This map shows the geology of the central and eastern parts of the Cascade Range in northern Oregon. The Quaternary andesitic stratovolcano of Mount Hood dominates the northwest quarter of the quadrangle, but nearly the entire area is underlain by arc-related volcanic and volcaniclastic rocks of the Cascade Range. Most stratigraphic units were emplaced since middle Miocene time, and all are Oligocene or younger. Despite the proximity of the map area to the Portland metropolitan area, large parts remained virtually unstudied or known only from limited reconnaissance until the late 1970s. A notable exception is the area surrounding Mount Hood, where mapping and chemical analyses by Wise (1969) provided a framework for geologic interpretation.
Mapping since 1975 was conducted first to understand the stratigraphy and structure of the Columbia River Basalt Group (Anderson, 1978; Vogt, 1981; J.L. Anderson, in Swanson and others, 1981; Vandiver-Powell, 1978; Burck, 1986) and later to examine the geothermal potential of Mount Hood (Priest and others, 1982). Additional mapping was completed in 1985 for a geologic map of the Cascade Range in Oregon (Sherrod and Smith, 1989). From 1987 to 1990, detailed mapping was conducted in three 15-minute quadrangles on a limited basis (D.R. Sherrod, unpublished mapping) (see fig. 1 for index to mapping). An ongoing volcanic hazards study of Mount Hood by the U.S. Geological Survey (Scott and others, 1994) has provided the catalyst for completing the geologic map of the Mount Hood 30-minute by 60-minute quadrangle.
As of June 1994, only two broad areas still remain largely unmapped. One of these areas, labeled 'unmapped' on the geologic map, lies in the Salmon River valley south of Zigzag along the west margin of the quadrangle. Although strata of the Columbia River Basalt Group in the Salmon River valley were mapped in detail by Burck (1986), the overlying middle and upper(?) Miocene lava flows, volcaniclastic strata, and intrusions have never been studied. The other poorly known area, the Mutton Mountains in the southeastern part of the map area, consists of Oligocene and lower Miocene volcanic and volcaniclastic rocks. Overlying lava flows of the Columbia River Basalt Group were mapped in some detail by Anderson (in Swanson and others, 1981).