Four depositional systems, spanning 6-7 m.y., make up the 14,000-m-thick Ridge Basin Group. These Miocene to Pliocene units dip to the north and are arranged in a composite shingled-onlapping pattern. In cross section, the basin forms an asymmetric half graben that grew through time. The basin consists of three different superposed small basins: Castaic, Ridge Route and Peace Valley, and Hungry Valley systems, formed adjacent to the San Gabriel fault. Each of the depositional basins is rectangular and consists of four structural elements: (1) San Gabriel fault, (2) Ridge Basin syncline, (3) structural arch, and (4) eastern boundary fault systems. Three structural arches occurred during Ridge Basin history and subdivided the basin into subbasins that influence sedimentation. These structural arches are the Sierra Pelona antiform (Castaic), Clearwater arch (Ridge Route and Peace Valley), and Liebre arch (Hungry Valley time). Common to these sedimentary basins is the Violin Breccia, which forms a 1.5-kmwide by 32-km-long sedimentary breccia deposited continuously along the San Gabriel fault. The Violin Breccia consists of three lithofacies: (1) fault gouge and talus that is highly brecciated and sheared adjacent to the fault (displaying steeply-dipping seismic doublelet diffractions), (2) disorganized talus and debris-flow beds that are poorly sorted and sheared (displaying chaotic to transparent reflections), and (3) organized beds that are moderately sorted and interfinger with the other depositional systems (displaying moderate-to high-amplitude downlapping wedges). Sedimentary facies include marine slope aprons, marine and lacustrine fan deltas, and alluvial fans. The Castaic depositional system occurs in an 8-by 30-km elongated basin that contains >4000 m of mainly marine sediment. The Sierra Pelona antiform subdivides this basin into broad shallow-marine shelf area to the south and deep-water area to the north with steep margins to the northeast. Sedimentary facies include (1) thin basal shallow-marine transgressive and nonmarine units to the northeast, (2) a lateral transition into a thick deep-marine slope, submarine fans, and basin plain deposits to the north and west, and (3) slope, fan, and deposits that shoal upward and laterally into shelf, deltaic, and fluvial sequences. The basal transgressive units onlap the east side of the basin at a 15°angle. The initial extension of the basin was to the southeast, and the basin axis was nearly symmetrical and near the center of the basin. With oblique slip along the San Gabriel fault, the basin's geometry became markedly more asymmetrical, and the slip direction became more closely parallel to the fault's 17