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Insights From Large-Scale Experiments: A New Depositional Model for Hybrid Beds Within Submarine Lobes


Distal regions of submarine lobes exhibit stacked successions of sand beds capped by silty horizons. These sand-silt packages are referred to as ‘hybrid beds’. One model proposes that hybrid beds are the product of originally dilute turbidity-currents that have entrained large quantities of silt along their multi-kilometer flow paths to the point of over-saturation. Over-saturation with fine grained sediment leads to: i) transportation of sands into the distal regions of lobes due to hindered settling velocities, and ii) damping of turbulence, which causes flow collapse and rapid deposition of silts. However, new experiments conducted within a 36 x 2.7 x 1.3m flume at the St. Anthony Falls Laboratory, University of Minnesota, have uncovered a secondary process for the construction of hybrid beds. These experiments investigated the entrainment and deposition of kaolin, and kaolin + sand, into overriding dilute turbidity-currents with maximum velocities ranging from 0.5-1ms-1. Flow processes were captured using a submerged side-view GoPro, overhead cameras, and an array of three 0.5 MHz Panametrics V301-SU transducers, whilst instantaneous suspended-sediment concentrations (~ 0.05 and 0.15m above the bed) were acquired using a LISST-Streamwise sampler. Findings from these experiments show that a secondary method to forming hybrid beds occurs from the entrainment of silts, or silts + sands, from a localized substrate source, which are then deposited as hybrid beds immediately downstream. In this model, the turbidity-currents do not become over-saturated with silts and sands resulting in flow collapse, but alternatively develop a partially entrained near-bed concentrated flow layer that deposits its sands and silts immediately downstream of initial entrainment. Importantly, the near-bed concentrated flow layer is not dense enough with silt to significantly hinder the settling velocity of sand particles. This allows for the basal deposits of these hybrid beds to be sandy, whereas their capping beds are muddy. With this knowledge there exists the potential to use the density and distribution of hybrid beds within distal lobe stratigraphy to understand in a relative sense when feeder channels across lobes turn on, and then off, as lobes are constructed.