New Compilation Geologic Map of the Cascade Range in Washington

Abstract

As part of its Geothermal Research Program, the U.S. Geological Survey is compiling the geology of the volcanic part of the Cascade Range in California, Oregon and Washington at a scale of 1:500,000. The three maps share the same explanation and emphasize Quaternary history of the range. To avoid problems of nomenclature and to emphasize the relation of rock units to volcanic cents, conventional stratigraphic units were not used. Rather, an interpretive approach was used, based on facies models of arc volcanism. The main criteria for subdivisions are compositions, age, and volcanic facies. The volcanogenic Cascade Range in Washington is divided into two segments by a northeastern trending line which passes north of Mount Rainer. The northern segment is more active seismically, but had a significantly lower rate of volcanic production, than the southern segment during the last m.y. Differences in the character of volcanic activity between segments persist back to at lease late Eocene time. A summary of Quaternary volcanic history materials produced by each stratovolcano are given as examples of the kinds of information that can be obtained from this map.