Origin of shore‐normal channels from the shoreface of Sable Island, Canada

Abstract

[1] A suite of modern, shore-normal channels was discovered on the southern shoreface of Sable Island, Canada. Repetitive multibeam and side scan surveys of these features, as well as box coring and sampling, showed that the depressions were perennial features of the shoreface. They occurred in fine and medium sand and were most abundant indepths less than 20 m. They were up to 1 m deep and 50 m wide, increased in size seawards, and generally followed the seabed slope. The channels dominated the inner shoreface becoming wider and deeper seawards to depths in excess of 40 m. There is a continuum of scales in width and depth of these channels, the smallest of which are equated with gutters preserved in the geological record. The internal structure of the shoreface sediments where the gutters are found is characterized by normally graded tempestites composed of cut-and-fill structures overlying an erosion surface. Thus, the environmental setting, internal structure, and scale are similar to ancient counterparts. Gutters have been interpreted to result from downwelling over palaeo-shorefaces during ancient storms [Myrow, 1992b]. The composition of gutter infill, as well as the stability, evolution, associated bed forms, and hydrodynamic conditions under which they formed, were examined to verify genetic interpretations of the ancient counterparts. The multisensor benthic lander RALPH was deployed within a field of shore normal channels for 17 days during late winter 1998. RALPH burst-sampled flow at hourly intervals while imaging the seabed. Resulting sequential imagery revealed the genesis of a group of five gutters during storm spin-down that was coincident with the formation of largescale wave ripples. This was followed by infilling of the gutters and wave ripple destruction within 20 hours by longshore sand transport. The gutters were located within, and parallel to, a larger channel (50 m wide and 0.50 m deep). They were formed by coastal set-up and subsequent downwelling that was triggered by the rotation of storm winds from seawards (onshore wind) to landwards (offshore wind). The gutters formed within 2-3 hours during spin-down of a winter storm under ''live-bed'' conditions of sand transport. Wave height and mean tidal flows were well below those of the storm peak and so they were not directly responsible for creating the gutters. The mechanism we propose for the evolution of gutters supports the geological interpretations; that is, a storm-driven, downwelling event as described by Swift and Niedoroda [1985]. However, the downwelling event appeared to take place suddenly (rapid acceleration in flow), briefly (few hours duration), and within narrow corridors guided by wider and deeper channels of the shoreface. High turbidity during the time of gutter formation leads us to believe that the phenomenon of inner shelf turbidity currents [as suggested by Walker, 1985] exists and may be instrumental in channel formation. INDEX TERMS: 4546

Sedimentology and reservoir properties of tabular and erosive offshore transition deposits in wave-dominated, shallow-marine strata: Book Cliffs, USA

“…These channels could also be interpreted as incised valley fills, but this seems unlikely due to that there are no evidence for this interval being subject to subaerial exposure, and no evidence for contemporaneous lowstand deposits. These channels could also be the deposits of shore-normal channels carved by storm-generated downwelling events (Héquette and Hill 1993;Amos et al 2003), but this does not explain that the subaqueous channels occur in areas in front of large distributary channels (Fig. 6), as this model would predict that these channels should be located throughout the study area.…”

Erosion and accretion processes in a muddy dissipative coast, the Chao Phraya River delta, Thailand

“…Furthermore, it is unlikely that rain caused the seabed erosion (Tolhurst et al, 2009) A possible mechanism of seabed erosion is enhanced wave dissipation when the tidal level was low and northeasterly winds caused wave breaking. This erosive process is similar to the formation of shore-normal channels reported by Amos et al (2003) in the sense that the erosion occurs during very short periods under less-energetic and low water level conditions. In the present case, turbid water induced by the enhanced dissipation may retreat across the intertidal zone along with ebb tides and transport sediment offshore ( Figure 9).…”

Sedimentology and reservoir properties of tabular and erosive offshore transition deposits in wave-dominated, shallow-marine strata: Book Cliffs, USA

“…These channels could also be interpreted as incised valley fills, but this seems unlikely owing to the fact that there is no evidence for this interval being subject to subaerial exposure, and no evidence for contemporaneous lowstand deposits. These channels could also be the deposits of shore-normal channels carved by storm-generated downwelling events (Héquette & Hill 1993;Amos et al 2003), but this does not explain that the subaqueous channels occur in areas in front of large distributary channels (Fig. 6), as this model would predict that these channels should be located throughout the study area.…”

“…These channels are interpreted to be turbidite-filled subaqueous channels linking river mouths to prodelta turbidite lobes, cut by river-fed hyperpycnal currents (Pattison 2005a, b;Pattison et al 2007). Several authors have described localized erosion near turbidite channels (Elliott 2000;Higgs 2004) and proximal turbidites (Amos et al 2003). Erosion due to bypassing turbidites is, therefore, a reasonable mechanism that may explain the presence of erosive offshore transition-zone deposits.…”

“…Amos et al (2003) observed a series of gutters (small channels less than 3 m wide and 0.5 m deep, and more than 40 m long) within irregular shore-normal channels, up to 1 m deep and 50 m wide, offshore Sable Island on the Scotian Shelf, Canada. These were interpreted to form by scour of turbidity currents during a downwelling event caused by coastal set-up during strong onshore winds.…”