RELATIVE AGES AND TIMING OF DEFORMATION OF THE SWAUK AND TANEUM FORMATIONS IN KITTITAS COUNTY, WASHINGTON

The Eocene Swauk and Taneum formations lie within or east of the Cascadia subduction related arc. The timing of accumulation of sediments in both formations as well the geometry and relative timing of the deformation affecting them are contentious topics. One hypothesis (Tabor et al., 1984) suggests that both the Swauk and Taneum were deposited prior to deformation; Tabor attributes deformation to movement along the Straight Creek fault. The contrasting hypothesis (Cheney, 1994) suggests that an episode of deformation followed deposition of the Swauk, but predated deposition of the Taneum, implying that the Taneum is deformed significantly less than the Swauk; Cheney attributes deformation to regional events. New geologic mapping, structural, petrographic, and Ar/Ar geochronologic investigations within the Swauk and Taneum formations along the eastern flank of the Cascades provide insight into the timing of deformation relative to sedimentation. The Swauk consists of angular coarse-grained, locally cross-bedded, variably thick-bedded, tan sandstone interbedded with lesser leaf fossil-bearing shale. Overlying the Swauk in angular unconformity is the Taneum, which is composed of 1 to 5 m-thick bedded basaltic to andesitic volcanic deposits ranging from volcanic breccia to pyroclastic flows to welded tuff. Cross-cutting the Swauk and Taneum formations is a swarm of fine-grained basalt dikes of the Teanaway formation. Bedding in the Swauk has been folded into NW-trending, shallowly NW-plunging, open to tight folds. Locally NE-dipping thrust faults cut these folds. The overlying Taneum is a homoclinal sequence dipping moderately to the southwest. Our field observations suggest that the Taneum is faulted to a lesser degree than the Swauk. On the basis of our field and structural data, we conclude that the Swauk experienced significant northeast-southwest contraction prior to the deposition of the Taneum formation. Subsequent to the deposition of the Taneum, both the Taneum and the Swauk were tilted moderately to the southwest. These observations support Cheney's hypothesis of significant deformation before and minor deformation after deposition of the Taneum. Constraints on the timing of deformation will be provided by Ar/Ar data.