Published by the American Geological Institute
of the Earth Sciences
up to dig down
Thirty-five million years ago, huge chunks of space debris careened into the eastern shore of what is now Virginia, gouging a crater 85 kilometers wide and over one kilometer deep. The damage is still felt today in the form of land subsidence and groundwater diversion. Now, scientists are conducting the most detailed seismic survey and core sampling yet of the Chesapeake Bay impact crater, including what some claim to be the first-ever analyses of gases trapped within impact breccia.
Researchers from the U.S. Geological Survey (USGS), the Hampton Roads Planning District Commission and Virginia have teamed up with colleagues at NASA’s Langley Research Center to study the crater. Researchers hope that the 2,700-foot core and seismic survey will uncover new structural details of the crater, which is the largest in the United States.
The first evidence for the impact structure was uncovered outside the crater in 1983, when C. Wylie Poag of the USGS was conducting research aboard the NSF research vessel Glomar Challenger. On that voyage, shipboard sedimentologists found tektites and shocked quartz in ejecta from a drill core taken 120 kilometers east of Atlantic City, N.J.
Mosaic of Landsat satellite imagery acquired from 1990-94
shows the location of the Chesapeake Bay impact crater,
with two lines representing the inner and outer rims. USGS.
Later studies by Poag, David Powars, also of USGS, and T. Scott Bruce of the Virginia Department of Environmental Quality revealed that the rock fragments were made of bits and pieces of sedimentary beds from southeast Virginia. Coupled with seismic profiles provided by Texaco and Exxon in 1993, the evidence convinced the researchers that an impact structure lay beneath the southern stretch of the Chesapeake Bay.
The researchers chose NASA’s Langley Research Center in Hampton, Va., as the coring site in part because it lies on the crater’s rim. “We have the materials facility to take these cores, crush them under very good vacuum conditions, and watch the gases that diffuse out,” says Joel Levine, a senior research scientist at NASA-Langley. “That’s something that hasn’t been done before.”
Levine was working as chief scientist on a project to send a small surveying airplane to Mars when Virginia coastal geology expert Gerald Johnson of the College of William and Mary called and suggested that NASA and USGS team up. As a result of the recent Mars lander mishaps, NASA canceled Levine’s airplane project, leaving him free to work on the Chesapeake Bay impact crater. “Ironically,” Levine says, “one of the things we were going to photograph [on Mars] was impact craters.”
Drilling began on July 23, and within weeks the Langley-USGS team had reached more than 750 feet below the surface. The researchers cored across a compaction fault lying over the buried crater and then hit the impact breccia near the crater rim.
Researchers from around the nation are signing on to help with analyses. According to Powars, the crater modeling will be done by one of the world’s experts on impact dynamics, Jay Melosh of the University of Arizona Lunar and Planetary Lab. Glenn Izett, research associate at William and Mary and an expert on impact mineralogy and structures, has also joined the team.
The seismic profiling will begin in September, and the data will have a resolution down to five meters. “We’re getting into uncored territory,”Powars says.
Geotimes contributing writer