STATEMAP GEOLOGIC MAPPING ALONG THE SNAKE RIVER, EASTERN IDAHO

The Idaho Geological Survey is conducting STATEMAP geologic mapping along the Snake River in the Blackfoot-Idaho Falls-Rexburg-Ashton area of Eastern Idaho. To date, preliminary versions of 12 7.5-minute quadrangles have been completed, providing 1:24,000-scale geologic mapping for the first time in this economically vital portion of the Snake River Plain.

The project area sits on the edge of the eastern Snake River Plain, a major crustal downwarp associated with the Yellowstone Hotspot. The oldest bedrock units are rhyolitic ash flow tuffs and flows of the Heise volcanic field erupted between 6.7 and 4.5 Ma when hotspot volcanism passed through the region. Subsequent Plio-Pleistocene basaltic volcanism and subsidence created the lava plains, shield volcanoes, and tuff cones characteristic of the eastern Plain. The 2.0 Ma Huckleberry Ridge Tuff, erupted from the Yellowstone caldera, is present in the northern end of the project area. The present-day Snake River drainage developed during this time, influenced by continuing subsidence of the Snake River Plain and extensional faulting along the Grand Valley fault zone. Basaltic volcanism occurred as recently as 5.6-6.4 ka with eruption of flows of Hell's Half Acre near Idaho Falls.

During the late Pleistocene, glaciation in the headwaters of the Snake River generated huge volumes of outwash. Thick quartzite-rich gravel deposits were deposited in the project area, forming both an important aggregate source and a major regional aquifer. Wind-blown loess, the source of soils in which Idaho's famous potatoes are grown, was sourced from both local glacial outwash and western Snake River Plain materials. Younger, coarser-grained eolian sands, probably derived from Bonneville Flood deposits, form linear sand dunes and sand sheets.

The mapping is supported by new and compiled geochemical analyses, paleomagnetic studies, and 40Ar/39Ar dates of volcanic rocks. Dating of loess and outwash with optically stimulated luminescence is being used to link these deposits with regional glaciation. Gravel isopachs and other subsurface information have been developed from thousands of domestic water wells. The maps also constitute an excellent source for K-12 education. Google Earth virtual field trips are being developed in consultation with local educators.