Neoproterozoic to early Paleozoic tectono-stratigraphic framework for central Idaho: Windermere Supergroup in the northern sector of the U.S. Cordillera

Abstract

ABSTRACT The Windermere Supergroup in southern British Columbia and its correlatives (such as the Pocatello Formation and lower Brigham Group in southeastern Idaho) along the western North American Cordilleran margin are an archetype of Neoproterozoic to early Paleozoic tectonic, sedimentary, and climatic processes. The central Idaho portion of the margin remains relatively understudied when compared to regions to the south in southeastern Idaho or to the north in northeastern Washington. This is in part a legacy of early workers, who identified the absence of Neoproterozoic and Cambrian strata in east-central Idaho across the Lemhi arch. However, Neoproterozoic and Cambrian rocks are indeed present west of the Lemhi arch within the central Idaho section of the Cordillera. Here, we summarize recent advances in our understanding of these strata within central Idaho and correlate the Pocatello Formation and Brigham Group rocks from northern Utah/southeastern Idaho through central Idaho to northeastern Washington. We also provide new constraints that link Cambrian strata from central Idaho across the Lemhi arch to southwestern Montana. Collectively, this emerging tectono-stratigraphic framework suggests extensive, some likely diachronous, stratigraphic boundaries and magmatic events relating to (1) widespread rifting ca. 720–680 Ma; (2) early and late Cryogenian (Sturtian and Marinoan) glacial sedimentation; (3) base-level drawdown and formation of incised valleys, previously correlated to the Marinoan glacial interval, but which now appear to be younger (ca. 600 Ma) and perhaps related to tectonic activity; (4) onset of the Sauk I transgression 560–530(?) Ma; (5) the ca. 515 Ma Sauk II lowstand, perhaps related to final rifting in southern Laurentia; and (6) the Sauk III lowstand coeval with exhumation of 500–490 Ma Beaverhead plutons within the Lemhi arch. Magmatism occurred ca. 680 Ma, 660 Ma, 600 Ma, and 500 Ma, providing age ties. These observations suggest that Neoproterozoic and lower Paleozoic strata in the central Idaho sector of the North American Cordillera record similar processes and sedimentation as strata elsewhere along the margin.