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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 thin-bedded, 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.

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