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Abstract

A prevailing hypothesis for the central Cascade Range of Washington State is that it underwent regional extension or transtension during the Eocene. This hypothesis is based on the idea that kilometers-thick, clastic, Eocene formations were deposited syntectonically in local basins. Our mapping and structural analysis indicate that these formations are preserved in fault-bounded, regional synclines, not in separate depositional basins. Thus, the type area for the hypothesis, the so-called Chiwaukum graben, is here renamed the Chiwaukum Structural Low. The Eocene arkosic Chum-stick Formation, which was thought to have been syntectonically deposited in the graben, is the proximal equivalent of the Roslyn Formation 25 km southwest of the graben. Because the name “Roslyn Formation” has precedence, the name “Chumstick Formation” should be abandoned. Additionally, several areas previously mapped as Chumstick Formation in the Chiwaukum Structural Low probably are parts of the older Swauk Formation and younger Wenatchee Formation.

The southwestern boundary of the Chiwaukum Structural Low includes the Leav-enworth fault zone, which consists of postdepositional, northwest-striking reverse faults with adjacent northwest-striking folds. The reverse faults place the regionally extensive early-Eocene, arkosic Swauk Formation over the mid-Eocene, arkosic Chumstick Formation. A diamictite, which previously was placed in the Chumstick Formation and inferred to have been syntectonically derived from the Leavenworth fault zone, is part of the older Swauk Formation. We mapped a 0.6–1-km-thick conglomerate-bearing sandstone as a robust marker unit in the Chumstick Formation; instead of being spatially related to the bounding faults, this unit has a >30 km strike length around the limbs of folds in the structural low. The northwest-striking reverse faults and fold hinges of the structural low are cut by north-striking strike-slip faults, which likely are late Eocene to Oligocene; these north-south faults partially bound the structural low. The Eocene folds and faults were reactivated by deformation of the Miocene Columbia River Basalt Group; this younger folding largely defines the regional map pattern, including the structural low.

A model to account for the above characteristics is that all of the Eocene formations, not just the Roslyn Formation, are kilometers thick and are remnants of regional unconformity-bounded sequences that were deposited on the Eocene margin of this part of North America. Their present distribution is governed by younger faults, folds, and erosion. Thus, the Eocene to Recent history of the central Cascade region is characterized not by crustal extension, but by episodes of folding (with related reverse faults) and strike-slip faulting.

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