GEOLOGIC MAP OF THE GREATER PORTLAND AREA, OREGON AND WASHINGTON

We present a new 1:63,360-scale geologic map of 51 7.5’ quadrangles encompassing the Portland-Vancouver metro area and its tectonically-active setting between the Coast Range and the Cascade volcanic arc. The map synthesizes complex geology that comprises Paleogene-Miocene strata of the Coast Range and their Siletzia basement (33 units); somewhat younger rocks of the western Cascade arc (19 units); regionally extensive Miocene Columbia River Basalt Group and associated rocks (21 units); complex late Cenozoic fill of the Portland, Tualatin, and northern Willamette basins, including the Pliocene-Pleistocene Boring volcanic field (15 units); and 21 unconsolidated units, including extensive Missoula flood deposits. Structure of this part of the Cascade fore-arc is dominated by the large, NW-trending, Gales Creek and Portland Hills dextral-oblique fault systems that appear to have driven basin subsidence and now drive local exhumation.

All source materials were in digital form. About 35% of the geology is previously unpublished and locally we have significantly reinterpreted published geology. We compiled the map in ArcGIS using the NCGMP09 data structure. Despite the availability of digital sources, we have redrawn extensively to resolve gaps and overlaps between quadrangles defined in NAD27 and NAD83, resolve map-boundary discontinuities, update decades-old published mapping, remove too-small polygons, generalize intricate Quaternary geology, and fix blunders. All line and point features are associated with a data source. We developed scripts to detect automation errors by testing the geologic validity of contact and fault topology, remove small island polygons, assist in the scale-dependent display of orientation data, and position dip annotation.

The map substantially improves our framework for studying earthquake hazards and groundwater and energy resources in this major metropolitan area. It aids our comprehension of terroir for some of the best vineyards in America and presents a modern portrayal of the southern Mist gas field. We hope our portrayal of incompletely understood stratigraphy and structure of the Portland Hills inspires further geologic study.