U-PB GEOCHRONOLOGY OF DETRITAL ZIRCONS FROM UPPER CRETACEOUS AND PALEOGENE STRATA ON THE KAIPAROWITS PLATEAU, SOUTHERN UTAH: IMPLICATIONS FOR THE TECTONIC AND PALEOFLUVIAL EVOLUTION OF THE SOUTHERN CORDILLERAN FORELAND BASIN

Upper Cretaceous-Paleogene strata exposed on the Powell Point and Yankee Meadows on the Kaiparowits Plateau of southern Utah provide a record of the evolution of the southern Cordilleran foreland basin and the transition between the Sevier and Laramide orogenies. We report here on the detrital zircon studies of the uppermost units exposed on the Kaiparowits Plateau. The Campanian Wahweap and Kaiparowits formations contain abundant Triassic (220 - 240 Ma), Jurassic (160-180 Ma), and Cretaceous (90-92 Ma and 77-84 Ma) volcanic zircons with a smattering of Proterozoic and Archean grains. The Capping Sandstone member of the Wahweap Formation contains, in addition, Paleozoic and Neoproterozoic grain populations (325-350 Ma, 450 Ma, 500-550, 600-680 Ma) likely recycled through Mesozoic Colorado Plateau aeolianites uplifted in the thrust belt to the southwest. Overlying samples of Campanian to Paleocene Canaan Peak, Paleocene Pine Hollow and Eocene Claron formations lie unconformably above a cobble to boulder conglomerate in the lower Canaan Peak, and contain radically different detrital zircon assemblages. Zircons have mostly Archean (2500 Ma), Paleo and Mesoproterozoic (900-1200 Ma, 1400-1500 Ma, 1600-1900 Ma) and Paleozoic to Neoproterozoic (450-700 Ma) ages, and are likely recycled through uplifted pre-Paleocene strata. Mesozoic zircon grains are sparse and no grains are within 20. m.y. of the latest Cretaceous and Paleocene depositional age. This provenance break eflects the change from southwest-derived thrust belt volcanogenic provenance to northeast or northwest derived local provenance, as suggested by Schmitt et al. (1991, GSA Sp. Paper 260) and Goldstrand (1994, JSP. V. 62). The proximal source areas were composed of Proterozoic and Paleozoic strata exposed in the Circle Cliffs and other Laramide uplifts. The youngest zircons in the fluvial sands of the Wahweap and Kaiparowits formations are progressively younger upward from 83 to 80 Ma, which is a few million years older than depositional age as constrained by Ar-Ar dates on bentonites. We suggest that these zircons are detrital and fluvially transported, rather than aeolian. This time lag provides an estimate of a <5 Ma residence time of volcanic zircons in the rapidly subsiding Kaiparowits foreland basin.