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 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 This guide describes a three-day field trip to the Paleogene sedimentary and volcanic rocks exposed between the Straight Creek–Fraser River and Entiat faults in the central Washington Cascades. These rocks record a history of deposition, deformation, and magmatism that can be linked to tectonic events along the North American margin using a robust chronology coupled with detailed sedimentological, stratigraphic, and structural studies. These events include deposition in a large sedimentary basin (Swauk basin) that formed in the forearc from <59.9–50 Ma; disruption and deformation of this basin related to the accretion of the Siletzia oceanic plateau between 51 and 49 Ma; the initiation, or acceleration of right-lateral, strike-slip faulting and the development of at least one strike-slip sedimentary basin (Chumstick basin) starting ca. 49 Ma; and the re-establishment of a regional depositional system after ca. 45–44 Ma (Roslyn basin) as strike-slip faulting was localized on the Straight Creek–Fraser River fault. These events are compatible with the presence of the Kula-Farallon ridge near the latitude of Washington ca. 50 Ma and its southward movement, or jump, following the accretion of Siletzia. This trip visits key outcrops that highlight this history and links them to regional studies of sedimentation, faulting, and magmatism to better understand the geologic record of this tectonic setting.