Grain-size fractionation influence recorded lobe in the deposits of the La Jolla Canyon-Fan System

Submarine lobe complexes consist of multiple sediment lobes constructed by sediment gravity flows and channel avulsions. To understand sediment transport processes and internal architecture it is critical to evaluate the impact of compensational stacking and channel avulsion on the resulting sediment record. Offshore Southern California, the La Jolla Canyon decreases in relief and confinement downstream, transitioning into an active lobe complex comprising channels, channel-forms, scours, and sediment waves. To characterize the upper ~10 m of lobe architecture, we integrated 1-meter-scale seafloor bathymetry and 1–6 kHz chirp sub-bottom profiler data with 18 sediment cores (20–130 cm) that were collected along two transects of the La Jolla lobe complex: one spanning the channel-to-lobe transition zone and one across the distal lobe complex. In the channel-to-lobe transition zone, at least three low-relief (~5 m) channels are present, and we focus on the broadest (>250 m wide), most continuous of these channels. The La Jolla canyon-channel system actively feeds the western channel where cores show a sandy fill in the upper 20 cm. The head of the eastern channel is disconnected from the main La Jolla channel and filled with muddier hemipelagic sediment, potentially representing abandonment of an older channel and lobe. Downstream, across the distal transect, cores were collected from lower-relief (<2 m) channel-forms between at least one lobe-shaped mound. CHIRP profiles show at least five lobes compensationally stacked from east to west. Cores show overall muddier deposits, i.e., mixtures of sand and mud with >25% silt and finer grains by volume. We interpret the proximal-distal change in grain size to reflect coarser deposition by sediment-gravity flows at the channel mouth, while finer grains are transported farther into the distal lobe complex. These observations of grain-size fractionation highlight the role of sediment gravity flow processes in controlling lobe formation, stacking, and internal architecture within an active submarine canyon-fan system.