How was the crater found?
http://meteor.pwnet.org/impact_event/impact_crater.htm
Identifying the impact crater has been a puzzle that has consumed thousands of hours of research. Since the crater is hidden from sight, tangled clues began to emerge during a search for water for thirsty Virginians, which slowly led to this unexpected discovery far out in the Chesapeake Bay.
Understanding the impact crater requires understanding the geology of Virginia. Virginia's Coastal Plain is a slopping series of sedimentary beds of sands and clays that were laid down in orderly fashion over millions of years. This subterranean layer cake has been studied for the past 200 years.
Within these layers lie aquifers. Nine aquifers have been identified and tapped for water for hundreds of years. It is from these aquifers that the freshwater needs of Virginians have been quenched.
After World War II, Virginia's growing population demanded more freshwater. Existing reservoirs and wells were not predicted to meet the growing demands. During the 1950s and 1960s, the state of Virginia began to sink test wells to determine the location and extent of the aquifers in the Coastal Plain.
T. Scott Bruce of the Virginia Department of Environmental Quality was on a well drilling team that in 1983 began examining the Newport News area of Virginia. The team expected to see neat layers of sand, clay, and the associated fossils in the core samples it collected at various depths. Instead, the samples showed layers that were jumbled and mixed. Figuring that these samples became mixed during collection, the core drilling team moved eastward looking for more favorable sites.
At about the same time, evidence of a bolide impact off the East Coast of the U.S. came to light. C.Wylie Poag was chief scientist on a drill ship, the Glomar Challenger. Ninety miles east of Atlantic City, New Jersey, Poag was working at a core site known as DSDP 612 . During routine examination of core samples, he began to recognize evidence of a meteorite. Analysis of the cores revealed the telltale evidence. Within the cores samples were shocked quartz, which are battered crystals that show multidirectional patterns of parallel fractures and are hallmarks of impacts from space. In addition, Poag's team detected microtektites, which are droplets of molten rock vapor blasted into theatmosphere after a massive impact. There were also microfossils from the late Eocene Epoch found in the debris of the impact indicating that the meteorite collided with the Earth 35 million years ago!
Poag reasoned that the core samples of microtektities were associated with the North American Tektite Strewn Field and that the source of these tektites was presumably a meteorite impact now buried to the west of the drilling site.
He furthered reasoned that the tektites were from an impact, but where? Not on the East Coast at the surface or on the Atlantic sea floor since these areas had been surveyed. The crater must be buried, hidden, and covered with sediments. But where? Poag began to use the USGScollection of offshore seismic reflection profiles to search for buried craters. Poag first hypothesized that the USGS seismic studies pointed to a small impact structure off the coast of New Jersey, now known as the Tom's Canyon crater. The crater is near DSDP 612, which was where he was examining the core samples at the time. Intrigued by the evidence, he learned about the work of David Powars of the U.S. Geological Survey and Bruce of the Virginia Department of Environmental Quality.
Powars was part of a research team drilling cores to study the subsurface layers across Virginia's Coastal Plain. The team was gathering hard data and was systematically drilling cores from Fredericksburg to the Atlantic Ocean. T. Scott Bruce's team needed a test hole drilled. Powars's team needed the core from such a hole. The groups joined forces in 1986 and,on a sultry August night, a very special core sample was brought to the surface. This was a core sample that none had every seen before.
Powars and Bruce noted the abnormalities, which were easy to spot, in the Exmore core sample. Instead of the expected clays and uniform layers of the Coastal sediments, the teams saw a twisted and confused mix of layers. It was the most exciting thing the teams had ever seen!
Powars thought that he had read of this jumbled layer before. In 1913, Samuel Sanford and John Cederstrom had recorded in careful detail the first report of Virginia's groundwater in the Coastal Plain. Drawing contour lines connecting known wells that contained equal concentrations of salt, he noted an "inland bulge" around thelower Chesapeake Bay, centered around the tiny town of Cape Charles on Virginia's Eastern shore.
The methods of core analysis were much cruder at the time. Coredrilling was expensive. Cederstrom had to content himself withliterally cataloging chips and pieces that were flushed up and out of wells that were being dug. He noted that the orderly layers of the Coastal Plain at one point became mangled and mixed. He called this strange batch of sediments the Mattoponi formation from a local Indian name. He also noted something else unusual, a low spot, perhaps afault, where southeastern Virginia subsurface layers seemed to dropaway.
What Cederstrom didn't find was freshwater. Aquifers flowing downfrom the west contained salt. Drilling 52 wells and connecting the salinity contents, Cederstrom noted what Powars would later recognize -- the "inland saltwater wedge."
Fellow scientists at that time did not accept Cederstrom's conclusion that the Mattoponi formation was somehow involved with the salinity in theaquifers, and Cederstrom's work was scorned as being inaccurate.
Saltwater aquifers had been known in the area since the Civil War. Union soldiers stationed at Fort Monroe were plagued by cisterns that dried upduring long hot summers. In 1864, the fort began to drill a well. Forfive years, the drillers labored. Finally, they reached water, but it wastoo salty to drink. They drilled deeper still, until they reached 907 feet. But the water was still too salty to drink, and they eventually gave up.
No one knew why the water was salty or why the James River took a sharp 90-degree turn to the northeast near Fort Monroe. Neither thewell nor the course of the river was following the usual patterns.
The normal course of rivers flowing from the mountains to the sea along the East Coast is predictable. They gently incline toward the Atlantic. However, the James, Rappahannock, and York rivers flow in this fashion until they near the coast. There, they bend abruptly and turn their open mouths north and east to face the tiny village of Cape Charles.
Poag, noting the findings of Powars and Bruce, analyzed some samples of what is now known as the Exmore breccia. Poag found age-scrambled rock fragments and shocked quartz in the breccia. Early on, he thought that the breccia was produced by a tsunami from the smaller impact he referred to as the Tom's Canyon Crater. However, additional data from the release of Texaco's seismic analysis of the Exmore area revealed exhilarating information. The Texaco seismic analysis revealed to Poag a huge crater buried beneath the Chesapeake Bay. Poag then collaborated with the National Geographic Society and the Lamont-Doherty Earth Observatory of Columbia University for new seismic surveys of the area. Through the use of gravity surveys, a much more detailed structure and the shape of a crater were revealed.
The hypothesis that a large meteorite impacted the Atlantic Ocean during the Eocene Epoch was tested and, through dogged research, scientists came to the inescapable conclusion that Virginia had been impacted and affected by an Earth-changing event.
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