THERMAL HISTORY OF THE CHESAPEAKE BAY IMPACT CRATER

The Chesapeake Bay impact crater is a 90 km wide structure that has been described as a complex peak-ring crater, created 35.7 million years ago in the then submerged unconsolidated sediments of the Atlantic coastal plain. An outer rim and annular trough are expressed by the disruption of pre-impact lithologic units. A 38 km inner basin with a peak ring and central uplift is inferred from seismic data. Based on published hydrocode models of the impact there may have been a significant melt sheet and certainly were elevated basement temperatures immediately following the impact in the inner basin. Numerical modeling based on reasonable post-impact temperatures shows heating of syn-impact sediment on the order of hundreds of degrees C for tens of thousands of years after the impact. However, published apatite fission track ages for crystalline basement within the annular trough are consistent with passive margin cooling, indicating maximum impact related temperatures at the top of the basement were under ~110 degrees C at these locations.

Reconstruction of the immediate post-impact thermal history of the Chesapeake Bay impact crater is approached through study of the thermal maturation of dispersed organic matter (DOM) found within syn- and post- impact sediments. More than 30 samples of core from one borehole just outside the crater, two within the annular trough, and one within the inner basin were processed for analysis. Two measures of thermal maturity are employed, thermal alteration index (TAI) and vitrinite reflectance. For TAI analysis, strewn slides are prepared with samples of concentrated organic matter derived from acid digestion and heavy liquid separation of core samples. Similar organic residue are mounted in epoxy pellets and polished for vitrinite reflectance analysis.

Early results indicate no thermal anomaly associated with syn-impact sediments located inside the annular trough. Mean random vitrinite reflectance values are between 0.2 and 0.3 % and consistent with the expected very low thermal maturity for relatively shallow Atlantic coastal plain sediments. After analysis of all samples is completed thermal modeling will be performed to evaluate varying post-impact temperature profiles and melt sheet geometries, using the maturity data as a constraint.