17:15 〜 19:15
[MIS01-P07] Submarine canyon wall-zone architectures, depositional patterns and formative processes: insights from subsurface and outcrop data
Previous canyon studies focus mainly on canyon infills, but unintentionally neglect canyon walls, leaving architecture and interpretation of canyon wall-zone strata unknown. We document, using outcrop and subsurface data, cases where the canyon wall is not the single surface that might be assumed from seismic images but rather a zone that includes slowly accumulated, fine-grained, draping stratified sediments. Our results suggest that submarine canyon ‘walls’ can have three main components: (1) a basal wall zone, sharply and unconformably truncating the underlying strata, containing one or more irregular erosional surfaces with muddy interbeds; (2) a middle-upper part of the canyon-wall zone that is also mud-prone, recognized on seismic data as a single high-amplitude peak or trough reflector of a mud-rich interval up to 10-30 m thick, but showing an upward decrease in gamma-ray value. These muddy beds and drapes are low-density turbidites (i.e., Bouma Tc, Td, and Te) with an upward increase in bed thickness and sand content. They therefore represent an upward-coarsening succession that downlaps onto the canyon wall and are interpreted in terms of the gradual expansion of cross-stream, dilute turbidity-current flows as they infill the canyon. It is the intermittent lateral progradation of marginal dilute turbidity-current tails that form the muddy (occasionally sandy) draping of the canyon walls; (3) an upper sharp surface to the canyon wall-zone is then overlain by onlapping thick, amalgamated turbidite beds of the main canyon fill. Our results challenge conventional viewpoints on the basal bounding surface of submarine canyons and therefore contribute to a more complete picture of submarine canyon stratigraphy and sedimentology.