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Error Bars to Equal Our Outcrops: Examples from the Denver Basin Show How High-Resolution U-Pb Geochronology Can Bring Focus to Fuzzy Old Problems in Terrestrial Stratigraphy and Paleontology

Johnson, Kirk R.1; Bowring, Samuel 2
1 Research and Collections, Denver Museum of Nature & Science, Denver, CO.
2 EAPS, MIT, Cambridge, MA.

As recently as 10 years ago, 2 sigma error bars on U-Pb and Ar40/39 radiometric dates were rarely better than 1%. For rocks of K-T boundary age, this equals an error of ±650,000 years. For paleontologists working in the Western Interior where sediment accumulation rates in Laramide synorogenic strata typically ranged from 100 to 150 meters per million years, this translated into error bars that were equivalent to ±65-98 meters of section in areas where surface outcrop exposures are rarely thicker than 50 meters. For this reason, radiometric dating was considered generally useful but not truly relevant to questions of high-resolution stratigraphy. For example, the K-T boundary occurs in an interval of reversed magnetic polarity known as C29R. The duration of this subchron, as estimated from the width of seafloor magnetic anomalies is on the order of half a million years but the subchron is represented in various terrestrial settings by something on the order of only 50 meters of section, a result in concordance with a sediment accumulation rate of 100 meters per million years. At an error of 1%, radiometric estimates of the age of the subchron had errors that were greater than the duration of the subchron, and as such, were useful for rough age estimates but not precise calibration. The recent dramatic increase in the precision and accuracy of U-Pb dating has now decreased error by more than an order of magnitude and it is now possible to achieve errors of less than 0.1%. With this advance, radiometric dating has now become a useful tool for asking questions at the outcrop scale where the error of the date can be represented by a few meters of section. This advance will now allow for precise radiometric calibration of the duration of magnetostratigraphic subchrons and much more precise calibration between terrestrial and marine sections. With precise calibration, we can now begin to compare marine and terrestrial paleotemperature proxies with a degree of confidence that was not possible merely a decade ago. The Late Cretaceous and early Paleocene strata of the Denver Basin are endowed with a rich paleontological record, a reliable magnetostratigraphy, good exposures of the K-T boundary layer itself, and abundant locally-sourced volcanic ashes. This combination provides a superb test bed for using radiometric dating to answer outcrop scale questions.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009