STRATIGRAPHIC RECORD OF THE MIDDLE EOCENE FAULT-BEND, STRIKE-SLIP CHUMSTICK BASIN, CENTRAL WASHINGTON, SYNCHRONOUS WITH AN EPISODE OF MAJOR STRIKE-SLIP FAULTING ACROSS THE CASCADES

Northwest Washington underwent a major episode of dextral, strike-slip faulting starting at 50 Ma. Faulting was accompanied by magmatism, metamorphism, mid-crustal exhumation, local dike swarms, and basin formation. The fault-bounded Chumstick basin formed in this setting and is composed of two subbasins. From the following evidence, the western Chumstick subbasin is interpreted as a fault-bend, strike-slip basin similar to the iconic Ridge basin in California. The western bounding Leavenworth fault has ~30 km of dextral offset that separates conglomerate clasts from their correlated source rocks. The eastern bounding Entiat fault is long, straight, subvertical and has many structures indicating strike-slip motion. Along the northern margin of the basin, the Leavenworth fault curves to the N and merges with the Entiat fault, similar to the intersection of the San Gabriel fault with the San Andreas fault along the Ridge basin. In the western subbasin, the Chumstick Formation is ~10 km thick, yet zeolites, vitrinite reflectance data, and paleogeothermal estimates by earlier workers suggest a maximum burial depth of 3.5 km and high basin temperatures of 60°-70°C/km, all indicating rapid sedimentation in a shingled basin. Moreover, 7 of the 18 tuffs were dated by U-Pb zircon TIMS (from ~1000 – 7000 m in the section) at 49.15 to 47.85 Ma, supporting rapid sedimentation rates of 6-7 m/ky in the lower section and 2-3 m/ky in the central section. Geologic mapping shows a west-dipping homocline in the western basin with strata younging to the NW. Paleocurrent data and facies analysis from earlier work and this study shows a two-sided, asymmetric basin of humid alluvial-fluvial deposits. A western conglomerate member has soft sediment deformation structures and lies in a narrow belt along the Leavenworth fault, interfingering to the east with the middle to uppermost part of the main fluvial unit in the basin center. This evidence suggests a shingled basin, with most sediment from the east, but the main depocenter near the bend in the western Leavenworth fault, much like the conveyor-belt model of deposition in the Ridge basin. Later folding during inversion of the western subbasin fits N-directed transpression on the Leavenworth fault, while the narrow eastern subbasin along the Entiat fault shows many strike slip traits.