U-PB GEOCHRONOLOGY OF THE BIGHORN DOLOMITE, WYOMING, USA: IMPLICATIONS FOR ORDOVICIAN WIND PATTERNS AND STRENGTH (Invited Presentation)

This study utilizes detrital zircon U-Pb geochronology from shallow-water carbonates of the Bighorn Dolomite in Wyoming, USA to provide robust evidence for long-distance eolian sediment transport during the Ordovician. The Bighorn Dolomite was deposited in a shallow water carbonate platform that developed approximately 10 degrees south of the Ordovician paleoequator on the western edge of Laurentia. Zircon grains were extracted from the basal Bighorn dolomite by traditional means of crushing, panning, heavy liquids and a Franz magnetic separator. U-Pb analysis was completed at the University of Arizona Lasercron Center. The zircon grains analyzed (n=57) range in size from 100-150 microns, are well rounded and have frosted surface textures. Thirty-nine zircons yield Trans-Hudson orogen ages, and range from 1804-1973 Ma. Two peak ages are evident in the data: 1836 and 1921 Ma. Nine zircons yield Paleoproterozoic ages between 2030-2090 Ma. The oldest fraction is dominated by very early Paleoproterozoic and Archean grains, and range in age from 2345-3017 Ma. Two grains yield Grenville (1051-1121 Ma) ages. The ages and textures of detrital zircons extracted from the Bighorn indicate that the zircon grains were transported by winds through saltation and suspension from the paleo east where rocks of the Proterozoic Trans-Hudson orogenic belt were exposed in present day Manitoba and Saskatchewan. Our interpretation of long-distance eolian transport is consistent with the paleogeography of Laurentia and expected prevailing wind directions, and draws on modern analogues where Saharan sediment is transported by trade winds for distances of more than 500 km.