Morphology, evolution and fill: Implications for sand and mud distribution in filling deep-water canyons and slope channel complexes

Abstract

A survey of the northeastern margin of the Rockall Trough on the Irish margin examined the transition from shelf edge to basin floor, in morphology and sedimentary activity, of a deeply incised submarine canyon system, the Donegal Bay submarine canyon. The survey produced superb 3D profiling of the canyon along its entire length, marking a transition from ‘cauliflower’ shaped head region with numerous tributary gullies feeding into one main canyon, to a single trunk canyon. This canyon, with an initial combined width and depth of > 17 km and > 800 m in the ‘cauliflower’ head area, decreases rapidly to > 4.5 km wide and > 450 m deep after the zone of tributary confluence. Eighteen kilometers further down dip, the canyon loses topographic expression as it approaches the lower rise and floor of the Rockall Trough. Degrees of recent sedimentary activity are evaluated by comparing side scan sonar systems of different frequency, and thus of different penetration sub sea, and by ground-truthing using drop (gravity) cores. The canyon was a very active system, dominated by sand transportation towards the floor of the Rockall Trough, along the slope as coarse-grained contourite, or as sand spillover from the shelf. Sand was also deposited as overbank deposits outside the main head region of the canyon, presumably by large volume turbidity currents and more active lateral gullies. The head area of the canyon system has been progressively cut off from sand source by progressive sea level rise since the last glaciation. Sand was locally deposited on terraces but not in the overbank area. Less frequent, lower volume and finer grained turbidity currents have become more common in the system. The initial sand and bypass-dominated system with small sediment waves, which may be gravels, has become dominated by muddy debrites in the lower reaches and by slumps in the upper reaches. Slumping in those upper reaches leads to ponding of sand in the head and upper reach areas, with only very occasional turbidity currents transporting sand further down the system in small channels. A model is produced to explain the mechanism and expression of backfilling in a large deep-water canyon system whose hinterland has been flooded back since glacial drawdown of sea level in the eastern Rockall Trough area. This model explains how sand may be trapped in large volumes in the upper reaches of a canyon system, due to slumping from the canyon margins and nearby upper slope regions. The focusing of sand deposition in areas where this is not usually expected will have important implications for hydrocarbon explorationists who wish to map the distribution of potential reservoir sand bodies within large, confined deep-water canyon systems.