Description

A suite of 40–50 seabed ridges occurs in water depths of typically 30–80 m on shallow banks surrounding the Summer Isles, NW Scotland (Fig. 1). The ridges comprise two morphological groups: a more delicate set with smaller dimensions and a more substantial set. Each ridge displays different planform morphology ranging from curvilinear to intricate (Fig. 1a, b). Occasionally ridges continue into water depths of c. 100 m, but the ridges are notably absent from basins and bathymetric depths of >100 m (Fig. 1a). The trend of the ridge crests is generally SSW–NNE and individual ridges vary in length. The ridges can typically be traced horizontally for 3–5 km, although shorter ridge fragments (<500 m in length) also occur. The more delicate ridges are notably more discontinuous than the larger, more substantial ridges. Most ridge crests are simple in planform, but a few bifurcate or display multiple crestlines. In places, the more delicate ridges overprint the larger less well-defined ridges (Fig. 1b). The longest more substantial ridges cross-cut seabed topography and traverse bathymetric ranges of 50 m or more.

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Fig. 1.

Multibeam bathymetry, geophysical profiles and geological samples from subaqueous moraines in the Summer Isles region, NW Scotland, UK. (a) Multibeam-bathymetric image showing suite of seabed moraines between Tanera Mor and Carn nan Sgeir. Note the intricate planform morphology and the absence of moraines in deeper-water basins. Acquisition system Geoswath. Frequency 125 kHz. Grid-cell size 5 m. (b) Multibeam image of seabed moraines c. 10 km NW of Tanera Mor. Same acquisition system. Note the small delicate De Geer moraines (m) overprinting the larger, older set of subaqueous moraines (SM). Location of seabed profile (lower panel) shown by line y–y′. (c) British Geological Survey seismic-reflection (boomer) profile of seabed moraines between Tanera Mor and Carn nan Sgeir. Acquisition system Applied Acoustics surface-tow boomer. Frequency 12 kHz. Note the asymmetric cross-profile of moraines. Bedrock/diamicton contact mapped where acoustically resolvable. SBM: seabed multiple. (d) Underwater photograph of subaqueous moraine in Summer Isles region at 35 m water depth showing large boulders on ridge crest. Squat lobster with outstretched claws c. 0.5 m wide. (Image: R. Shucksmith.) (e) Glacial diamicton recovered in BGS core 57-06/262 from flank of subaqueous moraine ridge (57.968° N, 05.357° W). (f) Location of study area (red box; map from GEBCO_08).

Horizontal spacing between ridges varies but is generally between 100 and 1000 m. Spacing between the more delicate ridges around the western headlands ranges from 100 to 500 m (Fig. 1b). Closer inshore, between Tanera Mor and Carn nan Sgeir, spacing between the larger ridges generally increases and can exceed 1000 m although ridge clustering occurs, especially between topographic highs (Fig. 1a). Occasionally smaller, more delicate ridges with spacing of 50–300 m are found between the more substantial ridges (Fig. 1a, b).

The ridges vary in height and width. The more delicate ridges further offshore are smaller, typically 2–4 m high and 30–50 m wide. The more substantial ridges closer to shore are generally 5–15 m high and c. 80–200 m wide (Fig. 1a, b). The delicate ridges are generally symmetrical in cross-section; the more substantial ridges tend to be asymmetrical with a gentler slope to the SE and a steeper slope to the NW (Fig. 1c), although asymmetry is sometimes lacking.

A core taken from the flank of one of the ridges recovered a massive, poorly sorted, matrix-supported diamicton with sandy-muddy matrix, and gravel and cobble grade clasts. Clasts were mostly of non-local lithology and some were faceted and striated (Fig. 1e). Dive photographs from ridge crests show large, lodged, subrounded to sub-angular boulders and cobbles set within a muddy matrix (Fig. 1d).

Interpretation

The seabed sediment ridges offshore of NW Scotland in the vicinity of the Summer Isles are interpreted to be subaqueous recessional moraines (some of which are De Geer moraines) formed at the grounded tidewater margin of an ice-sheet outlet glacier. Although no precise definition exists, the terms subaqueous moraine and De Geer moraine are sometimes used synonymously; the latter is, however, a subset of the former, based on geomorphological grounds. De Geer moraines are most commonly interpreted as marking former ice-front positions where sediment is dumped, squeezed or pushed up at grounded water-terminating glacier margins (Lindén & Möller 2005; Todd et al. 2007). As in terrestrial settings, the process of subaqueous moraine formation is normally associated with brief stillstands or minor readvances of the ice front. Preserved suites of subaqueous moraines therefore record punctuated retreat during overall glacier recession. There is strong evidence that, in some cases at least, small closely spaced De Geer moraines form annually during winter push events at the grounding line (Boulton 1986; Ottesen & Dowdeswell 2006).

The smaller seabed moraines c. 10 km NW of Tanera Mor exemplify De Geer moraines in nearshore waters around Scotland. With their delicate, superimposed, discontinuous form and small dimensions, these features probably formed at the grounding line of a partly buoyant (lightly grounded) tidewater ice-sheet margin retreating and stabilizing in shallower water (Stoker et al. 2009). Closer inshore, the more substantial, more laterally continuous subaqueous moraines with intricate planforms and asymmetric cross-profiles suggest an oscillating, highly crevassed tidewater ice margin strongly grounded on bathymetric/topographic highs, some of which probably acted as pinning points during overall ice-front retreat.

Importantly, the details revealed by new high-resolution marine-geophysical surveys around the UK continue to direct geological sampling and prompt new geological mapping, assisting in the reconstruction and refinement of the pattern, timing and former style of British–Irish Ice Sheet decay.