ABSTRACT

Since 1975, the U.S. Geological Survey has produced more than seventy reports dealing specifically with the geology of south-central Idaho. Approximately one-third of these reports are geologic maps. The rest are largely stratigrphic and structural studies. Precambrian, Paleozoic, and early Mesozoic (LowerTriassic) rocks exposed in the region are all or nearly all allochthonous, having been moved eastward or northeastward on a series of overlapping thrust sheets emplaced during Sevier deformation. Lower Cretaceous granitic rocks of the Idaho batholith at Deer Creek intrude the western thrust sheets indicating that thrusting took place before that time. Eastern thrust sheets override Upper Cretaceous and Paleocene synorogenic deposits.

The stratigraphic record documents several disturbances in the area before the Sevier orogeny These include: (1) a probably Proterozoic Z disturbance; (2) An Ordovician disturbance; (3) The Antler orogeny; (4) Extensional deformation contemporaneous with the rise of the ancestral Rocky Mountains, and (5) Uplift and nondeposition or erosion during the Sonoma orogeny.

Cenozoic rocks in the area record first the eruption of the intermediate flows and tuffs of the Eocene Challis volcanics, then the inception of basin and range and/or block faulting, followed closely by the downwarping and faulting of the northern margin of the eastern Snake River Plain. Thick rhyolitic caldera fill of the plain is overlain by the relatively thin basalt cover of the Snake River Group. Some basalt flows associated with rift zones are as young as about 2,000 years, and indicate that the eastern Snake River Plain may well be the site of future basalt eruptions.