Geologic and Tectonic History of the Western Snake River Plain, Idaho and Oregon Spencer H. Wood 1 and Drew M. Clemens 2 Editors’ note: The manuscript was submitted in July 1998 and has been revised at the authors’ discretion. 1 Department of Geosciences, Boise State University, Boise, ID 83725 2 U.S. Army Corps of Engineers, CENAE-EP-HC, 696 Virginia Road, Concord, MA 01742 Wood, S.H., and D.M. Clemens, 2002, Geologic and tectonic history of the western Snake River Plain, Idaho and Oregon, in Bill Bonnichsen, C.M. White, and Michael McCurry, eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 69-103. ABSTRACT The western Snake River Plain is a Neogene-aged intracontinental rift basin, about 70 km wide and 300 km long, trending northwest across the southern Idaho batholith. Its southeastern end merges with the northeast- trending eastern plain, a structural downwarp associated with extension along the track of the Yellowstone hot spot. Orientation of the western plain rift is parallel to several regional northwest-trending crustal discontinuities, such as the Olympic-Wallowa lineament and the Brothers fault zone, suggesting that the rift failed along zones of litho- spheric weakness, as the lithosphere was softened by the passing hot spot. Crustal refraction data and gravity show that the rift is not simply underlain by granitic rock, de- spite its appearance of having broken and extended the southern end of the Idaho batholith. Instead, the crust beneath 1 to 2 km of basin fill is mostly of mafic compo- sition down to the top of the mantle, about 42 km deep beneath the plain. North and south of the plain, the upper crust has velocities more typical of granitic rock. South of the plain, beneath the 9-11 Ma Bruneau-Jarbidge erup- tive center of silicic volcanics, is a zone of slightly high seismic velocity at a depth of 23 km that could be restite or an underplate of basalt related to formation of the si- licic magma. In this paper we show that some (12-10 Ma) rhyolite flows and domes erupted near the margins of the plain, but that thick rhyolite does not occur in deep wells in the subsurface of the plain northwest of Boise. For this rea- son, we suspect that much of the area of the plain was an upland and not a large depositional basin during the pe- riod of silicic volcanism. Geochronology of volcanic rocks on both sides indi- cate major faulting began about 11 Ma and was largely finished by 9 Ma. Since about 9 Ma, slip rates have been low (less than 0.01 mm/year) with the exception of a short (about 10-km) segment of late Quaternary faulting in the Halfway Gulch-Little Jacks Creek area on the south side. Earliest sediment of the plain is associated with ba- salt volcanism and high rates of faulting. Interbedded arkose, mudstone, and volcanic ash constitute this earli- est sediment mapped as the Chalk Hills Formation. Lo- cal basalt lava fields (dated 10-7 Ma) occur at several levels in the Chalk Hills Formation. An active rift envi- ronment is envisioned with lakes interconnected at times by a river system. The faulted and tilted Chalk Hills Formation is dis- sected by an erosion surface at the basin margins, indi- cating a regression of lakes to the deeper basins. Deposi- tional records of the regression are generally absent from the margins, but we suggest that the east Boise fan aqui- fer sediments and deep basin fill might be such a record. Nothing is known of the cause of the regression of the Chalk Hills lake. A transgressive lacustrine sequence encroached over slightly deformed and eroded Chalk Hills Formation on the plain margins, locally leaving basal coarse sand, or a thin beach pebble layer now iron-oxide cemented. The upper part of this transgression deposited shoreline oolitic sand deposits, indicating increased alkalinity of a closed lake. In the Boise foothills, much of the exposed sedi- ment appears to be this transgressive lacustrine sequence