Aberystwyth University Debris Transport in a Temperate Valley Glacier

Aberystwyth University Debris transport in a temperate valley glacier: Haut Glacier d'Arolla, Valais, Switzerland Goodsell, Rebecca; Hambrey, Michael J.; Glasser, Neil F. Published in: Journal of Glaciology DOI: 10.3189/172756505781829647 Publication date: 2005 Citation for published version (APA): Goodsell, R., Hambrey, M. J., & Glasser, N. F. (2005). Debris transport in a temperate valley glacier: Haut Glacier d'Arolla, Valais, Switzerland. Journal of Glaciology, 51(172), 139-146. https://doi.org/10.3189/172756505781829647 General rights Copyright and moral rights for the publications made accessible in the Aberystwyth Research Portal (the Institutional Repository) are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the Aberystwyth Research Portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the Aberystwyth Research Portal Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. tel: +44 1970 62 2400 email: [email protected] Download date: 25. Sep. 2021 Journal of Glaciology, Vol. 51, No. 172, 2005 139 Debris transport in a temperate valley glacier: Haut Glacier d’Arolla, Valais, Switzerland B. GOODSELL,* M.J. HAMBREY, N.F. GLASSER Centre for Glaciology, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK E-mail: [email protected] ABSTRACT. This paper considers the role of structural glaciology in debris entrainment, transport and deposition at Haut Glacier d’Arolla, a temperate valley glacier in Valais, Switzerland. Sedimentological descriptions and clast analysis have been used to identify relationships between ice structure and debris transport. Relationships identified are (1) debris associated with crevasse traces, (2) the folding of rockfall material incorporated within primary stratification to form medial moraines and (3) dirt cones and englacial debris layers associated with reactivated crevasse traces. A conceptual model is introduced to summarize the manner in which ice structures at Haut Glacier d’Arolla control entrainment and deposition of debris. 1. INTRODUCTION hydrology (e.g. Mair and others, 2001), and the develop- The aim of this paper is to investigate the influence of glacier ment of high-resolution glacier flow modelling (Hubbard structure on debris transport and deposition at Haut Glacier and others, 1998; Hubbard and Hubbard, 2000). The d’Arolla, Switzerland, a small temperate alpine valley formation of medial moraines and sediment transport glacier currently in a state of recession. pathways have also been described at the glacier (Small The importance of ice structures and ice deformation and others, 1979; Gomez and Small, 1985; Small, 1987). processes in debris transport has been highlighted in several recent papers on polythermal glaciers in Svalbard (e.g. 3. METHODS Boulton and others, 1999; Hambrey and others, 1999; Hambrey and Glasser, 2003). Previous studies of temperate Structural mapping glaciers either consider debris transport as part of a larger- The structure of Haut Glacier d’Arolla was determined by scale glacier system approach (e.g. Boulton and Eyles, 1979; field and aerial photograph observations, from which a Drewry, 1986; Small, 1987; Kirkbride, 1995; Evans, 2003), series of structural maps were produced. Field observations or are concerned only with specific aspects of debris were made over three seasons between 1999 and 2001. transport, for example the development of medial moraines (Eyles and Rogerson, 1978; Small and others, 1979; Gomez Sedimentary descriptions and clast analysis and Small, 1985; Anderson, 2000) or dirt cones (Boulton, Sedimentary descriptions were made in the field based on 1967; Drewy, 1972; Kirkbride and Spedding, 1996). In this Hambrey’s (1994) modification of Moncrieff’s (1989) non- paper we concentrate on the relationship between debris genetic classification of poorly sorted sediments. Clast transport processes and glacier structure, in particular the analysis is used to differentiate between sediment sources, role of folding in medial moraine formation, the formation of transport histories and depositional environments in the dirt cones from englacial debris layers, and the role of glacial setting (Boulton, 1978; Benn and Ballantyne, 1994). crevassing. The approach used in this study is to compare samples of known transport or known depositional environments with 2. FIELD SITE samples of unknown or uncertain transport or depositional environments. Haut Glacier d’Arolla lies at the southern end of the Arolla Thirty-eight sediment samples of 50 clasts each were valley, which forms the western branch of the Val d’He´rens collected from a variety of locations on and around Haut in the Valais canton of Switzerland. The rapidly receding Glacier d’Arolla (Fig. 1) and classified into groups according glacier is 4 km long, and descends from 3500 m a.s.l. at its to where the sample was collected. For each clast the headwall to 2560 m a.s.l. at its snout. It has a wide following were recorded: (i) lithology, (ii) shape (relative accumulation area comprising two main basins feeding a lengths of the a (long) axis, b (intermediate) axis and c (short) narrow tongue (Fig. 1). The largest of the basins is the Mont axis), (iii) roundness (smoothness or angularity of the edges), Brule´ basin (2.5 km2) forming the southeastern area, with a and (iv) texture (surface roundness such as faceting and smaller (1km2) basin between La Vierge and L’Eveˆque. striations). Clast sizes were recorded using the Wentworth There is a substantial literature covering many aspects of (1922) scale of grain-size. Clast roundness was estimated Haut Glacier d’Arolla, for example glacier geometry (Sharp visually using the six roundness classes according to and others, 1993), subglacial hydrology (e.g. Nienow and Krumbein (1941) and Powers (1953). Clast data are others, 1998), glacial meltwater chemistry (e.g. Brown and presented in a modified RA/C plot (Benn and Ballantyne, others, 1996), interactions between glacier dynamics and 40 1994), which is a covariant plot of the RA index (modified in this study to be the percentage of very angular, angular and *Present address: Department of Geography, University of Canterbury, subangular clasts) and the C40 index (percentage of clasts Private Bag 4800, Christchurch, New Zealand. with a c : a axial ratio of 0.4). 140 Goodsell and others: Debris transport in a temperate valley glacier Fig. 1. Location map of Haut Glacier d’Arolla, showing place names mentioned in text, location of sediment sample sites and locations of photographs shown as subsequent figures. Goodsell and others: Debris transport in a temperate valley glacier 141 Fig. 3. Debris emerging to form the eastern medial moraine. (a) Debris emerging from primary stratification at the top of the glacier tongue to form one lens of the eastern medial moraine. Ice- axe right of centre for scale. Ice flow top left to bottom right. (b) Rose diagram displaying the a axis (long) orientation of clasts emerging in situ from the ice, forming one lens of the eastern moraine. The orientation of fold axes measured along the plane of emergence is 0048, dipping at 118 N. The a axis of the majority of clasts is orientated at a similar angle. The observations made in this section are independent of this previous work, but comparisons are made in the discussion. Haut Glacier d’Arolla has two main medial moraines: an eastern and a western (Fig. 1). The relationship between ice Fig. 2. Simplified structural evolution of Haut Glacier d’Arolla, structure and medial moraine formation is different at each showing the development of the key structures that are associated with debris entrainment, transport and deposition. of the moraines. The point of emergence of the eastern medial moraine has moved up-glacier, in step with the rapid ablation and lowering of the surface of the glacier. The eastern medial moraine consists of a series of elliptical or V- 4. SUMMARY OF THE STRUCTURAL EVOLUTION shaped lenses, orientated roughly parallel to primary OF HAUT GLACIER D’AROLLA stratification and longitudinal foliation. At some locations, Figure 2 demonstrates the structural evolution of the glacier debris emerges along planes parallel to primary stratification in a simplified schematic form. The first event in the (Fig. 3a). Debris in each lens is of a distinct lithology, a structural evolution of Haut Glacier d’Arolla is the de- variety of either meta-pelite or meta-granite. The a axis of position of snow, forming primary stratification in the upper clasts emerging from the primary stratification tends to be glacier basin, which, following firnification, forms glacier parallel to fold axes measured along the plane of emergence ice (Fig. 2a). The upper basin is divided into subflow units (Fig. 3b). Once debris has emerged from the ice, it creates a according to where the ice originated. As the stratification variable debris cover ranging from 20% to 100%. Down- moves down-glacier it becomes progressively folded, as a glacier, the lenses become one joined-up mass, with denser result of compression as the wide upper glacier feeds into a debris cover creating an ice-cored ridge. narrow tongue (Fig. 2b). The western medial moraine is situated on the boundary The glacier tongue is dominated by longitudinal foliation, between the main Haut Glacier d’Arolla flow unit and the which forms both from folding and eventual transposition of La Vierge tributary glacier. In some places, the ice core of the primary stratification, and any other structures formed in the western medial moraine is exposed by cross-cutting cre- upper reaches of the glacier (Fig.

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