Tectonic and climatic forcing of widespread U.S. Rocky Mountain conglomerates at the Cretaceous/Paleogene boundary: Application of detrital zircon geochronology to basin analysis

There are a number of distinctive, penecontemporaneous, regionally distributed conglomeratic units in the central and southern U.S. Rocky Mountains whose origin and relationship to regional tectonics, climate change and basin evolution are unclear. These units -- the Dark Canyon sequence of the Wasatch Formation in the Book Cliffs of Utah, the Ohio Creek Member of the Mesaverde Group in the Piceance Creek Basin of Colorado, the Arapahoe Conglomerate of the Denver Formation (D1 sequence) in the Denver Basin, the Canaan Peak Formation in the Kaiparowits 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 abruptly prograde out across underlying units. They are broadly similar in composition, containing dominantly chert and quartzite clasts reworked from local Mesozoic and Paleozoic sources they were deposited by gravelly river systems they are unconformity-bounded and they were all deposited at or around the Cretaceous/Paleogene boundary. While these units have been broadly dated using palynology, interpretation of their origins has been difficult given the scarcity of sample material and the large age ranges for some recovered palynomorphs.Young peak ages of U-Pb detrital zircon spectra, interpreted to represent a maximum depositional age, are used in conjunction with traditional basin analysis techniques and published age data to determine the depositional history of these conglomeratic units. Subsidence analysis indicates that the latest Cretaceous and early Paleogene were times of reduced subsidence rates in these basins, and that the conglomerates were deposited after the initiation of Laramide-related subsidence in these basins, or late-stage Sevier thrusting in the case of the Canaan Peak Formation. While reported ages of deposition of these units cover a large time span, it is permissible that deposition was synchronous between &sim66 and &sim64 Ma, given the overlap between published ages and U-Pb dates of detrital zircons. The overall similarities in depositional style and timing suggest that gravel dispersal at the Cretaceous/Paleogene boundary could have been a result of regional climate change. Published Global Climate Models and stable isotope studies suggest that the Rocky Mountain monsoon had begun on the eastern flanks of the Sevier belt by the Campanian and had moved east to the broken Laramide foreland by the Paleogene. The depositional model proposed by this study concludes that a period of tectonic quiescence in the rising Laramide uplifts, evidenced by a reduction in subsidence rates and basin-wide unconformities, in latest Cretaceous and early Paleogene time led to overall reduced subsidence rates that, when coupled with an abrupt increase in seasonal precipitation during the Rocky Mountain monsoon, promoted a basinward shift in facies and created erosional surfaces in the proximal parts of basins. Renewed tectonism and subsidence in late Paleocene through Eocene time caused a subsequent retraction of lithofacies, leading to onlap of coarse-grained material (the conglomerates in this study) onto these erosional surfaces and subsequent deposition of finer-grained deposits. Thus, tectonics coupled with climate change lead to the deposition of the widespread conglomerates found around the Cretaceous/Paleogene boundary in the central and southern Rocky Mountains.