Recessional moraines in nearshore waters, northern Scotland

Abstract

Suites of relatively small, similar-sized and regularly spaced sediment ridges are commonly found fronting the termini of present-day tidewater glaciers in fjords and marine embayments. These ice-flow transverse subaqueous moraine ridges, sometimes referred to as De Geer moraines, record the incremental retreat of the ice front grounding-line over time (De Geer 1889;Lindén & Möller 2005). The distribution and pattern of wellpreserved transverse moraines in nearshore waters around Scotland has given valuable insight into the pattern, timing and style of retreat of the last British-Irish Ice Sheet (Bradwell et al. 2008a, b;Stoker et al. 2009). DescriptionA suite of 40 to 50 seabed ridges occur in water depths typically from 30 to 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 ~100 m, but the ridges are notably absent in basins and bathymetric deeps >100 m (Fig. 1a). The trend of the ridge crests is generally southsouthwest-north-northeast 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 ridges 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 the seabed topography and traverse bathymetric ranges of 50 m or more.The horizontal spacing between ridges varies but is generally between 100 to 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 to 300 m, are found between the more substantial ridges (Fig 1a, b).The seabed ridges vary in height and width. The more delicate ridges further offshore are smaller, being typically 2 to 4 m high and 30 to 50 m wide. The more substantial ridges closer to shore are generally 5 to 15 m high and approximately 80 to 200 m wide (Fig. 1a,b). The delicate ridges are generally symmetrical in cross section; the more substantial ridges tend to be asymmetric with a gentler slope to the southeast and a steeper slope to the northwest (Fig. 1c), although asymmetry is sometimes lacking.A core taken from the flank of one of the seabed ridges recovered a massive, poorly sorted, matrix-supported diamicton with sandy-muddy matrix, and gravel and cobble grade clasts. Clasts were mostly of no...

Autonomous underwater vehicle (AUV) observations of recent tidewater glacier retreat, western Svalbard

“…These could be the result of seasonal glacial readvances across the bay (Linden and Möller, 2005;Dowdeswell et al, 2016b). Similar De Geer moraines have been noted elsewhere from the Kongsfjord-Krossfjord (Howe et al, 2003;Streuff et al, 2015) and the Scotian shelf (Todd et al, 2007) and as relict features occurring inshore on the UK shelf (Van Landeghem et al, 2009;Dove et al, 2015;Bradwell and Stoker, 2018). The complex shape is here interpreted as being the product of a dynamic grounding line beneath the ice with possible modification by crevasse squeeze and meltwater discharge (Zilliacus, 1989).…”

High-resolution landform assemblage along a buried glacio-erosive surface in the SW Barents Sea revealed by P-Cable 3D seismic data

“…The second group of ridges shows a similar geomorphology to suites of seabed moraines in Northern Scotland (Bradwell and Stoker, 2016) and NW Spitsbergen (Burton et al, 2016), where these landforms have been interpreted as recessional and retreat moraines. Ridges with steeper ice-proximal slopes have been documented from a surging ice cap in Svalbard .…”

Introduction: an Atlas of Submarine Glacial Landforms

“…Occasionally, use is also made of bottom photographs and sediment-core material to illustrate specific submarine landforms (e.g. Bøe et al 2016;Bradwell & Stoker 2016a).…”