Researchers at the University of Texas at Austin have determined that the Ames impact structure in Oklahoma formed about 370 million years ago (Late Devonian), nearly 100 million years later than the previous estimate of around 467 million years ago (Ordovician).

The buried crater (roughly 14- 16 km in diameter) lies beneath the small town of Ames in northwest Oklahoma (Major County). It is a significant oil and gas reservoir because the impact fractured the rock, creating traps for hydrocarbons.

It was previously linked to the Ordovician Meteor Event (a proposed cluster of impacts around 467- 468 million years ago, possibly related to a fragmented L- chondrite asteroid and even a temporary debris ring around Earth).

That age rested mainly on biostratigraphy: conodont teeth (from ancient eel- like creatures) found in the crater fill that date to the Ordovician. Researchers now conclude those fossils were likely already millions of years old and were simply reworked and preserved during the impact.

The new age comes from U- Pb dating of zircon crystals in impact- affected granites/granodiorite from the site. Multiple techniques (secondary ion mass spectrometry and laser ablation: ICP -MS) consistently yielded a youngest weighted mean age of about 369.7 ± 5.9 Ma.

Imaging (cathodoluminescence and electron backscattered diffraction, including work with NASA) confirmed impact- related textures in some zircons, such as recrystallization features from shock. The host rock itself is much older (Mesoproterozoic, ~1.4 billion years). Argon dating of plagioclase reflected later thermal overprinting tied to burial and hydrocarbon maturation rather than the impact itself.

The revised timing places the Ames impact near the Frasnian- Famennian (Upper Kellwasser) mass extinction boundary (~372 million years ago), a major marine extinction event.

This removes it from the Ordovician Meteor Event and raises questions about whether impacts contributed to the Late Devonian die- off (alongside other potential drivers such as volcanism or environmental changes).

Lead author Elizabeth Catlos (UT Department of Earth and Planetary Sciences) noted that accurate timelines help distinguish extraterrestrial vs. terrestrial causes of major events. Co- author Danny Stockli (Jackson School dean) highlighted zircon’s value for both precise dating and recording impact shock.

The peer- reviewed study by Catlos and colleagues appeared in Meteoritics & Planetary Science (published online around July/September 2026). It builds on earlier sample collection and analysis by the late graduate student Andrew Parisi, with additional contributions from other UT researchers.

In short, improved radiometric dating of impact- shocked zircons has shifted a key North American crater’s place in geologic history and its potential links to mass extinctions.

The Ordovician Meteor Event (OME), also called the Ordovician meteorite event or Great Ordovician Meteorite Shower, was a prolonged period of dramatically elevated meteorite influx to Earth during the Middle Ordovician, beginning around 467.5 ± 0.28 million years ago (or ~465.8- 468 Ma in some calibrations) and lasting roughly 40 million years.

It is widely attributed to the catastrophic breakup of a large L- chondrite parent body (roughly 100- 150 km in diameter) in the asteroid belt between Mars and Jupiter. This collision scattered fragments into Earth-crossing orbits.

Supporting evidence includes:

• Shock ages in many modern L- chondrite meteorites (about one- third of meteorites that fall to Earth today are L chondrites linked to this event).
• Abundant fossil L-chondrite meteorites (up to ~20 cm across) preserved in Middle Ordovician limestone, notably at Thorsberg Quarry (Kinnekulle), Sweden.
• Elevated concentrations of ordinary chondritic chromite grains in sedimentary rocks of this age across multiple continents (Sweden, Russia, China, etc.).

A 2024 study noted that many associated craters formed within ~30° of the paleo- equator. This distribution is inconsistent with a random asteroid- belt source and has led to the hypothesis that a large asteroid may have passed within Earth’s Roche limit, broken apart, and temporarily formed a debris ring; material from the ring then deorbited and produced the observed impacts. Ring shading has even been speculated as a possible contributor to later Ordovician cooling (Hirnantian glaciation), though this remains debated.

The flux of dust, micrometeorites, and larger meteorites was estimated at ~100 times higher than today, with a corresponding increase in the rate of larger impacts.

This is recorded in:

• Fossil meteorites and chromite spikes in marine sediments.
• An excess of impact craters dated to roughly 470- 440 Ma (an order- of- magnitude higher density than expected in well- preserved regions such as parts of North America and Scandinavia).

Several structures have been linked (with varying confidence) to the OME, including:

• Decorah (Iowa, USA)
• Rock Elm (Wisconsin, USA)
• Slate Islands (Ontario, Canada)
• Crooked Creek (Missouri, USA)
• Others in North America, Europe, and elsewhere

(Note: The Ames structure in Oklahoma was previously included on biostratigraphic grounds but was recently redated to ~370 Ma and removed from the OME.)

Some researchers have proposed that the influx of dust and impacts may have contributed to cooling that helped trigger or coincide with the Great Ordovician Biodiversification Event (GOBE), a major radiation of marine life. However, high- resolution dating studies have questioned a direct causal link, showing that the main meteorite bombardment post- dates the onset of cooling and the start of diversification in some records. The event is not associated with a major mass extinction.

In summary, the Ordovician Meteor Event represents the largest documented asteroid disruption and subsequent Earth- impact shower of the past ~3 billion years, leaving a clear geochemical, sedimentological, and cratering signature in the Middle- Late Ordovician record.

The Frasnian- Famennian (F- F) mass extinction, also known as the Upper Kellwasser event (or part of the broader Kellwasser Crisis), was one of the “Big Five” Phanerozoic mass extinctions. It occurred at the boundary between the Frasnian and Famennian stages of the Late Devonian Period, approximately 372 million years ago.

Precise numerical ages place the F- F boundary around 371.87 ± 0.11 Ma (or constrained to roughly 371.8-372.4 Ma in various high- resolution studies), based on U- Pb dating of bentonite ash beds combined with cyclostratigraphy.

The crisis consists of two distinct pulses of environmental stress and extinction, recorded as widespread organic- rich black shales and limestones (the Kellwasser horizons, named after exposures in the Harz Mountains, Germany):

• Lower Kellwasser Event (late Frasnian, early Late rhenana Zone): lasting roughly 90 kyr.
• Upper Kellwasser Event (latest Frasnian, Late linguiformis Zone, coinciding with the F- F boundary): lasting roughly 110 kyr and generally more severe and widespread.

These were separated by a few hundred thousand years (estimates ~350- 800 kyr). The Upper Kellwasser is the main extinction pulse associated with the stage boundary.

Key features include:

• Widespread marine anoxia/euxinia (oxygen- depleted to sulfide- rich bottom waters), expanding even into shallow- water and reef settings during the Upper Kellwasser.
• Positive carbon- isotope (δ¹³C) excursions reflecting changes in the global carbon cycle.
• Sea-level fluctuations (often transgressive pulses followed by regression).
• Possible short- term climatic cooling or warming pulses, with evidence for elevated CO₂ in some records and glacio- eustatic sea- level falls in others.

No single cause is universally accepted; the crisis was likely multi- factorial and stepwise. Leading hypotheses include:

• Oceanic anoxia driven by eutrophication, climate change, or changes in ocean circulation (widely supported by the black- shale record).
• Volcanism, particularly activity linked to large igneous provinces (e.g., Viluy Traps) or other magmatic pulses, evidenced by mercury anomalies.
• Climate shifts (cooling or greenhouse spikes related to forest expansion, weathering, or volcanism).
• Possible contributions from sea- level change, nutrient runoff from expanding terrestrial vegetation, or tectonic factors.
• Extraterrestrial impacts have been suggested (e.g., Siljan crater), but high- precision dating generally shows they do not precisely coincide with the main extinction pulses and are not considered primary drivers.

The Upper Kellwasser anoxia appears more of an epicontinental (shallow- sea) phenomenon in many regions rather than a fully open- ocean event.

Recovery was slow in many ecosystems, especially reefs. A second major Devonian crisis (the Hangenberg event) occurred ~13 million years later near the Devonian- Carboniferous boundary.

In the context of recent research (such as the redating of the Ames impact structure in Oklahoma to ~370 Ma), the F- F extinction timing has prompted renewed discussion of whether impacts played any supporting role alongside terrestrial drivers, though anoxia and climate remain the dominant explanations.

The Ames impact structure, Oklahoma: New radioisotopic constraints and implications for North American impact chronology

The Ames impact structure in Oklahoma was long thought to have formed during the Ordovician Meteor Event, based on conodont biostratigraphy of its crater fill.

New U- Pb zircon dates from the impact-melt portion (using secondary ion mass spectrometry and laser ablation, inductively coupled plasma, mass spectrometry; n = 37 spots) yield:

• A Mesoproterozoic emplacement age for the impacted granodiorite of 1401.2 ± 8.1 Ma (±2σ, upper Concordia intercept).
• A weighted mean age from the youngest zircon dates of 369.7 ± 5.9 Ma (n = 10/11; MSWD = 0.71; p(χ²) = 0.7).

Cathodoluminescence and electron backscattered diffraction imaging show that most zircons (including the youngest Devonian- age grains) retain primary oscillatory zoning and lack deformation.

Two grains, however, display impact- related textures: regions of low- angle grain boundaries within microcracks and discrete arrays of granular zircon that crosscut oscillatory growth zoning.

Plagioclase ⁴⁰Ar/³⁹Ar analyses (n = 6 samples) give Late Carboniferous (~310.5 Ma) and Permian (~250.5 Ma) approximate total- fusion dates. These overlap the timing of heating and hydrocarbon maturation in the crater, indicating the argon system records post-impact thermal overprinting rather than the impact itself.

Based on the youngest zircon dates, the Ames structure is interpreted as Late Devonian and potentially linked to activity near the Frasnian- Famennian boundary, contemporaneous with other North American impacts.

Main conclusions from the paper

• Biostratigraphic (conodont) evidence previously suggested a late Ordovician (Chatfieldian) age tied to the Ordovician Meteor Event. The new zircon data instead support a minimum age of ~370 Ma.
• This places Ames near the Frasnian- Famennian (Upper Kellwasser) mass extinction (~372 Ma), raising the possibility of a contribution from bolide impacts to that event.
• The discrepancy between radioisotopic and biostratigraphic ages illustrates ongoing challenges in impact geochronology. The authors emphasize the need for additional material suitable for modern techniques and openness to alternative age hypotheses.
• Supporting data (geochronology and microstructural datasets) are available via the Texas Data Repository (DOI: 10.18738/T8/G5UENJ).

The full text is behind a Wiley paywall, but the abstract and extensive excerpts (including tables of zircon dates, petrographic descriptions, and discussion of microstructures such as planar deformation features) are publicly summarized in the sources above. The UT Austin Jackson School press release provides additional accessible context on the broader implications.

Published: Meteoritics & Planetary Science (first published 7 July 2026)

DOI: 10.1111/maps.70191

Provided: University of Texas at Austin

Authors: Elizabeth J. Catlos, Andrew F. Parisi, Michael E. Brookfield, Timmons Erickson, Sean P.S. Gulick, Axel K. Schmitt, Daniel F. Stockli, Daniel P. Miggins, Ben Ruchte, Mark Cloos

Abstract

The Ames impact structure (Oklahoma) is thought to have formed during the Ordovician Meteor Event, based on conodont biostratigraphy of its crater fill. Here, U–Pb zircon dates from its impact-melt portion, conducted using secondary ion mass spectrometry and laser ablation–inductively coupled plasma–mass spectrometry (n = 37 spots), yield a Mesoproterozoic emplacement age for the impacted granodiorite (1401.2 ± 8.1 Ma, ±2σ, upper Concordia intercept). However, the youngest zircon dates define a weighted mean age of 369.7 ± 5.9 Ma (n = 10/11), with MSWD = 0.71 and p(χ²) = 0.7. Cathodoluminescence and electron backscattered diffraction images reveal that most zircons, including the youngest Devonian-age grains, show primary oscillatory zoning and lack deformation. However, two have impact-related textures, including regions of low-angle grain boundaries within microcracks and discrete arrays of granular zircon crosscutting oscillatory growth zoning. Plagioclase (n = 6 samples, ⁴⁰Ar/³⁹Ar) yields Late Carboniferous (~310.5 Ma) and Permian (~250.5 Ma) approximate total fusion dates that overlap the timing of heating and hydrocarbon maturation in the crater, suggesting the argon system records postimpact thermal overprinting. Based on the youngest zircon dates, the Ames impact structure may record activity near the Frasnian–Famennian boundary, contemporaneous with other North American impacts.