Middle Jurassic landscape evolution of southwest Laurentia using detrital zircon geochronology

Abstract

The Middle Jurassic Wanakah Formation in western Colorado is a poorly understood unit in terms of depositional environment and absolute age of deposition; however, a refined interpretation of the provenance has important implications for understanding the landscape evolution of southwestern Laurentia during Mesozoic rifting of the supercontinent Pangea and the opening of the Gulf of Mexico. This study presents the first U/Pb age dating of detrital zircons from the Middle to Late Jurassic Entrada Sandstone, Wanakah Formation, and Tidwell and Salt Wash Members of the Morrison Formation. Detrital zircon geochronology results show a marked increase in ca. 523 Ma grains (compared to most Mesozoic sediments on the Colorado Plateau) that begins abruptly in the Wanakah Formation and continues into the basal Marker Bed A of the Tidwell Member of the Morrison Formation. U/Pb ages and petrography suggest that the Wanakah Formation was sourced, in large part, from the McClure Mountain syenite on the southwestern flank of the Ancestral Front Range. This abrupt change in provenance occurred due to stream capture and drainage reorganization that input a large amount of water into the basin and caused a shift in depositional environment from the eolian Entrada Sandstone to the hypersaline lake environments of the Wanakah Formation and the Tidwell Member. Additionally, stratigraphic, petrological, and detrital zircon analyses suggest that the contact between the Wanakah Formation and the Tidwell Member of the Morrison Formation is conformable, and the previously interpreted J-5 unconformity is likely not present in western Colorado. The stream capture and drainage reorganization that created the lake system recorded in the Wanakah Formation and the Tidwell Member likely evolved into the major fluvial system that deposited the Salt Wash Member of the Morrison Formation. The evolution of paleodrainages and provenance are important to understand because they help to constrain landscape evolution across southwestern Laurentia, and these insights can help to illuminate the influence of tectonic and sediment controls on depositional environment.