LATE PLEISTOCENE-HOLOCENE ALLUVIAL HISTORY OF SANTA ROSA ISLAND, CHANNEL ISLANDS NATIONAL PARK, CALIFORNIA

At 215 km² (83 mi²), Santa Rosa Island is the second largest of the California Channel Islands. One of its more striking landscape features is a series of incised alluvial valleys that characterize most of the larger drainages on the island. North of the E-W trending, oblique-slip Santa Rosa Island Fault, streams flow across an old marine terrace surface, forming broad, trough-shaped valleys containing inset alluvial floodplains that have been re-incised to form relict terraces standing as much as 12 m above narrow, steep-walled arroyos, or barrancas. South of the fault, the more rugged topography is characterized by narrower, steep, v-shaped valleys, in which alluvial terraces are discontinuous along the length of the valley. Alluvial aggradation began at least 24 ka cal BP and continued in response to rising base level conditions as sea level rose from its Last Glacial Maximum lowstand, which locally was 95 m (311 ft) below its current level. The highly erodible marine sedimentary bedrock and steep valley side slopes are prone to landslides and slope wash that contribute large amounts of sediment to the channels. In several of the larger canyons the alluvial fill exhibits a distinctive sequence of alternating lighter and darker layers of sediment. The darker layers generally contain silty clay or clay, whereas the lighter-colored layers are composed primarily of silt and sand. The lighter-colored sediments probably represent channel margin or floodplain deposits, whereas the darker sediments likely represent either paleosols that developed during relatively stable and (or) wetter periods, or in-stream wetlands that developed within the channels of the larger perennial or nearly-perennial streams. Channel-fill sequences containing sand and gravel are locally found cutting into or through the floodplain deposits. Arroyos that cut through Chumash shell middens and burial grounds indicate that re-incision of the streams likely occurred sometime in the past few hundred years.