The Chesapeake Bay bolide impact: a convulsive event in Atlantic Coastal Plain evolution

Abstract

Until recently, Cenozoic evolution of the Atlantic Coastal Plain has been viewed as a subcyclical continuum of deposition and erosion. Marine transgressions alternated with regressions on a slowly subsiding passive continental margin, their orderly succession modified mainly by isostatic adjustments, occasional Appalachian tectonism, and paleoclimatic change. This passive scenario was dramatically transformed in the late Eocene, however, by a bolide impact on the inner continental shelf. The resultant crater is now buried 400–500 m beneath lower Chesapeake Bay, its surrounding peninsulas, and the continental shelf east of Delmarva Peninsula. This convulsive event, and the giant tsunami it engendered, fundamentally changed the regional geological framework and depositional regime of the Virginia Coastal Plain, and produced the following principal consequences. (1) The impact excavated a roughly circular crater, twice the size of Rhode Island (∼6400 km 2) and nearly as deep as the Grand Canyon (∼1.3 km deep). (2) The excavation truncated all existing ground-water aquifers in the target area by gouging ∼4300 km 3 of rock from the upper lithosphere, including Proterozoic and Paleozoic crystalline basement rocks and Middle Jurassic to upper Eocene sedimentary rocks. (3) Synimpact depositional processes, including ejecta fallback, massive crater-wall failure, water-column collapse, and tsunami backwash, filled the crater with a porous breccia lens, 600–1200 m thick, at a phenomenal rate of ∼1200 m/hr. The breccia lens replaced the truncated ground-water aquifers with a single 4300 km 3 reservoir, characterized by ground water ∼1.5 times saltier than normal sea water (chlorinities as high as 25,700 mg/l). (4) A structural and topographic low, created by differential subsidence of the compacting breccia, persisted over the crater at least through the Pleistocene. In the depression are 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 strata. Many of these faults appear to reach the bay floor, and may be potential 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-floor depression over the crater appears to have predetermined the location of Chesapeake Bay. (7) As large impact craters are principal sources for some of the world's precious metals, it is reasonable to expect 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 widespread implications for traditional interpretations of certain structural and depositional features of the Atlantic Coastal Plain, particularly in southeastern Virginia.