Avulsion, inner bend bank failure, and lateral accretion: Holocene deep marine Lucia Chica channel complex, offshore central California

The youngest channel of the Lucia Chica channel system (LCCS), located offshore the Sur coast of central California, USA, is imaged with closely spaced Chirp reflection seismic profiles integrated with core and radiometric ages. Data record inner bend bank failure deposits, outer bank erosion, and younger depositional wedge deposits, less than 6 m thick, that resemble deep marine Lateral Accretion Packages (LAPs). As is typical of LAPs, the two cored depositional wedges at LCCS consist of coarse-grained, amalgamated sands beneath interbedded muds and fine-grained sands.

The failed portions of the inner bend levees form bathymetric terraces that extend 130 to 300 m parallel to the loop axis, 200 to 600 m perpendicular, and 12 to 20 m thick. Avulsion occurred between Loops 3 and 4. At the most distal loop of the daughter channel, Loop 5, the down-flow side of the inner bend consists of a series of offlapping deposits that result in at least 300 m of channel sweep and down-flow levee growth. The combined contributions of avulsion, outer bank erosion and inner bank deposition result in increased thalweg sinuosity and decreased levee crest sinuosity with time. Evolutionary changes of the studied portion of the LCCS correlate well with changes in eustatic sea level with avulsion prior to 27 kybp, early in oxygen isotope stage 2, to abandonment about 6.5 kybp, during stage 1.