The Real McCoy: Great Unconformity source-to-sink on the rifted passive margin of Laurentia

The Great Unconformity separates Archean and Proterozoic basement rocks from overlying Paleozoic sedimentary strata. Hypotheses for the formation of the Great Unconformity make predictions that can be tested within a source-to-sink depositional framework: kilometer-scale exhumation from Neoproterozoic to Cambrian tectonics and dynamic topography predicts protracted delivery of sediments to North American marginal basins, whereas exhumation driven by Cryogenian snowball Earth erosion predicts rapid syn-glacial deposition outboard of snowball Earth ice grounding lines. However, testing these hypotheses in North America is complicated by syn-glacial tectonism. Here, we develop a new depositional model, age model, and tectonic framework for the Trout Creek Sequence and McCoy Creek Group of northeast Nevada and western Utah. We combine geological mapping, stratigraphy, geochemistry, and U-Pb zircon geochronology to demonstrate that these strata represent some of the most distal Neoproterozoic marine deposits preserved on the western margin of Laurentia, and that they were deposited outboard of Cryogenian ice grounding lines. Subsidence modeling supports a ca. 656 Ma Cryogenian rift-drift transition, without requiring an Ediacaran-Cambrian reactivation, which was previously invoked to accommodate the Sauk transgressive sequence on the western Laurentian margin. Nonetheless, sedimentation rates during the Cryogenian snowball Earth glaciations were low, providing a negative test of the sub-glacial erosion hypothesis. Apparent deepening across the glacial intervals, and high sedimentation rates in their aftermath, suggest that Laurentian marginal basins were starved during snowball Earth glaciations. 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 a 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. These data are consistent with thermochronological data documenting diachronous erosion and exhumation across Laurentia over hundreds of millions of years along the Great Unconformity.