PALEOHYDROLOGY OF THE CHESAPEAKE BAY IMPACT STRUCTURE--EVIDENCE FROM THE ICDP-USGS EYREVILLE COREHOLE

The presence of brine (>35 ppt salinity) within the central Chesapeake Bay impact structure has led previous investigators to suggest that the brine generation may be associated with the impact itself, and that ground water within the central crater may not have been flushed out since the time of the impact. We have collected and analyzed pore-water chemistry for >100 samples along the entire vertical extent of the ICDP-USGS deep core hole near Eyreville, VA in order to further investigate the distribution and origin of the brine in the crater. Both major-ion and stable-isotope analyses are proving useful in interpreting the paleohydrology of the impact structure. Unlike previous studies that yielded salinities up to 40 ppt in a few isolated locations, the ICDP-USGS core hole has yielded salinities of 40-64 ppt over most of the 1,300-m-thick section of impact deposits.

Chloride and ¹⁸O values in the post impact sediments show very steady trends with depth that indicate mixing of fresh and saline waters. A one-dimensional ground-water flow and transport model was applied in an attempt to reproduce these trends. Simulated processes included molecular diffusion, upward advection from compaction, and an upper boundary condition salinity that varied with time based on a simplified sea-level curve. Although the Cl and ¹⁸O profiles have a diffusion character to them, simulations with diffusion only or with diffusion and advection could not reproduce their subtler features. Including the transient-salinity upper boundary condition, however, yielded very good matches to both the Cl and ¹⁸O profiles. We expect that a further model refinement that includes compaction more directly and a formal sensitivity analysis will help to constrain the parameters of the model and their confidence limits.

Other geochemical indicators have also been informative. For example, the Ca/Mg ratio increases regularly with depth, suggesting water-rock interaction related to increasing temperatures with depth. Very low Mg values in some of the deep samples are similar to high-temperature altered seawater extruding from deep-sea ocean vents. Overall, the chemistry suggests that much of the ground water in the central crater has been present since the time of impact, and that much of the brine was likely present in the coastal plain sediments before impact.