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Glaciers and Glaciation > Week 9: Glacial Sediments > Flashcards

Week 9: Glacial Sediments Flashcards

1
Q

Components of the debris cascade

A

SOURCE

TRANSPORT

DEPOSITION

diagram

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2
Q

Primary sources

A

Subglacial
Extraglacial
Aeolian

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3
Q

Secondary sources

A

Pre-existing

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4
Q

What does transport type depend on?

A

Clast morphology

Size

Fabric

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5
Q

What does deposition type depend on?

A

Bedding

Structure

Grain size

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6
Q

Fabric =

A

Relationship between clast orientation in response to direct imparting of strain/stress by ice

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7
Q

Proglacial transport path =

A

In front of ice

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8
Q

Paraglacial transport path =

A

Valley side or system/floor interacting with glacial processes

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9
Q

Subglacial transport path =

A

Transports actively, reworked, often deformed

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10
Q

Different transport paths give different signals on clast e.g.

A

Striae/faceting/grounding

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11
Q

High level transport =

A

DEBRIS NOT IN CONTACT WITH BED

Above ELA debris buried beneath successive layers of snow
Below ELA debris stays on surface until reaches snout
= minimal alteration

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12
Q

Low level transport =

A

BED CONTACT

Material from subglacial erosion/crevasse fall/downward ice flow
Debris deposited subglacial or to glacier surface at snout brought by compression

= basal debris is modified

diagram

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13
Q

How can debris be brought to surface at snout by compression?

A

At glacier margine = switch from extensional to compressional states = elevates sediment to sub/englacial

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14
Q

Primary glacigenic deposits (tills)

A

Glacitectonite

Subglacial traction till

Subglacial melt out till

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15
Q

Glacifluvial deposits

A

Plane bed deposits

Ripple cross-laminated facies

Cross-bedded facies

Gravel sheets

Hyperconcentrated flow

Silt and mud drapes

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16
Q

Gravitational mass movement (subaerial and subaqueous)

A

Scree/debris fall deposits

Slide and slump

Debris flow deposits

Turbidites

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17
Q

Suspension settling and ice bergs

A

Cyclopsams/cyclopels

Varves

Iceberg rafted debris/diamicton

Dropstone diamicton

Under-melt diamicton

Iceberg turbate

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18
Q

Is subglacial often:

a) primary
b) secondary

sediment

A

Primary

- others get reworked and follow second pathway

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19
Q

Glacitectonite =

A

Originally something else, deformed but recognisable (unlike till)

I.E. HAS SIGNAL OF PRE-EXISTING MATERIAL WITHIN

= rock/sediment deformed by subglacial shearing (deformation) but retains some of structural characteristics of parent material which may consist of ig/met/sed rock or unlithified sediments (Benn and Evans 1996)

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20
Q

Sedimentary signal reflects…

A

The energy of the system

- action of water sorts the material

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21
Q

“Mass flow deposits spectrum”

A

Material that has flowed (poor sorting) –> flows with high water content –> flows themselves –> turbidity flows

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22
Q

Diamict =

A

mixture of different grain size

e.g. cobbles/gravel/clay/silt

Subglacial tills tend to be diamicts

N.B. difficult to differentiate subglacial till from debris flow

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23
Q

Till =

A

sediment deposited DIRECTLY from glacier ice which has not undergone significant disaggregation but may have undergone glacially induced deformation

PRIMARY GLACIAL DEPOSIT

“…is a sediment and is perhaps more variable than any sediment known by a single name” (Flint 1957)

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24
Q

What sediment is deposited indirectly from glacier ice?

A

Sediment settled through water/mass movement

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25
Q

Relating tills and diamictons

A

All tills are diamictons but not all diamictons are tills

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26
Q

How does till form?

A

Lodgement

Deformation

Ploughing

Melt-out

N.B. Most tills are hybrids with complex interactions between these processes

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27
Q

Subtraction till =

A

Encompasses all processes that may result in the production of till through time

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28
Q

Lodgement =

A

plastering of debris from base of sliding glacier to the rigid/semi-rigid bed

Frictional drag between clast and bed > shear stress due to moving ice = clast lodged and ice flows around

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29
Q

Lodgement in soft beds

A

Clast ploughs through substratum

Once ploughed sediment provides sufficient resistance to retard forward movement = lodgement

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30
Q

Preferential lodgement

A
  1. Of large (>10cm) clasts?
  2. When thicker > deforming layer + clast log jams + boulder pavements
    = bridge between ice and more rigid substratum
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31
Q

How do you transition from lodgement till to deformation till?

A

Occurs as you thicken sediment
Controlled by water trapped in layer of till
More saturated = more able to deform

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32
Q

Melt-out =

A

passive subglacial release of debris by melting of glacier ice

= ‘thaw consolidation’/volume reduction due to debris content of ice (high = slight vol reduction = delicate englacial structures preserved)

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33
Q

Melt-out preservation potential

A

Balance between meltwater production and drainage away from site

High pore P in sediment pile/slurries = low preservation potential

If sediment structures complete suggests PASSIVELY let out

Secondary processes:
- slope processes
- stream reworking sediment
- winds reworking sediment
= lower
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34
Q

Effect of foliation on melt-out preservation

A

Horizontal foliation = foliation preserved

Dipping foliation = foliation dip modified

diagram

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35
Q

Subglacial mass foliage (cavity fill) =

A 
common where ice flows over irregular bedrock - deposition due to gravity
- slurry
- fall
- flow
... in subglacial cavities
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36
Q

Subglacial deformation =

A

When applied shear stress > material yield strength = sediments form mobile deforming layer beneath glaciers flowing over SOFT substrates

37
Q

What is the deforming layer in subglacial deformation made of?

A

Pre-existing sediment
Soft bedrock
Sediment deposited in syn-sediment context then sheared

“MOSAIC”

38
Q

Continuum of glacitectonite

A

From lightly deformed parent material –> heavily sheared deposits

39
Q

Type A glacitectonite

A

Penetrative

Original structures truncated by glacitectonic elements (banded/laminated)

40
Q

Type B glacitectonite

A

Non-penetrative
Small cumulative strains
Preserve pre-deformational structures

41
Q

Subglacial traction till =

A

sediment deposited by glacier sole sliding over +/ deforming its bed, having been released directly from ice by P melting +/ liberated from substrate then disaggregated and completely/largely homogenised by shearing (Evans et al 2006)

Completely homogenised by subglacial shear = uniform till appearance

42
Q

Subglacial tills =

A

hybrids; lodgement/deformation/melt-out occur across glacier bed at various spatial and temporal scales

43
Q

Melt-out till =

A

Sediment released by melting/sublimation of stagnant/slowly moving debris-rich glacier ice and directly deposited without subsequent transport/deformation (Benn and Evans 1998)

44
Q

Melt-out till at Matanuska Glacier, Alaska layers

A

Lawson 1981

a) Structureless pebbly, sandy silt
b) Discontinuous laminae, stratified lenses and pods of texturally distinct sediment in massive pebbly silt

c) Layers of texturally-, compositionally- or colour-contrasted sediment
Layers may drape clasts

45
Q

Is sediment in hollows affected by stress?

A

No = preserved

46
Q

Communition =

A

Slow break down and mixing of one material into another

“commented gradation”

47
Q

Subglacial till mosaic horizontal info

A

DIAGRAM
Evans et al 2006

Area of pre-existing lake sediments

Area of till sliding (striated/polished bedrock)

Lee-side cavity

Descending hydro fracture tills

Melt out till hollow re-coupled with bed

Overridden glacilacustrine sediments

Ascending hydro fracture tills

48
Q

Glacifluvial system controls

A

Topographic setting

Buried ice

Total sediment in transport

49
Q

Buried ice effect on glacifluvial system

A

Melts = smooth, flat braided landscape –> chaotic environment i.e. kame and kettle

50
Q

What determines the total sediment in transport?

A

1) DISCHARGE

2) SEDIMENT SUPPLY (cut and fill)

51
Q

Typical landform-sediment assemblages

A

SANDUR

KAME AND KETTLE TOPOGRAPHY

ESKERS

52
Q

Sandur =

A

braided outwash plain

53
Q

Esker =

A

sinuous looking ridge close to margin/sub-margin

54
Q

Coarse/fine sediment and where its found

A

Coarse = proximal = cut and fill/scour = channelise landscape

Fine = distal = low energy channels and deposition dominates

55
Q

Bedform definition

A

Grain size and flow velocity give rise to form, wavelength and height of bedform

diagram

56
Q

Hyperconcentrated flow =

A

complete mixture of sediment in suspension from boulders to clay, en mass laminar flow

57
Q

How do bedforms form?

A
  1. Suspended load, hyper concentrated flows, v high discharge
  2. Bedload (sliding, rolling, saltating particles)
58
Q

Froude no. >1

A

= supercritical flow (fast and shallow)

59
Q

Froude no. <1 =

A

Subcritical flow (deep and slow)

60
Q

Higher energy bedforms are…

A

Closer to ice

61
Q

Main types of bedform

A

Ripple cross-lamination (current ripples)

Planar and trough cross cross bedded gravels/sands

Gravel sheets

Hyperconcentrated flow (debris slurry)

Silt and mud drapes (waning discharge)

62
Q

Nature of subaqueous glacial sediments dictated by:

A
  1. Water density and stratification
  2. Bathymetry
  3. Density of inflowing meltwater vs absent lake/seawater
63
Q

Stratification occurs due to

A

T/salinity

64
Q

Bathymetry =

A

glacier/water interface

Indicates whether glacier fed/ice contact

65
Q

Position of inflow in water common due to…

A

Relative densities

Greater difference = less mixing = PLUME

66
Q

Overflow

A

Most common in glacimarine

Low density plume

Warm meltwater

Low levels debris

67
Q

Interflow

A

Glacimarine

Similar densities

68
Q

Underflow

A

High density plume (due to low T)

High concentration suspended sediment

Sinks = turbidity current

69
Q

Ice berg drop, dump and grounding structures e.g. ice berg dump

A

Ice berg tips, sediment on top falls and creates cone on sea floor

70
Q

Deltas =

A

Sediment masses built into standing water by fluvial transported debris delivered to shore

Tool for when ice terminated in water body and fixing former SL because always trades into UPPER body of water

71
Q

Delta graded sequence

A

Fluvial gravels (topsets)

Dipping sand/gravel (foresets)

Merge distally

Finer grained (bottomsets)

72
Q

Deltas; water level =

A

Fluvial aggradation

73
Q

Deltas; delta front =

A

Progradation

74
Q

Grounding line fans =

A

aka subaqueous outwash fans

= fan shaped depocentres where sub and englacial streams debouch subaqueously

75
Q

Debouch =

A

Emerge from confined space into wide, open area

76
Q

Processes in grounding line fans

A

Meltwater processes/facies dominate

Mass flow also important for remobilisation of unstable ice proximal sediments

Streams exiting grounding line immediately dump bedload

Sediment gravity flow = underflow = turbidity current (starts to sort)

Also rising turbulent plume:

1) settling layer/suspension settling
2) turbid overflow plume

Decrease in flow competence/sedimentation rate away from inflow points

77
Q

Proximal sediments in grounding line fans

A

Coarse gravelly subaqueous outwash

Gravelly mass flows

78
Q

Distal sediments in grounding line fans

A

Settling out of suspension = laminar stratified silts/clays
Proximal cyclopsams and distal cyclopels
Bioturbated massive mud
Glacilacustrine varves

79
Q

Cyclopsams/cyclopels =

A

couplets of graded sediments related to tide

80
Q

Grounding line fans vs turbidity currents

A

Difficult to distinguish

81
Q

qSubaqueous and subaerial mass movement =

A 
Fall
Slide
Slump
Debris flow
Turbidity currents

Important agents of sediment transfer

82
Q

Matrix cohesion effects

A

Strongly influences resulting flow behaviour/deposits

83
Q

Debris flows vs turbidity currents

A

Debris flows = high concentration plastic slurries

Turbidity currents = rapidly moving turbulent underflows

Transitions between flow types due to changes in water content/strength during flow

84
Q

Creep =

A

Slow inter granular frictional sliding with quasi-static grain contacts

85
Q

Slide =

A

Transiational/rotational; coherent mass with minor internal deformation

86
Q

Slump =

A

Coherent mass with considerable internal deformation

87
Q

Flow (with plastic behaviour) =

A

Remoulded mass

Non-turbulent bus possibly with transient large scale turbulent churning

88
Q

Flow (with fluid behaviour) =

A

Fully turbulent

89
Q

Fall =

A

Solitary grains/loose grain assemblages

Glaciers and Glaciation (29 decks)

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