LARGE-SCALE RHEOMORPHIC STRUCTURES AND BASALT STRATIGRAPHY OF THE NEWDALE AND LINDERMAN DAM 7.5-MINUTE QUADRANGLES, EASTERN SNAKE RIVER PLAIN, IDAHO

Detailed geologic mapping of the Newdale and Linderman Dam 7.5 minute quadrangles supported by paleomagnetic and geochemical data: document the distribution of large-scale gravity sliding and secondary deformation structures within the 2.06 Ma Huckleberry Ridge Tuff (Qyh) and underlying sediments, basalts and rhyolites; document geologic causes and effects of the Teton Dam failure; and refine the post Qyh basalt stratigraphy.

A sheet consisting of ~130 m of Qyh and at least 30 m of underlying units has moved ~1 km horizontally. Large scale structures in the detached sheet include diapiric antiforms >100 m in amplitude, strike slip faults with up to 1 km of displacement, and an arcuate 1 km by 12 km tectonically denuded valley. Detachment and deformation apparently occurred within the underlying sediments as a result of overloading by the rapid emplacement of the Qyh. Movement occurred after compaction and welding but before devitrification of the tuff. Paleomagnetic data confirm that folding of the Qyh took place above the maximum blocking temperature of magnetite (~580^oC). Infiltration of water along numerous open joints and shear zones produced during this deformation of the tuff contributed to the failure of the Teton Dam in 1976. Numerous landslides resulted from rapid drawdown of the reservoir when the dam failed. These slides involve ~500 acres or 34% of the submerged canyon slopes.

Basalt units in the area are compositionally diverse, consistent with having formed near the edge of the Snake River Plain during active caldera formation in the area. Compositional and paleomagnetic data are used to distinguish units and correlate isolated outcrops and vents. Field relations indicate that at least three lava flows of the Pleistocene basalt of Moody entered the Teton Canyon in rapid succession from the south and dammed the river at progressively higher levels. Pillow lavas and hyaloclastite deposits at the base of each flow indicate that the reservoir level reached the top of each lava dam, before subsequent flows were emplaced. The river finally breached and cut completely through all three flows.

Geologic mapping funded by USGS (1976-78), USGS National Cooperative Geologic Mapping Program and the State of Idaho (2009-10), and BYU-Idaho.