Subsurface Geology of the Raft River Geothermal Area, Idaho

Abstract

The Raft River Valley occupies an upper cenozoic structural basin filled with nearly 1,600 m of fluvial silt, sand, and gravel. Rapid facies and thickness changes, steep initial dips (30°C), and alteration make correlation of basin fill depositional units very difficult. Hydrothermal alternation products in the form of clays and zeolites, and deposition of secondary calcite and silica increase with depth. The abundance of near vertical open fractures also increase with depth, allowing greater movement of hydrothermal fluids near the base of the Cenozoic basin fill. The basin is floored with quartzites and schists that can be correlated with formations in the nearby Raft River Range. Beneath theses rocks is a Precambrian adamellite basement complex. The Raft River geothermal systems is a hot water convection system relying on deep circulation of meteoric water in a region of high geothermal gradients and open fractures near the base of the Tertiary basin fill.