Description

Figure 1a, c illustrate seafloor features that occur SW of Banks Island in the northwestern part of Amundsen Gulf (Fig. 1b). Water depths in this part of the axial trough range from 350 to 435 m. Prominent seafloor landforms include parallel ridges and grooves that trend northwesterly across the region. Depressions up to 135 m wide and 45 m deep occur as irregular linear and arcuate-shaped interruptions in many of the ridges. The linear interruptions extend across adjoining ridges along trends that lie approximately normal to the trend of the ridges (Fig. 1a, c). Arcuate-shaped depressions (moats) connect with wider inter-ridge valleys trending to the NW. In many instances the linear ridges, apparently undiminished in height adjacent to a moat, progressively decrease in width and height for up to 1.5 km NW of the moat, thereby producing a drumlinoid-shaped feature (Fig. 1c, d). Modifications of linear features such as illustrated here have only been observed in the northwestern part of Amundsen Gulf and appear to be absent in the southeast.

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Fig. 1.

(a, c) Bathymetric images showing MSGLs and moat features in northwestern Amundsen Gulf, Canadian Arctic Archipelago. Erosion has cut breaks across successive MSGLs normal to the orientation of the lineations (c), and has modified segments of some ridges into drumlinoid features with moats at their heads (a, c). Multibeam acquisition systems Kongsberg EM 300 (2003–08) and EM302 (2009–12). Frequency 30 kHz. Grid-cell size 10 m. (b) Location of study area (red box; map from IBCAO v. 3.0). (d) Sub-bottom profile x–x′ along one of the modified ridges. VE×15. Sub-bottom profile acquisition system Knudsen K320R chirp sonar. Frequencies between 2 and 7 kHz with a nominal frequency of 3.5 kHz.

Interpretation

Parallel ridges and grooves that trend northwesterly along the floor of Amundsen Gulf (Fig. 1a, c) are interpreted as mega-scale glacial lineations (MSGLs) formed beneath a fast-flowing glacial ice stream such as those described by Clark (1993) and Ó Cofaigh et al. (2002). The implied direction of ice movement is to the NW (from right to left), parallel to the axis of the sound. The ridges are composed of deformable sediments from the bed of the ice stream that were moulded and aligned by the advancing ice (e.g. Dowdeswell et al. 2004; Ó Cofaigh et al. 2005).

Numerous linear and arcuate breaks occur in the MSGLs in this region. The orientation of the linear breaks, being approximately normal to the direction of ice-stream flow, indicates that they are erosional features formed by subglacial meltwater rather than by a subglacial mechanical process. This origin is also consistent with that of the arcuate- shaped moats that have developed at many of the breaks in ridges, the widening of the valleys, and the narrowing and lowering of the ridges in the adjacent downstream area (Fig. 1a, c–d). In some instances this has resulted in the formation of drumlinoid features, high at the upstream end and tailing off downstream (Fig. 1c, d). The erosional episode that caused these modifications clearly post-dated formation of the MSGLs.

A similar origin is interpreted for several drumlinoid features in a drumlin field that occurs nearby in northwestern Amundsen Gulf described by Bennett et al. (2007) and Blasco et al. (2008). However, some of the other drumlinoid features there seem to be of constructional origin. Subglacial conditions and processes here appear to resemble some of those present beneath the Rutford Ice Stream in the Antarctic reported by Smith et al. (2007), King et al. (2007, 2009) and Smith & Murray (2009). Results of studies by these researchers indicate that while depositional and erosional processes are both active beneath the Rutford Ice Stream, drumlins observed there are depositional features.

No evidence of erosion or modification of seafloor features by subglacial meltwaters was found in the southeastern, upstream part of the area traversed by the Amundsen Gulf Ice Stream. Such modification has been found only in the NW downstream area, where meltwater would be expected to be more prevalent. This observation agrees with findings by King et al. (2007) where erosion by meltwater was found in the lower, faster-moving part of the Rutford Ice Stream. Moat-like features associated with drumlins have also been reported from the seafloor of West Antarctica and Greenland (Graham & Hogan 2016).