IS HIGH SALINITY CRETACEOUS SEAWATER TRAPPED IN THE DEEP ATLANTIC COASTAL PLAIN, USA?

Groundwater that is saltier than modern seawater was recently discovered in the Chesapeake Bay Impact Crater (CBIC) in the Atlantic Coastal Plain of Virginia, USA. Pore water from drill core at depths from 800 to 1,600 m in the crater-fill material was found to have salinity levels about twice modern seawater. The origin of this high salinity in and adjacent to the CBIC has been the subject of much speculation. Evaporite dissolution, and heating and vaporization from the impact have been previously suggested as causes. The chloride-bromide concentration ratios in the crater pore water, however, are indicative of a seawater, rather than an evaporite origin. Also, the amount of heat given off from the melt rock beneath the crater was calculated to be insufficient to evaporate enough water to have created the elevated salinity. Thus the origin of the brine has remained an enigma.

Groundwater from two wells installed deep in the crater during the drilling was found to have very high helium concentrations (exceeding 400,000 mccSTP/kg). Another recent helium and groundwater-age investigation of several wells in the deep coastal plain aquifers of Maryland revealed a range of in situ accumulation rates of helium in the Coastal Plain groundwater with an average value of 3.5x10^-12 ccSTP/g/yr. Dividing the total helium concentration by this average rate gives an age of the deep pore water of about 120 million years. This age coincides with the Early Cretaceous age of the Potomac Formation from which the pore water originated. In addition, the elevated salinity is consistent with conditions in the Early Cretaceous Atlantic Ocean, which was fringed by a number of evaporite deposits, and connected to the rest of the world’s oceans only by two or three narrow straits. These results would indicate that Early Cretaceous seawater is still trapped today as connate water in the deep Atlantic Coastal Plain, and that it had a salinity of about twice that of modern seawater.