GEOLOGIC MAPPING IN THE SOUTHERN BEAVERHEAD MOUNTAINS, EASTERN IDAHO (WEST SIDE): NEW STRUCTURAL INTERPRETATIONS

Geological mapping by the BYU-Idaho field camps on the west side of the southern Beaverhead Mountains in eastern Idaho has changed our structural interpretation of the range. The Beaverhead Mountains are a late Miocene to Recent, northwest-trending Basin and Range horst within the Cretaceous Sevier thrust belt. Structurally, the range shows a consistent pattern of east- to northeast-vergent asymmetric fault-bend and fault-propagation folds. The folds occur from outcrop to range scale, and stratigraphic throw on the core thrusts increases with structural depth. At the southern end of the range, the Copper Mountain thrust trends north-south, not parallel to the horst. Stratigraphic throw on the thrust decreases northward with decreasing structural exposure. Exposed strata of the southern Beaverhead Mountain Range comprise primarily a thick Paleozoic carbonate succession with scattered sandstones and mudstones underlain by a thick interval of Proterozoic sandstone. At the north end of the mapped area, the Beaverhead pluton intrudes apparently between the Proterozoic Gunsight Formation and the Ordovician Kinnikinic Quartzite. The Kinnikinic overlies the pluton, and stoped blocks of the Kinnikinic are present within the pluton.