Chapter 7: Middle and Late Proterozoic rocks and Late Proterozoic tectonics in the southern Beaverhead Mountains, Idaho and Montana: A preliminary report

Abstract

Two unconformity-bounded, pre-Ordovician packages of sedimentary rocks in the southern Beaverhead Mountains of southwest Montana and adjacent Idaho are recognized by differences in lithology, texture, bedding characteristics, and sedimentary structures. One package of thin-bedded arkoses and subarkoses is equivalent to rocks of the Middle Proterozoic Lemhi Group of Idaho. The other package contains thick-to thinbedded, locally feldspathic and glauconitic, sublitharenites of shallow marine depositional fades that are assigned to the Late Proterozoic and Lower Cambrian Wilbert Formation. The Wilbert Formation resembles and correlates with the Late Proterozoic Mutual Formation and the overlying Late Proterozoic and Lower Cambrian Camelback Mountain Quartzite of the Brigham Group in Idaho south of the Snake River Plain. Distribution of rocks of the Lemhi Group and Wilbert Formation in three Mesozoic Cordilleran thrust sheets of the southern Beaverhead Mountains, the Hawley Creek, Fritz Creek, and Cabin-Medicine Lodge sheets, and the presence of only Archean(?) rocks east of the Cabin-Medicine Lodge thrust system suggest that the Wilbert Formation was deposited in a north-trending extensional basin flanked by Middle Proterozoic Lemhi Group arkoses on the northwest and by Archean(?) metamorphic rocks on the east. This basin possibly was a northern arm of a Late Proterozoic and Early Cambrian rift basin that formed at the time of the opening of the paleo-Pacific Ocean (ca. 570 Ma) and widened toward the Cordilleran miogeocline south of the Snake River Plain. Subsequent Middle to Late Cambrian uplift (Skull Canyon disturbance) in the Beaverhead Mountains could have been generated by aesthenospheric upwelling in axial parts of the Wilbert basin.

500–490 Ma detrital zircons in Upper Cambrian Worm Creek and correlative sandstones, Idaho, Montana, and Wyoming: Magmatism and tectonism within the passive margin

“…On the southeastern side of the Lemhi arch in the southern Lemhi and Beaverhead Ranges, latest Neoproterozoic and Cambrian Wilbert Formation overlies the unconformity (Fig. 2; Skipp and Link, 1992). In central Idaho along the Salmon River southwest of Challis, Cambrian and Ordovician carbonate and siliciclastic rocks of the Bayhorse assemblage (Hobbs and Hays, 1990;Hobbs et al, 1991) were deposited on the downfaulted western side of the Lemhi arch .…”

Extra-basinal fluid infiltration, mass transfer, and volume strain during folding: Insights from the Idaho-Montana thrust belt

“…The deformation in the Lemhi Range is characterized by large-scale recumbent folding and large-displacement thrusts (Beutner, ms, 1968). Farther east, in the Beaverhead and Tendoy Ranges, basement, thinner Paleozoic section, and synorogenic conglomerates are folded and imbricated within the Cabin, Medicine Lodge, Four Eyes Canyon and Tendoy thrust sheets (Lowell, 1965;Ruppel and Lopez, 1984;Skipp, ms, 1985;Skipp and Link, 1992;Lonn and others, 2000;others, 2004a, 2004b).…”

“…3. Generalized geologic map of the study corridor showing sample localities, modified from Skipp and Link (1992), Wilson and Skipp (1994), and Lonn and others (2000). Cross section is a schematic reconstruction of the Sevier thrust belt along the study corridor for the Late Cretaceous, prior to the onset of extension.…”

Allochthonous impact-shocked rocks and superimposed deformations at the Beaverhead site in southwest Montana

“…Some petrologic and sedimentologic features of the rocks, however, suggest to Skipp and Link (1992) that the quartzite sequence may at least be partly correlative with the Late Proterozoic(?) and Lower Cambrian Wilbert Formation.…”

“…The age of the shatter-coned sandstones is crucial to establishing an older limit for the age of the structure: if the sandstone is Middle Proterozoic, as we now believe, the age of the structure is younger than about 900 Ma and older than Ordovician. On the other hand if Skipp and Link (1992) are correct, and the sandstone correlates with the Late Proterozoic to Lower Cambrian Wilbert Formation (about 700 to 550 Ma), then the age of the structure is probably Middle or Upper Cambrian, or about 530 to 505 Ma. One of the authors (R. B. Hargraves), however, traversed the Wilbert Formation in its type section in the southern Lemhi Range (Ruppel, 1975) and saw no resemblance to the shocked strata in the Medicine Lodge Valley.…”