A buried crater in northern Oklahoma is about 100 million years younger than it has been recorded as being, according to new dating published in the journal Meteoritics & Planetary Science.
The Ames impact structure is roughly nine miles across and lies entirely beneath the town of Ames and the farmland around it. It was identified in 1991 by an oil company, and the rock it disturbed is a significant oil and gas producer.
Until now the impact had been placed at about 467.5 million years ago, in the Middle Ordovician. A team led by Elizabeth Catlos, an associate professor in the Department of Earth and Planetary Sciences at the University of Texas at Austin, dated zircon crystals from granite that the impact had altered, using uranium-lead radioisotopic dating. The youngest grains give a weighted mean age of 369.7 million years, with an uncertainty of 5.9 million.
That is the Late Devonian. The results were checked using cathodoluminescence and electron backscatter diffraction imaging, work carried out in collaboration with NASA.
"No matter what technique we used, it was coming back to this younger signal," Catlos said.
The revision touches two separate pieces of Earth history. The older date had made Ames a member of the Ordovician Meteor Event, a cluster of North American impacts around 467 million years ago that has been read as evidence of a sharp rise in meteorite arrivals, and which has supported the suggestion that the planet briefly carried a ring of debris in the manner of Saturn.
The new date moves Ames to within a few million years of the Frasnian-Famennian mass extinction, about 372 million years ago, one of the most destructive episodes in the history of marine life.
The team also offers an explanation for the old reading. The impact churned up older material and mixed the fossils into the rocks while preserving them, so the fossils recovered there are genuinely ancient. They record the age of the ground that was hit rather than the moment of collision.
This is the most honest failure in the geological record. The fossils did not mislead anybody. They answered a question nobody had asked them, accurately, for thirty-five years.
The Questionable Times notes the consequence for the Ordovician Meteor Event, which is now one crater short. The event itself has not changed. No new rock has been examined and no impact has been undone. What has happened is that a member has left, which makes a cluster of impacts an administrative category rather than a physical one.
The ring follows from this. Earth's possible Saturn-like ring was inferred from the number of craters of a particular age in North America, and that number has just been reduced by paperwork. The planet's rings are therefore subject to revision at any time, in either direction, by anybody with a mass spectrometer.
Our analysts observe that the new placement is the better one commercially. Ames has moved from being one of several craters that jointly evidenced a ring to being a single nine-mile structure sitting two to three million years before a mass extinction of marine life, which is a far stronger position for one crater to hold, and which the dating alone does not establish.
The Department of Unnecessary Research has asked for one clarification before the chronology is reprinted. If the fossils inside the crater record the age of the ground rather than the age of the event, it would like to know what else in the geological record is currently dating the floor it is standing on.