Stephen P. Reidel

Emplacement of Columbia River flood basalt

Evidence is examined for the emplacement of the Umatilla, Wilbur Creek, and the Asotin Members of Columbia River Basalt Group. These flows erupted in the eastern part of the Columbia Plateau during the waning phases of volcanism. The Umatilla Member consists of two flows in the Lewiston basin area and southwestern Columbia Plateau. These flows mixed to form one flow in the central Columbia Plateau. The composition of the younger flow is preserved in the center and the composition of the older flow is at the top and bottom. There is a complete gradation between the two. Flows of the Wilbur Creek and Asotin Members erupted individually in the eastern Columbia Plateau and also mixed together in the central Columbia Plateau. Comparison of the emplacement patterns to intraflow structures and textures of the flows suggests that very little time elapsed between eruptions. In addition, the amount of crust that formed on the earlier flows prior to mixing also suggests rapid emplacement. Calculations of volumetric flow rates through constrictions in channels suggest emplacement times of weeks to months under fast laminar flow for all three members. A new model for the emplacement of Columbia River Basalt Group flows is proposed that suggests rapid eruption and emplacement for the main part of the flow and slower emplacement along the margins as the of the flow margin expands.

A lava flow without a source: the Cohassett flow and its compositional components, Sentinel Bluffs Member, Columbia River Basalt Group.

The eruption of Sentinel Bluffs Member lava flows at approximately 15.5 Ma marked the end of Grande Ronde Basalt volcanism, the most voluminous period of the Columbia River Basalt Group. More than 10,000 km3 of lava erupted from northerly trending feeder dikes in eastern Washington and northern Oregon and flowed westward down an ancestral paleoslope covering more than 169,700 km2 of the flood‐basalt province. The Sentinel Bluffs Member consists of six phases or types defined by their compositions. Lava flows having the first compositional type were the most voluminous and reached the Pacific Ocean. The volume of later lava flows having the other compositional types declined with time until the final eruption produced the second‐largest volume of basalt. At the source area, compositional variation in Sentinel Bluffs lava flows is relatively small. This homogeneity allows the individual compositional types and, thus, phases of the eruption to be easily recognized. Farther west, however, compositional heterogeneity increases with several compositional types occurring in individual lava flows. The Cohassett flow has more compositional heterogeneity than any Sentinel Bluffs Member lava flow. It is interpreted to be a local lava flow that formed as one compositional type after another was injected into the first and inflated it to form a lava flow with compositional zoning reflecting the sequence of eruptions. The compositions of the layers remain intact except for mixing along their contacts. Thin vesicular horizons separate compositional layers in the upper part of the lava flow but not in the lower part. A thick vesicular horizon called the interior vesicular zone marks the boundary between the last two compositional types to be injected. Other Sentinel Bluffs flows appear to have formed in a similar way. The compositional types and the field relations are best explained by rapid changes in magma composition feeding and inflating the flows and suggest rapid eruption and emplacement of the lava flows.

The Columbia River flood basalts and the Yakima fold belt

This is describes the field geology of the Columbia River Basalt Group and Yakima Fold Belt in the central Columbia Basin