Ongoing evolution of submarine canyon rockwalls; examples from the Whittard Canyon, Celtic Margin (NE Atlantic)

Carter, Gareth D.O., Huvenne, Veerle A.I., Gales, Jennifer A., Lo Iacono, Claudio, Marsh, Leigh, Ougier-Simonin, Audrey, Robert, Katleen and Wynn, Russell B. (2018) Ongoing evolution of submarine canyon rockwalls; examples from the Whittard Canyon, Celtic Margin (NE Atlantic). Progress in Oceanography, 169, 79-88. (doi:10.1016/j.pocean.2018.02.001).

Abstract

During the CODEMAP 2015 research expedition to the Whittard Canyon, Celtic Margin (NE Atlantic), a Remotely Operated Vehicle (ROV) gathered High Definition video footage of the canyon rockwalls at depths of approximately 412–4184 m below sea level. This dataset was supplemented by predominantly carbonate rock samples collected during the dives, which were subsequently tested for key physical property characteristics in a geotechnical laboratory. The high-resolution video footage revealed small-scale rockwall slope processes that would not have been visible if shipboard geophysical equipment was solely relied upon during the survey. Of particular interest was the apparent spalling failure of mudstone and chalk rockwalls, with fresh superficial “flaking” scars and an absence of sessile fauna possibly suggesting relatively recent mass-wasting activity. Extensive talus slopes, often consisting of coarse gravel, cobble and occasionally boulder-sized clasts, were observed at the foot of slopes impacted by spalling failures; this debris was rarely colonised by biological communities, which could be an indicator of frequent rockfall events. Bio-erosion was also noted on many of the walls prone to this form of rock slope failure (RSF). As in subaerial equivalents, internal fracture networks appear to control the prevalence of RSF and the geometries of blocks, often resulting in cubic and tabular blocks (0.2–1.0 m scale) of bedrock toppling or sliding out of the cliff face. Tensile strength parameters of carbonate rock samples were determined and these may affect the mass wasting processes observed within the canyon. It was found that carbonate samples which appeared to have a higher mud content, and reduced porosity, produced significantly higher tensile strength values. It is proposed that these stronger, “muddy” carbonate units form the overhanging ledges that often provide an ideal setting for sessile species, such as Acesta excavata clams, to colonise whereas the weaker “pure” carbonate units are more easily eroded and therefore form the undercutting, receding sections of the rockwall. By combining the ROV observations, basic discontinuity assessments (estimation of fracture orientations) and laboratory testing results, an understanding of the geomechanical properties of the bedrock can be obtained and linked with past and ongoing rock slope processes within the Whittard Canyon. These conclusions will have a wider implication for ongoing geomechanical processes within submarine canyons on a global scale.

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Contributors

Author: Veerle A.I. Huvenne

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