Geologic Map Day | October 16, 2020

Geologic Map of the Crane Creek Reservoir Quadrangle, Washington County, Idaho

current mapping

The geologic map of the Crane Creek Reservoir 7.5’ quadrangle depicts the rock units exposed at the surface or underlying a thin cover of soil or colluvium. This map is a result of field work conducted in the summer and autumn of 2017 by the authors. This map is an indication of what the middle Miocene (13 to 16 Ma) in west-central Idaho would have looked like. The map area is dominated by sheet flow volcanic rocks of the Grande Ronde Formation, part of the larger Columbia River Basalt Group, and the more chemically diverse Weiser Volcanics. The Weiser Volcanics flows span the geochemical range from high silica rhyolite to mafic basalt. Coeval with the volcanic rocks is sand, silt, clay, and volcanic ashes of the Payette Formation.

Sequoia fossil leaves

Sequoia sp. fossil leaf imprints in a volcanic ash from the Crane Creek Reservoir 7.5’ quadrangle.

Two high precision U-Pb zircon TIMS dates from volcanic ashes found in the Payette Formation help pinpoint the timing of events in the area. Crane Creek Reservoir is semi-arid terrain, the surficial deposits are dominated by reworked pebbles, sand, silt, and clay alluvial deposits of small to ephemeral streams and playa and lake deposits of the man-made Crane Creek and Paddock Valley reservoirs. Also depicted on the map is a wide network of mima mounds referred to here as patterned ground. The structure of the map area is predominated by northwest trending normal faults, however it bucks the trend of other maps produced in the area by revealing anomalous east-west trending normal faults. There is no evidence of recent offset on any of the faults. This map is part of a larger regional study to understand the origin and evolution of the hydrocarbon deposits in southwest Idaho.