Clast Shape, Till Fabrics and Striae

This section is taken from Bethan Davies’ PhD thesis.

Clast Angularity-Roundness | Clast Macro-Fabric Analysis | Striae Orientation | References | Comments |

Clast Angularity-Roundness

Clasts inherit their shapes from the surrounding environment; erosion, transportation and weathering give clasts distinctive geomorphological signatures1. Angularity-roundness is simple to measure in the field when undertaking till-fabric analysis. Descriptive criteria are used to assign clasts to a roundness category (Table 1). A semi-quantitative approach is used, considering the whole shape of the clast. The sharpest edge may not be representative of the whole roundness. Clasts are therefore assigned to categories based on descriptive criteria2.

Table 1: Descriptive clast-roundness categories. From Benn (2007)1.

Very Angular (VA) Edges and faces unworn, sharp, delicate protuberances.
Angular (A) Faces and edges unworn.
Subangular (SA). Faces unworn, edges worn
Subrounded (SR) Faces and edges worn but clearly distinguishable
Rounded (R) Edges and faces worn and barely distinguishable
Well Rounded (WR) No edges or faces distinguishable

Clast Macro-Fabric Analysis

The measurement of the arrangement of clasts within a diamicton can be a powerful tool in the analysis of Quaternary sediments3, and it is traditionally used, in conjunction with striae data, as a standard quantitative tool in the analysis of past ice flow directions. More recently, till fabric data has been used to infer process4. Till fabric data can be used together with striae data to reconstruct ice flow direction, and can be used to help interpret depositional processes. The resulting data are three mutually orthogonal eigenvectors (V1, V2 and V3), with the principal eigenvector, V1, being parallel to the axis of maximum clustering in the data. V3 is normal to the preferred plane of the fabric. The degree of clustering about the eigenvectors is given by the eigenvalues S1, S2, and S3, with their relative magnitudes reflecting the fabric shape5.

A-axis fabric data has a long history of research3,6,7. Coulomb plastic behaviour involves slippage between clasts and the surrounding, faster-flowing matrix. Therefore, more elongate clasts assume a minimum cross-sectional area, orientating the a-axis parallel to main stress direction. This makes strong, consistent till macro-fabrics in tills useful in interpreting palaeo ice-flow directions8.

Jæren, south-west Norway, forms the onshore border of the Norwegian Channel ice stream. The Jæren escarpment, separating Low Jæren from High Jæren, was formed by erosion by the ice stream, which occupied the Norwegian Channel on multiple occasions during Pleistocene glaciations8. Early analysis of the till macrofabrics indicated a strong westerly to south-westerly flow direction, but in Stavnheim, further south in Jæren, till fabrics measured a northwest to west component9. Andersen et al. (1987) argued that the glacier in Low Jæren moved north-westwards in an earlier phase, and then later moved in a westerly direction. Jónsdóttir et al. (1999) analysed till macro-fabrics and striations, aiming to delineate the pattern of regional glacial movements using macro-fabrics and clast lithology. They interpreted the glacigenic sediments as lodgement tills. The upper till had a strong, unimodal clustering of clast axes around the mean axis, resulting in a high significance value. The clast fabric from the lower till had a weak, equatorial, near random orientation of clast axes8. The direction of maximum clustering (V1 157° to 161°) coincided with the direction of the Jæren escarpment axis. Jónsdóttir et al. (1999) interpreted the upper fabric as representing palaeo ice flow direction as towards the northwest. The lower fabric was, however, probably influenced by cobbles and boulders, leading to a local fabric probably unrelated to glacier flow.

Recently, eigenvalues (S values) and vectors (V values) have been used to infer the genesis of glacial materials, indicating factors such as the rheology of the sediment. For example, debris-rich basal ice subjected to high cumulative strains tends to have strongly clustered clast macro-fabrics, whereas tills formed under low strain can have either strongly clustered or highly variable clast macro-fabrics3. Other researchers have found strong fabrics at low strains10,11.

Figure 1: Schematic diagram illustrating (A) Jeffery Rotation and (B) March Rotation. In Jeffery Rotation clasts are continually rotated as a result of vertical velocity gradients, whereas in March Rotation (B), clasts passively trace the deformation of the surrounding medium (from Benn, 2007b)

Hicock et al. (1996) advise caution in using till fabrics to infer genesis of sediments, and suggest that they only be used as a starting point. Eigenvalues cannot be used alone, given the complexity of the subglacial environment7. Some researchers have argued that Jeffery-type rotation (Figure 1A) is incompatible with the deforming bed hypothesis12. March-type rotation (Figure 1B) through plastic deformation has been identified as the dominant mode of clast orientation in deforming tills13. Weak clast macro-fabrics have often been reported as typical of deforming bed tills14, suggesting that particles are here free to rotate in a viscous medium15. Inhomogeneous deformation may produce a range of clast macro-fabric strengths, and localised fabric patterns reflect the deformation history and local strain conditions of the sediment15.

Carr and Rose (2003) concluded that “particle orientations in subglacial diamictons reflect the strain response of the sediment to the applied total stress during subglacial deformation”, and that particles of different size are rarely consistently orientated in relation to ice flow direction. Therefore it is important to limit the size range in the sample4.

To obtain the clast measurements, clasts in the approximate size range 8-32 mm should be excavated and the long axis (a-axis) and dip angle of 50 clasts per exposure recorded, using a compass-clinometer (Benn, 2007b). The data are presented in equal-area stereonets and rose diagrams, according to procedures in Evans and Benn (2004) and Benn (2007b). Clasts should be sampled from a 2 m2 area. Only clasts with elongate a-axes were measured, with ratios of >1.5:13. All three eigenvalues should be given.

Striae Orientation

Striae are used in conjunction with till-fabric analysis to reconstruct past ice-flow directions. Striae on individual in situ clasts and boulders were measured using a compass-clinometer. Up to 50 striae sets were collected per exposure, and at least 10 per clast. If a clast showed several sets of striae, then these were also noted. The data were collated and presented in rose diagrams.

Striae orientation has been often been used successfully in conjunction with clast macro-fabric analysis to determine palaeo ice flow directions16. In addition, striae on bedrock forms can be used to infer palaeo ice flow directions17-19. Striations are important as they provide an independent evidence for ice flow3.

Citation

Davies, B.J., 2009. British and Fennoscandian Ice-Sheet Interactions during the Quaternary, Unpubl. PhD Thesis. Department of Geography, Durham University, Durham, 502 pp.

Bethan Davies Thesis (Zipped PDFs – 70MB)

References


1.            Benn, D.I. Clast Form Analysis. in Encyclopedia of Quaternary Science (ed. Elias, S.A.) 904-909 (Elsevier, Oxford, 2007).

2.            Benn, D.I. Fabric strength and the interpretation of sedimentary fabric data. Journal of Sedimentary Research A64, 910-915 (1994).

3.            Benn, D.I. Macrofabric. in A practical guide to the study of glacial sediments (eds. Evans, D.J.A. & Benn, D.I.) 93-114 (Arnold, London, 2004).

4.            Carr, S.J. & Rose, J. Till fabric patterns and significance: particle response to subglacial stress. Quaternary Science Reviews 22, 1415-1426 (2003).

5.            Hubbard, B. & Glasser, N.F. Field Techniques in Glaciology and Geomorphology, 412 (Wiley, 2005).

6.            Lawson, D.E. A comparison of the pebble orientations in ice and deposits of the Matamuska Glacier, Alaska. Journal of Geology 87, 629-645 (1979).

7.            Hicock, S.R., Goff, J.R., Lian, O.B. & Little, E.C. On the interpretation of subglacial till fabric. Journal of Sedimentary Research 66, 928-945 (1996).

8.            Jónsdóttir, H.E., Sejrup, H.P., Larsen, E. & Stalsberg, K. Late Weichselian ice-flow direction in Jæren, SW Norway; clast fabric and clast lithology evidence in the uppermost till. Norweigian Journal of Geography 53, 177-189 (1999).

9.            Andersen, B.G., Wangen, O.P. & Østmo, S.R. Quaternary geology of Jæren and adjacent areas, south-western Norway, 55 (Norges Geologiske Undersøkelse Bulletin, 1987).

10.          Iverson, N.R., Jansson, P. & Hooke, R.L. In-situ measurement of the strength of deforming subglacial till. Journal of Glaciology 40, 497-503 (1995).

11.          Hooyer, T.S. & Iverson, N.R. Clast fabric development in a shearing granular material: implications for subglacial till and fault gauge. Geological Society of America Bulletin 112, 683-692 (2000).

12.          Piotrowski, J.A., Mickelson, D.M., Tulaczyk, S., Krzyszkowski, D. & Junge, F.W. Were deforming subglacial beds beneath past ice sheets really widespread? Quaternary International 86, 139-150 (2001).

13.          Benn, D.I. & Evans, D.J.A. Glaciers & Glaciation, 802 (Hodder Education, London, 2010).

14.          Hart, J.K. The relationship between drumlins and other forms of subglacial glaciotectonic deformation. Quaternary Science Reviews 16, 93-107 (1997).

15.          Evans, D.J.A., Phillips, E.R., Hiemstra, J.F. & Auton, C.A. Subglacial till: Formation, sedimentary characteristics and classification. Earth-Science Reviews 78, 115-176 (2006).

16.          Hicock, S.R. & Fuller, E.A. Lobal interactions, rheologic superposition, and implications for a Pleistocene ice stream on the continental shelf of British Columbia. Geomorphology 14, 167-184 (1995).

17.          Haavisto-Hyvärinen, M. Pre-crag ridges in southwestern Finland. Sedimentary Geology 111, 147-159 (1997).

18.          Ballantyne, C.K. Maximum altitude of Late Devensian glaciation on the Isle of Mull and Isle of Jura. Scottish Journal of Geology 35, 97-106 (1999).

19.          Roberts, D.H., Dackombe, R.V. & Thomas, G.S.P. Palaeo-ice streaming in the central sector of the British-Irish Ice Sheet during the Last Glacial Maximum: evidence from the northern Irish Sea Basin. Boreas 36, 115-129 (2007).

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