IS THE CHIWAUKUM GRABEN ON THE EASTERN FLANK OF THE CASCADE RANGE OF WASHINGTON AN EOCENE EXTENSIONAL BASIN?

The Chiwaukum graben has long been considered an extensional basin in which the non-marine, predominantly arkosic Chumstick Formation (CFM) was deposited during Eocene strike-slip faulting. This transtensional model was advanced because CFM was interpreted to be ≤12 km thick, have syntectonic conglomerates along the bordering faults, and to have vertically interfingering facies.

Recent 1:24,000 mapping along the Leavenworth fault (LF), the southwestern bounding fault of the graben, reveals otherwise. A conglomerate-bearing unit in the middle of CFM is over a kilometer thick, has a strike length of ≥ 28 km, unconformably overlies the lower part of the formation, and shows that CFM is ≤ 6.5 km thick. The conglomerate along LF is the Tronsen Creek Member of the older Eocene Swauk Fm., not CFM. Mapping and analysis of slickensides show that the LF system consists, not of normal or strike-slip faults, but of high-angle reverse faults. LF places pre-Eocene crystalline basement over the Swauk Fm.; the nearby Camas Creek fault places Swauk Fm. over CFM. N-S dextral faults offset both reverse faults and the northwesterly striking folds in CFM

We extend into older rocks the well known, major northwesterly trending regional folds in the adjacent Miocene Columbia River Basalt Group (CRBG). Consequently, the Naneum Ridge anticline has pre-Eocene crystalline basement and the Swauk Formation in its core. The northeastern limb has basaltic volcaniclastic rocks and minor basalt (previously assigned to CFM) and CFM in the same stratigraphic order as the basaltic Teanaway Fm. and the overlying arkosic Roslyn Fm., respectively, on the southwestern limb.

The Chiwaukum graben is the major syncline northeast of the Naneum Ridge anticline. CFM is the lateral equivalent of the Roslyn Fm.; it was preserved, not deposited, in the syncline. The Swauk "extensional basin" to the south is part of the regional Kittitas Valley syncline, and this syncline also has a reverse fault, the Easton Ridge thrust, on its southwestern limb. Both synclines formed in the Eocene and were reactivated by the post-CRBG folds; the latter determine the regional outcrop patterns of the Eocene formations.