CHESAPEAKE BAY IMPACT CRATER – DEVELOPMENT OF “BRIM” SEDIMENTATION

In the late Eocene (~35 Ma), the 85-95 km diameter Chesapeake Bay impact crater was formed by an asteroid or comet impact into a shallow sea on the continental shelf of eastern North America. The impact excavated an “inverted sombrero”-shaped structure, with a deep central crater surrounded by a shallower brim of collapsed and transported sediments. Tsunamis and earthquakes were generated, and shockwaves travelled outward and downward into an asymmetric, multi-layered target. The target consisted of 200-500 m of water above an E-NE dipping section of 30-50 m of Paleogene marine sediments, 0-40 m of Cretaceous marine sediments, and 400-1150 m of Cretaceous fluvial-deltaic sediments, above Neoproterozoic to Paleozoic crystalline basement. This asymmetry significantly affected the complex filling of the crater. A combination of glauconite, phosphate, and mixed-aged fossils and chaotic clasts characterizes the uppermost impact debris unit, the Exmore Formation. We compiled results of core studies across a radial transect of the crater brim (Watkins School, Langley, and Bayside cores) to reconstruct the history of sedimentation within minutes to days after impact.

The transect shows that the pre-impact sediments were truncated to increasingly greater depths inward across the inner brim, primarily by ocean-resurge currents, and that impact-generated shaking created soft-sediment deformation and sand injections in parautochthonous Cretaceous sediments in the outer part of the brim. There is a correlateable distribution of debris types across the cores of the traverse. The lowermost part of the Exmore is rich in Cretaceous clasts, in normal and inverted positions, probably deposited as avalanches and slides remobilized by partly-concurrent ocean resurge. The upper part of the Exmore is rich in Paleogene clasts, and depositional features indicate multiple resurge and mass-flow events, in contrast to the lower part. Locally, the original pre-impact stratigraphy is crudely shown in sediment-clast distributions; elsewhere it is chaotic. The Exmore thickens inward across the brim and locally contains rare to relatively abundant melt particles and other ejecta clasts. Comparable sections of turbidites and debrites cap the brim section.