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.

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 research^3,6,7. 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 strains^10,11.

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 directions^17-19. 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)