POSSIBLE HYDROTHERMAL ACTIVITY FOLLOWING THE CHESAPEAKE BAY BOLIDE IMPACT

Ground water sampled from a deep (300 m) well within the upper part of the Exmore beds of the inner crater of the Chesapeake Bay impact structure has yielded a salinity 30 percent greater than sea water. Moreover, ground water of near sea-water concentration in the outer crater is depleted in ²H and ¹⁸O with respect to modern seawater, a condition consistent with a non-ice-age, pre-Pleistocene seawater. I propose that much of the ground water within the inner crater may consist of seawater emplaced when the impact occurred 35 Ma. A calculation of ground-water velocity using Darcy’s law suggests flow rates are insufficient to move the water out of the crater within that time period. The combination of low permeability and a low long-term average hydraulic gradient yields an average velocity of less than 1 mm/yr. A similar calculation using Fick’s law demonstrates that solutes cannot have escaped by molecular diffusion since the impact. For typical diffusion coefficients, over 100 million years would be required for most of the dissolved salts to diffuse upward into the shallow aquifers.

Results of simulations from other investigators using shock-physics hydrocodes on the Chicxulub crater in Mexico illustrate that the crust would have been vaporized and melted down to 20 km during that impact. Extrapolating from these simulations, and given a bolide of 1-2 km in diameter, one can estimate that the top 2-3 km of crust at the Chesapeake crater was vaporized or melted. After the initial crustal rebound, the molten and heated crust was quickly overlain by up to 1,000 m of cold, seawater-filled tsunami breccias of the Exmore beds. The hot crust then acted as a heat source, dissipating its heat over the next several ten’s of thousands of years upward through the sediments. The location of the molten crust at a depth of only 1 km or so depth would likely have led to the creation of a steam phase within the breccias. Thermal pressurization of the relatively low permeability deposits may then have driven off some of the steam phase. Given the properties of the crater fill material, it is possible the remaining saline fluid is still found today as the brine present in the inner crater. This hydrothermal evolution scenario could be further verified and constrained by obtaining additional water and sediment samples from wells and cores within the inner crater.