DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF MIDDLE TO UPPER JURASSIC STRATA IN THE PARADOX BASIN: INSIGHTS INTO TECTONIC UPLIFT OF THE WET MOUNTAINS AND SOUTHERN FRONT RANGE, COLORADO, U.S.A

The Middle to Late Jurassic in the western United States marked a period of active tectonic processes on the Cordilleran foreland triggered by the subduction of the Farallon Plate and related magmatism on the western continental margin of Laurentia. Eolian and fluvio-lacustrine sediments were deposited in the Paradox Basin during this time. Rapid changes in the paleolandscape occurred during the Middle to Late Jurassic and led to a major shift in provenance from terranes in the southern and eastern Appalachian highlands to local sources within central Colorado. While general tectonic influences on the Mesozoic sedimentary basins of the western continental interior are well understood, further work is needed to constrain specific source terranes.

In this study, we present a dataset of 7,887 detrital zircon U-Pb ages from 31 sandstone samples collected from three successive Middle to Late Jurassic units within the Paradox Basin: Entrada Sandstone, Wanakah Formation, and Morrison Formation. Diagnostic detrital zircon age peaks that occur at 516–529 Ma, 1440–1448 Ma, and 1690–1708 Ma correspond to established ages of the Precambrian basement rocks and suites of granitic plutons associated with the southern Front Range and Wet Mountains Complex in central Colorado. The Wet Mountains Complex is an unusual contribution to the typical Mesozoic sediment sources on the Colorado Plateau. The unusually high fractional intensity of sediments derived from these Cambrian and Proterozoic sources suggests an unroofing episode that lasted for ~ 20 Ma, at which time the Paradox Basin was still evolving.