PLIO-PLEISTOCENE INTERACTIONS INVOLVING BASALT FLOWS AND THE SOUTH FORK OF THE SNAKE RIVER BETWEEN SWAN VALLEY AND RIRIE, IDAHO: A DAM STORY

This study reports the results of detailed geologic field mapping along the South Fork of the Snake River. The study emphasizes the history of interactions between basalt flows and the river and builds on previous mapping and age dating. Two periods of basaltic volcanism are recorded in Swan Valley. The ~4 Ma basalt of Swan Valley erupted onto the valley floor after the eruption of Kilgore tuff and before the eruption of Huckleberry Ridge tuff (Anders et al., 1989). These eruptive episodes formed a broad constructive edifice in the Conant Valley area which separated the Pine Creek from the Snake River. At about 1.5 Ma, following deposition of the Huckleberry Ridge tuff, basalt erupted on Antelope Flat (Anders et al., 1989). This eruption dammed the ancient Snake River and Pine Creek, filled their channels with basalt, and created a reservoir that extended ~32 km to the south into Swan and Star Valleys. Dam structures are dominantly composed of variably-palagonitized hyaloclastite with intercalated basaltic lava flows that locally have pillow basalt bases. Overtopping of the Conant Valley dam caused the capture of the paleo-Pine Creek drainage by the Snake River and the formation of the current river channel.