Geologic Framework


The CPRAS covers approximately 44,000 mi² of Idaho, Oregon, and Washington (fig. 1A). Kahle and others (2009) provided the context and interpreted geologic framework used in this report.

The Columbia Plateau is an intermontane basin between the Rocky Mountains and the Cascade Range that is filled with mostly Cenozoic basalt and sediment. The CRBG consists of a series of flows that erupted during various stages of the Miocene Age, 17 million to 6 million years ago. The basalt lava flowed from fissures and vents in Idaho, Oregon, and Washington. The number, extent, and thickness of flows vary depending on many factors, including proximity to vents, the volume of eruption, lava viscosity, cooling process, erosion, and topography over which the lava flowed (Swanson and others, 1979c; U.S. Geological Survey, 2009a). More than 300 flows have been identified, with individual flows ranging in thickness from 10 to more than 300 ft (Tolan and others, 1989; Drost and others, 1990). Total thickness of the series of flows may be greater than 15,000 ft in the central part of the study area near Pasco, Washington (Reidel and others, 2002) (fig.1A). Typically, lava erupted quickly and advanced away from the fissure or vent as a single, uniform sheet of lava; however, towards the margin of the CPRAS, depositional style commonly changed from sheet-flow to intra-canyon flows as the volume of lava decreased and the terrain became rugged enough to funnel flows into valleys and canyons. 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, then a sedimentary interbed occurred between flows.


The CPRAS can be divided into four structural regions (Reidel and others, 2002), with two of these, the Yakima Fold Belt and the Palouse Slope, contained within the Columbia River basin proper (fig. 1B). The Yakima Fold Belt is included in the western and central parts of the Columbia Plateau and consists of a series of anticlinal ridges and synclinal valleys. The Palouse Slope in the northern and eastern parts of the plateau is much less deformed and dips gently westward. The Blue Mountains, a composite anticlinal structure, bounds the Yakima Fold Belt and Palouse Slope to the southeast. The Clearwater Embayment marks the eastward extent of the CPRAS along the foothills of the Rocky Mountains.


Stratigraphy


The majority of rocks exposed in the region are the CRBG, intercalated sedimentary rocks of the Ellensburg Formation, younger sedimentary rocks and deposits, Pleistocene cataclysmic flood deposits, eolian deposits, terrace gravels of modern rivers, and other localized deposits. The simplified stratigraphy used here was summarized by Kahle and others (2009).


Sediment Stratigraphy


Within the Yakima Fold Belt, Miocene sedimentary deposits of the Ellensburg Formation underlie, intercalate, and overlie the CRBG and compose most of the thickness of the unconsolidated deposits in the basinal areas (Jones and others, 2006). These continental sedimentary deposits include fluvial sands and gravels, overbank deposits, lacustrine deposits, alluvial-fan deposits, sandstone, conglomerate, and interbedded volcaniclastic sediments. The Mabton Interbed includes deposits of the Ellensburg Formation that overlie the Wanapum Basalt and underlie the Saddle Mountains Basalt. In eastern Washington and west-central Idaho, sediment of the Latah Formation underlies, intercalates, and overlies the CRBG (Leek, 2006). The Latah Formation consists mostly of clay, silt, and sand deposited in drainages blocked by encroaching basalt flows. Both the Latah Formation and the Vantage Member of the Ellensburg Formation form the major sedimentary interbeds occurring between the Grande Ronde and Wanapum Basalts. Pleistocene to Holocene sediments overlying the CRBG include flood gravels and slack water sediments, terrace gravels of modern rivers, and eolian deposits that include the Palouse Formation.


Basalt Stratigraphy


The thickest, most extensive, and hydrologically most important geologic unit in the CPRAS is the CRBG (Whiteman and others, 1994). The CRBG has been divided into six geologic formations by Swanson and others (1979c): Imnaha Basalt, Picture Gorge Basalt, Prineville Basalt, Grande Ronde Basalt, Wanapum Basalt, and Saddle Mountains Basalt. These formations are divided into members and further subdivided into flow units on the basis of field mapping, well logs, aeromagnetic surveys, geochemistry, and magnetic polarity (U.S. Geological Survey, 2009a).


Flows belonging to the Imnaha Basalt, the oldest known in the CRBG, occur in western Idaho, eastern Washington, and Oregon (Kahle and others, 2009). The Picture Gorge and Prineville Basalt Formations are limited to areas in central Oregon defining the southern extent of the CRBG. The Imnaha Basalt and Grande Ronde Basalt (including Prineville and Picture Gorge) constitute 90 percent of the volume of the CRBG (Bjornstad and others, 2007). The remaining 10 percent includes the Wanapum and Saddle Mountains Basalts, which overlie the Grande Ronde Basalt. Flows of the Saddle Mountains Basalt are less widely distributed (fig. 1A). Folding and faulting of the basalts have occurred during the period of deposition and more recently. Distribution and thickness of younger basalt flows are controlled by structurally controlled valleys. During the Pleistocene, the surface of the basalt units was modified greatly during repeated catastrophic outburst flooding, which caused erosion of vast channels as well as removal and or deposition of overlying sediment.


First posted February 25, 2011