Researchers at The University of Texas at Austin have revised a key piece of Oklahoma's geologic history, with possible implications for how scientists interpret major events in the history of life on Earth.
Beneath the town of Ames, Oklahoma, lies a meteor impact structure that stretches for miles underground. Layers of sediment now cover the crater, but it remains significant both scientifically and economically. The Ames impact structure is also a major producer of oil and gas.
A Crater Long Linked to an Ancient Meteor Event
For years, the Ames crater was thought to belong to a cluster of major meteor impacts across North America dating to roughly 467.5 million years ago. That period is known as the Ordovician Meteor Event.
Because so many impact structures appear to date from around the same time, some researchers have proposed that Earth may once have been surrounded by a Saturn-like ring of asteroid debris during the Middle Ordovician.
New work from UT researchers now shows that the Ames impact does not belong to that episode.
By dating zircon crystals taken from granite altered by the impact, the team determined that the meteorite struck about 370 million years ago during the Late Devonian. That makes the crater nearly 100 million years younger than previously believed.
"No matter what technique we used, it was coming back to this younger signal," said lead author Elizabeth Catlos, associate professor at UT's Department of Earth and Planetary Sciences.
The research was published in July in Meteoritics & Planetary Science.
Why the Earlier Date Was Misleading
Before this study, the Ames impact had only been dated using biological evidence. Researchers had found teeth from an ancient eel-like animal called a conodont preserved in the rock. Those fossils came from organisms that lived during the older Ordovician period.
But Catlos said the teeth were probably already millions of years old by the time the asteroid struck. The impact likely churned up older material and mixed the fossils into the rocks while still preserving them.
The zircon dating provides a very different timeline. It shows that the Ames crater could not have formed during the Ordovician Meteor Event.
Instead, its new age places it close to the Frasnian-Famennian mass extinction event, which occurred about 372 million years ago and wiped out a large proportion of marine life on Earth.
Tiny Zircon Crystals Preserve the Impact
Danny Stockli, dean of the Jackson School of Geosciences and a co-author of the study, said zircon U-Pb dating offers one of the most accurate ways to determine when events occurred deep in Earth's past.
Zircon crystals can also preserve microscopic structures created by the intense pressures generated during an impact.
"These small crystals allow us to go back in time and learn about the major changes to Earth's ancient landscapes," Stockli said. "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."
To confirm that the zircons had actually been affected by the meteor strike, the team worked with NASA to image the crystals using cathodoluminescence and electron backscatter diffraction.
When zircon experiences the extreme conditions of an impact, it recrystallizes in a distinctive way. Those changes can be detected with these imaging techniques, allowing researchers to verify that the crystals recorded the collision itself.
A New Piece of a Mass Extinction Puzzle
According to Catlos, establishing more precise dates for mass extinctions and other major events is essential for understanding how Earth has changed over time.
One key question is whether extinction events were triggered primarily by forces from space, such as meteor impacts, or by processes within Earth, including episodes of massive volcanic activity.
"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,'" she said.
The project was initiated by former Jackson School of Geosciences graduate student Andrew Parisi, who graduated in 2018 and has since passed away. Parisi traveled to Oklahoma to obtain the Ames rock core from the Oklahoma Geological Survey, extracted zircon crystals from the material, and helped determine their ages.
Co-author Michael Brookfield, an affiliated researcher at the school, also passed away before the paper was published.
Research Professor Sean Gulick and Professor Emeritus Mark Cloos at the Jackson School also contributed to the research.
