Description

The seafloor of Rijpfjorden on Nordaustlandet in eastern Svalbard (Fig. 1f) is dominated by crudely streamlined bedrock with south–north to SSW–NNE long-axis orientations (Fig. 1a, b). In the outer fjord and on the inner shelf north of Rijpfjorden, this streamlined morphology is superimposed on regional east–west elongated bedrock undulations whose form is probably structurally controlled, based on the rugged morphology, with a locally ‘plucked’ appearance of the elevated surfaces. The elevated areas are 1–2 km across and the distance between them is about 1–2 km. The full extent of the undulating bedrock morphology is not known, however, because of the limited coverage of bathymetric data.

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Fig. 1.

(a) Swath-bathymetric image of outer Rijpfjorden and the inner continental shelf, north of Nordaustlandet, Svalbard (located in (f)). Acquisition system Kongsberg EM 300. Frequency 30 kHz. Grid-cell size 10 m. (b) Crag-and-tails and profile locations. (c) Cross-section along the long axis of a crag-and-tail. VE×3. (d) A cross-section perpendicular to crag-and-tails. VE×8. (e) Acoustic sub-bottom chirp profile of a crag-and-tail showing generally poor penetration, implying hard seafloor typical of bedrock. VE×5. (f) Location of study area (red box; map from IBCAO v. 3.0). V, Vestfonna; A, Austfonna.

The surface of the bedrock undulations here is fully covered by clusters of parallel, south–north to SSW–NNE orientated, streamlined landforms with relatively high and steep southern parts and somewhat lower, gentler and longer tapering tails pointing towards the north (Fig. 1a–d). The tails extend into the low-lying, sediment-covered areas between the elevated parts of the undulating bedrock (Fig. 1a, b). The lower-lying bedrock areas between the higher areas are covered with a 5–8 m thick layer of acoustically transparent sediments (Fig. 1b, e). The streamlined landforms are 900–1100 m long, 90–160 m wide and 15–30 m high (Fig. 1a, d). Spacing between the individual streamlined landforms within clusters varies from 30 to 200 m (Fig. 1b, d). One of the most prominent landforms has dimensions of 1100× 160×30 m (length×width×height) and an elongation ratio of 7:1 (Fig. 1b, c).

Interpretation

The south–north to SSW–NNE orientated and streamlined features on the inner shelf north of Nordaustlandet and in outer Rijpfjorden are interpreted as crag-and-tails. Crag-and-tails form subglacially where glacier-ice flows around bedrock obstacles to produce a characteristic landform morphology through erosion and deposition. Crag-and-tails comprise a larger and blunt ice-proximal protuberance or ‘crag’ and a narrow and tapering distal tail (Benn & Evans 2010). The blunt side often consists of glacially eroded bedrock, whereas the tail is made of less-resistant substrate or sediments that have been deposited in subglacial cavities formed in the lee of the crag (Benn & Evans 2010).

The presence of abundant ice-flow-parallel crag-and-tail landforms superimposed on the elevated parts of the seafloor undulations suggests that a highly erosive ice stream flowed northwards onto the shelf through Rijpfjorden from an ice dome over Nordaustlandet or in the Barents Sea (Hogan et al. 2010a). The identification of this ice stream draining north from Nordaustlandet supplements the mapping of Late Weichselian ice streams around northern Svalbard by Ottesen et al. (2007) and Hogan et al. (2010b). The fast-flowing ice stripped the seafloor of sediments, eroded around crags on the bedrock highs and moulded sediments into tails behind them in the lower-lying bedrock areas. The exposed bedrock of the structurally controlled undulations supports this interpretation through being largely devoid of sediment cover (Fig. 1e).

The ice stream in Rijpfjorden originated from the Svalbard–Barents Sea Ice Sheet that covered the whole of Svalbard and the Barents Sea area during the Last Glacial Maximum (Svendsen et al. 2004; Hughes et al. 2016). Ice retreated from the shelf north of Svalbard at c. 20–17 ka. The remaining ice masses covering Nordaustlandet today form the two large ice caps Vestfonna (2500 km²) and Austfonna (8000 km²); the latter is the largest ice cap in the Eurasian Arctic (Fig. 1f) (Dowdeswell 1986).