Virginia

The Discovery

The Discovery

Early in his career, Poag spent seven years exploring for oil and gas with Chevron Oil Company, and four years teaching geological oceanography at Texas A&M. In 1974, he went to work for the U.S. Geological Survey (USGS), specializing in the reconstruction of the geological history of continental margins, using microfossils and seismic-reflection surveys.  After spending a year helping the National Science Foundation (NSF) to manage its Deep Sea Drilling Project, Poag was offered the opportunity to lead a research team on the National Science Foundation’s drilling ship Glomar Challenger, which he graciously accepted. Poag stated, “I virtually stumbled across the Chesapeake Bay impact crater. Giant impact craters don’t show up in your backyard every day, but that is essentially what happened to me.”    It was a beautiful day in 1983 and Poag was serving as co-chief scientist on the Glomar Challenger. Poag, along with an international group of marine scientists, had a mission to drill a series of deep holes into the core of the seabed 90 miles east of Atlantic City, New Jersey. Site 612, where Poag and the rest of the crew on the Glomar Challenger were drilling, marked the key corehole, located in 1,400 meters of water near the edge of the continental shelf. It was at this particular site where Poag and his colleagues turned up a very thin layer of ejecta, (debris that shoots out of an impact site when a crater forms) from the still-unknown impact. The ejecta included unusual components consisting of microtektites and microkrystites (molten droplets of impact glass blasted into the atmosphere after a massive impact), along with shocked quartz, stishovite, and coesite (minerals that show the results of being hit by tremendous forces). The cores brought to the surface at Site 612 provided evidence that a bolide had struck somewhere in the vicinity of 150 to 300 miles away from where the crew was drilling. Because of the combination of microfossils in the ejecta layer and a radiometric date from the impact glass, Poag was able to figure out that the impact dated back to the late Eocene Epoch, 35 million years ago. The crew also found microfossils from various different geologic ages mixed together, most likely scrambled during the initial impact. 

Having uncovered the initial evidence of a possible impact, Poag wondered whether or not a crater lay buried close by, perhaps under the continental shelf. Even though his colleagues at the USGS had been imaging the subsea geological structure of the Atlantic continental shelf with seismic reflection surveys for more than 10 years, no one in Poag’s Woods Hole group (from the USGS Science Center at Woods Hole, Massachusetts) was a crater expert, which raised the possibility of overlooking the crater even though they may have imaged it. So, Poag decided that he was going to really scrutinize his seismic archives. Simultaneously, Poag’s colleagues - - Bob Mixon and David S. Powars from the USGS headquarters in Virginia and T. Scott Bruce with Virginia State Water Control Board -  - pulled up impact ejecta from coreholes drilled on the east/west margins of the Chesapeake Bay. Powars was part of a research team that was drilling cores to study the subsurface layers across Virginia’s coastal plain. The team needed hard data, so they were systematically drilling cores between Fredricksburg and the Atlantic Ocean. Bruce’s team needed a water test hole drilled and Powars’s team needed the core sample from the hole, so they joined forces. While drilling, they expected to see neat, horizontal layers of sand and clay combined with associated fossils. However, what they had expected to see was not what they discovered in their cores. The team noticed that the layers were scrambled together, and thought at first they had most likely mixed during collection. Not quite knowing what they found, but realizing that the sample was an unusual mixture of older rock fragments, Poag’s colleagues asked him to analyze the microfossils in the mixture in order to determine the geologic age. Surprisingly, Poag determined that the Virginia cores were composed of the same microfossils that he had seen in the ejecta from Site 612 off New Jersey. Because of the similarity, Poag suspected a connection between the onshore and offshore deposits, and wondered if the Virginia cores were also impact ejecta. So close to discovering what would soon be known as the largest impact crater in the United States and the sixth largest in the world, Poag needed some clarification as to whether or not his discoveries pointed toward the fact that there was an impact crater hidden somewhere nearby. In order to make sure, Poag sent some core samples to a geochemist at the University of Vienna, Austria, named Christian Koeberl, who indeed confirmed that what Poag had sent over did contain impact-shocked minerals. Koeberl’s confirmation brought Poag one step closer to unraveling the mystery as to where the unknown impact crater might lie. However, in 1993, the final piece was put into the puzzle and the mystery was solved when Texaco, Inc. released seismic-reflection profiles their geologists had collected in Chesapeake Bay. “The seismic data blew our minds!” Poag explained The Texaco seismic analysis revealed to Poag that a huge impact crater lay right smack in the middle of the bay, buried 350 meters below sediment. It was only by chance that two of the coreholes had cracked into the fragmented rock that filled the crater.

In 1994, Poag, Powars and Bruce announced their find article in the professional journal Geology, which was a hit around the world. These three geologists had developed a hypothesis that the Atlantic Ocean had been disrupted by a large meteorite during the late Eocene Epoch and what is now the state of Virginia was affected by this Earth-changing event. But questions remained. What size was this bolide that left such an immense crater in the bottom of the Chesapeake Bay, and how did it change the world from the moment it struck?