Database for the Geologic Map of Three Sisters Volcanic Cluster, Cascade Range, Oregon

A database of geologic map of Three Sisters Volcanic Cluster as described in the original abstract: The geologic map represents part of a late Quaternary volcanic field within which scores of eruptions have taken place over the last 50,000 years, some as recently as ~1,500 years ago. No rocks of early Pleistocene (or greater) age crop out within the map area, although volcanic and derivative sedimentary rocks of Miocene and Pliocene age are widespread to the east and west and are certainly buried beneath the younger volcanic field. Of the 145 volcanic map units described herein, only 22 are certainly older than late Pleistocene (>126 ka), and 12 are postglacial (<15 ka). The oldest unit identified yields an age of 532+/-7 ka, and the second oldest, 374+/-6 ka. Compositionally, 10 percent of the units are true basalt; 36 percent, basaltic andesite; 20 percent, andesite; 21.5 percent, dacite; and only 12.5 percent, rhyodacite or rhyolite. Most of the 145 volcanic map units described herein are newly defined, although equivalents of several were described by Taylor, 1978, 1987; Scott, 1987; and Scott and Gardner, 1992. Each is an eruptive unit derived from a single vent or fissure. Some are simple flow units, but many are shields, cones, or stacks of several lava flows that have chemical and mineralogical coherence. Each unit was delineated by field mapping on foot and its integrity confirmed, challenged, or revised by chemical and microscopic work in the laboratory. Definition of a few units required iterative acquisition of field and lab data over a period of years, providing a firm basis for subdividing, lumping, or correlating slightly heterogeneous sequences of lavas. Most units have narrow compositional ranges, but some show zoning or heterogeneity spanning ranges of a few percent SiO2. Orignial map citation: Hildreth, Wes, Fierstein, Judy, and Calvert, Andrew T., 2012, Geologic map of Three Sisters volcanic cluster, Cascade Range, Oregon: U.S. Geological Survey Scientific Investigations Map 3186, pamphlet 107 p., 2 sheets, scale 1:24,000. (Available at https://pubs.usgs.gov/sim/3186/.)