Late Pleistocene to Holocene environmental history of Devereux Slough

摘要

Environmental histories of coastal regions are important for providing historical perspectives on restoration projects as well as evaluating the risks due to tsunami and other coastal hazards. Currently, little is known about the Late Pleistocene through Holocene tsunami history of the highly populated Southern California coast. Devereux Slough is a flooded stream valley near the Southern Californian city of Santa Barbara. In this study I analyze six new cores from Devereux Slough containing sediments spanning the last 16 ka in order to provide insights into past coastal hazards and environmental conditions of the estuary. Five facies representing four main environments are identified. These facies include a pebbly sandy mud, a bioturbated mud, a laminated silt, a well sorted sand, and a brown silt. The succession of environments represented by these facies largely reflects the late Pleistocene/Holocene transgression following the last glacial maximum ~20 ka. The laminated silt resembles deposits from modern alluvial fans feeding the slough from gullies on its margins. The temporal distribution of this facies reflects that of floods preserved in marine cores from the Santa Barbara Basin and likely records periods of terrestrial flooding across the landscape. Interbedded well-sorted sand and laminated mud date to 3930+/-410 cal BP and likely represents alternating storm overwash and terrestrial floods. No evidence of marine inundation at the time of proposed large earthquakes on the nearby Pitas Point Thrust were found suggesting that either the fault was not tsunamigenic, the tsunami produced was less than 2 m in elevation, or the tsunami was confined to the eastern-most portions of the Santa Barbara Channel. Two sea-level index points based on radiocarbon ages from the intertidal gastropod Cerithidea californica provide rates of vertical motion for two locations within the slough. One index point suggests uplift at a rate of 0.25+/-0.39 mm/yr and the second subsidence at a rate of 0.05+/-0.39. As both are within error of zero and I have not accounted for compaction, the slough is likely on the uplifting side of the Moore Ranch Fault placing the fault to the north of the slough.