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.

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 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 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).