Abstract

Two ash beds from the Bishop Conglomerate along the south flank of the Uinta Mountains have given ⁴⁰Ar/³⁹Ar laser fusion eruption ages of 30.54 ± 0.22 Ma (ash of Diamond Mountain Plateau) and 34.03 ± 0.04 Ma (ash of Yampa Plateau).

Age, mineral composition, and zircon morphology of the ash of Diamond Mountain Plateau strongly suggest that it is correlative with the Cottonwood Wash Tuff of the Needles Range Group in western Utah/eastern Nevada. The Cottonwood Wash Tuff eruption has also been suggested as the source for the Upper Whitney ash bed in the Brule Formation of Nebraska and Wyoming (Blaylock, 1998). The presence of widespread pyroclastic fall deposits from the Cottonwood Wash Tuff, but not for other more voluminous eruptions of the Needles Range Group (Lund and Wah Wah Springs tuffs) may suggest that eruption conditions were different for the Cottonwood Wash Tuff compared to the other eruptions.

The ages obtained from the Bishop Conglomerate suggest that it is time-equivalent to the Starr Flat Member of the Duchesne River Formation. Because these two units are also lithologically similar, the Starr Flat Member of the Duchesne River Formation should perhaps be eliminated and all of these sediments placed under the formal name of the Bishop Conglomerate.