The application of plate-tectonics theory to the geologic history of the continental crust cannot succeed without detailed structural and radiometric data that precisely define the timing and extent of local crustal deformation. Although the geology of the Pacific Northwest is known in reconnaissance fashion, much remains to be accomplished in the realm of detailed geologic mapping and structural interpretation.

The Chiwaukum graben is an Eocene structure cut into crystalline basement rocks and the Swauk Formation on the eastern flank of the Cascade Range in central Washington (Willis, 1953; Whetten, 1976, 1977a, 1977b; Gresens, 1977) and it contains nonmarine sedimentary rocks of Eocene to Oligocene age (Fig. 1). The entire sedimentary section of the Chiwaukum graben and adjacent areas once was considered a single unit – the Swauk Formation of Paleocene (?) to early Eocene age (Russell, 1900; Smith, 1904). The term “Swauk” now is used in a more restricted sense as the oldest of several Tertiary formations (Gresens and others, 1977; Tabor and Frizzell, 1977). The late Eocene (45 m.y.) Chumstick Formation and the early Oligocene (34 m.y.) Wenatchee Formation are newly defined (Gresens and others, 1977). Several deformational episodes occurred during the time interval spanning deposition of the Swauk Formation to deposition of the Columbia River Basalt Group (which overlies the folded Wenatchee Formation). This has yielded a wealth of structural data and tectonic interpretation.

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