This seminar by was part of the University of Leeds, School of Geography seminar series. The audience was a mix of human and non-glacial geographers as well as a few specialists in glaciology, rheology and microstructural work.
Please feel free to email any comments.
The micromorphological slides shown would unfortunately take up too much memory space to present here at the present time at a resolution that would do them justice.
The broad structure of the seminar is...
A Snow patch builds up and its internal pressure converts it to ice. This moves under its own mass. Freeze thaw and plucking lead to sediment being
incorporated into the glacier. Eventually this reaches the bed and leads to gouging of the sediment into the material below, and the clasts break up to form till if it not evacuated by meltwater. To this is added reworked preglacial sediment from the valley floor.
Till deformation?
Glaciers were said to move by internal creep and basal sliding (comprised of creep of the ice around large obstacles and regelation around the smaller). However, Boulton and Jones (1979) suggested bed deformation could account for a significant proportion of glacial speed, the stress of the glacier, and the meltwater it produced, causing the till beneath to deform allowing faster movement than otherwise would be possible.
Models of such a process have been developed from soil deformation equations based on a combined Mohr-Coulomb / Viscous fluid approach, whereby sediments have a yield strength passed which they deform faster. While these aren't the only models used they show the principles involved...
Strain rate = (stress over a yield strength) x (a pore fluid pressure representation)
where the yield strength is determined by the cohesiveness of the sediment plus its internal friction angle multiplied by a pore fluid pressure representation such that increased pore fluid pressure leads to a lower strength.
Thus, we can see that if the pressure of fluid in the pores rises, the ease of deformation increases. That is, the movement and quantity of fluid in the till is critical to understanding how the dynamics of glacier above is effected.
Large scale variation in till deformation.
This is determined by the melt rate and evacuation rate - calculations for valley glaciers have shown that the water is too great to be evacuated by seepage and a subglacial hydrology develops. Plainly the changing response in this system will have a large effect on the water pressure in the till, for example low pressure tunnel valley systems that don't close up rapidly enough (ie. ice flow is less than change in flow rate) may draw fluid from surrounding till making the interfluves stronger.
All these models presume a warm bed of homogeneous particles, saturated or near saturation with no irregularities. Plainly in natural systems this is asking too much: meteorological and terrain changes cause glaciers to vary in base temperature, basal shear stress and fluid pressure, all of which change the till underneath and, therefore, change its response to stress, the way it deforms.
If we look at till it has a microstructural variation which often appears as well on the macroscale. This heterogeneity is caused by the variations in conditions outlined above, and therefore reflects on them. However, they also go on to effect the way the till moves, that is, they feedback to affect further variation. If you happen to believe in cause and effect, they fall like a shadow somewhere between the two and can be interpreted from either angle.
Therefore I intend to examine microstructures over the next 1/2 hour first from the point of view of this prickly pair; their causes and how we can use them to say something about the environment they were formed in, and secondly from the point of view of their effects - how they change those environments when they are formed in.
If we can determine the causes of microstructures we can say something about the glacial processes acting in the subglacial environment in any area. We can estimate causes by comparison with structures formed in other environments and by lab tests at near subglacial conditions. Not only can we say something about the processes, but if repeated processes do not wipe clean the till, we can see the history of the area in much the same manner as hard rock geologists analyse deformation events. A few case studies will reveal the process...
Case study one - the Criccieth boulder.
This work is presented in more detail in the online poster
Microstructures associated with clast
lodgement.
At Criccieth on the Lleyn Peninsular, North Wales there is a buried 'drumlin', into the side of which a large boulder has been lodged by ice flow from the Westwards. Immediately to the East of the clast is an area ~2m long and 50cm thick of mixed till and sand lenses which overlies a more continuous resistant layer. Thin sections from the mixed sediment layer reveal that it is composed of three sets of sediments.
The boundaries between the different bodies were undulatory and only showed shear at the tops of the topographic highs. This suggests deposition was by flow. This is backed up by the mixing of the sands and silts where the latter overruns the former. The sands are aligned horizontally and the clean pores suggest slow flowing water washed them. The stratigraphic position of the quartz, particularly its absence in the sand, suggests that it was winnowed out of the sand material.
I would like to suggest that this material was deposited by meltout from the bottom of the ice, with its restriction to the area up-ice of the boulder on the macroscale being due it representing the process of clast lodgement, thus,
Thus micromelanges in this situation represent, generally, a wet bedded and warm ice lodgement environment. It has a low effective pressure so the till deforms but is warm therefore doesn't represent an ice marginal environment. The till is stiff enough to lodge but still deforms under pressure.
Case study two - Yorkshire pebbles.
The next example is from the type-site for the last glaciation at Dimlington in Holderness. Here two glacial sequences are separated by a silt bed. Thin sections were taken from the top of the lower of the two till sequences at the base of the laminar silts. The microstructure was found to be graded away from the bed of the silts suggesting a glaciofluvial origin. Close to the base of the silts the microstructure was one of 'till pebbles', reworked spheres of local till with a matrix of local till. These disappeared within ~2 cm of the boundary and the matrix graded into glacial microstructural shears. Till pebbles are a reasonably common microstructure, and it has been suggested by Van Der Meer in Holland that they represent shear breakup of the material. However, this is the first time that such structures can be tied to a definite set of conditions, that is,
Thus, despite the fact that the fluid flow of till formed the microstructures in both cases the glacial environments are very different. In pebblized regions it is possible the till is too weak to support clasts for example.
Drainage
If we deform a homogeneous sediment of irregular particles the strain in the sediment, that is the amount of deformation, may be of two sorts - some will be pervasive ; grains will move over each other and to a certain extent be compressed together if the intervening fluid can escape, and some movement will be in shear zones ( there is some evidence that pervasive movement is many small shear zones). The amount of each will depend on the water content of the sediment - the more water the less discrete shear.
If we measure the amount of water flowing through such a sediment during deformation by pushing water in the top we see three distinct stages...
The more densely packed the grains to start off with the more dilation it takes to form shear layers where all the grains can smoothly move over one another. In underconsolidated till, like that showing pebblization there is almost no need for the drawing of water into dilated zones as grains can easily slip passed each other. In very consolidated material there is equally no need for such events as the grains are already aligned. The likelihood of dilatency is thus controlled by the grain packing and the angle of the long axis of grains and the likelihood of such dilatency collapsing with further shear is determined by the flatness of the grains.
Plainly, if the grains are rounded dilatency will be the end state and if the grains are platy shear alignment will be the end state. Both states have been seen on a macro and micro scale in subglacial sediments.
So what? you may ask. The cannier amongst you will have noticed that the amount of fluid involved at the end of deformation is only that stored or expelled from the shear zones. The amount of actual fluid passing through the sediment during to deformation is only important if the flow velocity is greater than the yield strength of the sediment and matrix grains are intrained into it.
The important thing about these flows during deformation is that the ease of movement of fluid into and out of shear zones controls their formation - ie the permeability of the sediment next to the shear zones controls their formation. And shear zones, once caused, effect the further deformation of the sediment because they are lines of weakness and change the local permeability.
Permeability in such sediments is determined by two factors that are changed by the shear process. Porosity and tortuosity. Porosity is the size of the gaps between grains and tortuosity is a measure of the length of path the pore fluid has to flow along to escape the sediment. Tortuosity is the ratio of length of flow in reality over the length as the crow flies. If we have an hollow space the path length is the space's side length, but if we stuff the space full of straws we can see that the path length is much larger.
Dilation increases the porosity along the shear zone and decreases the tortuosity, shear collapse decreases the tortuosity along the shear zone but massively increases it perpendicular to the shear zone. This means that once shear zones are set up, whether they are dilated or aligned the permeability along the shear zone will be much larger than the permeability perpendicular to it. Experimental data suggests tortuosity has a larger effect than porosity as in aligned zones permeability is increases even though porosity is very low. It seems likely that porosity mainly controls the flow through the action of the smallest pores as bottle necks and larger pores are irrelevant unlike the situation presumed for Darcian flow.
The effect of these zones is highly significant - Murray found hydraulic conductivity to increase by 100-300x for dilated material and Arch found changes of 400x for tortuosities of x20 in collapsed areas. As Murray pointed out this has significant effects - if the glacier is on an impermeable bedrock the flow to the front through the till is enhanced and less till thickness deforms, the shear strength local to the shear zones increases and any pervasive movement slows - the shear zone will slow as the local water pressure drops. If the drainage is into the bed the effect will depend of whether the shear is dilated - in which case drainage will be the same or greater with a proportion of the flow going horizontally, or if the shear is aligned, in which case the drainage may actually be reduced and fluid pressure build up above the shear zone causing faster flow.
I hope at some stage to look at the though flow of fluid through a deforming sediment to gain a quantifiable insight into these processes in real sediment - at the moment much of the work has been conducted by models or qualitatively on sediments for which the dynamic processes were not understood. I have developed a program which sends 'worms' through till thin sections to calculate the tortuosity and I hope to use real till deformed in the lab under controlled conditions and known permeability changes to further investigate the processes quantitatively. Much of the work so far has been completed on silts and clays and there is no understanding of the workings of the processes in diamicts where several grain sizes exist. It is not known, for example, the effect larger grains have on horizontal shear zones or vertical dilation development.
Clast support
Shear tests for the material strength of till by Kamb have lead to the acknowledgement that till rheologies in the lab are too soft to support the glacial basal shear stresses found. This has lead some people to suggest that clast - matrix and clast - clast interactions are controlling the till flow, in a similar manner to the importance which people are starting to attribute to sediment in rivers. Such an effect may well be due to the interference of the processes I have just discussed.
Graph of till strength vs. clast stress
The dropout velocity of clasts in sub-glacial silts is too great. While autosuspension may be partly responsible this graph doesn't take into account the fact that fast glaciers may produce more melt softening the till, therefore this approach is not strictly justified - though we do use the effective pressure Ice Stream B in Antarctica, which is fairly soupy. Whatever, the clasts are not supportable therefore we need other mechanisms, and microstructures may go some distance to aiding this process in maintaining clasts in the till so that they can slow glacier movement.
Microshears
In the stiff till environment we saw the production of sand layers and layers of odd ordered material these are more likely to dilate and therefore provide a better drainage capacity in the till when they are mixed in by further ploughing. This should lead to stronger sediment and increased lodgement. In the final deposit clasts will be supported by this strong till.
The semi-fluid till with pebbles is less likely to form long shears and the matrix movement path is lengthened, therefore, the sediment doesn't drain as well and stays fluid. However, the pebbles still maintain the clasts in suspension.
In the former case dilation necessitates the removal of fluid from surroundings, whereas this is not so in the second as the scale is larger and matrix flow can occur. Plus, in the second case there is no chance of realignment as the particles are spheres therefore water will not be lost to anisotropic drainage.
Thus, while both situations are intimately connected with the flow processes of till, and both environments are self-sustaining, they represent very different beds which the glacier must overrun. Even more difficult is that they are can both be seen to produce the characteristic 'Boulder-clay' bimodal till which is characteristic of subglacial sediments on an outcrop scale. Thus, it is of vital importance for the reconstruction of ice masses in the past, and present where material is available, that thin section analysis is carried out. On top of this, it is vital for such studies that microstructures are tied to more definite processes of formation, and that the feedbacks on the rheology of the sediments in question is considered and investigated.
Allen, J.R.L., 1977. The possible mechanics of convolute lamination in graded sand beds. Journal of the Geological Society of London. 134, 19 - 31.
Bagnold, R.A., 1954. Experiments on a gravity - free dispersion of large solid spheres in a Newtonian fluid under shear. Proceedings of the Royal Society of London, Series A, 225, 49 - 63.
Bagnold, R.A., 1962. Auto - suspension of transported sediment: turbidity currents. Proceedings of the Royal Society of London, Series A, 265, 315 - 319.
Boulton G.S., and A.S. Jones, 1979. Stability of temperate ice caps and ice sheets resting on beds of deformable sediment. Journal of Glaciology. 24 (90), 29 - 43.
Kanb, B., 1991. Rheological non-linearity and flow instability in the deforming bed mechanism of ice stream motion. Journal of Geophysical Research. 96 (B10) 16585 - 16595.
Murray, T., 1990. Deformable glacier beds: measurement and modelling. Unpub. Ph.D. thesis, University of Wales, Aberystwyth, 321pp.
Jump to...
Introduction
What is a glacier? How does till form?Analysis
Causes
Causes shed light on the mechanisms and history of glaciodeposition
This suggests that till pebbles seen in subglacial material may well be formed under similar conditions, that is, near fluid tills moving under a very low shear stress. Such semi-fluid conditions have been suggested to account for dropped out layers of boulders known as 'clast pavements', and such till has been seen being ejected from crevasses in the later stages of movement of the cyclically rapid surge-type glaciers. Effects
I shall discuss two ways in which micro structures in till effect the subglacial environment - through effecting the drainage of sediments and aiding the suspension of clasts.
Graph of autosuspension capacities of several glaciers based on the work of Bagnold and Allen.
Pebblization
Cause and effect - a synthesis
We saw before two different subglacial environments - one in which the till was stiff (lodging boulder)) the other in which the till was fluid (till pebbles). In what way can we say that the microstructures formed by such environments effect these situations to make them stable or unstable and how is it that each looks so similar on the macroscale?Bibliography and References
Arch, J., 1988. An experimental study of deformation microstructures in soft sediments. Unpub. Ph.D. thesis, University of Wales, Aberystwyth, 387pp.