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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 lithospheric 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, despite 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 composition 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 eruptive 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 silicic 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 reason, we suspect that much of the area of the plain was an upland and not a large depositional basin during the period of silicic volcanism.

Geochronology of volcanic rocks on both sides indicate 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 basalt volcanism and high rates of faulting. Interbedded arkose, mudstone, and volcanic ash constitute this earliest sediment mapped as the Chalk Hills Formation. Local basalt lava fields (dated 10-7 Ma) occur at several levels in the Chalk Hills Formation. An active rift environment is envisioned with lakes interconnected at times by a river system.

The faulted and tilted Chalk Hills Formation is dissected by an erosion surface at the basin margins, indicating a regression of lakes to the deeper basins. Depositional records of the regression are generally absent from the margins, but we suggest that the east Boise fan aquifer 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 sediment appears to be this transgressive lacustrine sequence where it is mapped as the Terteling Springs Formation, with shoreline sands and small deltas interfingering basinward with lake muds. The lake rose to its highest elevation of about 3,600 feet (1,100 m) in a period of less than a few million years. At that highest level, it overtopped the spill point into ancestral Hell’s Canyon and the Columbia-Salmon river drainage. Reliable geochronology constrains the time of overflow between 6.4 and 1.7 Ma and is in need of better resolution. The rise in lake level may have been indirectly caused by regional tectonic movement of the migrating uplift of the Yellowstone hot spot, as an associated Continental Divide migrated about 200 km eastward from the Arco area to its position in Yellowstone National Park over the period 6 Ma to present. In doing so, the catchment area of the Snake River may have increased as much as 50,000 square km. Captured runoff associated with the shifting topographic divide is hypothesized to have caused the level of Lake Idaho to rise to its spill point about 4 million years ago.

Downcutting of the outlet was apparently slow (about 120 m/Ma) during which time sandy sediment eroded from the basin margins and filled the remaining lake basin with interbedded mud and sand of lacustrine delta systems. This sedimentary sequence of a slowly lowering base level constitutes most of the Glenns Ferry Formation and the main sand-bearing aquifer section of the western plain. It is represented in the Boise foothills by a 60-m-thick unit of coarse sand with Gilbert-type foreset bedding called the Pierce Park sand. Subsequently, fluvial systems with gradients necessary to produce braid-plain sandy gravel deposits flowed to the outlet region near Weiser. These gravel deposits should decrease in age and altitude to the northwest, and at Weiser these oldest gravels occur at elevation 2,500 feet.

During the late stages of the draining of Lake Idaho, basalt volcanism resumed in the western plain, focusing along a line of vents that trends obliquely across the plain at about N. 70º W., named here the Kuna-Mountain Home volcanic rift. Both sublacustrine and subaerial volcanoes erupted and built a basalt upland with elevations of highest shields to 3,600 feet over the last 2.2 million years. Aligned vents and fissures of these volcanoes indicate the present orientation of the principal tectonic stress is N. 70º W., contrasting with the N. 45º W. boundary of the plain and the N. 30º W. alignment of vents in the eastern plain. This N. 70º W. alignment is similar to the same vent features of Quaternary basalt fields in eastern Oregon, suggesting that a province of similar tectonic stress orientation includes the western plain and much of eastern Oregon.

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This document was originally published in Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin by the Idaho Academy of Science. Copyright restrictions may apply.