AGE-DATING OLIGOCENE AND MIOCENE SANDSTONES AND CONGLOMERATES USING DETRITAL-ZIRCON U-PB AGES: A CASE STUDY OF BISHOP CONGLOMERATE AND BROWNS PARK FM. DEPOSITS FROM SOUTHWESTERN WYOMING AND NORTHWESTERN COLORADO

Age determinations of non-marine sandstones and conglomerates generally rely on fossils and/or direct dating of volcanic ash. However, in fluvial deposits where air-fall tuff comprises a significant proportion of the sediment, U-Pb dating of detrital zircons can provide ages accurate to within 1 to 2 million years of depositional ages. This suggestion stems from several new U-Pb ages of detrital zircons acquired from a suite of conglomerate and sandstone samples from the Oligocene Bishop Conglomerate and Miocene Browns Park Formation in southwestern Wyoming and northwestern Colorado. The oldest maximum depositional U-Pb ages (MDA) for these samples are from Bishop Conglomerate outcrops in the southern Green River Basin (ca. 35-34 Ma). The Bishop is overlain by tuffaceous sandstone and siltstone that produced an MDA of ca. 33-31 Ma. In northwestern Colorado, Bishop Conglomerate is overlain by tuffaceous sandstone and siltstone of the Browns Park Formation with MDAs ranging from ~24-8 Ma.

Collectively, the samples show consistently younger MDAs upsection. Tuffaceous components of Bishop Conglomerate and Browns Park deposits are air-fall ash derived from Basin and Range caldera complexes in southern Utah and Nevada. Radiometric ages of Basin and Range tuff units help constrain interpretations of Bishop and Browns Park MDAs. For example, the ca. 35-34 Ma MDA of the Bishop Conglomerate in Wyoming is consistent with the ~35.8 Ma tuff of the Stone Cabin Formation in the Basin and Range. However, zircon grains from the younger 31.7 Ma tuff of the Windows Butte Formation are absent from the 35-34 Ma Bishop Conglomerate sample, but are abundant in the overlying 33-31 Ma Bishop tuffaceous sandstone.

In summary, we suggest that the MDAs from these deposits generally closely represent their depositional age. The quasi-continuous nature of Oligocene through Miocene caldera eruptions in the Basin and Range Province provided a continuous supply of air-fall ash that drifted across Utah, southern Wyoming and northern Colorado. Subsequent reworking of volcanic ash by rivers produced tuffaceous sandstones and conglomerates. These results suggest that other geologic periods characterized by quasi-continuous volcanic input, such as the Late Cretaceous of the western U.S., may permit similar dating of siliciclastic rocks.