Columbia River Basalt Stratigraphy in the Pacific Northwest

In Cooperation with the Oregon Water Resources Department

Exposure of the Winter Water member of the Columbia River Basalt Group in the Umatilla Basin, Oregon (photograph by Terrence Conlon)

Stratigraphy

The CRBG has been divided into six formations: Imnaha Basalt, Picture Gorge Basalt, Prineville Basalt, Grande Ronde Basalt, Wanapum Basalt, and Saddle Mountains Basalt by Swanson and others (1979). These formations are divided into members and further subdivided into flow units based on field mapping, well logs, aeromagnetic surveys, geochemistry, and magnetic polarity.

Flows belonging to the Imnaha Basalt, the oldest known in the Columbia River Basalt Group, are found in western Idaho and eastern Washington and Oregon. The Picture Gorge and Prineville Basalt formations are limited to areas in central Oregon defining the southern extent of CRBG. The Grande Ronde Basalt comprises about 80% of the CRBG by volume and covers most of the area where the CRBG is found. Flows of the less voluminous, but widely distributed Wanapum Basalt commonly overlie the Grande Ronde Basalt in most areas. Few flows of the Saddle Mountains Basalt are as widely distributed, but one flow, the Pomona Member, did reach the Pacific Ocean, and it crops out along the lower Columbia River. The number, extent, and thickness of flows vary depending on many factors, including proximity to and volume of eruption, lava viscosity, cooling process, erosion, and topography over which the lava flowed.

Notes on geologic logs of the Columbia River Basalt Group:

Field mapping, subsurface well logs and samples, aeromagnetic surveys, and paleomagnetic and geochemical studies are used to identify and assign individual flows to formations, members, and flow units in the Columbia River Basalt Group. With detailed study and mapping of the CRBG, revisions are made in the classification of individual basalt flows.

The naming convention used in this website is based on work by Swanson and others (1979a) with revisions by subsequent investigators. The diagram of formations, members, and units represents the current stratigraphic nomenclature of the CRBG (Reidel and others, 2002). The naming classification shown provides a framework to identify and group individual basalt flows. It contains informal and formal geologic names.

Vantage interbed, Umatilla Basin, Oregon (photograph by Terrence Conlon)

The Ellensberg Formation includes unidentified sedimentary interbeds within the CRBG. Where the interbed is identified, the interbed name, for example, "Vantage," is used. (See photo at right.)

Within the detailed subdivisions of the CRBG, multiple flows may occur. Individual flows are numbered consecutively within the subdivision starting with the uppermost flow in a series of flows. The numbering sequence is unique to each site and is not intended for correlation of individual flows between sites.

The geologic logs and geochemical tables are presented as received from the geologists who interpreted the geologic data. The geologists assigned the geologic names and elevation of formations. In some cases, the elevation of the top and bottom of geologic units is referenced to sea level and in other cases is referenced to land surface.

References

Reidel, S.P., Johnson, V.G., and Spane, F.A., 2002, Natural gas storage in basalt aquifers of the Columbia Basin, Pacific Northwest USA--A guide to site characterization: Richland, Washington, Pacific Northwest National Laboratory.

Swanson, D.A., Wright, T.L., Hooper, P.R., and Bentley, R.D., 1979, Revision in the stratigraphic nomenclature of the Columbia River Basalt Group: U.S. Geological Survey Bulletin 1457-G, 59 p.