A2.3GQ3 Glacial and Quaternary Geology
LECTURE 4
GLACIOFLUVIAL AND
GLACIOLACUSTRINE DEPOSITS
SUMMARY
 Introduction
 Glacial meltwater streams
 Morphology of glaciofluvial deposits
 Sedimentology of glaciofluvial
deposits
 Glaciolacustrine sediments
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Introduction
 The proglacial area receives sediment by
several groups of processes


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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 icerafting.
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Glacial meltwater streams
 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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Markarfljot
Iceland
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Photo: J.W.Merritt
 Factors leading to braiding:

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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
 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.
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Glaciofluvial complex, Eokuk
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Kaimiform deposits, Lake o’Laws, Nova Scotia
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Treig delta complex near Fersit
 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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Eskers
Breidamerkurjökull
Iceland
Photo: J.W.Merritt
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Eskers
Breidamerkurjökull
Iceland
Photo: M.A.Paul
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Carstairs Esker
Lanarkshire, Scotland
BGS 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).
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Skeiderarsandur, Iceland
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Sedimentology of glaciofluvial deposits
 Despite their wide range of morphologies,
these deposits share several characteristic
features:
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rapid variation of facies;
presence of sandy-muddy matrix, leading to matrix
supported gravels in extreme cases;
sheet-like gravel deposits interbedded with sandmud sheets, due to waning from high peak flows.
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Breidamerkursandur
Iceland
Photo: 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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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
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Trollheim
Scott
Donjek
Platte
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 Collectively they represent:
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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 glaciofluvial 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
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Glaciolacustrine Sediments
 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 Bay
Canadian 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.
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Ice marginal delta, Cape Breton
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Ice-marginal subaqueous
sediments
Achnasheen
Photo 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 rhythmite
Sweden
Photo: M.A.Paul
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Glacilacustrine rhythmite
Sweden
Photo: M.A.Paul59
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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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Jokulsarlòn
Breidamerkurjökull
Photo: 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
 Morphology of glaciofluvial deposits
 Sedimentology of glaciofluvial
deposits
 Glaciolacustrine sediments
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THE END
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