Variation in canyon morphology on the Great Barrier Reef margin, north-eastern Australia: The influence of slope and barrier reefs
Résumé
New high-resolution bathymetry, seismic reflection profiles, and existing sidescan data have revealed the presence of a series of submarine canyons in the slope off the Noggin Passage region, north-eastern Australia. The morphology of the Noggin Canyons contrasts with that of the canyons in the Ribbon Reef region, further north along the north-eastern Australia margin. The Noggin Canyons are mostly slope-confined, with canyon heads located at water depths between 200 and 400. m. These narrow and straight canyons show a decrease in canyon relief with depth, and have incision values and canyon gradients lower than the Ribbon Reef Canyons. New findings on the Ribbon Reef Canyons reveal an increase of canyon relief with depth in the shelf-incised canyons, as well as complex relationships between geomorphic parameters, such as canyon gradient, incision, canyon width and canyon wall gradient. The main factors controlling the differences in canyon morphology are the shape of the continental slope and the presence of barrier reefs at the shelf-edge. Steep exponential and linear slope profiles, and the presence of an extensive shelf-edge barrier system in the Ribbon Reef region, are related to large shelf-incised canyons. In contrast, the slope-confined canyons of the Noggin region are linked to sigmoidal slopes, and more open outer-shelf conditions lacking barrier reefs. These conditions allow higher overall sediment supply to the upper slope, and the resulting formation of sigmoidal slope profiles. In the Ribbon Reef region, the physical barriers provided by the shelf-edge barrier reefs reduce the amount of shelf-to-basin sediment transport, thereby forming exponential slopes. Further, sediment gravity flow deposition through the canyons is more prominent in the Ribbon Reef region, as a direct consequence of the more frequent breaching of the shelf by the canyons, otherwise infrequent in the Noggin region. Our results highlight this particular relationship between canyon and slope morphology, and the importance of the variable shelf-edge morphology in controlling the shelf-to-basin sediment transport. This aspect is especially relevant for understanding the margin development in modern and ancient mixed carbonate-siliciclastic continental settings.