Detrital Zircon U-Pb geochronology of the Ordovician Lander Sandstone, Bighorn

Abstract

The Ordovician Lander Sandstone, which occurs unconformably above the Cambrian Gallatin Limestone and beneath the Bighorn Dolomite, occurs in the Bighorn, Powder, and Wind River basins of Wyoming. The Lander ranges from 0-10 m in thickness and consists of texturally and compositional mature, cross bedded quartz arenite. This study uses detrital zircon U-Pb geochronology to elucidate its provenance. Samples were collected from two localities along the eastern flank of the Bighorn Mountains near Buffalo, Wyoming: a roadcut on US 16 just west of the Clear Creek thrust and from along Crazy Woman Canyon Road. The results showed a statistical similarity between the two samples, and that zircon ages are predominantly Proterozoic in age (~75%) while the minority ages were Archean (25%). Probability density plots of the two-source areas show that the peak ages for Crazy Woman Canyon (n=90) are ~1840, 2075 and 2695 Ma and the US 16 peak ages (n=141) are ~1825, 2075, and 2725 Ma. The detrital zircon age spectra for these samples indicate that the Lander was not derived from local Archean basement and was not recycled from the underlying Cambrian. The Lander has a provenance in either the Trans-Hudson Province and adjacent rocks in present day Saskatchewan and Manitoba more than 1000 km to the north or from the Peace River Arch, an early Paleozoic highlands in northwestern Alberta and northeastern British Columbia. The Lander zircons have a similar provenance to eolian zircons in the Bighorn Dolomite and to other Ordovician sandstones on the Cordilleran Continental margin and central Idaho. The Lander provenance is distinct from the Ordovician St. Peter Sandstone, which occurs extensively east of the Transcontinental Arch. We interpret that the Lander was derived on the late Ordovician shoreline, and then transported via prevailing winds across the Laurentian shelf from east to west during sea level low stand, and then distributed throughout the shelf by currents.