Geologic framework of the Columbia Plateau Aquifer System, Washington, Oregon, and Idaho

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The Columbia Plateau in eastern Washington, north-central and northeastern Oregon, and western Idaho covers more than 70,000 square miles underlain chiefly by basalt belonging to the Columbia River Basalt Group. The Plateau is a large structural basin whose deepest part lies near Pasco, Washington, where the total thickness of basalt may exceed 14,000 feet. The Columbia Plateau regional aquifer system is a major aquifer system that consists chiefly of a great thickness of basalt belonging to the Columbia River Basalt Group, of minor interbedded sedimentary materials, and of overlying undivided consolidated and unconsolidated sediments. For hydrologic purposes, these rocks have been subdivided along stratigraphic boundaries into four hydrogeologic units. These hydrogeologic units are from oldest to youngest: the Grande Ronde, Wanapum, Saddle Mountains, and overburden units. Structure contour and thickness maps have been prepared for each unit and for selected sedimentary interbeds in this study. The Grande Ronde is the thickest most extensive hydrogeologic unit; each of the overlying younger units in turn cover less area and is less voluminous. Thicknesses of each unit range from a minimum of zero to a maximum of about 14,000 feet for the Grande Ronde, 1,200 feet for the Wanapum, and 800 feet for the Saddle Mountain hydrogeologic unit. The maximum thicknesses for each of the hydrologeologic units occur near the deepest part of the structural basin near Pasco, Washington. The thickest overburden, 2,000 feet, occurs in the Grande Ronde Valley near La Grande, Oregon, but is generally much thinner elsewhere. Dominant geologic structures in the Yakima Fold Belt in the western part of the Plateau are long, narrow, east-west to east-southeasterly trending anticlinal ridges, that are sharply folded and faulted, with intervening broad synclinal basins. Basalt units in the Palouse subprovince are gently inclined toward the southwest, but are gently warped in places by broad northwest and southwest trending low amplitude folds. The Blue Mountains province in the southeast part of the Plateau is a broad, deeply dissected, uplifted region crossed by northeast trending folds and by north to northwest trending normal faults and lineaments.