Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 40-15
Presentation Time: 9:00 AM-3:30 PM


WELLS, Ray E.1, HAUGERUD, Ralph A.2, NIEM, Alan3, NIEM, Wendy A.4, EVARTS, Russell C.5, O'CONNOR, Jim E.1, MA, Lina6, MADIN, Ian P.6, SHERROD, David R.7, BEESON, Marvin H.8, WHEELER, Karen L.9, HANSON, William B.10 and SAWLAN, Michael G.11, (1)U.S. Geological Survey, 2130 SW 5th Ave., Portland, OR 97201, (2)U.S. Geological Survey, c/o Dept Earth and Space Sciences, University of Washington, Box 351310, Seattle, OR 98195, (3)Department of Geosciences (Prof. emeritus), Oregon State University, Corvallis, OR 97331, (4)Pacific Northwest Geology, LLC, 6325 B Avenue, Otter Rock, OR 97369, (5)U.S. Geological Survey, Menlo Park, CA 94025, (6)Oregon Department of Geology and Mineral Industries, 800 NE Oregon St. #28 Suite 965, Portland, OR 97232, (7)Cascades Volcano Observatory, U.S. Geological Survey, Vancouver, WA 98683, (8)Department of Geology, Portland State University, PO Box 751, Portland, OR 97201, (9)U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, (10)57334 Timber Road, Vernonia, OR 97064, (11)U.S. Geological Survey, 345 Middlefield Rd MS-973, Menlo Park, CA 94025

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in an ArcGIS geodatabase (USGS GeMS; Geologic Map Schema) at 1:24,000 scale, including 18 quadrangles of previously unpublished mapping.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.