Flutes, megaflutes and erosional bedforms: a reappraisal of their dynamics

Erosional bedforms in deep-sea environments record the passage of highly erosive turbidity currents that are often voluminous and fully bypassing. This investigation focuses upon a subset of erosional bedforms that are elliptical in shape, and utilises a multidisciplinary approach to reappraise the occurrence and dynamics of the bedforms across all scales. The investigation provides highly detailed analyses for three scales of form: centimetre-scale flutes, metre-scale megaflutes, and large metre-to-kilometre scale scours. The results confirm that deep-water scours form a continuous spectrum from centimetre- to kilometre-scale. Furthermore, the investigation effectively ‘bridges the scale gap’ between traditional scales of analysis of outcrop and modern system. Laboratory modelling of the erosion of mud beds provides the first experimental generation of flutes upon a smooth mud-bed in open channel flows. High-resolution bathymetric data taken in time-series during experiments demonstrate that bedforms are stable, migrate downstream with time, and form in association with features not previously generated under experimental conditions, including pock-marks, gullies and potholes. The study also revisits the outcrops of the Carboniferous Ross Sandstone (SW Ireland) to address the stratigraphic occurrence, morphology and lithology of megaflutes and associated thickening-upward packages. This has identified that packages form via prograding lobe elements, deposited by mud-rich sands, and record a vertical trend from distal- to proximal-deposits accompanied by an increasing frequency of megaflutes and broad erosional surfaces. Existing process-models for these packages are re-examined, and a six-stage model for lobe-element evolution is proposed that documents successive phases of deposition, sediment bypass, erosion and lobe abandonment. High-resolution AUV data and well-constrained core control present the bathymetry (2x2 m pixel size), sedimentology and chronology of four large-scale modern deep-water canyon/channel systems along the northeast Atlantic continental margin. Scours range from 40-3170 m wide and 8-48 m deep and occur in zones of rapid flow expansion, such as channel mouths or overbank areas.