This section is taken from Bethan Davies’ PhD thesis.
Introduction to micromorphology
Micromorphology, or thin-section analysis, is the microscopic examination of the composition and structure of sediments. It was originally developed in soil science, with concepts of plasmic fabric and morphological features and structures dating from the early 1960s1. The undisturbed sediments are examined for a range of recognised microstructures, such as those first described by van der Meer (1993) and Menzies and Maltman (1992)2,3. They introduced key terms and concepts still used today, such as ‘plasmic fabrics’, and identified key structures indicative of subglacial deformation, such as rotational structures, necking structures, and crushed grains4-7. These structures can be used to account for the origins of a sediment, its transport pathways, and the processes of deposition and deformation8,9. Micromorphology now provides detailed information to aid the interpretation of sediments that are often massive at macroscale, and can give valuable information regarding genesis, deformation history and strain rates10.
Later developments attempted to quantify micromorphology and introduce guidelines into its application to glacial sediments10,11. Structural geology recently influenced the development of micromorphology, and analysis of structural features in subglacially and proglacially deformed materials can additionally identify different types of characteristic subglacial deformation12-15. Systematic structural analysis gives a deeper understanding of tectonostratigraphic sequences in soft sediments, and the glacier-induced stresses responsible for their development16.
Understanding depositional processes
Now, the combined use of sedimentology and micromorphology is important in determining the processes of deposition, post-depositional deformation, and porewater fluctuations in glacigenic sediments. It can be used carefully to discriminate between macroscopically similar diamictons, such as debris flows, traction tills, and glaciomarine and glaciolacustrine sediments11,17-20. Furthermore, thin-section microfabrics give valuable information regarding genesis and strain directions in the absence of other directional features, such as clast macro-fabrics11. Micromorphology can be used to account for the origins of a sediment, its transport pathways, and the processes of deposition and deformation. These can be combined to create an understanding of sediment-landform associations and landsystems.
Sample Collection and Preparation
Thin sections were sampled using Kubiena tins. Representative (and replicate samples where pragmatically possible) samples were collected from each lithofacies. These undisturbed samples were then prepared according to standard techniques2,21,22.
The unlithified, undisturbed samples should be analysed at multiple magnifications under petrological microscopes. The optical properties and relative orientations of the particles can determine the genetic stress history of the sediments. Using both plane- and cross-polarised light highlights the textural and structural characteristics of the sample. Thin sections were investigated at low magnifications between x10 and x100, as higher magnifications observe individual grains, which may not be helpful for structural interpretation.
A systematic description
The analysis of thin sections must employ a systematic, standardised description to be used meaningfully11, such as that outlined in Table 1. Presentation of all data in a single table allows easy comparison between samples. A table of symbols used in the presentation of data is given in Figure 1. A glossary of terms used in analysis of thin sections is given in Table 2. Where possible, photomicrographs and scans show distinct features. Where this is not possible due to the magnification of the image, features that cannot be seen have been marked on to show their position and orientation. Arrows on rotational structures have no inferred direction.
Recent research has expanded greatly on van der Meer’s (1993) classic interpretations, and the development of criteria to identify different depositional environments. Hiemstra and Rijsdijk (2003) used artificially induced strain in potter’s clay to investigate typical features found in subglacial diamictons. They found a close relationship between unistrial plasmic fabrics and rotational structures (Figure 2), and both increased in number with increasing strain. Grain lineations commonly occur in association with rotational structures such as turbates23.
Glossary of common terms
Table 2: Glossary of common terms used in micromorphology (after van der Meer, 1993; Perkins, 1998; Carr, 2001; Carr, 2004a; Menzies et al., 2006; Hiemstra, 2007). Refer also to Figure 2.
|Anisotropic||The anisotropic skeleton grains and plasmic matrix of the slide transmit plane polarised light, but under cross-polarised light they extinguish (i.e. transmit no light) four times per complete rotation, every 90°. The refractive index therefore varies with direction.|
|Birefringence||Optical property in which interference colours become visible by turning the stage of the microscope; cause by double refraction of light under crossed polarisers and consequent polarising of bundles of light.|
|Cross polarised light (XPL)||When passing plane polarised light through a second filter at 90° to the first (the upper polariser), we see the light through crossed polars. Cross-polarised light is used to determine properties such as dispersion, birefringence, and extinction.|
|Domain||Small zones in which clay particles are orientated parallel to each other, causing them to behave (optically) as a single crystal. Subglacial tills may exhibit multiple domains with banding and stratification.|
|Edge-to-edge crushing||Clast fragments touching at the edges with visible breakage contacts. Breaking and grinding may have occurred in response to high stress levels resulting in significant grain-to-grain contacts along grain edge asperities. More common in areas with low pore water content.|
|Galaxy / turbate / Rotational structure||Circular alignments of grains around cores of consolidated sediment or larger grains; indicative of rotation. Closely associated with planar features. For example, van der Meer (1993) and Hart (2007).|
|Grain stacks||Edge-to-edge grains forming to support developing stresses. Develop perpendicular to the stress field. For example, Menzies (2000) and Menzies et al. (2006).|
|Grain alignments||Preferred long axis of skeleton grains. Numerous grains in a row with aligned long axes. E.g., Hiemstra & Rijsdijk (2003).|
|Interference colours||The colour of anisotropic minerals under crossed polars varies, and the same mineral shows different colours depending on crystallographic orientation. These colours are on Newton’s Scale, divided into several orders:
|Isotropic||Isotropic minerals remain black in all positions when viewed under cross-polarised light. They have random atomic structures, so that structure and refractive index are the same in all directions.|
|Lineations||Lines of skeleton grains with aligned long axes. May indicate shear zone. For example, Hart (2007).|
|Microfabric||Skeleton grains commonly show preferred long axis. The vertical arrangement of skeleton grains. For example, Carr (2001).|
|Necking structure||Squeezing of plasma between skeleton grains. Indicative of matrix flowage.|
|Plane polarised light (PPL)||In normal, unpolarised light, waves vibrate in all directions. Filtering the light beam in the microscope with the lower polariser makes all the light waves vibrate in one direction, parallel to a particular plane.|
|Plasma (matrix)||Grains of colloidal size (< 2 μm); may consist of clay minerals, oxides and hydroxides of Fe, Al and Mn, soluble salts, etc. Often used to refer to matrix – all material smaller than the thickness of the thin section. Individual particles cannot be seen.|
|Plasmic fabric:||Birefringence models of the plasma. Based on optical properties of the particles as well as the optical properties caused by the orientation of particles relative to each other. For example, see Khatawa & Tulaczyk (2001) and Carr (2001).|
Detailed micromorphological study has highlighted the importance of grain size variation in the production of rotational structures24. Individual larger clasts may generate perturbations, allowing characteristic rotational structures to develop. Increases in grain size allow for more perturbations. As a result, a poorly sorted, coarse grained till will be more micromorphologically inhomogeneous than a finer-grained till.
Stringer initiation and deformable clasts with tails also indicate syntectonic rotation in a ductile, shearing medium25. The combination of lateral shear26 and rotational movement results in a variable response to the applied stress field according to grain size. Hart et al. (2004) argued that the rotational process mobilises particles by incorporating grains from the subjacent undeformed bed, as evidenced by van der Meer’s ‘Till Pebbles’ (van der Meer, 1993).
Hart et al. (2004) therefore proposed that subglacial deformation tills contain associations of rotational features, such as skelsepic plasmic fabrics, orientations of smaller skeleton grains around larger ones, and rotated, augen-like features with tails, with intermediate and linear features24. Linear features include inclined clasts, lines of grains (lineations) and fragmentation of clasts. Intermediate features include mini-shear zones with internal rotation, and clay intraclasts with internal plasmic fabric24. They argued that deformation tills contain a juxtaposition of rotational and linear features. This is a result of dynamism within the subglacial deforming till layer at the microscale, and is related to temporal and spatial variations in porewater content and pressure. The overprinting of ductile and brittle deformation within the same area of till is evidence of sudden phase changes related to fluctuating porewater pressure6.
These structures are illustrated in the conceptual diagrams (Figures 2 and 3). Figure 3 was developed from work by Van der Meer (1993), Menzies (2000), Hiemstra and Rijsdijk (2003), and Menzies et al. (2006). The structures are categorised by their genesis. These images only reflect structures observed in glacigenic sediments at Whitburn Bay, Warren House Gill, and in various boreholes in the North Sea18-20,27.
Davies, B.J., 2009. British and Fennoscandian Ice-Sheet Interactions during the Quaternary, Unpubl. PhD Thesis. Department of Geography, Durham University, Durham, 502 pp.
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