Late Pleistocene to Holocene environmental history of Devereux Slough, California

Zachary Nelson, Alexander R. Simms, & Matthew E. Kirby

In Preparation August 1, 2025, SCEC Contribution #14371

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 coastal environmental history of the highly populated Southern California coast. In this study we use six new cores from Devereux Slough, a flooded incised valley near Santa Barbara, California, to better understand past coastal hazards and environmental changes along the southern California Coast. 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 is similar to the record of floods preserved in marine cores from the Santa Barbara Basin and likely record terrestrial floods. 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; it was not preserved due to environmental factors, 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 Cerithideopsis (formerly 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 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.

Key Words
Santa Barbara, Moore Ranch Fault

Citation
Nelson, Z., Simms, A. R., & Kirby, M. E. (2025). Late Pleistocene to Holocene environmental history of Devereux Slough, California. The Holocene, (in preparation).

Related Projects & Working Groups
Ventura Special Fault Study Area