Deformation of the continental flood-basalt in the westernmost portion of the Columbia Plateau has resulted in regularly spaced anticlinal ridges. The periodic nature of the anticlines is characterized by dividing the Yakima fold belt into three domains on the basis of spacings and orientations: (1) the northern domain, made up of the eastern segments of Umtanum Ridge, the Saddle Mountains, and the Frenchman Hills; (2) the central domain, made up of segments of Rattlesnake Ridge, the eastern segments of Horse Heaven Hills, Yakima Ridge, the western segments of Umtanum Ridge, Cleman Mountain, Bethel Ridge, and Manastash Ridge; and (3) the southern domain, made up of Gordon Ridge, the Columbia Hills, the western segment of Horse Heaven Hills, Toppenish Ridge, and Ahtanum Ridge. The northern, central, and southern domains have mean spacings of 19.6, 11.6, and 27.6 km, respectively, with a total range of 4 to 36 km and a mean of 20.4 km ( n = 203). The basalts are modeled as a multilayer of thin linear elastic plates with frictionless contacts, resting on a mechanically weak elastic substrate of finite thickness, that has buckled at a critical wavelength of folding. Free slip between layers is assumed, based on the presence of thin sedimentary interbeds in the Grande Ronde Basalt separating groups of flows with an average thickness of roughly 280 m. Many of the observed spacings can be explained by this model, given that: (1) the ratio in Young’s modulus between the basalt and underlying sediments E/E o ⩾ 1,000, (2) the thickness of the Grande Ronde Basalt was between 1,200 and 2,300 m when the present wavelengths were established, and (3) the average thickness of a layer in the multilayer is between 200 and 400 m. The lack of well-developed anticline-syncline pairs in the shape of a sinusoid may be the result of plastic yielding in the cores of the anticlines after initial deformation of the basalts into low amplitude folds. Elastic buckling coupled with plastic yielding confined to the hinge area could account for the asymmetric fold geometry of many of the anticlines.

ABSTRACT In portions of Washington, Oregon, and Idaho, the Columbia River Basalt Group (CRBG) hosts a regional aquifer system that is the primary, and in many cases the only, water supply for numerous communities, small water systems, individual homes, industry, and agriculture. In much of the semiarid Columbia Plateau, portions of the CRBG aquifer system have seen significant water-level declines and do not appear to receive significant, if any, natural recharge. Aquifer horizons within the Columbia River basalt generally are associated with intraflow structures at the top (e.g., vesicular flow-top breccias) and bottom (e.g., flow-foot breccias, pillow lava and hyaloclastite complexes) of sheet flows. The interiors of thick sheet flows (in their undisturbed state) have extremely limited permeability and act as aquitards, typically creating a series of stacked, confined aquifers within the Columbia River basalt aquifer system. The dominant groundwater flow follows horizontal to subhorizontal pathways along individual, laterally extensive, interflow zones. Vertical groundwater movement through undisturbed basalt flow interiors is greatly restricted except where basalt flow interiors are disturbed (such as by folds or faults), truncated (such as by flow pinchouts and erosional windows), or where they are cross-connected by wells.

At several localities in southwestern Nevada and adjacent California the Mississippian-Pennsylvanian boundary coincides with the contact between the Eleana Formation and the overlying Tippipah Limestone, or their equivalent strata. Near Red Canyon southeast of the Eleana Range, southern Nye County, Nevada, a thin limestone in the Eleana containing the ammonoids Cravenoceras hesperium Miller and Furnish and C. merriami Youngquist of late Chester age is separated by approximately 100 ft of shale and minor quartzite from platy limestone of the Tippipah, containing Diaboloceras aff. D. neumeieri Quinn and Carr of Morrow age. Three inches of conglomeratic limestone marks the base of the Tippipah. In the hills northwest of Frenchman Flat, southern Nye County, 14 ft of shale separates a limestone in the Eleana containing late Chester brachiopods from the basal Tippipah containing the ammonoids Bisatoceras, Diaboloceras, and Stenopronorites. In the hills southwest of Indian Springs, northwestern Clark County, Nevada, we refer a similar sequence (the Indian Springs Member of Longwell and Dunbar, 1936, of the Bird Spring Formation) to rocks equivalent to part of the Chainman Shale and Tippipah Limestone. The Chainman equivalent is about 85 ft thick and contains Mississippian (Chester) brachiopods 25 ft below the top. The basal Tippipah equivalent contains Bisatoceras, Diaboloceras, Stenopronorites, and Syngastrioceras. One foot of conglomeratic limestone marks the base of the Pennsylvanian. The same lithologic sequence is recognized on the northeast flank of the Nopah Range, southeastern Inyo County, California. The Tippipah ammonoids appear closest to late Morrow forms, indicating a hiatus at the base of the Pennsylvanian in this region. A Mississippian-Pennsylvanian disconformity is further suggested by the abrupt lithologic change and the conglomeratic limestones.