RODINIA, RIFTING, AND THE RAMSHORN SLATE: GEOLOGIC MAPPING OF THE BAYHORSE AREA, CENTRAL IDAHO REASSIGNS THE AGE OF POORLY UNDERSTOOD STRATIGRAPHY FROM ORDOVICIAN TO NEOPROTEROZOIC

Recent geologic field mapping focused on the structurally complex Bayhorse area, west of Challis in central Idaho includes a slightly metamorphosed yet intact ~2.4 km thick stratigraphic section consisting of a lower argillaceous and dolomitic sequence that is overlain by quartzite. Previous mapping designated the rocks as late Cambrian to Middle Ordovician in age. However, the ages were partly based on a miscorrelation to Ordovician rocks to the south. A borehole drilled in the northeastern corner of the field area was initially interpreted to intersect the same stratigraphy exposed immediately to the west in a north-south trending anticline. The hole ended in a ~664 Ma subaqueous volcanic tuff. Our field mapping has documented continuity in surface geology adjacent to and within strata where the borehole was drilled, precluding significant fault offset between the borehole and the exposed strata previously thought to be Ordovician in age. Instead of previously interpreted relations that involve major thrust faults, our mapping also documents that stratigraphically higher quartzites lie above shales with an approximately 200 m gradational stratigraphic contact that consists of upward-coarsening shale and siltstone to interlayed quartzite, shaly quartzite, and finally predominantly quartzite. Additionally, U-Pb detrital zircon data from thick siliciclastic units at stratigraphically higher levels in the western portion of the mapping area are dominated by Grenville-age zircons (1.0-1.3 Ga) near the base, and a strong 1780 Ma age-peak in the top. These DZ signatures are consistent with relations in the Ediacaran to middle Cambrian Pocatello Formation and overlying Brigham Group in southeastern Idaho and would be anomalous in a post-late Cambrian Cordilleran unit. These preliminary interpretations make the upper quartzite (the Clayton Mine Quartzite) the uppermost unit in a relatively intact stratigraphic section with the 664 Ma tuff at the base. The identification of Cryogenian to pre-late Cambrian strata at Bayhorse is a new finding and suggests the Rodinian rift margin has a continuous northwest trend from northwest of Pocatello to Edwardsburg in central Idaho. Correlations from the Bayhorse assemblage to the Pocatello Formation and Brigham Group in SE Idaho will be tested by additional geochronology.