Rogen Moraine Paper
Total Page：16
File Type：pdf, Size：1020Kb
Morphological characteristics, formation and glaciological significance of Rogen moraine in northern Scotland Andrew G. Finlayson* and Tom Bradwell British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA, UK * Corresponding author. E-mail: [email protected]. Abstract Rogen moraine are enigmatic landforms whose exact origin is still debated. We use NEXTMap digital surface models and aerial photographs to map the distribution of previously unreported fields of Rogen moraine in the vicinity of Loch Shin, northern Scotland. Existing models of formation are tested against detailed morphological Rogen moraine characteristics obtained from the remote sensing data and field observations. Detailed morphometric analyses combined with their geographical setting lead us to postulate a likely mechanism of formation. Rogen moraine appear to have formed in areas where there were strong basal ice-flow velocity gradients. Thrusting by compression, or fracturing by extension of preexisting partially frozen sediment probably occurred in these areas, resulting in Rogen moraine formation. A general down-ice increase in ridge crest spacing suggests that the latter process may have been dominant, and is consistent with the location of Rogen moraine in the lee of topographic obstructions, in areas that experienced overall extensional ice flow. We also suggest that at least one field of Rogen moraine formed where lateral basal ice-flow velocity gradients were strongest – possibly in a subglacial shear margin setting. Given their location, the landforms may be consistent with formation during headward scavenging of the Moray Firth palaeo-ice stream into a shrinking core of cold-based ice. Keywords: digital surface models; transverse ridges; ice-flow velocity; ice stream 1 1. Introduction Rogen moraine form a landscape of curved to sinuous ridges at approximately right angles to ice movement. Their formation has been linked to a range of ice-sheet behaviour, including thawing beds (Hättestrand, 1997; Hättestrand and Kleman, 1999), freezing beds (Sollid and Sørbil, 1994), ice-stream onset (Dyke et al., 1992), ice-stream shutdown (Stokes et al., 2006), 90° changes in ice-flow direction (Boulton, 1987), and subglacial outburst floods (Fisher and Shaw, 1992; Shaw, 2002). As a result, the exact genesis of these landforms remains open to debate. Rogen moraine are typically, though not exclusively, found in core areas of former ice sheets. Individual ridges are shown to vary in internal composition and include disaggregated bedrock, subglacial till, and stratified sand and gravel (Hättestrand, 1997). A characteristic feature of classic Rogen moraine is the transformation into, or superimposition of, fluted or drumlinoid forms (Lundqvist, 1989, 1997). Where such features are absent, the North American term ribbed moraine (Hughes, 1964) is used in the literature to describe the ridges. Early researchers considered Rogen moraine formation to take place in an ice- marginal environment (Frödin, 1925; Cowan, 1968); however, observation of superimposed eskers and drumlins led other workers to favour a subglacial origin (e.g., Lundqvist, 1969). Since the late 1970s, four main mechanisms for Rogen moraine formation have been proposed: (i) cavity infilling of megaripples eroded into basal ice during subglacial outburst floods (Fisher and Shaw, 1992; Shaw, 2002); (ii) remoulding of preexisting landforms such as ice marginal moraines (Möller, 2006), or drumlins and flutes following a ~ 90° change in ice- flow direction under deforming bed conditions (Boulton, 1987); (iii) shearing and stacking or folding of debris-rich basal ice or preexisting sediments during compressive ice flow (Shaw, 1979; Aylsworth and Shilts, 1989; Bouchard, 1989; Lindén et al., 2008); and (iv) fracturing and extension of sediment sheets during a transition from cold- to warm-based ice conditions (Lundqvist, 1969; Hättestrand, 1997; Hättestrand and Kleman, 1999; Sarla, 2006). This range of hypotheses currently restricts confidence in palaeoglaciological reconstructions based on any one mechanism of formation (e.g., Kleman and Hättestrand, 1999). Based on unimodal distributions of ribbed moraine characteristics for over 33,000 ridges, Dunlop and Clark (2006) suggest a single formation mechanism may exist. However, Lindén et al. (2008) argue that the landforms are probably polygenetic. The wide range in morphological characteristics presented by Dunlop and Clark (2006) suggests differing processes may have operated in their formation, highlighting the need for detailed, location-specific data on these landforms in areas where they are developed. 2 To date, Rogen moraine have been described from North America (e.g., Hughes, 1964; Aylsworth and Shilts, 1989; Bouchard, 1989; Fisher and Shaw, 1992; Stokes et al., 2006), Fennoscandia (e.g., Lundqvist, 1969, 1989, 1997; Hättestrand, 1997; Raunholm et al., 2003; Möller, 2006), and Ireland (e.g., Knight and McCabe, 1997; Clarke and Meehan, 2001), while only sparse accounts exist of the landforms in Britain (e.g., Golledge and Merritt, 2005; Golledge, 2006; Bradwell et al., 2008). This paper presents clear examples of Rogen moraine from the Loch Shin area in northern Scotland (Fig. 1) – the first to be described in detail from Britain. Landform characteristics are presented to assess current hypotheses of formation and to explore implications for ice-sheet conditions during the latter stages of the Main Late Devensian glaciation in northern Scotland. 2. Study area and glaciological context The region around Loch Shin in northern Scotland forms a relatively low-lying area to the SE of the mountainous terrain of Assynt (Fig. 1). A number of wide, shallow valleys trend in a SE direction, draining toward the Dornoch Firth. The local bedrock chiefly comprises Neoproterozoic psammites and semipelites belonging to the Moine Supergroup. Using high- level weathering limits, Ballantyne et al. (1998) suggested that the ice-sheet surface at the Last Glacial Maximum (LGM) lay between 750 – 800 m over Assynt, with an ice divide lying near or slightly east of the present drainage divide. To the SE, ice coalesced to feed a major grounded ice stream in the Moray Firth during ice-sheet deglaciation (Merritt et al., 1995; Bradwell et al., in press). During the Loch Lomond (Younger Dryas) Stadial (12.7 – 11.5 ka cal BP), an icefield formed over the mountains of Assynt and NW Sutherland reaching the northern tip of Loch Shin (Lukas, 2005; Bradwell, 2006). 3. Methods Fields of transverse ridges in the vicinity of Loch Shin (Coir’ an Laoigh, Strath Grudie, and Strath Tirry, shown on Fig. 1) were mapped in a spatially attributed ESRI ArcMap GIS using a digital surface model (DSM) with 1.5-m vertical and 5-m horizontal resolution, derived from NEXTMap Britain topographic data. The DSM was illuminated from various directions, limiting the bias introduced by relief-shading and enabling subtle landforms to be identified (cf. Smith and Clark, 2005). Black and white stereoscopic air photos at 1:25,000 scale and georectified colour monoscopic air photos at 1:10,000 scale were also examined to provide further landform detail. Scanned, georectified field maps from early twentieth 3 century geological surveys (Geological Survey of Scotland, 1921a, 1921b, 1921c) of the area were consulted in the GIS to aid identification of areas where bedrock lies at, or close to the surface. Individual ridges were delimited by clear breaks in slope at their bases. Spatial dimensions for twenty randomly selected ridges in each of the fields shown in Fig. 1 were measured in the GIS. Heights and cross profiles were obtained by viewing geometric data extracted from the DSM as profile graphs. For each field of transverse ridges, crest spacing was measured in the GIS along transects parallel to palaeo-ice flow (inferred from regional streamlined landforms and glacial striations). Ridge crest spacing was measured along three regularly spaced transects at Coir’ an Laoigh and Strath Tirry. However, only one transect was used at Strath Grudie due to the narrowness of the field. On the ground, a walk-over survey of each field of transverse ridges was conducted to verify the remote sensing data. Natural sections are small and sparse, but where found sedimentological observations were made, including descriptions of texture, compactness and sedimentary structures. Absence of larger natural sections has precluded in depth sedimentological investigation at this stage. 4. Results The results of the geomorphological mapping are shown in Figs 2, 3, and 4. Each location is described in turn below. 4.1. Coir’ an Laoigh Coir’ an Laoigh is a shallow valley to the SE of an elevated bedrock outcrop (Figs. 2A, B). The valley continues toward flatter ground at ~ 180 m above UK Ordnance Datum (OD), which eventually drops down into the deeper valleys of Glen Cassley to the east and Glen Oykel to the south. A SE trending, 1.5-km-wide field of transverse ridges extends from Coir’ an Laoigh across the flatter ground, disappearing on the steeper slopes. The ridges are typically 150 – 420 m long, 75 – 270 m wide, and 4 – 12 m high (Table 1). Many are concave downvalley and possess an asymmetrical cross-profile with gentle upvalley and steeper downvalley slopes (Fig. 5A). Aerial photographs show NW to SE aligned streamlined forms superimposed on some of the ridges (Fig. 2C), particularly toward the southeastern end of the field. Ridge crest spacing is relatively regular for the upper 2 km of the field, generally between 100 and 300 m (Fig. 6A). However, downvalley ridge spacing 4 becomes much more variable. The longest transect, A2-A2’, shows an increase in frequency of ridges > 300 m apart from 3 km onwards. Crest spacing in transect A1-A1’ increases to > 400 m from 1.5 to 2.8 km and then decreases to < 200 m at 3.5 km down the field, before increasing to > 600 m at 4.5 km. Transect A3-A3’ shows highly variable crest spacing between 100 and 650 m from 2.5 to 4.5 km. Natural exposures in the transverse ridges are sparse with small (c.