PROVENANCE ANALYSIS OF THE BASAL AMSDEN FORMATION OF WYOMING USING DETRITAL ZIRCON GEOCHRONOLOGY BY LA-ICP-MS

The basal sandstone member of the Pennsylvanian (Morrowan) Amsden Formation in central Wyoming is a widespread terrigenous red and brown quartz arenite that unconformably overlies the Mississippian Madison Limestone. U-Pb analyses on detrital zircons were conducted on two samples from the basal Amsden sandstone for provenance analysis. One sample was collected along Highway 14 west of Dayton, WY and the other at the Tensleep Fish Hatchery on Highway 16 (n=705 total). K-S analyses indicate that the two age spectra are statistically identical. U-Pb ages of about 90% of the zircons match ages indicating an easterly source with a strong Appalachian signature, including Grenville (1.3-1.0 Ga), Pan-African (620-530 Ma), Taconic (490-440 Ma), Acadian (420-350 Ma), and Alleghenian (330-270 Ma) ages. Smaller populations of zircon ages include the Midcontinent Granite-Rhyolite Province (1.5-1.3 Ga), the Yavapai and Mazatzal Provinces (1.8-1.6 Ga), Penokean and Trans-Hudson Orogens (1.9-1.8 Ga) and Superior Province (>2.5 Ga) ages. These detrital zircon age spectra are similar to the age spectra for sandstones of that age in the Illinois and Forest City Basins 1000 km to the east. Local Archean and Mesoproterozoic rocks were not significant sediment producers during Amsden deposition. The pre-Grenville grains present here were likely recycled from lower Paleozoic sandstones in the midcontinent. These data indicate that the basal Amsden sandstones were derived from source areas in the Appalachian Orogen to the east. Fluvial systems, perhaps as long as 2000 km, were present on the North American continental interior prior to the transgression of the Absaroka seas.