DETRITAL ZIRCON U-PB GEOCHRONOLOGY AND PROVENANCE OF THE MIOCENE BEAR VALLEY FORMATION, SOUTH-CENTRAL UTAH, USA

The Bear Valley Formation is as thick as 300 m and extends over an area of >2500 km² in the Black Mountains and Markagunt Plateau, UT. The Bear Valley Formation is composed of tuffaceous sandstone interbedded with tuff, mudstone and polymict volcanic breccias. The sandstone beds are lithic and arkosic arenite that occur as massive beds with large-scale cross bedding interpreted to be of eolian in origin. The Bear Valley is either autochthonous, or it is allochthonous and emplaced as part of the Markagunt and Black Mountain gravity slides. The age of the Bear Valley Formation provides essential constraints to the emplacement age of these gravity slides. We conducted LA-ICPMS analysis of detrital zircon geochronology (s=5, n=686) on the autochthonous upper Bear Valley at Kane Springs in the Black Mountains, the allochthonous Bear Valley at Sandy Creek in the central Markagunt plateau, and in the Jackrabbit Hills. We also sampled the lower Bear Valley (also allochthonous) and a clastic dike in the Jackrabbit Hills. The three samples of upper Bear Valley sandstone are statistically indistinguishable, having 90% Tertiary zircons, unimodal age peaks of ~23.0-23.5 Ma, and an MDA of ~23.0 Ma, which makes the unit Miocene in age. The upper Bear Valley is interpreted to consist of primary zircons derived from volcanic source areas to the west. The lower Bear Valley consists of about 80% Tertiary zircons, has an MDA of 23.7 Ma, and the zircon age spectrum is bimodal, with age peaks of 23.7 and 29.4 Ma. The lower Bear Valley consists of both primary zircons and zircons recycled out of the underlying Wah Wah Springs Formation and is also sourced from the west. The clastic dike has a more complex zircon age spectrum, with more than 90% being Tertiary in age. Two age peaks of ~30.0 and ~31.4 are present. The clastic dike was derived from the underlying Wah Wah Springs and Brian Head formations during gravity slide emplacement.