FACIES AND PROVENANCE CHANGES IN THE CAP-CARBONATE INTERVAL (CA. 685-<665 MA) OF THE NEOPROTEROZOIC POCATELLO FORMATION, SCOUT MOUNTAIN AREA, SOUTHEAST IDAHO

A newly analyzed diamictite-cap carbonate interval in the Neoproterozoic Pocatello Formation exposed on Scout Mountain, Idaho, records ca. 685-665 Ma syn- to post-glacial deposition and exhibits changes in facies and provenance as compared to correlative units northward. The massive diamictite lithofacies consists of poorly sorted quartzitic and granitic clasts up to 20 cm embedded in a sandy matrix. The diamictite is overlain by a distinctive 12.3 m-thick, upward fining, stratified unit of moderately sorted quartz arenite and laminated green argillite. The contact with the overlying cap carbonate is gradational and marked by cm-scale interlayered green siltite and dolostone, with the siltstite beds becoming thinner and less abundant upwards. The cap-carbonate unit (> 2.0 m thick) comprises thinly laminated, pink to tan dolostone with thin silty interbeds. Above the exposed cap carbonate unit is a covered interval (27 m) overlain by >7.5 m of m-scale, fine-grained feldspathic sandstone–argillte fining upward cycles with rare dolomite interbeds. The sandstone exhibits asymmetric ripples and soft sediment deformation. U-Pb geochronology of detrital zircons from the quartz arenite reveal three main peaks: ca 685 Ma grains related to syn-rift volcanic activity that constrain maximum depositional age (n=13 out of 100 grains), Mesoproterozoic grains (1.0 to 1.4 Ga) that may be recycled from older Neoproterozoic strata, and 2.45-2.7 Ga grains sourced from basement rocks and/or reworked from diamictite. The wide variety of detrital-zircon grain ages and sources is interpreted to record rapid influx of siliciclastic sediment during deglaciation, which slightly preceded to overlapped with rapid deposition of the dolostone. Dolostone δ¹³C values range from -4.0 to -4.9 per mil PDB, similar to other cap carbonate units locally and globally. The facies present at this locale show similarities as well as differences with other cap carbonate exposures in SE Idaho, particularly the sub-cap quartzite-argillite interval that is not present 20 km to the north at the other main exposures of this interval (Portneuf Gap). Varying thickness and facies changes are interpreted to reflect a dynamic depositional environment and rapid influx of sediment from multiple sources during glacial retreat.