THERMAL MATURITY IN THE CHESAPEAKE BAY IMPACT STRUCTURE: DATA FROM THE EYREVILLE DEEP CORES, VIRGINIA, USA

Vitrinite reflectance data from the ICDP-USGS Eyreville deep core in the central crater of the Chesapeake Bay impact structure, plus previously published thermal data from the Cape Charles test hole on the crater's central uplift, show patterns of maximum-temperature distribution in the crater fill and overlying post-impact sediments with implications for impact-related thermal history.

In the Eyreville cores, thermal maturities increase linearly downward through the post-impact sediments (0- 444 m) and the upper 80 m of the syn-impact Exmore diamicton to 0.36% R_m (mean random vitrinite reflectance). At 525 m depth, in a transition zone where the Exmore diamicton interfingers with the underlying sediment blocks, the slope of reflectance versus depth increases, and strata are marginally mature (0.42-0.52% R_m). These reflectances are also higher than those measured and modeled at equivalent depths in Baltimore Canyon (COST) and Coastal Plain boreholes. However, within 20 m of the top of the suevite (1,398 m depth), reflectance in the sediment-clast breccia still does not exceed 0.6% R_m.

To date, thermal data from the 150-m-thick suevite and underlying fractured schist and pegmatites put only upper limits on maximum effective syn- to post-impact temperatures of those units. In the suevite, between 1,490 and 1,510 m depth, reflectances from black shale clasts average 5% R_m. It is not yet clear whether this maturity is impact-related or inherited from Paleozoic sub-greenschist metamorphism. However, it does suggest that impact-related heating that would have affected organic maturation did not exceed 300-350˚C within the suevite. Below the suevite, Ar⁴⁰/Ar³⁹ age spectra data from two muscovite samples separated from pegmatite at 1,697 and 1,702 m yielded cooling ages of 244.2 and 243.9 Ma with no evidence of impact heating >350˚C .

In the Cape Charles hole on the central uplift, ten km from Eyreville, published peak fluid inclusion homogenization temperatures are 235-257˚C at 747 m depth. Modeled reflectances over 1.5% R_m for these conditions suggest, not unexpectedly, higher post-impact temperatures at shallower depths at Cape Charles due to conductive heating from advected isotherms and hydrothermal upwelling controlled by central uplift morphology.