Postimpact heat conduction and compaction-driven fluid flow in the Chesapeake Bay impact structure based on downhole vitrinite reflectance data, ICDP-USGS Eyreville deep core holes and Cape Charles test holes

Abstract

Vitrinite refl ectance data from the International Continental Scientifi c Drilling Program (ICDP)–U.S. Geological Survey (USGS) Eyreville deep cores in the centralcrater moat of the Chesapeake Bay impact structure and the Cape Charles test holes on the central uplift show patterns of postimpact maximum-temperature distribution that result from a combination of conductive and advective heat fl ow. Within the crater-fi ll sediment-clast breccia sequence at Eyreville, an isorefl ectance (~0.44% Ro) section (525–1096 m depth) is higher than modeled background coastal-plain maturity and shows a pattern typical of advective fl uid fl ow. Below an intervening granite slab, a short interval of sediment-clast breccia (1371–1397 m) shows a sharp increase in refl ectance (0.47%–0.91% Ro) caused by conductive heat from the underlying suevite (1397–1474 m). Refl ectance data in the uppermost suevite range from 1.2% to 2.1% Ro. However, heat conduction alone is not suffi cient to affect the temperature of sediments more than 100 m above the suevite. Thermal modeling of the Eyreville suevite as a 390 °C cooling sill-like hot rock layer supplemented by compactiondriven vertical fl uid fl ow (0.046 m/a) of cooling suevitic fl uids and deeper basement brines (120 °C) upward through the sediment breccias closely reproduces the measured refl ectance data. This scenario would also replace any marine water trapped in the crater fi ll with more saline brine, similar to that currently in the crater, and it would produce temperatures suffi cient to kill microbes in sediment breccias within 450 m above the *Current address: Department of Geology and Environmental Geosciences, Lafayette College, Easton, Pennsylvania 18042, USA; lovem@lafayette.edu. Malinconico, M.L., Sanford, W.E., and Horton, J.W., Jr., 2009, Postimpact heat conduction and compaction-driven fl uid fl ow in the Chesapeake Bay impact structure based on downhole vitrinite refl ectance data, ICDP-USGS Eyreville deep core holes and Cape Charles test holes, in Gohn, G.S., Koeberl, C., Miller, K.G., and Reimold, W.U., eds., The ICDP-USGS Deep Drilling Project in the Chesapeake Bay Impact Structure: Results from the Eyreville Core Holes: Geological Society of America Special Paper 458, p. 905–930, doi: 10.1130/2009.2458(38). For permission to copy, contact editing@geosociety.org. ©2009 The Geological Society of America. All rights reserved. on 17 November 2009 specialpapers.gsapubs.org Downloaded from