Discriminating glacier thermal and dynamic regimes in the sedimentary record

Sedimentary Geology 251-252 (2012) 1–33

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Invited Review

Discriminating glacier thermal and dynamic regimes in the sedimentary record Michael J. Hambrey ⁎, Neil F. Glasser Centre for Glaciology, Institute of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, Ceredigion SY23 3DB, Wales, UK

a r t i c l e

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Article history: Received 11 November 2011 Received in revised form 15 January 2012 Accepted 16 January 2012 Available online 26 January 2012 Editor: J. Knight Keywords: Glacial sediments Thermal regime Temperate glaciers Polythermal glaciers Cold glaciers Clast shape

a b s t r a c t This paper provides a description and evaluation of the sedimentary facies and environments associated with a range of glacier thermal and dynamic regimes, with additional consideration given to the tectonic context. New and previously published data are evaluated together, and are presented from modern terrestrial and marine glacial sedimentary environments in order to identify a set of criteria that can be used to discriminate between different glacier thermal regimes and dynamic styles in the sedimentary record. Sedimentological data are presented from a total of 28 glaciers in 11 geographical areas that represent a wide range of contemporary thermal, dynamic and topographic regimes. In the context of “landsystems”, representatives from terrestrial environments include temperate glaciers in the European Alps, Patagonia, New Zealand, the Cordillera Blanca (Peru), cold glaciers in the Dry Valleys of Antarctica and the Antarctic Peninsula region, and polythermal valley glaciers in Svalbard, northern Sweden, the Yukon and the Khumbu Himal (Nepal). The glaciomarine environment is illustrated by data from cold and polythermal glacier margins on the East Antarctic continental shelf, and from a polythermal tidewater glacier in Svalbard, along with general observations from temperate glaciers in Alaska. These data show that temperate glacial systems, particularly in high-relief areas, are dominated by rockfall and avalanche processes, although sediments are largely reworked by glaciofluvial processes. Debris in polythermal glaciers is both thermally and topographically influenced. In areas of moderate relief, debris is mainly of basal glacial origin, and the resulting facies association is dominated by diamicton. In high-relief areas such as the Himalaya, the debris load in polythermal glaciers is dominated by rockfall and avalanche inputs, resulting in extensive accumulations of sandy boulder-gravel. Cold glaciers are dominated by basal debris-entrainment, but sediments are little modified from the source materials, which are typically sandy boulder-gravel from older till, and sand (from glaciofluvial, glaciolacustrine and aeolian sources). Similar facies associations, but with different facies geometry and thickness occur in equivalent glaciomarine settings. Application of these concepts can aid the interpretation of glacier thermal regime (and hence palaeoclimate) in Quaternary and ancient glacial systems. © 2012 Elsevier B.V. All rights reserved.

1. Introduction It is now forty years since Boulton (1972a,b) first described the important role of glacial thermal regime in determining the processes responsible for glacial debris-entrainment, transfer and sedimentation. Although other workers have developed and refined these concepts through both theoretical studies of debris entrainment and transport (e.g. Alley et al., 1997) and site-specific sedimentological case studies (e.g. Lian and Hicock, 2000), no rigorous criteria exist for identifying former glacier thermal or dynamic regimes in the sedimentary record. A large amount of effort has been expended in recent years in attempts to establish the nature and dynamic significance of deformed subglacial sediments (e.g. Boulton and Hindmarsh, 1987; Hart, 1995; Benn and Evans, 1996, 2010; Boulton, 1996; Marshall et al., 1996; Maltman et al., 2000; Hoffmann and Piotrowski, 2001). However, it can be argued

⁎ Corresponding author. Tel.: + 44 1970 621860; fax: + 44 1970 622659. E-mail address: [email protected] (M.J. Hambrey). 0037-0738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2012.01.008

that this effort has been at the expense of understanding wholesale patterns of sediment production by glaciers and their relationship with glacier thermal regime (e.g. Glasser and Hambrey, 2001a,b). Without an understanding of the spatial and temporal distribution of the sedimentary facies produced by glaciers of different thermal regimes, our ability to interpret correctly the sedimentary record of former glaciers and palaeoclimate is severely impaired. The aim of this paper, therefore, is to present data from a wide range of representative glacial sedimentary environments or “landsystems” in order to identify a set of criteria that can be used to discriminate between different glacier thermal regimes and dynamic styles in the sedimentary record. Field data are presented from a range of terrestrial glacial environments, primarily from valley glaciers and ice cap outlet glaciers that represent the diverse range of thermal and dynamic conditions present on Earth today. A more limited data-set from glaciomarine environments is also evaluated since, despite being relatively understudied, they are much better represented in the ancient record owing to their better preservation potential than their terrestrial counterparts. Data are presented using a facies approach to enable direct comparison

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between field sites, and to facilitate comparisons with former glacier forefields and with sediments preserved in the geological record. It is apparent that, apart from thermal regime, both topographic setting and the controlling tectonic regime, play an important role in determining the sedimentary facies associations that will be preserved; from this perspective this paper covers areas with low, moderate and high relief. Sedimentological data are presented from a total of 28 glaciers in 11 geographical areas (Fig. 1, Table 1): (1). temperate glaciers in the European Alps, Patagonia, New Zealand, the Cordillera Blanca (tropical Andes, Peru); (2). cold glaciers in the Antarctic Peninsula region and in the Dry Valleys, Antarctica; and (3). polythermal valley glaciers in areas of moderate relief, including Svalbard, northern Sweden and the Yukon, and high-relief areas of the Khumbu Himal (high-Himalaya, Nepal) (Fig. 1). Collectively, these glaciers represent a wide range of thermal regimes and dynamic styles determined by the latitudinal and altitudinal variations in terrestrial glacier distribution on Earth today (Fig. 2). Although several approaches have been adopted for the study of glacigenic sediments, and methods of sedimentological analysis clearly defined (Eyles, 1983a,b; Dreimanis, 1989; Brodzikowski and Van Loon, 1991; Hambrey, 1994; Menzies, 1995, 1996; Miller, 1996; Krüger and Kjaer, 1999; Hubbard and Glasser, 2005; Benn and Evans, 2010), there have been few attempts to assemble a consistent sedimentological data-set from modern environments that can be used as a basis for interpreting the Quaternary and pre-Quaternary glacial record. However, it is recognised that a wide range of over-lapping sedimentary processes operate in most environments, but to different degrees.

Hence, it is the relative proportions of the different facies that can be used to discriminate between glacier thermal and dynamic regimes.

2. Climate, glacier thermal regime and debris entrainment processes 2.1. Climatic influences The spatial distribution of glacier ice on earth is controlled by variations in mass balance, which, in turn, are a function of climate, especially atmospheric temperature and precipitation. Benn and Evans (Ch. 2, 2010) have distinguished three basic types of mass balance cycle: (1) winter accumulation type with well-defined accumulation and ablation seasons; (2) summer accumulation type, with both maxima of accumulation and ablation occurring simultaneously in summer; and (3) year-round ablation type, with one or two accumulation maxima coinciding with wet seasons. Since atmospheric temperature decreases with altitude, there is a direct relationship between temperature and elevation in polar, temperate, equatorial and subtropical climates (Fig. 3). The altitudinal distribution of precipitation, however, varies between polar, temperate, equatorial and subtropical climates. In polar environments, precipitation is generally low and declines linearly with elevation. In temperate environments, precipitation generally increases with elevation before declining at very high altitudes. In both subtropical and equatorial environments, precipitation is generally high, initially rising with elevation to a maximum of around 1000 to 2000 m above sea level, thereafter decreasing with elevation. When combined with the global distribution of land at different latitudes on Earth, these

Fig. 1. The location of the glaciers studied in this paper.

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Table 1 The glaciers from which data are presented in this paper, with key attributes and references to selected published studies. Region

Glaciers studied

Location

Glacier thermal regime/topography

Dominant bedrock lithology of catchment

Source

European Alps

Bas Glacier d'Arolla Haut Glacier d'Arolla Soler Leones Tasman Mueller Hooker Fox Franz Josef Raucolta Ahueycocha Artesonraju

45°58′N, 7°32′E

Temperate High relief Temperate High releief Temperate High relief

Gneiss, schist

Goodsell et al. (2002) Goodsell et al. (2005a,b) Glasser and Hambrey (2002)

Chilean Patagonia

46°53′S, 73°14′W 46°45′S, 73°14′W Aoraki/Mt Cook, 43°41′S, 170°12′E New Zealand 43°44′S, 170°03′E 43°40′S, 170°07′E 43°28′S, 170°01′E 43°28′S, 170°11′E Cordillera Blanca, 8°54′S, 77°44′W Peru 8°53′S, 77°38′W 8°59′S, 77°39′W Dry Valleys, Wright Lower 77°26′S, 162°47′E Antarctica Victoria Lwr. 77°22′S, 162°21′E Suess 77°39′S, 162°45′E Taylor 77°44′S, 162°18′E James Ross I., “Grooved” 64°21′S, 57°19′W Antarctic Peninsula “Humpback” 64°22′S, 57°19′W “Niche” 64°21′S, 57°21′W “Rabot Point” 64°16′S, 57°22′W Lazarev Sea, Various ice shelves, ice cliffs Antarctic continental Antarctica and ice streams shelf between 70°S, 12°′E and 70°30′S, 7°′W Svalbard Midre Lovénbreen 78°53′N, 12°04′E

Temperate V. high relief Cold Moderate Relief

Granite, schist, dolerite Granite, schist Greywacke, argillite Greywacke, argillite Greywacke, argillite Schist Schist Granite, gneiss

Hambrey and Ehrmann (2004)

Hambrey, Glasser, Quincey, Richardson (unpubl. data)

Granite, gneiss Granite, gneiss Granite, gneiss Granite, gneiss Basalt, hyaloclastite

Hambrey and Fitzsimons (2010)

Cold and polythermal (marine) Polythermal

Varied

Kuhn et al. (1993)

Phyllite, quartzite,

Hambrey et al. (1999); Glasser and Hambrey (2001a, 2003); Midgley (2001); Graham (2002) Bennett et al. (1998) Glasser and Hambrey (2001b) Etienne et al. (2003) Glasser et al. (2003) Hambrey and Clarke (unpublished data) Hambrey et al., 2009

Cold Moderate Relief

Austre Lovénbreen Kronebreen/Kongsvegen Kongsfjorden icebergs

78°53′N, 12°04′E 78°51′N, 12°31′E 78°58′N, 12°27′E

Moderate Relief

schist, marble, amphibolite, chert, sandstone, limestone

Northern Sweden

Storglaciären

67°54′N, 18°35′E

Yukon

Trapridge Glacier

61°14′N,140°20′W

Gneiss, metasediments, amphibolite Gneiss, metasediments, volcanics

Khumbu Himal, Nepal

Khumbu

27°56′N, 86°49′E

Imja Lhotse

27°54′N, 86°55′E 27°55′N, 86°54′E

Polythermal Moderate relief Polythermal Moderate relief Polythermal (inferred) V. high relief

generalisations explain the contemporary global distribution of glacier ice (Fig. 4). 2.2. Glacier thermal regime The thermal regime of a glacier is a fundamental control on glacier movement, meltwater production and routing, styles of erosion and deposition, and rates of geomorphological activity (Kleman and Glasser, 2007). Debris entrainment and transport are thus strongly related to glacier thermal regime (Bogen, 1996; Hallet et al., 1996; Alley et al., 1997). Debris is entrained either at the bed or on the surface of a glacier, and the transport paths and textural characteristics of glacial sediments are therefore related to the thermal and dynamic characteristics of the glacier (Boulton, 1972a,b, 1978). It therefore follows that if the nature of the sedimentary products deposited by specific glacier types can be tightly constrained, then it is possible to make inferences concerning the thermal and dynamic regimes of former glaciers. Boulton (1972b) defined four theoretical boundary conditions for the thermal regime at the sole of a glacier: a zone of net basal melting, a zone in which there is a balance between melting and freezing, a zone in which sufficient meltwater freezes to the glacier sole to maintain it at the melting point, and a zone in which the glacier sole is below the pressure melting point. However, the individual zones representing these boundary conditions are difficult to recognise in most glaciers and ice sheets and a simpler three-fold classification of thermal regimes is commonly adopted: temperate, cold and polythermal. The terms “polar” and “subpolar” are also commonly used to describe thermal regime, but we prefer “cold” and “polythermal” since they avoid any geographical connotation.

Granite, schist, psammite

Hambrey and Smellie (unpublished data)

Granite, schist, psammite Granite, schist, psammite

2.3. Temperate glaciers Temperate (“warm”) glaciers are those in which the ice at the bed is at the pressure-melting point throughout. These glaciers are typical of alpine regions with maritime climates and high snow accumulation rates (e.g. Patagonia, New Zealand, Iceland, southern Norway, parts of Alaska). Temperate glaciers are also a feature of higher mountain regions with strongly seasonal climates, such as the summer-dominated precipitation regimes of the tropical Andes. Temperate glaciers ablate through melting on the surface, internally and at the base. However, because most water discharges from the base at the glacier terminus they are commonly referred to as wet-based. Basal meltwater typically facilitates rapid glacier motion by enhancing glacier sliding, sub-sole deformation, or a combination of both. These glaciers are normally regarded as being efficient agents of glacial erosion. Temperate glaciers normally carry little debris at the bed (e.g. basal debris-rich layers of the order of centimetres to tens of centimetres) because of the substantial volumes of meltwater flowing there. In contrast, the surface commonly has extensive areas of supraglacial debris derived from rockfall activity on surrounding valley walls and slopes (Whalley et al., 1996). Exceptions to this generalisation are temperate glaciers in strongly compressive tectonic settings, such as at a number of Alaskan glacier margins, where greater thicknesses of basal debris may build up (Powell and Molnia, 1989). Here values of between 3 and 15 m of debris-rich ice have been reported, with debris concentrations varying from 3 to over 40% by volume. Debris entrainment in temperate glaciers is also possible by thrusting, but normally only where glaciers encounter a reverse bedrock or moraine slope (Glasser and Hambrey, 2002), where winter-frozen sediment is incorporated

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Fig. 2. Representative glaciers from different thermal regimes: (A) Temperate Grosser Aletschgletscher, Swiss Alps. (B) Temperate Oberaargletscher, Swiss Alps, showing typical angular supraglacial debris. (C) Cold Wright Lower Glacier, Dry Valleys, Antarctica, with ramp of deformed fluvial sediment with an Aeolian drape. (D) Polythermal Trapridge Glacier, Yukon, Canada, soon after termination of a ‘slow surge’. (E) Polythermal glaciomarine environment, Conwaybreen, NW Spitsbergen. (F) Byrd Glacier, an outlet glacier from the East Antarctic Ice Sheet (lower), feeding through the Transantarctic Mountains into the Ross Ice Shelf (upper); glacier width is 25 km (U.S. Geological Survey, Landsat-7 satellite image). (G) Grounded ice cliff at the edge of the East Antarctic Ice Sheet, Ingrid Christensen Land.

into the glacier by compressive flow against a cold outer margin (Harris and Bothamley, 1984), or where ice impinges on an already established moraine (Croot, 1988). Glacial landforms and sediments from temperate glaciers are extremely varied, reflecting the complexity of the sediment transport and depositional processes operating in temperate and tropical climatic regimes (Eybergen, 1986; Schlüchter and Wohlfarth-Meyer, 1986; Krüger, 1993, 1994; Benn, 1994; Kirkbride and Spedding,

1996; Alley et al., 1997; Kjær, 1999; Glasser and Hambrey, 2002; Spedding and Evans, 2002; Benn et al., 2003; Evans, 2003a,b; Swift et al., 2006; Cook et al., 2007). Previous studies on high-alpine valley glaciers have emphasised the role of supraglacial sedimentation, derived principally from valley-side debris ranging from bedrock masses, valley-side fans and soils, to fluvial sediments transported by supraglacial streams (Eyles, 1979, 1983a,b). Processes occurring during supraglacial debris-transport have a profound impact on the

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ablation. Typically, the glaciers which have created these moraines are mantled with supraglacial debris, and their down-wasting and recession sometimes leads to the formation of potentially hazardous moraine-dammed lakes, and the production of outburst flood sediments (Hubbard et al., 2005). Similar latero-terminal moraine complexes also occur in the high-Himalaya, but the glaciers here are believed to be polythermal. 2.4. Cold-based glaciers

Fig. 3. The relationship between precipitation/temperature and elevation in polar, temperate, equatorial and subtropical climates (modified from Barry, 1992).

distribution of sediment at the ice margin, influencing the form and structure of supraglacial debris features, and leading to the morphology and sedimentary facies of features as debris is deposited beyond the margins (Boulton and Eyles, 1979; Eyles, 1979; Owen et al., 2003). Supraglacial debris is often organised into medial and lateral moraines, both of which are important features of high-level sediment transport by these glaciers (Small and Clark, 1974; Eyles and Rogerson, 1978a, b; Small et al., 1979; Gomez and Small, 1983; Rogerson et al., 1986; Small, 1987; Smiraglia, 1989; Goodsell et al., 2005a,b). Many alpine regions (e.g. the European Alps and Southern Alps of New Zealand) as well as mid-latitude regions such as Scanadinavia and Iceland, have a climate in which most accumulation takes place in winter and most ablation in summer. Based on a study of two Icelandic glaciers in an area of moderate relief, Evans and Twigg (2002) recognised three depositional domains: (1) areas of extensive, lowamplitude marginal, dump, push and squeeze moraines; (2) incised and terraced glaciofluvial forms; and (3) subglacial landform assemblages of flutes, drumlins and overridden push moraines. In contrast, the most striking of all temperate glacier moraines are the lateroterminal complexes of the Peruvian Andes, which commonly exceed 100 m in height. This region experiences a climate with seasonal preciptitation, predominantly in summer, but with both winter and summer

Cold-based glaciers are those in which ice is below the pressure melting point throughout and, as a result free-flowing basal meltwater is absent. These glaciers can exist only in cold arid climates in high latitudes where rates of snow accumulation are low and mean annual temperature is well below 0 °C (Chinn, 1991). In the past, the Antarctic and Greenland ice sheets were regarded as cold-based, but coring to the base and ice-sheet modelling have shown that extensive areas are at the pressure melting point (e.g. Pattyn, 2010). This is because, even in the coldest Polar Regions, a mean annual temperature of −60 °C is insufficient to overcome the geothermal heat-flow and raise the basal temperature of thick glaciers and ice sheets to the pressure melting point. Hence, glaciers are cold-based only where the ice is thin in polar regions, and these are typically at the margins of the ice sheet. Cold-based glaciers in the Dry Valleys of Antarctica are among the best known, and typical basal temperatures range from −18 °C, the mean annual temperature, near the margin, to −15 °C near the middle (Holdsworth and Bull, 1970). Any subglacial meltwater produced from local pressure melting refreezes within the ice. Limited amounts of supraglacial meltwater may be generated by absorption of solar radiation on dust-covered surfaces, even in sub-zero temperatures, and flow off the glacier front or sides. Since no meltwater exists at the bed, these glaciers are commonly referred to as dry-based. Moreover, being frozen to their beds, cold glaciers move predominantly by internal deformation concentrated in the lowermost several metres of the ice mass and in the underlying sediment or bedrock. It has been reported that basal sliding and sub-sole deformation are either absent or extremely slow (Shreve, 1984). However, basal sliding at −5 °C has been reported from the Pamirs (Echelmeyer and Wang, 1987) and at −17 °C from the Dry Valleys (Cuffey et al., 1999). Consequently, cold glaciers have generally been regarded as having little ability to erode their beds. This assumption of non-erosion or “glacial protection” has been challenged by recent studies of debris-entrainment mechanisms

Fig. 4. Geographical variation of the highest summits and the highest and lowest snowlines with latitude on a latitudinal cross-section from pole to pole (modified from Barry, 1992). Glaciers can exist in all areas where there is land above the limit of the lowest snowline.

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beneath certain Antarctic glaciers (Cuffey et al., 1999). Moreover, Atkins et al. (2002) and Lloyd Davies et al. (2009) have produced evidence from Allan Hills, Victoria Land, Antarctica, in the form of abrasion marks (scrapes), subglacial deposits (crushed bedrock debris, sandstone and siltstone breccia), glaciotectonically deformed substrate, isolated blocks, ice-cored debris mounds, and boulder trains to support the notion that a cold-based glacier can indeed erode its bed and release sediment. In addition, basal processes not only remain active at subfreezing temperatures, but can significantly influence glacier motion. This is particularly the case in glaciers where sub-freezing basal thermal conditions coincide with the presence of fine-grained, ice-rich subglacial sediments (Tison et al., 1993; Waller, 2001). Cold-based glaciers commonly have substantial basal debris loads, especially near their margins where stacking of debris sequences are a result of regelation (Knight, 1997). Significant thicknesses of debrisrich ice can be entrained where glaciers over-ride and incorporate a frontal apron composed of ice and debris produced by dry calving (Shaw, 1977a,b; Lloyd Davies et al., 2009). This process may generate basal debris layers up to 5 m thick. Debris is typically stratified, reaching concentrations of 40–50% by volume. Deposition is dominated by removal of ice by sublimation in these cold arid environments. The structural properties of tills arising from this process, such as fissility and foliation, clast orientation and dip have a much higher preservation potential in arid than humid environments (Shaw, 1977a). Landforms associated with cold-based glaciers include thrust-block moraines composed of sand, gravel and organic silt entrained as frozen blocks of lacustrine sediment (Fitzsimons, 1990, 1996, 1997a,b, 2003; Humphreys and Fitzsimons, 1996; Fitzsimons et al., 2001). Commonly, these blocks retain their original depositional structures and, in general, basal glacial sediments deposited in arid polar environments retain more of the attributes of the pre-entrainment and transportation phases than do those deposited in the humid polar environment where meltwater may be present. The interaction of the basal ice zone of cold valley glaciers with fluvial and aeolian sediment, as well as older tills also produces a suite of distinctive landforms. The basal ice is laden with sand, strongly foliated, boudinaged and has undergone thrusting. The associate landforms include ice-contact sediment ramps comprising foliated ice and tilted masses of glaciofluvial sand and gravel, draped with aeolian sediment (Hambrey and Fitzsimons, 2010). Thus, as cold glaciers have become better known, their role as active geomorphological agents is increasingly being recognised (Fitzsimons, 2003). 2.5. Polythermal glaciers Polythermal valley glaciers are an intermediate type of glacier, commonly having a complex thermal structure. Blatter and Hutter (1991) defined a polythermal ice mass as consisting of several disjointed regions, each exhibiting distinct flow characteristics: cold, if its temperature is below the local melting point; temperate if at the pressure melting point. However, these authors pointed out that this simple concept is complicated by impurities in polycrystalline ice, intergranular flow of water and ice-permeability. Polythermal glaciers are associated especially with maritime high-latitude climates in both the Arctic and Antarctic (e.g. Canadian High-Arctic, Svalbard, Russian Arctic and the Antarctic Peninsula). Typically, the snout, lateral margins and surface layer of a polythermal valley glacier are below the pressuremelting point, whereas thicker, higher-altitude ice in the accumulation area is warm-based (Blatter, 1987; Holmlund and Eriksson, 1989; Blatter and Hutter, 1991; Hagen and Saetrang, 1991; Ødegard et al., 1992; Björnsson et al., 1996; Holmlund et al., 1996; Jansson, 1996; Jansson et al., 2000; Moore et al., 2009). This mix of thermal regimes makes the dynamics of polythermal valley glaciers complex. A typical polythermal glacier slides on its bed or experiences subsole deformation where warm-based ice dominates in the accumulation zone, and moves only by internal deformation where it is cold-based ice at the snout and lateral margins. Polythermal glaciers tend to carry a high

basal debris load, with debris-rich basal ice zones between 1 and 3 m in thickness, and a debris concentration of up to 50% (Dowdeswell, 1986). The surface of such glaciers rarely has a substantial cover of debris, although medial moraines are commonly observed in their lower ablation areas (Hambrey et al., 1999; Hambrey and Glasser, 2003a,b). Meltwater tends to follow supraglacial and englacial routes and well-developed basal hydrological networks are rare (Vatne et al., 1996; Hodgkins, 1997; Rippin et al., 2003). Descriptions of the debris-entrainment and transport processes associated with polythermal glaciers indicate that there are strong structural glaciological controls on landform and sediment development (Weertman, 1961; Swinzow, 1962; Hooke, 1973; Clapperton, 1975; Hudleston, 1976; Boulton, 1970ab, 1972ab, 1978; Hambrey and Müller, 1978; Harris and Bothamley, 1984; Hagen, 1987, 1988; Croot, 1988; Evans, 1989a, b; Van der Wateren, 1995; Boulton et al., 1996, 1999; Zdanowicz et al., 1996; Hambrey et al., 1997, 1999, 2005; Bennett et al., 1998, 1999; Glasser et al., 1998, 1999, 2003; Ó Cofaigh et al., 1999, 2003; Glasser and Hambrey, 2001a,b; Etienne et al., 2003; Hambrey and Glasser, 2003a). Drawing on data from two Svalbard valley glaciers, Usherbreen and Erikbreen, Etzelmüller et al. (1996) also suggested that the deformation of permafrost is important in the formation of ice-cored moraines and push moraines. Stresses beneath the advancing glaciers are transmitted to the proglacial sediments and can be sufficient to cause proglacial deformation of the permafrost layer (Hambrey and Huddart, 1995; Huddart and Hambrey, 1996; Etzelmüller and Hagen, 2005). Folding, thrust-faulting and overriding of proglacial sediments are possible under these conditions. The nature of the deformation is controlled by the mechanical properties of the sediment, which is influenced by the water content and thermal condition, whether it be frozen or unfrozen. One region where basal processes are not the dominant mechanism of debris-entrainment is the high-Himalaya. Here, the extremely steep relief over-rides thermal characteristics and results in the valley glaciers receiving large volumes of supraglacial debris as a result of avalanches and rockfall (Owen et al., 2003; Hambrey et al., 2009). Few data are available on the thermal structure of Himalayan glaciers, but from hydrological characteristics many are believed to be polythermal with a cold surface and temperate base. On a larger scale, the Greenland and Antarctic ice sheets are also probably polythermal. In the case of the Antarctic Ice Sheet, calculations and numerical modelling of thermal regime show that as much as 55% of the grounded part of the ice sheet is at the pressure melting point (Pattyn, 2010). The wet-based areas include not only outlet glaciers and ice streams (e.g. Stearns et al., 2008), but also zones with subglacial lakes and basal drainage (Siegert et al., 2005; Smith et al., 2009). 2.6. Glaciomarine sediments and climate Glaciomarine sedimentation takes place in a wide range of climatic and environmental settings (Dowdeswell and Scourse, 1990; Dowdeswell et al., 1998; Dowdeswell and Ó Cofaigh, 2002). Temperate tidewater glaciers are characteristic of the fjords of SE Alaska and southern Chile, whilst polythermal tidewater glaciers are a feature of the Canadian Arctic, Greenland, Svalbard, the Russian Arctic islands and the Antarctic Peninsula. In contrast, the coast of East Antarctica is dominated by slow-moving cold-based glaciers, ice shelves (some with basal freeze-on) and fast-flowing wet-based ice streams (Liu and Powell, 1996), these representing the coldest conditions under which glaciers interact with the ocean. In terms of sedimentology, glaciomarine sedimentation in Alaska is dominated by meltwater processes (e.g. Powell, 1990, 2003), whereas iceberg and biogenic sedimentation is dominant in polar settings (Dowdeswell et al., 1994, 1998; Anderson, 1999; Domack et al., 1999). During glacial advances across a continental shelf, the resulting sediment/landform assemblages resemble those of a terrestrial ice

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sheet (e.g. Ó Cofaigh et al., 2002). However, it is at the continental shelf edge that most of the glacigenic sediment accumulates, in the form of prograding wedges or aprons, known as trough-mouth fans (e.g. Vorren, 2003). These fans can record tens of millions of years of accumulation from multiple ice advances to the continental shelf break, e.g. the Prydz Trough-Mouth Fan that has accumulated since ice sheets first became established on East Antarctica, 34 m.y. ago (Cooper et al., 1991; Hambrey et al., 1991; Wise et al., 1991). 2.7. Role of tectonics and topography Tectonic setting is an important influence on rates of glacial erosion and sedimentation (Eyles, 1993). Glacierised regions subjected to rapid uplift and the creation of unstable areas of high relief include Alaska, the Andes, the Himalaya and the Southern Alps of New Zealand. Uplift is accompanied by rapid erosion of mountain peaks and large-scale rockfalls, sometimes covering entire glacier tongues with coarse angular debris. Apart from some parts of the Andes, these areas also receive considerable amounts of precipitation, so that meltwater and rain are the major influences on sediment reworking. Rates of sedimentation of tens of centimetres per day have been recorded in grounding-line fans in Alaskan fjords (Powell, 2003). Thus, as grounding-line fans build up above sea-level, marine glacial systems can convert to terrestrial glacial systems over very short timescales (e.g. decades). In regions of exceptionally high relief, such as the Himalaya and Andes, tectonically controlled topography is the first-order control on sediment entrainment, which in these regions is dominated by rockfalls and avalanches. Thus, thermal regime is less important here than tectonic uplift and erosion. 2.8. Glacier thermal regime—summary The distinction between these different glacier types is important because glacier thermal regime, determined principally by the prevailing environmental and climatic conditions, is a first-order control on the dynamics of a glacier. Consequently, glacier thermal regime largely dictates the processes of ice movement (sliding, internal deformation and subsole deformation), the nature of sediment entrainment and transport, and ultimately the sedimentary products released by the glacier. Successful interpretation of the sedimentary record of former glaciers and ice sheets therefore requires knowledge of the sedimentary products of modern ice masses. Entrainment of debris at the bed is by a combination of processes, mainly involving cycles of pressure melting and refreezing (regelation), to create a basal debris layer (Alley et al., 1997; Knight, 1997; Hubbard et al., 2009). Basally derived debris is subject to comminution at the ice/bedrock interface, and typically is dominated by clasts up to boulder-size, with subangular and subrounded shapes, faceted surfaces, and striations (if the lithologies are fine-grained) (Boulton, 1978). Much clay- or silt-grade sediment is produced by abrasion (Hindmarsh, 1996). In contrast, debris that falls on the glacier surface as a result of frost shattering of the overlooking cliffs is generally very angular to angular, and the proportion of fines is generally small (Owen et al., 2003). Glacial deposition involves the release of debris that has been transported on or within glacier ice. Debris is modified during transport primarily by basal processes (e.g. abrasion and quarrying during intra-clast collision, subglacial sediment deformation), and by water in subglacial, englacial and supraglacial stream channels. Debris that follows a passive transport path (supraglacially or englacially) tends to retain its primary characteristics. The texture and clast characteristics of the resulting sedimentary products are also influenced by lithology. Hard, coarse-grained crystalline rocks such as granite or gneiss are resistant to comminution and yield sediments with sand-dominated matrix and clasts with few signs of faceting and striations. In contrast, soft, fine-grained sedimentary rocks abrade easily, produce more silt and clay and more striated and facetted clasts (Hambrey and Ehrmann, 2004).

7

In glaciomarine environments, the proportion of different lithofacies reflects their terrestrial counterparts. However, the geometries of the facies associations or ‘landsystems’ are different; key features are grounding-line fans, morainal banks, trough-mouth fans, iceberginfluenced muds and biogenic sediments (Powell, 2003; Vorren, 2003). Similarly, the thicknesses of the sedimentary sequences will be greater in marine settings, amounting to tens or even hundreds of metres since the accommodation space for accumulation is generally greater, e.g. in deep fjords or on subsiding/prograding continental shelf margins.

3. Definitions and terminology It is important to discriminate between descriptive and genetic terms when evaluating sediments in glacial environments, or indeed any other sedimentary environment. Definitions of the terminology used in the description of glacigenic sediments are required to ensure consistency between different workers in this field. Description of sorted glacigenic sediments, such as glaciofluvial sediment, is generally straightforward, and in this paper we follow the classification of Miall (1978). However, since many glacial sediments lack sorting, a nongenetic classification of poorly sorted sediments is required for an objective study of sedimentary facies before process-related terms such as ‘till’ are used. The terms diamicton (unlithified), diamictite (lithified) and diamict (both) are now well established in the glacial sedimentological literature for ‘a non-sorted or poorly sorted terrigenous sediment that contains a wide range of particle sizes’ (Flint et al., 1960). However, this definition hides a vast range of textures, and the literature abounds with conflicting ideas about what constitutes a ‘diamicton’. In addition, the term “diamict” is often used to mean “diamicton”, according to the original definition (e.g. Eyles, 1983a,b). A textural classification, based on the relative proportions of sand and gravel, was developed by Moncrieff (1989), a modified version of which is illustrated in Table 2 (from Hambrey and Glasser, 2003a,b). All field descriptions of sedimentary facies in this paper follow this classification. In essence, this classification is a compromise between geologists who have argued that diamictons(ites) are poorly sorted sediments with a high proportion of gravel (e.g. clast-supported diamicts of Eyles, 1983a,b; Benn and Evans, 2010), and those who have opted for a lower percentage of gravel, such as the offshore drilling community (e.g. Barron et al., 1989; Hambrey et al., 2002). We also define the following terms that are commonly used, and still regarded as acceptable, in the naming of glacigenic sediments on a genetic basis: Glacigenic sediment Any sediment of glacial origin, used to embrace all sediments that have been influenced by glacier ice. Glacial debris material being transported by a glacier. Till An “unsorted deposited with a wide range of grain sizes deposited directly from glacier ice, without subsequent disaggregation and flow”. Basal till or subglacial till glacial sediment released from basal ice beneath a glacier. Supraglacial till glacial sediment let down onto the substrate as a glacier down-wastes or recedes. These and other terms were adopted formally by INQUA (Dreimanis, 1989), but are in need of revision as the applicability of more refined terms, that emphasis specific processes, such as “lodgement till”, “melt-out till” and “deformation till” has been questioned recently. In a wide-ranging evaluation of the microstructural attributes of tills, Van der Meer et al. (2003) argued that all subglacial tills are the product of processes active within deforming glacier beds. These authors have therefore suggested that subglacial tills are not depositional but structural sediments and ought to be termed “tectomicts”. Evans et al.

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Table 2 Non-genetic classification of poorly sorted sediments, modified from that proposed by Moncrieff (1989). Note that the maximum proportion of gravel in a diamicton in this classification is 50%. Terminology refers to both unconsolidated sediments and their lithified equivalents (from Hambrey and Glasser, 2003a). Percent gravel (<2mm) in whole sediment Mud <0.06mm

Trace ( < 0.01) MUD (STONE)

0.11

Sandy MUD (STONE)

<1%

1−5%

Sandy MUD (STONE) with dispersed clasts

Sand/mud ratio of 1 matrix

9 SAND (STONE) Sand 2.0-0.06 mm

50 − 95 %

Muddy SAND (STONE) with dispersed clasts

SAND (STONE) with dispersed clasts

> 95 %

Muddy GRAVEL/BRECCIA/ CONGLOMERATE clast-poor muddy DIAMICT (ON/ITE)

clast-rich muddy DIAMICT (ON/ITE)

0

20

33 clast-poor 50 intermediate DIAMICT (ON/ITE)

Muddy SAND (STONE)

5 − 50 %

MUD (STONE) with dispersed clasts

GRAVEL/ BRECCIA/ CONGLOMERATE

clast-rich intermediate DIAMICT (ON/ITE)

40 Percent sand in matrix 60

66 clast-poor sandy DIAMICT (ON/ITE)

clast-rich sandy DIAMICT (ON/ITE) SANDY GRAVEL/BRECCIA/ CONGLOMERATE

Gravelly SAND (STONE)

(2006a) argued that many glacier beds consist of mosaics of deforming and sliding conditions that vary in time and space. Consequently, the sedimentary signature of subglacial or basal tills is characterised by changes in till texture and proportions over short distances that reflect a wide range of processes, notably lodgement, frictional retardation, melt-out, deposition by gravity, subglacial shearing of rock and sediment, cavity-filling and boulder-pavement formation (Evans et al., 2006; Benn and Evans, 2010, Ch. 10). The all-embracing term advocated by these authors is “subglacial traction till”. This term is adopted in this paper. Specifically, lodgement till, deformation till and melt-out till, as in earlier studies, would be grouped together under this new term. 4. Facies analysis Traditionally, genetic labels (e.g. “till”) have been given to poorly sorted sediments that are inferred to be glacigenic, without provision of the evidence for processes of deposition. This approach is flawed and contrasts with the usual descriptive approach used in mainstream sedimentology. Facies analysis, as an objective means of dealing with sediments, was originally used in the oil industry and successfully applied to fluvial sediments (Miall, 1978, 1992; Reading, 1996). Facies analysis consists of (i) defining and describing lithofacies, (ii) recognising recurring facies associations, (iii) interpreting the depositional environment, and (iv) developing conceptual models of depositional systems. One of the earliest applications to glacial systems was that of Eyles et al. (1983), who proposed a set of lithofacies codes for describing glacial sediments. Although widely used, this approach is highly prescriptive and contains interpretative elements. Rather, we advocate the use of customised facies designations for each glacigenic succession based on a range of sedimentological parameters (Table 3), which are validated in contemporary glacigenic settings.

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processes responsible for these deposits and the likely spatial variability of these sedimentary facies. The concept of glacial sediment/landform associations (the “landsystems” approach) provides a useful means of summarising these data. The approach rests on the assumption that, at the large scale, it is possible to identify areas of land with common attributes, distinct from the surrounding areas, which can be related to the processes involved in their development (Benn and Evans, 1998; Evans, 2003a,b). The landsystems approach was pioneered for glaciated terrain by Boulton and Paul (1976) in an attempt to identify tracts of land created by similar till-forming processes and therefore of similar geotechnical properties. The glacial landsystem concept was then applied to former ice-sheet beds. Originally, three distinct landsystems were defined: The subglacial landsystem, in which the dominant glacial sediment/ landform associations are formed at the glacier bed. The supraglacial landsystem, in which the dominant glacial sediment/ landform associations are largely composed of a drape of supraglacial debris. Table 3 Principal sedimentological attributes used in analysis of glacigenic facies. Characteristics

Specific attributes

Bedding

Thickness Boundary relationships Sedimentary structures Particle-size distribution • Proportions of gravel, sand and mud • Sorting • Grain orientation Lithology Powers roundness Shape: RA/C40 index Surface features • Striae • Chattermarks • Facets Fabric Ductile: folds, foliation, strain shadows Brittle: faults, shear zones, crushed grains Microfabric: plasmic fabric

Texture

Clast morphology

5. Glacial sediment/landform associations In their discussion of the physical processes and deposits of glacial sedimentary environments, Ashley et al. (1985) focused intentionally on small- and intermediate-scale features (up to several km) by describing the physical characteristics of glacigenic deposits, the

100

50 % gravel

Deformation structures

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9

in the High Arctic (Bennett et al., 1997) and is extended here to a wide range of glacial settings.

The glaciated valley landsystem, in which the dominant glacial sediment/landform association is that formed by mountain glaciation.

7. Facies and landsystems associated with temperate glaciers As the concept has developed, other landsystems have been added to this simple list, partly because the full complexity of sediment transport processes in glaciated valleys is now recognised (Spedding and Evans, 2002). These include landsystems formed in different thermal regimes, in proglacial and glaciomarine environments, as well as those found in settings where surge-type glaciers are common (Evans, 2003b). A major challenge is to relate landsystems to glacier thermal regime. Here, those landsystems that are related to temperate, polythermal and cold-based glaciers, both in terrestrial and marine environments, are identified.

7.1. Lithofacies characteristics The principal attributes of the lithofacies associated with alpine valley glaciers, embracing regions such as the European Alps, the Southern Alps of New Zealand, and the Andes of Peru and Patagonia are summarised in Table 4 and illustrated in Fig. 5. Focusing on alpine valley glaciers, the principal visible inputs to the glacier system are evident supraglacially as scree, discrete rockfalls, stream-generated fans and rock-ice-snow avalanche cones. Angular boulder-gravel is the characteristic lithofacies resulting from these processes, with boulders up to several metres in diameter being common. Clast shapes range from very angular to subrounded, with angular being dominant. Co-variant plots show RA/C40 values typically of 60/90 (Fig. 6). Sand is a minor constituent of these deposits. During transport supraglacial sediment is reworked to some extent by supraglacial streams, producing more sand, the dominant lithofacies being sandy boulder-gravel with shapes ranging from very angular to rounded, but angular remaining dominant. They may become mixed with more rounded and striated material from the bed where uplift occurs on shear zones or thrusts. Stream-reworked supraglacial debris also includes a variety of gravel lithofacies and moderately well-sorted sand and silt, especially where water collects in ponds and abandoned channels. Sandy boulder-gravel near glacier margin may show signs of rounding as a result of abrasion against the valley sides. In contrast to supraglacial debris, basal debris consists of sandy gravel dominated by subrounded and subangular clasts dominant with low RA/C40 values. This lithofacies is interpreted to have been reworked from glaciofluvial sediments and incorporated into the basal zone of traction of the glacier. Direct deposition from the ice releases sandy boulder-gravel as lateral moraines. Today, many exposed faces of the inside of these moraines are visible following down-wasting of the glacier surface from the Neoglacial limit. These faces may exceed 100 m above the modern ice surface and are prone to collapse by rotational failure and continuous degradation. Given their largely supraglacial and icemarginal derivation the clasts within these moraines range from very angular to rounded, with angular remaining dominant. A small proportion of striated clasts are found within suitable fine-grained lithologies. On a decadal timescale the preservation potential of lateral moraines is high, but eventually they degrade to a bench or sluice off the oversteepened rock walls. In alpine regions, these moraines curve into end-moraine complexes up to 50 m high, although in many cases they are destroyed by glaciofluvial processes. The lithofacies in these landforms is more varied, reflecting the variable substrate over which the glacier has flowed; they include sandy gravel (interpreted as glaciofluvial), and stratified sand and mud or laminites (interpreted as

6. Data acquisition: field areas and methods Field data are presented from contemporary glaciers in Svalbard (Norwegian High-Arctic), Storglaciären (northern Sweden), Haut Glacier d'Arolla (Swiss Alps), Soler Glacier (Chilean Patagonia), the Mount Cook area (New Zealand), the Khumbu Himal (Nepal), the Cordillera Blanca (Peru), the Dry Valleys of Antarctica, James Ross Island (northern Antarctic Peninsula), and the East Antarctic continental shelf (Figs. 1, 2). These glaciers and their key attributes are shown in Table 1. On land, samples were collected from debris in transport on and within the glaciers, as well as from depositional landforms in close proximity to the ice margins where depositional environments are well-constrained. In Svalbard glaciomarine sediments that have been reworked into onshore moraines with little mixing were also examined. On the East Antarctic continental shelf gravel samples were collected by dredging and coring from the German research vessel, RV Polarstern, from areas under the direct influence of icebergs. Lithofacies are described according to the sedimentary attributes listed in Table 3. They are considered in terms of (i) inputs to the glacial system, (ii) transport on, within and at the base of the glacier, (iii) direct deposition from the glacier, and (iv) reworking after deposition by the glacier. Sediments were classified in the field on the basis of the Hambrey and Glasser (2003b) modification of the Moncrieff (1989) classification of poorly sorted sediments (Table 2). Particle-size distributions were determined using a combination of mechanical sieving techniques (gravel and sand fractions) and a Micromeritics SediGraph 5100 particle-size analyser (for the silt and clay fractions). Clast morphology was analysed for samples of 50 clasts, including clast roundness on a modified Powers (1953) scale, and the measurement of a, b and c axes for clast shape. These data were analysed using the approach of Benn and Ballantyne (1993, 1994) in which the RA index (% of angular and very angular clasts) is plotted against the C40 index (% of clasts with c/a axial ratio ≤0.4) on a co-variant plot. This method has been shown to provide good discrimination between glacial facies

Table 4 Typical lithofacies, interpretation and relative abundance at temperate glaciers. Key to relative abundance: ***** = dominant ← → * = rare. Lithofacies

Clast-rich muddy or sandy diamicton Sandy boulder-gravel Rounded gravel Angular gravel Sandy gravel Sand Laminite Silt

Interpretation

Abundance

% striated clasts

% faceted clasts

% clasts

Matrix (%) sand

silt

clay

Sorting coefficient

Sorting category

Basal glacial

*

6

12

5–30

50

40

10

1.05–1.8

Poorly-sorted

Ice marginal Glaciofluvial Supraglacial Reworked glaciofluvial Reworked glaciolacustrine Reworked ice-proximal glacio-lacustrine Glaciolacustrine

**** ** ***** ***** ** ** **

2 0 0 2 – – –

8 0 0 4 – – –

50–70 90–100 90–100 80–100 – – –

60 90 95 70 10 10 0

30 10 5 30 90 10 100

10 0 0 0 0 80 0

1.3–1.7 0.35 0.45 0.5–0.6 0.37–0.5 0.4 0.34–0.48

Poorly-sorted Well-sorted Well-sorted Mod. well-sorted Well-sorted Well-sorted Well-sorted

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Fig. 5. Representative lithofacies associated with temperate glaciers: (A) Supraglacial debris comprising angular rockfall material, Gornergletscher, Switzerland with the Matterhorn in background. (B) Basal debris accumulating on the limestone floor of a cavity beneath Glacier de Tsanfleuron, Switzerland; flow from back to front of cavity indicated by linear structures in the ice. (C) Latero-terminal moraine complex of Llaca Glacier, Cordillera Blanca, Peru; moraine comprises sandy boulder-gravel, and the intervening down-wasting ice is draped with supraglacial debris. (D) Glaciofluvial braid-plain of sandy gravel between Tasman Glacier with supraglacial debris cover (foreground) and Godley Glacier (background), New Zealand; water-flow is towards the camera. (E) crudely bedded sandy gravel with imbricated clasts of glaciofluvial origin, below Fox Glacier, New Zealand; height of section is about 25 m. (F) Ice-marginal lake accumulating laminated sand, surrounded by supraglacial debris, Soler Glacier, North Patagonian Icefield, Chile; pale coloured sediment ridge at right comprises uplifted glaciolacustrine sands.

glaciolacustrine). In the tropical Andes, where meltwater plays a lesser role, the end-moraine complexes are much larger; as the glaciers recede and down-waste they hold back moraine-dammed lakes that, in time, may fail, producing outburst flood deposits (Reynolds, 1992; Hubbard et al., 2005). Diamicton in the sense used in this paper is a relatively rare deposit in alpine environments. Where it does occur, it is of the clast-rich sandy or muddy variety, with subangular and subrounded clasts dominant, and low RA/C40 values. Striated and faceted clasts occur if fine-grained lithologies are available. The diamicton may have a sharp contact with bedrock, although commonly the lower boundary is not exposed. The upper contact is sharp and irregular, and overlain by angular debris. This facies is interpreted as subglacial traction till, overlying ice-abraded bedrock and underlying supraglacial meltout till. In areas of lesser relief, where the supraglacial input is reduced, such as

Iceland or Norway, diamicton has a much greater chance of surviving reworking by glaciofluvial processes. Reworking after deposition by the glacier's subglacial stream is an important process in temperate alpine environments. The dominant lithofacies is sandy gravel, with dominant clast sizes ranging from boulder, through cobble to pebble, with size decreasing as distance from the ice margin increases. Clasts range from angular to wellrounded with subrounded being dominant, and very low RA/C40 values. Striae and facets commonly survive within approximately 100 m of the ice margin, but rarely beyond. Mud is largely winnowed out, leaving a sandy matrix. Progressive rounding and fining downstream tends to be interrupted by additions of subglacial and ice-marginal facies from collapsing riverbanks. In general, these glaciofluvial sediments become more sandy downstream. Pockets of well-sorted clay and silt may

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Fig. 6. Clast shape data from modern temperate, cold and polythermal glaciers shown as a co-variant plot of the RA index (% of very angular and angular clasts within a sample) against the C40 index (% of clasts with a c/a axial ratio of ≤ 0.4). Each symbol represents a sample of 50 clasts. All samples were collected and analysed by the authors except for additional data for Midre Lovénbreen (Midgley, 2001) and Haut Glacier d'Arolla and Bas Glacier d'Arolla Goodsell et al. (2002, 2005a,b).

survive in slack-water areas. The basal contact of these glaciofluvial sediments is rarely observed, except where incised into subglacial till; the upper surface may represent the land surface. Glaciofluvial processes produce a range of facies associations (Miall, 1992). The thickness of

reworked glacigenic sediments can reach several tens or even a few hundred metres if suitable depocentres, such as glacially overdeepened basins exist. Late Quaternary examples are the sandar of Iceland and the valley trains of Alaska and New Zealand.

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7.2. Relative abundance of lithofacies In temperate alpine environments the dominant lithofacies are (i) boulder gravel with mainly angular clasts on the steep slopes above the glaciers, (ii) sandy boulder gravel with mainly angular and subangular clasts forming lateral and end moraines, and (iii) sandy boulder/cobble gravel with mainly subrounded clasts and sand, which represent glacially transported sediment reworked by braided rivers (Fig. 5). Diamicton forms only a minor component in contemporary alpine environments, as for example in the Southern Alps, where the main depocentres are filled with glaciofluvial sediment (Hambrey and Ehrmann, 2004). Unless an ice-proximal lake is present, finer grained facies form only a small percentage of the total sedimentary sequence. However, where present, as at Lake Pukaki downstream of Tasman Glacier, laminated sand, silt and clay may be reworked into endmoraine complexes when the glacier re-advances through a proglacial lake (Hart, 1996; Mager and Fitzsimons, 2007). Representative facies of high alpine environments are illustrated in Fig. 5. However, in areas of lesser relief debris deposited directly from ice has a better chance of surviving glaciofluvial reworking. Extensive sheets of diamicton are present, for example in the relatively flat proglacial areas of some Icelandic and Norwegian glaciers (e.g. Evans and Twigg, 2002). In the tropical Andes, the dominant glacigenic facies are of supraglacial derivation, and reworking by glaciofluvial activity is limited by the confined nature of the valleys.

7.3. Temperate glacier landsystems Temperate glacial landforms and sediments are varied, reflecting the complexity of the sediment transport and depositional processes operating in temperate glacial environments (Eybergen, 1986; Schlüchter and Wohlfarth-Meyer, 1986; Krüger, 1993, 1994; Benn, 1994; Kirkbride and Spedding, 1996; Kjær, 1999; Glasser and Hambrey, 2002; Spedding and Evans, 2002; Benn et al., 2003; Hambrey and Ehrmann, 2004). Therefore, no uniform facies association, nor a single landsystem model can apply to all temperate glacial environments. Examples of schematic vertical profile logs showing typical facies associations for a range of temperate glacier environments are presented in Fig. 7A–E, and a landsystem model for the alpine environment in Fig. 8. Previous studies on alpine glaciers in mountainous terrain have emphasised the role of supraglacial sedimentation, derived principally from valley-side debris ranging from bedrock masses, valley-side fans and soils, to fluvial sediments transported by supraglacial streams (Eyles, 1979, 1983a,b). Various authors have argued that supraglacial debris transport has a profound influence on the distribution of sediment at the ice margin, whilst the form and structure of supraglacial debris features are important controls on the distribution of landforms and sedimentary facies in front of these glaciers (Boulton and Eyles, 1979; Eyles, 1979) (Fig. 8). Supraglacial debris is often organised into medial and lateral moraines, both of which are important features of high-level sediment transport within these glaciers (Small and Clark, 1974; Eyles and Rogerson, 1978a,b; Small et al., 1979; Gomez and Small, 1983; Small et al., 1984; Rogerson et al., 1986; Small, 1987; Smiraglia, 1989; Goodsell et al., 2005a,b). Proglacial deposition is important in alpine terrain, leading to the development of extensive braided outwash plains (valley trains or sandar), which may become the most aerially extensive landform component. Subglacial deposition may also be an important process in a temperate alpine glacier setting, but normally the resulting sediments are reworked by streams. In contrast subglacial deposits, typically diamicton, are less subject to reworking in less topographically constrained areas, such as Iceland. Based on a study of two Icelandic glaciers, Evans et al. (1999) and Evans and Twigg (2002) recognised three depositional domains: (1) areas of extensive, low amplitude marginal, dump, push and squeeze moraines; (2) incised and terraced glaciofluvial forms;

(3) subglacial landform assemblages of flutes, drumlins and overridden push moraines. Terminal moraines, composed of a variety of sedimentary facies including diamicton (interpreted as a basal glacial deposit), sandy boulder-gravel, gravel and sandy gravel (collectively interpreted as reworked supraglacial, glaciofluvial and ice-marginal deposits depending on clast-angularity), and laminates and silts (interpreted as ice-proximal and reworked glaciolacustrine deposits) are present in many alpine and Andean regions. For example, around the North Patagonian Icefield, the heterogeneous nature of these sediments results from the reworking of pre-existing sediments during glacier advance (Glasser et al., 2009). Thus many of the terminal moraines are composed of sediments originating in over-ridden lacustrine basins and reworked glaciofluvial material. The sandar are strongly topographically confined, especially to the east of the icefield, and therefore not as laterally extensive as their Icelandic and Alaskan counterparts. Vertical accretion in the confined valleys appears to be more important, as a result of which thick, but not laterally extensive, deposits accumulate over time. Laterally extensive Icelandic-style sandar are only developed to the west of the icefield, where there is less topographic control on their expansion. 7.4. Temperate surging glaciers Large physical changes take place when a temperate glacier surges (Sharp, 1988). Specifically, marked changes in the hydrological conditions at the bed and in debris-entrainment processes at the ice margin are likely to be manifested in the sediment/landform assemblages. Nevertheless, no single landform can be used to identify the product of a glacier surge in the palaeo-record (Evans and Rea, 2003; Benn and Evans, 2010, Ch. 11). However, based on several studies on Icelandic glaciers with a known surge history, sediment/landform assemblages provide a strong signal of surging behaviour (Evans and Rea, 2003). The recognition of surging is based on defining three overlapping geomorphological zones: (i) an outer zone of thrust-block and push moraines; which grades up-flow into (ii) a zone of patchy hummocky moraine, and then into (iii) a zone of flutes, crevasse-squeeze ridges, concertina eskers, with areas of pitted, channelled or hummocky outwash, and occasional over-ridden thrust and push moraines. Complexities emerge for glaciers with a cyclic surge history, when an earlier surge sequence is over-ridden by a later one (Evans and Rea, 2003). This temperate surging glacier landsystem is clearly different from the valley glacier systems described above, and in fact bears a closer resemblance to polythermal glaciers in Svalbard of both surging and non-surging types (see below). 8. Facies and landsystems associated with cold glaciers 8.1. Lithofacies characteristics The range of lithofacies exhibited by cold glaciers is quite different from that associated with temperate and polythermal glaciers. In areas dominated by glacial erosion, e.g. Allan Hills, Victoria Land, Antarctica, deposits are represented by thin accumulations of poorly sorted sandstone and siltstone breccia with a friable structureless matrix, and mainly subangular clasts; this facies is interpreted as “till” (Atkins et al., 2002; Lloyd Davies et al., 2009). In the terrestrial depositional zone, the range of lithofacies associated with valley glaciers or outlet glaciers is limited. Sand, gravel and residual glacier ice are dominant, whilst diamicton is absent, as exemplified by Wright Lower Glacier in the Dry Valleys of Victoria Land, Antarctica (Hambrey and Fitzsimons, 2010) (Figs. 7G, 9). Additional clast data are presented from other glaciers in the Dry Valleys and from inferred cold glaciers on James Ross Island, a polar-maritime climatic regime in the northern Antarctic Peninsula region (Table 5). Recognising the deposits of cold glaciers in the geological record is

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13

Fig. 7. Idealised lithofacies associations for different thermal regimes in terrestrial and marine settings. Ornaments within key are lithofacies descriptions. Interpretations of depositional process are placed alongside the logs. Open shading for clasts indicates a supraglacial or fluvial aspect, shaded clasts are indicative of subglacial derivation. References are to data in papers from which the logs are drawn. Grain-size indicators: c = clay, s = silt, s = sand, g = gravel.

challenging because of the near-absence of clear indications of glacial transport. Much of the sediment incorporated and transported by glaciers in the Dry Valleys is derived from other sources, rather than created by the glaciers themselves. Supraglacially, very well sorted aeolian sand

is widespread but patchy, occupying hollows as a result of differential ablation. Minor pebble/cobble gravel from rockfall occurs if the glacier is bordered by steep cliffs. Clasts range from angular to subrounded with subangular being dominant, and RA values are low compared with rockfall-generated material elsewhere, although C40 values are

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Fig. 8. Conceptual model for a temperate alpine valley glacier landsystem.

similar (Fig. 6). This reflects that the source material is heavily weathered with clasts showing edge-rounding. Basal glacial sediment reflects the surrounding non-glacial material. It includes well-sorted sand that is faulted and boudinaged, and is interpreted as having been

incorporated from glaciofluvial sediment by regelation and glaciotectonic processes. Also in the basal zone is sandy boulder-gravel and gravelly sand, which is poorly sorted and contains clasts ranging from angular to rounded, in which subrounded is dominant. RA/C40 values

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Fig. 9. Representative lithofacies associated with cold glaciers: (A) Basal debris comprising well-sorted glaciofluvial sand, with beds strongly boudinaged, base of Wright Lower Glacier, Victoria Land, Antarctica. (B) Foliated basal ice with glaciofluvial sand, overlain by melted out sand, and capped by (pale) aeolian sand, Wright Lower Glacier. (C) Boulder-gravel at ice edge and sandy gravel in ridge beyond, representing melted out basal debris, Wright Lower Glacier. (D) Drape of aeolian sand reworked by supraglacial streams on the decaying snout of Wright Lower Glacier. (E) Gully and alluvial fan of sand associated with ice-marginal sediment apron, Wright Lower Glacier. (F) Moraine comprising unmodified clasts from a hyaloclastite (volcanic breccia) at margin of informally named “Humpback Glacier”, SE James Ross Island, Antarctic Peninsula.

Direct deposition from these cold glaciers produces lithofacies with little textural modification from the material that is in transport (Hambrey and Fitzsimons, 2010). The sandy boulder/cobble-gravel that makes up ice-marginal ridges and is a minor component of the

centre on 20/50. The origin of this lithofacies is the extensive sheet of Ross Sea Drift, thought to have been deposited by a wet-based ice sheet during the late Pleistocene Epoch (Denton et al., 1991). None of the clasts observed showed signs of striations.

Table 5 Typical lithofacies, interpretation and relative abundance at cold glaciers. Key to relative abundance: ***** = dominant ← → * = rare. Lithofacies

Sandy boulder-gravel Sandy cobble-gravel Gravelly sand Sand Sand Sand Sand Siliceous mounds and laminae Foliated glacier ice Firn

Interpretation

Abundance

% striated clasts

% faceted clasts

% clasts

Matrix (%) sand

silt

clay

Sorting coefficient

Sorting category (estimated)

Subglacial (reworked till)

****

0

0

50–80

95

5

0

n/d

Poorly sorted

Subglacial Subglacial Glaciofluvial Lacustrine Aeolian Stromatolites and algal mats Basal ice remnants Snow-filled channel or depression

** ***** ***** **** *** * ** *

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

5–10 0 0 0 0 0 0 0

98 100 100 95 100 n/d n/d n/d

2 0 0 5 0

0 0 0 0 0

n/d n/d n/d n/d n/d

Poorly sorted Poorly sorted Well sorted Well sorted Well sorted

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ice-contact aprons, shows a widely varying texture, and low RA and medium C40 values. This facies is reworked from the older Ross Sea Drift. Associated lithofacies within these landforms include glaciotectonised foliated basal ice and glacier ice, that have a complex internal structure. Other facies are well-sorted sand with lenses of firn, preserving original fluvial sedimentary structures such as trough cross-bedding and current ripples, although now glaciotectonically tilted, and a drape of rippled aeolian sand that is very well sorted. Well-developed aprons, thrust-block moraines and ice-contact ridges are not universally present, but are best developed at slowly advancing glaciers, such as Wright Lower and Suess glaciers. In contrast some advancing cold glaciers show few ice-contact landforms. For example, the ice-marginal cliffs of Rhone Glacier in Taylor Valley undergo drycalving, so the snout is fringed by avalanched ice blocks. The main evidence of geomorphological activity here is shallow ice-marginal stream courses. The adjacent Taylor Glacier, on the other hand, is of a polythermal type, with a prominent thrust-block moraine across part of its snout. This glacier lies in a saline lake basin, and as much as 50% of the bed of the lower ablation area may be melting (Robinson, 1984). Reworking following deposition occurs by fluvial, lacustrine and aeolian processes (Hambrey and Fitzsimons, 2010). Supraglacial runoff is the sole source of water and is the result of the radiation effect on dirty ice even at sub-zero temperatures. Streams flowing off the glacier dissect the ice-contact aprons and ridges, producing well-sorted sand in a series of alluvial fans (typical angle: 5–10°). Braided stream courses of low gradient (1–2°) grade from these alluvial fans, and the sand is recycled and forms small-scale bedforms. Where there is a supply of gravel, lags occur in channel floors and on bars, the clasts generally being of pebble-size, angular to subrounded (subangular dominant), with low RA values and medium C40 values. The braided streams pass via poorly defined deltas into the proglacial lakes, whose levels vary by tens of metres on a decadal time scale. These lakes are permanently frozen except for a summer-time moat around their margins. Sand from the braided streams is the main source of sediment, and this is supplemented by the growth of algal mats and stromatolites (up to a few 10s of centimetres high) in the shallows. Some of the stromatolites penetrate the lake-ice cover and their tops may be exposed. Wind is also a major recycling agent in the proglacial areas of cold glaciers. Very well-sorted sand is the main lithofacies, and forms the well-known dunes in Victoria Valley. Elsewhere, discontinuous drapes of wellrounded grains of sand form discontinuous drapes several tens of centimetres thick over existing landforms and the ice margin (Fig. 7G). Severe katabatic winds may also move gravel, occasionally creating dunes of pebble-sized material. On James Ross Island, most of the outlet glaciers from the central ice cap are polythermal, but small niche glaciers, nourished by wind-drifted snow also occur; data are presented for four of these niche glaciers (Fig. 6). Supraglacial debris comes from minor rockfalls from adjacent cliffs of volcanic breccia, so the shape of clasts is partly controlled by the shape of the fragments from this breccia, with high RA and low C40 values. Basal debris is incorporated by glaciotectonic processes such as thrusting and shows little modification of the source material, whilst small moraines also demonstrate similar shape characteristics. Other glacigenic facies are limited in association with these glaciers, and the minor meltwater runoff from the glacier surface tends to cut channels in the underlying soft Cretaceous sediment, rather than work the glacial debris. 8.2. Relative abundance of lithofacies Based on the distribution of sediments around the Wright Lower Glacier and other glaciers in the Dry Valleys, the range of lithofacies is much more limited than in temperate and polythermal glacier regimes, with sand and sandy gravel being dominant (Hambrey and Fitzsimons, 2010) (Table 5). Examples are shown in Fig. 9. Well-sorted sand is the dominant constituent of the basal debris layer, ice-contact aprons,

alluvial fans, braided streams and delta-tops. In addition, there are extensive deposits of very well-sorted aeolian sand. Sandy boulder/cobble gravel of the Ross Sea Drift is the next most abundant facies, and is reworked by modern glacial processes into ice-contact ridges. Minor facies are embedded glacier ice and firn, and well-sorted gravel, algal mats and stromatolites. 8.3. Cold glacier landsystems Since cold glaciers are frozen to their beds, they move predominantly by internal deformation. Basal sliding and subsole deformation have been assumed to be either absent or extremely slow (Shreve, 1984; Echelmeyer and Wang, 1987). As a result, cold glaciers have generally been regarded as having little ability to erode their beds (Cuffey et al., 1999). However, recent research has demonstrated that glacial erosion and glaciotectonic deformation may occur in certain circumstances (Atkins et al., 2002; Lloyd Davies et al., 2009). Furthermore, cold glaciers commonly carry substantial basal debris loads, especially near their margins where stacking of debris sequences due to regelation is common (Knight, 1997; Fitzsimons, 2003; Hambrey and Fitzsimons, 2010). Significant thicknesses of debris-rich ice can be entrained where a glacier over-rides and incorporates a frontal apron composed of ice and debris produced by dry calving (Shaw, 1977a,b). This process can form basal debris layers of up to 5 m thickness. Debris is typically stratified, reaching concentrations of 40–50% by volume. Landforms associated with cold-based glaciers include thrust-block moraines composed of sand, gravel and organic silt entrained as frozen blocks of lacustrine sediment (Fitzsimons, 1990, 1996, 1997a,b; Humphreys and Fitzsimons, 1996), abrasion marks, subglacial deposits (crushed bedrock debris, sandstone and siltstone breccia), and, rarely, glaciotectonically deformed substrate, isolated blocks, ice-cored debris mounds, boulder trains and ploughed boulders(Atkins et al., 2002; Lloyd Davies et al., 2009). Based on cold glaciers in the Dry Valleys, generalised lithological logs show a limited range of lithofacies. As documented above, these are dominated by sand and sandy gravel, plus slivers or remnants of glacier ice and even snow. The cryospheric component is conserved because of the intense permafrost conditions. Indeed, it has been argued that glacier ice as much as 8 m.y. old is preserved beneath a supraglacial debris cover in the Dry Valleys (Sugden et al., 1995). A cold glacier depositional landsystem model for the Dry Valleys (Fig. 10) shows the following components: (i) Ice-contact ridges and aprons, typically up to 10 m in height, comprising sandy boulder-gravel representing reworked older moraines (i.e. the Ross Sea Drift), well-sorted sand interpreted as reworked glaciofluvial and glaciolacustrine sediment. The internal structure of these ridges and aprons is characterised by folding, thrusting and zones of foliated debris-rich basal ice and snow lenses. Some, but not all, could be classified as thrustblock moraines (Fitzsimons, 2003). (ii) Ice-contact saline lakes in which the sediments comprise wellsorted sand, with algal films and stromatolites (e.g. Parker et al., 1982). (iii) Braided glaciofluvial outwash sands, also comprising well-sorted sand, with sandy pebble-gravel lags on bars. The water originates solely from supraglacial runoff. Stream activity is much less vigorous than in temperate and polythermal regimes. (iv) Discontinuous rippled aeolian sand drapes over much of the area, especially on ice-contact aprons. Sand dunes occur in some areas, the most impressive occurring in the lower Victoria Valley (e.g. Calkin and Rutford, 1974; Bristow et al., 2010). Typically, the ice contact forms (i) represent about 10–20% of the proglacial area, although below the surface there may be extensive older remnants, as well as subdued older landforms some distance

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Fig. 10. Conceptual model for a cold terrestrial glacier landsystem.

from the ice margin. The remainder of the proglacial zone is represented by (ii) and (iii), and along with the aeolian sand-drapes (iv), these components vary spatially to a considerable degree, depending on aspect (in relation to katabatic winds) and local topography. The preservation potential of the sediment/landform associations of cold glaciers is high if the climate remains stable. Landforms and sediments are locked in the permafrost which tends to lack a prominent active layer. The lack of vigour of fluvial processes, and only limited occurrences of mass-movement on slopes, results in terrain that is little-modified for millions of years, especially in the Dry Valleys (Sugden et al., 1995). It is within this landscape that even buried ice and snow can be preserved over this time-scale. Of course, if the climate warms, as is already happening in the Antarctic Peninsula, the ground becomes more saturated in summer, and mass-movement and stream activity become much more vigorous processes. 9. Facies and landsystems associated with polythermal glaciers 9.1. Lithofacies characteristics Data for polythermal glaciers come mainly from a suite of terrestrial glaciers in Svalbard, none of which exhibit evidence of former surge-type behaviour. It should, however, be noted that estimates of the proportion of surge-type glaciers in Svalbard range from 13% (Jiskoot et al., 1998), through 36% (Hamilton and Dowdeswell, 1996), to 90% (Lefauconnier and Hagen, 1991; Sund et al., 2009). Therefore, the possibility remains that the glaciers studied are surge-prone. Additional data are also presented from (i) Trapridge Glacier, a high-altitude polythermal glacier in the Yukon, which completed a ‘slow-surge’ in 2005, following a more normal surge in the 1940s (Frappé-Sénéclauze and Clarke, 2007), and (ii) Storglaciären in northern Sweden (Etienne et al., 2003; Glasser et al., 2003). The range of lithofacies is arguably the greatest for polythermal glaciers,

reflecting complex glacial transport processes, and deposition influenced by thermally variable bed conditions, glaciofluvial processes and periglacial activity (Fig. 11). In general, the facies associations are closer to temperate than to cold glaciers, and merge with them imperceptibly. Table 6 summarises the principal clast and matrix characteristics of lithofacies at Svalbard glaciers, together with relative abundance. Polythermal glaciers carry abundant supraglacial debris, normally as lateral and medial moraines, rather than mantling the whole surface as is common for temperate alpine glaciers. The distribution of this debris is strongly controlled by structural processes, including folding, incorporation within foliation (Hambrey and Glasser, 2003a,b) and thrusting (Hambrey et al., 1999), the latter bringing subglacial debris to the surface in the terminal regions of the glaciers. Supraglacial debris is typically sandy boulder-gravel, originating as scree, with predominantly angular shapes, and high RA and C40 values (Fig. 6). At the base of the glacier, large amounts of subglacial debris are transported, notably a variety of clast-rich diamictons interpreted as till; this material was entrained by regelation when the glacier was wetbased. However, over time, as the glacier thins, the wet-based area shrinks, and ultimately a large part of the glacier rests on frozen sediment. Depositional products from polythermal glaciers are dominated by diamictons of various types (Table 6). These texturally diverse lithofacies are characterised by angular to rounded clasts with subangular and subrounded being dominant. There is a strong lithological control on whether clasts become striated and faceted; within the same diamicton, soft fine-grained rocks such as limestone commonly bear these features, but metamorphic rocks do not – both exist in the Svalbard samples. The upper surface of the diamicton is commonly fluted (Christoffersen et al., 2005), and also draped with stripes of supraglacial debris. The diamicton plots as a broad cluster with low to moderate RA and C40 values (Fig. 6). Adjacent to the glacier margin there is mixing by sliding of debris off the ice, and pushing of

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supraglacial debris and basal material, giving rise to a heterogeneous sandy boulder-gravel, forming irregular, commonly ice-cored mounds. The dominant reworking processes are glaciofluvial, sediment gravity-flowage and prior glacial advances. Glaciofluvial processes do not generally affect much of the immediate proglacial area. Glacier hydrology is dominated by ice-marginal englacial to subglacial streams, which typically emerge at either end of the terminus, follow discrete channels and only emerge several hundred metres from the snout. Thus, much of the subglacial till zones is not influenced by streamreworking, although direct supraglacial run-off rinses out the fines from the surface, leaving a cobble lag. Glaciofluvial processes give rise to sandy gravel with subrounded clasts typically of cobble size, and low RA and C40 values. Eskers are occasionally found, and they carry the same attributes as other glaciofluvial facies. Within the Neoglacial

limits glaciofluvial sediments are of limited distribution, but beyond, large fans develop over coastal plains, and here sandy gravel becomes the dominant lithofacies. Poor drainage of ground that is ice-cored within the Neoglacial limits allows shallow lakes to form, and these become a repository for sand, silt and mud, which are commonly laminated. These lithofacies are commonly represented in moraine ridges, emplaced by thrusting (Hambrey et al., 1997). A range of gravity flow processes (slumping, creep, debris-flowage and small-scale turbidity flows in ponds) are widespread in the proglacial areas of Svalbard glaciers (Bennett et al., 2000). The development of an active layer in the permafrost, and slowly melting dead ice under a cover of readily deformable diamicton facilitates these processes. Debris-flows are particularly common, but texturally do not significantly change the diamicton, although flow-cessation leads to the flushing out of fines forming small mudflats. Reworking of all these lithofacies by

Fig. 11. Representative lithofacies associated with polythermal glaciers: (A) Proglacial zone of Midre Lovénbreen from the air, showing receding ice margin, hummocky moraine complexes, till plain with flutes and supraglacial debris stripes, and braided glaciofluvial sediments. (B) Folded layers of subglacial debris in the snout of surge-type Trapridge Glacier, Yukon; diamicton of an earlier basal till in the foreground. (C) Angular supraglacial debris, deposited from a medial moraine, proglacial area of Pedersenbreen, Svalbard. (D) Heterogeneous sandy gravel and diamicton of basal ice derivation adjacent to the 2005 surge limit of Trapridge Glacier, Yukon. (E) Diamicton overlain by muddy gravel, representing two stages of basal till deposition, proglacial area of Trapridge Glacier. (F) Lateral moraine, comprising sandy gravel, recently deposited during a surge, upper margin of Trapridge Glacier. (G) Heterogeneous mud, muddy gravel and diamicton of a debris-flow, developed on ice-cored moraine from Kongsbreen, Svalbard, the glacier in the background. (H) Cobble lag on diamicton, resulting of removal of fines by meltwater, Pedersenbreen, Svalbard. (I) Supraglacial debris on the debris-mantled tongue of Khumbu Glacier, Nepal. (J) Aeolian sand in the lateral morianic trough of Imja Glacier, Khumbu Himal, Nepal; the lateral moraine is on the right.

M.J. Hambrey, N.F. Glasser / Sedimentary Geology 251-252 (2012) 1–33

19

Fig. 11 (continued).

cover is commonly continuous in the ablation zone (Owen et al., 2003; Hambrey et al., 2009).

glaciotectonic processes during phases of glacier advance, notably the Neoglacial that peaked in this region about 1900 AD, was also an important process. The products are moraine-mound complexes that have been interpreted as the product of both englacial and proglacial thrusting (Hambrey et al., 1997, 1999). The range of lithofacies for the Swedish and Canadian examples, and their textural attributes are broadly similar to those in Svalbard. In terms of RA/C40 plots, there is less process-based clustering, although the fields for supraglacial debris and diamicton are similar (Fig. 6). The inferred polythermal glaciers of the Himalaya produce quite different lithofacies from those outlined above. Here, steep topography (as in tropical glaciers of the Andes) has a greater influence than thermal regime on debris-entrainment processes. Supraglacial debris

9.2. Relative abundance of lithofacies The definitive glacigenic sediment ‘diamicton’, is the dominant lithofacies at modern polythermal glaciers in regions of moderate relief, reflecting the importance of debris-entrainment at the glacier bed. Sandy gravels of various types, reflecting transport by subglacial and proglacial streams and fine-grained lacustrine sediments are also moderately important. These lithofacies form a complex mosaic in the proglacial zone. However, it is dead glacier ice and Aufeis, rather than sediment, that in volumetric terms is the dominant ‘lithofacies’ in the

Table 6 Typical lithofacies, interpretation and relative abundance at polythermal glaciers in high-Arctic regions of moderate relief. Key to relative abundance: ***** = dominant ← → * = rare. Lithofacies

Clast-poor intermediate diamicton Clast-rich sandy diamicton Clast-rich intermediate diamicton Clast-rich muddy diamicton Sandy boulder-gravel Sandy cobble/pebble-gravel Gravel (type 1) Gravel (type 2) Gravel with sand Sand and mud, incl. laminites

Interpretation

Subglacial till; glacigenic sediment flow Subglacial till; glacigenic sediment flow Subglacial till; glacigenic sediment flow Subglacial till glacigenic sediment flow Ice-marginal Glaciofluvial Fluvial Subglacial fluvial Glaciofluvial Lacustrine

Abundance

% striated clasts

% faceted clasts

% clasts

**

10

6

***

10

*****

Matrix (%)

Sorting coefficient

Sorting category

19

4.05

Extremely poorly-sorted

8–20

2–8

1.5–2.87

Very poorly sorted

41–83

5–43

11–20

2.91–4.27

Very poorly-sorted

30

15

78

7

1.22

Poorly-sorted

3 0 0 0 0

70 10–40 80–95 100 70–90

20 92–98 75–90 – 90–98

10 1–6 6–19 – 1–7

0 1–2 2–8 – 1–3

0.87–1.61 1.78–2.96 – 0.73–2.82

–

0–5

76–99

1–19

1–5

0.88–1.12

sand

silt

clay

5

48

33

8

30

75–90

12

4

25–35

**

14

6

*** *** *** ** **

2 0 0 0 0

***

–

Moderately-sorted Poorly-sorted Very poorly-sorted Well-sorted Moderately-sorted to poorly-sorted Moderately-sorted to poorly-sorted

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immediate proglacial zone of many polythermal glaciers, although this is commonly hidden by a veneer of sediment. In contrast, the Himalayan polythermal glaciers are dominated by sandy bouldergravel, either derived from rockfall or reworked by basal processes during lateral and end-moraine formation. 9.3. Polythermal glacier landsystems Recent decades have seen intensive research on polythermal Arctic valley glaciers, with particular emphasis on characterising sediment/ landform assemblages and on defining the link between structural glaciology and landform development. In these glaciers it is typical for the snout, lateral margins and surface layer of the glacier to be below the pressure-melting point, whereas thicker, higher-level ice in the accumulation area is often warm-based (Holmlund and Eriksson, 1989; Hagen and Saetrang, 1991; Ødegard et al., 1992; Björnsson et al., 1996; Holmlund et al., 1996; Jansson, 1996; Jansson et al., 2000). This mix of thermal regimes makes the dynamics of polythermal glaciers complex. The typical Arctic polythermal glacier is sliding on its bed or experiencing subsole deformation where warm-based ice dominates in the accumulation zone, and moving only by internal deformation where it is cold-based at the snout and lateral margins. Polythermal glaciers tend to carry a high basal debris load, with debris-rich basal ice zones several metres thick. For example, the basal debris layer in glaciers on Baffin Island is between 0.8 and 2.9 m in thickness, with a debris concentration of up to 50% (Dowdeswell, 1986).

As noted above, there are strong structural glaciological controls on landform and sediment development. Distinctive landforms include ice-thrust ridges recording ice-marginal and proglacial glaciotectonic deformation, “hummocky” moraines, kame and kettle topography and lateral meltwater channels. The deformation of permafrost can also be important in the formation of ice-cored moraines and push moraines, and folding, thrust-faulting and overriding of proglacial sediments are possible under these conditions (Etzelmüller et al., 1996). The lithostratigraphy of the proglacial areas of polythermal glaciers is complex, but rarely depicted. Glasser and Hambrey (2001a) illustrated several sections for the Svalbard glaciers that have supplied the data for this paper. A generalised example (Fig. 7F) shows the relationship between weathered bedrock and overlying diamicton and gravel, which is representative of large areas of subglacial till deposition that have not been modified by glaciofluvial and gravityflow processes. Moraine systems are represented by a complex “glacio-tectonostratigraphy”, that has been illustrated by Huddart and Hambrey (1996), Bennett et al. (1999) and Boulton et al. (1999). Based on observations primarily in Svalbard, but affirmed by those in Arctic Sweden and the Yukon, a polythermal glacier landsystem model (Fig. 12) shows the following components: (i) A glacier surface characterised by structurally controlled angular gravel of supraglacial origin and thrust ridges of muddy gravel of subglacial derivation.

Fig. 12. Conceptual model for a polythermal terrestrial glacier landsystem.

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(ii) An ice-contact zone where sandy boulder-gravel accumulates from a mix of supraglacial and subglacial debris, forming mounds. (iii) A proglacial zone dominated by a variety of diamictons deposited as subglacial till, in part fluted and draped with angular gravel debris stripes of supraglacial origin, occasional moraine-mound complexes of mixed lithofacies reflecting englacial thrusting, and lakes containing sand, silt and clay. Glaciofluvial streams from either side of the glacier flow across this zone in constrained channels (apart from some local braiding) depositing sandy cobble-gravel. These channels are commonly incised into bedrock forming anastomosing networks. In some case, the main glacial outlet stream emerges through dead glacier ice some distance from the active ice margin as an upwelling. (iv) A high lateral moraine ridge of mainly angular boulder-gravel forming a drape over an ice core of Neoglacial and earlier ages, but still connected to the glacier at its lateral margins. (v) A prominent outer ridge or moraine-mound complex of similar age in which stacking of multiple lithofacies and glacier ice has taken place by thrusting, but now disconnected from the glacier. The preservation potential of the polythermal glacier landsystem is high on a millennial time-scale, except that landforms underlain by, or partly constructed of, dead glacier ice will degrade as the ice core melts, but only if the permafrost disappears. The polythermal glacier landsystem associated with high-Himalayan glaciers is quite different from that described above. However, the thermal structure of such glaciers is, as yet, poorly known. The key attributes of the Himalayan landsystem are debris-mantled tongues bounded by high latero-terminal moraine complexes that date from the Little Ice Age and earlier ice-expansions. They are composed mainly of sandy boulder-gravel. Understanding the thermal structure and facies architecture of these moraines is important because, on recession, these moraines act as a dam, retaining glacial lakes that are prone to failure (Reynolds, 1999; Richardson and Reynolds, 2000). 9.4. Polythermal surge-type glaciers Distinguishing polythermal landsystems between surge-type and non-surge-type glaciers is challenging. In the High-Arctic, surges tend to be widely spaced in time, and last several years, so compared with the short-lived temperate glacier surges of just a few months, their role in landform development seems little different from non-surgetype glaciers. This problem is compounded by the fact that few polythermal glaciers have surged more than once in historically recorded time, and there is debate in Svalbard, at least, as to whether nearly all the glaciers there are surge-prone or not. An illustration of this problem is presented by the recent surge of Comfortlessbreen in NW Spitsbergen. Prior to the surge, the glacier lacked the structural and hydrological attributes normally associated with a surge, such as looped moraines and a well-developed stream network with potholes. Huddart and Hambrey (1996), on mapping the sediment/landform assemblage, inferred that the glacier was not surge-type. Certainly the older landforms, a thrust-moraine complex, did not need a surge to produce them, but new thrust moraines were being produced as the glacier advanced in 2009. By comparing the Svalbard glaciers that have noknown surge history with Trapridge Glacier in the Yukon, which does have a known surge history (both ‘normal’ and slow), it is evident that the sediment/landform assemblages are broadly similar. 10. Glaciomarine facies and “landsystems” 10.1. Lithofacies characteristics Glaciomarine facies associations are as equally varied as their terrestrial counterparts, although there is a paucity of textural and clast shape

21

data from modern environments. However, the multiple processes responsible for the facies are well documented, and their interactions have been helpfully represented graphically by Dowdeswell (1987). Data are presented from temperate glaciers in Alaskan fjords, for cold and polythermal glacier-dominated continental shelves in Antarctica and for polythermal glaciers in Svalbard (examples illustrated in Fig. 13). The most intensive work on the glaciomarine environment has been undertaken on the grounded temperate glaciers in Alaskan fjords (Powell, 2003). A wide range of interacting processes are evident: glacial, fluvial, calving and iceberg-rafting, marine currents and waves, sea ice, rock-fall from fjord walls, subaqueous mass flow, aeolian and biological. Climate, tectonics and sea-level change exert strong forces on local processes, and these may be recognised in geological records of ancient fjordal sequences. Lithofacies reflect the proximity of the ice margin, and the glacier itself may provide sufficient sediment to provide a platform that maintains some stability. Stratigraphy architecture is strongly influenced by bathymetry, and with near-horizontal bedding in more distal parts of the basin emphasises the importance of sediment gravity flows (Cowan et al., 2010). Over-deepened glacially eroded basins act as sediment depocentres, whilst bedrock sills act as pinning points where iceproximal depositional systems can accumulate (Benn and Evans, 2010, Ch. 12). Compared with most terrestrial glacial sedimentary sequences, those in fjords accumulate to depths of several hundred metres. For example, in inner Glacier Bay, Alaska, deposits below the most recent Neoglacial expansion that date from the Last Glacial Maximum are more than 300 m thick (Cowan et al., 2010). The sedimentary infill in fjords is generally expressed in terms of process of deposition, and textural characteristics have rarely been precisely defined. Nevertheless, based on the work of Powell (2003) and his colleagues, Benn and Evans (2010, Ch. 12) have generalised the sediment infill in terms of advance, ice-maximal and retreat depositional units (with additional amplification of lithofacies), each of which produces its own facies association (Powell, 1981; Boulton, 1986): (i) Advance phase sediments are typified by a diamicton, interpreted as subglacial till which thickens down-fjord. (ii) Maximum phase sediments are commonly located at pinning points that facilitate ice-marginal stability and the accumulation of push or thrust moraines, morainal banks, groundingline fans and deltas. The sediments are highly variable in these features with a mixture of angular boulder-gravel (of supraglacial origin), diamicton (of basal glacial origin, commonly reworked as debris flows), gravel (of subglacial fluvial origin), sand (from suspension or from gravity flows), which grade into laminated sand and mud (tidal rhythmite, otherwise referred to as cyclopsams and cyclopels) and massive stony mud (iceberg-rafting into basinal sediments derived from suspension, also known as bergstone mud) with increasing distance from the ice margin. (iii) Retreat phase sediments reflect the migration of the maximum phase sediments up-fjord. Thus progressively more distal facies are laid down on top of older units. Pauses in recession lead to the development of a series of, for example, grounding-line fans or morainal banks. Few data are available for defining clast characteristics in temperate glacier-influenced fjords, but intuitively they will resemble those of alpine valley glaciers, with emphasis on glaciofluvial sediment, supraglacial debris, subglacial sediment and iceberg-rafted debris (of both supraglacial and basal glacial origin), in that order. Marine sedimentary processes bordering a cold ice sheet, albeit drained by wet-based outlet glaciers and ice streams, have been examined on the Antarctic continental shelf, where rates of sedimentation are orders of magnitude less than for temperate glaciers in

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Fig. 13. Examples of glaciomarine environments. (A) Tidewater glacier with icebergs and grounding-line fan (right), Muir Glacier, Glacier Bay, Alaska. (B) Debris-laden iceberg near Muir Glacier, providing the material for bergstone mud facies. (C) The Brunt Ice Shelf at Halley Bay, East Antarctica, with partial fringe of sea ice; the vessel is RRS Bransfield. (D) Small fast-flowing ice stream, Chaos Glacier, emanating from the East Antarctic Ice Sheet. (E) Tabular iceberg with horizontal stratification, Lazarev Sea, East Antarctica.

fjords. The coastline of Antarctica has been quantified as consisting of the following components (Drewry, 1983): Ice shelf (floating) 44% Ice wall (cold-based, grounded on the sea bottom) 38% Ice stream (basal sliding or flowing over a deformable bed) 13% Rock 5% By far the most important contributors to sedimentation on the continental shelf are ice streams, despite their limited linear extent at the coast. Sea-bottom sediments have been sampled in many parts of Antarctica through a combination of piston, gravity and box coring. Early research over many parts of the continental shelf (Anderson et al., 1980) revealed that well-sorted terrigenous sand or gravelly sand with little biogenic material was widespread, although any stratification had been disrupted by bioturbation. An increase in the proportion of fines was reported offshore. The sandy lithofacies were referred to as “residual glacial marine sediments” as it was believed that fines had been winnowed out by current activity. The gravel component was inferred to be ice-rafted. Kuhn et al. (1993) reported rather more variable facies after sampling more than 1500 km of East Antarctic coastline. In the eastern Weddell Sea they found that the dominant

contemporary lithofacies was terrigenous gravelly and muddy sand with a low biogenic component. In the Lazarev Sea, two main lithofacies were reported: calcareous biogenic sediment, and well-sorted terrigenous sand with little biogenic material. These sediments were interpreted as iceberg-rafted glaciomarine sediments, from which the fines had been winnowed out (agreeing with Anderson et al., 1980). In both areas, these sediments overlie an over-consolidated diamicton that has been interpreted as a subglacial till dating from the Last Glacial Maximum, deposited when the ice sheet advanced across the continental shelf (Anderson et al., 1980; Elverhøi, 1981; Wellner et al., 2001; Ó Cofaigh et al., 2002; Dowdeswell et al., 2004). Clast analysis of modern continental shelf sediments offers the prospect of inferring mode of transport by the more dynamic (ice stream) components of the Antarctic Ice Sheet. Early studies focused on documenting roundness and sphericity of clast populations. However, although roundness is a good discriminator of transport processes alongside clast shape (using RA/C40 plots), sphericity is no longer deemed a good discriminator (e.g. Bennett et al., 1997). Following a benchmark paper by Boulton (1978) in characterising clast shape and sphericity from temperate and polythermal glaciers, Domack et al. (1980) documented clasts from modern sediments off the George V Coast of East Antarctica. He determined that most of the clasts were of

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basal glacial origin, broadly matching Boulton's subglacial sediment from Iceland and Svalbard. Developing this approach further, Kuhn et al. (1993) attempted to relate clast shape to glaciological regimes in the eastern Weddell and Lazarev seas. From the predominantly subangular and subrounded clasts, commonly with striations and facets (especially on fine-grained lithologies), most of the gravel component in the lithofacies described above is derived from the base of the ice sheet. We have reprocessed the raw clast data obtained for the Kuhn et al. (1993) paper in terms of the RA/C40 index. Plots for clasts in areas of the Lazarev Sea immediately offshore from ice shelves, ice tongues and grounded ice cliffs (ice walls) are shown in Fig. 14A. The values of both RA and C40 are low to medium, and their distribution differs from most other glacier regimes. These values reflect primarily transport at the base of a wet-based glacial system, and with little evidence of glaciofluvial reworking or supraglacial debris. Icebergs probably transported this material to areas otherwise lacking terrigenous sedimentation, namely ice shelves and grounded ice cliffs. Thus, most of the material probably originated from ice streams. Lithological analyses indicated that clasts are mostly derived from the adjacent onshore areas, with only minor mixing of exotic clasts, suggesting that most sediment is released before icebergs move distances up to several hundreds of kilometres. The sedimentary processes associated with polythermal tidewater glaciers have been investigated in various parts of the Arctic, notably in the fjords in Svalbard, East Greenland and Ellesmere Island, where characteristic facies associations can be defined. Elverhøi et al. (1983) investigated modern-day sedimentation in several fjords in western Spitsbergen, notably Kongsfjorden, using a combination of highresolution acoustic profiling and sediment sampling. The principal lithofacies identified were: (a) A blanket of soft homogeneous mud with dispersed gravel clasts accumulating particularly in basins where it is subjected to bioturbation. This was interpreted as resulting from settling from suspended sediments with the addition of iceberg-rafted debris. Sulphide layers within the mud were identified as being of

23

organic origin, originally derived from diatoms which bloom each spring. (b) Sandy gravelly mud associated with the limit of a surge in 1948 (see below), and also with the modern ice front, interpreted as of subglacial meltwater derivation. The modern context for this would be a grounding-line fan. (c) Interbedded sand and mud originating from different processes, including settlement from turbid sediment plumes and various types of gravity flow, and which has been subjected to reworking on sills. (d) A ‘compacted till’ (diamicton) which underlies the above lithofacies, which was deposited either as a basal till during Pleistocene advances to the fjord mouth or glaciomarine sediment compacted by an ice advance. The contemporary iceberg-rafted debris component of polythermal tidewater glaciers has been investigated in Kongsfjorden, NW Spitsbergen (Glasser and Hambrey, 2001b). Debris was analysed from twelve icebergs trapped in sea ice in the inner fjord. The dominant lithofacies extracted from debris layers in up-turned icebergs are clast-rich muddy and sandy diamictons, with lesser amounts of clast-poor muddy gravel and mud. A unique sample that showed the grooved nature of basal glacial sediment that had been in contact with the glacier bed, confirmed that these textural attributes are characteristic of subglacial sediment, and this lithofacies forms the bulk of material carried by icebergs. Clast shapes range from very angular to rounded with subrounded normally making up the dominant proportion. Low RA and moderate C40 values match the basal debris field from already deposited sediments on land (Bennett et al., 1997, 1999) (Fig. 14B). The above studies, however, only give a partial view of the wide range of lithofacies that accumulate in glacier-influenced fjords. Many tidewater glaciers in Svalbard, where the fjord waters are relatively shallow compared with Greenland, have reworked the proximal glaciomarine sediments and pushed them onshore, notably during surges (e.g. Boulton et al., 1996; Huddart and Hambrey, 1996; Bennett et al., 1999). By effectively sampling the sea floor, these glaciers have

Fig. 14. Clast shape data from a modern polythermal proximal glaciomarine environment shown as a co-variant plot of the RA index against the C40 index. Each symbol represents a sample of 50 clasts. All samples were collected and analysed by the authors from marine sediments emplaced on land by glaciotectonic processes, and from icebergs trapped in winter sea ice.

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provided the best opportunity to document in detail the characteristics of fjordal glaciomarine sediments associated with polythermal glaciers. The most detailed study has been undertaken in inner Kongsfjorden in NW Spitsbergen, where a surge in 1948 created thrust-moraine complexes comprising sea-floor sediments on both sides of the fjord (Bennett et al., 1999). The principal lithofacies documented (Fig. 15), which occur in the thrust moraines, are: Diamicton type 1 is massive muddy clast-rich to clast-poor. Clasts range from angular to rounded with subangular and subrounded dominant. RA values are low and C40 values moderate to high. A high proportion of the fine-grained lithologies are striated and faceted. This diamicton is interpreted as either a subglacial till or a basinal mud with iceberg-rafted debris (bergstone mud). Diamicton type 2 is massive sandy and clast-rich. Clasts range from very angular to subrounded with very angular and angular dominant. RA and C40 values are both moderately high. Few clasts are

striated. This lithofacies is interpreted as a mixture of iceproximal supraglacial and basal glacial debris, the latter having been elevated before deposition to high levels in the glacier by folding in associated with foliation-parallel ridges and thrusting (Glasser et al., 1998). Sandy gravel and gravelly sand type 1, which is commonly interbedded with sand and silt. Clasts range from subangular to well rounded, the subrounded being dominant. RA values are low and C40 moderately high. Foraminifera and broken shells are common. This lithofacies is interpreted as having formed in an ice-proximal glaciomarine environment. Sandy gravel and gravelly sand type 2 is an openwork pebblecobble gravel with trough and tabular cross-bedding. Clast roundness and shape characteristics are similar to sandy gravel type 1, and also contains foraminifera and shell fragments. This lithofacies is interpreted as having formed in a delta-top glaciofluvial setting.

Fig. 15. Representative lithofacies associated with polythermal tidewater glaciers, Kongsfjorden, NW Spitsbergen: (A) Transition from grounding-line fan to ice-contact delta, exposed over little more than a decade as the glacier (Kongsvegen) recedes; the surface of the feature is predominantly sandy gravel of glaciofluvial origin, with mud in the ponds. (B) Clast-rich muddy diamicton emerging from beneath the receding margin of Conwaybreen, seen at low tide. (C) Ridge of sandy gravel, now incorporated in a thrust–moraine complex, produced by the 1948 surge of Kongsvegen; this sediment was once part of a grounding-line fan, but is now uplifted several tens of metres above sea level. (D) Complex association of disturbed lithofacies that form part of a morainal bank in front of Kronebreen, including clast-rich muddy diamicton (dominant), laminated sand (cyclopsams) and mud; the smooth reddish ridge (diamicton) is ice-cored. (E) Sampling clast-rich muddy diamicton from iceberg, trapped in sea ice, in Kongsfjorden. (F). Gravelly mud comprising suspended sediment and iceberg-rafted debris, exposed at low tide near Conwaybreen; geese footprints for scale.

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Muddy gravel comprises moderately well sorted pebble/cobble gravel with silty coatings, commonly associated with sandy gravel type 2. The interpretation of this lithofacies is as a grounding-line fan in which both subaqueous discharge and suspension settling contribute to the lithofacies. Sand. There is a range of moderately well sorted sand lithofacies, which may be laminated or rippled. Internally sands are variable. Depositional processes range from accumulation in silting ponds, to glaciofluvial to glaciomarine. There is some evidence of gravity flowage. Mud, which is massive with variable amounts of sand and gravel. It is interpreted as having been laid down as a result of a combination of settling from suspension and iceberg-rafting, in other words it is a bergstone mud. Sand-mud laminite, which is rhythmically laminated and contains isolated gravel clasts. The interpretation of these laminites is that they are tidally induced cyclopsams and cyclopels (Cowan and Powell, 1990). In another example, from Ellesmere Island, Stewart (1991) documented broadly similar facies associations, also associated with a grounding-line fan (Fig. 7H). The above facies associations are typical of fjords where iceberg rafting is relatively small-scale. Where

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sedimentation is dominated by iceberg sedimentation, as in some Greenland fjords, the dominant lithofacies is massive diamicton (Fig. 7I). Dowdeswell et al. (1994) used a combination of acoustic profiling and coring to establish the distribution of glacigenic facies in the deep fjord of Scoresby Sund. With a huge flux of icebergs from outlet glaciers from the Greenland Ice Sheet at the head of this, the world's longest open fjord, ice-rafted debris is the main contributor to fjord-bottom sedimentation. The principal lithofacies, diamicton, covers 90% of the fjord bottom. It is interpreted as direct deposition from basal ice in icebergs with the addition of suspension settling of fines from glaciofluvial sources. Most of the remaining 10% are lenses of gravel and coarse sand, which are the products of iceberg-dumping. Much of this material has been subject to reworking by icebergs. In contrast to iceberg-dominated Scoresby Sund, the neighbouring Kejser Franz Josef Fjord has a much smaller iceberg input. Here, bottom sediments are dominated by mud, which is the product of sediment gravity flows and suspension settling from meltwater plumes, which overwhelms the input of ice-rafted debris (Evans et al., 2002). 10.2. Relative abundance of lithofacies There are marked differences in the relative proportions of lithofacies in the three glaciomarine environments described above

Table 7 Lithofacies and their attributes in glaciomarine environments. Data for temperate and cold glaciers are largely estimates based on non-specific descriptions in the literature. Key to relative abundance: ***** = dominant ← → * = rare. Lithofacies

Interpretation

Temperate tidewater glaciers (fjord) Diamicton (massive, clast- Subglacial till rich) Angular boulder gravel Supraglacially derived debris from ice cliffs Gravel Subglacial fluvial contributing to grounding-line fan Sand Sedimentation from suspension or turbidity currents Gravelly sand (massive) Basinal sediment from suspension with ice-rafted debris Tidal rhythmites with ice-rafted debris Laminated sand and mud (cyclopsams and cyclopels); dispersed clasts

Abundance Striated clasts

Faceted clasts

% clasts

Dominant clast roundness

RA/C40 values

Sorting category

***

Some

Some

Some

SA/SR

n/d

**

Few

Few

Few

A

n/d

Very poorly sorted Poorly sorted

****

None

Few

70–90 SR/R

***

–

–

0–5

****

Few

Few

**

Few

–

Moderately sorted Well sorted

10–25 SA/SR

n/d

Poorly sorted

Few

0–10

n/d

Variable

Some

Some

20–50 SA (SR)

n/d n/d

Very poorly sorted Moderately sorted

–

SA/SR

n/d

Cold & polythermal glaciers (continental shelf) Diamicton (massive, clast-rich, Subglacial till; rain-out at grounding-line ** sandy) Gravelly sand Winnowed glacier-derived marine sedi- **** ment with ice-rafted debris (“residual glaciomarine sediment”) Muddy sand Glacier-derived marine sediment ***

Some

Some

10–20 SA (SR)

–

–

Trace

–

–

Diatomaceous mud

Diatom settling onto muddy substrate

***

–

–

Trace

–

–

Diatom ooze

Predominantly biogenic sedimentation

**

–

–

Trace

–

–

Subglacial till; morainal bank; basinal mud with ice-rafted debris Mixed supraglacial and basal glacial debris in morainal bank; subaqueous debris flows Ice-proximal grounding-line fan; glaciofluvial on delta-top Grounding-line fan with suspension settling Glaciofluvial; turbidity flows Suspension settling with ice-rafted debris (bergstone mud) Tidal rhythmites with minor ice-rafted debris

*****

Common Common 10–40 SA-SR

**

Few

Few

20–50 VA/A

****

None

Few

*

Few

Few

50–90 SR 10–50 70–90 SA/SR

** **

– Few

– Few

0–5 0–5

**

Few

Few

0–5

Polythermal glaciers (fjord) Diamicton-1 (massive, clast-rich, muddy) Diamicton-2 (massive, clast-rich, sandy) Sandy gravel/gravelly sand Muddy gravel Sand Mud (with dispersed clasts) Sand-mud laminate (with dispersed clasts)

Moderately sorted Moderately sorted Well sorted

Low/ mod.-high High/high

Very poorly sorted Very poorly sorted

– SA/SR

Low/ mod.-high Low/ mod.-high – –

Moderately sorted Moderately sorted Well sorted Poorly sorted

SA/SR

–

Variable

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(Table 7). Diamicton, interpreted as subglacial till or having been derived from basal ice, occurs in all environments, but is dominant in association with polythermal fjord glaciers. A variety of sand and gravel illustrates varying proportions of inputs directly from subglacial streams. These are most prevalent in the temperate fjordal environment. On a polar continental shelf bordering an ice sheet, as in Antarctica, direct glacial and glaciofluvial input is currently limited, and the dominant facies are gravelly sands (the product of current winnowing and referred to as “residual glaciomarine sediment”, plus sand of aeolian origin) and siliceous-biogenic sediments.

10.3. Glaciomarine “landsystems” Although the term “landsystems” is a misnomer in a glaciomarine context, the concept itself is still valid (Powell, 2003; Benn and Evans, 2010). The key components of this “landsystem” for both temperate and polythermal glacier regimes have been defined by Powell (2003), there being different emphases on the importance of glaciofluvial versus supraglacial and iceberg inputs to the depositional system. Since the modern “landsystems” are under water and difficult to evaluate without geophysical and coring equipment, much of what we know of the internal architecture of glaciomarine landsystems comes from isostatically or tectonically uplifted sequences (e.g. Lønne, 1995, 2005; Hambrey and McKelvey, 2000). The most important sediment/ landform associations are morainal banks and grounding-line fans, which ultimately grade up into outwash deltas with basin-infilling up to the inter-tidal level. A series of grounding-line fans may be found on the fjord floor reflecting interrupted recession, with finer grained sediments with dropstones intervening (Powell et al., 2000; Hambrey et al., 2002). Re-advances may truncate an earlier fan and superimpose a new one on top (Powell et al., 2001). A generalised landsystem model for a polythermal fjordal glacier, based on a well exposed Palaeogene– Neogene facies association uplifted along the world's largest fjordal trough, the Lambert Graben in East Antarctica is illustrated in Fig. 16.

In contrast to the temperate and polythermal tidewater glacier regimes, which are associated with some of the highest sedimentation rates experienced on Earth, the cold glacier offshore “landsystem” is relatively starved of terrigenous sedimentation, particularly where it is bordered by ice shelves (Fig. 17) or ice walls. Ice streams flowing into an ice shelf will release some of their basal sediment as a grounding-line or grounding-zone wedge of sediment, similar to morainal banks of less harsh climatic regimes. Because these zones are inaccessible for sampling, the best indication of processes comes from ice streams that enter the sea directly. An innovative study using a remotely operated vehicle (Powell et al., 1996) beneath Mackay Glacier in the western Ross Sea demonstrated that the main lithofacies produced at the grounding-line was diamicton, derived and reworked from the basal debris layer of the glacier. Meltwater and sediment plume activity was not strongly in evidence, but seaward of the grounding-line biogenic sediment became more evident. Where ice shelves border exposed bedrock areas, aeolian sand is the main component of supraglacial transport. For example, much of the sediment on the sea floor of the western McMurdo Sound is derived from this source (Barrett et al., 1983). Many ice shelves accrete marine ice on their undersides by basal freeze-on; thus any basal sediment not released at the grounding-line is trapped within the ice shelf (Fig. 17). This sediment is released only when tabular icebergs are produced, drift away and melt, producing a muddy sand, commonly with a gravel component. Current activity commonly winnows out the fines, depositing them further offshore. Additionally, in the zone immediately adjacent to the ice shelf front, biogenic productivity may be high, and the muddy sand is diluted with diatoms settling on the sea bottom. Indeed, in many parts of the Antarctic margin the ice-marginal sediment is effectively diatom ooze (e.g. Domack et al., 1999) (Fig. 7J). Diatom remains and suspended mud may be carried beneath the ice shelf by currents but, beneath much of it, sedimentation rates are close to zero (Fig. 17). Ice cores from a site 450 km from its northern margin of the Ross Ice Shelf (J9) showed the basal 6 m of its 416 m thickness to be frozen sea water

Fig. 16. Conceptual model for a polythermal fjordal glacier landsystem.

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Fig. 17. Conceptual model for a cold glacier-influenced Antarctic continental shelf “landsystem”.

(Zotikov et al., 1980); cores from the sea floor revealed about 16 cm of Holocene mud on top of muddy Miocene diamict (Webb et al., 1979). The main sediment discharge routes for the Antarctic Ice Sheet are via ice streams which, because they are wet-based and commonly move over a deformable bed, behave like polythermal glaciers in terms of sediment delivery. A combination of geophysical data (especially side-scan sonar) and shallow coring has provided remarkable imagery illustrating the nature of these ice stream “landsystems” (e.g. Ó Cofaigh et al., 2002; Anderson and Fretwell, 2008; Dowdeswell et al., 2008). Submarine landforms show a transition in an offshore direction from glacially eroded bedrock with subglacial meltwater channels, through drumlin fields to mega-scale glacial lineations. The sedimentary material is characterised by a geotechnically overcompacted diamicton, interpreted as subglacial till. It is typically overlain by a few metres of mud, with dispersed iceberg-rafted clasts and a diatomaceous component. These features were produced by palaeoice streams. Similar processes are probably going on beneath modern Antarctic ice streams (Smith et al., 2007), but these conditions are not representative of modern shelf sedimentary processes. The suite of subglacial bedforms extends across the Antarctic continental shelf in linear belts; they indicate that at the Last Glacial Maximum ice was grounded to the shelf-break. At this point a troughmouth fan is typically developed. A well studied example is the Prydz Trough-Mouth Fan, which developed over the period from early Oligocene to Pleistocene. Here, a combination of seismic reflection studies and several hundred metres of core reveal the architecture of this body and the important role played by shelf processes on the top surface of the fan during recessions, and glacigenic gravity flows (diamictite) when grounded ice reached the edge of the shelf and prograded the fan (Cooper et al., 1991; Hambrey et al., 1991). However, large-scale trough-mouth fan development is not an important process today, as the ice sheet is now well back from the continental shelf break. The architectural complexity of glaciomarine landsystems in fjords and on continental shelves has led some authors to develop a sequence stratigraphic approach to help interpret the sediment/landform associations in terms of sea-level change and tectonic uplift or subsidence, beginning with Cooper et al. (1991). This approach is based on

describing the ‘motif’ of depositional sequences recording in drill cores on the Antarctic continental shelf, notably in the Cape Roberts cores of the Victoria Land Basin (Fielding et al., 2000, 2001; Naish et al., 2001, 2009; Powell and Cooper, 2002; Barrett, 2007). The main components are glacial systems tracts and glacial surfaces of erosion, the latter marked by unconformities. 11. Discussion 11.1. Glacial sediments in the geological record The recognition of glacigenic sediments in the rock record is of fundamental importance to palaeoclimatological and palaeoenvironmental reconstruction. Even where exposure is good, evidence for glaciation may be equivocal as, for example, in alpine regions where massmovement and fluvial processes may overprint direct evidence for glacial deposition, or in those areas where tectonic deformation has overprinted depositional features. However, detailed facies analysis yields sufficient criteria that can be taken together to form the basis of a glacial interpretation. Table 3 lists the principal criteria for establishing the glacial origin of a diamict-bearing succession. Although we recognise that modern ice-marginal settings may not always be close analogues for the geological record (cf. Zielinski and van Loon, 2002), the starting point is that descriptions of modern glacial sediments and processes are used to reconstruct former glacial environments. 11.2. Glacial sedimentary processes and sediment/landform associations The glacial geological community is well served with up-to-date textbooks concerning glacial processes in a wide range of environments and under different thermal regimes (Evans, 2003a,b; Bennett and Glasser, 2009; Benn and Evans, 2010). However, for the purposes of evaluating the relative importance of different depositional processes in the Quaternary and older record, such as the roles played by rockfall, direct glacial deposition by ice, reworking by glaciofluvial processes, or deposition in the marine environment, it is useful to present a conceptual model (Fig. 18). A summary of the relative importance of processes

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Fig. 18. Conceptual model of the varying climatic settings under which the principal glacial sedimentary processes take place, by region and latitude. The relative importance of each process is indicated by the width of the shaded zones.

during entrainment, transport and deposition in the three main types of thermal glacial system (cold, polythermal and temperate) is given in Table 8, along with the principal sedimentary facies that occur in each regime. In examining the evidence of past glaciations it is important to recognise that nearly all the processes described above occur under all thermal and tectonic regimes to a greater or lesser degree. The challenge is to use the relative importance of the different processes and facies to infer a particular thermal and depositional regime (Fig. 18). This is important because of the climatic contrasts between a temperate glacial regime and the coldest parts of Antarctica are considerable, and thus have bearing on how the geological record is interpreted. Identifying the role played by reworking of glacial sediments by other processes is one of the most controversial aspects of examining the importance of past glacial processes (e.g. Schermerhorn, 1974; Eyles and Januszczak, 2004). Glacigenic sediments are typically subject to syn-depositional and post-depositional modification by fluvial, mass-movement, and aeolian processes. In terrestrial settings, fluvial modification by proglacial streams is particularly important in temperate climates, and many temperate glaciers terminate at the head of large outwash or sandur plains (often called “valley trains”) composed almost entirely of reworked glacial sediments. Resedimentation by mass-movement processes is common in ice-cored terrain where water, released by the melting of buried glacier ice or permafrost, mixes with sediment to create glacigenic sediment flows. Aeolian modification involves the redistribution by wind of smaller, readily-entrained particles, particularly in more arid areas, creating ventifacts, deflation surfaces and small dunes. The extent to which each of these processes operates is controlled to a great extent by

the local topographic, meteorological and climatological conditions. Resedimentation by subaquatic gravity flows is also important in glaciomarine and glaciolacustrine environments, where large amounts of sediment may accumulate on relatively steep ice-contact slopes, which become unstable during recession. 11.3. Interpreting pre-Quaternary glaciations The data presented herein from contemporary glaciers should be of use to those interested in reconstructing both Quaternary and pre-Quaternary glacigenic sequences. Earth's pre-Quaternary glacial record has received considerable attention (Hambrey and Harland, 1981; Eyles, 1993; Deynoux et al., 1994; Crowell, 1999; Hambrey and Orombelli, 1999), although this represents only a fraction of the level of that received by the Quaternary Period. The oldest known glacigenic sediments are of late Archaean age (2600–3100 Ma) from South Africa. Extensive Palaeoproterozoic (2500–2000 Ma) tillites are known from South Africa, Australia and Finland. The most prolonged and globally extensive glacial era took place in Neoproterozoic time (1000–550 Ma), when there is evidence for glaciation on all continents. This has given rise to the ‘Snowball Earth’ hypothesis of global glacierisation and complete shut-down of the hydrological cycles (Kirschvink, 1992; Hoffman et al., 1998; Hoffman and Schrag, 2002; reviewed by Etienne et al., 2007, and Fairchild and Kennedy, 2007). Sporadic Early Palaeozoic glacial events are best represented by the late Ordovician to early Silurian deposits and erosional features of Gondwana, notably in Africa. The most prolonged and extensive phase of glaciation during the Phanerozoic Eon spanned about 90 Ma of the Carboniferous and Permian Periods, affecting all

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29

Table 8 Summary of the relative importance of different debris entrainment processes, transport position and release in cold, polythermal (excluding high relief areas) and temperate glacial environments. Source and location

Cold-based glaciers

Polythermal glaciers

Temperate glaciers

1. Debris entrainment processes Snow, ice and rock avalanches Rockfall Aeolian processes Regelation Thrusting Folding

* * ** ***** ** ****

** ** * *** ***** ***

**** ***** * ** * *

2. Debris transport position Supraglacial Englacial Basal glacial Glaciofluvial

* ** ***** *

** *** *** *

**** ** * *****

3. Debris release Supraglacial Englacial Basal glacial Ice frontal Glaciofluvial Typical facies associations (and interpretation)

* ** ***** *** * Sandy boulder-gravel (reworked basal till) Gravelly sand (reworked fluvial/aeolian material)

** *** **** * * Diamicton (basal glacial) Muddy/sandy boulder-gravel (ice-marginal or ice-frontal) Sandy gravel (glaciofluvial) Gravel (glaciofluvial) Angular gravel (supraglacial) Sand and mud (glaciolacustrine)

**** ** * * ***** Sandy boulder-gravel (ice-marginal) Sandy gravel (glaciofluvial)

Gondwana continents. Finally, after a phase of global warmth, with little evidence of ice on Earth, Cenozoic ice sheet-scale glaciations began in Antarctica around the Eocene/Oligocene transition (34 Ma). Northern Hemisphere glaciation followed, with minor icerafting events recorded in the North Atlantic from late Miocene time, until full-scale glaciations began in late Pliocene time (2.6 Ma). Of all these glaciations, it is those of the Neoproterozoic that are the most controversial, and where the sedimentological evidence is interpreted in the widest variety of ways. However, from the data presented above from modern environments, we are now in a strong position to evaluate the environmental and climatic significance of well-preserved Neoproterozoic and other ancient sequences. 12. Conclusions This review is the outcome of systematic facies analyses in a wide variety of modern glacial regimes. Data from high-Arctic, sub-Arctic, temperate, tropical and Antarctic climatic regimes, (both terrestrial and marine environments) can be used to draw the following conclusions. For temperate glaciers, debris-entrainment is predominantly from rockfall and avalanches onto the glacier surface; for polythermal glaciers, from thrusting, regelation and folding at the base; and for cold glaciers, by regelation and folding also at the base. Levels of transport in the glacier reflect these processes, with temperate glaciers transporting most of their debris at the surface, but also extensively reworking subglacial debris by glaciofluvial processes. Polythermal glaciers carry most debris in basal and englacial positions, and the proportion of supraglacial debris is relatively small. Cold glaciers carry most of their load at the base, but modify it very little during glacial transport (Fig. 18). The resulting facies associations from each type of glacier landsystem typically comprise the following lithofacies as found on land (listed in order of relative abundance along with their interpretation) (Table 8): • Temperate glaciers—boulder-gravel with predominantly angular clasts (supraglacial melt-out till), sandy boulder-gravel with mainly

Angular gravel (supraglacial) Sand and mud (glaciolacustrine) Diamicton (basal glacial)

subangular and subrounded clasts (lateral and terminal moraine deposits and minor basal till); sandy boulder-cobble-gravel with mainly subrounded and rounded clasts (glaciofluvial) and clastrich diamicton (also moraine and subglacial till). • Polythermal glaciers in regions of moderate relief—clast-rich diamicton (subglacial till), sandy boulder-cobble gravel with mainly subrounded and subangular clasts (glaciofluvial), and sand and mud (glaciolacustrine). In high-relief areas, steep topography is more important than thermal regime, and polythermal glaciers in these areas are influenced primarily by rockfall and avalanches. • Cold glaciers—sandy boulder-gravel with subrounded and subangular clasts (reworked older wet-based subglacial tills), sand (glaciofluvial), sand with stromatolites (glaciolacustrine), and sand with rounded grains (aeolian). Each of the above types of glacier may also be found in the marine environment. Here, the same sedimentary attributes are present, but with the addition of biogenic components. Mixing by oceanographic processes, including iceberg drift, results in complex facies associations. It is in these offshore realms that the best chance of long-term (multimillion year) preservation occurs. Acknowledgements We are grateful to the referees Peter Barrett and Matthew Bennett for their insightful comments on the manuscript. We also thank our field companions Matthew Bennett, Garry Clarke, Kevin Crawford, James Etienne, Werner Ehrmann, Sean Fitzsimons, Bryn Hubbard, David Huddart, Gerhard Kuhn, Duncan Quincey, Shaun Richardson and John Smellie. Nick Midgley and Becky Goodsell kindly provided additional data from Midre Lovénbreen (Svalbard) and Haut Glacier d'Arolla (Swiss Alps) respectively. Much of the work was funded by the UK Natural Environment Research Council, and Aberystwyth and Liverpool John Moores universities. The Antarctic components of the modern data-set were undertaken with the support of the Alfred Wegener Institut (Bremerhaven), Antarctica New Zealand and the British Antarctic Survey. A substantial part of this paper was written

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