Direct Glacial Sedimentation

Debris deposited directly by a glacier is known as till. A till is defined as sediment deposited by glacier ice, but one which has not been disaggregated, although it may have suffered glacially induced flow either in the subglacial or supraglacial environment. It normally consists of large pebbles, cobbles or boulders, referred to generally as clasts, set within a fine-grained matrix of silt and clay. Its characteristics are, however, highly variable, as eloquently expressed in the following statement 'till is a sediment and is perhaps more variable than any sediment known by a single name' (Flint, 1957, p. 105). The definition of till is important because it excludes situations where the sediment has settled through water, such as is the case where a glacier terminates in a lake or in the sea. Some authors have used the term waterlain till for till-like sediments deposited in such environments, but it is perhaps more appropriate to use the non-genetic term diamicton. Diamicton is a non-sorted or poorly sorted unconsolidated sediment that contains a wide range of particle sizes for which no genesis is presumed; all tills are diamictons, but not all diamictons are tills (Figure 8.1). The term diamict is preferred by some authors, although technically it is a collective term for a diamicton and diamictite, the latter being a lithified diamicton.

Glacial Geology: Ice Sheets and Landforms Second Edition Matthew R. Bennett and Neil F. Glasser © 2009 John Wiley & Sons, Ltd

Figure 8.1 Close up of a subglacial till or diamict. [Photograph: M. R. Bennett]

There are four primary processes by which debris in transport within a glacier may be deposited: (i) lodgement - this occurs when the frictional resistance between a clast in transport at the base of a glacier and the glacier bed exceeds the drag imposed by the overlying ice such that the clast ceases to move; (ii) meltout - the direct release of debris by melting; (iii) sublimation - vaporisation of ice causing the direct release of debris; and (iv) subglacial deformation - this involves the assimilation of sediment into a deforming layer beneath a glacier. The process of sublimation is currently only documented from Antarctica and occurs there due to the extreme cold and aridity of this environment.

Traditionally these process distinctions are used to recognise a range of different till types on the basis of their macromorphology, namely (Table 8.1): (i) lodgement till; (ii) subglacial meltout till; (iii) deformation till; (iv) supraglacial meltout and flow till; and (v) sublimation till. However, this distinction has increasingly been challenged over the past 10 years with the development of micromorphological analysis of till. Micromorphology involves the collection of undisturbed sediment blocks from the field, which are then impregnated with resin and thin-sectioned for study under a microscope. Careful analysis of the internal architecture of these sediments suggests that the traditional classifications are no longer valid and that most tills form by a combination of processes, especially in the subglacial environment. Distinguishing a lodgement till from a deformation till for example is not possible at a microscopic scale, because both show evidence of deformation. In fact it has recently been argued that tills should be referred to as tectomicts - the product of a complex suite of tectonic rather than depositional processes. Although such terminology has yet to be widely adopted it does draw attention to the problem of distinguishing lodgement till from meltout and deformation till in the subglacial environment. To address this issue here we recognise two basic till domains: (i) subglacial tills; and (ii) supraglacial tills.

Table 8.1 Summary table of the main sedimentary characteristics of the main types of till.

Lodgement till

Subglacial meltouttill

Deformation till

Particle shape

Particle size

Particle fabric

Particle packing

Particle lithology

Structure

Clasts show characteristics typical of basal transport: rounded edges, spherical form, and striated and faceted faces. Large clasts may have a bullet shaped appearance.

The particle-size distribution is typical of basal debris transport, being either bimodal or multimodal.

Lodgement tills have strong particle fabrics in which elongated particles are aligned closely with the direction of local ice flow.

Typically dense and well consolidated sediments.

Clast lithology is dominated by local rock types.

Massive structureless sediments, with well-developed shear planes and foliations. Sheared or brecciated clasts, smudges , may be present. Boulder clusters or pavements may occur within the sediment along with evidence for ploughing of clasts.

Clasts show characteristics typical of basal transport, being rounded, spherical, striated and faceted. These characteristics are less pronounced than those of lodgement till.

The particle-size distribution istypical of basal debris transport, being either bimodal or multimodal. Sediment sorting associated with dewatering and sediment flow may be present.

Fabric may be strong in the direction of ice flow, although it may show a greater range of orientations than that typical of lodgement till.

The sediment may be well packed and consolidated, although this is usually less marked than in a lodgement till.

Clast lithology may shows an inverse superposition.

Usually massive but if it has been subject to flow it may contain folds and flow structures. Crude stratification is sometimes present. The sediment does not show evidence of shearing and overriding during formation.

Dominated by the sedimentary characteristics of the sediment that is being deformed, although basal debris may also be present.

Diverse range of particle sizes reflecting that found in the original sediment. Rafts of the original sediment may be present causing marked spatial variability.

Strong particle fabric in the direction of shear, which may not always be parallel to the ice-flow direction. High-angle clasts and chaotic patterns of clast orientation are also common. Densely packed and consolidated.

Diverse range of lithologies reflecting that present within the original sediments.

Fold, thrust and fault structures may be present if the level of shear homogenisation is low. Rafts of undeformed sediment may be included. Smudges (brecciated clasts) may also be present.

Table 8.1 Continued.

Particle shape

Particle size Particle fabric

Particle packing Particle lithology

Structure

Supraglacial meltout (moraine) till Flow till Sublimation till

Usually dominated by sediment typical of high-level transport, but subglacially transported particles may also be present. The majority of clasts are not normally striated or faceted.

The size distribution is typically coarse and unimodal. Some size sorting may occur locally where meltwater reworking has occurred.

Clast fabric is unrelated to ice flow, is generally poorly developed and spatially highly variable.

Poorly consolidated, with a low bulk density.

Clast lithology is usually very variable, and may include far-travelled erractics.

Crude bedding may occur but generally it is massive and structureless.

Broad range of characteristics, but dominated by particles that are angular and have a non-spherical form. The majority of clasts are not striated or faceted.

The size distribution is normally coarse and unimodal, although locally individual flow packages may be well sorted.

Variable particle fabric. Individual flow packages may have a strong fabric, reflecting the former palaeoslope down which flow occurred.

Poorly consolidated with a low bulk density.

Variable, but may include far-travelled erractics.

Individual flow packages may sometimes be visible. Crude sorting, basal layers of tractional clasts may be visible in some flow packages. Sorted sand and silt layers may be common, associated with reworking by meltwater. Individual flow packages

Clasts typical of basal transport being rounded, spherical, striated, and faceted.

The particle size distribution is typical of basal debris transport, being either bimodal or multimodal.

Strong in the direction of ice flow, although it may show a greater range in orientation than a typical lodgement till.

Typically has a low bulk density and is loose and friable.

Clast lithology may show an inverse superposition.

The deposit is usually stratified and may preserve englacial fold structures.

8.1.1 Subglacial Till

Till may accumulate in a subglacial setting via a range of processes, including: (i) direct lodgement of debris in traction over the glacier bed; (ii) basal melting and debris release; (iii) deposition in basal cavities; and (iv) deformation and assimilation of overridden sediment.

1. Direct lodgement. In order to understand this process it is best to first consider a single clast in transport at the base of a glacier. A clast in transport at the base of a glacier need not move forward at the same speed as the basal ice: the ice may flow around the particle as it transports it. The particle will lodge or stop moving when its forward velocity is reduced to zero. This will occur whenever the friction between the particle and the bed exceeds the drag on the particle provided by the ice flowing over it. At this point the ice will simply flow around the particle without moving it forward. A particle may, therefore, lodge beneath flowing ice. Figure 8.2 shows several ways in which the velocity of an individual particle or mass of particles may be reduced to zero as they move over a rigid bed or plough through a soft one. In a simple abrasion model such as that proposed by Geoffrey Boulton (see Section 5.1.4; Figure 5.2) lodgement is part of a continuum, with erosion controlled by effective normal pressure. In this simplified scenario lodgement is controlled by variables such as: (i) an increase in ice thickness, which will increase the effective normal pressure; (ii) a fall in basal water pressure; and (iii) a fall in ice velocity.

2. Subglacial melting. Sediment is supplied to the sole of the glacier and released by basal melting. It may then be transported subglacially before it finally lodges. The rate of basal melting is determined by: (i) the geothermal heat flux; (ii) the amount of frictional heat generated by sliding and ice deformation, which increases with ice velocity towards the equilibrium line; (iii) ice thickness, because increasing the thickness of a glacier may increase basal ice temperature; (iv) the rate of advection of cold or warm snow; and (v) ice-surface temperatures. This is discussed in more detail in Section 3.4.

3. Cavity deposition. Subglacial cavities are known to occur where glaciers flow over irregular bedrock surfaces. Large cavities may form in the lee of bedrock obstacles, especially where the ice is thin and fast-flowing. Sediment can accumulate within these cavities in a variety of ways (Figure 8.3).

4. Subglacial deformation. Sediments can form a mobile deforming layer beneath glaciers flowing over soft substrates (see Section 3.3.3; Box 3.4). This layer may consist of soft preglacial sediment - either non-glacial or earlier glacial sediment - which has been overrun and deformed or it may consist of sediment deposited in a syn-sedimentary context by subglacial meltwater or by lodgement and meltout processes. The deformation of this sediment by subglacial shear may give rise to a homogeneous glacial till, even if it starts out as something very different. Think for a moment of mixing red jam into white yogurt; initially the two are distinct but as one mixes (i.e. one applies stress) the jam is drawn out into layers that are progressively folded into the yogurt until finally a homogeneous

A: Particle lodgement on a rigid substratum Ice

Frictional resistance increases and particle movement ceases

Resistance

Frictional resistance increases and particle movement ceases

B: Particle lodgement on a soft substratum 1 2

Resistance

Boulder

Ice Till

B: Particle lodgement on a soft substratum 1 2

Boulder

Ice Till

Till

Till

C: Lodgement of debris-rich ice mass Ice

C: Lodgement of debris-rich ice mass Ice

Debris-rich ice mass Vm = 0

Figure 8.2 Particle lodgement beneath a glacier. (A) Particle lodgement on a rigid substratum. The clast in transport stops moving when the frictional resistance between it and the bed exceeds the drag imposed on the clast by the flowing ice. (B) Particle lodgement on a soft substratum. Clasts plough through soft sediment and will be stopped when the sediment ploughed up in front of them provides sufficient resistance to retard forward movement. Subsequently other clasts may jam against the first, and in this way boulder pavements or concentrations may form. (C) Lodgement of a debris-rich ice mass. A debris-rich ice body may lodge beneath a glacier when the frictional resistance between it and the bed exceeds that of the ice above, which shears over the debris-rich ice mass. [Modified from: Boulton (1982) in: Research in Glacial, Glacio-Fluvial and Glacio-Lacustrine Systems (eds R. Davidson-Amott, W. Nickling and B.D. Fahey),

Debris-rich ice mass Vm = 0

Figure 8.2 Particle lodgement beneath a glacier. (A) Particle lodgement on a rigid substratum. The clast in transport stops moving when the frictional resistance between it and the bed exceeds the drag imposed on the clast by the flowing ice. (B) Particle lodgement on a soft substratum. Clasts plough through soft sediment and will be stopped when the sediment ploughed up in front of them provides sufficient resistance to retard forward movement. Subsequently other clasts may jam against the first, and in this way boulder pavements or concentrations may form. (C) Lodgement of a debris-rich ice mass. A debris-rich ice body may lodge beneath a glacier when the frictional resistance between it and the bed exceeds that of the ice above, which shears over the debris-rich ice mass. [Modified from: Boulton (1982) in: Research in Glacial, Glacio-Fluvial and Glacio-Lacustrine Systems (eds R. Davidson-Amott, W. Nickling and B.D. Fahey),

Cavity

Breidamerkurjokull, Iceland

Cavity

Breidamerkurjokull, Iceland

Glacier-

Glacier-

Fine slurry enters cavity from ice-rock interface

Basal debris-rich ice

Fine slurry enters cavity from ice-rock interface

Basal debris-rich ice

Basal debris-rich ice

Melting releases debris from glacier sole Sediment accumulates on cavity floor

Basal debris-rich ice

Melting releases debris from glacier sole Sediment accumulates on cavity floor

Clast expulsion

Enhanced pressure around clast in traction

Clast expulsion

Enhanced pressure around clast in traction

Clean

Basal debris-rich ice

Basal debris-rich ice

Subglacial fluviatile deposition in cavity

Clean

Figure 8.3 Observed mechanisms by which debris accumulates on the floor of subglacial cavities. [Modified from: Boulton (1982) in: Research in Glacial, Glacio-Fluvial and Glacio-Lacustrine Systems (eds R. Davidson-Arnot, W. Nickling and B.D. Fahey), Geo Books, figure 1, p. 4]

pink mixture results. Figure 8.4 illustrates this point in a geological context. Glaciotectonic deformation takes place whenever the stress imposed by a glacier exceeds the strength of the material beneath or in front of it. The material may be subject to both brittle (faults, thrusts) and ductile (folds) deformation depending on the pore-water pressure within the sediment. Ductile deformation is favoured by high pore-water pressures, which reduce the internal friction or strength of the material allowing it to deform. Deformation of this sediment proceeds in stages depending upon the amount of shear stress applied. At low levels of shear the sediment is simply folded and faulted. As the level of shear increases these structures slowly become attenuated and the nose of folds may be detached from their core, or derooted, to create boudins. Boudins are sausage-shaped blocks of less ductile material surrounded by a more ductile medium. In time they may become attenuated and drawn out at high levels of shear to form tectonic laminations (Figure 8.4). Sediments that experience very high-levels of shear become completely mixed and homogenised. The product of intense deformation is therefore a homogeneous diamict in which all the original sedimentary structures of the deposit have been destroyed. Some authors use the term glacitectonite for a sediment that retains some of the structural characteristics of the material from which it is derived after deformation; that is a sediment that has not been completely remoulded and homogenised.

A: Minor folding

A: Minor folding

B: Tectonic laminations and boudins

C: Homogenised diamicton

C: Homogenised diamicton

Figure 8.4 Schematic diagram showing the style of glaciotectonic structures associated with different levels of subglacial deformation. The insert shows how a fold may be attenuated to form a tectonic lamination. [Modified from: Hart and Boulton (1991) Quaternary Science Reviews,

Most subglacial sediment owes its origin to a combination of the above processes, although deformation beneath moving ice is probably the dominant process. Recognition of distinct subtypes of till is therefore not possible. This message has been repeatedly reinforced in recent years by micromorphological work, which reveals that most till contains a complex record of superimposed deformation events of varying styles (brittle versus ductile) and that the process history is therefore best interpreted in terms of a tectonic framework. The range of material incorporated into subglacial tills is one of the reasons for the diverse nature of this sediment. In some cases it may be dominated by glacially derived sediments produced by erosion and the comminution of clasts during transport. In other cases it may simply reflect the tectonic mixing of preglacial sediments under subglacial shear.

Given the importance of subglacial deformation to the formation of subglacial till it is worth exploring a little further the spatial and temporal controls on this process. Some of the fundamental principles of subglacial deformation are listed below.

1. Subglacial sediment will deform whenever the applied stress exceeds the strength of the material. The strength of the material is determined by the pore-water pressures within it; high pore-water pressures allow the grains to move more easily past one another and reduces the internal friction within the sediment. In sediment with low pore-water pressures, deformation will tend to occur in a brittle fashion along distinct failure surfaces such as fractures or faults, whereas at high pore-water pressure the same sediment may deform in a ductile fashion via a series of folds. Deformation will also occur along the weakest horizon within sediments with properties that are not homogeneous. It is also worth noting that as granular sediments deform they may expand or dilate, and that this dilated sediment deforms at a lower shear stress than that initially necessary to overcome the internal friction within the sediment at rest. Dilatancy will also control how easily pore-water can drain from sediment as stress varies. Finally, sediment properties may be modified during deformation, with changes to grain-size distributions due to clast-to-clast crushing, as well during the assimilation of new material into the deforming layer.

2. Sediment properties vary spatially and temporally beneath a glacier due to spatial facies variations across the glacier bed and temporal variations in subglacial hydrology. As a consequence not all areas of a glacier bed will be in motion at the same time due to deformation. Instead, it is better to envisage a mosaic of deforming and non-deforming (sticky spots) patches below a glacier. The distribution of these sticky spots may vary in time and will of course have an impact on the overall velocity component of a glacier due to subglacial deformation.

3. Sediment within a deforming layer will move forward under the applied stress, a process sometimes referred to as till advection. If the flow of deformable sediment into a given area equals the out flow of sediment from that area then deposition will not occur unless the geometry or dynamics of the glacier changes. However, if more sediment flows into an area than out, for example from an area of rapid transport to one of little transport, then sediment accumulation will occur. Alternatively if one considers a glacier with extending or accelerating ice flow downstream, then downcutting will occur because output will exceed the input of sediment. In these areas deformation will be evidenced only by a sharp basal slip or décollement surface. In contrast, in areas of compres-sional flow due to decreasing basal shear stress down-ice, subglacial deformation will also decrease down-ice and sediment accumulation may occur because output will be less than the input. The point is illustrated in Figure 8.5 where a

Figure 8.5 Progressive accumulation of a deformation till via individual tectonic slices in a compressive or deaccelerating flow regime. [Modified from: Boulton et al. (1991) Quaternary

International, 86, figure 2, p. 8]

Figure 8.5 Progressive accumulation of a deformation till via individual tectonic slices in a compressive or deaccelerating flow regime. [Modified from: Boulton et al. (1991) Quaternary

International, 86, figure 2, p. 8]

series of attenuated tectonic laminations accumulate as a series of slices, progressively one above the other. For an ice sheet flowing over a deforming bed of uniform character this relationship between extending and compressing flow will result in a pattern of erosion and deposition by subglacial deformation like that shown in Figure 8.6.

Figure 8.6 Patterns of subglacial erosion and deposition by subglacial deformation within the Greenland Ice Sheet. [Diagram reproduced from: Boulton (1987) in: Drumlin Symposium (eds J. Menzies and J. Rose), Balkema, figure 5, p. 37. Copyright © 1987, Taylor & Francis]

Deposition of a subglacial till involves the immobilisation of the deforming layer and is therefore essentially controlled by changes in applied stress as well as by the material properties of the deforming layer. It is important to note, however, that not all workers agree with the pervasive nature of subglacial deformation and some suggest that the properties of deformation may result simply from the ploughing of boulder and ice keels at the ice-sediment interface (Box 8.1). It is also possible that subglacial meltout may occur without deformation, for example below stagnant ice that has ceased to move. In these special situations debris may meltout sufficiently undisturbed to retain characteristics of the basal ice from which it is derived. These processes were once thought to be quite widespread, however, situations where ice melts out without being deformed either by movement or by the overburden are probably very limited and may not have good preservation potential.

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