A2.3GQ3 Glacial and Quaternary Geology LECTURE 5

1. A2.3GQ3 Glacial and Quaternary GeologyLECTURE 5 TIDEWATER GLACIERS

2. SUMMARY • Introduction • Dynamics of tidewater glaciers • Fjord-based deposition • Ice frontal sedimentation • Proximal sedimentation • Distal sedimentation • Fjord-based facies associations

3. Introduction

4. At the present day many glaciers terminate at sea-level in fjords. This creates a distinctive series of sediment associations, partly marine and partly terrestrial.

5. Kongsfjord Spitsbergen Photo: J.D.Peacock

6. Kongsvegen: SpitsbergenNorsk Polarinstitutt photo

7. Kongsfjord Spitsbergen Photo: M.A.Paul

8. Marine and lacustrine fronts display many similarities; differences arise from the relative densities of lake and seawater and from the absence of tidal motion in lakes. • This influences the behaviour of plumes (underflow vs overflow) and the drift of icebergs. • In turn, this controls the distribution of so-called rain-out deposits and of ice-rafted deposits.

9. Dynamics of tidewater glaciers

10. Tidewater glacier margins are defined by the position of the grounding line, and thus by ice dynamics and bed topography. • Ice will float in a water depth about 90% of the ice thickness. Thus water depth determines the position of the grounding line.

11. From the grounding line, the ice will advance until the rate of loss by calving equals the rate of ice discharge. • This typically occurs at a distance in front of the grounding line about equal to the ice thickness. • This defines the position of the calving line, which is thus slightly in advance of the grounding line.

12. The calving rate is roughly proportional to water depth. Thus the overall position of the glacier margin is strongly dependent on seabed topography. • In fjords, constrictions in the sidewalls and bed, termed pinning points, are likely to define possible stationary positions of the margin.

13. Fjord-based deposition Ice frontal sedimentation

14. Ice-front depositon in fjords is very similar to that seen at water-based margins in lakes. • If deposition occurs near to the grounding line, a mixture of sediment will be received from basal debris, supraglacial debris and meltwater. • The assemblage of grounding sediments is thus produced by a mix of subglacial, ice contact, gravity driven and water column processes.

18. Fjord-based deposition Proximal sedimentation

19. Proximal sediments are deposited near to the ice-margins by deposition through the water column. They show some similarities to ice-frontal sediments. • They are often till-like sediments that contain layers of sorted materials, which often show evidence of disturbance. • These have sometimes (misleadingly) been referred to as ‘water-lain tills’.

20. Sediment may fall directly into the water column by the slumping of supraglacial debris or by the rolling of icebergs. • This releases a shower of variously sized particles that settle through the water under gravity and produce a graded deposit.

21. Kongsfjord Spitsbergen Photo: J.D.Peacock

24. Proximal glaciomarine sediments Dicksonfjord, Spitsbergen Photo: M.A.Paul

25. Deposition can also occur by rain-out from hyperconcentrated plumes around the ice-front. • Some glaciomarine sediments are rhythmically graded due to the pulsed input of sediments. • Rhythmic proximal sediments are usually composed of sand and silt and are termed cyclopsams;

26. Probable glaciomarine cyclopsams Inverness area Photo: J.W.Merritt

27. Fjord-based deposition Distal sedimentation

28. The zone of distal sedimentation can extend for tens of kilometres from the ice margin. • It is dominated by suspension rain-out and ice rafting. • There is normally a proximal to distal fining of the bulk sediment, with a relative increase in the proportion of the fine material.

29. Continued rain-out can occur from sediment plumes that are generated at the inflow points of waterfalls and jets. • These plumes consist of a hyper-concentrated, fine-grained suspension that moves from the entry point due to density-, wind- and tide-driven currents.

30. During this movement the plume loses material by rain-out until it can mix freely with the surrounding water and lose its coherent identity. • Rainout is assisted by the processes of flocculation and by biogenic pelletisation within the water column.

31. The rained out material forms a sediment drape over the seabed. • In shallow water, where bioturbation is common, the sediment is typically a massive, distal-fining sandy diamicton or mud.

32. Where biogenic activity is suppressed, silts and clays separate to give upwards-fining units with sharp bases. These are often rhythmic and are termed cyclopels.

33. Probable glaciomarine cyclopels Inverness area Photo: J.W.Merritt

34. Ice rafting involves the transport of debris by floating ice, followed by release through the water column. • The distribution of this debris will follow the tracks of bergs, which are often constrained by bathymetric features and well-defined currents.

35. Kongsfjord Spitsbergen Photo: J.D.Peacock

36. Kongsfjord Spitsbergen Photo: M.A.Paul

37. Dropstones Vibrocore 60-05/51 Faeroe-Shetland Channel

38. Icebergs will often impact the seabed close to a calving margin, leading to ploughing and possible debris release.

40. Woodworth-Lynas & Guigné 1990

41. Fjord-based facies associations

42. Powell (1981) has described five fjord-based sediment associations based on examples from Alaska. • The key concepts are • Depth of water at the ice front • Speed of ice-front retreat • Whether the glacier actually enters the water

43. Association 1 is formed in deep water in which the ice front is likely to retreat rapidly. • It is dominated by so-called morainic bank deposits produced by oscillation sages,

45. Association 2 is formed in shallow water in which the ice front is likely to retreat slowly. • It is dominated by grounding line deposits produced by meltwater-driven input and subsequent disturbance and redistribution.

47. Association 3 is formed in very shallow water with little or no iceberg calving. • It is dominated by ice-contact deltaic beds (mainly foresets) composed of sand and gravel.

49. Associations 4 and 5 are formed when the ice no longer enters the water. In this case a true Gilbert-type delta is formed. • The distinction between associations 4 and 5 depends on the distance from the ice to the sea: • if close, the sediments will be relatively coarser; • if distant, the sediments will be relatively finer