Geologic map of the Spirit Lake West Quadrangle, Skamania and Cowlitz counties, Washington.

Abstract

The Spirit Lake West Quadrangle, located about 15 km north of Mount St. Helens, is underlain by Oligocene volcanic and volcaniclastic rocks locally veneered by Pleistocene glacial deposits and tephra derived from Mount St. Helens. The Tertiary rocks are dominantly basalt and basaltic andesite flows and associated fragmental rocks deposited in a subaerial environment. They have been gently tilted to the east and intruded by granite rock of the 21-Ma Spirit Lake Pluton. The volcanic rocks locally exhibit hydrothermal alteration to clay-carbonate assemblages; most of the altered areas appear to be closely associated with Tertiary faults.

27 samples from this reference