REGIONAL STRATIGRAPHIC IMPLICATIONS FROM NEW MAPPING OF THE NORTHERN BEAVERHEAD RANGE, MONTANA AND IDAHO

Three years of STATEMAP-supported geologic mapping by the MBMG and IGS in the Beaverhead Range along the Idaho-Montana border has identified five fault-bounded Mesoproterozoic sedimentary packages. The easternmost (1) contains >19,000 ft of east-facing coarse-grained quartzite and is tentatively assigned to the Missoula Group, Belt Supergroup. Separated from the eastern package by the eastern strand of the Beaverhead Divide fault (BDF) is (2), an east-facing, fine-grained feldspathic quartzite that grades from white to green to red through 15,000 ft of section. To the NW, package (3) consists of folded but largely east-facing fine-grained feldspathic quartzite. West of the Freeman thrust and the western strand of the BDF is folded but largely west-facing, carbonate-bearing, fine-grained feldspathic quartzite (4) that grades downward to carbonate-bearing siltite, argillite, and phyllite. The westernmost package (5) is folded, but largely east-facing siltite and argillite containing intervals rich in planar-laminated white quartzite. Reconnaissance work in the Lemhi Range to the SW suggests that the lower parts of (2) and (3) resemble the Gunsight Fm at its type locality. However, that Gunsight Fm is underlain by carbonate-bearing siltite and argillite of the Yellow Lake member of the Apple Creek Fm, and is capped by clean quartzite of the Swauger Fm, in contrast to the sequence seen in package (2). Strata in (4) apparently match type Big Creek Fm and underlying West Fork Fm. The westernmost strata (5) may be the Inyo Creek Fm and rocks below the Inyo Creek not exposed in the Lemhi Range. A key observation is that the Big Creek Fm SE of Lem Peak in the Lemhi Range is conformably underlain by clean quartzite mapped as Swauger. Possible explanations include: a) the Swauger mapped over type Gunsight is Neoproterozoic and rests unconformably on an incomplete Gunsight section; b) there is no Lem Peak fault SE of Lem Peak, and rocks correlated across it comprise a continuous stratigraphic sequence such that there are two Big Creek-like carbonate-bearing quartzites, two Apple Creek-like finer grained intervals, and possibly two Swauger-like clean quartzites. If correct, the strata of the Lemhi Range form a northeast-dipping homocline, and the Lemhi Group nomenclature in the region will require substantial revision.