The Columbia River Basalt Group in western Oregon; geologic structures and other factors that controlled flow emplacement patterns
The Columbia River Basalt Group in western Oregon; geologic structures and other factors that controlled flow emplacement patterns (in Volcanism and tectonism in the Columbia River flood-basalt province, Stephen P. Reidel (editor) and Peter R. Hooper (editor))
Special Paper - Geological Society of America (1989) 239: 223-246
- basalts
- Cascade Range
- Cenozoic
- chemical composition
- Coast Ranges
- Columbia River Basalt Group
- controls
- emplacement
- erosion
- evolution
- Grande Ronde Basalt
- igneous rocks
- lava flows
- Miocene
- Neogene
- Oregon
- paleomagnetism
- petrology
- physical properties
- Tertiary
- topography
- United States
- volcanic rocks
- Willamette Valley
- Yakima fold belt
- western Oregon
- Grouse Creek
- Downey Gulch
- Sand Hollow
- China Creek
- Bull Run Fault
- Portland Basin
- Sentinel Butte
- Silver Falls
- Portland Hills-Clackamas River Zone
- Sherwood Trough
- Untanum
- Bull Run Anticline
- Portland Hills Anticline
- Winter Water
- Slack Canyon
Topography and ground conditions were important factors in controlling the distribution of individual Columbia River Basalt Group (CRBG) flows in western Oregon. The Columbia trans-arc lowland, the Yakima fold belt, the Portland Hills-Clackamas River structural zone, and Cascadian volcanism largely controlled the distribution of CRBG flows across the Miocene Cascade Range. The first flows to cross the Miocene Cascades into the Willamette Valley encroached onto a low-relief topography generally consisting of eroded Tertiary-age marine sedimentary rocks deformed along northwest-trending structural zones, volcanic highs, and estuaries. No north-south trough affected the distribution and thickness of the CRBG in the Willamette Valley, but an incipient Coast Range acted as a leaky barrier to the Oregon coast. Water-saturated sediments rapidly extracted heat from advancing CRBG lava flows, producing narrow, abnormally thick lobes extending along existing topographic lows. Deformation along the northwest-trending Portland Hills-Clackamas River structural zone produced a major topographic barrier early and late in the incursion of CRBG flows. The CRBG thins across this zone from 600 to 150 m. This zone diverted the earliest Grande Ronde flows into and through the Portland Basin. Some of the succeeding R (sub 2) and N (sub 2) Grande Ronde flows were able to cross this zone and followed another structural low, the Sherwood trough, to the Oregon coast. The total thickness of CRBG along the Sherwood trough is approximately 300 m, about twice that on either side. Paleodrainage developed during time intervals between emplacement of CRBG flows. The positions of these drainage courses were influenced by the position of the CRBG flow margins and/or structural lows. A longer hiatus between flows (> 100,000 yr) enabled rivers to develop major canyons by headward erosion, which served to channelize subsequent CRBG flows.
