Columbia River Basalts
Brittany Hollon & Natalie Gentry
- Identify sheet flows, intracanyon flows, and compound flows, noting differences and similarities
- Classify distinct basaltic flows within the Columbia River Basalt Group stratigraphy
- Identify important structures key to the present geology of the CRBG
- Note contacts between crystalline basement and the much younger CRBG (Waterville, Washington)
Field Trip Stops: Various Days
- Observe the Columbia Hills Anticline and Mosier Syncline from the Rowena Overlook, which has been bisected by the Columbia River and displays clear stratigraphy
- Rattlesnake Mountain, an example of Yakima folding active during flood basalt eruption. – Rattlesnake Hill Gas Field, old gas field with a view of the Rattlesnake Mountain Thrust Fault
- Elephant Member of the CRBG, the last of the large volume basalt lavas (Benton City)
- U.S. 2, Waterville, Washington, the contact of crystalline basement with massive basalt flows
|Figure 1: Map of field trip stops.|
The Columbia River Flood Basalt Group are common over much of Washington, Oregon, and western Idaho, therefore field trip stops are spread out and will be visited over several days of the field trip. Rowena Overlook is conveniently located on exits off I-84 East from Portland. Benton City/Rattlesnake Mountain stops are not far from I-82 and will be visited during the drive from the Blue Mountains to Wenatchee, WA. The last stop is along U.S. 2 between Waterville and Entiat.
Regional Geology of the Columbia River Basalts
The Columbia River Basalt Group (CRBG) is a continental flood basalt province, similar to the Deccan Traps and Siberian Traps. A flood basalt is a “vast accumulation of horizontal flows, erupted from fissures in rapid succession over large areas” (Neuendorf, Mehl, and Jackson, 2005). The CRBG is among the youngest flood basalts known, ranging in age from 17-6 Ma. The CRBG cover more than 200, 000 km2 of area in Oregon, Idaho, and Washington.
|Figure 2: Generalized area of the extent of CRBG. Courtesy USGS.|
The basalts erupted into an intermontane basin, located between Precambrian craton in the east and Mesozoic accreted terranes in the west. These basalts are sheet flows or compound flows, but sheet flows are most common. The thickest part of these basalts are in the central and western-central part of Columbia River Plateau. Here the basalts are locally thicker than 3 km. The total area is over 200, 000 km2, and the total volume 224, 000 km3. Around 96% of the 224,000 km3 volume is thought to have been erupted in < 2.5 Ma span. During this time many of the eruptions emitted 1000 km3 to 5000 km3 in as little as 250, 000 years.
|Figure 3: Two types of lava flow associatted with the Columbia Flood Basalts.|
The Columbia River Plateau can be divided into several major structural sub-provinces, including:
- Yakima Fold Belt– most deformation
- Palouse Slope– least deformed, overlies craton rock
- Blue Mountains– CRBG overlies accreted terrains and volcaniclastic rocks.
Figure 4: Sub-provinces of the Columbia River plateau. Courtesy of Idaho State.
Field Trip Stop Descriptions
Stop 1: Rowena Crest Overlook (UTM 632454 E; 5060199 N)
Stratigraphy and Structure of the Columbia Hills Anticline and Mosier Syncline
The Rowena Crest Overlook (elevation 217 meters) offers views of the Columbia Hills anticline and flood basalt stratigraphy disected by the Columbia River. This water gap was originally incised by the ancestral Snake River before the Columbia River avulsed and channeled through the Walulla Gap. The Missoula Floods during the Pleistocene have carved the present gap, allowing for observation of the CRBG stratigraphy.
Figure 5: General stratigraphy of the CRBG. (Tolan et al., 2009)
The terraced stratigraphy observed is from Ice Age floods that carved out the Columbia River Gorge. Seeing specific CRBG members and structure may be difficult and depends on weather and time of day. On clear days, Mt. Adams can be seen to the north at a bearing of 346°.
Figure 6: Looking east up the Columbia River Gorge. Columbia Hills Anticline supposedly visible, unable to tell due to lighting. Photo by Lance Wilson. Figure 7: West towards Portland, terraced stratigraphy indicative of Missoula Floods. Gravel dunes near water’s edge also attributed to Missoula Floods. Photo by Lance Wilson. Figure 8: The hairpin curve road to Rowena Crest Overlook cuts through much of the CRB stratigraphy. Photo by Lance Wilson. Figure 9: Mt. Adams is visible to the north from Rowena Crest Overlook on clear days. Photo by Rachael Rutter.
Stop 2: Benton City, Whan Road and Old Inland Empire Road, Union Pacific Railroad grade (UTM 306138.99 E; 512779.13 N)
Elephant Mountain Member of the Columbia River Basalt Group
The Elephant Mountain basalt is one of the youngest major lava outpourings. It flowed down the ancestral Snake River into the Pasco Basin. The member consists of the older Yakima Bluffs flow and the younger Wards Gap flow. The railroad grade provides an excellent outcrop of both flows. To the east of the trail leading to the grade is a massive Wards Gap flow colonnade overlying the tope-flow breccias of the Yakima Bluffs flow. Further east the breccias become more vessiculated. In some places along the base of the Ward’s Gap flow are vesicle pipes that are bent in the flow’s westward direction.
Stop 3: Crest of Rattlesnake Mountain, Rattlesnake Mountain Road (UTM 299352.90 E; 5142416.64 N)
The drive up Rattlesnake Mountain is mostly on the Pomona Member of the CRBG, with the Ice Harbor Member and Elephant Mountain Member seen on the other side of the ridge. However, some Ice Harbor Member be seen before reaching the Elephant Mountain member. The Elephant Mountain member covered the mountain during eruption.
Rattlesnake Mountain exemplifies anticlines seen in the Yakima fold belt, which was actively forming as the last of the flood basalts were erupting. To the north the Palouse sub-province’s gentle dips contrasts with dips in the Yakima fold belt. The contact of the Palouse and the Yakima also denotes where western terranes were accreted on to the North American craton. Rattlesnake Mountain, a doubly plunging anticline, is prototypical of the Yakima fold belt because of a steeply dipping north nose and a gently dipping south nose. On clear days, Mount Stuart and Glacier Peak can be seen to the northwest.
Stop 4: U.S. 2, between Entiat, WA and Waterville, WA
Contact Between Crystalline Basement and Columbia Flood Basalts
Along U.S. 2, west of Waterville, one can observe the contact between the much older crystalline basement of the Northern Cascade Range and the Columbia Flood Basalts.
Neuendorf, K., Mehl, J. J., & Jackson, J. (2005). Glossary of Geology. Alexandria, VA: American Geological Institute .
Reidel, S. P., Barton, M. S., & Petcovic, H. L. (2003). The Columbia River flood basalts and the Yakima fold belt. In T. Swanson, Western Cordillera and adjacent areas (pp. 87-105). Boulder: Geological Society of America.
Tolan, T., Martin, B., Reidel, S., Anderson, J., Lindsey, K., & Burt, W. (2009). An introduction to the stratigraphy, structural geology, and hydrogeology of the Columbia River Flood Basalt Province: A primer for the GSA Columbia River Basalt Group Field Trip. The Geological Society of America Field Guide , 599-643.
Tolan, T., Martin, B., Reidel, S., Kauffman, J., Garwood, D. L., & Anderson, J. (2009). Stratigraphy and tectonics of the central and eastern portions of the Columbia River Flood Basalt Province: An Overview of our current state of knowledge. The Geological Society of America Field Guide 15 , 645-672.