All inlets and fjords on northern Baffin Island record glacial erosion by outlet glaciers (Shepard 1931; Pelletier 1966). However, their positions were probably influenced by prior fluvial erosion (Fortier & Morley 1956; Pelletier 1966; Gilbert 1982) or by graben-style faulting (Andrews & Miller 1979; Gilbert 1982; Dowdeswell & Andrews 1985; England 1987). Navy Board Inlet is a narrow waterway (about 10 km wide) located south of Lancaster Sound between Baffin Island and Bylot Island in Arctic Canada (Fig. 1). Lineations formed by the movement of glacial ice are preserved on the seafloor of this inlet.
Multibeam bathymetry, cross-sections and sub-bottom profiler data over crag-and-tail features in Navy Board Inlet, Nunavut, Canada. (a) Sun-illuminated multibeam-bathymetric data showing crag-and-tail features. Acquisition system Kongsberg EM300. Frequency 30 kHz. Grid-cell size 10 m. (b) Location of study area (red box; map from IBCAO v. 3.0). (c) Detail of crag-and-tail features located on the margin of a bedrock high. Locations of cross-sections indicated by labelled white lines. (d) Bathymetric profile parallel to the crag-and-tail features. VE×6. (e) Bathymetric profile normal to the crag-and-tail features. VE×10. (f) 3.5 kHz sub-bottom profile across crag-and-tail features. Acquisition system Knudsen 320R.
Description
Linear ridges are observed over much of Navy Board Inlet (Fig. 1). The ridges range from subtle individual features about 5 m high, 160 m wide and 700 m long to prominent, closely spaced parallel features further north (Fig. 1a). These latter lineations are up to 2700 m long, 600 m wide and 40 m high (Fig. 1a, e).
Bedrock outcrops occur at the seafloor throughout the study area as observed in the multibeam-bathymetric data. Sub-bottom profiles through this area suggest that the shallow geology is composed of bedrock and/or till due to the lack of acoustic penetration in the substrate (Fig. 1f). A very thin (c. 2 m) drape of sediment is observed over a small area at the south end of the sub-bottom profile in Figure 1f. There are no core samples available to ground-truth the type of sediment that comprises this drape, but it is likely that it is postglacial mud or perhaps a glacimarine deposit.
The closely spaced parallel lineations are located on the northern margin of a bedrock high (Fig. 1c) that trends NW–SE and rises 150 m above the surrounding seafloor. The lineations occurring on the margin of this bedrock high form a sediment wedge which decreases in height and width to the north as distance from the bedrock increases (Fig. 1d). A depression is observed along the south side of the bedrock high (Fig. 1a). Using the adjacent seafloor that appears to be unmodified as a reference for comparison, the sediment wedge is approximately 50 m thick and the depression is about 60 m deep.
Ice ploughmarks and pits are observed on the seafloor in about 230 m water depth at the northern limit of the study area (Fig. 1a, c).
Interpretation
All of the glacial lineations observed in this area of Navy Board Inlet are adjacent to bedrock highs or pinnacle outcrops on the seafloor. The lineations which flank the north side of the bedrock high form a wedge of till decreasing in height and width to the north, interpreted as crag-and-tail deposits (Holmes 1964). These features formed beneath glacial ice in the lee of bedrock highs as the ice flowed to the north. The depression on the south side of the large bedrock high (Fig. 1a, c) is interpreted to be the result of glacial erosion as the ice flowed over the bedrock obstruction.
The length-to-width ratio of the crag-and-tail deposits ranges from approximately 3:1 to 16:1 in Navy Board Inlet. Clark & Stokes (2003) attribute glacial lineations with a highly elongate shape (length:width >10:1) to ice streams. While the elongation of the features in Navy Board Inlet may not definitively indicate the presence of a glacial ice stream, it does suggest that a northwards-trending outlet for glacial ice was present in this area.
Glacial ice was up to 2200 m thick on northern Baffin Island during the Last Glacial Maximum (Peltier 2004) but only as thick as 1600 m in central Lancaster Sound (Klassen & Fisher 1988) and less in eastern Lancaster Sound (Peltier 2004). This difference in ice thickness would have produced ice draw-down and led to northward ice movement. Due to the narrow dimensions of Navy Board Inlet and the small and discontinuous glacial lineations found there, it is interpreted to have only played a minor role as an outlet for ice to move from Baffin Island to Lancaster Sound.
Iceberg pits and ploughmarks in northern Navy Board Inlet occur in water depths of about 230 m (Fig. 1a, c). Present-day icebergs in the Baffin Bay region typically have keel depths of 100–200 m (Dinsmore 1972) but are known to have become grounded in water depths up to 427 m (Milne 1969). It is most likely that the ice ploughmarks observed in this area are caused by modern ice processes rather than during previous glacial conditions.
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