Abstract Douglas Channel is a 140 km-long fjord system on Canada's west coast where steep topography, high annual precipitation and glacially over-deepened bathymetry have resulted in widespread slope failures. A 5 year project involving numerous marine expeditions to the remote area produced a comprehensive assessment of the magnitude and frequency of slope failures in the region. A classification scheme is presented based on morphology and failure mechanism: (1) debris flows are the most common in all parts of the fjord – they are often small with a subaerial component where fjord wall slope is very high or tend to exceed volumes of 10 6 m 3 where fjord wall slope is lower, allowing for accumulation of marine sediments; (2) large failures of oversteepened glacial sediments occurring at transgressive moraines and glaciomarine plateaus following deglaciation – the largest is at Squally Channel with an estimated volume of 10 9 m 3 ; (3) fjord wall failures that involve bedrock slump or rock avalanche; (4) translation of marine sediments; (5) composite/other slides; and (6) two scallop-shaped sackungen, or deep-seated gravitational slope deformations of granodiorite with volumes exceeding 60 × 10 6 m 3 . The postglacial marine sedimentary record shows evidence of large-scale slope failures of all styles that were especially active following deglaciation. The Holocene marks a transition to a lower frequency and change to primarily debris flows and smaller rock slides. Slope failures that may be capable of generating tsunamis and may be damaging to coastal infrastructure have occurred in all parts of Douglas Channel through much of the Holocene. Here we present a morphological analysis with volume estimates and age control using multibeam bathymetry, high-resolution sub-bottom data and sediment cores. The study details an extensive analysis of slope failures in a fjord network that can be extended to other fjord environments.

Abstract During the late Wisconsin Fraser Glaciation on the Pacific Margin of Canada, ice moved offshore from the Coast Mountains of the Canadian Cordillera and south into the Strait of Georgia, reaching a maximum extent at about 14 000 14 C bp . Most of the strait was ice-free by 11 300 14 C bp . Deglaciation was very rapid with regional downwasting and widespread stagnation. This resulted in a stratigraphy of thick till (30–60 m), overlain by ice-proximal glacimarine sediments and a thin and discontinuous ice-distal glacimarine unit. Glaciation of Queen Charlotte Basin reached a maximum sometime after 21 000 14 C bp . Deglaciation in this region began sometime after 16 000 to 15 000 14 C bp and ice had retreated fully onto mainland British Columbia by 13 500 14 C bp . Deglaciation was rapid, with the eastward retreat of an ice shelf. This resulted in a stratigraphy of a till up to 50 m in thickness, usually turbated by iceberg scour and overlain in some areas by thin, ice-proximal glacimarine sediments and much thicker (20 m) widespread ice-distal glacimarine sediments. A significant difference between these two regions is the deglacial relative sea-level history. Rapid regression of the outer Queen Charlotte Islands shelf occurred between approximately 14 600 and 12 500 14 C bp , primarily due to rapid isostatic rebound and contemporaneous with deglaciation of the continental shelf. Sea-level reached a maximum lowstand of greater than 150 m and remained low until approximately 12 400 14 C bp . In Georgia Basin, sea-level was at a relative high stand of 50 to 200 m during initial deglaciation, falling to between 0 to 50 m below present sometime after 10 000 14 C bp . We suggest that rapid emergence on the northern margin of the outer shelf was due to forebulge effects. Further, the very limited extent of glacial ice on the Queen Charlotte Islands and the exposure to the open Pacific forced the retreat of the Cordilleran ice-sheet margin eastwards thereby resulting in dominantly ice-distal glacimarine sedimentation. In contrast, the initial relative sea-level highstand during deglaciation between the Vancouver Island and Cordilleran glaciers in the Strait of Georgia resulted in significant ice-proximal deposition and limited ice-distal glacimarine deposition.