Recent research on the Chesapeake Bay impact structure, USA - Impact debris and reworked ejecta

Horton, Jr. , J. W.; Aleinikoff, J. N.; Kunk, M. J.; Gohn, G. S.; Edwards, L. E.; Self-Trail, J. M.; Powars, D. S.; Izett, G. A.

Abstract:

Four new coreholes in the western annular trough of the buried, late Eocene Chesapeake Bay impact structure provide samples of shocked minerals, cataclastic rocks, possible impact melt, mixed sediments, and damaged microfossils. Parautochthonous Cretaceous sediments show an upward increase in collapse, sand fluidization, and mixed sediment injections. These impact-modifi ed sediments are scoured and covered by the upper Eocene Exmore beds, which consist of highly mixed Cretaceous to Eocene sediment clasts and minor crystalline-rock clasts in a muddy quartz-glauconite sand matrix. The Exmore beds are interpreted as seawater-resurge debris flows. Shocked quartz is found as sparse grains and in rock fragments at all four sites in the Exmore, where these fallback remnants are mixed into the resurge deposit. Crystalline-rock clasts that exhibit shocked quartz or cataclastic fabrics include felsites, granitoids, and other plutonic rocks. Felsite from a monomict cataclasite boulder has a sensitive high-resolution ion microprobe U-Pb zircon age of 613 ?? 4 Ma. Leucogranite from a polymict cataclasite boulder has a similar Neoproterozoic age based on muscovite ⁴⁰Ar/³⁹Ar data. Potassium-feldspar ⁴⁰Ar/³⁹Ar ages from this leucogranite show cooling through closure (???150 ??C) at ca. 261 Ma without discernible impact heating. Spherulitic felsite is under investigation as a possible impact melt. Types of crystalline clasts, and exotic sediment clasts and grains, in the Exmore vary according to location, which suggests different provenances across the structure. Fractured calcareous nannofossils and fused, bubbled, and curled dinofl agellate cysts coexist with shocked quartz in the Exmore, and this damage may record conditions of heat, pressure, and abrasion due to impact in a shallow-marine environment. ?? 2005 Geological Society of America.

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Article

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70035498

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Horton, Jr. , J. W.; Aleinikoff, J. N.; Kunk, M. J.; Gohn, G. S.; Edwards, L. E.; Self-Trail, J. M.; Powars, D. S.; Izett, G. A.

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170

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384

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English

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Special Paper of the Geological Society of America

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Recent research on the Chesapeake Bay impact structure, USA - Impact debris and reworked ejecta; 2005; Article; Journal; Special Paper of the Geological Society of America; Horton, Jr. , J. W.; Aleinikoff, J. N.; Kunk, M. J.; Gohn, G. S.; Edwards, L. E.; Self-Trail, J. M.; Powars, D. S.; Izett, G. A.