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

(3) Seismic profiles 50 km further north reveal a more complex glacial stratigraphy. Distinct erosional surfaces within the underlying sediments can be traced across the main sea-floor trough. Many of these surfaces have an undulating corrugated expression (Fig. 3d). The exposed sea-floor corrugations are laterally equivalent to the mega-scale glacial lineations shown in Figure 3c. In Figure 3d, these lineations can be traced below an uppermost unit dated to the Late Devensian (MIS 2-1) by borehole 78/4. Thus, we interpret this mega-scale glacial lineation surface to have formed in association with the last BritishIrish Ice Sheet. The lower corrugated horizons are interpreted as earlier, buried, mega-scale glacial lineations surfaces based on their identical surface morphology and stratigraphie relationship with sea-floor features. Their exact age of formation remains uncertain.

(4) A series of submarine moraine banks are preserved on the outermost part of the northern Hebrides Shelf and merge into a trough-mouth fan on the adjacent slope (Stoker & Holmes 1991; Stoker 1995) (Fig. 3e). The most distinctive moraine bank occurs at the shelf edge, is 20-30 m thick and up to 4.5 km wide, and can be traced laterally for up to 70 km. The age of this bank is uncertain. It underlies the Wolstonian-lower Devensian strata proved in borehole 88/9,10, although its basal contact is unconformable on older glacigenic sediments preserved within the prograding slope apron of the SuIa Sgeir Fan (Fig. 2). The front (seaward) edge of the moraine bank interdigitates with the uppermost package of mass-flow deposits preserved on the fan. At least three major packages of glacially influenced mass-flow deposition are recognized in the SuIa Sgeir Fan, separated by slope-wide clinoform units consisting of glacimarine-marine strata deposited under more quiescent, interstadial-interglacial conditions (Stoker 1995). This implies that the development of the SuIa Sgeir Fan was episodic and related to specific rapid phases of sedimentation concomitant with an ice-sheet terminus at the continental shelf edge. This is consistent with trough-mouth fan development elsewhere (Vorren & Laberg 1997; � Cofaigh et al. 2003; Sejrup et al. 2003).

Discussion. The presence of highly elongate (7.5:1 to 25:1), streamlined, subglacial bedforms in NW Scotland, on land and on the sea floor, strongly suggests that a geographically restricted fast-flowing zone once drained the NW sector of the grounded British-Irish Ice Sheet. We term this fast-flowing zone The Minch palaeo-ice stream. This ice stream would have flowed west from a source area in the NW Scottish Highlands, and coalesced with eastward-flowing ice from Lewis (see Johnstone & Mykura 1989). It would then have flowed north along the topographic low of The Minch, and finally NW onto the continental shelf (Fig. 3a). The 'sink' for much of this ice stream's sediment budget was the SuIa Sgeir trough-mouth fan.

This interpretation is consistent with the set of geomorphological criteria proposed by Stokes & Clark (1999) based on present-day ice-stream characteristics. The geomorphological signature of The Minch palaeo-ice stream contains all five elements, to greater or lesser degrees, characteristic of contemporary ice streams (outlined by Stokes & Clark 1999).