TIGHTENING THE BELT: NEW AGE AND PROVENANCE CONSTRAINTS ON PROTEROZOIC STRATA INEAST-CENTRAL IDAHO

In east-central Idaho, Mesoproterozoic strata of the Lemhi Group and overlying rocks of the Lawson Creek and Apple Creek Formations are considered correlative to the Missoula Group of the upper Belt Supergroup. Limited data from prior work roughly places the Missoula Group between ~1450 and ~1370 Ma in age. The upper part of this succession is >5 km in thickness in east-central Idaho. Similar rocks in the northern Lost River Range near Leaton Gulch were previously described as “Ordovician(?) to Proterozoic(?)” (OZl), but were recently re-mapped; these rocks contain a green porcellanite bed (interpreted as a reworked tuff) that yielded a U-Pb zircon age of ~1336 Ma and are overlain in angular unconformity by Neoproterozoic and Cambrian rocks. Strata below the unconformity near Leaton Gulch are similar to nearby strata in the type-section of the Lawson Creek Formation. Recent structural analysis, as well as preliminary detrital zircon (DZ) geochronology and Lu-Hf geochemistry also support a tentative correlation with the Lawson Creek Formation. If the Lawson Creek and Apple Creek Formations are indeed correlative with rocks near Leaton Gulch, there are significant implications for the thickness and age of the upper Belt Supergroup. (1) Mesoproterozoic strata above the Lawson Creek Formation in east-central Idaho are >5 km thick and <1336 Ma, >30 m.y. younger than previously thought, which may indicate that this succession is stratigraphically higher than the Belt Supergroup. (2) The upper part of the Belt Supergroup succession in Idaho is thought to be correlative, in part, with the Garnet Range Formation of western Montana. If true, this requires that the Garnet Range Formation is also post-Belt in age. (3) Alternatively, if all rocks are included as part of the Belt Supergroup, then the younger Belt succession contains rocks deposited after ca. 1336 Ma, which requires substantial revision to timing and subsidence rate of the Belt basin. We will test these hypotheses with additional DZ geochronology and geochemistry, paleogeographic reconstructions in the context of a recent global Lu-Hf database, and newly measured stratigraphic sections, in order to better place our results in the context of regional stratigraphic correlations of Mesoproterozoic strata in the northern Rocky Mountains.