For decades, scientists linked Oklahoma’s Ames impact structure to a meteor event that swept across Earth nearly 470 million years ago. According to research published in Meteoritics & Planetary Science titled ‘The Ames impact structure, Oklahoma: New radioisotopic constraints and implications for North American impact chronology’, new evidence points to a very different timeline. By dating zircon from impact-melted granite, researchers may have found a distinct group of grains with an age of nearly 370 million years, suggesting the crater formed during the Late Devonian. That would place the impact close to the Frasnian-Famennian boundary and the Upper Kellwasser Crisis, a major extinction event.
How did new dating place the Ames impact near a major extinction
The Ames impact structure lies beneath Ames, Oklahoma, with an outer rim measuring 14-16 km in diameter. It was previously considered one of the largest impact structures associated with the Ordovician Meteor Event, which began about 466-468 million years ago. This connection was based on conodonts and moulds identified in material filling the crater.To test the age of the impact, researchers used radioisotopic dating on zircon and plagioclase from impacted granite. The youngest zircon population had a weighted mean age of nearly 370 million years.The researchers interpret this as a maximum age for the impact event. If correct, this would place the Ames impact in the Late Devonian rather than the Ordovician, changing its position in North American impact chronology.
How did zircon dating reveal the age of the Ames impact
Researchers analysed zircon from the impact-melt portion of the Ames structure using secondary ion mass spectrometry and laser ablation-inductively coupled plasma-mass spectrometry. They made 37 measurements.The youngest zircon dates formed a distinct population. 10 of the 11 analyses gave a weighted mean age of nearly 370 million years after one statistical outlier was excluded. Most zircons, including the youngest Devonian-age grains, retained their primary oscillatory zoning and showed no signs of deformation.Two zircons displayed impact-related textures, including low-angle grain boundaries within microcracks and granular zircon arrays crossing their growth zoning.
Why did the earlier fossil evidence place the Ames impact in the Ordovician
The earlier Ordovician age for Ames came from Middle Ordovician conodonts and moulds found in the Crater Shale. The study says this age was based on only four small chips and that the Crater Shale has poorly understood and debated stratigraphic relationships.The researchers propose that the impact excavated Middle to Upper Ordovician Simpson Group strata and disrupted fossil-bearing sediments. Those materials could then have been redeposited inside the crater during resurge and tsunami backwash.Under this interpretation, the conodonts came from older rocks struck by the impact and were later deposited inside the crater. Rapid redeposition and burial inside the crater may also have helped preserve the delicate fossil remains.
Ames impact dates to the period of the Late Devonian extinction
The nearly 370-million-year-old zircon age falls close to the Frasnian-Famennian boundary, which marks the Upper Kellwasser Crisis, an extinction event. The Ames result falls within the broader Middle to Late Devonian window of other North American impacts, though some are somewhat older. The paper lists Flynn Creek in Tennessee at about 382 million years, Nicholson Lake in Canada at about 385 million years, and the Alamo impact-related deposits in Nevada at 355-382 million years.If the zircon interpretation is correct, the Ames impact may provide additional evidence for one or more large bolide impacts during the Late Devonian. The researchers say this could inform discussions about links between extraterrestrial impacts and environmental disturbances associated with the Frasnian-Famennian mass extinction.