Columbia River Basalt Stratigraphy in the Pacific Northwest

In Cooperation with the Oregon Water Resources Department

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

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 (Swanson and others, 1979; Tolan and others, 1989). 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.

Over 300 flows have been identified, and at least 20 made their way through the Willamette Valley to the coast (Tolan and others, 1989). Individual flows range in thickness from 10 to more than 300 feet, and total thickness of the series of flows may be as great as 15,000 feet (Reidel and others, 1989a). Typically, lava was erupted rapidly and advanced away from the fissure or vent as a single, uniform sheet of lava. The top and bottom of individual flows may be vesicular and brecciated. When the hiatus between flows was sufficiently long, soil developed or sediments were deposited on the surface of a flow. If these sediments were preserved, a sedimentary interbed occurs between flows. The flow top, flow bottom, and an intervening sediment is called an interflow zone. Between interflow zones, flow interiors typically consist of dense, nonvesicular, glassy to crystalline basalt.

The CRBG 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.

Recognizing the stratigraphy of the CRBG is key to understanding deformation rates and internal structure of the aquifer. Although monotonous at first glance, flow packages, and in some cases individual flows, can be distinguished using subtle variations in appearance, chemical composition, and paleomagnetic direction. Field mapping, subsurface well logs and samples, aeromagnetic surveys, paleomagnetic and geochemical studies, and well testing are used to document individual flows, their distribution, deformation, and permeability.

References

Reidel, S.P., Fecht, K.R., Hagood, M.C., and Tolan, T.L., 1989, The geologic evolution of the central Columbia Plateau, in Reidel, S.P., and Hooper, P.R., eds., Volcanism and tectonism in the Columbia River flood-basalt province: Geological Society of America Special Paper 239, p. 247-264.

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.

Tolan, T.L., Reidel, S.P., Beeson, M.H., Anderson, J.L., Fecht, K.R., and Swanson, D.A., 1989, Revisions to the estimates of the areal extent and volume of the Columbia River Basalt Group, in  Reidel, S.P., and Hooper, P.R., eds., Volcanism and tectonism in the Columbia River flood-basalt province: Geological Society of America Special Paper 239, p. 1-20.