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Chicxulub 'impact layer' reinterpreted as volcanic ash

Controversy over the age of the Chicxulub Crater, and whether it truly represents the smoking gun of the end-Cretaceous impact, rumbles on - writes Ted Nield


Gerta Cottonmouth CreekDating impact craters with precision is a thorny problem, and when the stakes are high, even an acrimonious one.  Although since the public announcement  in 1991 of the Chicxulub structure offshore Yucatan most geologists have accepted the hypothesis that it represented the dinosaur-killing impact revealed by Walter and Luis Alvarez, Philip Asaro and Helen Michel in 1981 when they announced the discovery of the iridium anomaly from rocks in the Bottaccione Gorge, Italy. 

Picture:  Professor Gerta Keller points out the yellow clay layer, now thought to be a volcanic ash, Cottonmouth Creek.

However, others have remained unconvinced by the connection - notably Professor Gerta Keller, a distinguished micropalaeontologist and biostratigrapher from Princeton University, who has led a heretical campaign united around the belief that the Chicxulub structure represents a different impact, pre-dating the K/T boundary by some 300,000 years (subsequently reduced to 150,000 years with the newer time scale).


The massive disruption of sediments close to the impact structure itself creates considerable difficulties in precision stratigraphy required to resolve such questions, and so researchers have sought out sediments far enough away to be unaffected by physical disruption, but close enough to bear sedimentological witness to the cataclysmic event in an undisturbed sequence.

For that reason, geological investigations on the Gulf Coastal Plain (chiefly the states of Texas and Alabama) have become increasingly important, with attention focusing on river sections and cores in the Brazos River area (Falls County, Texas).

Attention has focused on a distinctive yellow claystone exposed in Cottonmouth Creek (picture) within the uppermost Maastrichtian, some way below the K/T boundary, and below the reworked impact spherule layers at the base of sandstone complex.  In previous work, Gerta Keller and her collaborators had interpreted this as the ‘original Chicxulub impact spherule layer’, believing it to consist of the alteration products impact-generated glass with no volcanic component.  However, new research published late last year by Hart et al. has re-examined the horizon and found feldspar phenocrysts and zircons, indicating a volcanic origin. 

A comparable, older volcanic ash has also been found, immediately north of the Highway 413 road bridge over the Brazos River.  Coming somewhat lower in the section, this indicates to Hart and colleagues that such ash falls were a ‘normal’ feature of the succession. Zircons in the Cottonmouth Creek claystone have been dated as 65.95±0.04 Ma, while those taken from the new locality are currently being dated by Brent Miller at Texas A&M University.

Keller, and her collaborator Thierry Adatte (Université de Lausanne) have also visited the ‘other’ exposure of the yellow claystone and they await confirmation of zircons.  Their sample is also being dated, at MIT.  Keller says: “We hope to get a good age for this layer”, agreeing that, with zircon present, the older layer must be volcanic in origin and Keller et al.’s idea (that it represented the first impact spherule layer) is “questionable and probably wrong”.


However, Keller maintains that Hart et al.’s further conclusion made in their paper – namely that the impact spherules at Brazos support the ‘traditional’ interpretation that the Chicxulub impact is KTB in age - is not supported by the evidence.  “The pre-KTB age of the Chicxulub impact is in no way dependent on the yellow clay layer, but on the fact that the actual reworked impact spherule layers  and sandstone complex are up to one matre below the KTB mass extinction, the first appearance of Danian species, and the KTB-characteristic Δ13C shift.   Reinterpreting the yellow clay as an ash layer does not add anything to the Chicxulub age story or the KTB; but if good age control can be gained from zircon dating it will provide a good lower age limit for the reworked impact spherule layers at the base of the sandstone complex that infills incised valleys in the Brazos area.” 

“I do not agree at all with Hart et al.’s claim that the fact this clay turns out to be volcanic ash, dated latest Maastrichtian, removes the need for a ‘pre-K/T boundary, pre-extinction’ impact event" says Keller.  “This has no bearing on the KTB - unless one employs circular reasoning - that the Chicxulub impact is defined by the spherule layer, and that therefore this impact is KTB in age.”

Hart told Geoscientist Online: “Our work in the Brazos River area continues, involving an expanding team of geoscientists. In October 2012, part-funding by a research award from the Geological Society of London has allowed this work to be continued and expanded into Alabama with Prof. Peter Harries (University of South Florida).  In the area to the west of Greenville (Alabama) several successions have been visited (Braggs, Mussel Creek and Moscow Landing).  These sections have been logged, sampled for microfossils and described in more detail than is currently available on a number of websites or in field guides: there are few research papers on these sections.”

Meanwhile the major international conference ‘Volcanism, Impacts, and Mass Extinctions: Causes & Effects’, taking place at the Natural History Museum, London on March 27-29 this year, is filling up fast with close on 150 registrations already.  “It is shaping up to be an amazing multidisciplinary meeting with key experts from fields across the spectrum of geosciences, palaeontology, astrophysics, and the atmospheric and climate sciences. We hope we will be lucky and some serious science journalists will try to understand that mass extinctions are not the result of single impacts or volcanism, but rather from the resulting complex interactions with the atmosphere, biosphere and oceans leading to toxic environments, ocean acidification, acid rain, carbonate crisis, ozone destruction and so on.”

Further reading

  • Hart, M.B., Yancey, T.E., Leighton, A.D., Miller, B., Liu, C., Smart, C.W. & Twitchett, R.J. 2012. The Cretaceous-Paleogene boundary on the Brazos River, Texas: new stratigraphic sections and revised interpretations. GCAGS Journal, 1, 69-80.
  • Keller, G. and Adatte, T. (eds) 2011. The KT Mass Extinction and the Chicxulub Impact in Texas. Society of Sedimentary Geology (SEPM) Special Publication 100, 313pp.