INITIAL PROVENANCE ANALYSIS OF THE LATE JURASSIC LACUSTRINE WANAKAH FORMATION, EASTERN UTAH AND WESTERN COLORADO

The Jurassic Wanakah Formation is exposed in western Colorado and eastern Utah and is interpreted as deposited in a hypersaline lacustrine environment. This unit overlies the Entrada Formation in these areas working as a seal to the eolian reservoir. Stratigraphically the Wanakah Formation underlies the Tidwell Member of the Morrison Formation and is thought to be broadly coeval with the Summerville Formation which holds the same stratigraphic position in southern Utah. The J-5 Unconformity is observed as an angular unconformity between the Summerville Formation and the overlying Tidwell Member of the Morrison Formation in the San Rafael Swell. Although there is no obvious angular unconformity between the Wanakah Formation and the Tidwell Member in eastern Utah and western Colorado, the J-5 unconformity is typically placed between these two units. This study investigates the hypothesis that the J-5 unconformity should be placed between the Entrada Sandstone and the Wanakah Formation in western Colorado to produce a more refined basin reconstruction of this petroleum system.

Samples (n=6) from the Entrada Formation, Wanakah Formation and the Tidwell Member were collected from Escalante Canyon in western Colorado for detrital zircon provenance analysis. U/Pb dating of the Entrada Formation grains (n=190) showed significant peaks in the typical North American Jurassic distributions for the Colorado Plateau with an atypically large peak at ~520Ma. In contrast the Wanakah Formation grains (n=207) present a nearly unimodal distribution at ~520Ma with very little in the traditional Colorado Plateau grains. The Tidwell Member grains (n=197) return to the wider age distribution seen in many Jurassic rocks of the Colorado Plateau. Detrital zircon analysis from the Entrada Sandstone, Wanakah Formation and the Tidwell Member suggest a shift in provenance between the Entrada Formation and the Wanakah Formation and a major shift in provenance from the Wanakah Formation to the overlying Tidwell Member of the Morrison Formation. Results suggest the J-5 unconformity is between the Wanakah Formation and the Tidwell Formation; however, an unconformity is also documented between the Entrada Sandstone and the Wanakah Formation. Understanding this shift in provenance will help to define this late Jurassic depositional basin.