SPECULATION ON THE AGES AND RELATIVE STRATIGRAPHIC POSITIONS OF BELT SUPERGROUP-EQUIVALENT ROCKS OF THE LEMHI SUBBASIN, EAST-CENTRAL IDAHO AND SOUTHWESTERN MONTANA

The Mesoproterozoic (ca. 1.45 to <1.39 Ga) Belt Supergroup is variably interpreted to record failed intracontinental rifting or breakup of the supercontinent Nuna. However, a lack of fossil constraints and few interbedded volcanic rocks have hindered age constraints on its deposition. Recent work on Belt basin rocks in central and east-central Idaho and southwestern Montana—deposited in the “Lemhi subbasin”—interprets that they represent a coarser grained age-equivalent to the highest stratigraphic levels (Missoula Group) exposed in the main Belt basin. However, a lack of stratigraphic marker horizons and structural disruption have hindered regional correlations across major faults within the Lemhi subbasin. Published ages of these strata come from ca. 1.38 Ga cross-cutting plutons in central Idaho and detrital zircon maximum depositional ages (MDAs) as young as ca. 1.39 Ga from rocks in the northern Lemhi Range. However, the relative stratigraphic positions of rocks crosscut by these plutons is uncertain and different parts of the section, tentatively interpreted by others to be lithostratigraphic correlatives, yield conflicting absolute and relative age constraints. Here, we summarize existing and new age data from Lemhi rocks using MDAs from a newly compiled Belt DZ database, as well as recent U-Pb zircon ages from interbedded horizons that we interpret as reworked tuffs. We then compare the relative structural positions of strata via a recently published regional-scale balanced cross section across the Lemhi subbasin. Recent U-Pb zircon ages of ca. 1.38 Ga from tuffs interbedded with Lemhi subbasin quartzites suggest that ca. 1.38 Ga plutons in central Idaho were emplaced concurrently with—and thus do not constrain the younger age limit of—Lemhi subbasin strata. Interpreted in conjunction with restoration of slip along thrusts and normal faults between Lemhi subbasin rocks in the Lemhi Range and Beaverhead Mountains, this suggests that strata crosscut by ca. 1.38 Ga plutons in central Idaho may represent an older part of the Lemhi subbasin whereas Lemhi strata in the Beaverhead Mountains are likely younger. Finally, we speculate that the ages of some Lemhi subbasin-correlative rocks are younger than previously thought, potentially spanning to <1.32 Ga.