Geologic Map of Upper Eocene to Holocene Volcanic and Related Rocks in the Cascade Range, Washington

Introduction

Since 1979 the Geothermal Research Program of the U.S. Geological Survey has carried out a multidisciplinary research effort in the Cascade Range. The goal of this research is to understand the geology, tectonics, and hydrology of the Cascades in order to characterize and quantify geothermal resource potential. A major goal of the program is compilation of a comprehensive geologic map of the entire Cascade Range that incorporates modern field studies and that has a unified and internally consistent explanation.

This map is one of a series presently being compiled that shows Cascade Range geology by fitting published and unpublished mapping into a province-wide scheme of lithostratigraphic units; map sheets of the Cascade Range in California and in Oregon complete the series. The complete series forms a guide to exploration and evaluation of the geothermal resources of the Cascade Range and will be useful for studies of volcanic hazards, volcanology, and tectonics.

For geothermal reasons, the maps emphasize Quaternary volcanic rocks. Large igneous-related geothermal systems that have high temperatures are associated with Quaternary volcanic fields, and geothermal potential declines rapidly as age increases (Smith and Shaw, 1975). Most high-grade recoverable geothermal energy is likely to be associated with silicic volcanism less than 1 Ma. Lower grade (= lower temperature) geothermal resources may be associated with somewhat older rocks; however, volcanic rocks older than about 2 Ma are unlikely geothermal targets (Smith and Shaw, 1975).

First posted October 16, 2014