Structural outer rim of Chesapeake Bay impact crater: Seismic and bore hole evidence

Abstract

Abstract— Nine seismic‐reflection profiles and four continuous core holes define the gross structural and stratigraphic framework of the outer rim of the Chesapeake Bay impact crater. The rim is manifested as a 90 km diameter ring of terraced normal‐fault blocks, which forms a ∼320 m–1200 m high rim escarpment. The top of the rim escarpment is covered by a 20 m–30 m thick ejecta blanket. The escarpment encircles a flat‐floored annular trough, which is partly filled with an ∼250 m thick breccia lens (Exmore breccia). The Exmore breccia overlies a 200 m–800 m thick interval of slumped sedimentary megablocks, which, in turn, rests on crystalline basement rocks. All postimpact strata (upper Eocene to Quaternary) sag structurally into the annular trough, and most units also thicken as they cross the rim into the crater. Postimpact compaction and subsidence of the Exmore breccia have created extensive normal faulting in overlying strata.

Energy, volatile production, and climatic effects of the Chicxulub Cretaceous/Tertiary impact

“…In contrast, Sharpton et al [1993,1996] analyzed gravity and core data from which they inferred a Df of 260-300 km and a minimum Dte of 120 km and a Sharpton et al [1996] as well, it has never been conclusively demonstrated that the major gravity feature at Chicxulub must correlate with the final crater rim. For example, the major gravity anomaly in the -•90 km diameter Chesapeake Bay crater lies well within the crater rim [Poag, 1996], which was also formed in sedimentary rocks like Chicxulub. Hildebrand et al [1995] equate the location of the transient crater rim with a gravity low that they interpret as the peak ring at a radius of 40-45 km.…”

Traces of catastrophe: a handbook of shock-metamorphic effects in terrestrial meteorite impact structures

“…About one-third of the presently known impact structures are subsurface (Grieve, 1991(Grieve, , 1997Grieve andMasaitis, 1994;Grieve et aL, 1995); (Canada) (Jansa andPe-Pipe,;]. Several large and relatively young buried impact structures have also been identified by geophysical techniques: the 90-km-diameter Chesapeake Bay Crater (USA) (Poag, 1996(Poag, , 1997; the larger (>180-kin diameter) Chicxulub structure (Mexico), which is associated with the K/T event (Hildebrand et aL, 1991;Sharpton et al, 1992;papers in Ryder et al, 1996); and the large (->70 km?) Morokweng structure (South Africa) (Corner et aL, 1997;Koeberl et al, 1997a).…”

“…If th : crater remains below the water level, it will immediately begin to fill with sediments, and its subsequent history will _.epend on whether it remains below water level (continuous sediment filling) or is uplifted at some future time (begim ing of erosion). A number of such submarine impact struc ures have now been recognized; some have subsequently be en raised above sea level [e.g., Lockne (Sweden) (Therrk ult and Lindstrom, 1995;Lindstro'm et al, 1996)] and others still remain buried [e.g., Montagnais (Canada) (Jansa and t¥-PiDer, 1987); the Chesapeake Bay Crater (USA) (Poag, 1996(Poag, , 1997; and the recently discovered Mjolnir structure (Norway) in the Barents Sea (DyDvik et al, 1996)]. …”

“…Until recently; the sea floor has been largely ignored in the search for impact structures, and only a few submarine impact structures have been identified (e.g., Jansa andPe-Pipe_;Poag, 1996;Dypvik el al., 1996). The special problems of submarine impact structures are finally receiving attention.…”

“…Seismic profiling studies are increasingly being used to determine the structural deformation present beneath large impact structures [e.g., Gosses Bluff(Australia) (Milton etal., 1972(Milton etal., , 1996b; Montagnais (Canada) (Jansa and Pc-Piper, 1987); Chesapeake Bay (USA) (Poag, 1996(Poag, , 1997 (Grieve, 1991;Grieve and Masaitis, 1994;Grieve et al, 1995). Surprisingly Geophysical studies will continue to play a critical role in the future discovery and study of impact structures.…”