PROGRESS IN GEOLOGIC MAPPING OF THE PORTLAND BASIN, OREGON AND WASHINGTON

The rapidly growing Portland-Vancouver metropolitan area lies within the Portland Basin, a topographic and structural depression between the Oregon-Washington Coast Range and the Cascade Range. The basin has a complex but poorly understood history and is filled with Neogene sedimentary and volcanic deposits. Shallow crustal seismicity characterizes the basin but most earthquakes cannot be attributed to known faults. The Pacific Northwest Urban Corridor project of the U.S. Geological Survey, in cooperation with the Oregon Department of Geology and Mineral Industries and the Washington Division of Earth Resources, has undertaken a mapping program to develop a better understanding of the structure of the Portland Basin in order to improve seismic risk assessments. Fifteen 7.5-minute quadrangles are being mapped in the northern half of the basin; fieldwork is completed in 10. The mapping is supported by and integrated with potential-field geophysics, paleomagnetic investigations, geochemical analyses, and radiometric dating. The maps are compiled as geodatabases, allowing for ready merging of individual maps at any desired scale and for comparison with a wide array of georeferenced datasets.

The new work indicates that northward translation of the Coast Range relative to the Cascade Range is accommodated by a major right-lateral fault zone along the Columbia River north of the Portland Basin. At the north end of the basin this structure splays into several northwest-striking faults that exhibit oblique-reverse slip, and the zone of deformation spreads eastward into the Cascades.

The new mapping also addresses other environmental issues. Landslides are widespread in weakly consolidated fluvial and glacial sediments, and identification of landslide-prone areas is of increasing importance as development expands into adjacent foothills. Basin fill stratigraphy is an essential input for evaluating groundwater resources, especially in Washington where virtually all municipal water supplies derive from local aquifers. Volcanic hazards are manifested not only by the stratovolcanoes of Mount St. Helens and Mount Hood to the east but also by Quaternary monogenetic vents within the basin itself; spatial and temporal patterns suggest the locus of intrabasin volcanism is migrating slowly northwestward.