PRELIMINARY 3-D GEOLOGIC MAP OF THE SCHOLLS 7.5' QUADRANGLE, OREGON

In an effort to characterize the subsurface geology of the Vancouver-Portland-Salem urban corridor, we used driller's logs from various sources and surface geologic mapping to produce a preliminary 3-D geologic map of the Scholls 7.5' quadrangle, located within the Tualatin basin between the Gales Creek-Mt. Angel and the Canby-Beaverton fault zones. We examined more than 100 drillers logs and input lithologic picks into a database. Although logs are of variable quality, it is possible to identify the contact between the Columbia River Basalt Group (CRBG) and overlying Neogene sediments. Almost all the wells that penetrated the CRBG indicate thick (as much as 30 m) red to brown clay, or paleosol, which we interpret as the weathered top surface of the CRBG, providing an error estimate in identifying this contact throughout the quadrangle. The top of the CRBG varies in elevation from 220 m above sea level on Cooper Mountain to nearly 300 m below sea level, and coincides with residual gravity anomalies. The contact appears to be folded and faulted along the E-W-striking Beaverton fault zone. South of the fault, the CRBG is exposed at the surface, where it forms the en echelon, E-W-striking Cooper and Bull Mountain anticlines. Across the Beaverton fault on the north side of Cooper Mountain, the CRBG is at 270 m below sea level. South of Cooper and Bull Mountain, the CRBG is folded into a narrow E-W syncline with a gently-dipping south limb formed by the Chehalem Mountains. The syncline has a pronounced 4-km northwestward jog that coincides with the course of Beef Bend in the Tualatin River, where the syncline may be offset by a northwest-trending dextral fault. The post-CRBG sedimentary section is dominated by fine grain size. Water, if present and noted, corresponds to intervals of sand and rare gravel, which are generally thin and not areally extensive. Individual flows within the CRBG have been identified in a few wells by Beeson and Tolan based on geochemistry of the cuttings. Comparison of lithologic descriptions in original logs with the geologic interpretations based on geochemistry and texture suggests that flow interiors might be distinguished from more permeable flow tops and bottoms. Correlation of flows, however, is hindered by uncertainty in well locations and interpretation of lithologic picks in the logs.