WHAT’S UNDER THE WESTERN SNAKE RIVER PLAIN? A REVIEW OF CRUSTAL GEOPHYSICS AND SUBSURFACE GEOLOGY

Data on the deep crust and mantle beneath the western plain is limited to the Boise-Elko refraction line of Hill and Pakiser (1967) and gravity measurements reported by Mabey (1976). The mantle is about 40 km deep beneath the plain. Lower crust P-wave velocity (17 to 40 km) is 6.8 to 7 km/s beneath the plain and relatively high compared to the margins. Velocity in the shallow crust (8 to 17 km) is also relatively high, 6.8 to 6.6 km/s. Despite its rift basin structure, the entire western plain is a broad gravity high of +25 to +60 milligals. The western plain cuts across the southern end of the Idaho batholith, but the above geophysical data suggests it is not simply underlain by downfaulted granite. We suggest that the upper crust beneath the plain has been so invaded by basaltic material that it no longer has the properties of granitic rock. An interesting high velocity layer of 7 km/s, several kms thick, is situated in the lower crust about 23 km deep along the track of the Yellowstone hot spot beneath the Bruneau-Jarbidge eruptive center, and interpreted as a remnant basaltic underplate or a mafic residue of the rhyolite systems. Deep wells in the center of the western plain drilled up to 3.5 kms of basalt beneath the 1 to 2 kms of sedimentary fill. No significant rhyolite occurs in wells in the central western part of the plain; however, sections of 2.4 km of rhyolite occur locally in outcrop and wells on the margins. Lack of significant rhyolite in the center of the western plain suggests much of it was an upland during the rhyolite eruptive period 9.6 to 15 Ma. Recently completed high-resolution seismic and older industry data image depositional systems of Neogene Lake Idaho, showing thick lacustrine muds, delta systems, and a channeled angular unconformity surface on the lacustrine Chalk Hills Formation, overlain by transgressive and regressive lake sediments of the Terteling Springs and Glenns Ferry Formations. These sedimentary features control the large groundwater resource system of the western plain.