Stratigraphy of individual basalt flows in the N 2 magnetostratigraphic unit of the Grande Ronde Basalt (GRB) within the central Columbia Plateau has been developed using data from seven surface sections and fifteen boreholes. Twenty-one individual flows have been identified and grouped into eight flow packages. The flow correlations were developed based on chemical composition, paleomagnetic vector direction, stratigraphic position, and thickness of individual flows. A multivariate statistical procedure, discriminant analysis, was used to test the validity of using chemical composition alone to define the flow packages. Results of the test show that samples can be correctly classified within one adjacent flow package in 94 percent of the cases. Application of discriminant analysis to chemical composition data indicates that within the Pasco Basin the upper two-thirds of the N 2 GRB contains 17 individual flows, of which only 9 to 15 may be present at any one location. Seven of these flows are present throughout the portion of the Pasco Basin studied. Correlation of flows between boreholes or surface sections means that units with the same stratigraphic position and diagnostic characteristics have been identified. In most cases this means that a single flow formed from one eruption. However, some flows may not be continuous, or some correlated flows may represent eruptions separated by as much as several thousand years but which gave rise to flows with identical stratigraphic positions and similar characteristics. The greatest number of flows occurs in the southeastern part of the basin, but the thickest total section occurs in the western and northwestern parts of the basin. The detailed flow correlations provide evidence of deformation during emplacement of the N 2 GRB. Variations of the thickness of individual flows and packages of flows disclose that subsidence was greatest in the western portion of the basin and that growth of the Yakima Ridges began at least by late GRB time. This timing of deformation of the Yakima Ridges is consistent with previous interpretations. We conclude that discriminant analysis applied to chemical composition of GRB flows provides a means of quantitatively defining correlation of flow packages. The method yields estimates of uncertainty in correlations but must be applied in the context of an appropriate stratigraphic framework. Discriminant analysis should be useful in a variety of volcanic terranes where subtle but consistent differences in composition occur between individual flows or flow packages. Successful application of the technique requires that variations in the chemistry of individual flows or packages of flows be generally less than chemical differences among flow packages and that a relatively large number of samples be available for classification.

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.

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 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.

Recent mapping of pre-basalt rocks along the northwestern Columbia River basalt margin and well logs from Shell Oil Company gas wells provide new information about the rocks and structure underlying the Yakima fold belt. Pre-basalt rocks along the margin range in age from Jurassic to lower Miocene, with early to middle Tertiary arkosic and volcaniclastic strata concentrated in three fault-bounded basins. With one exception, pre-basalt rocks cut by the Shell Oil Company wells (Yakima Minerals, Bissa, and Saddle Mountains) can be correlated with rocks found in the basins along the margin. These rocks extend under the Columbia River Basalt Group almost to the center of the Columbia Basin. Two major features, the Leavenworth–Hog Ranch cross-structure and the White River–Naches River fault zone, affect the distribution of sedimentary rock types. Based on well and geophysical data, the Columbia River Basalt Group thins across the Hog Ranch–Naneum Ridge structure, suggesting that this feature was active during Miocene time. The northwestern Columbia River basalt margin is the focus of major structural elements that converge on the Yakima fold belt, including the Olympic-Wallowa lineament (OWL), the Cle Elum–Wallula lineament (CLEW), the Hog Ranch–Naneum Ridge cross-structure, the Chiwaukum graben, and the White River–Naches River fault zone. In the area of CLEW, splays of the Straight Creek fault turn southeast and pass under the Columbia River Basalt Group, aligning with folds of the Yakima fold belt. Elsewhere along the margin, there is little expression of sub-basalt structure in the overlying Columbia River basalt. The Columbia River Basalt Group, at the margin, exhibits an absence of faulting and displays only broad, gentle folds. Closely spaced, tight folds and associated faults in the interior of the Yakima fold belt either die out before reaching the margin or become broad, gentle flexures.

Tectonic development and evolution of the central Columbia Plateau since middle Miocene time is a product of dynamic interplay among (1) the eruption and emplacement of the Columbia River Basalt Group (CRBG), (2) the subsidence of the area encompassing the Yakima fold belt subprovince, (3) the growth of the Yakima folds, and (4) the influence of regional structures transecting the fold belt, specifically the Hog Ranch-Naneum Ridge anticline and the Cle Elum–Wallula disturbed zone. Subsidence of the Yakima fold belt subprovince began prior to the eruption of the CRBG and has continued from Miocene time to the present. The rate of subsidence kept pace with the rate of CRBG flow emplacement, decreasing as CRBG volcanism waned. Simultaneously, anticlinal fold growth within the Yakima fold belt occurred under north–south compression and also decreased as the rates of subsidence and eruptions of lava declined. Paleomagnetic data indicate fold growth was accompanied by local clockwise rotation of basalt within the anticlines. The tectonic and volcanic histories of the central Columbia Plateau are interrelated and imply a common cause. The structural rotation and north-south compression, and thus fold growth, are interpreted to result from oblique subduction along a converging plate margin. The coincidence of the timing and rates of fold growth, subsidence of the central Columbia Plateau, and basalt production rates suggest that CRBG volcanism is primarily a product of oblique subduction off western North America.

ABSTRACT The geomorphology, soils, and climate of Columbia Basin vineyards are the result of a complex and dynamic geologic history that includes the Earth's youngest flood basalts, an active fold belt, and repeated cataclysmic flooding. Miocene basalt of the Columbia River Basalt Group forms the bedrock for most vineyards. The basalt has been folded by north-south compression, creating the Yakima fold belt, a series of relatively tight anticlines separated by broad synclines. Topography related to these structures has strongly influenced the boundaries of many of the Columbia Basin's American Viticultural Areas (AVAs). Water gaps in the anticlinal ridges of the Yakima fold belt restrict cold air drainage from the broad synclinal basins where many vineyards are located, enhancing the development of temperature inversions and locally increasing diurnal temperature variations. Vineyards planted on the southern limbs of Yakima fold belt anticlines benefit from enhanced solar radiation and cold air drainage. Most Columbia Basin vineyards are planted in soils formed in eolian sediment that is primarily derived from the deposits of Pleistocene glacial outburst floods. The mineralogy of the eolian sediment differs substantially from the underlying basalt. Vineyard soil chemistry is thus more complex in areas where eolian sediment is comparatively thin and basalt regolith lies within the rooting zone. The components of physical terroir that broadly characterize the Columbia Basin, such as those described above, vary substantially both between and within its AVAs. The vineyards visited on this field trip are representative of both their AVAs and the variability of terroir within the Columbia Basin.

A composite section of eight Grande Ronde Basalt flows delineates the margin of the Columbia River Basalt Group on this portion of the eastern flank of the Cascade Range. The Grande Ronde Basalt flows belong to the following units (in descending stratigraphic order): Sentinel Bluffs Member (Basalt of Museum 2 and Museum 1; Basalt of Stember Creek; and upper and lower flows of the Basalt of McCoy Canyon), Ortley member (informal), Grouse Creek member (informal “Meeks Table” flow), and Wapshilla Ridge Member. All these Grande Ronde Basalt flows display similar intraflow structures (cooling joint patterns) and lithology, but they are separable by chemical compositions (i.e., TiO 2 , MgO, P 2 O 5 , Cr, Ba, and Zr). Individual Grande Ronde Basalt flows can range in thickness from 8 to 180 m, with the maximum total thickness of the Grande Ronde Basalt section being 555 m. As the Grande Ronde Basalt flows advanced into the map area, they covered plains, filled stream-cut valleys and canyons up to 160 m deep, and surrounded extinct volcanoes 750 m tall. In post–Grande Ronde Basalt time, the Grande Ronde Basalt flows were deformed into a series of ENE-striking anticlines, synclines, and associated faults that define this portion of the Yakima Fold Belt. During this same time, transpressional deformation activity increased folding and thrust faulting in the Cle Elum–Wallula Lineament, a structural segment of the Olympic-Wallowa Lineament. In addition, series of NNW-striking, dextral strike-slip and normal faults were developed with displacements up to 4.5 km on the strike-slip faults and 1 km on the normal faults. The N-striking Goat Creek and NW-striking Indian Flat and Devils Slide faults merge with the White River fault to the west. These faults, along with the E-NE–striking Bethel Ridge anticline and NNW-striking Cleman Mountain anticline, form the major structures in this area.