Columbia River Basalt Stratigraphy in the Pacific Northwest

Science Center Objects

The Columbia River Basalt Group (CRBG) consists of a thick sequence of Miocene flood basalt that covered northern Oregon, eastern Washington, and western Idaho between 17 and 6 million years ago. It is an important regional aquifer system, and, in its folded and faulted flows, it records the late Cenozoic structural evolution of much of the Pacific Northwest.

This website contains stratigraphic information on the CRBG that is useful in many types of studies, including hydrogeologic, basin evolution, and geologic hazard investigations. A long-term goal is to compile geochemical and paleomagnetic data, and stratigraphic interpretations of the CRBG over its extent in three States.

 

Background

Although CRBG eruptive activity spanned an 11-million-year period from 17 million to 6 million years ago, most of the CRBG flows were emplaced over 2.5 million years, from 17 to 14.5 million years ago. The basalt lava issued from fissures and vents in eastern Washington, northeastern Oregon, and western Idaho. Some eruptions covered thousands of square miles, sending flows hundreds of miles from their source. The Columbia River Basalt Group occurs at land surface and has been shaped by tectonics and erosion to form the scablands of eastern Washington, the rolling hills of north central Oregon, cliffs along the Columbia River Gorge, uplands within the northern Willamette Valley, and headlands along the central and northern Oregon Coast.

Geologic Evolution and Hazards

Because the basalt lavas were typically sheet flows, they provided an ideal regional time horizon to understand paleoenvironments, paleodrainage systems during eruptions, and evolution of fault and fold structures. Location of faults, and timing and direction of fault movement can be inferred by compiling and analyzing the stratigraphy of the CRBG. The pattern of folding and faulting in the CRBG is consistent with contemporary deformation measured by GPS and provides an integrated picture of geologic strain in populated areas. This information is currently being used to map geologic structures that may pose hazards to people and infrastructure in the Pacific Northwest (see http://geomaps.wr.usgs.gov/pacnw/).

Groundwater Resources

Interflow zones, which consist of the top of one basalt flow, the bottom of the overlying flow, and any intervening sediment, if present, generally are permeable where the basalt is vesicular or brecciated. The permeable interflow zones within the Columbia River Basalt Group are an important source of water supply in the Pacific Northwest. The permeability of interflow zones varies because not all interflow zones are vesicular and brecciated. Between interflow zones, the dense flow interiors are relatively impermeable. Conceptually, then, the CRBG is a series of productive aquifers consisting of permeable interflow zones separated by less permeable flow interiors.

Although permeable interflow zones may yield large amounts of water initially, continued large withdrawals result in declines in water levels because of low storage properties and limited recharge of water reaching these productive zones. In order to understand and manage this important, but limited, groundwater resource, CRBG stratigraphy is used to identify interflow zones and map their lateral continuity. Once the interflow zones are mapped, the permeability and hydraulic connection of interflow zones can be determined and informed management options considered. The USGS and others use this information to improve the understanding of groundwater flow in the CRBG and provide information to agencies responsible for managing the water resources in the CRBG.

Further Information about CRBG

USGS Columbia River Interdisciplinary Science Explorer Website

USGS PNW Geologic Mapping and Urban Hazards

USGS Willamette Basin Ground Water Study

Geology of Lowland Aquifers in NW Oregon

USGS Cascades Volcano Observatory: Columbia Plateau, Columbia River Basalt Group

Washington State Department of Natural Resources—Geology

Mantle Plumes’ discussion of possible origins of Columbia River Basalt Group

Oregon State University—Columbia River Flood Basalt Province, Idaho, Washington, Oregon

Columbia River Basalt Province in Digital Geology of Idaho

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. 

This website contains stratigraphic information on the CRBG that is useful in many types of studies, including hydrogeologic, basin evolution, and geologic hazard investigations. A long-term goal is to compile geochemical and paleomagnetic data, and stratigraphic interpretations of the CRBG over its extent in three States.

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.

The Ellensberg Formation includes unidentified sedimentary interbeds within the CRBG. Where the interbed is identified, the interbed name, for example, "Vantage," is used.

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.