A2.3GQ3 Glacial and Quaternary Geology LECTURE 5 TIDEWATER GLACIERS

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

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: Spitsbergen Norsk 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.

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.

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.

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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

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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

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.

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40 Woodworth-Lynas & Guigné 1990

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,

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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.

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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.

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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

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51  In principle, a succession of associations from 1 to 5 will be seen as a glacier retreats up the fjord.  The associations will replace one another both laterally and vertically.

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53 SUMMARY  Introduction  Dynamics of tidewater glaciers  Fjord-based deposition  Ice frontal sedimentation  Proximal sedimentation  Distal sedimentation  Fjord-based facies associations

THE END

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