DETRITAL ZIRCON POPULATIONS IN THE EARLY CRETACEOUS CEDAR MOUNTAIN FORMATION, UTAH AND CLOVERLY FORMATION, WYOMING

LA-ICP-MS U-Pb ages of detrital zircons from the Early Cretaceous Cedar Mountain Formation in Utah and similar age Cloverly Formation in Wyoming show that sediment sources for these formations are consistent over a broad area, but with some interesting differences. Even though we picked only euhedral zircons in an attempt to find the depositional age of these formations, we still encountered a large number of older grains. At all of the localities, Precambrian age peaks of approx. 590 Ma, 1050 Ma, 1205 Ma, 1420 Ma, 1700 Ma, and 2600 Ma occur. The 1200 Ma peak is less pronounced and the 1425 Ma peak is more pronounced in the southernmost sections. The Paleozoic ages are more varied but include significant numbers of Cambrian (~500 Ma) grains in the Utah sections, Ordovician (~450 Ma) grains in the Wyoming sections, and Silurian (~420 Ma) and Devonian (~380-400 Ma) grains in both Utah and Wyoming sections. Mesozoic detrital zircon ages show consistent Jurassic peaks at 145 Ma, 160 Ma, and a broad Triassic peak between 210-240 Ma in all sections. The Cloverly sections have a strong Early Jurassic peak at 195 Ma, which does not appear in the Utah sections, while the Price River II and Mussentuchit localities do not show the Middle Jurassic, 167 Ma peak, found in the other sections. The Precambrian age peaks correlate to prominent episodes of granitic magmatism in western North America: Wyoming Province 2.9-2.6 Ga, Yavapai-Mazatzal 1.8-1.65 Ga, the anorogenic Granite-Rhyolite terrane (1.5-1.3), the Grenville-Llano province 1.3-1.0 Ga, and the Southern Oklahoma aulocogen at 550 to 525 Ma. The early Paleozoic grains may have originated in ash eruptions along the eastern margin of North America that were incorporated into older formations and have now been recycled into these formations. Devonian grains could originate from the Yukon-Tanana terrane, where magmatism of this age occurred—which at the time of ash eruption may have been directly west of this part of cratonic North America. The Triassic peak correlates broadly with the ashy deposits of the Chinle Formation. Triassic magmatic rocks generally peak at about 213 Ma. The 145-167 Ma peaks correlate with the Morrison/Carmel/Twin Creek ash deposits. The Early Jurassic peak at 195 Ma may relate to sparse plutonic rocks in Washington and Nevada.