DETRITAL ZIRCON PROVENANCE OF THE EOCENE-OLIGOCENE BRIAN HEAD FORMATION, MARYSVALE VOLCANIC PROVINCE, UTAH, USA

The Eocene-Oligocene Brian Head Formation is the basal volcanic unit throughout much of the southern Marysvale volcanic field and was involved in both the massive Markagunt, Sevier and Black Mountain gravity slides, where the Brian Head Formation is primarily found near the top of the autochthonous lower plate. The Brian Head Formation is as thick as 300 m and consists of light-colored volcaniclastic sandstone, conglomerate, breccia, siltstone, mudstone, and minor airfall tuff. These rocks were deposited in a slowly aggrading fluvial to lacustrine depositional environment south of known volcanic source areas of this age in central Utah. Here we sampled three sandstone units near the top of the formation near Haycock Mountain and along the southern and eastern flanks of the Sevier Plateau. The sandstones are compositionally and texturally immature and classified as lithic and arkosic wackes. Each sample was taken from autochthonous samples that rest beneath the Markagunt or Sevier gravity slide and thus provide important information of the regional setting prior to gravity slide emplacement. Zircons were separated by traditional gravitational and magnetic techniques, and U-Pb geochronological data was obtained by ICP-MS at the University of Arizona Laserchron Center. A total of 376 zircons were analyzed. Most zircons were pale, translucent, and euhedral. The detrital zircon age spectra are statistically indistinguishable. Each sample contained >80% Paleogene zircons and had prominent age peaks of ~34.5 Ma. The maximum depositional age is ~34 Ma. Older zircons range from Mesozoic to Archean in age. Thus, the upper Brian Head Formation has a consistent zircon provenance that reflects mostly local, primary plutonic and volcanic zircons derived from central Utah eruptive centers as well as zircons recycled from the underlying Mesozoic and Paleozoic strata.