Geological Survey Circular 838 Guides to Some Volcanic Terrances in Washington, Idaho, Oregon, and Northern California

D.A. Swanson, U.S. Geological Survey, Menlo Park, California 94025
and
T.L. Wright, U.S. Geological Survey, Reston, Virginia 22092

The Columbia River Basalt Group comprises a tholeiitic flood-basalt province of moderate size (fig. 1), covering an area of about 2 x 10⁵ km² with an estimated volume of about 2 x 10⁵ km³ (Waters, 1962). The group is the youngest assemblage of flood basalt known, with an age range from about 17 to 6 m.y. ago; most eruptions took place between about 17 and 14 m.y. ago (Watkins and Baksi, 1974; McKee and others, 1977). It is the only flood basalt province of Phanerozoic age in North America.

Figure 1. Index map showing approximate outcrop extent of the Columbia River Basalt Group (from Waters, 1961) and locations mentioned in text. See figure 7 for details in southeast Washington and northeast Oregon. (click on image for an enlargement in a new window)

Wide-ranging regional studies of the basalt, underway for the last 10 years, have been devoted primarily to defining stratigraphic and chemical relations for use in unraveling the history of the province and comparing the Columbia River Basalt Group with flood basalt elsewhere. Recently, these studies have been accelerated because of the need to know more about the stratigraphy and structure of the basalt as related to potential storage of nuclear waste within the basalt pile. A geologic map of the entire province is under preparation. Reconnaissance geologic maps of the basalt in most of Washington and northern Idaho have been completed (Swanson and others, 1979a) and eventually will be published in color (for example, Swanson and others, 1980). Studies in 1978 and 1979 were conducted by full-time and temporary personnel of the U.S. Geological Survey under Interagency Agreement No. EW-78-I-06-1978 with the U.S. Department of Energy in support of the Basalt Waste Isolation Program, administered by Rockwell Hanford Operations, Richland, Washington. Personnel involved in this mapping project have been: J.L. Anderson, R.D. Bentley, G.R. Byerly, V.E. Camp, J.N. Gardner, P.R. Hooper, D.A. Swanson, W.H. Taubeneck and T.L. Wright.

GENERAL ASPECTS

The Columbia River Basalt Group is characterized by moat features considered typical of flood-basalt provinces. Flows are voluminous, typically 10-30 km³ with a maximum volume of 700 km³, and many cover large areas, as much as 40,000 km². They generally advanced as sheetfloods, rather than as channelized or tube-fed flows, and form thick cooling units composed of one or more flows. Eruptions took place from fissure systems tens of kilometers long. Eruption rates were high, generally greater by 2-3 orders of magnitude than those of other Cenozoic basalt provinces, as determined by 1) theoretical considerations based on the relations among eruption volumes, dimensions of linear vent systems, and distance traveled without appreciable crystallization or cooling breaks, and 2) the absence of constructional shields even though viscosities calculated at constant temperature from dry-weight chemical analyses are equal to or higher than those for oceanic tholeiites containing similar amounts of MgO. Small spatter ramparts formed along fissures but are poorly preserved owing to bulldozing and rafting by flows and to later erosion. Cinder cones are very rare. In these and other features, basalt of the Columbia River Basalt Group contrasts with that produced by basaltic plains or oceanic volcanism (Greeley, 1977), as in the Snake River Plain, Iceland, and Hawaii.

The flows cover a diverse assemblage of rocks ranging in age from Precambrian to Miocene. The pre-basalt topography had considerable local relief, 1000 m or more in places, near the margin of the Columbia Plateau. Some of the prebasalt hills today stand high above the plateau surface, especially in the Spokane area. Little is known about the nature of the rocks or pre-basalt surface beneath the central part of the plateau. Sparse evidence from a drill hole 3.24 km-deep just west of the Pasco Basin suggests that a thick weathered or altered zone caps a sequence of lower Tertiary volcanic rocks of mafic and intermediate composition at least 1970 m thick (Raymond and Tillson, 1968; Newman, 1970; Jackson, 1975; Swanson and others, 1979b).