Sliding Bed Features: Overview of Glacial Geology

1. A2.3GQ3 Glacial and Quaternary GeologyLECTURE 6 SLIDING BED FEATURES

2. OVERVIEW • Introduction • Subglacial surfaces • Subglacial erosion • Subglacial deposition • Glaciotectonic structures

3. 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.

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.

7. 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 A14509-5)

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

18. 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.

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.

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.

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.

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).

31. 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.

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.

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.

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

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