PROVENANCE OF THE WASATCH FORMATION AND OHIO CREEK CONGLOMERATE IN THE PICEANCE CREEK BASIN (NORTHWEST COLORADO, U.S.A.)

The Piceance Basin in northwest Colorado is a Laramide structural basin surrounded by basement-involved uplifts. The earliest Cenozoic stratigraphy is represented by (1) the Ohio Creek Conglomerate, which is a coarse-grained, sheet-like sandstone unit that extends across much of the basin, and (2) the Wasatch Formation, which is dominated by fluvial sandbodies and overbank strata that contain paleosols of varying pedogenic development. The Wasatch Formation can be further subdivided into three members, the Atwell Gulch, Molina, and Shire members, which display dramatic up-section changes in fluvial sandbody stacking patterns. The lower Atwell Gulch and upper Shire members are dominated by single- to multi-storied, lenticular sandbodies with abundant fine-grained overbank deposits whereas the intervening Molina Member is dominated by tabular fluvial sandbodies and proportionally less overbank deposits. We determined new and compiled legacy U-Pb detrital zircon radiometric dates from sandbodies distributed through the Ohio Creek Conglomerate and Wasatch Formation to assess major changes in provenance. Our data includes 1265 ages, as well as assessments of sandstone composition using standard petrographic techniques. A comparison of the age spectra with one another suggest a largely similar provenance amongst the units that is also similar to the underlying Upper Cretaceous fluvio-deltaic and marine strata. Sandstones from the Ohio Creek Conglomerate and Wasatch Formation are litharenites and feldspathic litharenites with a similar composition to underlying Upper Cretaceous strata. These results document a distinct difference from the provenance of coeval strata in the Uinta Basin immediately to the west of the Piceance Basin, which were determined by previous studies. The different provenance between the two basins indicates that the massive paleo-California River that filled the Uinta Basin was unable to prograde into the Piceance Basin during the earliest Paleogene. The data also indicate that it is unlikely that crystalline basement was exposed in the Sawatch and Uncompahgre uplifts surrounding the Piceance Basin. Instead the Ohio Creek Conglomerate and Wasatch Formation represent recycling of Upper Cretaceous strata during the early uplift and unroofing of these Laramide structures.