Umtanum Ridge is one of the best-exposed Yakima ridges formed by folded basalt flows in south-central Washington. An analysis was made of the structural geometry and strain distribution in the deformed basalt layers exposed on the ridge at Priest Rapids Dam. The purpose of the analysis was to gain an understanding of the distribution and orientations of the small-scale structures (faults, breccias, joints) around the anticlinal structure. From this we can assess the relative strain intensity and distribution around the fold, and use this information, along with the mapped profile shape of the fold and associated faults, to construct a balanced section leading to constraints on the tectonic models of the Columbia Plateau. The strain distributions and structural geometries within Umtanum Ridge accord well with an asymmetric kink-fold geometry with predominantly flexural strains in the steep limb; however, the internal cataclastic flow is not penetrative at field-observation scale. Discrete flexural slip has occurred, both within and along flow contacts, as well as some internal shatter brecciation and faulting between and across the flow-parallel faults. The Umtanum fault, a large reverse fault, is associated with the anticline and, on the basis of the reconstructed section, is conjectured to have formed out of the kink-like fold at depth. Slickenside striae orientations on faults developed during folding are generally perpendicular to the fold axis. This is interpreted to mean that the dominant movement of basalt layering during folding was perpendicular to the fold axis. The mechanical continuity between the anticline and the adjacent syncline to the north is interpreted to have not been disrupted until late in the fold history. Because of this hypothesized continuity and because the dominant relative movement direction of displacement was perpendicular to the fold axis, movement on the Umtanum fault is intepreted to have been predominantly dip-slip. It is further inferred that if any regional strike-slip component was present in the Pasco Basin, it does not manifest itself obviously in the Umtanum fold kinematics.

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 This field trip guide covers a two-day trip to examine the characteristics of Columbia River Basalt Group flows and the Yakima fold belt. This field trip focuses on the main physical characteristics of the lavas, compositional variations, and evidence for their emplacement, and on the geometry of the anticlinal ridges and synclinal valleys of the fold belt and deformational features in the basalts.

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.

ABSTRACT The Miocene Columbia River Flood-Basalt Province is one of the youngest and perhaps the best studied flood-basalt province on Earth. This field guide describes a three-day field trip through the central, eastern, and western portions of the Columbia Plateau region of this province, visiting field localities that have been key to understanding the geologic and structural history of this province. The guide provides a brief summary of our current understanding of the geologic and tectonic evolution of this flood-basalt province. Recent refinements in Columbia River basalt stratigraphy have confirmed the huge size of many of the Columbia River basalt flows (1000– 5000 km3 in volume) and a wide range of emplacement rates. The emplacement rate estimates range from as low as one to two months to as high as three to four years. Many aspects of Columbia River basalt volcanism appear to be associated with regional-scale deformation (e.g., regional-scale subsidence, folding, and faulting).

Abstract Eocene sedimentary and volcanic rocks on the eastern flank of the Cascade Range consist of five regional, unconformity-bounded formations of the Challis synthem. These formations define a series of northwesterly striking folds. Five anticlines are 9 to 28 km apart, have pre-Tertiary crystalline rocks in their cores, high-angle reverse faults on their steeper northeastern limbs, and pass down-plunge into more gentle folds in the Neogene Columbia River Basalt Group (CRBG). Such northwesterly trending folds extend from east of the Columbia River across the Cascade Range to the Puget Lowland. The Chiwaukum graben and Swauk basin, which heretofore were thought to be local, extensional, depositional basins, are, instead, the major northwesterly trending synclines in this series of folds. The Eocene formations were preserved, not deposited, in these synclines. Dextral, N-S faults cut the reverse faults and the pre-CRBG portion of some of the folds. The post-CRBG folds control the regional distribution of the Eocene formations. The Cascade Range is a southerly plunging, post-CRBG anticline. Clasts in the Thorp Gravel indicate that this anticline began to rise ca. 4 Ma. The anticline has an amplitude of ∼3.5 km, and it causes the plunges of the northwesterly striking post-CRBG folds. The northerly and northwesterly post-CRBG folds form a regional interference pattern, or “egg-crate,” that dominates the present topography of Washington State.