PROJECT HOTSPOT – THE SNAKE RIVER SCIENTIFIC DRILLING PROJECT: A PROGRESS REPORT

Project Hotspot – the Snake River Scientific Drilling Project – seeks to understand the long-term volcanic and sedimentary history of the Snake River volcanic province, its relationship to the Yellowstone hotspot, and its potential for the development of geothermal power. Project Hotspot drilling commenced in September 2010. Three deep wells are planned: Kimama, on the axial volcanic zone of the central SRP was completed at 1912 meters below surface (mbs) in January 2011; Kimberly, on the southern margin of the Twin Falls eruptive complex, was started in February 2011, with a planned total depth of 1800 mbs; Mountain Home, in the western SRP, is scheduled to begin in late April 2011.

The Kimama drillhole is sited over an extensive plume of warm ground water. We expected to intersect rhyolite at less than 1200 mbs; in reality, rhyolite was never encountered– silt and clay rich sediment intercalations 20-50 m thick were found at 180 mbs, 1220 mbs, and 1700 mbs, with thinner layers scattered throughout. This is in contrast to well WO-2 at the Idaho National Lab, 110 km to the NE, where the basalt-rhyolite contact lies at 1150 mbs, and the Wendell-RASA well, 70 km to the W, where circa 5 Ma basalts are found at 335 mbs. The base of the SRP aquifer, as defined by the temperature inflection at its base, lies at 980 mbs – more than twice as thick as expected, with a very steep thermal gradient below the aquifer. Basalt stratigraphy is strikingly revealed by neutron logs, with high neutron absorption in the porous flow tops, and low absorption in the massive flow centers. Basalt alteration increases downward, from calcite-nontronite to saponite to quartz-zeolites. Fracture zones may be marked by their log signature.

Stratigraphy in the Kimberly drill hole is also unexpected: rhyolite ash flows from the Twin Falls eruptive complex are underlain by basalt flows with intercalated sediments at depths around 240-335 mbs. More rhyolite may lie beneath these basalts, and a complete stratigraphic section will be available by the time of the meeting. Our results so far suggest that standard models for development and deformation of the Snake River volcanic province need to be re-evaluated to explain the extraordinary thickness of basalt at Kimama, and the intercalation of basalt and rhyolite at Kimberly.

Hotspot Team: B. Utley, K. Lowry, L. Garcia, R. Bradshaw, T. Forbush, J. Stokes, Utah State Univ.