (4) A structural and topographic low, c reated by differential subsidence of the compacting breccia, persisted over the crater at least through the Pleistocene. In the depression a re preserved postimpact marine lithofacies and biofacies (upper Eocene , lower Oligocene, lower Miocene) not known elsewhere in the Virginia Coastal Plain. (5) Long-term differential compaction and subsidence of the breccia lens spawned extensive fault systems in the postimpact st rata. Many of these faults appear to reach the bay floor, and may be p otential hazards for motion-sensitive structures in population centers around Chesapeake Bay. Near-surface fracturing and faulting generated by the impact shock may extend as far as 90 km from the crater rim. ( 6) Having never completely filled with postimpact sediments, the sea-f loor depression over the crater appears to have predetermined the loca tion of Chesapeake Bay. (7) As large impact craters are principal sour ces for some of the world's precious metals, it is reasonable to expec t that metal-enriched sills, dikes, and melt sheets are present in the inner basin of the crater. In addition to these specific consequences , the crater and the convulsive event that produced it, have widesprea d implications for traditional interpretations of certain structural a nd depositional features of the Atlantic Coastal Plain, particularly i n southeastern Virginia.