A2.3GQ3 Glacial and Quaternary Geology LECTURE 4

1. A2.3GQ3 Glacial and Quaternary GeologyLECTURE 4 GLACIOFLUVIAL AND GLACIOLACUSTRINE DEPOSITS

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

3. 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.

7. 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.

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.

14. 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

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).

27. Skeiderarsandur, Iceland

28. 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

39. 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.

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.

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).

46. De Geer moraines: Hudson Bay Canadian Geological Survey photo A14882-91

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.

50. Ice marginal delta, Cape Breton