AT THE INTERFACE OF BIOSTRATIGRAPHY AND ISOTOPIC GEOCHRONOLOGY—EXPLORING THE CRETACEOUS WORLD

Over their lengthy careers with the U.S. Geological Survey, Bill Cobban and John Obradovich have contributed greatly to our knowledge of the Cretaceous fossil record, to the stratigraphy of marine and coastal deposits during the Upper Cretaceous. They determined the chronology of both fossils and rock more precisely than for any comparable period of Earth’s history. What made their collaboration so productive was their mastery of complementary fields of inquiry, and their prompt investigations when the technique of laser-fusion ⁴⁰Ar/³⁹Ar geochronology first became available to date bentonites (volcanic ash beds) interbedded with marine sediments containing the ammonites and inoceramids. Cobban’s biostratigraphic work is unique in its lateral geographic extent, as in many cases it was linked to geologic mapping of many co-authors throughout the Rocky Mountain and northern Great Plains. Cobban and his colleagues integrated field geologic mapping with intensive paleontological collecting that by depicting ammonite zones, subdivided thick Western Interior shales (e.g. Pierre Shale in Colorado, 5000-8000’ thick). Significant biostratigraphic collaborations include those with Glenn Scott, John Reeside, Jim Gill, ‘K’ Molenaar, and Al Merewether, among many others. John Obradovich assiduously pursued every bentonite that might yield sanidines to date. He worked with 2 or 3 ash beds for each one that yielded datable sanidine. In 1993, he summarized the isotopic chronology of Upper Cretaceous bentonite and ammonites. This work yielded a precise chronology, related to ammonite biostratigraphic zones and international stages, that remains the best-dated upper Cretaceous sequence, from 99.6 Ma to 65.5 Ma. Critical to securing the multidisciplinary legacy of Cobban and Obradovich is work on the prodigious Denver Mesozoic fossil collection of over 14,000 ammonite and inoceramid localities which includes archiving the sanidine separates and documenting the precise field geographic and stratigraphic location of each sample.