The Growth and Collapse of a Volcanic Field: Detrital Zircon U/Pb Geochronology and Provenance of the Oligocene Bear Valley Formation, Southwest Utah, USA

The Bear Valley Formation (Fm.) is a distinctive eolian sandstone interbedded with thick volcanic rocks of the Marysvale volcanic field of southwest Utah, the southern part of which failed during eruptive activity along three mega-scale gravity slides. The formation is as thick as 300 m and extends over an area of >2,500 km 2 in the Black Mountains and Markagunt Plateau. The Bear Valley Fm. is composed of tuffaceous sandstone interbedded with tuff, conglomerate, and polymict volcanic mudflow breccias. The sandstone beds are lithic arenite and lithic wacke that occur as massive beds with large-scale cross bedding. The Bear Valley Fm. occurs in the upper plate of the Markagunt gravity slide and is in both the upper and lower plates of the Black Mountains gravity slide. We used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to acquire U/Pb dates of detrital zircons (N = 3, n = 346) from the autochthonous Bear Valley Fm. at Kane Spring and Jako Wash in the Black Mountains and the allochthonous Bear Valley at Sandy Wash in the central Markagunt Plateau. All samples are dominated by Oligocene zircons with maximum likelihood ages for deposition ranging from 23.6 to 24.0 Ma. The western-most sample from Jako Wash also preserves a slightly older group of zircons, indicating derivation from either the underlying Wah Wah Springs Fm. or another unit erupted from the Indian Peak caldera complex to the west. Thus, the upper Bear Valley Fm. was deposited within ~400 kyr before the emplacement of the Markagunt gravity slide at 23 Ma, reflecting accelerated uplift of the northern Marysvale complex that ultimately resulted in collapse and slide emplacement.