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HDSO Glaciers > Techniques in Glacial Sedimentology > Flashcards

Techniques in Glacial Sedimentology Flashcards

1
Q

Roundness

A 
RA - % clasts very angular or angular
RWR - % clasts rounded or well rounded
C40 index -% clasts c/a axis less than 0.4	
Can compare with standards or 
Measure the axis of the clasts
A > B > C
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2
Q

Why are subglacial class rounded?

A

They are picked up and moved underneath the glacier and so undergo abrasion.

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3
Q

What is till fabric analysis?

A

The investigation of the orientation and dip of clasts (how the stones are oriented in the till).

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4
Q

What is till fabric analysis used to establish?

A

Ice direction and till genesis (how the till is formed)

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5
Q

Data Collection and analysis

A

Collect orientation and dip of at least 25 samples, but more normally 50.
Avoid surface
Try and take more than set of 25 samples at each site.

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6
Q

Different ways of interpreting/ presenting

2D analysis of till fabric

A

Rosa diagram

2D histogram

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7
Q

Different ways of interpreting/ presenting

3D analysis of till fabric

A

Polar diagram
Dip 0o outside
90o inside

Equal area Stereonet
dip - 0o outside
90o inside
Contoured by Kamb’s method
Higher the contour the more clustered they are
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8
Q

2D Use of Till Fabric

A
  • Until 1980’s most workers used till fabric analyses to reconstruct palaeo ice flow
  • Need to be careful, only certain types of till genesis reflect ice flow
  • Rose diagram is sufficient
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9
Q

Jeffrey 1922 - why do clasts rotate in a flowing medium?

A

– where the velocity is low the particals will orientate parallel with flow
– where the velocity is high they will move to a transverse position (perpendicular with flow)

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10
Q

Supraglacial

Orientated with ice direction?

A

No

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11
Q

Supraglacial

Strength of orientation

A

Low

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12
Q

Subglacial melt-out

Orientated with ice direction?

A

No

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13
Q

Subglacial lodgement

Orientated with ice direction?

A

Yes

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14
Q

Subglacial deformation

Orientated with ice direction?

A

Yes

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15
Q

Subglacial melt-out

Strength of orientation

A

Low

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16
Q

Subglacial lodgement

Strength of orientation

A

Low

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17
Q

Subglacial deformation

Orientated with ice direction?

A

Yes

18
Q

Subglacial deformation

Strength of orientation

A

Medium to high

19
Q

Flow till

Orientated with ice direction?

A

Slope

20
Q

Flow till

Strength of orientation

A

Medium to high

21
Q

Subaqueous proximal till

Orientated with ice direction?

A

Slope

22
Q

Subaqueous proximal till

Strength of orientation

A

Medium to high

23
Q

Subaqueous distal till

Orientated with ice direction?

A

No

24
Q

Subaqueous distal till

Orientated with ice direction?

A

Low

25
Q

Dowdeswell and Sharp (1986), pioneered the use of till fabric to interpret till genesis

How?

A
  • Used eigen values from Mark (1973)

* Calculates main clustering around three major vectors; V1, V2, V3 - initialised vectors; S1, S2, S3.

26
Q

Deformation till

A

Initial research showed ‘deformed’ lodgement till was less oriented than ‘undeformed’ lodgement till
The most deformed were the least well oriented

27
Q

Two models for clast behaviour in deforming layer

Active rotation

A

All moving and when they stop they’re all in different directions
They also knock each other

28
Q

Two models for clast behaviour in deforming layer

Passive strain marker

A

They’re relatively stationary with material moving around them
Increasing deformation = more orientated

29
Q

Factors that influence continuous rotation (weak fabric)

A

Simple shear
Interaction between inclusions
A higher inclusion concentration

30
Q

Factors acting for the stable clasts (strong fabric)

A

Thin shear zone, inclusion/matrix
Slippage
Elongated clasts
Pre-existing matrix fabric

31
Q

Melt out till fabrics

A
  • Although Dowdeswell and Sharp (1986) originally suggested a distinct fabric based on Lawson’s (1979) research,
  • …subsequent research on the debris- rich basal ice (Hart, 1998), has shown a similar fabric pattern to that found from deformation till
32
Q

Subaqueous tills

A
  • Distal Ice Rafted Debris(IRD) deposits ‘drop stones’- very weak orientation
  • Proximal sediment gravity flows depend on water content and slope
33
Q

Eigen values and Till Genesis

A
  • Many people have used D&S original work uncritically
  • Bennett et al (1999) suggested don’t use at all!
  • Useful tool, but must be used with other evidence
34
Q

Micromorphology

A
  • Investigation of ‘thin sections’
  • Microstructures and microfabric
  • Ideas from originally from soil science (Brewer, 1976), adapted by van der Meer (1983) for till interpretation
35
Q

Micromorphology
Sample collection

Sample preparation

A
  • Collect undisturbed samples in Kubiena tins from the field
  • Difficult and time consuming to collect
  • Samples are left to dry
  • Soaked in a bed of acetone
  • Cut into thin slices and mounted on glass slides
  • Examined under the microscope
36
Q

Two Approaches

A

• Van der Meer (1983) - soil science based approach
- Useful pioneering approach, but soil science terminology can be a problem.

• Menzies (1998, 2012) - soil science/structural geology approach

37
Q

Microfabric

A 
2D
plotted on a rose diagram
strength of orientation is calculated
Tukey Test – chi- squared (Hart et al, 2004)
Curray method – Carr (2002)
38
Q

What is sediment fabric?

A

A description of the 3D disposition of the individual grains and pebbles which make up a deposit.

39
Q

What two ways can sediment fabric be measured?

A
  1. By reference to the orientation of the sedimentary particles.
  2. By analysis of the sedimentary particles dip.
40
Q

In order to describe the orientation of a clast, two values must be specified:

A

The orientation of the longest dimension (long axis) of the clast in the horizontal plane (the trend).

The orientation of the longest dimension (long axis) of the clast in the vertical plane (the dip).

HDSO Glaciers (22 decks)

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