Sequences

Where thick sequences of supraglacial debris cover the glacier surface multiple layers of till - lodgement tills, meltout tills and flow tills - may be superimposed. In certain situations these may be separated by units of sand and gravel. A typical section consists of a till layer at the base, above which there is a sequence of sand and gravel, capped by a second till layer. This type of section is often referred to as a tripartite till sequence. Traditionally such sequences were interpreted as the product of multiple glaciations. The ice advanced to deposit the first till then retreated, depositing the sand and gravel, before readvancing to deposit the upper till. Many of these sequences have now been reinterpreted in terms of a single episode of glaciation. Ice advances over the area to deposit a basal lodgement till. As the ice retreats a topography of ice-cored ridges develops between which glaciofluvial sands and gravels are deposited. As the ice retreats further these outwash rivers become abandoned and the glaciofluvial deposits are covered by flow till derived from the adjacent ice-cored ridges. Meltout of the buried ice inverts the topography to give a tripartite till sequence. The correct interpretation of each till layer and of the facies present is therefore very important. One of the first sequences to be reinterpreted in this way was that at Glanllynnau in North Wales (Boulton, 1977). This represents an important step forward in the interpretation of till sequences and glacial stratigraphy.

Source: Boulton, G.S. (1977) A multiple till sequence formed by a late Devensian Welsh ice cap: Glanllynnau, Gwynedd. Cambria, 4, 10-31. [Modified from: Addison et al. (1990) North Wales: Field Guide. Quaternary Research Association, Cambridge, figure 17, p. 41.].

^ Debis-rich ice |< Sandy upper till | | Sand | | Organic mud & peat

Buried ice | | Sand & gravel Clay-rich lower till Movement of flow till

^ Debis-rich ice |< Sandy upper till | | Sand | | Organic mud & peat

Buried ice | | Sand & gravel Clay-rich lower till Movement of flow till

Interpretation

Glaciofluvial outwash.

Flow till units.

Interbedded outwash (glaciofluvial sands) may show fining upward sequences associated with filling and abandonment of meltwater channels.

Flow tills. Erosional base to unit often with a concentration of clasts. Individual flow unit 'packages' maybe identified.

Glaciofluvial outwash.

Flow tills.

Contorted and partially reworked lenses of flow till in glaciofluvial sediments.

Crudely stratified meltout till, may show evidence of subglacial flow.

Lodgement t

Fld

A-—'

Dms(r)

A

Dms(r)

A^a

Dms(r)

^ A

St St

Dms/Sm

Figure 8.16 Typical vertical log and facies model for a glacier with a mixed thermal regime or high englacial debris content. [Modified from: Eyles (1983) Glacial Geology, Pergamon Press,

This picture may be complicated for valley glaciers with thick supraglacial debris layers due to rockfall from adjacent valley sides. This supraglacial debris gives rise to a thick irregular drape of coarse, angular supraglacial meltout till, which is often reworked as flow tills and by fluvial action. This drape of debris transported at high levels within the glacier will mantle the subglacial sediment facies.

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