The Swauk Basin lies between the Straight Creek and Entiat–Leavenworth fault systems in the central Cascades of Washington and is one of several early Tertiary strike-slip basins in the Pacific Northwest. In this paper, seven informal stratigraphic units are recognized in the lowest portion of the basin fill—the lower and middle Eocene Swauk Formation—in the eastern part of the basin. These units have a stratigraphic thickness of more than 4800 m and were deposited in alluvial fan, braided river, meandering river(?), lacustrine-deltaic, and lacustrine environments. Rapid facies changes and reversals in paleocurrent directions indicate numerous tectonically controlled drainage reorganizations. Sediments were mainly derived from crystalline rocks to the east, north, and west (?), and sediment accumulation rates were high (about 64 cm/1000 years). The Leavenworth fault has a complex history and, periodically, formed the eastern margin of Swauk Basin. During the final phases and shortly after deposition of the Swauk Formation, the vertical sense of slip on the Leavenworth fault reversed itself and the trace of the fault shifted westward, leading to formation of the Chiwaukum Graben. Following deposition, the Swauk Formation was deformed into west-northwest-trending folds and then intruded by north–northeast-trending dike swarms. Based on similarities with other well-documented strike-slip basins, we conclude that strike-slip faulting was the main control on the deposition and deformation of the Swauk Formation.