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

The location of overdeepened basins along a pathway across the continental shelf, clearly depicted by the present-day 10Om isobath (Fig. 1), is consistent with a cross-shelf trough, common to many glaciated margins (Piper et al. 1990; Davies et al. 1997). The main body of the trough extends from The Minch to the shelf edge, a distance of 150km, with an average width of 30-40 km. The connection with the SuIa Sgeir Fan is clearly marked by a re-entrant at the shelf edge, shown by the landward deviation of the 150 m isobath (Fig. 1).

New geomorphological mapping data. Aerial photographs and field mapping were used to interpret the Quaternary geology and geomorphology in parts of Wester Ross, NW Scotland. Highresolution 1 kJ (150-750 Hz) sparker seismic reflection and UDI AS350 dual sidescan-sonar data (50 kHz), acquired by the BGS as part of the regional mapping programme of the UK continental shelf, were used to examine the marine landscape. By combining these data, subglacial bedforms and other features attributed to glaciation have been identified in several locations along a hypothesized ice-stream flow line (Fig. 3a).

(1) Bedrock mega-scale glacial lineations occur on the present-day watershed immediately NE of Ullapool in NW Scotland (Fig. 3b). These subparallel features are cut in hard Proterozoic Moine Supergroup psammites and range in length from 500 to 5000 m. The primary features are erosional megagrooves, ranging from 10 to 50m wide and 5 to 30m deep, with average spacings of 100-200 m. Their overall trend is east-west, parallel to former ice sheet flow, as evidenced by erratics and glacial striae (Read et al. 1926). The intervening bedrock landforms are highly elongate streamlined drumlinoid ridges. The longest of these ridges have elongation ratios (widthrlength) of 15:1 to 25:1. The whole landform assemblage is a striking example of ice-directional mega-scale glacial lineations formed by subglacial erosion processes. Similar land_-systems have been attributed to ice-stream onset zones (see � Cofaigh et al. 2003;Bradwell 2005).

(2) Seismic investigations and sidescan-sonar surveys have revealed large-scale ridge-groove structures on the sea floor in The Minch, between the NW Scottish mainland and Lewis (Fig. 3c). These strongly parallel, highly elongate features are interpreted as mega-scale glacial lineations formed at the palaeo icesheet bed (e.g. Clark 1993). In cross-section, on seismic profiles, these lineations appear as a hummocky or corrugated sea floor with a relief of 5-15 m and ridge spacings of c. 100-500 m. In plan view, they extend beyond the limit of the dual sidescansonar record, which covers a horizontal distance of about 750 m. This equates to minimum elongation ratios of 7.5:1. However, the fact that such lineations occur consistently on numerous seismic lines, throughout The Minch, implies that they are likely to measure thousands of metres in length. Very similar features have been identified on the sea floor in Hudson Bay, Canada (Josenhans 1997), offshore Norway (Sejrup et al. 2003; Stalsberg et al. 2003) and Antarctica (� Cofaigh et al. 2002; Dowdeswell et al. 2004). Sidescan-sonar images reveal the strong north-south orientation and high density of the ridge-groove structures in The Minch.