Post-Impact Heat Conduction and Compaction-Driven Fluid Flow in the Chesapeake Bay Impact Structure, Based on Downhole Vitrinite Reflectance

Downhole vitrinite reflectance measurements, coupled with heat transport modeling for the central crater moat and central uplift of the Chesapeake Bay impact structure (CBIS), have significant implications for the post-impact thermal history of the structure.

Vitrinite reflectance data from the ICDP-USGS Eyreville deep coreholes (central crater moat) show effects of advective and conductive heating. Although thermal maturity of post-impact Coastal Plain sediments (0-444 m) is typical for the region (0.2-0.31%R_o), maturity in underlying sedimentary breccias is above background. From 444 to 525 m depth, maturity increases, then remains constant (~0.44%) to 1,096 m, the top of a basement-derived granite body. This isothermal pattern is typical of vertical advective fluid flow. Below the granite, reflectance increases from 0.47% to 0.59%R_o in a thin sedimentary breccia interval (1,371-1,397 m), due to conductive heat from underlying suevites and clast-rich melt rocks (1,397-1,474 m). Reflectances in the uppermost suevite are 1.2-1.4%. Modeling the Eyreville suevite as a 390˚C cooling “sill” accompanied by compaction-driven vertical fluid flow (0.046 m/year) from the suevite and deeper 120˚C basement brines upward through the sediment breccias for 10,000 years closely reproduces the measured reflectance values.

Reflectances from the Cape Charles test hole (central uplift) show a similar pattern of a shallower isoreflectance section (~0.41%) in the upper sediment clast breccia and reflectance increasing exponentially with depth from 0.41% to 0.96% in a 90-m contact metamorphic zone above crystalline-clast breccias with suevite. Previous data from coreholes in and near the outer annular trough revealed no impact-related thermal effects.

Groundwater salinities greater than seawater are associated with the CBIS. Modeled vertical heated fluid flow is consistent with patterns of microbe and brine distribution. The results demonstrate the importance of compaction-driven fluid flow in distributing heat in impact structures, particularly marine impact structures having undercompacted sedimentary fill overlying fractured basement rocks.