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Total Ral alive Strain

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Glacial Lithofacies Pictures

Figure 2.1 1 Details of subglacial sediment sequences at the margins of Icelandic glaciers showing typical vertical transitions from glacitectonized stratified sediments to overlying tills. A) Composite stratigraphic log from Skalafellsjokull showing various lithofacies and interpreted relative strain curve (from Evans, 2000a). B) Vertical profile log of section on the south shore of BreiSarlon on the BreiSamerkurjokull foreland showing a till with a possible A and B horizon superimposed on the B horizon of an eroded underlying till (from Evans and Twigg, 2002). Both tills have sheared rafts of underlying materials at their bases including peat from a nearby outcrop of in situ organics. The fabric shape triangle (Benn, 1994) includes envelopes for upper and lower tills (A and B horizons, respectively) based upon data collected by Dowdeswell and Sharp (1986) and Benn (1995). The cannibalization of underlying outwash by the deforming layer is apparent in the inheritance of clast shapes and lack of striae on clasts in the lower till.

Figure 2.1 1 Details of subglacial sediment sequences at the margins of Icelandic glaciers showing typical vertical transitions from glacitectonized stratified sediments to overlying tills. A) Composite stratigraphic log from Skalafellsjokull showing various lithofacies and interpreted relative strain curve (from Evans, 2000a). B) Vertical profile log of section on the south shore of BreiSarlon on the BreiSamerkurjokull foreland showing a till with a possible A and B horizon superimposed on the B horizon of an eroded underlying till (from Evans and Twigg, 2002). Both tills have sheared rafts of underlying materials at their bases including peat from a nearby outcrop of in situ organics. The fabric shape triangle (Benn, 1994) includes envelopes for upper and lower tills (A and B horizons, respectively) based upon data collected by Dowdeswell and Sharp (1986) and Benn (1995). The cannibalization of underlying outwash by the deforming layer is apparent in the inheritance of clast shapes and lack of striae on clasts in the lower till.

necessary to the development of subglacial fracture fills, the possibility of a frozen bed at the glacier margin for significant periods of time clearly has implications for subglacial processes such as those envisaged by van der Meer et al. (1999) at Slettjokull. Just as supercooling in overdeepenings may produce abnormal quantities of debris-rich basal ice for an active temperate glacier, so short-lived ice-marginal permafrost may impart its own diagnostic structures on subglacial deformation tills beneath temperate ice. The exact significance of shallow permafrost at the snouts of temperate glaciers, whatever its age, is still not fully understood but its geological impact at the margins of Icelandic glaciers appears to be restricted to secondary subglacial till structures and the production of large push moraines by stacking of frozen till slabs (see above).

Generally, the characteristics of tills on temperate glacier forelands strongly suggest a subglacial deformation and lodgement origin. They comprise materials derived from pre-existing stratified sediments in addition to localized abrasion of rock surfaces and are generally thin (<2 m). Thicker sequences are constructed by the sequential plastering of several till layers onto stratified sediments and bedrock (e.g. tectonic/depositional slices of Boulton et al., 2001). This is similar in depositional style to the rheologic superposition proposed by Hicock (1992), Hicock and Dreimanis (1992), and Hicock and Fuller (1995), and the till/stratified interbed successions of Eyles et al. (1982), Evans et al. (1995), and Benn and Evans (1996). Complex till sequences, often associated with larger push moraines, are constructed where glacier margins become stationary for substantial periods. It is evident that the construction or stacking of such complex till sequences at contemporary glacier snouts is a sub-marginal process and this knowledge may be applied to the interpretation of till architecture at regional scales (e.g. Boulton, 1996a, b; Johnson and Hansel, 1999). Although subglacial deformation and lodgement (and perhaps ploughing) are predominant till deposition processes in temperate glaciers, the delivery of subglacial material to the glacier margin and the construction of moraines involve the freeze-on of till slabs for at least part of the year. Where frozen submarginal conditions persist for long periods, possibly perennially, discontinuous permafrost must be considered in models of marginal till delivery and moraine construction even though lodgement and deformation processes still dominate the subglacial environment immediately up-ice.

2.2.3 Glacifluvial and Glacilacustrine Domain

The monitoring of glacifluvial landforms and proglacial lakes at temperate glacier margins provides valuable insights into process/form relationships, particularly during glacier recession (e.g. Welch, 1967; Howarth, 1968, 1971; Price, 1969, 1971, 1973, 1980, 1982; Price and Howarth, 1970; Churski, 1973; Gustavson, 1975; Smith and Ashley, 1985; Boulton, 1986; Evans and Twigg, 2000, 2002). Proglacial outwash streams drain either away from glacier margins to produce sandur fans or parallel to the margin due to topographic constraints to produce ice margin-parallel outwash tracts and kame terraces (Fig.s 2.12—2.15). Additionally, glacier recession uncovers large depressions in which proglacial lakes can evolve (e.g. Thorarinsson, 1939; Howarth and Price, 1969; Price and Howarth, 1970; Bjornsson, 1976; Price, 1982). The nature and size of depo-centres and sediment thicknesses in such lakes vary according to their longevity and ratio of glacier recession to sedimentation rate (Shaw, 1977c; Teller, Chapter 14).

Sandur fans are prograded from subglacial or englacial meltwater portals, often resulting in linkages between fan apices and eskers (Price, 1969; Fig. 2.13). The ice contact nature of sandur fans results in the development of pitted ice-proximal fan surfaces and steep ice-contact faces. During extended periods of glacier marginal stability, extensive and thick sandur fans may coalesce. An excellent example is BreiSamerkursandur located beyond the Little Ice Age maximum moraines of BreiSamerkurjokull where the routeways of proglacial streams have not been channelized by pre-existing topographic high points (Evans and Twigg, 2000, 2002). Such thick ramps of sediment are difficult for later proglacial meltwater and subglacial erosion to rework and hence proglacial drainage and then glacier flow is diverted around them. Stratigraphic evidence indicates that temperate glaciers often advance over considerable tracts of glacifluvial outwash without disrupting their general form. For example, Boulton (1986, 1987) and Evans and Twigg (2002) have reported that the forms of pre-advance sandur fans are still

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