NEWLY-DISCOVERED, ANCIENT EXTRUSIVE RHYOLITE IN THE SALTON SEA GEOTHERMAL FIELD, IMPERIAL VALLEY, CALIFORNIA

Abstract

Thick (150-300 m), ancient rhyolites and tuffs unambiguously erupted onto a paleosurface have been discovered beneath 1.7 km of clastic sedimentary strata in the eastern part of the Salton Sea geothermal field. The rhyolites are aphyric and flow-banded, and consist entirely of micropoikilitically devitrified glass. The tuffs contain accretionary lapilli, blocky glass shards, and sedimentary debris; they are interpreted as phreatomagmatic.

Assuming an average sedimentation rate of 2.24 mm/yr for this part of the Salton trough (a figure based on occurrence of the petrographically distinctive, 0.76 Ma Bishop Tuff fallout deep in the nearby State 2-14 scientific borehole), the age of the new rhyolite is calculated to be about 0.73 Ma. A potentially valuable marker horizon, the new rhyolite is envisioned as part of a much larger buried dome field, perhaps analogous to the one now exposed above the Coso geothermal system about 390 km to the north.