Braided submarine channels produced with experiments suggest scale independent controls on planform morphology similar to rivers

Abstract:

There are two dominant planform morphologies in channelized aggradational sedimentary systems; meandering and braided. Within marine settings meandering channels formed from turbidity and density currents are the most abundant. Braided channels are rare, and it is unclear if the few documented cases are generated by spontaneous bar deposition or are an artifact of erosive events or seafloor bathymetry. This is in contrast to fluvial systems wherein both meandering and braided planforms are common, and led us to the question if submarine channels require different conditions from river systems to obtain braided planform morphologies. We ran two experimental series wherein we provided density currents with an initial channel geometry known to produce braiding in natural and experimental river systems. Fluvial braiding occurs across a wide range of scales, and we predicted the same would hold true for subaqueous systems. The two experiments had initial width to depth ratios of 1000:1 and 350:1 and similar sediment to saline water discharge ratios of 0.02 and 0.03, and a saline current with a density contrast similar to natural currents. The experiments freely evolved a network of individual channels separated by depositional bars. Moreover, existing stability model theory for fluvial systems successfully explains both braided and meandering submarine channels from a compilation of lab and field examples. This suggests the aspect ratio of the flow determines the planform morphology, and that it is a scale independent phenomenon. The rarity of braided submarine channels in nature is more likely explained by factors that inhibit channel widening in submarine systems as compared to fluvial systems. We suggest the cause is a combination of the relatively thicker flow depths of turbidity currents that require commensurately wider flows, and a variety of feedbacks that tend to yield higher overbank sedimentation rates in submarine channels compared to fluvial systems.