AGE AND ORIGIN OF VOLCANIC ASHES IN THE BISHOP CONGLOMERATE, SOUTH FLANK OF THE UINTA MOUNTAINS, UTAH

The Oligocene Bishop Conglomerate is a key unit in understanding the uplift and erosion history of the Uinta Mountains. Pyroclastic ash fall deposits within the formation have been analyzed in order to better constrain its age and to identify the sources for the ash. To accomplish this we have studied their chemical composition, mineralogy and age and then compared them with likely correlative ash-flow and other ash fall deposits in the western United States. Sample DJ-1 from a dacitic ash fall deposit near the top of the formation, contains some detrital K-feldspar, but gives a single-crystal laser-fusion (SCLF) sanidine ⁴⁰Ar/³⁹Ar eruption age of 31.80 ± 0.46 Ma (1σ; Data relative to Fish Canyon Sanidine at 27.84 Ma) and multi-crystal laser-fusion (MCLF) sanidine age of 30.47 ± 0.25 Ma. DJ-1 strongly correlates through similar age, hornblende and biotite compositions, and zircon morphology to the Cottonwood Wash Tuff that erupted at 30.81 ± 0.18 Ma from the Indian Peak caldera complex on the Utah-Nevada border and subsequent correlative ash falls of this eruption. A second ash collected from two localities near the base of the formation, DJ-8 and DJ-9, does not correlate to volcanic rocks from the Indian Peak caldera complex. These rhyolitic samples are older, contain more sanidine, are not hornblende bearing, and have very different zircon morphology compared to DJ-1. DJ-8 and DJ-9 give SCLF sanidine ages of 34.46 ± 0.26 and 33.81 ± 0.32 Ma, respectively, and MCLF sanidine ages of 34.00 ± 0.06 and 33.73 ± 0.11 Ma, respectively. We have so far been unable to identify its source. An additional ash has been collected from the formation and is in the process of being analyzed.