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Influence of the glacier bed lithology on the formation of a subglacial till sequence — ring-shear experiments as a tool for the classification of subglacial tills
Quaternary Science Reviews 20 (2001) 1113}1125
In#uence of the glacier bed lithology on the formation of a subglacial till sequence * ring-shear experiments as a tool for the classi"cation of subglacial tills B.U. MuK ller*, C. SchluK chter Institute of Geology, University of Berne, Baltzerstrasse 1, CH } 3012 Berne, Switzerland
Abstract A glaciogenic sediment sequence deposited by a local glacier in the Rhine catchment area near Sargans (Eastern Swiss Alps) was investigated in order to examine the shear behaviour of two di!erent tills. In addition to ring}shear experiments, examination of clast lithology, clay mineralogy and physical properties of all samples were determined. SEM}analyses of arti"cially produced deformation patterns in the till samples show only a very thin shear zones of 2}3 m thickness, despite long shear distances of up to 145 mm. A clear relationship between local bedrock and the formation of a thin basal layer of heavily deformed till can be demonstrated. This locally derived till layer exhibits distinctly di!erent shearing properties compared to the massive overlying till unit that is composed of more distantly transported lithic material from the upper catchment area of the glacier. The di!erence in the angle of internal friction between the two tills is '103 at residual shear strength, although the clay mineralogy of both tills is similar. It can further be demonstrated that after the deposition of a 10}15 cm thin till layer directly on bedrock, the #ow dynamics of the glacier tongue may have changed due to di!erences in glacier bed debris lithology. Di!erences in the shear behaviour of the sediments, allied to microscopic analyses, point to an important environmental change from a deformation till to an overlying `non}deformed tilla. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction It has been suggested that glaciers may develop a soft basal deformation layer (Alley et al., 1987, 1988, 1991; Boulton, 1979, 1996; Hart and Boulton, 1991). However, there are areas of past glacial environments where this model remains controversial, e.g., the southern lobes of the Laurentide ice sheet, whose sediments have been interpreted by some researchers as deforming-bed tills (e.g. Alley, 1991; Hicock and Dreimanis, 1992), whilst others have argued for a melt-out till origin (e.g. Clayton et al., 1987, 1989; Ronnert and Mickelson, 1992). While considering a soft deformation glacial-bed model, it remains, for example, unexplained how the glaciers of the Last Glacial Maximum (LGM) in some of the Alpine valleys in Switzerland removed soft-sediment bodies of up to 500 m in thickness (MuK ller, 1993, 1995). A recent model on glacial erosion by a deformable bed
* Corresponding author. Tel.: #41-31-631-8771; fax: #41-31-6314843. E-mail address: [email protected] (B.U. MuK ller).
mechanism (Boulton, 1996) may not be able to produce such amounts of erosion since the model has been developed primarily for ice sheets and not for valley glaciers. Another observation that seems to con#ict with a soft bed model is the absence of basal till covers in the bottom of many overdeepened valleys in Switzerland. It may therefore be reasonable to consider a predominantly basal erosional model for Alpine ice masses during most of the last glacial cycle. In assessing glaciodynamic models more reliance on glaciogenic material properties should be considered. For example, the sensitivity of sediment strain-rate to the possible variations in the applied shear stress may reveal new evidence indicative of subglacial processes. This would also be the case where most of the glacier movement consists of basal sliding (Harper et al., 1998). Scott (1976) argued that the lithology of the glacier bed and its in#uence on the physical properties of subglacial tills may point to particular glaciodynamic models. It follows therefore that the experimental data can help further to explain mechanisms at the glacier bed and may assist in understanding the erosional and depositional patterns found in the Alps.
0277-3791/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 1 4 1 - 4
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There is a "eld evidence that suggests that not all subglacial tills were formed in a deformable-bed environment. Most tills found on the northern slope of the Alps (Rhone glacier, Rhine glacier) do not show any macroscopic or microscopic evidence of deformation (MuK ller 1996, 1998). As these tills normally contain a high percentage of quartz sand derived from Tertiary sandstones it is possible that they have a deformation style, which does not favour the formation nor preservation of deformation structures. Investigations on the shear behaviour of tills have been made, for example, by McGown and Derbyshire (1977), and more recently by Iverson et al. (1997, 1998). However, a direct relationship between the #ow dynamics of an ice body over its bed and the shear strength of the sediments accumulated at the glacier bed can only be partly developed. The investigations reported here, all follow standard geotechnical methods. This paper presents the "rst set of results on a sediment pro"le in the Eastern Swiss Alps near Sargans (Fig. 1). The lithological properties of the sequence, as well as descriptions of macroscopic properties of the sediments, along with a paleoglaciological reconstruction have been previously published (MuK ller, 1996).
2. Geological background The till under study was deposited during the LGM advance by the local Weisstannen Glacier, which reached the main valley of the Rhine earlier than the Rhine glacier (Fig. 1a). This till is the only evidence of the build-up or the maximum of the LGM in the region. The site at Sargans illustrates a stratigraphic sequence from bedrock to subglacial tills to proglacial lacustrine sediments. Bedrock consists of unweathered, glacially polished Permian shales, dipping at a low angle (0553/0203) to the northeast. In the immediate area, red Permian shales dominate the catchment area of the local Weisstannen Glacier. These are easily recognised and distinguished from the dark greyish sediments derived from Rhine Glacier. Above bedrock, two tills, related it to the local Weisstannen Glacier, occur that can be recognised on the basis of colour and petrographical content. A basal till layer can be subdivied into an upper till unit overlying a "ne-grained, heavily deformed lower till unit. This basal till layer is remarkably homogenous without any apparent bedding, banding or layering. The tills of the local Weisstannen glacier in the lowermost part of the sequence (Fig. 1b) have not been deformed by the later Rhine glacier advance. Overlying the till layer is a sequence of local proglacial debris #ow sediments derived from the retreating Weisstannen Glacier. Following this retreat phase, the main Rhine glacier advanced and dammed a small ice-marginal lake in this tributary valley. A coarsening-upwards sequence of laminated clays and
silts to well-sorted gravels reveals the evidence for the in"lling of this proglacial lake. Following the in"lling phase, intense proglacial and subglacial glaciotectonic deformation in the lacustrine sediments occurred possibly due to Rhine glacier advance into the tributary valley. As a consequence of the main glacier advance, a thin "ll layer covering the whole sequence was deposited (MuK ller, 1996). The investigations presented here concentrate on the basal till unit and on comparison between the lower thin, basal, "ne-grained and the upper thicker till units.
3. Methods The following laboratory analyses were carried out to measure (1) the shear strength of the two lower and upper till units found in the basal till sequence described above; (2) to compare the shear behaviour of the two till units in terms of their respective lithological and physical properties; (3) to evaluate the in situ deformation structures in the basal till unit with those generated mechanically by the shear experiments; and (4) to attempt to explain the genesis of the till sequence and the processes by which the two individual units were formed and deposited. Laboratory analyses included grain size composition, bulk density, Atterberg limits, porosity, and the natural moisture content. Laboratory analyses were performed following SNV procedures (Verein Schweizerischer Strassenfachleute, 1959), directly comparable to DIN procedures (Deutsches Institut fuK r Normung e.V., 1993). Additionally, the calcium carbonate content of the (0.5 mm fraction was determined by colometric methods (CS-mat 5500 by StroK hlein, following the laboratory standards of the University of Berne). The clast lithology of the 8}63 mm size fractions was also determined in combination with clast morphometric analyses. Bulk rock mineralogy and clay mineralogy were determined using XRF at the University of Neuchatel. The shear tests were generated in a ring-shear device (Type Wille Geotechnik) based on the DIN-Norm 18137 Part 3 (Draft, October 1995). This device is computerised and driven electro-mechanically permitting normal stress and shear stress levels to be set and measured in very small increments, shear strain being measured at the lower shear ring (Fig. 2). The samples for shear stress determination have a 96 mm outer diameter and 50 mm inner diameter, with a surface area of 50 cm. Sample thickness varies between 14 and 18 mm depending on the degree of consolidation. With a maximum normal stress and shear stress of 500 kN/m, experiments can be carried out under water in order to ensure full water saturation during testing. It is di$cult to cut poorly sorted tills as undisturbed samples to required ring-shaped sample size. Therefore, it was necessary to develop a sample preparation method
B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
Fig. 1. (a) Location of Vorderes SchloK ssli investigation site.
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for disturbed samples. According to Holtz and Ellis (1961) gravel contents of less than 40% has little e!ect on shear strength, therefore, it was decided to remould the tills which could not be cut to the sample size needed. These samples were "rst sieved at 0.5 mm to remove the coarse grains. For the construction of the ring-shaped samples the homogenised fraction ((0.5 mm) was used and poured into the shear sample container up to a height of 20 mm. The dry powder was saturated with water and consolidated in incremental steps (25}50}100}200}500 or 125}250}500 kN/m) depending on the Atterberg limits of the sample. Normal stress during consolidation was switched to a higher value when the measured settlement of the sample was less than 0.01 mm/15 min. After reaching the end of the maximum consolidation step, each sample was given one hour of relaxation time before the next shear test began. Of the samples analysed for this investigation 90% were sheared under overconsolidated conditions. In otherwords, the normal stress during the test has to be equal or lower than the highest consolidation stress produced during consolidation. Shear distance ranged between a minimum of 50 mm and to a maximum of 295 mm. Shear speed varied from 0.01 to 0.05 mm/min depending on the Atterberg limits (Table 1). A high plasticity index dictated low shear speeds. It should be noted that these speeds (10 to 50/ma) are comparable to typical velocities of some larger present-day Alpine
Fig. 1. (b) Schematic overview of the Vorderes SchloK ssli pro"le.
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Fig. 2. Outline of the sample holder of the ring-shear device type `WILLE Geotechnika.
Table 1 Applied displacement speeds during the shear experiments depending on the Plasticity index (I ) of the sample Plasticity index I (%)
Maximal shear speed (mm/min)
(10 (20 20}30 '30
0.05 0.03 0.02 0.01
glaciers (Maisch et al., 1999). For drained shear tests, very low shear speeds are required to ensure that total drainage of the shear plain develops in the sample during the test. Following each shear experiment, the sample water content, and wet and dry bulk densities were calculated and the compaction of each sample due to shearing was also determined. If slickensides appeared on the experimentally developed shear plain, they were examined under SEM. Finally, for most samples, thin sections were made in order to microscopically examine any deformation patterns present on the shear planes.
4. Results Seven samples from the basal till layer were analysed, "ve from the lower till unit (VS-3, VS-3a, VS-12, VS-12N and VS-13) and two from the upper till unit (VS-1 and VS-8). 4.1. Lithology and petrography At `Vorderes SchloK sslia the till directly overlies the Permian `SchoK nbuK hl-Schiefera, a very "ne-grained, pinkish shale. The shales occur only locally for about 2.5 km up-valley and 1 km down-valley from the study site. Petrographical analyses of the 8}63 mm fraction of the basal till layer show that all the till samples were of similar composition, derived from the local catchment of the Weisstannen Valley and were incorporated homogeneously into the basal till sequence (Fig. 3). A higher percentage of the Permian `SchoK nbuK hla shales can be found in the lower till unit (sample VS-3, Fig. 3). Heavy minerals populations within the basal till layer including zircon, tourmaline, rutile and sphene (MuK ller, 1995) demonstrate that this basal layer is exclusively derived from the local Permian and Triassic rocks of the Helvetic nappes.
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4.2. Clay mineralogy To better understand the shear tests it was decided to analyse the bulk rock composition and the clay fraction of the samples by XRD (Table 2). All bulk rock samples of the basal till units show a strong correlation to the bedrock lithology (Richter, 1968). Samples VS-3 and VS-13 from the lower till unit show a mineral content
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with up to 80 % phyllosilicates, the remaining 20% being quartz, plagioclase and calcium carbonate. Sample VS-1 contains a smaller percentage of phyllosilicates and more quartz and plagioclase. The carbonate content for all the samples analysed is similar to the bedrock (Richter, 1968). The 2}16 m fraction shows a similar composition to the bulk rock samples, although there is slightly more quartz. Within the clay fraction ((2 m) the mineralogical composition is mostly fresh, unweathered chlorite and mica with small amounts of quartz. No reworked clay minerals from soils can be found in these tills in contrast to those observed in other samples from the northern Alpine Foreland (Peters, 1969). 4.3. Grain size distributions Five samples were analysed by sieving and in the Atterberg cylinder (Verein Schweizerischer Strassenfachleute, 1959). The results are given in Fig. 4. 4.4. Physical properties The water content, bulk densities before and after the shear test, and the Atterberg limits of all the samples are shown in Table 3. 4.5. Shear strength determination (ring-shear testing)
Fig. 3. Lithological composition of the pebble fraction (8}63 mm) of the till deposited by the Weisstannen Glacier.
The samples from both the lower and upper till units of the basal till layer show di!erent deformation properties (Fig. 5). Sample VS-1 was taken from the upper till unit 0.8 m above bedrock. In the shear strain/shear path diagram, no decrease in shear strain to residual shear
Table 2 The lithologic composition (determined by XRD) of the bulk rock samples of the fraction 2}16 m and the clay mineral content (fraction (2 m) of the samples from Vorderes SchloK ssli Bulk rock Sample no.
Phyllosilicates (%)
Quartz (%)
K-feldspar (%)
Plagioclase (%)
Calcite (%)
Dolomite (%)
VS-1 VS-3 VS-13
55.0 75.7 81.4
19.8 7.7 8.9
0.0 0.0 0.0
11.1 9.2 7.3
13.0 5.0 2.4
1.1 2.4 0.0
Quartz
K-Feldspar
Plagioclase
Mica (in % of phyllosilicates)
Chlorite (in % of phyllosilicates)
Fraction 2 } 16 m Phyllosilicates (relative abundance) Sample no. VS-1 VS-3 VS-13
*** *** ***
* * *
* * **
** ** **
70 65 68
30 35 32
Fraction (2 m (%) Sample no.
Smectites
Chlorite
Mica
I/S mixed layers
Kaolinite
Quartz
VS-1 VS-3 VS-13
* * *
28 25 22
72 75 78
* * *
* * *
? ? ?
B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
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Fig. 4. Grain-size distributions of "ve till samples from the Vorderes SchloK ssli in a summary diagram. Table 3 Geotechnical properties of VS-3, VS-12 and VS-13 from the basal till layer, and VS-1 and VS-8 from the upper till unit Sample no.
Water Content (%)after shear test (saturation)
Bulk density (g/cm) (after shear test, wet)
Bulk density (g/cm) (after shear test, dry)
Liquid limits (%)
Plastic limit (%)
Plasticity index (%)
VS-1 VS-3 VS-8 VS-12 VS-13
18.0 23.4 n.m. 17.4 17.0
2.29 2.29 n.m. 2.32 2.36
1.95 1.86 n.m. 1.91 1.95
18.0 25.3 18.1 24.5 24.0
13.0 16.1 12.0 15.0 15.1
5.0 9.2 6.1 9.5 8.8
n.m.-not measured.
strength was observed during the experiment. After failure occurred, shear strain increased slowly and continued even after long shear distances of up to 150 mm were used. Sample VS-3a, taken from the lower till unit, exhibits a much lower strain for all experiments under identical testing conditions. It is not only the di!erence in absolute shear stress at the point of failure of the sample which di!ers widely, but also the further development of the stress curves towards the state of residual strength that is totally di!erent. Sample VS-3a shows a wellde"ned decrease in shear stress while Sample VS-1 shows shear stress increasing slowly after the point of failure. This di!erent behaviour can also be expressed using the brittleness index (I ) (Bishop, 1967) I (%)"T !T /T ,
(1)
where T is the maximal shear strength and T the resid ual shear strength.
A high brittleness index (I ) implies a sharp decrease of the shear stress after the failure of the sample generally caused by either a high clay mineral content or by a high overconsolidation ratio (MuK ller-Vonmoos et al., 1985). As a general trend it can be seen that samples from the lower till unit have higher values for I than those from the upper till unit (Table 4). An average shear line, calculated from all till sample data points (Fig. 6a and b), provides an approximation of how well the shear experiments on di!erent samples (from the same till) "t and the accuracy of the tests. With correlation coe$cients ranging between 0.94 and 0.99, the di!erent test results can be used to calculate average angles of friction and cohesion for the two basal till units. It is readily apparent that the di!erence between the maximum shear strength (red data points) and the residual shear strength (blue data points) in the lower till unit is larger than in the upper till unit (Table 5). The results show strong evidence for the di!erent behaviour of the two tills with a di!erence of 9.8% for
B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
maximum shear strength and of 10.9% for the angle of inner friction. Cohesion is 20% higher at failure (maximum shear strength) for the lower till unit and drops sharply during the tests. Likewise, the residual shear strength is 36% lower in the lower till unit than in the upper till. 4.6. Macrofabrics One macrofabric was analysed in the upper till unit approximately 0.7 m above bedrock, and 0.55 m above the top of the lower till unit. Woodcock statistics reveal a very strong fabric where for 24 measurements: C"4.36, and k"0.795; and for 19 measurements: C"4.67 and k"0.65. Calculating the Bingham statistics for eigenvalues generates the following results: S1: 0.864; S3: 0.011; with a mean: 70.6. Due to the poor outcrop, it was not possible to complete macrofabric measurements in the lower till unit. Based on Hart (1994) the above values could provide evidence for only a thin deformation layer; however, a di!erent interpretation of these results will be discussed later where it will be
Fig. 5. Diagram of shear-strain versus shear distance of tested samples VS-3A and VS-1.
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suggested that no deforming layer occurred within the upper till unit of this basal till layer. 4.7. Micromorphology and SEM At both the macroscopic and microscopic scale, the lower till unit shows extremely strong internal deformation. Deformation style could be described best as homogeneously chaotic. Numerous very small slickensides can be observed exhibiting no preferred orientation and are distributed regularly some contorted or even folded. Due to the abundance of small-scale slickensides, the till matrix shows a characteristic polished surface appearance. The upper till unit does not exhibit slickensides nor other planar deformation structures although the clast fabric is strong (Fig. 7). Thin sections display evidence of considerable deformation. Brittle deformation seems to dominate as many zones of discrete shear planes display unistrial, plasmic fabric (van der Meer, 1993; Hiemstra and van der Meer, 1997) (see VS-13; Fig. 8b). Additional evidence for brittle deformation in the lower till unit at `Vorderes SchloK sslia are broken grains `#oatinga in the "ne-grained matrix (Fig. 8a). Other deformational features such as rotational structures are rare but occasionally observed. There is a dominance of brittle deformation in comparison to ductile deformation. It is possible that brittle deformation structures were produced at a later stage of till deformation and therefore, overprint any earlier deformation patterns. It might be expected that the grain-size composition of these sediments would favour ductile deformation; however, if sediment moisture content was below the sediment's plasticity limit during deformation only brittle failure would occur. Thin sections from the upper till unit show no macroscopic nor microscopic deformation although the sediments are poor in calcium carbonate and rich in clay fraction, which might favour the visibility of deformation structures. A comparison was made between in situ deformation structures observed in the tills at `Vorderes SchloK sslia with those structures produced in the laboratory. Despite the long shear paths in the ring shear device (50}295 mm) there is surprisingly little evidence of deformation in the
Table 4 The shear parameters of the samples from Vorderes SchloK ssli Sample no.
Angle of friction (max shear strength) (3)
Cohesion (max shear strength) (kN/m)
Angle of friction (resid Cohesion (resid shear shear strength) (3) strength) (kN/m)
Brittleness Index (I ) (%)
VS-1 VS-3 VS-3A VS-8 VS-12 VS-12N VS-13
28.5 24.0 18.8 30.8 20.9 20.3 20.6
19.5 11.8 47.7 9.4 11.4 11.9 23.7
27.7 20.6 13.8 29.4 15.5 12.8 17.8
2.9 22.2 24.8 !1.4 28.1 28.4 30.1
19.1 1.1 39.3 16.1 6.8 19.8 !1.01
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Fig. 6. (a) Average shear-line plot for the upper till unit (samples VS-1, VS-8) and (b) average shear-line plot of the lower till unit (samples VS-3, VS-3A, VS-12 and VS-13) from the Vorderes SchloK ssli.
Table 5 Summary of the mean shear parameters for the two till units Till unit
Angle of friction (maximal shear strength) (3)
Cohesion (maximal shear strength) (kN/m)
Angle of friciton (residual shear strength) (3)
Cohesion (residual shear strength) (kN/m)
Regression quality R (s/r)
Basal unit (samples VS-3, VS-3A, VS-12, VS-13) Upper unit (samples VS-1, VS-8)
18.6
27.2
16.2
16.8
0.975/0.946
28.4
21.8
27.1
26.0
0.995/0.992
deformation was noted in the thin sections. However, using SEM (Figs. 10}12) striking evidence of `polishinga of the slickensided shear plane was observed (Fig. 10). Most of the planar particles are very clearly orientated parallel to the shear plane (Fig. 11). In a transverse view of the same shear plane a small zone is developed of oriented particles of only two to four particle layers with a total thickness of 3}5 m (Fig. 12).
5. Methodological constraints
Fig. 7. Till macrofabric of the upper part of the till of the local Weisstannen glacier at Vorderes SchloK ssli. Pebble long axes orientation: scatter and contour plot.
arti"cially produced sediments when seen in thin section. On the macroscopic scale slickenside structures on he shear plane are observed (Fig. 9) but no trace of
The in#uence of variable pore water pressure on the mechanical behaviour of the glacigenic sediments is important; however, it could not be simulated in our experiments because it is not possible yet to carry out undrained testing in a ring-shear device. Any model dealing with basal deformation of glacial sediments has to permit pore water pressure changes over time at any given locality. By conducting, in the "rst instance, shear tests with a ring-shear device, long shear path distances could be acheived that allow the determination of residual shear strengths approximating conditions at the base of a glacier. Positive pore water pressures can also be achieved in the shear plane using this direct-shear device
B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
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Fig. 8. Photomicrographs, (a) crushed grain (sample VS-13d); and (b) thin section from sample VS-13 with zones of discrete shear with unistrial plasmic fabric visible in bright grey.
Fig. 9. Photograph of the two shear-plane-halves produced during shear experiment VS-12N.3.
by increasing shear speed. In this latter case the drainage of the shear plane is not possible and due to the rearrangement of the sediment particles, positive pore water pressures can develop. This procedure, however, is very di$cult to verify, as local increased pore water pressure values cannot be measured.
Experimentally developed deformation structures in the samples seem to be di!erent from the ones found in nature. Since particular brittle deformation structures are small and therefore discrete shear planes without parasitic branches may not be visible in thin section. Shear planes with well-developed slickensides sometimes
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Fig. 10. SEM photograph of a shear plane from the sample VS-13. View of the slickensided shear plane with planar particles showing a strong orientation parallel to the shear plane.
Fig. 11. SEM picture of a cut through an opened shear plane of sample VS-3. On the upper edge of the sample, particles show an orientation parallel to the shear-plane. In the central and in the lower part of the picture there are di!erent domains of particles with aberrant orientation.
consist of 1}3 layers of orientated particles (see results) measuring in total only 2}4 m in thickness. The location of shear planes generated in a ring-shear device may be caused also by the e!ects of the sample container and might therefore be instrument speci"c. In every direct shear experiment the position of the shear planes are strictly determined by the sample holder.
6. Discussion and conclusions The quanti"cation of the deformation behaviour of glaciogenic sediments by ring-shear tests is possible. The
Fig. 12. SEM photograph showing a cut through the unopended sample VS-3 with the very thin sheared zone in the middle of the sample (centre of the picture).
data from `Vorderes SchloK sslia show a strong relationship between physical properties and the deformation behaviour of the tills under laboratory conditions. It seems likely that the di!erence in the clay fraction (10}25%) between in the two till units may be the cause of the di!erent deformation styles noted. The clay-rich till samples tend to develop slickensides, while the sandrich tills do not show any detectable deformation structures. Therefore, it can be stated that the deformation style of the tills is directly in#uenced by the bedrock from which the till is derived. Maximum shear strength, especially residual shear strength, seems to be dependent on the grain size distribution and grain shape. Holtz and Ellis (1961) and Maha (1994) found similar results from testing arti"cial, bimodal mixtures of clay and pebbles. Our data demonstrate that the same pattern occurs in a matrix ((0.5 mm) of natural till sediment. Lithologically, the two till units are essentially identical. It appears that a di!erence of 10}25% in the planar-shaped clay size particles between the lower and upper till units is su$cient to change the shear behaviour of the sediments. It can also be shown, that relatively high brittleness indices (I '25) in shear tests are possible without the presence of sensitive clay minerals in the samples. The lower 10}15 cm thick layer of the Weisstannen Glacier till sheet has a very low shear strength. The angle of inner friction is 103 in the lower till unit ( f"18.63) than in the upper till unit ( f"28.43) at failure; while the residual shear strength di!erence is even more pronounced with f"27.13 in the upper and f"16.23 in the lower unit. However, cohesion seems to be the most important di!erence between the two layers. For the upper unit, a small increase in cohesion between
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the breaking shear strength and the residual shear strength is observed, while in the lower unit, a strong drop in cohesion in the order of 39% occurred. Following the suggestions and calculations of Iverson et al. (1998) for their ring-shear experiments on similar materials, there is no evidence from their test performed of a viscous behaviour in either till unit. These results suggest that the lower till unit of the Weisstannen Glacier reacted di!erently to the stress caused by the glacier during advance in comparison to the higher parts of the till sheet. 6.1. The lower till unit The genetic evolution of this particular sediment is closely related to the underlying shaley bedrock. The lithological composition of the unit consists exclusively of crushed local bedrock. The dominantly planar shape of the matrix particles leads to an particular geotechnical behaviour similar to sediments with a high content of sensitive clay minerals. Distinct slickensides present only in the basal unit re#ect the strong shear this layer has undergone. The sediments were most probably left by the glacier in a state of residual shear strength due to intense deformation. During this deformation process the formerly thicker sediment layer may have been thinned substantially. Comparing the high number of discrete shear planes and their well-developed aspect to arti"cially formed shear planes, long shear paths ('10 m?) have to be taken into account for the deformation of this unit. It is, therefore, suggested that this lower till unit can be interpreted as a typical deformation till. 6.2. The upper till unit As soon as the shaley bedrock was e!ectively sealed by the thin, lower, deformation till unit the lithic composition of the till changed. The glacier was unable to erode material from the underlying local bedrock and isolation of the shale terminated the massive in#ow of planar shaped, clay size grains to the glacier base. More sandy detritus from the sand-rich bedrock facies up-glacier became incorporated into this zone of "ll formation so that the physical properties of the till also changed. The upper till unit reacted di!erently to the strain induced by the moving glacier and due to the changed grain-size composition the freshly accumulated subglacial sediments could not be easily deformed. The strain needed to reach the point of failure of the sediments in this upper till unit had to be 30}80% higher than in the underlying lower till unit. Even after failure of the upper till unit, cohesion increased constantly thus requiring even higher strain levels to e!ectively deform. Showing no decrease of shear strength with continuing deformation the upper till became further stablised when sheared, requiring higher
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strain to induce deformation. Assuming that the strain rate a glacier can induce at its base is more or less constant for a given dynamic state or glacier extension, the freshly formed upper till unit was more di$cult or impossible to deform by the same glacier. Small-scale shearing over short, horizontal displacement paths ((2 mm), which do not reach the point of failure of the sediments during the deposition of the sediment, leads already to an increase in cohesion of the till. Sediments such as the upper till unit do not fail at a de"ned stress level but develop increasing resistance to the strain induced (Fig. 5, red curves). This behaviour of the sediments at the glacier base could either slow down the glacier or it could lead to an increased ongoing accretion of till material at the glacier base, since transport of till materials along the ice-sediment interface has become more di$cult under these circumstances. With sediment accreting at the glacier base, the zone of basal movement at the ice-bed interface would move slowly upward during this process, resulting in a homogenous and weakly sheared stack of sediment. Under these circumstances accretion and compaction of the till material is more or less continuously ongoing and there is little or no material transported along the glacier base by shearing or bulldozing. Similar sequences were described by Banham (1977) and Hart et al. (1990), the latter de"ning this process as `constructional deformationa. The only di!erence in the pro"le described in this paper is the complete lack of deformation in the upper part of the sequence. 6.3. Till facies Deformation structures are only to be found in the lower till unit, which directly overlies bedrock. Generally, more sandy upper till unit which is without clay minerals and has less planar-shaped particles does not show shear planes, slickensides nor other deformational patterns. The only `deformationala structures of the upper till are the clast fabrics. It is suggested here that subglacial sediments without any deformation visible either in thin section or with SEM have been formed by a mechanism of continuous accretion due to short shear paths and increasing sediment cohesion. According to the classi"cation of van der Meer and Menzies (1997) tills with no visible deformation structures should not be classi"ed as lodgement tills. Due to the absence of any deformational structures in these tills we suggest therefore a new classi"cation for subglacial tills without any deformation at any scale be termed: `non-deformable tillsa. One diagnostic criterion is the grain-size composition, with a clay fraction content (10% and with an important percentage of coarse silt and sand (35}60%). Not only are the physical and lithological characteristics important for the formation and preservation of a non-deformable till but it is essential that drainage of the subglacial sediments is
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B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
maintained during their accretion. As soon as pore water pressure increases, the sediments at the glacier bed may switch to a deforming state. Since the site under investigation is located in a steep gorge with a strong #ow gradient changing pore water pressure may not have played a signi"cant role. Even if temporary increases of the pore water pressure occurred, the upper till unit was not sensitive to them because of an improved permeability of the order of 1}2 magnitudes in comparison with the lower till unit. Di!erent grain-size compositions produced di!erent till facies at `Vorderes SchloK sslia. The observed progression from bedrock erosion to the deposition of a deformation till and subsequently to the accumulation of a non-deformable till are related to the ice dynamics. The lower till unit is most likely the product of a rapidly advancing glacier. The upper, and much thicker, till unit is interpreted as a non-deformable till of a slowly advancing or even stable glacier. This lithological and geotechnical interpretation of the sediments "ts with the paleoglaciological explanation (MuK ller, 1996) of a stable or slowly retreating local glacier which was later `overruna by the much larger Rhine glacier.
Acknowledgements We wish to express many thanks to everyone who helped us in producing data for this paper. Thieny Adatte from University of Neuchatel for the XRD analyses and their to interpretation; Marco Herwegh from the Geological Institute of University of Berne for his great help with the SEM pictures and thin section work and for the stimulating discussions; Heidi Haas for the carbonate content measurements. Many thanks to Prof. John Menzies for all the encouraging comments and for revising the manuscript and to the reviewers of this article and to the editor.
References Alley, R.B., 1988. Glacial deposits as (lack of ) evidence for deforming beds. Society of Economic Paleontologists and Mineralogists Annual Midyear Meeting Abstracts, Vol. 2, Columbus, Ohio. Alley, R.B., 1991. Deforming bed origin for southern Laurentide till sheets? Journal of Glaciology 37, 67}76. Alley, R.B., Blankenship, D.D., Bentley, C.R., Rooney, S.T., 1987. Till beneath Ice Stream B. 3. Till deformation: evidence and implications. Journal of Geophysical Research 92 (B9), 8921}8929. Banham, P.H., 1977. Glacitectonites in till stratigraphy. Boreas 6, 101}105. Bishop, A.W., 1967. Discussion session 2. Shear strength of sti! clay. Proceedings of the Geotechnical Conference, Oslo, pp. 35}46. Boulton, G.S., 1979. Processes of glacier erosion on di!erent substrata. Journal of Glaciology 23, 15}38.
Boulton, G.S., 1996. Theory of glacial erosion, transport and deposition as consequence of subglacial sediment deformation. Journal of Glaciology 42, 43}62. Clayton, L., Attig, J.W., Mickelsen, D.M., 1987. Evidence against deformable bed material as the cause of southern Laurentide ice streams and surges. INQUA 1987 programme with abstracts, p. 145. Clayton, L., Mickelsen, D.M., Attig, J.W., 1989. Evidence against pervasively deformed bed material beneath rapidly moving lobes of the southern Laurentide ice sheet. Sedimentary Geology 62 (3}4), 203}208. Deutsches Institut fuK r Normung e.V., 1993. Erkundung and Untersuchung des Baugrundes. DIN-Taschenbuch, Vol. 113. Beuth, Berlin. Harper, J.T., Humphrey, N.F., Pfe!er, W.T., 1998. Three-dimensional deformation measured in an Alaskan glacier. Science 281, 1340}1342. Hart, J.K., 1994. Till fabric associated with deformable beds. Earth Surface Processes and Landforms 19, 15}32. Hart, J.K., Boulton, G.S., 1991. The interrelationship between glaciotectonic deformation and glaciodeposition. Quaternary Science Reviews 10, 335}350. Hart, J.K., Hindmarsh, R.C.A., Boulton, G.S., 1990. Di!erent styles of subglacial glaciotectonic deformation in the context of the Anglian ice sheet. Earth Surface Processes and Landforms 15, 227}242. Hicock, S.R., Dreimanis, A., 1992. Deformation till in the Great Lakes region: implications for rapid #ow along the south-central margin of the Laurentide ice sheet. Canadian Journal of Earth Sciences 29, 1565}1579. Hiemstra, J.F., van der Meer, J.J.M., 1997. Pore-water controlled grain fracturing as an indicator for subglacial shearing in tills. Journal of Glaciology 43, 446}454. Holtz, W.G., Ellis, E., 1961. Triaxial shear characteristics of clayey gravelly soils. 5th International Conference on Soil Mechanics and Foundation Enginering, Vol. 1, Paris, pp. 143}149. Iverson, N.R., Baker, R.W., Hooyer, T.S., 1997. A ring-shear device for the study of till deformation: tests on tills with contrasting day contents. Quaternary Science Reviews 16, 1057}1066. Iverson, N.R., Hooyer, T.S., Baker, R.W., 1998. Ring-shear studies of till deformation: Coulomb-plastic behaviour and distributed strain in glacier beds. Journal of Glaciology 44, 634}642. Maha, D., 1994. Bodenmechanische Eigenschaften Schweizerischer Tone and toniger Kiese bei Kontakt mit DeponiesickerwaK ssern. VeroK !entlichungen des Institutes for Geotechnik der ETH ZuK rich 207. Maisch, M., Wipf, A., Denneler, B., Battaglia, J., Benz, C., 1999. Die Gletscher der Schweizer Alpen. Schlussbericht NFP 31. Hochschulverlag AG, Zurich. McGown, A., Derbyshire, E., 1977. Genetic in#uences on the properties of tills. Quarterly Journal of Engineering Geology 10, 389}410. Meer, J.J.M., van der, Menzies, J., 1997. Micromorphology of glacigenic sediments - Manual of the Second International Technical Workshop on the Micromorphology of Glacial Sediments. Revised 2nd printing. St. Cathrines/Canada. MuK ller, B.U., 1993. Zur QuartaK rgeschichte des Seeztales (Kt. St. Gallen). Dissertation ETH-Nr. 10184, 249 S. MuK ller, B.U., 1995. Des Walensee-/Seeztal (SG) - Eine Typusregion alpiner Talgenese. Vom Entstehen and Vergehen des grossen Rheintal-ZuK richsees. Eigenverlag, SarganserlaK nder Druck AG, Sargans. MuK ller, B.U., 1996. Letzteiszeitliche Glazitektonik im Weisstannental (SG) - eismechanische und klimatostratigraphische Implikationen. Eclogae Geologicae Helvetiae 8913, 1065}1075. MuK ller, B.U., 1998. Scherdeformationen an glazialen Lockergesteinen - In der Natur und im Laborversuch. Bulletin fuK r angewandte Geologie 312, 183}197. MuK ller-Vonmoos, M., Honold, P., Kahr, G., 1985. Des Scherverhalten reiner Tone. Mitteilungen des Instituts fuK r Grundbau und Bodenmechanik ETH-ZuK rich 122, 3}22.
B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125 Peters, T., 1969. Tonmineralogie einiger Glazialablagerungen im schweizerischen Mittelland. Eclogae Geologicae Helvetiae 62/2, 517}525. Richter, J., 1968. Die Geologie der Guscha-Gruppe im St. Galler Oberland. Dissertation ETH ZuK rich. Ronnert, L., Mickelson, D.M., 1992. High porosity of basal till at Borroughs glacier, southeastern Alaska. Geology 20, 849}852.
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Scott, J. S., 1976. Geology of Canadian tills. In Legget, R.F. (Ed.), Glacial Till. Royal Society of Canada Special Publication 12, 50}66. Van der Meer, J.J.M., 1993. Microscopic evidence of subglacial deformation. Quaternary Science Reviews 12, 553}587. Verein Schweizerischer Strassenfachleute, 1959. Norman Nr. 670 005, 670 008, 670 335, 670 340, 670 345, 670 810a, 670 812a, 670 816. Schweizerische Normenvereinigung (SNV), ZuK rich.
In#uence of the glacier bed lithology on the formation of a subglacial till sequence * ring-shear experiments as a tool for the classi"cation of subglacial tills B.U. MuK ller*, C. SchluK chter Institute of Geology, University of Berne, Baltzerstrasse 1, CH } 3012 Berne, Switzerland
Abstract A glaciogenic sediment sequence deposited by a local glacier in the Rhine catchment area near Sargans (Eastern Swiss Alps) was investigated in order to examine the shear behaviour of two di!erent tills. In addition to ring}shear experiments, examination of clast lithology, clay mineralogy and physical properties of all samples were determined. SEM}analyses of arti"cially produced deformation patterns in the till samples show only a very thin shear zones of 2}3 m thickness, despite long shear distances of up to 145 mm. A clear relationship between local bedrock and the formation of a thin basal layer of heavily deformed till can be demonstrated. This locally derived till layer exhibits distinctly di!erent shearing properties compared to the massive overlying till unit that is composed of more distantly transported lithic material from the upper catchment area of the glacier. The di!erence in the angle of internal friction between the two tills is '103 at residual shear strength, although the clay mineralogy of both tills is similar. It can further be demonstrated that after the deposition of a 10}15 cm thin till layer directly on bedrock, the #ow dynamics of the glacier tongue may have changed due to di!erences in glacier bed debris lithology. Di!erences in the shear behaviour of the sediments, allied to microscopic analyses, point to an important environmental change from a deformation till to an overlying `non}deformed tilla. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction It has been suggested that glaciers may develop a soft basal deformation layer (Alley et al., 1987, 1988, 1991; Boulton, 1979, 1996; Hart and Boulton, 1991). However, there are areas of past glacial environments where this model remains controversial, e.g., the southern lobes of the Laurentide ice sheet, whose sediments have been interpreted by some researchers as deforming-bed tills (e.g. Alley, 1991; Hicock and Dreimanis, 1992), whilst others have argued for a melt-out till origin (e.g. Clayton et al., 1987, 1989; Ronnert and Mickelson, 1992). While considering a soft deformation glacial-bed model, it remains, for example, unexplained how the glaciers of the Last Glacial Maximum (LGM) in some of the Alpine valleys in Switzerland removed soft-sediment bodies of up to 500 m in thickness (MuK ller, 1993, 1995). A recent model on glacial erosion by a deformable bed
* Corresponding author. Tel.: #41-31-631-8771; fax: #41-31-6314843. E-mail address: [email protected] (B.U. MuK ller).
mechanism (Boulton, 1996) may not be able to produce such amounts of erosion since the model has been developed primarily for ice sheets and not for valley glaciers. Another observation that seems to con#ict with a soft bed model is the absence of basal till covers in the bottom of many overdeepened valleys in Switzerland. It may therefore be reasonable to consider a predominantly basal erosional model for Alpine ice masses during most of the last glacial cycle. In assessing glaciodynamic models more reliance on glaciogenic material properties should be considered. For example, the sensitivity of sediment strain-rate to the possible variations in the applied shear stress may reveal new evidence indicative of subglacial processes. This would also be the case where most of the glacier movement consists of basal sliding (Harper et al., 1998). Scott (1976) argued that the lithology of the glacier bed and its in#uence on the physical properties of subglacial tills may point to particular glaciodynamic models. It follows therefore that the experimental data can help further to explain mechanisms at the glacier bed and may assist in understanding the erosional and depositional patterns found in the Alps.
0277-3791/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 1 4 1 - 4
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There is a "eld evidence that suggests that not all subglacial tills were formed in a deformable-bed environment. Most tills found on the northern slope of the Alps (Rhone glacier, Rhine glacier) do not show any macroscopic or microscopic evidence of deformation (MuK ller 1996, 1998). As these tills normally contain a high percentage of quartz sand derived from Tertiary sandstones it is possible that they have a deformation style, which does not favour the formation nor preservation of deformation structures. Investigations on the shear behaviour of tills have been made, for example, by McGown and Derbyshire (1977), and more recently by Iverson et al. (1997, 1998). However, a direct relationship between the #ow dynamics of an ice body over its bed and the shear strength of the sediments accumulated at the glacier bed can only be partly developed. The investigations reported here, all follow standard geotechnical methods. This paper presents the "rst set of results on a sediment pro"le in the Eastern Swiss Alps near Sargans (Fig. 1). The lithological properties of the sequence, as well as descriptions of macroscopic properties of the sediments, along with a paleoglaciological reconstruction have been previously published (MuK ller, 1996).
2. Geological background The till under study was deposited during the LGM advance by the local Weisstannen Glacier, which reached the main valley of the Rhine earlier than the Rhine glacier (Fig. 1a). This till is the only evidence of the build-up or the maximum of the LGM in the region. The site at Sargans illustrates a stratigraphic sequence from bedrock to subglacial tills to proglacial lacustrine sediments. Bedrock consists of unweathered, glacially polished Permian shales, dipping at a low angle (0553/0203) to the northeast. In the immediate area, red Permian shales dominate the catchment area of the local Weisstannen Glacier. These are easily recognised and distinguished from the dark greyish sediments derived from Rhine Glacier. Above bedrock, two tills, related it to the local Weisstannen Glacier, occur that can be recognised on the basis of colour and petrographical content. A basal till layer can be subdivied into an upper till unit overlying a "ne-grained, heavily deformed lower till unit. This basal till layer is remarkably homogenous without any apparent bedding, banding or layering. The tills of the local Weisstannen glacier in the lowermost part of the sequence (Fig. 1b) have not been deformed by the later Rhine glacier advance. Overlying the till layer is a sequence of local proglacial debris #ow sediments derived from the retreating Weisstannen Glacier. Following this retreat phase, the main Rhine glacier advanced and dammed a small ice-marginal lake in this tributary valley. A coarsening-upwards sequence of laminated clays and
silts to well-sorted gravels reveals the evidence for the in"lling of this proglacial lake. Following the in"lling phase, intense proglacial and subglacial glaciotectonic deformation in the lacustrine sediments occurred possibly due to Rhine glacier advance into the tributary valley. As a consequence of the main glacier advance, a thin "ll layer covering the whole sequence was deposited (MuK ller, 1996). The investigations presented here concentrate on the basal till unit and on comparison between the lower thin, basal, "ne-grained and the upper thicker till units.
3. Methods The following laboratory analyses were carried out to measure (1) the shear strength of the two lower and upper till units found in the basal till sequence described above; (2) to compare the shear behaviour of the two till units in terms of their respective lithological and physical properties; (3) to evaluate the in situ deformation structures in the basal till unit with those generated mechanically by the shear experiments; and (4) to attempt to explain the genesis of the till sequence and the processes by which the two individual units were formed and deposited. Laboratory analyses included grain size composition, bulk density, Atterberg limits, porosity, and the natural moisture content. Laboratory analyses were performed following SNV procedures (Verein Schweizerischer Strassenfachleute, 1959), directly comparable to DIN procedures (Deutsches Institut fuK r Normung e.V., 1993). Additionally, the calcium carbonate content of the (0.5 mm fraction was determined by colometric methods (CS-mat 5500 by StroK hlein, following the laboratory standards of the University of Berne). The clast lithology of the 8}63 mm size fractions was also determined in combination with clast morphometric analyses. Bulk rock mineralogy and clay mineralogy were determined using XRF at the University of Neuchatel. The shear tests were generated in a ring-shear device (Type Wille Geotechnik) based on the DIN-Norm 18137 Part 3 (Draft, October 1995). This device is computerised and driven electro-mechanically permitting normal stress and shear stress levels to be set and measured in very small increments, shear strain being measured at the lower shear ring (Fig. 2). The samples for shear stress determination have a 96 mm outer diameter and 50 mm inner diameter, with a surface area of 50 cm. Sample thickness varies between 14 and 18 mm depending on the degree of consolidation. With a maximum normal stress and shear stress of 500 kN/m, experiments can be carried out under water in order to ensure full water saturation during testing. It is di$cult to cut poorly sorted tills as undisturbed samples to required ring-shaped sample size. Therefore, it was necessary to develop a sample preparation method
B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
Fig. 1. (a) Location of Vorderes SchloK ssli investigation site.
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for disturbed samples. According to Holtz and Ellis (1961) gravel contents of less than 40% has little e!ect on shear strength, therefore, it was decided to remould the tills which could not be cut to the sample size needed. These samples were "rst sieved at 0.5 mm to remove the coarse grains. For the construction of the ring-shaped samples the homogenised fraction ((0.5 mm) was used and poured into the shear sample container up to a height of 20 mm. The dry powder was saturated with water and consolidated in incremental steps (25}50}100}200}500 or 125}250}500 kN/m) depending on the Atterberg limits of the sample. Normal stress during consolidation was switched to a higher value when the measured settlement of the sample was less than 0.01 mm/15 min. After reaching the end of the maximum consolidation step, each sample was given one hour of relaxation time before the next shear test began. Of the samples analysed for this investigation 90% were sheared under overconsolidated conditions. In otherwords, the normal stress during the test has to be equal or lower than the highest consolidation stress produced during consolidation. Shear distance ranged between a minimum of 50 mm and to a maximum of 295 mm. Shear speed varied from 0.01 to 0.05 mm/min depending on the Atterberg limits (Table 1). A high plasticity index dictated low shear speeds. It should be noted that these speeds (10 to 50/ma) are comparable to typical velocities of some larger present-day Alpine
Fig. 1. (b) Schematic overview of the Vorderes SchloK ssli pro"le.
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B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
Fig. 2. Outline of the sample holder of the ring-shear device type `WILLE Geotechnika.
Table 1 Applied displacement speeds during the shear experiments depending on the Plasticity index (I ) of the sample Plasticity index I (%)
Maximal shear speed (mm/min)
(10 (20 20}30 '30
0.05 0.03 0.02 0.01
glaciers (Maisch et al., 1999). For drained shear tests, very low shear speeds are required to ensure that total drainage of the shear plain develops in the sample during the test. Following each shear experiment, the sample water content, and wet and dry bulk densities were calculated and the compaction of each sample due to shearing was also determined. If slickensides appeared on the experimentally developed shear plain, they were examined under SEM. Finally, for most samples, thin sections were made in order to microscopically examine any deformation patterns present on the shear planes.
4. Results Seven samples from the basal till layer were analysed, "ve from the lower till unit (VS-3, VS-3a, VS-12, VS-12N and VS-13) and two from the upper till unit (VS-1 and VS-8). 4.1. Lithology and petrography At `Vorderes SchloK sslia the till directly overlies the Permian `SchoK nbuK hl-Schiefera, a very "ne-grained, pinkish shale. The shales occur only locally for about 2.5 km up-valley and 1 km down-valley from the study site. Petrographical analyses of the 8}63 mm fraction of the basal till layer show that all the till samples were of similar composition, derived from the local catchment of the Weisstannen Valley and were incorporated homogeneously into the basal till sequence (Fig. 3). A higher percentage of the Permian `SchoK nbuK hla shales can be found in the lower till unit (sample VS-3, Fig. 3). Heavy minerals populations within the basal till layer including zircon, tourmaline, rutile and sphene (MuK ller, 1995) demonstrate that this basal layer is exclusively derived from the local Permian and Triassic rocks of the Helvetic nappes.
B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
4.2. Clay mineralogy To better understand the shear tests it was decided to analyse the bulk rock composition and the clay fraction of the samples by XRD (Table 2). All bulk rock samples of the basal till units show a strong correlation to the bedrock lithology (Richter, 1968). Samples VS-3 and VS-13 from the lower till unit show a mineral content
1117
with up to 80 % phyllosilicates, the remaining 20% being quartz, plagioclase and calcium carbonate. Sample VS-1 contains a smaller percentage of phyllosilicates and more quartz and plagioclase. The carbonate content for all the samples analysed is similar to the bedrock (Richter, 1968). The 2}16 m fraction shows a similar composition to the bulk rock samples, although there is slightly more quartz. Within the clay fraction ((2 m) the mineralogical composition is mostly fresh, unweathered chlorite and mica with small amounts of quartz. No reworked clay minerals from soils can be found in these tills in contrast to those observed in other samples from the northern Alpine Foreland (Peters, 1969). 4.3. Grain size distributions Five samples were analysed by sieving and in the Atterberg cylinder (Verein Schweizerischer Strassenfachleute, 1959). The results are given in Fig. 4. 4.4. Physical properties The water content, bulk densities before and after the shear test, and the Atterberg limits of all the samples are shown in Table 3. 4.5. Shear strength determination (ring-shear testing)
Fig. 3. Lithological composition of the pebble fraction (8}63 mm) of the till deposited by the Weisstannen Glacier.
The samples from both the lower and upper till units of the basal till layer show di!erent deformation properties (Fig. 5). Sample VS-1 was taken from the upper till unit 0.8 m above bedrock. In the shear strain/shear path diagram, no decrease in shear strain to residual shear
Table 2 The lithologic composition (determined by XRD) of the bulk rock samples of the fraction 2}16 m and the clay mineral content (fraction (2 m) of the samples from Vorderes SchloK ssli Bulk rock Sample no.
Phyllosilicates (%)
Quartz (%)
K-feldspar (%)
Plagioclase (%)
Calcite (%)
Dolomite (%)
VS-1 VS-3 VS-13
55.0 75.7 81.4
19.8 7.7 8.9
0.0 0.0 0.0
11.1 9.2 7.3
13.0 5.0 2.4
1.1 2.4 0.0
Quartz
K-Feldspar
Plagioclase
Mica (in % of phyllosilicates)
Chlorite (in % of phyllosilicates)
Fraction 2 } 16 m Phyllosilicates (relative abundance) Sample no. VS-1 VS-3 VS-13
*** *** ***
* * *
* * **
** ** **
70 65 68
30 35 32
Fraction (2 m (%) Sample no.
Smectites
Chlorite
Mica
I/S mixed layers
Kaolinite
Quartz
VS-1 VS-3 VS-13
* * *
28 25 22
72 75 78
* * *
* * *
? ? ?
B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
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Fig. 4. Grain-size distributions of "ve till samples from the Vorderes SchloK ssli in a summary diagram. Table 3 Geotechnical properties of VS-3, VS-12 and VS-13 from the basal till layer, and VS-1 and VS-8 from the upper till unit Sample no.
Water Content (%)after shear test (saturation)
Bulk density (g/cm) (after shear test, wet)
Bulk density (g/cm) (after shear test, dry)
Liquid limits (%)
Plastic limit (%)
Plasticity index (%)
VS-1 VS-3 VS-8 VS-12 VS-13
18.0 23.4 n.m. 17.4 17.0
2.29 2.29 n.m. 2.32 2.36
1.95 1.86 n.m. 1.91 1.95
18.0 25.3 18.1 24.5 24.0
13.0 16.1 12.0 15.0 15.1
5.0 9.2 6.1 9.5 8.8
n.m.-not measured.
strength was observed during the experiment. After failure occurred, shear strain increased slowly and continued even after long shear distances of up to 150 mm were used. Sample VS-3a, taken from the lower till unit, exhibits a much lower strain for all experiments under identical testing conditions. It is not only the di!erence in absolute shear stress at the point of failure of the sample which di!ers widely, but also the further development of the stress curves towards the state of residual strength that is totally di!erent. Sample VS-3a shows a wellde"ned decrease in shear stress while Sample VS-1 shows shear stress increasing slowly after the point of failure. This di!erent behaviour can also be expressed using the brittleness index (I ) (Bishop, 1967) I (%)"T !T /T ,
(1)
where T is the maximal shear strength and T the resid ual shear strength.
A high brittleness index (I ) implies a sharp decrease of the shear stress after the failure of the sample generally caused by either a high clay mineral content or by a high overconsolidation ratio (MuK ller-Vonmoos et al., 1985). As a general trend it can be seen that samples from the lower till unit have higher values for I than those from the upper till unit (Table 4). An average shear line, calculated from all till sample data points (Fig. 6a and b), provides an approximation of how well the shear experiments on di!erent samples (from the same till) "t and the accuracy of the tests. With correlation coe$cients ranging between 0.94 and 0.99, the di!erent test results can be used to calculate average angles of friction and cohesion for the two basal till units. It is readily apparent that the di!erence between the maximum shear strength (red data points) and the residual shear strength (blue data points) in the lower till unit is larger than in the upper till unit (Table 5). The results show strong evidence for the di!erent behaviour of the two tills with a di!erence of 9.8% for
B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
maximum shear strength and of 10.9% for the angle of inner friction. Cohesion is 20% higher at failure (maximum shear strength) for the lower till unit and drops sharply during the tests. Likewise, the residual shear strength is 36% lower in the lower till unit than in the upper till. 4.6. Macrofabrics One macrofabric was analysed in the upper till unit approximately 0.7 m above bedrock, and 0.55 m above the top of the lower till unit. Woodcock statistics reveal a very strong fabric where for 24 measurements: C"4.36, and k"0.795; and for 19 measurements: C"4.67 and k"0.65. Calculating the Bingham statistics for eigenvalues generates the following results: S1: 0.864; S3: 0.011; with a mean: 70.6. Due to the poor outcrop, it was not possible to complete macrofabric measurements in the lower till unit. Based on Hart (1994) the above values could provide evidence for only a thin deformation layer; however, a di!erent interpretation of these results will be discussed later where it will be
Fig. 5. Diagram of shear-strain versus shear distance of tested samples VS-3A and VS-1.
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suggested that no deforming layer occurred within the upper till unit of this basal till layer. 4.7. Micromorphology and SEM At both the macroscopic and microscopic scale, the lower till unit shows extremely strong internal deformation. Deformation style could be described best as homogeneously chaotic. Numerous very small slickensides can be observed exhibiting no preferred orientation and are distributed regularly some contorted or even folded. Due to the abundance of small-scale slickensides, the till matrix shows a characteristic polished surface appearance. The upper till unit does not exhibit slickensides nor other planar deformation structures although the clast fabric is strong (Fig. 7). Thin sections display evidence of considerable deformation. Brittle deformation seems to dominate as many zones of discrete shear planes display unistrial, plasmic fabric (van der Meer, 1993; Hiemstra and van der Meer, 1997) (see VS-13; Fig. 8b). Additional evidence for brittle deformation in the lower till unit at `Vorderes SchloK sslia are broken grains `#oatinga in the "ne-grained matrix (Fig. 8a). Other deformational features such as rotational structures are rare but occasionally observed. There is a dominance of brittle deformation in comparison to ductile deformation. It is possible that brittle deformation structures were produced at a later stage of till deformation and therefore, overprint any earlier deformation patterns. It might be expected that the grain-size composition of these sediments would favour ductile deformation; however, if sediment moisture content was below the sediment's plasticity limit during deformation only brittle failure would occur. Thin sections from the upper till unit show no macroscopic nor microscopic deformation although the sediments are poor in calcium carbonate and rich in clay fraction, which might favour the visibility of deformation structures. A comparison was made between in situ deformation structures observed in the tills at `Vorderes SchloK sslia with those structures produced in the laboratory. Despite the long shear paths in the ring shear device (50}295 mm) there is surprisingly little evidence of deformation in the
Table 4 The shear parameters of the samples from Vorderes SchloK ssli Sample no.
Angle of friction (max shear strength) (3)
Cohesion (max shear strength) (kN/m)
Angle of friction (resid Cohesion (resid shear shear strength) (3) strength) (kN/m)
Brittleness Index (I ) (%)
VS-1 VS-3 VS-3A VS-8 VS-12 VS-12N VS-13
28.5 24.0 18.8 30.8 20.9 20.3 20.6
19.5 11.8 47.7 9.4 11.4 11.9 23.7
27.7 20.6 13.8 29.4 15.5 12.8 17.8
2.9 22.2 24.8 !1.4 28.1 28.4 30.1
19.1 1.1 39.3 16.1 6.8 19.8 !1.01
B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
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Fig. 6. (a) Average shear-line plot for the upper till unit (samples VS-1, VS-8) and (b) average shear-line plot of the lower till unit (samples VS-3, VS-3A, VS-12 and VS-13) from the Vorderes SchloK ssli.
Table 5 Summary of the mean shear parameters for the two till units Till unit
Angle of friction (maximal shear strength) (3)
Cohesion (maximal shear strength) (kN/m)
Angle of friciton (residual shear strength) (3)
Cohesion (residual shear strength) (kN/m)
Regression quality R (s/r)
Basal unit (samples VS-3, VS-3A, VS-12, VS-13) Upper unit (samples VS-1, VS-8)
18.6
27.2
16.2
16.8
0.975/0.946
28.4
21.8
27.1
26.0
0.995/0.992
deformation was noted in the thin sections. However, using SEM (Figs. 10}12) striking evidence of `polishinga of the slickensided shear plane was observed (Fig. 10). Most of the planar particles are very clearly orientated parallel to the shear plane (Fig. 11). In a transverse view of the same shear plane a small zone is developed of oriented particles of only two to four particle layers with a total thickness of 3}5 m (Fig. 12).
5. Methodological constraints
Fig. 7. Till macrofabric of the upper part of the till of the local Weisstannen glacier at Vorderes SchloK ssli. Pebble long axes orientation: scatter and contour plot.
arti"cially produced sediments when seen in thin section. On the macroscopic scale slickenside structures on he shear plane are observed (Fig. 9) but no trace of
The in#uence of variable pore water pressure on the mechanical behaviour of the glacigenic sediments is important; however, it could not be simulated in our experiments because it is not possible yet to carry out undrained testing in a ring-shear device. Any model dealing with basal deformation of glacial sediments has to permit pore water pressure changes over time at any given locality. By conducting, in the "rst instance, shear tests with a ring-shear device, long shear path distances could be acheived that allow the determination of residual shear strengths approximating conditions at the base of a glacier. Positive pore water pressures can also be achieved in the shear plane using this direct-shear device
B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
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Fig. 8. Photomicrographs, (a) crushed grain (sample VS-13d); and (b) thin section from sample VS-13 with zones of discrete shear with unistrial plasmic fabric visible in bright grey.
Fig. 9. Photograph of the two shear-plane-halves produced during shear experiment VS-12N.3.
by increasing shear speed. In this latter case the drainage of the shear plane is not possible and due to the rearrangement of the sediment particles, positive pore water pressures can develop. This procedure, however, is very di$cult to verify, as local increased pore water pressure values cannot be measured.
Experimentally developed deformation structures in the samples seem to be di!erent from the ones found in nature. Since particular brittle deformation structures are small and therefore discrete shear planes without parasitic branches may not be visible in thin section. Shear planes with well-developed slickensides sometimes
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Fig. 10. SEM photograph of a shear plane from the sample VS-13. View of the slickensided shear plane with planar particles showing a strong orientation parallel to the shear plane.
Fig. 11. SEM picture of a cut through an opened shear plane of sample VS-3. On the upper edge of the sample, particles show an orientation parallel to the shear-plane. In the central and in the lower part of the picture there are di!erent domains of particles with aberrant orientation.
consist of 1}3 layers of orientated particles (see results) measuring in total only 2}4 m in thickness. The location of shear planes generated in a ring-shear device may be caused also by the e!ects of the sample container and might therefore be instrument speci"c. In every direct shear experiment the position of the shear planes are strictly determined by the sample holder.
6. Discussion and conclusions The quanti"cation of the deformation behaviour of glaciogenic sediments by ring-shear tests is possible. The
Fig. 12. SEM photograph showing a cut through the unopended sample VS-3 with the very thin sheared zone in the middle of the sample (centre of the picture).
data from `Vorderes SchloK sslia show a strong relationship between physical properties and the deformation behaviour of the tills under laboratory conditions. It seems likely that the di!erence in the clay fraction (10}25%) between in the two till units may be the cause of the di!erent deformation styles noted. The clay-rich till samples tend to develop slickensides, while the sandrich tills do not show any detectable deformation structures. Therefore, it can be stated that the deformation style of the tills is directly in#uenced by the bedrock from which the till is derived. Maximum shear strength, especially residual shear strength, seems to be dependent on the grain size distribution and grain shape. Holtz and Ellis (1961) and Maha (1994) found similar results from testing arti"cial, bimodal mixtures of clay and pebbles. Our data demonstrate that the same pattern occurs in a matrix ((0.5 mm) of natural till sediment. Lithologically, the two till units are essentially identical. It appears that a di!erence of 10}25% in the planar-shaped clay size particles between the lower and upper till units is su$cient to change the shear behaviour of the sediments. It can also be shown, that relatively high brittleness indices (I '25) in shear tests are possible without the presence of sensitive clay minerals in the samples. The lower 10}15 cm thick layer of the Weisstannen Glacier till sheet has a very low shear strength. The angle of inner friction is 103 in the lower till unit ( f"18.63) than in the upper till unit ( f"28.43) at failure; while the residual shear strength di!erence is even more pronounced with f"27.13 in the upper and f"16.23 in the lower unit. However, cohesion seems to be the most important di!erence between the two layers. For the upper unit, a small increase in cohesion between
B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
the breaking shear strength and the residual shear strength is observed, while in the lower unit, a strong drop in cohesion in the order of 39% occurred. Following the suggestions and calculations of Iverson et al. (1998) for their ring-shear experiments on similar materials, there is no evidence from their test performed of a viscous behaviour in either till unit. These results suggest that the lower till unit of the Weisstannen Glacier reacted di!erently to the stress caused by the glacier during advance in comparison to the higher parts of the till sheet. 6.1. The lower till unit The genetic evolution of this particular sediment is closely related to the underlying shaley bedrock. The lithological composition of the unit consists exclusively of crushed local bedrock. The dominantly planar shape of the matrix particles leads to an particular geotechnical behaviour similar to sediments with a high content of sensitive clay minerals. Distinct slickensides present only in the basal unit re#ect the strong shear this layer has undergone. The sediments were most probably left by the glacier in a state of residual shear strength due to intense deformation. During this deformation process the formerly thicker sediment layer may have been thinned substantially. Comparing the high number of discrete shear planes and their well-developed aspect to arti"cially formed shear planes, long shear paths ('10 m?) have to be taken into account for the deformation of this unit. It is, therefore, suggested that this lower till unit can be interpreted as a typical deformation till. 6.2. The upper till unit As soon as the shaley bedrock was e!ectively sealed by the thin, lower, deformation till unit the lithic composition of the till changed. The glacier was unable to erode material from the underlying local bedrock and isolation of the shale terminated the massive in#ow of planar shaped, clay size grains to the glacier base. More sandy detritus from the sand-rich bedrock facies up-glacier became incorporated into this zone of "ll formation so that the physical properties of the till also changed. The upper till unit reacted di!erently to the strain induced by the moving glacier and due to the changed grain-size composition the freshly accumulated subglacial sediments could not be easily deformed. The strain needed to reach the point of failure of the sediments in this upper till unit had to be 30}80% higher than in the underlying lower till unit. Even after failure of the upper till unit, cohesion increased constantly thus requiring even higher strain levels to e!ectively deform. Showing no decrease of shear strength with continuing deformation the upper till became further stablised when sheared, requiring higher
1123
strain to induce deformation. Assuming that the strain rate a glacier can induce at its base is more or less constant for a given dynamic state or glacier extension, the freshly formed upper till unit was more di$cult or impossible to deform by the same glacier. Small-scale shearing over short, horizontal displacement paths ((2 mm), which do not reach the point of failure of the sediments during the deposition of the sediment, leads already to an increase in cohesion of the till. Sediments such as the upper till unit do not fail at a de"ned stress level but develop increasing resistance to the strain induced (Fig. 5, red curves). This behaviour of the sediments at the glacier base could either slow down the glacier or it could lead to an increased ongoing accretion of till material at the glacier base, since transport of till materials along the ice-sediment interface has become more di$cult under these circumstances. With sediment accreting at the glacier base, the zone of basal movement at the ice-bed interface would move slowly upward during this process, resulting in a homogenous and weakly sheared stack of sediment. Under these circumstances accretion and compaction of the till material is more or less continuously ongoing and there is little or no material transported along the glacier base by shearing or bulldozing. Similar sequences were described by Banham (1977) and Hart et al. (1990), the latter de"ning this process as `constructional deformationa. The only di!erence in the pro"le described in this paper is the complete lack of deformation in the upper part of the sequence. 6.3. Till facies Deformation structures are only to be found in the lower till unit, which directly overlies bedrock. Generally, more sandy upper till unit which is without clay minerals and has less planar-shaped particles does not show shear planes, slickensides nor other deformational patterns. The only `deformationala structures of the upper till are the clast fabrics. It is suggested here that subglacial sediments without any deformation visible either in thin section or with SEM have been formed by a mechanism of continuous accretion due to short shear paths and increasing sediment cohesion. According to the classi"cation of van der Meer and Menzies (1997) tills with no visible deformation structures should not be classi"ed as lodgement tills. Due to the absence of any deformational structures in these tills we suggest therefore a new classi"cation for subglacial tills without any deformation at any scale be termed: `non-deformable tillsa. One diagnostic criterion is the grain-size composition, with a clay fraction content (10% and with an important percentage of coarse silt and sand (35}60%). Not only are the physical and lithological characteristics important for the formation and preservation of a non-deformable till but it is essential that drainage of the subglacial sediments is
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B.U. Mu( ller, C. Schlu( chter / Quaternary Science Reviews 20 (2001) 1113}1125
maintained during their accretion. As soon as pore water pressure increases, the sediments at the glacier bed may switch to a deforming state. Since the site under investigation is located in a steep gorge with a strong #ow gradient changing pore water pressure may not have played a signi"cant role. Even if temporary increases of the pore water pressure occurred, the upper till unit was not sensitive to them because of an improved permeability of the order of 1}2 magnitudes in comparison with the lower till unit. Di!erent grain-size compositions produced di!erent till facies at `Vorderes SchloK sslia. The observed progression from bedrock erosion to the deposition of a deformation till and subsequently to the accumulation of a non-deformable till are related to the ice dynamics. The lower till unit is most likely the product of a rapidly advancing glacier. The upper, and much thicker, till unit is interpreted as a non-deformable till of a slowly advancing or even stable glacier. This lithological and geotechnical interpretation of the sediments "ts with the paleoglaciological explanation (MuK ller, 1996) of a stable or slowly retreating local glacier which was later `overruna by the much larger Rhine glacier.
Acknowledgements We wish to express many thanks to everyone who helped us in producing data for this paper. Thieny Adatte from University of Neuchatel for the XRD analyses and their to interpretation; Marco Herwegh from the Geological Institute of University of Berne for his great help with the SEM pictures and thin section work and for the stimulating discussions; Heidi Haas for the carbonate content measurements. Many thanks to Prof. John Menzies for all the encouraging comments and for revising the manuscript and to the reviewers of this article and to the editor.
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