Initial Results from Drillholes PLV-1 and PLV-2 in the Western Moat of the Long Valley Caldera

Abstract

Core holes PLV-1 and PLV-2 are long the first intermediate depth geothermal exploration holes to be located in the western moat of the Long Valley caldera. PLV-1 was drilled through a sequence of moat rhyolite lava flows and tuffs and terminated in hydrothermally altered tuffs of the early rhyolite. PLV-2 was drilled through the moat basalt flows, a telescoped moat rhyolite sequence, and into the early rhyolite flows and tuffs. The temperature profile from PLV-1 shows irregular and isothermal intervals created by downward moving meteoric water through hard fractured lavas to a depth of 1800 feet. Below 1800 feet the temperature gradient abruptly increase in soft impermeable tuffs to 36°F/100'. PLV-2 has an irregular temperature profile which can not be used for extrapolation purposes. PLV-1 offers the first direct evidence that a geothermal reservoir is located to the west of Casa Diablo Hot Springs, however little yet can be said about the location, size, or depth of this reservoir.