SURFICIAL GEOLOGIC MAPPING OF THE PORTLAND METROPOLITAN AREA, OREGON

Recent, natural events affecting numerous areas of the globe (e.g. the Tohoku, Japan earthquake and tsunami, the Haiti earthquake, or the eruptions of Iceland’s volcanoes) serve as a reminder of the potential for similar events occurring in Oregon, and around the Pacific Northwest. Bearing this in mind, a re-assessment of the geology was required to gain a better understanding of local or regional earthquake hazards, such as ground motion, amplification or liquefaction. Comprehensive geologic data is necessary to promote hazard mitigation and risk reduction. Other issues such as landslide hazards, water quality, or urban planning studies can also be addressed from improved mapping.

New, detailed surficial mapping encompassing the ever-growing Portland metropolitan area is in progress at the Oregon Department of Geology and Mineral Industries (DOGAMI) . This mapping will be produced using digitally compiled information from previous geologic mapping, interpretation of high-resolution lidar data, borehole data, and well log data (courtesy of the Oregon Water Resources Department), the National Resources Conservation Service's soil surveys, and some field data collection. Quaternary deposits resulting from landslides or mass-wasting, fluvial (streams and river) processes, the cataclysmic Missoula floods and man-made disturbances dominate the urban landscape. Outcrops of underlying lavas from the Miocene Columbia River Basalt Group and Pliocene-Pleistocene Boring volcanics can be found at the surface, producing relief of the higher elevations in the fore-arc basin where Portland is situated.

All or parts of twenty 7.5’ quadrangles of new mapping will be compiled and published as a surficial geologic map and assembled into an ESRI ArcGIS geodatabase by DOGAMI. Additionally, the collection of subsurface geologic data at DOGAMI will be compiled into a single geodatabase format, which should provide a base for 3D modeling of geotechnical parameters in the Portland area. The surficial mapping was also carried out in conjunction with the USGS’ Pacific Northwest Urban Corridor Geologic Mapping Project to produce a larger regional-scale geologic map of the Portland and Tualatin Basins in northwest Oregon and part of southwest Washington.