GLACIAL SEDIMENTS

Materials that are being or have been transported by glacier ice usually excluding debris that is subsequently reworked (see redeposition). Glacigenic sediment is usually used in a broader sense to include sediments with a greater or lesser component derived from glacier ice. A comprehensive classification of terrestrially deposited glacigenic sediments was developed for the International Union for Quaternary Research (INQUA), based on our understanding of the processes of transport and deposition but is constantly being refined. Sediment carried by the glacier is located (1) in the basal ice layer, where it is referred to as basal glacial debris; (2) on the surface, where it is derived primarily from rockfall, referred to as supraglacial debris and (3) in the interior if ingested from the surface via crevasses or meltwater conduits, or from the bed via folds and thrust faults, where it is referred to as englacial debris. Commonly, ice also moves over a soft deformable bed of subglacial debris.

The principal genetic types of subglacially deposited sediment are as follows. Sediment deposited by uniquely glacial processes without subsequent disaggregation and reworking is termed till. This is further subdivided into lodgement till, which results from active plastering onto the bed of sediment by the glacier, and meltout till, which is released by melting from relatively inactive debris-rich ice. This debris is highly mobile and the term glacigenic sediment flow (formerly referred to as flow till) is applied. Deformation till comprises weak rock and sediment that has been detached by the glacier from its source and its original structural integrity destroyed, although it retains many of its primary characteristics. Together, all these are grouped under the name subglacial till (or basal till). In recent years, there has been a tendency to avoid the terms lodgement and meltout tills and to refer to them together as subglacial traction tills. This is because the glacier bed experiences a wide range of processes and the resulting sediments are a complex mosaic of different depositional products.

Sediment released directly by glaciers or from a concentrated cluster of icebergs into a waterbody is referred to as ice-proximalglaciomarine deposits (or [Page 469]glaciolacustrine deposits), formerly termed waterlain till. Farther from the ice in the waterbody can be found ice-distal glaciomarine (or glaciolacustrine) sediment.

All these sediments are generally poorly sorted, with material potentially ranging in size from clay or silt to boulders or cobbles. Except for ice-distal glaciomarine sediment, all these types of glacigenic sediment usually lack bedding, and can only be distinguished on the basis of detailed analysis of sediment texture (e.g. grain-size analysis), particle shape and roundness, clast surface features (e.g. striations and friction cracks) and fabric analysis. Ice-distal deposits include material derived directly from icebergs, mixed with sediment formerly in suspension, and other types of marine sediments or lacustrine sediments. The well-defined practice of facies analysis has been established to interpret sedimentary sequences and, in particular, glacigenic sediments. Different sediment types (lithofacies) are first described objectively and rigorously, and only then interpreted (e.g. a diamicton may be interpreted as a till). Lithofacies are grouped into facies associations to aid interpretation of the overall environment. By combining facies analysis with descriptions of landform morphology, sediment/landform associations can be formally defined. From these, in specific geographical contexts, and based on modern examples, a variety of glacial landsystems have been defined.

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