AUTHIGENIC-MINERAL EVIDENCE FOR POST-IMPACT DIAGENESIS AND ALTERATION IN THE CHESAPEAKE BAY IMPACT STRUCTURE: A PROGRESS REPORT

The late Eocene Chesapeake Bay impact structure provides a unique opportunity to investigate post-impact diagenetic and alteration processes in a marine impact structure. The structure formed in poorly consolidated coastal plain sediments and underlying granitic and metamorphic basement rocks overlain by seawater. 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), demonstrate changes in mineralogy due to post-impact diagenesis and hydrothermal alteration. The mineralogy of impact-disrupted and impact-generated sedimentary deposits in the annular trough, including the Exmore breccia, is mildly affected by diagenesis. Petrographic observations in samples from the Bayside and Langley cores in the annular trough indicate that authigenic smectite, illite, and vermicular kaolinite are intergrown within the intergranular spaces of the sediments. The smectite generally has 10% or less interlayered illite. Other authigenic minerals include Fe-calcite, pyrite, and quartz, the latter of which is present as grain overgrowths with depth. Given these mineralogical constraints, the deepest sediments sampled in the annular trough experienced temperatures no greater than ~90°C. In contrast, the mineralogy of crater-fill deposits from the partially cored USGS Cape Charles test hole, near the center of the central crater, shows prominent diagenetic changes in the upper sediment-clast breccia and evidence for hydrothermal alteration in the lower crystalline-clast breccias. Smectite, illite, kaolinite, and high-Fe chlorite are present in samples of the upper sediment-clast breccia, and the smectite has as much as 20% interlayered illite. Authigenic quartz, calcite, and pyrite are also present. The lower crystalline-clast breccias in the Cape Charles test hole are altered to chlorite, illite, quartz, albite, calcite, and epidote, a typical low-temperature hydrothermal alteration assemblage. The upper sediment-clast breccia in the Cape Charles test hole likely experienced temperatures near 100°C, whereas the lower crystalline-clast breccias experienced temperatures greater than 200°C, based on the presence of epidote and absence of smectite.