A MAP DATABASE OF ISOTOPICALLY DATED CRETACEOUS BENTONITE LOCALITIES IN THE WESTERN INTERIOR U.S

Upper Cretaceous marine sedimentary rocks cover vast areas of the Great Plains, Northern Rockies, and Colorado Plateau and range in thickness from a few hundred meters to as much as 2-3 km. These dominantly clastic sediments were deposited in the Western Interior Seaway (WIS) that extended from the Gulf of Mexico to the Arctic, and they contain a detailed record of biostratigraphic, paleoclimatic, and volcanic history representing >30 million years (99.6-69 Ma) of Earth history. W.A. Cobban, collecting for over 60 years, has provided a richly detailed history of the paleontology of ammonites and inoceramid fauna in the WIS, and John Obradovich has determined dozens of ⁴⁰Ar/³⁹Ar bentonite ages for this biostratigraphy (Cobban and others, USGS OFR 2006-1250-online), making it the chronologic reference standard for Upper Cretaceous correlation.

Geospatial locality information for the bentonites containing sanidine that have been successfully separated and isotopically dated is summarized in a new map database. Location data originally published as PLSS (section-range-township and quarters) have been digitized from the most detailed map and GPS locations available. Biostratigraphic context (index ammonites and inoceramids) related to these bentonite age points have also been digitized as part of a digital curation effort for the USGS Denver Mesozoic (D-number) paleontology collections. Future ⁴⁰Ar/³⁹Ar sanidine and U/Pb zircon geochronology investigations will build on the framework of these localities, and add improved precision to the pattern of Cretaceous climate and sea-level change, and the biologic response to these changes. Initial new ⁴⁰Ar/³⁹Ar isotopic ages determined at the University of Wisconsin Rare Gas Laboratory have refined the precise ages of closely spaced bentonites in the Greenhorn Limestone near Pueblo, CO, while confirming the accuracy of Obradovich (1993) and Cobban’s previous geochronology, when calculated using the same ⁴⁰Ar/³⁹Ar monitor ages. The chronology of this key WIS section can be astronomically calibrated, thus providing additional geochronologic constraints for interpreting Upper Cretaceous biostratigraphy worldwide.