GEOLOGIC MAP OF THE BAYHORSE ANTICLINE, CUSTER COUNTY, IDAHO: WINDERMERE SUPERGROUP WEST OF THE LEMHI ARCH

Correlative Neoproterozoic and Lower Cambrian strata are recognized along almost the entirety of the western Laurentian margin from Sonora, Mexico to Yukon, Canada. However, along the inferred continuation of the Laurentian margin, northwest of the Snake River Plain in east-central Idaho, Neoproterozoic and Cambrian rocks are missing across a feature known as the Lemhi arch. Approximately 50 km southwest of this recognized Lemhi arch unconformity, within the central Idaho fold-thrust belt, our geologic mapping of the Bayhorse Anticline has identified a slightly metamorphosed yet intact, ~1.5 km thick stratigraphic section of argillaceous and dolomitic rocks, which overlies a ~668 Ma tuff. This 1.5 km-thick section is overlain, in gradational contact, by a ~1 km thick quartzite unit. The entirety of this ~2.5 km thick section was previously thought to be Ordovician in age. Gabbroic sills preliminarily dated here (in situ U–Pb SIMS on baddeleyite) at 608 ± 33 Ma, intrude the gradational contact, constraining the depositional age of the lower ~1.5 km of strata to the Cryogenian and Ediacaran. Additional U-Pb detrital zircon data indicates primarily 1200-1000 Ma age-populations in the lower argillaceous and dolomitic strata. In the overlying ~1 km thick quartzite unit, a distinct shift in detrital zircon populations, from dominantly 1200-1000 Ma age-populations to dominantly Paleoproterozoic-age populations (~1780 Ma age-peak), consistent with other Ediacaran to Middle Cambrian quartzites along the western Laurentian margin, south of British Columbia, is recognized. This ~2.5 km thick section is deformed within a presumably Cretaceous, east-verging, open anticline, with smaller amplitude parasitic folds. Stereonet analysis indicates that pervasive pressure solution cleavage in the lower argillaceous strata was folded around the same axis as the bedding, suggesting that layer-parallel shortening occurred prior to significant folding, likely during emplacement of the overriding Copper Basin thrust sheet, ~15 km to the west. A series of N-S to NE-SW striking normal faults crosscut these folds and are presumed to be Eocene in age.