Techniques in Glacial Sedimentology Flashcards
Roundness
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
Why are subglacial class rounded?
They are picked up and moved underneath the glacier and so undergo abrasion.
What is till fabric analysis?
The investigation of the orientation and dip of clasts (how the stones are oriented in the till).
What is till fabric analysis used to establish?
Ice direction and till genesis (how the till is formed)
Data Collection and analysis
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.
Different ways of interpreting/ presenting
2D analysis of till fabric
Rosa diagram
2D histogram
Different ways of interpreting/ presenting
3D analysis of till fabric
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
2D Use of Till Fabric
- 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
Jeffrey 1922 - why do clasts rotate in a flowing medium?
– 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)
Supraglacial
Orientated with ice direction?
No
Supraglacial
Strength of orientation
Low
Subglacial melt-out
Orientated with ice direction?
No
Subglacial lodgement
Orientated with ice direction?
Yes
Subglacial deformation
Orientated with ice direction?
Yes
Subglacial melt-out
Strength of orientation
Low
Subglacial lodgement
Strength of orientation
Low
Subglacial deformation
Orientated with ice direction?
Yes
Subglacial deformation
Strength of orientation
Medium to high
Flow till
Orientated with ice direction?
Slope
Flow till
Strength of orientation
Medium to high
Subaqueous proximal till
Orientated with ice direction?
Slope
Subaqueous proximal till
Strength of orientation
Medium to high
Subaqueous distal till
Orientated with ice direction?
No
Subaqueous distal till
Orientated with ice direction?
Low
Dowdeswell and Sharp (1986), pioneered the use of till fabric to interpret till genesis
How?
- Used eigen values from Mark (1973)
* Calculates main clustering around three major vectors; V1, V2, V3 - initialised vectors; S1, S2, S3.
Deformation till
Initial research showed ‘deformed’ lodgement till was less oriented than ‘undeformed’ lodgement till
The most deformed were the least well oriented
Two models for clast behaviour in deforming layer
Active rotation
All moving and when they stop they’re all in different directions
They also knock each other
Two models for clast behaviour in deforming layer
Passive strain marker
They’re relatively stationary with material moving around them
Increasing deformation = more orientated
Factors that influence continuous rotation (weak fabric)
Simple shear
Interaction between inclusions
A higher inclusion concentration
Factors acting for the stable clasts (strong fabric)
Thin shear zone, inclusion/matrix
Slippage
Elongated clasts
Pre-existing matrix fabric
Melt out till fabrics
- 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
Subaqueous tills
- Distal Ice Rafted Debris(IRD) deposits ‘drop stones’- very weak orientation
- Proximal sediment gravity flows depend on water content and slope
Eigen values and Till Genesis
- 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
Micromorphology
- Investigation of ‘thin sections’
- Microstructures and microfabric
- Ideas from originally from soil science (Brewer, 1976), adapted by van der Meer (1983) for till interpretation
Micromorphology
Sample collection
Sample preparation
- 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
Two Approaches
• 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
Microfabric
2D plotted on a rose diagram strength of orientation is calculated Tukey Test – chi- squared (Hart et al, 2004) Curray method – Carr (2002)
What is sediment fabric?
A description of the 3D disposition of the individual grains and pebbles which make up a deposit.
What two ways can sediment fabric be measured?
- By reference to the orientation of the sedimentary particles.
- By analysis of the sedimentary particles dip.
In order to describe the orientation of a clast, two values must be specified:
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).
