Age

Although its general stratigraphy and paleontology appeared to define the Chicxulub structure as Cretaceous, geologists argued about its exact age. Officer, Meyerhoff, and their colleagues said that since Cretaceous strata lay above it, the structure must be older than latest Cretaceous and therefore could not be the K-T impact crater. They based their conclusions on the earlier identification by Meyerhoff that the fossils in the breccia blocks above the melt rock were late Cretaceous. Subsequently, however, these fossils were redated and found to be from the Tertiary period. In any case, impact ejecta often contains blocks of older rocks that were excavated during a cratering event and blasted into the air, from whence they resettle, landing on top of younger ones in a way reminiscent of the upside-down-cake stratigraphy that Shoemaker found at Meteor Crater.23 Ejecta from Ries Crater contains blocks of all sizes (some over 1 km) derived from rocks much older than the Miocene age of the crater. A single Chicxulub drill core could encounter such an older, out-of-place block on the way down and lead to the erroneous conclusion that, for that site, the K-T impact crater was falsified.

The first radiometric age report dated the Haitian tektites at

64.5 ± 0.1 million years, and for comparison, a feldspar from the K-T boundary at Hell Creek, Montana, source of T. rex skeletons, at

64.6 ± 0.2 million years.24 These two ages do not quite overlap the 65-million-year age of the K-T boundary within their error bands. However, such bands reflect only the "intralaboratory" error, that is, they give the probable range within which the age would fall if measured again in the same laboratory. But the argon-argon method also requires reference to an interlaboratory standard, which can introduce small differences when different laboratories analyze the same sample, enough to bring the Haitian tektite ages into conformity with the K-T boundary age. Chris Hall of the University of Michigan and colleagues confirmed Izett's results in their own laboratory, obtaining an age of 64.75 million years for four separate Haitian tek-tites.25 They noted that the ages of the tektites measured within their lab agreed so well that they "would make an excellent [argonargon] standard"; for an isotope geochemist, this is the ultimate compliment. Izett's measurement represented the first time that a K-T impact product, if that is indeed what the Haitian tektites are, had been directly and absolutely dated.

Next came measurement of the age of the Chicxulub melt rock itself, and from two different laboratories. First to report were Carl Swisher and colleagues from the Institute of Human Origins at Berkeley.26 They measured the age of the Chicxulub igneous rock and obtained 64.98 ± 0.05 million years, establishing the Chicxulub event as of exact K-T age. They also dated tektite glass from Haiti and glass embedded in rocks of K-T age at Arroyo el Mimbral in northeast Mexico (rocks that Smit and others believe were generated by the K-T event), and obtained almost exactly 65 million years for both. A few weeks later, Buck Sharpton of the Lunar and Planetary Institute and his co-workers reported an argon-argon age of 65.2 ± 0.4 million years for a different sample of the Chicxulub melt rock.27 All of these measurements are consistent and show that the Chicxulub event, the Haitian tektites, and at least one K-T ejecta deposit, date to precisely 65 million years.

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