a2.3gq3 glacial and quaternary geology lecture 4 glaciofluvial and glaciolacustrine deposits

A2.3GQ3 Glacial and Quaternary Geology LECTURE 4

GLACIOFLUVIAL AND GLACIOLACUSTRINE DEPOSITS

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SUMMARY

Introduction

Glacial meltwater streams

Morphology of glaciofluvial deposits

Sedimentology of glaciofluvial deposits

Glaciolacustrine sediments

Introduction

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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;

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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.

Glacial meltwater streams

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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

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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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MarkarfljotIceland

Photo: J.W.Merritt

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

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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.

17Glaciofluvial complex, Eokuk

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Kaimiform deposits, Lake o’Laws, Nova Scotia

19Treig delta complex near Fersit

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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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EskersBreidamerkurjökullIcelandPhoto: J.W.Merritt

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EskersBreidamerkurjökullIcelandPhoto: M.A.Paul

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Carstairs EskerLanarkshire, ScotlandBGS Photo

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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).

27Skeiderarsandur, Iceland

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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.

30BreidamerkursandurIcelandPhoto: M.A.Paul

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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.

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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

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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.

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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.

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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

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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

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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

Glaciolacustrine Sediments

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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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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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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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De Geer moraines: Hudson BayCanadian Geological Survey photo A14882-91

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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.

50Ice marginal delta, Cape Breton

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Ice-marginal subaqueous sedimentsAchnasheenPhoto J.W.Merritt

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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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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.

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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.

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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.

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Glacilacustrine rhythmiteSwedenPhoto: M.A.Paul

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Glacilacustrine rhythmiteSwedenPhoto: M.A.Paul

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

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JokulsarlònBreidamerkurjökullPhoto: M.A.Paul

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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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SUMMARY

Introduction

Glacial meltwater stream



Glaciolacustrine sediments

THE END

a2.3gq3 glacial and quaternary geology lecture 4 glaciofluvial and glaciolacustrine deposits

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