A NEWLY IDENTIFIED NEOPROTEROZOIC AND CAMBRIAN RIFT-RELATED SEDIMENTARY SUCCESSION IN THE BAYHORSE REGION OF CENTRAL IDAHO: LATERAL AND TRANSVERSE VARIATIONS IN THE LAURENTIAN MARGIN

Geologic mapping and detrital zircon analysis of the structurally complex Bayhorse, Bayhorse Lake, and Clayton Quadrangles in central Idaho has identified a slightly metamorphosed yet intact ~1.5 km thick stratigraphic section of Neoproterozoic argillaceous and dolomitic rocks (Garden Creek Phyllite, Bayhorse Dolomite, and Ramshorn Slate), which overlie in apparent stratigraphic continuum a 664 +/- 6 Ma tuff in a borehole. This sequence can be correlated to the upper Pocatello Formation and lower Brigham Group of southeast Idaho. This fine-grained succession is overlain by a gradational contact with a ~1 km thick coarse siliciclastic unit (Clayton Mine Quartzite) that has significant Grenville-age zircons (1.0-1.3 Ga) near the base, and a strong 1780 Ma age-peak near the top, interpreted to record the rise of the Transcontinental Arch at ~540 Ma. We correlate the Clayton Mine Quartzite with the upper Brigham Group of southeast Idaho and northern Utah, which spans the Neoproterozoic/Cambrian boundary. The Clayton Mine Quartzite is overlain in a likely unconformable contact by Middle Ordovician fossiliferous dolostone and the Kinnikinic Quartzite, suggesting that much of the Upper Cambrian and Lower Ordovician section is missing here. Less than 10 km to the northwest, in Squaw Creek, and across an inferred thrust fault, a similar detrital zircon provenance shift is preserved in a >650 m thick package of sandstone and carbonate that conformably underlie a Middle Cambrian shale. This shale is variably preserved beneath an unconformity and an Ordovician dolostone. We interpret the Squaw Creek strata to be a western and more complete stratigraphic succession. ~50 km northeast of Bayhorse, in the Lemhi Range, Middle Ordovician Kinnikinic Quartzite lies in angular unconformity above Mesoproterozoic Belt Supergroup rocks, forming the Lemhi Arch. Prior work demonstrates that this unconformity is at least partially Late Cambrian in age and possibly contemporaneous with the newly documented unconformity in the Bayhorse region. Overall, these results suggest correlative Cryogenian and Ediacaran sedimentation through central and southeastern Idaho. However, during final rifting, central Idaho experienced a period of enigmatic Cambrian tectonism and exhumation.