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article imageLargest impact crater in the U.S. was buried for 35 million years

By Karen Graham     Aug 17, 2019 in Science
About 35 million years ago, an asteroid hit the ocean off the East Coast of North America. Its impact formed a 25-mile diameter crater that now lies buried beneath the Chesapeake Bay, an estuary in Virginia and Maryland.
The location of the crater in the Chesapeake Bay is now completely covered by younger sediments. It was discovered in the early 1990s by marine geophysical surveys and subsequent drilling. It is the largest known impact crater in the U.S. and the 15th-largest on Earth.
When the asteroid hit, the nearby area experienced fires, earthquakes, falling molten glass droplets, an air blast and a devastating tsunami. The falling molten glass droplets were part of an impact ejecta layer, which includes tektites (natural glass formed from debris during meteorite impacts) and shocked zircon crystals which were thrown out of the impact area.
Today, scientists call this area the "North American tektite strewn field," which covers an area of close to 4 million square miles, about 10 times the size of Texas. Some of the ejecta landed on land while the rest immediately cooled on contact with seawater and then sank to the ocean floor.
Researchers have found clear traces of the impact and dated them for the first time using the uraniu...
Researchers have found clear traces of the impact and dated them for the first time using the uranium-thorium-helium technique. ODP 1073 on the map refers to the ocean drilling project site where the sample material for this study was collected.
GEBCO world map 2014
A group of scientists, including Arizona State University School of Earth and Space Exploration scientist and lead author Marc Biren, along with co-authors Jo-Anne Wartho, Matthijs Van Soest and Kip Hodges, obtained drilling samples from the Ocean Drilling Project site 1073 and dated them with the "uranium-thorium-helium technique" for the first time.
Their research was recently published in the international journal Meteoritics & Planetary Science.
"Determining accurate and precise ages of impact events is vital in our understanding of the Earth's history," Biren said. "In recent years, for example, the scientific community has realized the importance of impact events on Earth's geological and biological history, including the 65 million years old dinosaur mass extinction event that is linked to the large Chicxulub impact crater."
To determine the age of the crater, the researchers studied zircon crystals. This is because they preserve evidence of shock metamorphism, which is caused by shock pressures and high temperatures associated with impact events. The crystals they dated were very small, about the thickness of a human hair.
The location of the crater in Chesapeake Bay. It is now completely covered by younger sediments  but...
The location of the crater in Chesapeake Bay. It is now completely covered by younger sediments, but was discovered in the early 1990s by marine geophysical surveys and subsequent drilling. It is the largest known impact crater in the U.S. and the 15th-largest on Earth.
Powars et al. 2015, Christoph Kersten / GEOMAR
"Key to our investigation was zircon -- or to be more precise: zirconium silicate -- crystals that we found in the oceanic sediments of a borehole, which is located almost 400 kilometers (250 miles) northeast of the impact site, in the Atlantic Ocean," says co-author Wartho, who began the study when she was a lab manager at the Mass Spectrometry Lab at ASU, according to Science Daily.
The worth of the uranium-thorium-helium dating method was demonstrated when Biren was able to identify and process the shocked zircon fragments for imaging and chemical analysis with an electron microprobe, according to Technology Networks.
“This research adds a tool for investigators dating terrestrial impact structures,” Biren said. “Our results demonstrate the uranium-thorium-helium dating method’s viability for use in similar cases, where shocked materials were ejected away from the crater and then allowed to cool quickly, especially in cases where the sample size is small.”
More about impact crater, Chesapeake bay, geological history, zircon crystals, Science
 
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