Iceberg ploughmarks are common features of glaciated continental margins worldwide. They form when iceberg keels plough through sediments on the ocean floor. They occur in various sizes and shapes, but generally exhibit a linear or curvilinear geometry. The SW Barents Sea is a heavily glaciated margin and its seafloor shows numerous ploughmarks among other glacial landforms (Andreassen et al. 2008). We present seafloor features formed during the late Weichselian glaciation (Solheim et al. 1990) imaged using high-resolution P-Cable 3D seismic data. P-Cable 3D seismic data have a spatial resolution of 6×6 m which is better than many hull-mounted multibeam systems, particularly in deep water. However, the cumulative seismic response of the seafloor is slightly different due to the lower-frequency bandwidth (30–350 Hz) of P-Cable compared to multibeam systems (up to 500 kHz). In addition, amplitude information is readily available and complements grid-based interpretations.
Description
Numerous linear and curvilinear depressions occur on the seafloor near Loppa High, SW Barents Sea, between 310 and 350 m water depth (Fig. 1). They appear to be erosional features and are up to about 3 km long, between 10–150 m wide and up to 12 m deep (Fig. 1a, c, f). Although these linear depressions criss-cross the seafloor, their general orientation is ENE–WSW and a large number are roughly parallel to each other (Fig. 1a). Some of the wider depressions show elevated banks about 3 m higher than the surrounding seafloor (Fig. 1c). A few of the depressions cut across these existing features, which indicates a younger age of formation. The largest depression observed (c. 150 m wide) is curvilinear and occurs in a nearly orthogonal orientation to the ENE–WSW linear depressions (Fig. 1a, c).
(a) Ploughmarks in the SW Barents Sea from high-resolution 3D seismic-reflection data. (b) RMS amplitude map showing amplitude anomalies. (c) Distance–depth profiles across a large ploughmark showing elevated banks or berms. VE×8. (d) Location of study area (red box; map from IBCAO v. 3.0). (e) 3D seismic-reflection profile showing the V-shaped expression of ploughmarks. Acquisition P-Cable 3D system. Frequency 20–250 Hz. Grid-cell size 6.25 m. (f) Distance–depth profile along a ploughmark, highlighting the corrugation ridges within. VE×8.
Small, sub-parallel ridges up to about 1.5 m high are present within and at the edges of some depressions (Fig. 1a, f), perpendicular to the ploughmark orientation. They occur in small groups within a few ploughmarks and are not widespread in the area. Another set of features in the NE part of the study area includes linear depressions which are deeper (c. 12 m) and shorter (c. 500 m) than most of the other depressions observed (Fig. 1a).
On the seismic-reflection data, the sub-parallel ENE–WSW-trending depressions appear as V-shaped reflections (Fig. 1e). Enhanced reflections are present at the base of some of the V-shaped reflections (Fig. 1e), which are highlighted in the root-mean-square (RMS) amplitude map (Fig. 1b) derived at the seafloor surface (9 ms window). The enhanced reflections occur near longer depressions and seems to be confined to the walls and floors of the depressions in most cases (Fig. 1b).
Interpretation
The linear and curvilinear depressions observed are interpreted as iceberg ploughmarks resulting from the ploughing of the seafloor by iceberg keels (Elverhøi & Solheim 1983). Similar cross-cutting ploughmarks are a common seabed feature in the Barents Sea (e.g. Vorren et al. 1988) and on other glaciated margins (e.g. Barnes & Lien 1988; Dowdeswell et al. 1993).
The general ENE–WSW orientation of the ploughmarks matches the major ice-movement direction in the Bjørnøya cross-shelf trough (Andreassen et al. 2008). However, some of the ploughmarks which do not follow the general direction might have formed by ice drifting seawards from Norway (Winsborrow et al. 2010). Younger ploughmarks cutting across other features represent multiple phases of iceberg ploughing. Large icebergs could create linear, mostly parallel ploughmarks when their movement was constrained. This could occur when a large volume of icebergs calve during early deglaciation. Isolated, multi-directional depressions could have formed by icebergs subject to more open-water conditions, such as those during deglaciation, and could be younger in age (<20 ka).
Small, sub-parallel ridges occurring within some ploughmarks (Fig. 1f) are interpreted to represent corrugation ridges, similar to those reported from Antarctica (Jakobsson et al. 2011). They are probably formed by the squeezing of sediments into ridges by icebergs due to tidal action, although several alternative mechanisms have also been discussed (Graham et al. 2013). Enhanced reflections on the floors and walls of ploughmarks represent harder material than the surroundings, exposed by the ploughing action of the keels.
- © 2016 The Author(s). Published by The Geological Society of London. All rights reserved
References
Iceberg ploughmarks in the SW Barents Sea imaged using high-resolution P-Cable 3D seismic data
