U/PB ZIRCON AGES FOR TWO FOSSIL LOCALITIES IN THE UPPER JURASSIC MORRISON FORMATION, WESTERN COLORADO AND EASTERN WYOMING: IMPLICATIONS FOR CORRELATIONS ACROSS THE DEPOSITIONAL AREA

The Upper Jurassic Morrison Formation of the Western Interior of North America is one of the most prolific fossil-bearing rock units in the world, and it has been studied in detail across its vast depositional area. Long-distance correlations within the formation have been difficult, however, due to the inherent variability of terrestrial systems, the lack of biostratigraphically useful fossils, and the absence of definitive marker horizons in this rock unit. The commonly used lithostratigraphic framework has been shown to have problems as well. Radiometric dating has the potential to help overcome these issues and to aid in correlations across the depositional area.

New, geographically diverse U/Pb ages, along with legacy ⁴⁰Ar/³⁹Ar ages (recently recalculated due to the recalibration of the Fish Canyon Tuff sanidine standard to the astronomical timescale), allow significantly improved long-distance correlations. In addition, techniques such as CA-TIMS and the use of an ultra-low lab blank are allowing the University of Wyoming Geochronology Lab to achieve greater precision and accuracy in dating single small ashfall zircons. These crystals often have such a low level of radiogenic lead that they would previously have been deemed un-dateable. The ages being reported here are the result of a combination of these techniques.

These new ages, from two geographically disparate fossil localities in the Morrison Formation, can now be used to test previously published correlations of fossil-bearing localities. The ages, from Reed’s Quarry 9 at Como Bluff in Albany County, Wyoming and Mygatt-Moore Quarry at Rabbit Valley in Mesa County, Colorado, are both older that expected based on these earlier correlations. These new data support the concept that long-distance correlations of the Morrison Formation based on lithostratigraphy, including a change in the dominant clay mineralogy, should be used with caution in the absence of radiometric dates.