For decades, scientists linked Oklahoma’s Ames impact structure to a meteor event that swept across Earth nearly 470 million years ago.
asserted
that → link → Earth
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.
uncertain
evidence → accord → 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.
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crater → date → Devonian
That would place the impact close to the Frasnian-Famennian boundary and the Upper Kellwasser Crisis, a major extinction event.
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That → place → boundary
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.
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structure → place → 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.
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which → consider → Event
This connection was based on conodonts and moulds identified in material filling the crater.
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connection → base → crater
To test the age of the impact, researchers used radioisotopic dating on zircon and plagioclase from impacted granite.
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researchers → test → granite
The youngest zircon population had a weighted mean age of nearly 370 million years.
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population → have → years
The researchers interpret this as a maximum age for the impact event.
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researchers → interpret → 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.
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this → place → 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.
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Researchers → reveal → spectrometry
They made 37 measurements.
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They → make → measurements
The youngest zircon dates formed a distinct population.
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dates → form → population
10 of the 11 analyses gave a weighted mean age of nearly 370 million years after one statistical outlier was excluded.
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outlier → give → years
Most zircons, including the youngest Devonian-age grains, retained their primary oscillatory zoning and showed no signs of deformation.
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zircons → include → deformation
Two zircons displayed impact-related textures, including low-angle grain boundaries within microcracks and granular zircon arrays crossing their growth zoning.
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zircons → display → zoning
Why did the earlier fossil evidence place the Ames impact in the Ordovician
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evidence → place → Ordovician
The earlier Ordovician age for Ames came from Middle Ordovician conodonts and moulds found in the Crater Shale.
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age → come → 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.
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Shale → say → relationships
The researchers propose that the impact excavated Middle to Upper Ordovician Simpson Group strata and disrupted fossil-bearing sediments.
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impact → propose → sediments
Those materials could then have been redeposited inside the crater during resurge and tsunami backwash.
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materials → redeposit → backwash
Under this interpretation, the conodonts came from older rocks struck by the impact and were later deposited inside the crater.
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conodonts → come → crater
Rapid redeposition and burial inside the crater may also have helped preserve the delicate fossil remains.
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redeposition → help → 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.
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which → date → Crisis
The Ames result falls within the broader Middle to Late Devonian window of other North American impacts, though some are somewhat older.
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some → fall → impacts
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.
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paper → list → 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.
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impact → provide → Devonian
The researchers say this could inform discussions about links between extraterrestrial impacts and environmental disturbances associated with the Frasnian-Famennian mass extinction.
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this → say → extinction
The Aporia
