Exploring a new breadth of cyclic steps on distal submarine fans

Research on the depositional record of submarine fans and related turbidite systems has highlighted the importance of channel, lobe and lev´ee–overbank architectural elements as fundamental building blocks. However, many of the characteristics and processes of deposits left by flows traversing those fans remain elusive, because flows seem to be able to go unconfined for long distances. Offshore southern California (USA), the La Jolla Canyon decreases in relief to become an approximately U-shaped channel across the basin floor of the San Diego Trough. The La Jolla Channel gradually loses confinement and transitions to a network of scours, some of which align to form incipient channels, and fields of bedforms. High-resolution seafloor topography, CHIRP seismic-reflection data, sediment cores and hydrodynamic flow analysis are used to explore these features. The focus is on two regions of bedforms: (i) a field of net-depositional, concentric bedforms across the eastern lev´ee–overbank upstream from the terminus of the La Jolla Channel; and (ii) a linear train of more erosional bedforms approximating an incipient channel adjacent to the present mouth of the La Jolla Channel. These bedforms are interpreted to be among a class of upper-flow-regime bedforms called cyclic steps, which were formed by densimetric Froude supercritical turbidity currents that spilled out of the present La Jolla Channel. The highresolution data for the La Jolla Fan provide valuable insights into the characteristics of supercritical bedforms likely common to distal submarine fans, as well as on sedimentary processes likely important for submarine fan growth into sedimentary basins. In particular, the pattern of evolution of the La Jolla Fan suggests that cyclic steps with wavelengths on the order of tens of metres to a few hundreds of metres could be fundamentally important for the evolution of the distal submarine fans with relatively low-relief main channels.