Unlocking the Secrets of the Chesapeake Bay Impact Crater and Virginia's Inland Saltwater Wedge

By Kathleen Gohn and Randy McFarland

Since the 1940's, scientists and water managers have been puzzled by the presence of salty ground water extending inland beneath Chesapeake Bay into the aquifers that supply water to parts of southeastern Virginia. During the early 1990s, scientists studying the geology of the Atlantic Coastal Plan made an important discovery that has dramatically altered our understanding of this saltwater wedge. By drilling deep coreholes and collecting geophysical data about the underlying rocks, USGS and Virginia Department of Environmental Quality scientists identified a large impact crater near the mouth of the Chesapeake Bay.

We know now that about 35 million years ago, a large meteorite struck shallow ocean waters in the area that is now southeastern Virginia, blasting through the water column and thousands of feet of rock and sediment layers. The meteorite was 1-2 miles in diameter and was traveling at thousands of miles per hour when it hit. The result is an enormous filled crater, 56 miles wide and about 6,000 feet deep--as deep as the Grand Canyon.

Within minutes from the instant of impact, billions of tons of ocean water were vaporized, and millions of tons of sediment, debris, and seawater were thrown thousand of feet into the atmosphere and scattered along much of the East Coast. Mile-high tsunami rushed far inland, possibly even overtopping the Blue Ridge Mountains. The seawater then backwashed with equal violence into the void left by the blast, to fill the crater with a chaotic mix of fluidized sand and chunks of clay, limestone, and sand that range in size from a few inches to as large as battleships. Normal conditions gradually returned during the weeks and months that followed, and sedimentation resumed, depositing layers of sand and clay that buried the crater about a thousand feet deep over the next 35 million years.

Since the time of the impact, the area of the crater along with the rest of the East Coast has emerged from and been re-submerged by the ocean many times. Seawater that entered the sediments by submersion has normally been flushed out by fresh ground water during emergent periods. So today, most areas along the east coast contain fresh ground water. Those disrupted sediments inside the crater a thousand feet below the surface, however, are affecting the normal pattern of ground-water flow. The location of the crater coincides directly with the salty ground water beneath southeastern Virginia, which chemical evidence indicates is seawater trapped within the crater fill, possibly from as far back as the impact itself. This ancient origin of the saltwater wedge provides an important new insight that is relevant to the management of ground-water resources in the fast-growing Hampton Roads area.

Since late 1999, the USGS has been part of a multidisciplinary, multi-agency scientific study of the crater, with partners including the Virginia Department of Environmental Quality, the Hampton Roads Planning District Commission, the NASA Langley Research Center, and the College of William and Mary. An international scientific workshop was held in Reston in September 2003 to develop plans for drilling a mile-deep corehole through the center of the crater in 2005. Data from this core and others already drilled near the crater margin will provide detailed information about how impact craters are formed, and specifically about how this crater led to salty groundwater in the Hampton Roads area of Virginia. A better understanding of the crater's geology is critical to the development by USGS scientists of a new regional ground-water flow model. Municipal and regional agencies will use this model in planning for future water supplies for the rapidly growing population of this coastal area.

Through this study of one of Earth's largest known impact craters, USGS scientists and partners are increasing our scientific understanding of cratering processes, and at the same time addressing the critical issue of water availability in southeastern Virginia.

For more information on the Chesapeake Bay Impact Crater studies, see http://geology.er.usgs.gov/eespteam/crater/. This project and other USGS monitoring and scientific investigations that have been instrumental in the understanding of the critical role that ground water plays in sustaining coastal populations, economies, and ecosystems throughout the Atlantic coastal zone are described in the recent report "Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast" (USGS Circular 1262, http://water.usgs.gov/pubs/circ/2003/circ1262/).