DETRITAL ZIRCON GEOCHRONOLOGY OF LATE ORDOVICIAN SANDSTONE IN WYOMING SUGGEST TALSON-THELON OROGEN WAS A MAJOR SOURCE OF SILICICLASTIC SEDIMENT ALONG THE TRANSCONTINENTAL ARCH

The Middle to Late Ordovician Eureka Quartzite (and its equivalents) are a unique orthoquartzite deposited along the otherwise carbonate-dominated Lower Paleozoic passive margin of the western North American Cordillera. Previous detrital zircon provenance studies indicate the sole source of sediment is the Peace River Arch, British Columbia (Gehrels et al., 1995; Gehrels and Dickinson, 1998; Gehrels, 2000). These detrital zircon populations are unique in the western Cordillera because they commonly are dominated by a large group of 1.8-2.0 Ga grains with lesser peaks from 2.2-2.8 Ga and Grenville age grains (1.0-1.2 Ga) are uncommon. According to this long-distance transport model siliciclastic sediment was transported over 2000 km by southerly long-shore currents along the passive margin. The long-distance transport model implies that potentially exposed basement source areas on the Transcontinental Arch east of the passive margin, as well as recycling of underlying Mesoproterozoic to Lower Palezoic siliciclastic rocks supplied little or no detritus to the passive margin even though these potential sources were positioned near the equator during the Middle to Late Ordovician and likely subjected to intense equatorial weathering.

New U-Pb geochronologic analyses of two Middle to Late Ordovician Winnipeg Group quartzite and carbonate-cemented sandstone in the Williston Basin, WY (equivalent in part to the upper Eureka Quartzite) detrital zircons by the LA-ICPMS provide a test for the single-source long-distance transport model. These quartzose units in Wyoming were deposited directly on Early Ordovician sedimentary rocks. The detrital zircon population for these sandstones are nearly identical to populations reported for the Eureka Quartzite with an abundance of 1.8-2.0 Ga grains, a few 2.2-3.0 Ga grains and no Grenville age grains. The similarity of these zircon populations from widely disparate locations suggests they had a common sediment source. The most abundant local source of 1.8-2.0 Ga zircons is the Talston-Thelon Orogen that was likely exposed along the Transcontinental Arch during the Late Ordovician.