SANTA BARBARA BASIN, CALIFORNIA FLOOD LAYER PROVENANCE: MAPPING FLOODING HAZARD BY SEDIMENT SOURCE REGION

Flood deposit provenance investigations can be used to identify the sources of sediment eroded during extreme precipitation events, and thus indicate areas likely to be at-risk during future flood events. In southern California, Santa Barbara Basin (SBB) sediment contains a continuous Holocene flood record, and lithologic diversity of catchments discharging into SBB create unique sedimentary fingerprints. Here we identify the provenance of SBB flood layers using quantitative mineralogy and elemental and isotope geochemistry.

Twenty-four stream bed sediment sediment samples were collected within the five catchments (Santa Clara River, Ventura River, Santa Ynez River, Santa Ynez Mountains, and the Northern Channel Islands) that discharge into SBB, and separated into 63-250 μm and <63 μm size fractions. Additionally three recent SBB flood layers (A.D. 1861-62, 1761, and 33-53) were sampled from marine cores SPR0901-04BC (34º 16.895’ N, 120º 02.489’ W, 588 m water depth) and MV0811-14JC (34º 16.906’ N, 120º 02.162’ W, 582 m water depth). Quantitative mineralogy, elemental concentrations, and radiogenic Nd and Sr isotopic ratios were determined. These complementary techniques enable distinction between source areas. Mass balance and mixing models are used to quantify catchment source contribution to SBB flood layer sediment.

Flood layers contain smectite (>30%), quartz (14%), plagioclase (>11%), illite (>9%), and potassium feldspar (>7%), with minor chlorite (5-6%) and kaolinite (3-4%). The <63 μm size fractions of stream bed sediment samples contain 10-35% smectite and <13% illite. Chlorite and kaolinite abundances differ among the catchments. Chlorite is present only in catchments draining the southern flanks of Santa Ynez Mountains. This may indicate SBB flood layer sediments are derived primarily from the southern Santa Ynez Mountains, including the highly populated lower Santa Clara River, Ventura River, and Santa Ynez Mountains catchments.