PROVENANCE AND AGE OF THE BROWNS PARK BASAL CONGLOMERATE AND BISHOP CONGLOMERATE BASED ON U-PB DETRITAL ZIRCON AGES, NORTHWESTERN COLORADO

New U-Pb detrital-zircon ages show that the Bishop Conglomerate and basal conglomerate of the Browns Park Fm. are broadly equivalent in age (early Oligocene), and probably reflect a pulse of post-Laramide tectonism. Deposition of these fluvial conglomeratic units coincided with extensive Oligocene Basin and Range volcanism, which produced abundant Oligocene-aged zircon grains. These zircon-rich air-fall deposits were reworked into these coarse-grained fluvial sediments, and provide a reasonable means for age dating the conglomeratic units.

The Bishop Conglomerate of northwestern Colorado ranges in age from ca. 30 to 27 Ma. U-Pb detrital-zircon age spectra and analysis of gravel provenance show that Bishop represents fluvial channels that drained the eastern Uinta Mountains. The basal conglomerate of the Browns Park Fm. ranges in age from ca. 30 to 28 Ma. U-Pb detrital-zircon age spectra, paleocurrent measurements of cross-bedded fluvial sandstones, and analysis of gravel provenance show that the basal conglomerate of the Browns Park Fm. represents ancient rivers that drained both the west flank of the Park Range as well as portions of central Colorado, including perhaps the White River Uplift and Sawatch Range. Collectively, these rivers flowed northward towards central Wyoming.

Oligocene erosion beveled both Laramide mountain ranges as well as adjacent basin fill, and produced regional erosion surfaces. We interpret this widespread erosion to have been driven by regional Oligocene uplift of the Rocky Mountain region. Subsequent extension led to the cessation of widespread conglomeratic sedimentation ca. 28 Ma, and led to the subsequent accumulation of thick tuffaceous Miocene basin-fill units including the sandstones and siltstones of the Browns Park Fm.