Landform Sediment Assemblages in Active Temperate Glacier Forelands

Previous studies of landform-sediment associations on the recently deglaciated forelands of active temperate glaciers have highlighted three dominant depositional domains with characteristic landform-sediment assemblages. First, areas of extensive, low-amplitude marginal dump, push and squeeze moraines (Price, 1970; Krüger, 1987; Evans et al., 1999a; Evans and Twigg, 2000, 2002) are derived largely from material on the glacier foreland and often record annual recession of active ice (Sharp, 1984; Boulton, 1986; Krüger, 1995). Short periods of readvance or stability can produce larger push moraines (Krüger, 1993). This can be a product of stacking of sub-marginally frozen sediment slabs (Humlum, 1985; Krüger, 1985, 1987, 1993, 1994a, 1996; Matthews, et al. 1995), or dump, squeeze and push mechanisms operating at the same location over several years. The incremental thickening of ice-marginal wedges of deformation till has also been proposed (e.g. Johnson and Hansel, 1999). Second, subglacial landform assemblages of flutings, drumlins and overridden push moraines dominate the areas between ice-marginal depo-centres (Krüger, 1987; Benn, 1995; Evans et al., 1999a, b). These features have been linked to subglacial deforming layers, particularly in previous Icelandic research on active temperate glaciers (Boulton, 1987; Boulton and Hindmarsh, 1987; Benn, 1995; Hart, 1995; Benn and Evans, 1996). Their ubiquity in temperate glacier forelands attests to the sparsity of supraglacial sediments let down onto the substrate during deglaciation. Third, large areas of glacifluvial forms such as recessional ice-contact fans (Boulton, 1986; Price, 1969) and hochsandur fans (Krüger, 1997) cut through, or are directed by, moraine ridges. These outwash forms are often associated with simple and complex, anabranched eskers (Price, 1966, 1969) and small areas of pitted outwash (Price, 1969, 1971). Within the enclosed depressions on the foreland, proglacial lakes will expand and contract in response to the evolving drainage networks, acting as temporary storage for glacifluvial sediments. In addition to the three major depositional domains, anomalies are to be expected due to the signatures of erratic/non-cyclic or site-specific processes, and these are reported elsewhere. For example, jokulhlaup (Maizels, 1997; Russell and Knudsen, 1999; Fay, 2002) and surge (Evans and Twigg, 2002) features may be represented in the landform record of active temperate glaciers. The three major depositional domains are now reviewed in turn by concentrating on their diagnostic landform-sediment associations.

2.2.1 Marginal Morainic Domain

The most extensive and sharply defined moraines on active temperate glacier forelands are the small push moraines, often clearly linked to annual advances (Price, 1970; Boulton, 1986; Fig. 2.2). They comprise a wide range of sediments including:

• subglacially derived diamictons with large numbers of striated and faceted clasts

• glacifluvial sediments that have been reworked from pre-advance outwash

• glacitectonized slabs of laminated sands and muds originating in proglacial lakes.

Figure 2.2 Push moraines at the margins of Icelandic active temperate glaciers. A) Aerial photograph of push moraines at the margin of Fjallsjokull, showing sawtooth plan forms and partial overriding (Landm^lingar Islands and University of Glasgow, 1965); (scale bar represents 750 m). B) Large composite push moraine at the margin of Flaajokull, constructed by the snout during a period of ice-marginal stability in the early 1990s.

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