Feather-like submarine channels: a unique imprint of overbank flows imaged by 3D seismic data

Abstract

We document for the first time the extensive occurrence of “feather-like channels” in the 19000-year-old glacigenic submarine strata in the North Sea Fan (NSF). We describe these features in the uppermost deposits of the NSF, predominantly on the surface that marks the end of the period of shelf-edge glaciation, using over 14000 km2 of high-resolution 3D seismic reflection data (vertical resolution of 2m and bin size of 6.25 x 18.75m).  These channels are a few 10’s of meters wide and depths on the limit of seismic resolution (~2 m). They lack clear cross sections, mostly presenting as disruptions in the otherwise readily traceable reflections, which make them easier to identify in amplitude maps rather than structure maps and seismic profiles. These “feather” channels occur exclusively in association with larger, deeper channels. The “feathers” diverge from the margins of the main channel, forming an obtuse angle with the flow direction of the main channel, becoming progressively sub-parallel further downstream, similar to a bird’s feather, with the divergence from the main channel axis in the downstream direction. They run for varied distances, as short as a few 100’s of meters and up to 7 kilometers. It’s also important to highlight that they occur extensively throughout the surface, with a small spacing of 10’s of meters between each other. Similar features have been described by others as lineations formed at the base of debris flows. This was credited to circular depressions found at the end of such lineations and the fan shape that those features would create at the end of a main channel body. This description is clearly different from what we have described in this study, where both circular depressions and fan-shaped terminations were absent. Here, we interpret them as the record of overbank flows from the main channel, due to their geometry and dimensions, representing the large pulses meltwater coming from the shelf. Our investigation into the “feather-like channels’’ reveals a unique seismic geomorphology, in a well understood palaeogeographical setting. The exclusive association of these channels with larger, deeper counterparts, their small spacing, and varied distances emphasize their pervasive nature. This research not only refines our understanding of submarine sedimentary dynamics, but also highlights the indispensable role of high-resolution 3D seismic data in understating the subsurface.