Researchers study largest impact crater in the US, buried for 35 million years
Phys.org - Tue 13 Aug 23:00 GMT

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. From this impact, the nearby area expe…

  The 21 shocked and unshocked zircon crystals dated in this study were separated from this ~30 cubic centimeters of unconsolidated late Eocene sediment obtained from Ocean Drilling Project site 1073, hole A. Credit: Biren/ASU About 35 million years ago, an asteroid hit the ocean off the East Coast of North America.

  When the asteroid hit, it also produced 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.

  A team of researchers, 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, has obtained drilling samples from the Ocean Drilling Project site 1073 and dated them with the "uranium-thorium-helium technique" for the first time.

  "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."

  Credit: Powars et al. 2015, Christoph Kersten / GEOMAR "Key to our investigation were 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.

  For this study, Biren worked with co-authors Wartho (now working at GEOMAR Helmholtz Centre for Ocean Research Kiel), Van Soest and Hodges to prepare samples for analysis and to date zircon crystals with the uranium-thorium-helium dating method.

  information: M. B. Biren et al, (U-Th)/He zircon dating of Chesapeake Bay distal impact ejecta from ODP site 1073, Meteoritics Planetary Science (2019).