Columbia River Basalt Stratigraphy in 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.

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

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