Oklahoma’s Buried Crater May Be About 100 Million Years Younger Than Thought

Ancient Fossils Inside Oklahoma Crater May Have Given Scientists the Wrong Age

A crater buried under Oklahoma may be roughly 100 million years younger than scientists have long believed, a gap about as wide as the distance from today back to a time when dinosaurs still roamed. New dating of tiny crystals from the crater’s melted rock points to an impact around 370 million years ago instead of 470 million, which would drop the Ames structure into a different geologic period altogether.

Ames sits thousands of feet below the town of Ames, Oklahoma, with a rim roughly 9 to 10 miles across. The shattered rock left behind also holds oil and gas, with estimates of recoverable oil running from 25 million to 145 million barrels. The crater has long been linked to a barrage of space rocks that pummeled Earth starting roughly 466 to 468 million years ago, after a large asteroid broke apart.

A team led by Elizabeth Catlos of the University of Texas at Austin reports in Meteoritics & Planetary Science that the crater’s timing looks very different. The authors stop short of calling it settled, saying Ames “may record activity” near a major extinction period in the Late Devonian, a stretch of geologic time around 370 million years ago.

Ordovician Fossils Inside the Ames Crater May Predate the Impact

Until now, the crater’s Ordovician age, about 470 million years, has rested on fossils. Researchers found conodonts, tiny feeding parts from an extinct animal, in the black shale that fills the crater. No high-precision dating had ever backed up that age, and the fossil-based date came from just four small chips of shale.

Catlos and colleagues read those fossils differently. Their proposal is that the impact tore older fossils out of the rocks it struck, then swept them back into the crater as seawater rushed in and tsunami waves washed backward. The fossils would still be genuinely ancient, but they would record the age of the ground that was hit rather than the moment of collision, a bit like finding an old coin in a freshly dug hole and dating the hole by the coin.

Zircon Crystals in the Ames Crater Cluster Near 370 Million Years

For a more direct date, the team turned to zircon, a tough mineral that works like a stopwatch. When a zircon crystal forms, it traps uranium, which slowly turns into lead at a steady, known rate. Measuring how much has changed reveals when the crystal’s clock started or was last reset.

Samples came from a drill core near the crater’s center, roughly 9,000 feet down, in rock that melted during the impact. Crystals were scarce, since only a one-inch plug is allowed from each foot of core, but the team separated 16 zircon grains and took 37 measurements with two lab methods, one using lasers and the other an ion beam.

Most readings pointed back to roughly 1.4 billion years, matching the granite the space rock slammed into. A smaller group pointed elsewhere. Ten of the eleven youngest readings agreed with one another, averaging 369.7 million years, give or take about 6 million, and both methods found Devonian dates in some of the same crystals.

Those grains showed orderly growth layers and no shock damage. The authors interpret them as crystals whose clocks were wiped clean, or that grew fresh, in the heat of the impact. Later tectonic activity could explain the young dates instead, but the authors judged that unlikely, partly because the crater sits well away from major fault systems.

Only Two of Ten Mapped Zircons Showed Clear Shock Damage

To check for impact damage, the team mapped ten grains with an electron microscope technique. Eight showed only minor bending. Two carried dense microcracks and tiny granular patterns cutting across their layers, signs consistent with a violent shock. Combined with shock features that earlier studies found in quartz, they confirm the crater came from an impact, a point that had been debated.

Oddly, those two battered crystals gave older, mixed dates rather than the Devonian one. Shocked zircon often keeps its pre-impact age, the authors note, so the damaged crystals and the reset ones need not be the same grains.

Argon Dates Point to Later Heating, Not the Impact

A separate test, using argon gas trapped in a common rock mineral, gave messy results ranging from about 310 million to 250 million years ago, all younger than the zircon result. The authors say those dates do not mark the impact. Most fall within a stretch when buried rock in the region heated up and oil and gas formed, and heat can make argon leak out and scramble the clock.

If the Devonian age holds, Ames would join other North American craters of similar age, including Flynn Creek in Tennessee and Nicholson Lake in Canada, both roughly 380 million years old, plus impact deposits near Alamo, Nevada. The timing also sits near a mass extinction about 372 million years ago, though the study does not show that Ames caused or contributed to it.

Ames now has two clocks that disagree by about 100 million years. The crystals are strong enough evidence to put the Ordovician label in doubt, but locking in a Devonian date will take more zircon. A search is now on in nearby rock for the debris the blast flung outward, which cooled quickly and could give a cleaner date.

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Disclaimer: This article summarizes a peer-reviewed scientific study for general informational purposes. It does not capture every detail or limitation of the original research, and readers are encouraged to consult the full paper for complete information.

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