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
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.