UC Santa Barbara

Heightened storm activity, drought, and large earthquakes have the potential to impact daily life in Southern California and transform its coastal landscape. To better understand the nature and frequency of these events, the stratigraphy and seismic architecture of University of California, Santa Barbara (UCSB), Campus Lagoon (referred to as Campus Lagoon hereafter), a flooded river valley incised into a marine isotope stage (MIS) 3 marine terrace near Santa Barbara, California, was examined. The valley fill provides a record of late Holocene environmental change for the Santa Barbara region as well as a potential archive of coseismic uplift within the western Santa Barbara Fold Belt (SBFB). Nine sediment cores and 2.70 km of high-resolution seismic data from Campus Lagoon were examined. From the collected sediment cores, six sedimentary facies were identified. These facies span three broad depositional environments including intertidal, estuarine, and lower supratidal environments. The oldest deposits sampled in this study were coarse-grained gray sand facies (GS). Radiocarbon ages of the GS facies suggest deposition started at least ~5 ka and lasted until at least ~4 ka. The clayey silt with shell fragments facies (CS-SF) is found interbedded with the GS facies from ~4.5 ka and ~3.7 ka. Clayey silts with and without evaporites, respectively referred to as CS-E and CS facies, occur throughout the lagoon and typically reside atop the GS facies. Gypsum lamina occurs throughout Campus Lagoon between ~3.2 ka and ~0.6 ka. The CS-E and CS facies are atop the GS facies and are occasionally interbedded with packages of sandy silt facies (SASL), which lack coarse pebbles, rip-up clasts, and shell fragments. The GS facies is typically also found in the stratigraphic top of sediment cores in the southeast region of the lagoon. Deposition of the upper beds of the GS facies occurs starting ~0.8 ka and ~0.7 ka in the southeast region of Campus Lagoon. The upper GS facies laterally grades into the SASL facies in the central region of the lagoon. The SASL facies was concurrently deposited with the GS facies starting ~0.6 ka and is interpreted to represent the distal sands associated with the GS facies. A laminated sandy silt facies (LSS) farther inland was concurrently deposited with the GS and SASL facies ~0.6 ka. The LSS facies is interpreted to be terrestrial input from sheetwash in the northwest region of Campus Lagoon based on its geographic location, lack of marine signatures, finer sediment grain size, and higher charcoal and wood fragment abundance. Deposition of the GS facies between at least ~5 ka and until at least ~4 ka is interpreted to represent the onset of heightened El Niño Southern Oscillation (ENSO), which contributed to the migration of sands within the Santa Barbara littoral cell from the west to the east. The CS-E and CS facies represent evaporative mudflat environments, which are interpreted as responses to periods of late Holocene aridity, specifically the Late Holocene Dry Period (LHDP) and Medieval Climate Anomaly (MCA). The return of the GS facies, as well as deposition of the SASL and LSS facies, between ~0.7 ka and ~0.6 ka coincides with the onset of stormy conditions associated with the Little Ice Age (LIA). In addition to providing information related to paleoclimate, coastal depositional environments have the potential to preserve tectonic events and document changes in relative sea level (RSL). Two separate downward shifts in onlapping seismic reflections observed within the southeast region of Campus Lagoon suggest at least two potentially rapid falls in RSL between ~3.7 ka and ~0.6 ka. Seismic unit (SU) 1 consists of the GS facies and is overlain by the CS-E / CS facies. SU 2 onlaps SU 1 at a lower elevation, suggesting a possible transition from an intertidal zone to a lower supratidal mudflat environment. A second possibly abrupt RSL fall is observed between SU 3 and SU 4 in which SU 4 onlaps SU 3 at a lower elevation. Possible RSL falls may have been triggered by large magnitude earthquakes that uplifted the lagoon. As RSL fell due to potential coseismic uplift of the land surface, marine water circulation between the ocean and lagoon may have been hindered and possibly exposed the middle and back regions of Campus Lagoon to drought-like conditions associated with the LHDP and MCA during the late Holocene.