The Chiwaukum graben in central Washington is bounded by the Entiat fault on the east and the Leavenworth fault on the west. The graben is largely underlain by the middle Eocene Chumstick formation, an unusually thick (> 5,800 m? or 19,000 ft?) sequence of sandstone, shale, conglomerate, and minor tuff. Five lithofacies are recognized in the Chumstick formation. They are interpreted as debris-flow and stream-generated alluvial fan deposits, braided and meandering stream deposits, and lacustrine deposits. Stratigraphic relationships within the basin are highly variable and include lateral facies changes, pinchouts, and interfingering of units. Basaltic dikes and sills intrude Chumstick sediments throughout the graben.

Structural data from the Leavenworth fault zone show highly variable orientations of minor fault planes, and dominance of subhorizontal slickensides. The fault zone has an irregular trace, and contains braided and en echelon fault segments which separate lenses of conglomerate. The Entiat fault has a highly linear trace. Fault surfaces with slickensides of variable orientation, fractured and folded mylonites, and pods of fractured conglomerates within the Entiat fault zone indicate a complex history of faulting. Within the graben, folds, normal faults, and fracture zones trend northwest, parallel to the graben-bounding faults; strike-slip faults in the graben trend northeast.

It is proposed that the Chiwaukum graben formed as a rhombochasm or possibly as a composite pull-apart basin. Consistent with this interpretation are the rhomb-shape of the graben; the thickness of basin fill; sedimentation patterns, including cyclic upward-coarsening sequences adjacent to uplifted fault blocks, and rapid facies changes; the presence of extension-generated(?) volcanism synchronous with sedimentation; and evidence for syndepositional deformation in the graben and on the basin margins. Orientations of structures in the graben are not consistent with a transtensile model; however, they may be the result of strike-slip and normal faulting in an asymmetrical shear zone.

Strike-slip models for the origin of the Chiwaukum graben are consistent with Ewing's tectonic synthesis of the Pacific Northwest during the Eocene.

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