GEOLOGIC MAPS OF THE GALES CREEK FAULT ZONE AND WESTERN TUALATIN BASIN, NORTHWEST OREGON

New USGS geologic mapping provides a framework for hazard assessments in NW Oregon. Of 56 7.5’ quadrangles underway, 16 maps cover the Gales Creek fault and extend into the central Tualatin basin. We present a compilation of these maps and focus particularly on the Carlton and Buxton quads, which both lie on the east flank of the Coast Range. Mapping status is at: http://geomaps.wr.usgs.gov/pacnw/new.html

The Carlton quad is about 60 km southwest of Portland. The area is largely underlain by Eocene marine strata tilted northeast by uplift of the Coast Range. Upper middle Eocene Yamhill Formation siltstone and sandstone are intruded by Eocene basalt and diabase sills. The Yamhill is in turn overlain by shallow marine, micaceous sandstone of the upper Eocene Spencer Formation, and tuffaceous siltstone of the upper Eocene Keasey Formation. Pleistocene Missoula Flood deposits of sand, silt, and clay blanket surfaces at elevations below 120 m. Three folding episodes are recognized: 1, NNE-trending post-Yamhill and pre-Spencer; 2, WNW-trending post-Spencer and pre-Columbia River Basalt Group (CRBG), and 3, WNW-trending post-CRBG. WNW-trending faults parallel the later two groups of folds and may act as Riedel shears, accommodating clockwise rotation of Coast Range rocks. The Buxton quad is about 40 km west of Portland. Paleogene marine strata of upper Eocene Keasey and Pittsburg Bluff Formations, and Oligocene Scappoose Formation are tilted SE into the Tualatin basin by uplift of the Coast Range. The lower to middle Miocene CRBG unconformably overlies the Paleogene strata, dips more gently eastward into the Tualatin Basin and is in turn unconformably overlain in the area by Plio-Pleistocene fluvial strata of the Hillsboro Formation. This Eocene-Oligocene marine section is what likely underlies CRBG in the nearly 5 km deep Tualatin Basin. A remnant of the oldest CRBG flows in the Willamette Valley are exposed on Wildcat Mountain, where 3 N1 normal polarity flows provisionally correlated with Downey Gulch member of the Grande Ronde underlie reversely polarized Wapshilla Ridge member.