Recently Identified Mesoproterozoic Strata in South‐Central Idaho Document Late‐Stage Rifting of the Nuna Supercontinent in Western Laurentia.

Sedimentary basins are valuable archives of tectonic processes involved in continental rifting. The northern Rocky Mountains preserve the Belt Supergroup, one of the most complete records of Mesoproterozoic strata on Earth; however, debate remains about its tectonic origin. We investigated a recently identified package of Mesoproterozoic strata at Leaton Gulch near Challis, Idaho, using a combination of traditional and newer sedimentological tools. Results suggest that the Leaton Gulch stratigraphic section was deposited in a fluvial setting ca. 1,380–1,317 Ma, spanning the poorly documented interval between late Belt Supergroup deposition at ∼1,370 Ma and recently characterized Deer Trail Group strata that are less than 1,300 Ma. Detrital zircon age distributions from Leaton Gulch demonstrate a similar provenance signature to Missoula Group rocks of the upper Belt Supergroup; however, Leaton Gulch strata are up to ∼70 Ma younger than most prior age constraints on Belt Supergroup rocks. Regional metabentonites (interpreted as metamorphosed reworked tuffs) found within Leaton Gulch and 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 metabentonite from Leaton Gulch suggest a 1,450–1,300 Ma geochemically consistent and moderate–high silica melt source. Collectively, the strata of Leaton Gulch record basin sedimentation during a critical window of Mesoproterozoic time. We speculate that sedimentation during late‐stage Belt Supergroup deposition thickened and stepped westward, abandoning the main Belt basin, culminating with breakup of the Nuna Supercontinent. Plain Language Summary: Sedimentary basins serve as important records of continental breakup. One of the most complete basin records in North America is the 1470–1370 million year old Belt Supergroup, but its tectonic origin is still poorly understood. We investigated recently identified Mesoproterozoic sedimentary rocks near Challis, Idaho. By measuring the sedimentary rocks and determining the age and chemistry of detrital (transported as ancient sediments) zircons within them, we conclude that the rocks at Leaton Gulch were deposited in a river setting between 1380 and 1317 million years ago. Leaton Gulch rocks therefore span a poorly documented interval between the Belt Supergroup and younger (∼1,300 Ma) Mesoproterozoic rocks of the Deer Trail Group preserved in eastern Washington. Detrital zircons from Leaton Gulch rocks record a sediment provenance similar to that of upper Belt Supergroup rocks. Regionally distributed metabentonites (ancient volcanic ash), found both at Leaton Gulch and in Belt Supergroup rocks, have chemical signatures consistent with continental rifting. Leaton Gulch rocks therefore record the sedimentation and rifting after deposition of the Belt Supergroup predicted by paleogeographic reconstructions. Late‐stage Belt Supergroup rocks thickened and stepped westward through time, ultimately abandoning the main Belt basin, culminating with the breakup of one of Earth's earliest supercontinents, Nuna. Key Points: Fluvial deposition occurred <1,380–1,317 Ma between Belt Supergroup deposition and late‐stage breakup of the Nuna supercontinentLeaton Gulch strata record continued sediment sourcing from the Belt Supergroup or local recycling of Belt Supergroup sedimentary rocksTiming of deposition and detrital zircon results are consistent with westward stepping of rifting after deposition of the Belt Supergroup [ABSTRACT FROM AUTHOR]