CHESAPEAKE BAY IMPACT CRATER: PETROGRAPHIC AND GEOCHEMICAL INVESTIGATIONS OF THE IMPACT BRECCIA FILL

The Chesapeake Bay impact structure is, at 85 km diameter, one of the largest known impact structures on Earth. Its importance stems from the facts that it is the only fully marine impact structure that has been extensively studied, by both reflection seismic surveys and drilling. This impact event took place at 35.7 Ma, and has been related to one of the two late Eocene microtektite strewnfields. In addition, this impact structure is of major environmental importance - with regard to both hydrogeology and neotectonics – to millions of inhabitants of the eastern seaboard of the United States. The excellent state of analysis of this very large, complex impact structure has contributed essential knowledge about impacts into the marine realm.

A first petrographic and geochemical study of samples from the impact breccia fill, the so-called Exmore breccia (imaged on seismic profiles in the environs of the central uplift with a maximum thickness of 1.2 km), showed that the breccia is composed of a range of clastic components (the various pre-impact sediments and crystalline granitoid basement) set into fine-grained clastic matrix of the same components. Shocked clasts are very rare; observed shock effects include shock fracturing in quartz, PDFs in both quartz and feldspar, rare impact melt and glass fragments, and rare and scattered occurence of impact glass spherules (proximal [!] microtektites). Co-existence of these shocked particles with other minerals demonstrates that most of the particles with shock deformation are derived from crystalline basement. U-stage analysis of crystallographic orientations of PDFs as well as relative frequencies of PDFs per grain indicate that most particles with this shock effect were deformed at 10-20 GPa. XRF and INA analysis of clasts and breccia samples shows that the felsic sediments of the Potomac, Aquia, Nanjemoy, and Piney Point Formations provided most of the breccia material. This and the granitoid basement seem to be the only significant contributing components. Our siderophile element and Ir analysis did not show any trace of a meteoritic component in Exmore breccia.