Glacial landforms on German Bank, Scotian Shelf: evidence for Late Wisconsinan ice-sheet dynamics and implications for the formation of De Geer moraines

Todd, B J; Valentine, Page C.; Longva, Oddvar; Shaw, John

doi:10.1080/03009480600992050

Cited by 23 publications

(29 citation statements)

References 54 publications

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“…Todd et al, 2007), the 405 orientation of the ridges (sub-parallel to underlying streamlined landforms) here is perhaps morecompatible with this hypothesis than a morainic origin. Unfortunately, we have no ground-truthing 407 data from these features to determine ridge composition, so further work is required to ascribe an 408 origin to these features.…”

Section: Sinuous Ridges 397mentioning

confidence: 71%

“…As we have interpreted these minor transverse ridges as De Geer 578 moraines, and recognizing the overdeepened bathymetry of the Sound of Jura, we hypothesize that 579 retreat likely occurred along tidewater margins (e.g. Todd et al, 2007). A radiocarbon age from the 580 Sound of Jura, published by Peacock (2008), indicates that ice retreat was complete by 13.1 14 C ka BP 581 (approximately 15 cal ka BP).…”

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confidence: 99%

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Submarine glacial landforms record Late Pleistocene ice-sheet dynamics, Inner Hebrides, Scotland

Dove

Arosio

Finlayson

et al. 2015

Quaternary Science Reviews

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We use ∼7000km 2 of high-resolution swath bathymetry data to describe and map the submarine glacial geomorphology, and reconstruct Late Pleistocene ice sheet flow configurations and retreat dynamics within the Inner Hebrides, western Scotland. Frequently dominated by outcrops of structurally complex bedrock, the seabed also comprises numerous assemblages of well-preserved glacigenic landforms typical of grounded ice sheet flow and punctuated ice-margin retreat. The occurrence and character of the glacially streamlined landforms is controlled in part by the shallow geology and topography, however these factors alone cannot account for the location, orientation, and configuration of the observed landforms. We attribute the distribution of these elongate streamlined landforms to the onset zone of the former Hebrides Ice Stream (HIS) – part of a major ice stream system that drained 5–10% of the last British–Irish Ice Sheet (BIIS). We suggest this geomorphic signature represents the transition from slow ‘sheet flow’ to ‘streaming flow’ as ice accelerated out from an environment characterized by numerous bedrock obstacles (e.g. islands, headlands), towards the smooth, sediment dominated shelf. The majority of streamlined landforms associated with the HIS indicate ice sheet flow to the southwest, with regional-scale topography clearly playing a major role in governing the configuration of flow. During maximal glacial conditions (∼29–23ka) we infer that the HIS merged with the North Channel-Malin Shelf Ice Stream to form a composite ice stream system that ultimately reached the continental shelf edge at the Barra-Donegal Trough-Mouth Fan. Taken collectively however, the pattern of landforms now preserved at seabed (e.g. convergent flow indicators, cross-cutting flow sets) is more indicative of a thinning ice mass, undergoing reorganization during overall ice sheet retreat (during latter stages of Late Weischselian glaciation). Suites of moraines overprinting the streamlined landforms suggest partial stabilization of the HIS prior to the ice sheet retreating to more isolated, topographically confined troughs and basins. Retreat from the shelf towards, and back into the Inner Hebrides may have been rapid due the prevalence of overdeepened troughs. Within the near-shore fjord-like troughs and deeps, basin-aligned streamlined landforms indicate the subsequent flow of thinner topographically partitioned ice masses, and overprinted moraines record further ice margin retreat, potentially along tide-water margins. This work provides the first geomorphological constraints for this large marine-influenced sector of the former BIIS. We also shed new light on the glacial geomorphic record found at the transition from terrestrial to marine continental-shelf settings, and examine the interplay between substrate geology, bed topography/bathymetry, and grounding-line positions – relationships which are important for characterizing contemporary marine ice sheet margins

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Section: Sinuous Ridges 397mentioning

confidence: 71%

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confidence: 99%

Submarine glacial landforms record Late Pleistocene ice-sheet dynamics, Inner Hebrides, Scotland

Dove

Arosio

Finlayson

et al. 2015

Quaternary Science Reviews

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“…The series of minor transverse ridges between the larger recessional moraines appear similar in form and spacing to De Geer moraines (Fig. 2 inset) (Todd et al 2007), which would indicate sub-aqueous deposition. This, together with the convex 'upglacier' inclination across the trough, and the over-deepened bathymetry of the Sound of Jura, suggests that ice-sheet retreat occurred at a tidewater margin, grounded in relatively shallow water, but with floating ice shelves perhaps extending over deeper water (Fig.…”

Section: Discussionmentioning

confidence: 99%

Deglacial landform assemblage records fast ice-flow and retreat, Inner Hebrides, Scotland

Dove

Finlayson

Bradwell

et al. 2016

Memoirs

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High-resolution bathymetry data have been central to many recent advances in the understanding of past dynamics of the former British-Irish Ice Sheet (BIIS). As approximately two thirds of the former BIIS was probably marine-based during the Last Glacial Maximum (LGM) (~29-23 ka), geomorphic observations of the seabed are increasingly required to understand the extent, pattern, and timing of past glaciation. Until recently, glacial reconstructions for the Inner Hebrides, offshore western Scotland, have been based primarily on terrestrial observations and data. Previous workers have proposed generalised reconstructions in which the Inner Hebrides are located within a significant former ice-sheet flow pathway that drained the western Scottish sector of the BIIS, feeding the Barra Fan during the LGM and earlier glaciations (Fig. 1). Results from numerical ice-sheet modelling suggest that former ice-flow velocities within the region were on the order of hundreds to thousands of metres per year, but also yield further insight by demonstrating how dynamic binge/purge cycles affected ice-sheet mass balance over time (Hubbard et al. 2009). Following the LGM, ice-sheet retreat through the area is estimated to have been in the order of 20 m per year (Clark et al. 2012). Here we present swath bathymetry data from the Inner Hebrides that provide in situ constraints on the ice-sheet flow, and subsequent retreat dynamics from within this important sector of the BIIS. DescriptionWithin the Inner Hebrides (Fig 1a.), Howe et al. (2012) used high-resolution swath bathymetry to identify the first direct evidence relating to subglacial processes and fast ice-flow on the inner shelf. These data have been compiled together with newer bathymetry, as well as terrestrial airborne radar data, to form a near-continuous offshore-onshore digital elevation model (DEM) (horizontal resolution -5 m) that covers the majority of the Inner Hebrides region (Dove et al. 2015). These data cover an extensive an area (~7,000 km²), and our interpretation of the style, location, and pattern of glacial landform assemblages reveals new evidence on the configuration and relative timing of past phases of ice-sheet flow and retreat in the region.On multiple submarine rock platforms and within overdeepened troughs, a diverse assemblage of streamlined ridges and elongate landforms oriented broadly parallel, or sub-parallel to major basin axes is observed (Figs. 1, 2). These streamlined landforms occur in areas of both bedrock and unlithified sediment and are up to several kilometres in length. Sedimentary features range in height up to 20 m, with elongation ratios between 2:1 and 10:1. Streamlined landforms comprising some

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“…Although no precise definition exists, the terms subaqueous moraine and De Geer moraine are sometimes used synonymously -with the latter being 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 waterterminating 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.…”

Section: Discussionmentioning

confidence: 99%

Recessional moraines in nearshore waters, northern Scotland

Bradwell

Stoker

2016

Memoirs

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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...

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Glacial landforms on German Bank, Scotian Shelf: evidence for Late Wisconsinan ice-sheet dynamics and implications for the formation of De Geer moraines

Cited by 23 publications

References 54 publications

Submarine glacial landforms record Late Pleistocene ice-sheet dynamics, Inner Hebrides, Scotland

Submarine glacial landforms record Late Pleistocene ice-sheet dynamics, Inner Hebrides, Scotland

Deglacial landform assemblage records fast ice-flow and retreat, Inner Hebrides, Scotland

Recessional moraines in nearshore waters, northern Scotland

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