THE UPPER MIOCENE MOONSTONE FORMATION OF CENTRAL WYOMING: LINKING VERTEBRATE BIOSTRATIGRAPHY AND U-PB GEOCHRONOLOGY WITH POST-LARAMIDE TECTONISM

Miocene strata are relatively scarce in central Wyoming and poorly dated. The upper Miocene Moonstone Formation provides an opportunity in which mammalian biochronology and U-Pb geochronology can be used to date post-Laramide tectonism in the region. The crest of the Sweetwater arch, formed during the Laramide orogeny, began to subside as an east- to west-oriented syncline early in the Miocene, during deposition of the fluvial/eolian Split Rock Formation. Downwarp continued into the mid and late Miocene and allowed saline lakes to form among the sandy, generally fine-grained sediments, soils, and air-fall tuffs that became the Moonstone Formation. These strata yield the diagnostic late Miocene mammals Plionictis ogygia, Copemys pisinnus, Phelosaccomys hibbardi, Pronotolagus albus, and Goniodontomys disjunctus, indicating late Barstovian to early Hemphillian ages. The biostratigraphic ages of the faunas are supported by two U-Pb zircon radiometric dates of 11.30 ± 0.48 and 8.39 ± 0.24 Ma from interbedded ashes. After early Hemphillian time (ca. 6 Ma), conglomerates were deposited above the Moonstone Formation, derived from nearby mountains and heralding onset of normal faulting. The Granite Mountains graben, within the core of the much larger Sweetwater arch, formed a block 160 km long that dropped 400–600 m adjacent to original southern margins of the Sweetwater arch. Differential downdrop of the Granite Mountains graben led to tectonic origin of the modern Seminoe and Ferris mountains to the south. Other evidence that normal faulting post-dated ca. 6 Ma involves the presence of the Black Shale Member (BSM) in the Moonstone Formation at levels that predate input of conglomerates from nearby mountains. The BSM contains pristine but detrital pollen and megaspores transported northward along a shallow stream gradient in the late Clarendonian. These grains probably were reworked from Upper Cretaceous strata of the northern Hanna Basin, suggesting that the intervening Seminoe Mountains remained covered at the time of transport. Stratigraphy, mammalian fossils and U-Pb geochronology have established that graben development along with topographic origin and erosion of the Seminoe Mountains postdates the early Hemphillian.