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Show Spencer H. Wood Geosciences, Boise State University Boise, Idaho 83725 swood@ trex. idbsu. edu LATE PLIOCENE DRAINAGE OF LAKE IDAHO: GEOMORPHOLOGY OF THE OUTLET AREA AND SUBSURFACE DATA FROM THE WESTERN SNAKE RIVER PLAIN, IDAHO AND OREGON A great lake occupied the western Snake River Plain basin from about 9 million until 2 million years ago. Wheeler and Cook's ( J. of Geol., 1954) concept of capture and drainage of Lake Idaho through Hells Canyon to the Columbia- Salmon River drainage is supported by subsequent studies. Lacustrine sediments of the Chalk Hills and Glenns Ferry Formation suggest the lake at one time may have covered as much as 13,500 km . Seismic stratigraphic data indicates depths up to 255 meters ( Wood, 1994, AAPG Bull.). Maximum thickness of 2 km of sediment is detected by seismic data near Caldwell, Idaho. A moderate angular unconformity with angular discordance of less than 5 degrees is locally observed within sediments on the north and south margins of the western plain, and has been taken by some investigators as the boundary between the Glenns Ferry and Chalk Hills Formations. These unconformities may indicate local tectonic movement and may not be time correlative over the basin. Data from deep wells and seismic reflection surveys indicate a major shift in the nature of sediments in the basin is associated with a level, now buried 300 to 370 m beneath the central plain ( Figure 1). Beneath that level are delta, prodelta, and deep lake sediments of the lacustrine system, while above that level are mixed fluvial and lacustrine sediments. The level is important because significant sorted- sand aquifers are above that level, while below that level, sediments are mostly mudstones of low permeability. The present concept is that the level represents the beginning of lowering base level as a result of capture and outlet of the lake through Hells Canyon. In the Boise foothills the level is overlain by Gilbert- delta sequences, some with foresets 25 meters thick prograding northwestward. Apparently the slowly lowering base level caused erosion of accumulated sands on the lake margins, and progradation of sand bodies into the basin, filling the lake basin and locally giving way to fluvial systems. Downcutting of the spillway area and regrading ancestral Hells Canyon near and downstream of Huntington, Oregon was probably slow, on account of resistant Miocene volcanic bedrock at the outlet and the crystalline Paleozoic and Mesozoic bedrock of the river bed in Hells Canyon. Known rates of valley incision of bedrock by major rivers is in the range 9 to 30 cm / 1000 years ( Schumm and Ethridge, 1994, SEPM Spec. Publ. # 21). At a typical rate of 15 cm / 1000 years, the lowering of the spillway and grade of Hells Canyon by 200 meters may have required at least 1.3 million years. At the outlet near Cobb Rapids, the Snake River canyon is a V- shaped incision into basalt about 215 meters deep. Near the outlet, widespread gravel terrace deposits beveling lake deposits occur only to about 140 meters above river level indicating the level when through- going river systems finally reached the outlet. Lake level and sedimentation of the uppermost Idaho Group were governed mostly by this slow lowering of base level. Lake level fluctuations due to climatic change, compaction of sediment in the basin, and local tectonic movement were probably superposed on this major |