A2.3GQ3 Glacial and Quaternary Geology LECTURE 6 SLIDING BED FEATURES

2 OVERVIEW  Introduction  Subglacial surfaces  Subglacial erosion  Subglacial deposition  Glaciotectonic structures

Introduction

4  ‘Sliding bed’ refers to those glaciers that move in part by basal sliding.  If the bed is of low strength it may become deformed by this motion and some movement may thus occur within the bed itself.  This is usually the case when the bed is impermeable (e.g. clays) and subglacial water pressure is high.

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6  Sliding bed glaciers show two major differences from frozen bed glaciers:  Lowered surface slope  Increased flow rate  These both result from their lower basal shear stress as compared with a frozen bed glacier.  In an extreme case the glacier may surge, so producing an unsustainable discharge for a limited period.

Streamlined surfaces

8  Sliding bed glaciers produce a number of characteristic landforms, many of which are streamlined in the direction of flow.  These landforms may be constructed from a variety of materials, predominantly subglacial lodgement till.  They also produce ice-frontal features by squeezing or pushing. These were described in an earlier lecture.

9  Lodgement till surfaces show a hierarchy of streamlined forms that are generically termed drumlinoid (true drumlins are an end-member).  They are essentially mobile bedforms, analogous to ripples and dunes, that are controlled by ice dynamics and till rheology.  Erosional forms on hard substrates are also streamlined in their upflow direction. The down flow direction is usually broken by cavitation and stress- release.

10 Breidamerkurjökull Photo: M.A.Paul

11 Drumlin field: Saskatchewan, Canada (Canadian Geological Survey photo A )

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13 Streamlined lodgement till, Forth lowlands Photo: M.A.Paul

14  Lodgement till surfaces exposed in front of modern glaciers also show secondary flutings.  These are usually created by flow into a cavity in the lee of an obstruction such as a boulder.  Once initiated they become self-generating.

15 Breidamerkurjökull Photo: M.A.Paul

16 Breidamerkurjökull Iceland Photo: M.A.Paul

17 Blomstrandbreen Spitsbergen Photo: M.A.Paul

Deposition from sliding ice

19  Deposition takes place by three groups of processes:  Frictional lodgement (produces lodgement till)  Melt-out (produces melt-out till)  Tractive deformation (produces deformation till)  Subglacial traction can also cause glaciotectonic disturbance of the existing bed.  These tills can be seen as end-points in a continuum of primary glacial deposits.

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21 Lodgement till (hard bed)  Subglacial debris in contact with the bed will experience frictional retardation.  Lodgement occurs when the tractive force imposed by the moving ice is unable to overcome this friction.  The sediment so formed is known as lodgement till. It is probably the most common glacial sediment.

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23 Lodgement till (soft bed)  Debris is also released against a soft bed by frictional lodgement and also by ploughing into the soft substrate.  The whole bed may begin to deform when the stress level in the substrate rises above some critical value.

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26  Melt-out till is formed when debris-bearing ice becomes stagnant and melts in situ.  It is believed that englacial structures such as banding or folding will be preserved to some extent.  Melt-out till is probably relatively unusual since high pore pressures will usually occur within the debris.  This will lead to secondary disturbance by flowage.

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29  Some debris may melt out from the roof of the cavity and collect as a poorly sorted mass on the cavity floor.  This is often termed lee-side deposition and the deposits are known as lee-side deposits (or lee-side till).

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Glaciotectonics

32  Observations below modern glaciers have shown that till can become mobile to part or full-depth.  This has led to the recognition of two layers:  The A-layer: the mobile deforming upper layer  The B-layer: the rigid static lower layer  The boundary between the layers can migrate in response to changes in glacial traction and subglacial effective stress.

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34  The substrate becomes moble more easily if there is a high subglacial water pressure.  This occurs when the bedrock is of low permeability or there is rapid sliding on a soft subglacial layer.  Examples include clay bedrock or over-ridden marine or lake clays.

35  Defromation is thus closely coupled to the conditions of subglacial drainage. There are two basic drainage models:  one-dimensional: the subglacial sediments are underlain by an aquifer at shallow depth and the drainage is vertical;  two dimensional: the subglacial sediments are underlain by an aquiclude and the drainage is horizontal.  These give rise to fundamentally different patterns of subglacial pore pressure and resultant deformation.

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38  In the one-dimensional model, drainage is largely vertical from the glacier base to the aquifer. The pore pressure gradient in the till thus reduces downwards.  This stabilises the till and supresses the formation of the A-layer.  Till formed under these conditions is principally lodgement till (B-horizon).

39  In the two dimensional model, vertical subglacial drainage is prevented. Thus meltwater is forced to migrate laterally, either within the till, or as a discrete film, or in channels.  In such a case a thick A-layer is produced. This occupies the subglacial bed either partly or entirely and the product is deformation till.  Under some conditions the deformation may extend into underlying, non-glacial deposits and produce glaciotectonic structures.

40  In this case we expect a vertical sequence in which the disturbance increases upwards.

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43  At the base of the sequence we find brittle fracture and fragmentation of the substrate, with some incorporation of the fragments into the overlying till.  These fragments may be very large, in which case they are known as bedrock rafts.

44 Chalk raft, Sidestrand

45 Deformed subglacial sediments West Runton, Norfolk Photo: M.A.Paul

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47  Above this we find extensive shear deformation that may involve a large thickness of the subglacial sediment.  Individual structures such as folds can be identified.

48 Probable deformation till Norfolk Photo: M.A.Paul

49 Deformed subglacial sediments West Runton, Norfolk Photo: M.A.Paul

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51  Above this level we find extensive shear deformation of the subglacial sediments, leading first to shear banding and then to complete mixing, so creating a deformation till.

52 Deformed subglacial sediments West Runton, Norfolk Photo: M.A.Paul

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54 Probable deformation till Holderness Photo: B.F.Barras

55  It can be argued that many tills originated under deforming bed conditions even if they now appear completely mixed.  This may be a fundamental distinction between the tills of lowland areas and those of highland areas, which are more likely to be the products of rigid-bed, grain-by-grain lodgement.

56 Summary  Introduction  Subglacial surfaces  Subglacial erosion  Subglacial deposition  Glaciotectonic structures

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