Oklahoma's Buried Ames Crater Is 100 Million Years Younger, Tying It to a Mass Extinction

A meteor crater hidden beneath the small town of Ames, Oklahoma, is nearly 100 million years younger than geologists believed, according to researchers at the University of Texas at Austin. The new date moves the impact out of a famous cluster of ancient strikes and places it right next to one of the largest mass extinctions in Earth's history.

The Ames impact structure spans miles but is buried under layers of sedimentary rock, so it can't be seen from the surface. It matters to Oklahoma's economy because it is a major oil and gas producer. Until now it was grouped with a series of North American impacts dated to around 467.5 million years ago, during the Middle Ordovician period. So many craters date to that window that some researchers have proposed Earth briefly had a Saturn-like ring of asteroid debris that rained down on the planet, a hypothesis known as the Ordovician Meteor Event.

The UT team dated tiny zircon crystals from granite that was shattered by the impact. Uranium in zircon decays to lead at a steady rate, which makes it one of the most accurate clocks in geology. The crystals put the strike at about 370 million years ago, in the Late Devonian. "No matter what technique we used, it was coming back to this younger signal," said lead author Elizabeth Catlos, an associate professor in UT's Department of Earth and Planetary Sciences. The study was published in the journal Meteoritics & Planetary Science.

The old date came from fossils. Researchers had found teeth from conodonts, small eel-like animals, in the rock and used their known age range to date the crater. Catlos said those teeth were probably already millions of years old when the asteroid hit and were simply churned up and preserved in the debris. To prove the zircons they dated had actually been hit by the impact, the team worked with NASA to image the crystals using cathodoluminescence and electron backscatter diffraction. Shock waves from an impact make zircon recrystallize in a distinctive way that those methods can reveal.

The new age lines up with the Frasnian-Famennian mass extinction about 372 million years ago, which wiped out a large share of marine life, including many reef-building species. Scientists still debate what drove that extinction, with candidates ranging from volcanic eruptions to ocean oxygen loss to impacts. "With this research, we're basically taking a major pawn out of the Ordovician Meteor Event and dumping it into the Frasnian-Famennian event, and saying, 'This is where this impact belongs,'" Catlos said. She said knowing whether extinctions were triggered from space or from inside the Earth is crucial to understanding how the planet works.

Danny Stockli, dean of UT's Jackson School of Geosciences and a co-author, said zircon records both the timing and the shock pressure of an impact. "It would be great to do this for more of the meteor impact sites across the continent so we could get a more accurate timeline for these major events," he said.

The project was started by former Jackson School graduate student Andrew Parisi, who collected the Ames rock core from the Oklahoma Geological Survey, extracted the zircons and helped date them. Parisi graduated in 2018 and has since died. Co-author Michael Brookfield also died before the paper was published. Research professor Sean Gulick and professor emeritus Mark Cloos contributed to the work.