The Prairie Creek Formation, a sedimentary unit of limited outcrop extent (150 km2) on the southern Cascadia forearc in northern California, provides geologic data that constrain the style of tectonic activity in this portion of the forearc during the time interval 4(?) Ma to about 0.5 Ma. The formation records deposition of approximately 550 m of mostly fluvial sediment during a period of subsidence of the forearc. The formation was deposited by a large braided river, most likely an ancestral Klamath River, that prograded into the forearc. Sedimentation mainly consisted of sand and well-rounded conglometratic alluvium. However, within the lower half of the section, there occur several thin horizons of coarse angular conglomerate of a distinctive lithology. The only likely provenance for this anomalous conglomerate is fault scraps intermittently exposed near the area of deposition. The tectonic setting of this portion of the sourthern Cascadia forearc in late Pliocene of early Pleistocene through mid-Pleistocene was therefore one of general forearc subsidence with intermittent uplift due to faulting. Intercalation of sand and gravel braidplain deposits in the fluvial section suggests periodic changes in the net subsidence rate. Subsidence culminated in a brief period of submergence followed shortly by uplift to as much as 500 m in the late Pleistocene. The transition to uplift records a significant change in tectonic regime. This change may be due to the deformational effects of either or both of the following tectonic processes: northward encroachment of the Mendocino triple junction during the late Pleistocene, or a late Pleistocene change in the direction and/or rate of convergence of the Juan de Fuca plate relative to the overriding North American Plate. ¿ American Geophysical Union 1989