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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
Relating tills and diamictons
All tills are diamictons but not all diamictons are tills
26
How does till form?
Lodgement Deformation Ploughing Melt-out N.B. Most tills are hybrids with complex interactions between these processes
27
Subtraction till =
Encompasses all processes that may result in the production of till through time
28
Lodgement =
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
29
Lodgement in soft beds
Clast ploughs through substratum | Once ploughed sediment provides sufficient resistance to retard forward movement = lodgement
30
Preferential lodgement
1. Of large (>10cm) clasts? 2. When thicker > deforming layer + clast log jams + boulder pavements = bridge between ice and more rigid substratum
31
How do you transition from lodgement till to deformation till?
Occurs as you thicken sediment Controlled by water trapped in layer of till More saturated = more able to deform
32
Melt-out =
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)
33
Melt-out preservation potential
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 ```
34
Effect of foliation on melt-out preservation
Horizontal foliation = foliation preserved Dipping foliation = foliation dip modified **diagram**
35
Subglacial mass foliage (cavity fill) =
``` common where ice flows over irregular bedrock - deposition due to gravity - slurry - fall - flow ... in subglacial cavities ```
36
Subglacial deformation =
When applied shear stress > material yield strength = sediments form mobile deforming layer beneath glaciers flowing over SOFT substrates
37
What is the deforming layer in subglacial deformation made of?
Pre-existing sediment Soft bedrock Sediment deposited in syn-sediment context then sheared "MOSAIC"
38
Continuum of glacitectonite
From lightly deformed parent material --> heavily sheared deposits
39
Type A glacitectonite
Penetrative Original structures truncated by glacitectonic elements (banded/laminated)
40
Type B glacitectonite
Non-penetrative Small cumulative strains Preserve pre-deformational structures
41
Subglacial traction till =
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
Subglacial tills =
hybrids; lodgement/deformation/melt-out occur across glacier bed at various spatial and temporal scales
43
Melt-out till =
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
Melt-out till at Matanuska Glacier, Alaska layers
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
Is sediment in hollows affected by stress?
No = preserved
46
Communition =
Slow break down and mixing of one material into another "commented gradation"
47
Subglacial till mosaic horizontal info
**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
Glacifluvial system controls
Topographic setting Buried ice Total sediment in transport
49
Buried ice effect on glacifluvial system
Melts = smooth, flat braided landscape --> chaotic environment i.e. kame and kettle
50
What determines the total sediment in transport?
1) DISCHARGE | 2) SEDIMENT SUPPLY (cut and fill)
51
Typical landform-sediment assemblages
SANDUR KAME AND KETTLE TOPOGRAPHY ESKERS
52
Sandur =
braided outwash plain
53
Esker =
sinuous looking ridge close to margin/sub-margin
54
Coarse/fine sediment and where its found
Coarse = proximal = cut and fill/scour = channelise landscape Fine = distal = low energy channels and deposition dominates
55
Bedform definition
Grain size and flow velocity give rise to form, wavelength and height of bedform **diagram**
56
Hyperconcentrated flow =
complete mixture of sediment in suspension from boulders to clay, en mass laminar flow
57
How do bedforms form?
1. Suspended load, hyper concentrated flows, v high discharge 2. Bedload (sliding, rolling, saltating particles)
58
Froude no. >1
= supercritical flow (fast and shallow)
59
Froude no. <1 =
Subcritical flow (deep and slow)
60
Higher energy bedforms are...
Closer to ice
61
Main types of bedform
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
Nature of subaqueous glacial sediments dictated by:
1. Water density and stratification 2. Bathymetry 3. Density of inflowing meltwater vs absent lake/seawater
63
Stratification occurs due to
T/salinity
64
Bathymetry =
glacier/water interface Indicates whether glacier fed/ice contact
65
Position of inflow in water common due to...
Relative densities Greater difference = less mixing = PLUME
66
Overflow
Most common in glacimarine Low density plume Warm meltwater Low levels debris
67
Interflow
Glacimarine Similar densities
68
Underflow
High density plume (due to low T) High concentration suspended sediment Sinks = turbidity current
69
Ice berg drop, dump and grounding structures e.g. ice berg dump
Ice berg tips, sediment on top falls and creates cone on sea floor
70
Deltas =
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
Delta graded sequence
Fluvial gravels (topsets) Dipping sand/gravel (foresets) Merge distally Finer grained (bottomsets)
72
Deltas; water level =
Fluvial aggradation
73
Deltas; delta front =
Progradation
74
Grounding line fans =
aka subaqueous outwash fans = fan shaped depocentres where sub and englacial streams debouch subaqueously
75
Debouch =
Emerge from confined space into wide, open area
76
Processes in grounding line fans
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
Proximal sediments in grounding line fans
Coarse gravelly subaqueous outwash | Gravelly mass flows
78
Distal sediments in grounding line fans
Settling out of suspension = laminar stratified silts/clays Proximal cyclopsams and distal cyclopels Bioturbated massive mud Glacilacustrine varves
79
Cyclopsams/cyclopels =
couplets of graded sediments related to tide
80
Grounding line fans vs turbidity currents
Difficult to distinguish
81
qSubaqueous and subaerial mass movement =
``` Fall Slide Slump Debris flow Turbidity currents ``` Important agents of sediment transfer
82
Matrix cohesion effects
Strongly influences resulting flow behaviour/deposits
83
Debris flows vs turbidity currents
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
Creep =
Slow inter granular frictional sliding with quasi-static grain contacts
85
Slide =
Transiational/rotational; coherent mass with minor internal deformation
86
Slump =
Coherent mass with considerable internal deformation
87
Flow (with plastic behaviour) =
Remoulded mass | Non-turbulent bus possibly with transient large scale turbulent churning
88
Flow (with fluid behaviour) =
Fully turbulent
89
Fall =
Solitary grains/loose grain assemblages

Glaciers and Glaciation (29 decks)

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