PREVIOUSLY UNRECOGNIZED MESOPROTEROZOIC STRATA IN SOUTH-CENTRAL IDAHO DOCUMENT LATE-STAGE RIFTING OF THE NUNA SUPERCONTINENT IN WESTERN LAURENTIA

The North American Intermountain West preserves the Belt Supergroup, one of the best records of Mesoproterozoic strata on Earth; however, debate remains about first-order interpretations of basin formation in the region. We investigated a recently identified package of Mesoproterozoic strata at Leaton Gulch near Challis, Idaho, using a combination of traditional and newer sedimentological tools including measuring and characterizing sedimentary lithofacies and lithofacies associations, sandstone petrography, as well as detrital zircon U-Pb geochronology, Lu-Hf, and trace element geochemical analysis. Results suggest the Leaton Gulch stratigraphic section was deposited in a fluvial setting <1400–1300 Ma, spanning the poorly documented interval between cessation of the Belt Supergroup at ~1370 Ma and recently characterized Deer Trail Group strata at <1300 Ma. Detrital zircon age distributions from Leaton Gulch demonstrate a similar sediment provenance signature to Missoula Group rocks of the upper Belt Supergroup based on detrital zircon U-Pb data; however, Leaton Gulch strata are up to ~70 Ma younger and could be recycled from proximal Belt Supergroup sources. Though <1370 Ma ages reported by recent work suggest that upper Missoula Group strata may be younger than previously thought, Leaton Gulch strata are still at minimum 15–30 My younger than the Missoula Group. Regional porcellanites (interpreted as metamorphosed reworked tuffs) found within Leaton Gulch and surrounding Missoula Group strata show dominantly radiogenic εHf_(t), with a range of -8 to +15, interpreted as a mix of primary mantle and remelted metasedimentary sources. Zircon trace element data of the porcellanite from Leaton Gulch suggests a melt source that was high in silica and of consistent chemistry from 1450–1300 Ma. Collectively, we interpret that the strata of Leaton Gulch record basin formation during continental extension that was coincident with breakup of the Nuna Supercontinent, spanning a critical window of previously unrecognized time between deposition of the Belt Supergroup and the Deer Trail Group.