DETRITAL ZIRCON STUDIES IN RELATION TO PROVENANCE OF THE JURASSIC WANAKAH FORMATION AND ADJACENT ROCK UNITS IN WESTERN COLORADO

The Jurassic Wanakah Formation is a regionally important, well-exposed, but poorly studied mudrock that extends throughout western Colorado, Utah and New Mexico. Stratigraphically, the Wanakah Formation and its equivalent – the Summerville Formation, are placed between the Middle Jurassic Entrada Sandstone and the Tidwell Member of the Upper Jurassic Morrison Formation in western Colorado and southeastern Utah, respectively, in the Paradox and Piceance Basins. The Wanakah Formation is reported to be bounded on the top by a regional unconformity; the J-5 Unconformity. The environment of deposition, provenance, and absolute age of the Wanakah Formation are not yet determined substantively; additionally, the stratigraphic placement of the J-5 unconformity is currently a subject of debate. The J-5 is an angular unconformity between the Tidwell Member and Summerville Formation during the Oxfordian stage of deposition in the northern San Rafael Swell and it is placed typically between the Tidwell Member and Wanakah Formation in western Colorado. Preliminary studies including section measurements of outcrops in study areas suggest a conformable contact between the Tidwell Member and Wanakah Formation in western Colorado. Additionally, petrologic data infer that the regional scour surfaces between the Entrada Sandstone and Wanakah Formation might represent the J-5 unconformity. U-Pb ages for 100 detrital zircon grains per representative sample from different sections of Wanakah Formation and Tidwell Member show correlation of age peaks in the range of 525 - 515 Ma, suggesting similarity in provenance and a conformable unroofing sequence that slowly wanes into the Tidwell Member. Available data from literature posits that these Cambrian age grains were derived from varied sources; grains eroded from the uplifted basement rocks of the Ancestral Rocky Mountains province, and reworked Triassic sands eroded from the Amarillo-Wichita uplift.