THE REAL MCCOY: NEW CONSTRAINTS ON THE TECTONIC EVOLUTION OF THE LATE NEOPROTEROZOIC WESTERN LAURENTIAN MARGIN FROM THE MCCOY CREEK GROUP OF NORTHERN NEVADA

The rifting of the supercontinent Rodinia, resulting in the birth of the western margin of the Laurentian craton, is recorded by thick Neoproterozoic successions of the Windermere Supergroup and equivalent strata preserved throughout the North American Cordillera. Integration of these stratigraphic archives has been limited by uncertainty in correlations between shelf strata in California and deeper-water successions in Nevada, resulting in multiple proposed models for the age and duration of rifting along the nascent western Laurentian margin. Here, we develop a new age and facies model for the outer-slope-to-basinal strata of the Ediacaran McCoy Creek Group of northern Nevada in order to constrain the timing and tempo of tectonic subsidence along this portion of the margin. The basal McCoy Creek Group hosts a diamictite-cap carbonate couplet associated with the Marinoan snowball earth glaciation, and is superseded by nearly three kilometers of deep-water siliciclastic facies. Cross-stratified arenitic quartzites within this interval, previously interpreted to be fluviodeltaic, are likely associated with the delivery of coarse material to deeper basinal environments via transport through incised submarine canyons. At the top of the McCoy Creek Group, the Osceola Formation hosts carbonate δ¹³C values consistent with the deeply negative δ¹³C profile of the Shuram carbon isotope anomaly, chemostratigraphically constraining the age of this interval to c.a. 575-565 Ma. A subsidence model, informed by these new age constraints and stratigraphic observations, identifies a rift-drift transition along the Cryogenian western Laurentian margin without the need for a second Ediacaran-Cambrian rift. Additionally, contrary to hypotheses that invoke subglacial erosion during the Cryogenian snowball earth glaciations as a primary driver of the Great Unconformity (and which predict the rapid infilling of pericontinental accommodation space in the wake of the Marinoan glaciation), the McCoy Creek Group records the gradual Ediacaran-Cambrian infilling of a sediment-starved basin developed along a Cryogenian rifted margin.