Minch palaeo-ice stream, NW sector of the British-Irish Ice Sheet, The

Journal of the Geological Society, May 2005 by Stoker, Martyn, Bradwell, Tom

Geophysical data from the UK continental shelf off NW Scotland reveal strongly parallel subglacial bedforms on the sea bed. Field mapping on the adjacent landmass has identified closely spaced bedrock megagrooves and highly elongate drumlinoid forms. All these mega-scale glacial lineations probably formed beneath the same fast-flowing zone of the last British-Irish Ice Sheet, The Minch palaeo-ice stream. This ice stream drained the NW sector of the British-Irish Ice Sheet during the Late Devensian Glaciation (marine isotope stage 2) and terminated near the edge of the continental shelf. The size of the adjacent trough-mouth Sula Sgeir Fan and the presence of buried mega-scale glacial lineations within the Quaternary stratigraphic record imply that fast ice-sheet flow in The Minch has been a feature of several mid- to late Pleistocene glaciations.

Keywords: British-Irish Ice Sheet, Late Devensian, subglacial environments, bedforms, ice streams.

Ice streams are fast-flowing zones within grounded ice sheets, responsible for discharging the bulk of the ice mass (Paterson 1994; Stokes & Clark 2001). They are critical to the stability and dynamics of contemporary ice sheets, such as those in Greenland and Antarctica. Palaeo-ice streams were equally significant in controlling the dynamics and evolution of Quaternary ice masses (Stokes & Clark 2001; � Cofaigh et al. 2003), and their identification in the geological record helps shed light on the behaviour of former ice sheets. It is therefore imperative that scientists accurately determine the location and extent of former ice streams so as to make better-informed models of the future response of ice sheets to climatic change.

In this paper we identify streamlined subglacial bedforms on the land surface, sea bed and in the Quaternary stratigraphic record associated with the British-Irish Ice Sheet. We demonstrate that these bedforms are associated with a fast-flowing palaeo-ice stream in The Minch (Fig. 1) that flowed NW across the northern Hebrides Shelf. The bedforms are located in a broad shallow trough, defined mainly by the 100m isobath, which extends from the fjords of the NW Scottish mainland to the continental shelf break. At the trough terminus is the PlioPleistocene SuIa Sgeir Fan, covering an area of about 3750 km^sup 2^, which slopes from the shelf break into the deep-water Rockall Trough. Between early Pliocene and mid-Pleistocene time (c. 4-0.44 Ma), the SuIa Sgeir Fan prograded seaward as a shelf-margin deltaic system; however, from the mid-Pleistocene, deposition on the fan was dominated by glacial processes (Stoker 1995).

Glacial setting. Ice-contact processes have shaped the continental shelf off NW Scotland for the last 0.44 Ma (Stoker 1995). Seismic reflection profiles across the northern Hebrides Shelf reveal an irregular glacial unconformity with troughs cut into the relatively soft Mesozoic and Cenozoic strata, whereas the harder Precambrian crystalline basement rocks remain largely upstanding (Fig. 2). Locally thick accumulations (50-150m) of sub-glacial and proglacial sediments, including basal till, multiple ice-contact sequences, stratified proglacial outwash and glacimarine sediments, are preserved in overdeepened basins, such as the North Minch and North Lewis basins (Figs 1 and 2) (Fyfe et al. 1993; Stoker et al. 1993).

The architecture of the glacial succession indicates a general landward stacking (younging) of strata, which is borne out by several key British Geological Survey (BGS) boreholes located between the shelf edge and The Minch (Figs 1 and 2). On the basis of amino-acid diagenesis, undisturbed proglacial glacimarine deposits recovered in borehole 88/9,10, taken from the uppermost unit on the outermost shelf (Fig. 2), have been broadly dated as Wolstonian to Early Devensian in age (Stoker et al. 1993), spanning marine isotope stages (MIS) 10 to 4. In the mid-shelf region, borehole 77/8 penetrated a stacked succession of diamictons assigned a pre-Late Devensian (pre-MlS 2) age on the basis of amino-acid diagenesis and the erosional truncation of the interbedded proglacial sediments (Stoker et al. 1993) (Fig. 2). In The Minch, biostratigraphy and radiocarbon dating of the uppermost sea-bed unit, sampled by borehole 78/4 (Figs. 2 and 3d), proved a Late Devensian-Holocene (MIS 2-1) sequence of glacimarine and marine strata, at least as old as 12785� 150 ^sup 14^C years bp (Graham et al. 1990), corresponding to c. 15 ka cal. bp (Calib 4.4; Stuiveref al. 1998).

On the Hebrides Slope, large volumes of subglacial sediment were supplied to the slope apron of the SuIa Sgeir Fan, where about 330 km^sup 3^ of middle-upper Pleistocene sediment accumulated (in excess of 200 m thick), largely through mass-flow processes (Stoker 1995). Such fan-shaped, diamicton-dominated sediment accumulations located at the trough mouth are commonly referred to as 'trough-mouth fans' (Vorren & Laberg 1997; Stokes & Clark 1999). An Anglian to Devensian (MIS 12-2) glacial record has been proved from the Hebrides Slope, south of the SuIa Sgeir Fan (Stoker 1995), although its correlation with the fan remains equivocal.