An unfrozen unlithified sediment bedgeological implications

Structures reflecting subglacial shear deformation of sediments, locally to a depth of metres, are widespread, although not universal, in both modern and formerly glaciated regions. Figure 2.9 shows an informative example, typical of many, observed on the Island of Funen in Denmark, during a 2001 field trip led by Jorgensen and Piotrowski in a zone of a former ice stream (Jorgensen & Piotrowski, 2003). Three zones can be readily distinguished, which are equivalent to the deformation zones of Boulton (1987).

1 A lower zone (C), in which proglacial fluviatile sediments are largely undisturbed;

2 An intermediate zone (B), in which strongly overturned shear folds occur in a sequence of till, sand and gravel, but which are either rooted in the sediments of zone C, or, if detached, can be recognized as derived from them. By allowing for tectonic thickening and thinning of these beds, but particularly by following the limbs of shear folds that have not been de-rooted, it is possible to reconstruct the approximate net strain in this zone. It suggests a tectonic transport at the top of zone B of about 70 m.

3 An upper zone (A), in which a diamicton containing numerous elongated lenses and wisps of sand, which are sometimes

Figure 2.9 Deformation structures in a quarry at Davinde on the Island of Funen, Denmark in a section parallel to the direction of tectonic transport. The stippled ornament with triangles shows till, the unpatterned stratum is predominantly sand, in which lines show bedding planes. The circle ornaments show gravelly beds. Zone A, of the upper till and sandy masses that have been incorporated by folding, contains very highly attenuated folds reflecting the largest tectonic transport. Zone B is a zone of overturned folds reflecting lesser tectonic transport. Zone C is a zone of little deformation. The lower figure shows the estimated minimum strain in each of these three zones. There is d├ęcollement at the interfaces between the three zones, but the magnitudes cannot be determined. It is likely to be much larger at the A-B interface than at B-C.

Figure 2.9 Deformation structures in a quarry at Davinde on the Island of Funen, Denmark in a section parallel to the direction of tectonic transport. The stippled ornament with triangles shows till, the unpatterned stratum is predominantly sand, in which lines show bedding planes. The circle ornaments show gravelly beds. Zone A, of the upper till and sandy masses that have been incorporated by folding, contains very highly attenuated folds reflecting the largest tectonic transport. Zone B is a zone of overturned folds reflecting lesser tectonic transport. Zone C is a zone of little deformation. The lower figure shows the estimated minimum strain in each of these three zones. There is d├ęcollement at the interfaces between the three zones, but the magnitudes cannot be determined. It is likely to be much larger at the A-B interface than at B-C.

folded, appears to lie unconformably on those of zone B. The estimated shear strain in zone A is an absolute minimum, derived by estimating the finite shear strain in individual isoclinal folds in this zone. However, as these folds are generally de-rooted or difficult to trace back to distant roots through extreme shear thinning, it is clear that the shear strain in zone A is far larger than the minimum.

The potential significance of this sequence can be best understood by referring to the pattern of monitored shear strain shown in Fig. 2.6. The cumulative shear strain in the uppermost 1.0m is 3.5m in 12 days (Fig. 2.6b). This approximates to a strain rate of 106 per year or 1060 in 10yr, an extremely large finite shear strain. Applying this to the section illustrated in Fig 2.9, any sandy units from zone C or B, folded because of local stress concentrations (Fig. 2.5a) into a deforming mass such as that shown in Fig. 2.6b, would be enormously attenuated, recognized only as thin, subhorizontal sandy lenses or wisps. It is on this basis that the diamic-ton in zone A is suggested to be a deformation till that has formed in a zone of shearing such as that in the topmost 0.5 m in Fig. 2.6. Moreover, if local stress concentrations, or local increases in fric-tional resistance or consolidation on the surface over which it shears, produce folding, these laminae will tend to become progressively more strongly mixed into the deforming mass, to produce a homogenized sediment.

If such mixing by folding occurs, horizons that originally lay at the base of the rapidly shearing mass could be translated to the top, and vice versa. Under such circumstances, we could translate the average shear strain through the deforming bed in Fig. 2.6 into an average velocity of 53myr-1. Clearly such a mass could travel far beyond the sediment source area from which it was derived, and would be added to by incorporated materials from further down ice. The unconformable relationship between the sediments in zone A and zone B would be a reflection of such strong de-rooted transport, particularly as the amount of time the site was last glaciated prior to final ice retreat in the last phase of late Weichselian glaciation in the area was probably about 2000-3000yr (Houmark-Nielsen, 1999). This would be sufficient to permit material incorporated at this site at the time of initial glacier overriding to have been transported a very long distance, although the zone-A sediments at the site may themselves have been incorporated into the flow only recently, shortly before deglaciation. It is also possible that the A-B unconformity has been a surface of strong erosion from which sediment was incorporated into the shearing nappe above it. Given the potentially long period over which deformation may have occurred, the apparently large strains in zone B may reflect only very small strain rates, and may have lain in the lower zone of Fig. 2.6 in which the period of measurement was so short that only slight deformation was recorded.

Figure 2.10a & b shows a model of an advancing ice sheet with a deforming bed based on the theory of Boulton (1996b). In the zone up-glacier of the equilibrium line, the inner zone of accelerating flow, there is erosion of the bed. Down glacier of this zone, the outer zone of deccelerating flow produces a thickening till mass, which is itself eroded as the glacier advances over it, to produce an advancing wave of deposition (compression) and erosion (extension). As discussed above, a shearing nappe can be highly erosive through folding-incorporation of underlying sediments (Fig. 2.5a). Even if the deforming layer remains of

(c) Decreasing Deforming velocity and sediment nappe discharge \ depositional condition

Increasing velocity and discharge erosional condition Original surface

-____

Till on uneroded substrate

Till on eroded substrate

Deforming sediment on eroding substrate

Figure 2.10 (a & b) A modelled till wave generated as the glacier advances. It is derived from the theory of till transport by subglacial deformation (Boulton, 1996b), but could apply equally to basal ice transport and lodgement. Zones of deposition and erosion extend outwards as the glacier advances. (c) A schematic diagram showing the zone of accelerating flow (extension) up-glacier of the equilibrium line, in which a thin deforming sediment nappe erodes the bed by incorporation of underlying sediment; and the zone of decelerating flow (compression) down-glacier of the equilibrium line which creates net deposition from the deforming nappe and thickening of the deposited till.

constant thickness, a down-glacier increase in velocity will permit the discharge of deforming sediment to increase, thus leading to incorporation (erosion) of yet more sediment in the deforming horizon (Fig. 2.10c). As a consequence, the sediments underlying a deforming sediment nappe may suffer aggregate deep erosion as successive deforming masses continuously move its surface, incorporating successive increments of sediment from it. As we pass into the terminal zone of a terrestrial glacier, where we expect basal velocities in general to decrease, the discharge of sediment in a deforming horizon of constant thickness will decrease and till will begin to be deposited from the base of the deforming horizon (Boulton, 1996b). From this point on, a thickening till stratum will form from successive increments of deposition from the base of the deforming horizon (Fig. 2.10c). Clark et al. (2003b) have suggested that because many tills are in excess of a metre in thickness and deforming horizons tend to be thin, that deformation cannot be an important source of erosion. This confuses the thickness of a deposited till with the thickness of the deforming horizon.

It is frequently observed that a sharp interface separates till and apparently undisturbed underlying sediment. This is most likely to be a product of an erosive deformation process. The existence of a soft, deforming till at the ice-bed interface, acting as a buffer between the glacier and an underlying stable bed, and able to incorporate irregularities that form local stress concentrations on the underlying surface by erosively folding them into the shearing nappe, is a means of creating an apparently undeformed, planar surface.

2.5 The origin of till and its properties

As with any other sediment, the thickness of a till is a product of the rate of transport into the zone of deposition and the period of time over which the rate is sustained. There are three principal modes of deposition of till: lodgement, deformation and melt-out.

2.5.1 Lodgement till

This is assumed to be deposited when the frictional drag between clasts transported in the basal ice and the bed is sufficient to halt the clasts against the bed. Lodgement is a cumulative process in which debris is continually imported into the region and progressively accumulated on the bed. In principle, a long period of till accumulation could produce a considerable till thickness. The till surface will bear streamlined features such as flutes and drum-lins. Notwithstanding the many tills that have been ascribed to a lodgement process (e.g. Benn & Evans, 1998), we have no direct documentation of the process and no unequivocal demonstration of a lodgement origin for any deposited tills. Hart (1995b) has doubted that lodgement is a significant process by which till is finally deposited.

2.5.2 Deformation till

The way in which deformation tills might either be associated with underlying deformation structures or might overlie an undisturbed sediment across a planar interface as a consequence of erosion at its base has been described above. The till effectively acts to absorb stress at the base of the glacier, and can protect an underlying interface against deformation. If, for example, a melt-out till overlaid pre-existing sediments, the underlying surface would be a surface over which ice had flowed, and more deformation would be expected at the interface than in the case of a deformation till.

The till-creating potential of the deformation process does not depend upon the thickness of the deforming horizon, but on the sediment discharge in the deforming horizon. For example, a relatively thick (0.45 m) deforming horizon at Breidamerkurjokull had a two-dimensional discharge of about 25.7 m2yr-1, whereas a thinner (0.3 m) deforming horizon at Trapridge Glacier had a discharge of 314m2yr-1 (Boulton et al., 2001), simply because the flow velocity of the ice-sediment system is greater in the latter case. Even if the active deforming horizon is thin, the ultimate till that is deposited from it may be relatively thick. The ways in which the pattern of erosion and till depositon may vary through a glacial cycle based entirely on changes in the transporting power (Boulton, 1996b) are illustrated in Fig. 2.11. As in the case of lodgement till, a deformation till surface will be characterized by streamlined drumlin and flute forms.

2.5.3 Melt-out till

Melt-out till is the inevitable consequence of the slow melting out of debris-rich stagnant ice that is buried beneath a supraglacial sediment overburden. Simple thermodynamic considerations suggest that melting out will almost invariably be on the surface of the buried ice mass rather than beneath it. As this till represents the melting out of debris from a stationary ice mass, its ultimate thickness is limited by the mass of debris in a column of ice. It is not continuously being transported to the place of deposition as are the other two till types. This inevitably limits the thickness of melt-out tills, as the total debris content of a vertical column of ice is rarely enough to create more than a few decimetres and exceptionally metres of till (Table 2.1). However, their role in preserving buried stagnant ice, which then intercepts glacial drainage to create hummocky kame landscapes, is important, and some subpolar glaciers with relatively large debris loads, such as those of Spitsbergen, and some subpolar glaciers with relatively large debris loads can create melt-out tills with thicknesses in excess of a metre and potentially be a major source of supraglacial debris flows (flow tills) (van der Meer, 2004). Melt-out till deposition will tend to be associated with hummocky rather than streamlined glacial topography, although Monro-Stasiuk and Sjogren (this volume, Chapter 5) have suggested that 'hummocky terrain' can be of erosional origin, a puzzle that demands further analysis.

2.5.4 The state of consolidation of subglacial tills

It was formerly supposed that state of consolidation of tills and their tendency to overconsolidation was determined by the ice

EROSION/DEPOSITION

(b) STRATIGRAPHY

GLACIAL SURFACE

SURFACE

Figure 2.11 A schematic diagram of erosion/deposition through a simple glacial cycle. (a) Advance and retreat of an ice sheet in a glacial cycle. The longitudinal pattern of erosion/deposition along a specific timeline is as shown in Fig 2.10b. The three vertical lines show the sequence of events at specific locations through time. An early phase of till deposition (the 'till wave' as in Fig. 2.10b) is succeeded by a period in which this till is progressively eroded. Only at the rightmost location does erosion occur for a period long enough to remove the earlier deposited till entirely and then to erode into the pre-till surface. Till is deposited on the eroded surface during the last phase of glacial retreat. At the middle location, earlier deposited till is eroded but not completely removed before the retreat-phase till is deposited, producing an erosion surface within the till, often marked by a boulder pavement and a lithological contrast (Boulton, 1996b). At the left-most location, till is deposited continuously. (b) The structure of the resultant till, including the location of internal erosion surfaces and timelines.

overburden pressure, and therefore that measured pre-consolidation values from tills could be used to infer former ice loads at the glacial maximum (e.g. Harrison, 1958). This involves three related assumptions: that ice load alone is the determinant of pre-consolidation pressure; that tills were necessarily present beneath the glacier at the maximum of glaciation to receive the imprint of contemporary pre-consolidation; and that the measured

SURFACE

Safety factors

OflOlOO 0*00*00 OOO OlOOlOO

<OKOCN<0 <OKOCN<0 LO O <0 *0KOCN<0

OflOlOO 0*00*00 OOO OlOOlOO

<OKOCN<0 <OKOCN<0 LO O <0 *0KOCN<0

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135

Distance from datum (m)

Figure 2.12 Safety factors at the topmost transducers in the till at 12 m, 30 m, 65 m, 85 m and 125 m along the transect shown in Fig. 2.3. The heavy line shows the glacier margin through time. The maximum ice thickness was achieved at about 195-200 days. The smallest safety factors (strongest deformation) occur early on during the advance; the largest safety factors (maximum effective pressures and pre-consolidation) occur after the period of maximum loading, when a more efficient drainage system had been established.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135

Distance from datum (m)

Figure 2.12 Safety factors at the topmost transducers in the till at 12 m, 30 m, 65 m, 85 m and 125 m along the transect shown in Fig. 2.3. The heavy line shows the glacier margin through time. The maximum ice thickness was achieved at about 195-200 days. The smallest safety factors (strongest deformation) occur early on during the advance; the largest safety factors (maximum effective pressures and pre-consolidation) occur after the period of maximum loading, when a more efficient drainage system had been established.

pre-consolidation reflects conditions at the glacial maximum. The first is very unlikely under any circumstances and there is no reason to believe that the second and third are commonly true.

The data in Fig. 2.4 show conditions that influence the state of consolidation of tills and associated subglacial sediments as they are overridden by a glacier. Using an assumed shear stress derived from the average gravitational shear stress, and assuming a Coulomb yield criterion, a safety factor (strength/stress) is calculated, in which values > 1 indicate stability, and values < 1 indicate failure. The results shown in Fig. 2.12 indicate some important conclusions.

1 The effective pressure varies both diurnally and seasonally, primarily in response to variations in recharge to the sub-till aquifer and to the top of the till from water draining to the bed from the glacier surface.

2 The maximum effective pressure (which would be the value of pre-consolidation recorded by the till provided that it is not remoulded by shear), which occurs after day 210, does not coincide with the maximum ice load, which occurs on about day 190. The maximum effective pressure is determined by late-stage drainage of the system. 3 Shearing in the till (safety factor < 1) occurs early during the glacier advance, whereas the maximum effective pressures occur later.

Measured pre-consolidation values large enough to inhibit failure cannot be used, as Hooyer & Iverson (2002) have done, to infer that the till could not have deformed. In the Breidamerkurjokull case, the period after the very active advance is a period of drainage reorganization that leads to a general fall in water pressures. A similar sequence is suggested to have occurred during the surge of Sefstrombreen in Spitsbergen in 1882-86 (Boulton et al., 1996), where there is strong geological evidence and contemporary observation that the glacier surged forward on a deforming carpet of marine sediment and till, which would have required very low effective pressures in the sediment. Now, however, the sediments and tills are relatively heavily pre-consolidated, which is suggested to have occurred when the glacier stagnated after the surge and a reorganization of drainage permitted water pressures to be reduced.

These observations reinforce the need to understand how far beneath an ice sheet surface water can penetrate to the bed. Is it merely in the terminal zone, or is it far from the margin as Arnold & Sharp (2002) suggest? If the latter, then a large variety of short-term, seasonal drainage effects would be important in driving highly variable subglacial processes. If the former, we would expect slowly varying hydraulic, geotechnical and depositional conditions beneath ice sheets.

2.6 Large-scale patterns of sediments and landforms and inferences drawn from them

An important current focus of glacial geological study is the reconstruction of the large scale properties of former ice sheets and the way in which they have evolved through a glacial cycle (e.g. Kleman et al., this volume, Chapter 38). The advent of satellite imagery and broad swathe bathymetric devices has permitted coherent reconstructions to be made of landform systems that show very large scale patterns of distribution both on land and beneath the sea, rather than having to depend upon a fragmental patchwork of field surveys. There are currently three large-scale patterns that have been established for the European ice sheet, from which palaeoglaciological inferences can be drawn:

1 large scale drift lineations (drumlins and flutes);

2 relict landscapes;

3 esker distributions and tunnel valleys;

and a fourth one, the distribution of till thickness, which is less well known and possibly less diagnostic of origin.

In some cases, large-scale fossil features have been used to infer processes that occur beneath ice sheets, rather than merely being explained by reference to modern process studies.

Was this article helpful?

0 0

Post a comment