Mass flow and hydrofracturing during Late Devensian moraine emplacement, NE Scotland
Introduction
There are a range of processes, including bulldozing/pushing, squeezing, freeze-on, melt-out and mass flowage of sediments, operating at the snouts of contemporary glaciers which result in the building of ice-marginal moraines on a variety of scales (e.g., Benediktsson et al., 2008; Benn and Evans, 2010; Bradwell et al., 2013). Ice marginal pushing leading to glacitectonism of penecontemporaneous or pre-existing sediments is generally regarded as the dominant process in the formation of annual recessional moraines. Such moraines are formed in response to the minor readvance of the ice margin during the winter, even though the glacier is undergoing overall retreat (e.g., Price, 1970; Sharp, 1984; Krüger, 1995; Evans and Twigg, 2002; Benn and Evans, 2010; Bradwell et al., 2013). However, melting of the ice at the snout during the summer months can lead to the deposition (dumping) of debris released from the ice to form ice marginal aprons and fans, which can themselves become deformed during a subsequent readvance (e.g., Hewitt, 1967; Boulton and Eyles, 1979; Lukas, 2005, 2012). The sediment being released from the ice through melt-out will be subjected to remobilisation by mass flowage, gravity driven fall or even fluvial transport by meltwater streams. These so called 'dump moraines' form were the ice remains stationary during the accumulation of the debris and their size will depend on the length of this still stand and volume of material being released from the ice (Benn and Evans, 2010). Consequently, the dominant moraine forming process can change both spatially and temporally along the ice margin leading to the construction of potentially complex ice-marginal landforms comprising both undeformed and highly tectonised glacigenic sediments. As a result the ice-marginal landforms preserved within the geological record may owe their origin to the complex interaction of a number of processes.
Although micromorphology has become a well-established technique and increasingly used by glaciologists and Quaternary geologists as a primary tool for the analysis of deformed glacigenic sequences and as an aid to understanding the processes occurring beneath glaciers, very few studies use this method to investigate the processes occurring in ice-marginal settings (e.g., Lachniet et al., 1999, 2001; Menzies and Zaniewski, 2003; Phillips, 2006; Reinardy and Lukas, 2009; Skolasińska et al., 2016). Consequently, our understanding of the microscale structures developed in response to ice-marginal processes (e.g., mass flow, freeze-thaw, desiccation and dewatering, fluvial reworking) remains limited. This is critical when these processes can strongly modify or even overprint any pre-existing features present within the sediments released from within, or beneath, the retreating ice. Furthermore, this absence of evidence becomes increasingly important when several published studies have tried to use micromorphology to determine the depositional setting of massive diamictons preserved within the geological record (van der Meer, 1987; Carr et al., 2000; Carr, 2001; Carr et al., 2006; Menzies et al., 2006; Kilfeather et al., 2010).
This paper contributes to our knowledge of the processes which led to the development of a Late Devensian (Weichselian, Marine Isotope Stage (MIS) 2) recessional moraine near Loch Killin in the Monadhliath Mountains to the east of Fort Augustus, NE Scotland (Fig. 1). Macroscale sedimentological and structural field observations are combined with micromorphological and micro-structural analysis to investigate the ice-marginal processes which led to the construction of this landform. Detailed mapping of the microstructures within the stratified diamictons and sands mantling the glacitectonised core to the moraine reveal a complex history of microfabric development formed as a result of ductile shearing during the emplacement of these mass flow deposits. Furthermore, thin sections taken from a system of sediment-filled hydrofractures which cut the moraine record the escape of pressurised meltwater from beneath the ice. Combining the results of both the macro- and microscale studies has allowed a detailed model of ice-marginal landform development to be established. This involved glacitectonism as a result of ice-push during glacier readvance, followed by mass flow of sediments released as a result of melting of the snout during a period of still stand, followed by hydrofracturing and water escape accompanied by extensional fault on the up-ice side of the moraine probably during the initial stages of renewed glacier retreat. The range of microstructures found within the mass flow deposits (diamictons) are comparable to those found within subglacially deformed traction tills which has important implications for anyone trying to use micromorphology, on its own, to establish the depositional setting of glacigenic diamictons.
Section snippets
Regional setting and location of study area
The exposed section examined during this study (UK National Grid Reference [NH 5190 1211]; latitude-longitude 57°10′32.32″ N, 4°27′04.15″ W) is located on eastern side of the River Fechlin which flows out of the northern-end of Loch Killin, located to the west of Fort Augustus, NW Scotland (Fig. 1a and b). Loch Killin is an approximately NE-SW-trending, elongate lake filling a prominent steep-sided glaciated valley (Fig. 1c and d) on the north side of the Monadhliath Mountains. The bedrock
Methods
Prior to sampling for micromorphology, the sedimentary sequence was logged and the macroscale characteristics, such as sedimentary lithofacies and architecture, and macroscopic deformation structures, were described and sketched in detail (Fig. 2, Fig. 3), following the procedures prescribed in Evans and Benn (2004), Phillips and Lee (2011) and Evans, (2018). The samples were collected using 10 cm square, aluminium Kubiena tins, which were cut into the face of each exposure in order to limit
Sedimentology and macroscale deformation structures within the moraine
The sedimentary and structural interpretation of the section exposed on northern-side of the River Fechlin is shown in Figure 2. The section cuts through a thin (2–4 m thick) sequence of variably disrupted sands, gravels and diamictons, mapped as glacifluvial ice-contact deposits, which are overlain by hummocky glacial deposits (Fig. 1d). At the ESE-end of section the lower part of the sequence is composed of a folded and thrust sequence of sands and gravels (Fig. 2, Fig. 3a). These sands and
Microscale structures within the sediment-filled hydrofractures
Three samples (N13916; N13917; N13918) were collected from a steeply inclined sediment-filled hydrofractures cutting through the glacitectonised glacilacustrine sequences within the core of the moraine (Fig. 4e). All three thin sections are composed of finely laminated clay, silt and sand filling a c. 3–8 cm wide, steeply ESE-dipping vein (75 °ESE/019°; Fig. 4b) which cross-cuts an apparently massive, micaceous, matrix-poor sand (Fig. 5, Fig. 6, Fig. 7). In thin section the sand is fine-grained
Microstructures within the stratified diamictons and sands
Thin section N13919 is composed of weakly to moderately, thinly bedded coarse-sand, silty and silty sand which can be divided into four main units (Fig. 11): (i) occurs at the bottom of the thin section and comprises poorly sorted coarse silt with graded silt to clay laminae indicating that the sequence is the right-way-up; (ii) the overlying unit of matrix-supported, poorly sorted and weakly laminated coarse-grained silty sand; (iii) a laterally discontinuous, lenticular layer of laminated
Sedimentation in response to fluid flow within the hydrofractures
The laminated nature of the sediment-fill and the presence of locally well-developed/preserved sedimentary structures (Fig. 8, Fig. 9, Fig. 10) clearly indicate that the steeply inclined hydrofractures which cut the ice-marginal moraine at the northern-end of Loch Killin accommodated several phases of fluid flow. Comparable with other sediment-filled hydrofracture systems in glacial environments (e.g., van der Meer et al., 2009; Phillips et al., 2013; Phillips and Merritt, 2008; Phillips and
Interplay between glacitectonics, hydrofracturing and ice marginal sedimentation during construction of a recessional moraine
The dominant moraine forming process can change both spatially and temporally potentially leading to the construction of complex ice-marginal landforms which owe their origin to the complex interaction of a number of processes. The results presented here of a macroscale (field) sedimentological and structural study, combined with a detailed micromorphological analysis, have provided valuable insights into the ice-marginal processes which interacted during the construction of the moraine exposed
Conclusions
The results of a macroscale (field) sedimentological and structural study, combined with a detailed micromorphological analysis, have enabled a number of conclusions to be made regarding the ice-marginal processes which occurred during the construction of a complex ice-marginal moraine exposed near Loch Killin:
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The stratified diamictons and sand sequence mantling the north-western, down-ice side of the moraine were laid down as a result of ice marginal sedimentation. The diamictons are
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
Jonathan Lee and Andrew Finlayson are thanked for their constructive comments on an earlier version of this paper. John Fletcher at the BGS thin section laboratory is acknowledged for his expertise in making the thin sections of the unlithified sediments examined during this study. The two anonymous reviewers and the editor Malcolm Hart are thanked for their very constructive reviews. This paper is published with permission of the Director of the British Geological Survey.
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