TESTING PROVENANCE AND REGIONAL CORRELATIONS OF NEWLY RECOGNIZED MESOPROTEROZOIC STRATA IN EAST-CENTRAL IDAHO

Mesoproterozoic strata exposed in east-central Idaho are >10 km thick east of the Salmon River and are considered correlative to the Missoula Group of the upper Belt Supergroup. Age constraints on these rocks are hindered by a lack of fossils, but rare crosscutting and interbedded igneous rocks constrain Missoula Group-equivalent strata to between ~1450 and ~1370 Ma in age. Establishing a more robust upper bound of Mesoproterozoic sedimentation is essential for testing basin dynamics and ultimately the breakup timing of one of Earth’s early supercontinents, Nuna. Recent work demonstrated that the Mesoproterozoic Deer Trail Group, which stratigraphically overlies Belt Supergroup rocks in northeastern Washington, was deposited ~1300 Ma. Deer Trail Group-correlative rocks have not been identified elsewhere in the northern Rockies. North of Leaton Gulch, Idaho, rocks previously mapped as Proterozoic to Ordovician were recently re-mapped and measured. These rocks are lithologically similar to the Missoula Group correlated Lawson Creek Formation. Both the Lawson Creek and the Leaton Gulch units contain green porcellanite beds that we interpret to be reworked tuffs. Detrital zircon (DZ) U-Pb geochronology of a porcellanite at Leaton Gulch reveals a prominent DZ age mode of ~1336 Ma, suggesting deposition younger than the Belt Supergroup. New Lu-Hf zircon data from Leaton Gulch strata suggest a non-radiogenic (juvenile) source. The DZ ages are also bimodal, consistent with sediment sourcing from the ~1650-1750 Ma Yavapai-Mazatzal terranes and ~1450-1400 Ma tuffs within the Missoula Group. DZ analysis of additional porcellanites in southwestern Montana, northern Idaho, and a preliminary sample collected from mapped Lawson Creek Formation are also bimodal but yield youngest DZ ages of ~1410 Ma. These ages are not consistent with results from Leaton Gulch, suggesting the ~1336 Ma Leaton Gulch porcellanite may not be regionally distributed. If strata at Leaton Gulch are correlative with the Lawson Creek Formation, then the east-central Idaho succession—and correlative formations of the upper Missoula Group—must be <1336 Ma and therefore >30 m.y. younger than previously thought. Alternatively, the Leaton Gulch unit is stratigraphically above the Belt Supergroup and correlative with the Deer Trail Group.