GEOLOGIC MAP OF THE GREATER PORTLAND METROPOLITAN AREA AND SURROUNDING REGIONS, OREGON AND WASHINGTON

We present a new geologic map of the greater Portland-Vancouver metropolitan area, which occupies the tectonically active lowland between the Coast Range and Cascade volcanic arc. The map synthesizes the geology of 51 7.5’ quadrangles, mapped at 1:24,000 scale in a cooperative effort among the USGS, Oregon Department of Geology and Mineral Industries, and Washington Geological Survey since 1989. The map covers Paleogene marine strata and accreted basalt basement of the Coast Range; younger rocks of the western Cascade arc; flood basalt of the Miocene Columbia River Basalt Group (CRBG); post-CRBG fill of the Portland, Tualatin, and northern Willamette basins; and the Quaternary Boring volcanic field. Missoula flood deposits and mega-landslide complexes are widespread. We compiled the map from digital sources in an ArcGIS geodatabase (USGS GeMS; Geologic Map Schema) at 1:24,000 scale, including 18 quadrangles of previously unpublished mapping. We resolved gaps and overlaps, fixed mapping errors, and removed tiny polygons to produce a 1:63,360-scale map. The structure of the map area is dominated by the Holocene-active, NW-trending Gales Creek and Quaternary-active Portland Hills dextral-oblique fault systems that accommodate northward motion of the Cascadia fore-arc. The 60-km-long Gales Creek Fault, about 35 km west of Portland, forms the boundary between the Coast Range and the Tualatin basin, which is at least 5 km deep based on companion geophysical surveys. The Portland Hills fault system bounds the NW-striking Tualatin Mountains uplift that separates the Portland Basin from the Tualatin basin. Continued subsidence of the Tualatin and Portland basins is suggested by Columbia River Basalt at 300-400 m below sea level in the basins. This map provides a framework for an improved understanding of a variety of earth science issues; including earthquake hazards from crustal faulting and strong ground shaking, aquifer storage and recovery systems in the CRBG, natural gas storage at the Mist gas field, Columbia River ecosystems, and the terroir of well-known American Viticultural Areas in northwest Oregon.