Submarine flow tills at Victoria, British Columbia: Discussion

Abstract

Hicock et al. describe complex submarine outwash cones from Quarternary glacimarine exposures. I do not question their interpretation of the diamicton sediments in the cones as being submarine flows or slumps from glacial debris; indeed, their descriptions are invaluable for tidewater glacier facies modelling. However, I have some comments on the suggested origin of the flows. Several different configurations of tidewater ice fronts may exist, but few allow the formation and preservation of sediment gravity flow deposits that originate subaerially and flow into the sea. Under polar conditions where glaciers cannot maintain themselves as floating ice tongues or ice shelves, they end in very shallow water and a beach is constructed at the base of their cliffs (e.g., Sugden and John 1976, Fig. 11.94. Most debris is slowly melted out, but rarely flowage may occur. However, such material is highly likely to be reworked by waves and, further, the ice margin in Puget Sound that Hicock et al. describe was probably not under polar-type conditions. In my experience of modem temperate tidewater glaciers in Glacier Bay, Alaska, only restricted conditions allow thick accumulation of supraglacial melt-out tills from englacial debris layers. Ablation of tidewater glaciers occurs more by berg calving rather than upper-surface melting, and ice fronts (at least above tide level) are nearly vertical cliffs. Ice is very crevassed back from the face, and the free face with consequent extensive flow commonly creates large transverse crevasses. Englacial debris layers are exposed across an ice face rather than transversely across the upper surface. However, the sides of each layer may intersect the upper surface of a glacier snout to form supraglacial melt-out debris parallel to the valley walls. Other supraglacial debris is primarily from rockfalls off valley walls and is rubble rather than diamicton. Rarely, some debris flows down the face from the upper edge, but the steepness of slope ensures sorting and separation of particles as the flow proceeds. The larger mass of pebbles and coarse-grained sand particles causes them to fall faster