Impactites and Basement-Derived Rocks in the ICDP-USGS Eyreville Cores, Chesapeake Bay Impact Structure, USA

The 1,766-m deep ICDP-USGS Eyreville coreholes in the late Eocene Chesapeake Bay impact structure provide one of the most complete geologic sections ever obtained from an impact structure, including: a sequence of basement-derived mica schists (containing fibrolitic sillimanite and graphite), granite, pegmatite, and local mylonites, with impact-breccia dikes (1,766–1,551 m); an impactite sequence including polymict breccias with blocks of cataclastic gneiss (1,551–1,474 m) overlain by suevites and clast-rich impact melt rocks (1,474–1,397 m); gravelly quartz sand containing an amphibolite block and boulders of cataclasite and suevite (1,397–1,371 m); allochthonous unshocked granite (gneissic-to-massive, fine-to-coarse-grained; 1,371–1,096 m); and crater-fill sediments (1,096–444 m). The suevites and lithic impact breccias have abundant shocked quartz, and Raman spectra confirm the presence of coesite and reidite. Silicate glasses of different composition have meniscus-like textures, indicating immiscible melts at quench, and they are partly smectite-altered; coexisting sulfide melts appear as tiny Fe+S±Ni spheres. Chrome spinel, baddeleyite, and corundum in silicate glass indicate high-temperature crystallization under silica undersaturation. The impactites may record a transition from lithic-block-rich ground surge to melt-rich fallback from the ejecta plume. A lack of shock metamorphism in the allochthonous granite suggests inward transport during the transient-cavity collapse. Post-impact heating in the deepest sequence was <~350°C based on undisturbed ⁴⁰Ar/³⁹Ar plateau age spectra of muscovites. Temperatures in sand above the suevite were <~150°C, based on ⁴⁰Ar/³⁹Ar age spectra of detrital microcline. In addition to impact effects, the basement-derived rocks provide important information about deeply buried Appalachian terranes beneath the Atlantic Coastal Plain. The pre-Mesozoic basement-derived rocks include multiply-deformed middle-amphibolite-facies schists, greenschist-facies metamorphic clasts and sub-greenschist sedimentary-rock clasts in the suevites and lithic impact breccias, amphibolite, and Neoproterozoic and Permian granites. Structural partitioning of basement-derived rocks is attributed mostly to the late modification stage of cratering.