PROVENANCE ANALYSIS OF THE PASS PEAK, PINYON, AND HAREBELL FORMATIONS USING U/PB DETRITAL ZIRCON GEOCHRONOLOGY

The preponderance of synorogenic quartzite cobble conglomerates where Sevier-Laramide structures converged in northwest Wyoming is striking, as is the thickness (1-4 km) of some units, and their presence on mountain tops. There is no obvious source area for any of these quartzite clasts in the Harebell, Pinyon, or Pass Peak Formation, although paleocurrents all suggest a source from the now-eroded ancestral Targhee uplift to the northwest. The goal of this research is to determine the provenance of these units using U/Pb detrital zircon geochronology. Eight samples (#602 total zircons) of conglomerate matrix from the Upper Pinyon (2 localities, #139), Lower Pinyon (2 localities, #176) Harebell (3 localities, #242), and Pass Peak (1 locality, #45) Formations were collected in Sublette and Teton counties, WY. The Pass Peak, Harebell, and Lower Pinyon formations were dominated by Mesozoic (13-68%) and Yavapai-Mazatzal (38-56%) zircons. Each of these zircon populations indicate a westerly source, with primary sediment being derived from various parts Idaho Batholith and older zircons being recycled from Neoproterozoic quartzites of the Ancestral Targhee uplift. Archean zircons are present in each sample as well (7-20%), but the paucity of such zircons indicates that the Laramide basement-cored uplifts were not as yet significant source areas. The Upper Pinyon samples were dominated by Eocene (20-37%) and Archean (24-46%). These zircons are interpreted to be derived largely from the north from the Absaroka volcanic succession and the Laramide Beartooth basement-cored uplift, respectively. Mesozoic (8-10%) and Yavapai-Mazatzal (15-16%) zircon populations are still present, which could indicate some sediment supply from the west, or alternatively could have been recycled from older conglomeratic units. The presence of significant Eocene zircons in the upper Pinyon Formation makes the Pinyon Formation younger than previously thought, and Pinyon deposition continued, at least locally, during the early stages of Absaroka volcanism.