Glacial Sediment Flashcards
What is a till?
- It’s a type of sediment that is only deposited by glaciers.
- Type of diamicton (made of many different sized particles).
- Usually unstratified (no layers) and unsorted (not sorted by size due to energy)
- Usually contains far-travelled material (erratics)
What are till pinnacles?
Larger stones protect some of the tills underneath resulting in pillars
Usually has some sort of calcium carbonate component to it (can bare its own weight)
Types of Glacial Till
Supraglacial till
Sub-glacial tills
Sub glacial till types…?
Melt out till (commonly forms within cavities)
Lodgement till (“lodged” by the ice onto the substrate)
Deformation till (deformation of soft non-glacial sediments)
Supra-glacial till types…?
Supra-glacial meltout till (e.g., lateral and medial moraines)
Flow till (from flow of supraglacial material at the ice front)
Sublimation till (form only in very cold regions, not common)
What does fabric mean?
Orientation of the stones in a till
What supra glacial till?
Debris on the surface of the glacier, the ice melts out and a till remains on the land surface.
Very random in the orientation of the grains - may be slow or fast.
Generally chaotic fabric, also called ablation till.
May be affected by sediment gravity flows (then called flow till).
What are flow tills?
Wet sediment sliding down slope.
Any till that has flowed downslope.
Lodgement till
Till deposited in the direction of ice flow from a moving glacier.
Friction retardation against a hard rock bed.
Where the glacier is moving up obstacles, then the glacier is in contact with the bed.
The stones could stick through friction.
How would you identify something as a lodgement till?
Stoss and lee clasts (minor’s roche mouteneees with a braided side and a jagged side).
Strong fabric in ice direction producing striated clasts
No evidence of deformation
What is ploughing?
Stones are getting pushed up against some kind of surface.
Pushes up the material in front of the clasts.
Makes the bed a little bit more active.
Deformation till
3 elements to deformation till
Stones are being removed at the ice till interface (an element of meltout).
As the ice moves forward, so the till moves forward (from one glacial environment to another).
You can work out the thickness of this deforming layer.
The base in the deforming layer is changing, material beneath is being deposited because it’s not moving any more. Where there is soft bed material there is potential for sediment to get deformed.
Styles of deformation
Low shear strain, vertical strain markers are overturned in the ice direction
Ice wedge casts
Faults
Shows you how much deformation has gone on
What was originally vertical has been overturned
Folds and boudinage
When there is a rheological difference between the pre-glacial sediments and the till, boudins will form
Boudinage
Extensions of beds of different competance
Folds are rare in
shear zones, except in protected low pressure zones, e.g. on the distal side of a boudin.
Constructional Deformational Sequence
Thin deforming bed / high advection
Builds up a sequence of preserved layers
Deformation increases upwards
• E.g. Gt Blakenham
Excavational Sequence
- Thick deforming bed / lower advection
- Deforming layer moves down into preglacial sediments
- Deformation similar throughout
- e.g. West Runton
What is a sediment gravity flow?
Wet sediment sliding down slope easily.
What is flow till?
Any till that has flowed downslope.
How do cavities form?
If you’re in a hard bedrock environment, the bed of the valley is going to be very irregular.
The glacier has to go up and down these obstacles.
As it does so it makes a cavity behind the obstacle.
What does the fact that within the cavity is slightly warmer material?
The material will slightly melt out, and so does the debris.
Melt out till
What is simple shear?
Much like a stack of cards if you sheer the top when the bottom stays in place, the whole rest of deck stretches.
This thin zone of deformation is called the shear zone.
Clast shape of subglacially transported tills
Sub-glacially transported tills typically have sub-rounded to rounded, striated, faceted and spherical (not elongated) clasts, with a bimodal distribution (silt/clay and coarse).
Clast shape of supra glacially transported tills
Supra glacially transported tills typically have angular, non- spherical clasts with a coarse unimodal grain-size distribution (this example from Whistler is not unimodal!)
What are melt out tills formed from?
Formed from the melting out of the debris rich basal ice layer without deformation
Debris on the surface is frozen into the ice layer
Slightly warmer in the cavity so that material will slightly melt out and so will the debris (unconsolidated material)
Where are melt out tills best preserved?
Best preserved in cavities, in hard rock environments
Flat bedrock environments are unlikely to contain melt out tills
What is effective pressure continuum?
The difference between the weight of the glacier and the water pressure in the till
Regarding effective pressure, at the rigid bed there is…
High effective pressure
Regarding effective pressure, at the deforming bed there is…
Low effective pressure
Where does subglacial deformation occur?
What is essential?
In a shear zone.
A wet sediment deforming bed is essential.
What happens during shearing?
Irregularities get stretched out and when it does it forms a lamination (line).
If you stretch (attenuate) something so much you produce boudins. when it snaps.
Once boudins form laminations form around them.
In a lodgement till, stones are being lodged against the bedrock oriented with the direction.
In the deforming layers the clasts are…
Rotating
What are the three zones in deformation till?
Homogoenous zone
Sheared zone
Overturned zone
What is a subaqueous till?
What are dropstones?
Debris in the ice of icebergs that are released as the iceberg melts into a waterbody.
