AUTHIGENIC-MINERAL EVIDENCE FOR POST-IMPACT DIAGENESIS AND ALTERATION IN THE CHESAPEAKE BAY IMPACT STRUCTURE: PRELIMINARY RESULTS FROM THE EYREVILLE BOREHOLE CORE

The late Eocene Chesapeake Bay impact structure (CBIS) provides a unique opportunity to investigate post-impact diagenetic and alteration processes in a marine impact structure. Petrographic and X-ray diffraction analyses of drill-core samples from five locations, four within the structure (Bayside, Cape Charles, Kiptopeke, USGS-NASA Langley) and one outside it (Jamestown), have been completed and provide evidence for post-impact diagenesis and low-temperature hydrothermal alteration in the crater-fill deposits. We report below on samples from the Eyreville borehole core, drilled in fall 2005, which provides a continuous record of the sedimentary fill within the inner crater of the CBIS. Preliminary results of hand-sample, petrographic, and bulk X-ray diffraction studies of samples from the Eyreville core, ranging from 349 to 1375 m depth, suggest post-impact conditions similar to those documented in the upper part of the Cape Charles borehole. Bulk X-ray diffraction studies indicate the samples are composed primarily of quartz, feldspars (microcline and albite), muscovite, smectite, and chlorite with variable quantities of pyrite and calcite. In thin section, the Exmore breccia shows mainly smectite matrix with dispersed pyrite lepispheres and fine calcite rhombs; some feldspar dissolution and albitization and glauconite recrystallization are evident. The matrix in the underlying Cretaceous megablock breccia contains chlorite intergrown with smectite and pyrite lepispheres. Feldspars show partial dissolution and partial to complete albitization, and micas show deformation and partial chloritization. One breccia sample was obtained below the granite megablock and above the suevite; it is dominated by smectite cement and feldspar dissolution and generally shows less alteration than those above the granite block. Overall, preliminary observations suggest that a pronounced monotonic increase in alteration is not present in the sedimentary breccias in the Eyreville borehole. Rather, the most evident alteration is at depths of 800 to 1000 m. At present it is unclear whether this pattern originates from lateral fluid flow from the central peak, where hydrothermal alteration is well documented, or other potential factors influencing alteration or a sampling bias.