Abstract A ruling hypothesis for the central Cascade Range in Washington is that the Eocene arkosic formations, which are kilometers thick, were deposited in local grabens, such as the Chumstick Formation in the putative Chiwaukum graben. However, the formations are regional in extent and are preserved in less extensive northwesttrending synclines. The Chumstick Formation in the Peshastin syncline is a more proximal equivalent of the Roslyn Formation, which is preserved in the Kittitas Valley syncline 25 km to the southwest. The Chiwaukum structural low is partially bounded on the southwest by the Leavenworth fault zone, which consists of northwesterly striking, northeasterly verging reverse faults (with associated northwest-striking folds). The reverse faults and the hinges of the folds are cut by N-S, dextral strike-slip faults, which also partially bound the Chiwaukum structural low. Conglomeratic units in the Chumstick Formation are not proximal to either set of bounding faults. The Leavenworth fault occurs on the steeper northeastern limb of a northwesterly trending, basement-cored anticline. The Eagle Creek and Ainsley Canyon anticlines also have reverse faults on their steeper northeastern limbs. In the Puget Lowland, the Seattle reverse fault is in a similar anticline. The regional distribution of the Eocene formations and uplift of the Cascade Range are caused by folding of the Miocene Columbia River Basalt Group since 4 Ma. The remnant of a 4 Ma andesite on Natapoc Mountain shows that the present low topography of the Chiwaukum structural low is erosional and young.

Abstract This paper summarizes exploration efforts along the Wenatchee gold belt (WGB), and shows that the belt may have a strike length greater than 9.5 miles (Figure 1). Exploration drilling by Asamera Minerals (U.S.) Inc. and joint-venture partner Breakwater Resources Ltd. has shown that hydro thermal alteration and gold-silver mineralization extends approximately 4.5 miles southeast and 5 miles northwest of the Cannon mine. The WGB is a northwest-trending zone within the Chiwaukum graben. The graben also trends northwesterly, and is bound by the Entiat fault on the eastern margin and the Leavenworth fault zone on the western margin. Within the graben the Eagle Creek fault zone is adjacent and parallel to the Entiat fault. Gold-silver mineralization along the WGB is apparently directly associated with the Eagle Creek fault zone (Figure 2). Although Miocene Columbia River flood basalts conceal the southern portion of the Chiwaukum graben, data from a series of oil and gas wells drilled during the 1980’s in the Columbia basin, suggest that some graben structures can be projected 50 miles southeast of Wenatchee. The WGB hosts two gold-silver mines, the inactive L-D (Lovitt) mine and the active Cannon mine. The L-D mine produced 0.4 million ounces of gold (Patton and Cheney, 1971), and the Cannon mine will produce approximately 1.2 million ounces of gold by its closure in late 1994. Arkosic sediments of the Chumstick Formation host mineralization at both mines. The mineralization is directly associated with zones of silicification and/or argillic alteration and associated quartz stockworks.

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.