STRUCTURAL AND VOLCANIC DEVELOPMENT OF THE WEISER EMBAYMENT, WEST-CENTRAL IDAHO

The Weiser Embayment in west-central Idaho is a structural bound basin geographically bordered by the Salmon River Mountains, Seven Devils Mountains, and the Snake River Canyon. The southern part of the embayment is truncated by northwest-trending faults associated with graben development of the western Snake River Plain. This area has experienced two styles of deformation since middle Tertiary time. The earliest structures included regional tilting, broad warping, folding, and local reverse faults that initiated features such as the Indian Valley syncline and the Sturgill Peak anticline, Younger structural style is typically expressed by high-angle extensional features such as the Squaw Butte, Long Valley, and Paddock Valley fault systems. Basement rocks to the Tertiary volcanics and sediments include Permian to Jurassic accreted metasedimentary and metavolcanic rocks, Mesozoic and Cenozoic intrusive complexes, and Eocene volcanics. The volcanic rocks in the area include Imnaha and Grande Ronde formations of the Columbia River Basalt Group, including the ferro-andesite Fiddlers Hell Member of the Grande Ronde. These rocks are overlain by Payette Formation sediments and members of the Weiser basalts. Volcanic rocks in the area include basalts, andesites, and rhyolites that range in age from 16-12.5 Ma. Recent and ongoing geologic mapping by the Idaho Geological Survey, partly funded by the USGS StateMap program, has succeeded in identifying new units and working towards developing a regional stratigraphy for the Weiser Embayment.