A2.3GQ3 Glacial and Quaternary Geology LECTURE 4 GLACIOFLUVIAL AND GLACIOLACUSTRINE DEPOSITS

2 SUMMARY  Introduction  Glacial meltwater streams  Morphology of glaciofluvial deposits  Sedimentology of glaciofluvial deposits  Glaciolacustrine sediments

Introduction

4  The proglacial area receives sediment by several groups of processes  Mass wasting of debris covered ice  Glaciofluvial processes that require the involvement of flowing water derived from glacier ice;  Glaciolacustrine processes that involve a lake of glacial origin;

5  These processes create a range of sediments:  stagnant ice bodies allow direct deposition of unsorted sediments by mass flow (flow tills);  constrained melt streams that occupy englacial or supraglacial positions lead to mounded deposits that are channelised to a greater or lesser extent;  unconstrained melt streams allow the construction of laterally extensive sandar by river braiding;  glacial lakes allow deposition by stream inflow, subaqueous jets, suspension rain-out and ice- rafting.

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Glacial meltwater streams

8  Glacial melt streams are characterised by:  strongly variable discharge of water and sediment (both spatial and temporal);  high peak flows  frequent migration of discharge patterns

9  Bedload dominated due to abundant available coarse sediment from mass wasting.  High competence during peak flows creates mobile bed conditions over wide areas.  Broad, shallow floodplain, containing a braided pattern of distributary channels.

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11 Markarfljot Iceland Photo: J.W.Merritt

12  Factors leading to braiding:  abundant coarse sediment  steep long profiles  lack of vegetation  fluctuating discharge.  Shallow, broad channel allows secondary helical flows that create longitudinal bars and scour pits.

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Morphology of glaciofluvial deposits

15  Glaciofluvial processes create a range of landforms which depend on the shape and extent of any containing ice.

16  Constructional mounds - often generically termed kames or kamiform.  These originate in hollows between ice blocks.  Removal of the supporting ice creates a variety of final shapes, which may be either flat topped or rounded.  Intervening hollows are termed kettle holes - these may contain kettle lakes.

17 Glaciofluvial complex, Eokuk

18 Kaimiform deposits, Lake o’Laws, Nova Scotia

19 Treig delta complex near Fersit

20  Deposition in elongate englacial or supraglacial channels creates linear deposits termed eskers  These follow the lines of englacial/supraglacial streams and form when sediment is available.  They are underlain by ice and subsequent collapse creates a sharp crested morphology

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22 Eskers Breidamerkurjökull Iceland Photo: J.W.Merritt

23 Eskers Breidamerkurjökull Iceland Photo: M.A.Paul

24 Carstairs Esker Lanarkshire, Scotland BGS Photo

25  When no lateral restriction is present the meltwater flows as a wide braided stream.  This creates an unconstrained spread of sediment termed an outwash fan or sandur (pl. sandar).

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27 Skeiderarsandur, Iceland

Sedimentology of glaciofluvial deposits

29  Despite their wide range of morphologies, these deposits share several characteristic features:  rapid variation of facies;  presence of sandy-muddy matrix, leading to matrix supported gravels in extreme cases;  sheet-like gravel deposits interbedded with sand- mud sheets, due to waning from high peak flows.

30 Breidamerkursandur Iceland Photo: M.A.Paul

31 Classical braided model of Miall (1977)  Peak flows build gravel bars  Declining flows allow upwards fining, exposure cuts secondary channels in bar surface  Low flows deposit sand units in main channels  Very low flows allows ponding in which mud drapes are deposited.

32 Sedimentology of glaciofluvial deposits  Miall also introduced a classification of overall architectures using a series of type areas based on North American rivers.  These type sequences are known as the  Trollheim  Scott  Donjek  Platte

33  Collectively they represent:  the transition from proximal to distal settings  a relative change from gravel to sand deposits  a change from mass flow to fluvial mechanisms.  These facies architectures can be classified into a generalised sequence in the seawards direction.

34  Dominated by massive, clast supported gravels (Gm) and matrix supported gravels (Gms)  Represent the products of braid bars and debris flows repectively, with subsidiary channel flow deposits  Characteristic of very high energy, proximal glacio- fluvial environments.

35  Dominated by massive, clast supported gravels and cross-bedded gravels  Represent the products of successive longitudinal bars with minor waning flow deposits  Characteristic of fluvially dominated proximal sandar

36  Dominated by discrete, upward-fining sequences with erosional bases  Represent the products of separate, migrating channels with occasional sheet flow  Characteristic of sandy intermediate sandar

37  Dominated by superimposed sand units with various styles of bedding  Represent the products of migrating bedforms within numerous distributaries  Characteristic of sandy reaches of lower sandar and non-glacial braided rivers

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Glaciolacustrine Sediments

40  Glaciolacustrine sediments are produced by episodic inflow into non-saline, standing water.  Deposition may occur directly from ice in association with a water-contact ice front, from an inflowing stream or by sedimentation from the lake itself.  This produces a wide range of landforms and sediments.

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42 Ice-contact deposits  Direct deposition occurs at or close to the ice- front grounding line, whose position fluctuates as a result of ice dynamics.  Sediment is released by melting, pressurised ‘jet’ flow or by flowage under gravity.  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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44 Ice-contact deposits  Active ice bedforms include streamlined forms, large scale push/dump ridge complexes and transverse squeeze/push ridges (termed de Geer moraines).

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46 De Geer moraines: Hudson Bay Canadian Geological Survey photo A

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48  Deltaic accumulations occur near inlets, often possessing classic delta-front avalanche, foreset and topset deposits.  This then allows the inflow to advance further into the water as the sediment pile becomes established.

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50 Ice marginal delta, Cape Breton

51 Ice-marginal subaqueous sediments Achnasheen Photo J.W.Merritt

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53  Pressure-driven input of sediment (jet flow) close to the bed creates distinctive sediment mounds termed grounding-line fans.  These are typically composed of coarse sediment, with a variable admixture of fines that depends on the local strength of the jet efflux.  The majority of fines are removed as plumes that form density underflows, inflows or surface overflows, depending on the sediment concentration and water density.

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55  The style of fan is determined by the strength of the discharge.  This determines the detachment point of the plume from the bed and the velocity and distance of travel across the fan.

56  low discharge is associated with the immediate loss of coarse sediment and detachment of the plume, possibly avalanching on the distal face;  intermediate discharge is associated with erosion on the fan surface and a defined traction layer at the base of fan units;  high discharge is associated with channelling and erosion of the fan surface, production of dune bedforms on the fan at the point of plume detachment.

57  In the body of the lake finer sediments undergo rhythmic settling (not necessarily annual) from suspension.  This creates draped layers with a variety of on-lapping or off-lapping relations to subjacent sediments.

58 Glacilacustrine rhythmite Sweden Photo: M.A.Paul

59 Glacilacustrine rhythmite Sweden Photo: M.A.Paul

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61  Reworking by currents and by gravity flowage is significant around virtually all water-contact ice-margins.  These currents may be tidal or density driven.  Gravity flowage arises from the (usually) rapid rate of deposition, accumulation on unstable slopes and from the generation of internal pore pressure.

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63  In deeper lakes, floating ice is able to introduce ice-rafted debris and dropstones into finer sediments;

64 Jokulsarlòn Breidamerkurjökull Photo: M.A.Paul

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66  Glacial lakes can be short-lived, due to frequent switching of drainage patterns and collapse of ice dams.  They often show evidence of changes in lake level (former shorelines) and incision into older sediments is common.

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68 SUMMARY  Introduction  Glacial meltwater stream  Morphology of glaciofluvial deposits  Sedimentology of glaciofluvial deposits  Glaciolacustrine sediments

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