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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.

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