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Straight, Asymmetric Channels and Longitudinal Bars Within Channelized Seafloor Areas: Example From The Modern Seafloor


Submarine channels occur in many physiographic domains of the deep-sea environment. They have a large spectrum of planforms, dimensions, internal elements and hierarchic significance. This paper focuses on a type of submarine channel found in different environmental settings of the modern seafloor in the Tyrrhenian Sea. The channels are straight or very gently curving and vary in width from 100 to 500 m, with a maximum relief in the order of 10–15 m. Their main feature is a noticeable asymmetry resulting from a steep, abrupt margin and a gentler, more gradual one. Channels are never isolated, but are rather part of extensive channelized areas. Within the channelized areas, elevated longitudinal areas separate the channels. The elevated areas have sediment waves and scours indicative of flows that from their crests point toward the adjacent channels. Sidescan sonar data show that along the gentle channel margin a clear passage between the channels and the bars cannot be picked up. On the contrary, an abrupt limit is observed in coincidence with the steep channel side. Subbottom profiles indicate that the elevated areas are depositional, with deposits that continue along the gentle channel side and, although often with a different seismic facies, also represent the channel infill. Thus, the elevated areas are not erosional remnants between fully erosional channels. Upslope from the channelized areas the seafloor is flat indicating that it is the site of laterally widespread flows. Therefore, we also infer that the elevated areas are not levees, since they are not formed by flows which overbank the channels. They are rather formed from flows that occupy the whole channelized areas and behave differently in the channels and in the elevated areas. We conclude that the elevated areas are better interpreted as bars. As a whole, we explain the channel as being migrating elements with an erosional cut bank and a depositional one where lateral accretion is occurring from the longitudinal bars. The channels and longitudinal bars are located downslope from slope channel mouths in transient fans, or compose channel belts confined within slope channels. They develop on seafloor with a gradient in excess of 1° and where sampled, they consist of sandy or gravelly sediments. The Tyrrhenian sea data, suggest therefore that laterally migrating channels and longitudinal bars are components of channelized, high-gradient, coarse grained systems connected to deeper base levels.