Abstract Eocene right-lateral displacements occurred along several major fault zones in western and central Washington, including the Straight Creek fault, the Entiat-Leavenworth fault system, and probably the Puget fault, a covered north-trending structure in the Puget Lowland. Within this strike-slip framework, nonmarine sediments accumulated in the Chuckanut, Puget-Naches, Chiwaukum graben, and Swauk Basins to form some of the thickest (more than 6,000 m) alluvial sequences in North America. To varying degrees, the basins are characterized by (1) high sediment-accumulation rates, implying rapid subsidence; (2) abrupt local stratigraphic thickening and thinning; (3) intrabasinal and basin-margin unconformities; (4) abrupt facies changes; (5) fault-induced drainage re-organization; (6) intermittent internal drainage; and (7) interbedded and intrusive relationships with extension-related(?) volcanic rocks. Similar sandstone petrography between basins suggests a common sediment source with possible local or temporary connection between basins. Sedimentation and deformation throughout the province were diachronous, and the basins experienced rapid alternating subsidence and uplift. Orientations of folds and faults are consistent with regional right-lateral shear. The 90-km-wide composite Chuckanut-Puget-Naches Basin probably formed as a pull-apart basin between the Straight Creek and Puget faults. The 50-km-wide Swauk Basin may have formed as a fault-wedge basin between the Straight Creek and Entiat-Leavenworth faults. These basins are large when compared to most modern and ancient basins controlled by strike-slip faults, suggesting that processes of strike-slip basin formation operate at a variety of scales. The 20-km-wide Chiwaukum graben probably formed as a pull-apart basin between the Entiat and Leavenworth faults. Eocene strike-slip faulting was probably driven by oblique convergence of the Kula plate below North America. Although strike-slip basins in Washington occupied a forearc or possibly an intra-arc tectonic setting in this ancient continental margin, they differ significantly from typical forearc or intra-arc basins.

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 The Chumstick Formation is a middle to late Eocene nonmarine unit that was strongly affected by syndepositional tectonism and magmatism. The unit is mostly found between the Leavenworth and Entiat fault zones (Fig. 1), although there are sedimentary units related to the Chumstick Formation that are found outside of this basin (“Chiwaukum graben”), both west of the Leavenworth fault (“sandstone facies of Red Hill” ofTaylor et al. (1988)), and east of the Entiat fault (Evans, 1988, 1994a). The basin is divided into two subbasins by a third fault zone, the Eagle Creek fault zone, which parallels and partly overlaps the Entiat fault zone. The eastern and western subbasins had different depositional and deformational histories (Evans, 1988, 1994a). The data base for these interpretations includes geologic mapping, detailed measurement of 4,225 m of section at 27locali ties (Fig. 1), 1 ,514 paleocurrent measurements, 41 thin-section point counts ( 400-500 grains per slide), 35 conglomerate clast counts (4,625 conglomerate clasts), and new zircon fission track ages from a tuff and a sandstone. In addition, 78 coal samples were collected, and vitrinite reflectance values calculated for the author by the Washington Division of Geology & Earth Resources and by Shell Oil Company. The results offacies analysis and depositional interpretations were reported earlier (Evans, 1991a). The identification of34 plant macrofossil taxa and 19 plant microfossil taxa, interpretation of paleosols, and identification of trace fossils made possible paleoclimatic interpretations reported earlier (Evans, 1991 b).

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.

Abstract The Tertiary sedimentary and volcanic rocks of the Cascade Range unconformably overlie a crystalline basement of previously accreted terranes. The Tertiary strata are parts of four synthems, or interregional unconformity-bounded sequences of tectonic origin. Thus, the formations in these synthems were not deposited in local basins. The 55-38 Ma Challis synthem has five regional unconformity-bounded formations; the names with precedence are (from the base up) Swauk, Taneum, Teanaway, Roslyn, and Naches. Near Blewett Pass (nee Swauk Pass), the Challis fluvial and arkosic Swauk Formation is ~5 km thick and has several members in a generally upward-fining succession. The members of the Swauk do not interfinger, and some are separated by unconformities. The Oligocene to mid-Miocene andesitic and rhyolitic Kittitas synthem is almost absent in the area. The most voluminous lithostratigraphic unit in the mid-Miocene to Pliocene Walpapi synthem is the Columbia River Basalt Group. Clasts of Columbia River Basalt Group and older rocks in the ca. 4 Ma Thorp Formation of the High Cascade synthem record initial uplift of the Cascade Range to the west. North of Blewett Pass, the northwesterly segment of the Leavenworth fault is the Camas Creek reverse fault that places Swauk and Teanaway in the Blushastin anticline over a syncline in the Roslyn Formation. Northerly striking faults in the Leaven-worth fault zone are parts of a younger system that cuts the Camas Creek thrust and northwest-striking folds in Challis rocks. In style, scale, and age, the Camas Creek fault resembles the Easton Ridge thrust south of Cle Elum, the Eagle Creek fault in the Chiwaukum graben, and the Seattle fault in the Puget Lowland. These faults are on the steeper northeastern limbs of major anticlines in Challis rocks. Down plunge, these folds are more gentle in Walpapi rocks. These folds and faults are part of the regional Seattle-Wentachee-Kittitas fold-and-thrust belt. The Straight Creek fault is a major, north-south, dextral fault in the northern Cascade Range. The fault offsets all five of the Challis unconformity-bounded formations. The southeasterly curving discontinuity along which it was mapped east of Easton is due to unconformities at the base of the Taneum and Teanaway, not a fault. The Straight Creek fault is 2.7 km west of Easton and passes southward beneath Kittitas rocks. Although the fault dextrally displaces pre-Tertiary units >90 km, Tertiary displacement is &55 km. This may indicate two (or more) periods of displacement. Perhaps the displaced portion of the fault underlies Puget Sound. Two sets of post-Walpapi folds deform the Tertiary synthems. The Seattle-Wentachee-Kittitas fold-and-thrust belt is part of a set of northwest-striking folds. One of several north-trending regional anticlines causes the Cascade Range. The Cascade Range anticline, with an amplitude of ~3.5 km, has risen in approximately the past 3.5 m.y. This anticline causes the plunges of the Seattle-Wentachee-Kittitas fold-and-thrust belt folds. The two sets of folds cause a regional interference, or “egg-crate,” pattern that dominates the present topography of the Pacific Northwest.

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.