APPLICATION OF DETRITAL ZIRCON GEOCHRONOLOGY IN BASIN ANALYSIS TO EVALUATE TECTONIC VS. CLIMATIC ORIGIN FOR WIDESPREAD CONGLOMERATES AT THE K/T BOUNDARY

The integration of detrital zircon geochronology with basin analysis provides a means of determining the origin and significance of a number of distinctive, regionally distributed conglomeratic units in the central and southern US Rocky Mountains. These units – the Dark Canyon sequence of the Wasatch Formation in the BookCliffs of Utah, the Ohio Creek member of the Williams Fork Formation in the Piceance Creek Basin of Colorado, the Canaan Peak Formation in the Kaiparowitz Plateau of southwestern Utah, and the Ojo Alamo Formation in the San Juan Basin of Colorado and New Mexico – are generally thin and widespread, and are not simply parts of overall coarsening upwards sequences. They are broadly similar in composition, containing dominantly chert and quartzite reworked from local Mesozoic sources; they were deposited by gravelly braided river systems flowing to the northwest, north and east; they are stratigraphically abrupt and unconformity-bounded; and they were all deposited at or around the K/T boundary. The overall similarities in depositional style and timing suggest that gravel dispersal at the K/T could have been a result of regional climate change. However, young peak ages of U-Pb detrital zircon spectra that are interpreted to represent a maximum depositional age, along with published ages, indicate that deposition was not synchronous, thus precluding a climatic interpretation. These U-Pb dates, which indicate that deposition took place between the Campanian (82 Ma) and early Paleocene (64 Ma), more likely represent the beginning of Laramide deformation in each basin. Subsidence analysis indicates that the K/T was a time of reduced subsidence rates in these basins. These data suggest that widespread gravel progradation occurred during incipient uplift of nearby Laramide structures, prior to significant thrust load emplacement, which would tend to accelerate subsidence and trap gravel close to the bounding uplift.