Geologic map of upper Eocene to Holocene volcanic and related rocks of the Cascade Range, Oregon

Abstract

Since 1979, Earth scientists of the Geothermal Research Program of the U.S. Geological Survey have carried out multidisciplinary research in the Cascade Range. The goal of this research is to understand the geology, tectonics, and hydrology of the Cas­ cades 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 con­ sistent explanation. This map is one of three in a series that shows Cascade Range geology by f i t t ing publ ished and unpublished mapping into a province-wide scheme of rock units distinguished by composition and age; map sheets of the Cascade Range in Washington (Smith, 1993) and California will complete the series. The complete series forms a guide to exploration and evaluation of the geothermal resources of the Cas­ cade Range and will be useful for studies of volcano 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 f ie lds, and geothermal potential declines rapidly as age increases (Smith and Shaw, 1975). Most high-grade recov­ erable geothermal energy is likely to be associated with silicic volcanic systems active in the past 1 million years. Lower grade (= lower temperature) geothermal resources may be associated with somewhat older rocks; however, volcanic rocks emplaced prior to 2 million years ago are unlikely geothermal targets (Smith and Shaw, 1975). Rocks older than a few million years are included on the maps because they help to unravel geologic puzzles of the present-day Cascade Range. The deeply eroded older volcanoes found in the Western Cas­ cades physiographic subprovince1 are analogues of today’s snow-covered shield and composite volcanoes. The fossil hydrothermal systems in the roots of Eocene to Pliocene vents, now exposed, provide clues to processes active today beneath the Pleistocene and Holocene volcanic peaks along the present-day crest of the Cascade Range. Study of these older rocks aids in developing models of geothermal systems. These rocks also give insight into the origins of volcanic-hosted mineral deposits and even to potential volcanic hazards. Historically, the regional geology of the Cascade Range in Oregon has been interpreted through recon­ naissance studies of large areas (for example, Diller, 1898; Williams, 1916; Callaghan and Buddington, 1938; Williams, 1942, 1957; Peck and others, 1964). Ear ly s tudies were hampered by l imited access, generally poor exposures, and thick forest cover, which flourishes in the 100 to 250 cm of annual precipi­ tation west of the range crest. In addition, age control was scant and limited chiefly to fossil flora. Since then, access has greatly improved via a well-developed network of logging roads, and isotopic ages— mostly potassium-argon (K-Ar)—have gradually solved some major problems concerning timing of volcan­ ism and age of mapped units. Nevertheless, prior to 1980, large parts of the Cascade Range remained unmapped by modern studies. Geologic knowledge of the Cascade Range has grown rapidly in the last few years. Luedke and Smith (1981, 1982) estimated that, when their maps were made, more than 60 percent of the Cascade Range lacked adequate geologic, geochemical, or geochro­ nologic data for a reliable map at 1:1,000,000 scale. Today only about 20 percent of the Cascade Range is too poorly known to show reliably at the larger 1:500,000 scale of this map. In Oregon the poorly known areas include Oligocene and Miocene rocks in the Western Cascades physiographic subprovince and parts of the Columbia River Gorge. The present series of maps of the Cascade Range is not merely a reworking of previously published data. Geologic interpretations shown here are based largely on newly published and unpublished geologic maps and isotopic age determinations, including our own, done since 1980. To assign all map units their correct composition and age, we also reevaluated older published maps and incorporated recently determined chemical analyses and isotopic ages.