EVIDENCE FOR A SINGLE CRYOGENIAN RIFT-DRIFT TRANSITION FROM THE TROUT CREEK SEQUENCE AND MCCOY CREEK GROUP OF NEVADA AND UTAH

Reported stratigraphic thicknesses from the Neoproterozoic Trout Creek Sequence and McCoy Creek Group of Nevada and Utah have been incorporated into tectonic subsidence models that have been interpreted to depict a failed Cryogenian rift on the western Laurentian margin, followed by a second rift-drift transition in the Ediacaran-Cambrian transition in order to accommodate thick Cambrian successions associated with the Sauk transgressive sequence. Here, we present a new stratigraphic framework, depositional model, and age model for the Trout Creek Sequence and McCoy Creek Group of northeast Nevada and western Utah, and demonstrate that previous workers dramatically underestimated the thickness of the Ediacaran McCoy Creek Group. We combine geological mapping, stratigraphy, geochemistry, and U-Pb zircon geochronology to illustrate that these strata represent some of the most distal Neoproterozoic marine deposits preserved on the western margin of Laurentia. A tectonic subsidence model, utilizing our updated stratigraphic framework and new age model, supports a single ca. 656 Ma Cryogenian rift-drift transition, without requiring an Ediacaran-Cambrian reactivation to accommodate the Sauk transgressive sequence on the western Laurentian margin. Detrital zircon age spectra through the early Ediacaran McCoy Creek Group display limited up-section variability, consistent with the development and long-term stability of an Ediacaran western Laurentian passive margin and the progressive erosion and redeposition of the Proterozoic sedimentary cover of North America. The dearth of Grenville-age zircon in the uppermost McCoy Creek Group and appearance of jasper clasts derived from Proterozoic units in the Midcontinent, along with detrital mica and carbonate that are the products of first-generation weathering of crystalline basement, are interpreted to be associated with the ca. 570 Ma emergence of the Transcontinental Arch.