The micromorphology of unconsolidated sediments

Sedimentary Geology 238 (2011) 213–232

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Sedimentary Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s e d g e o

Review

The micromorphology of unconsolidated sediments Jaap J.M. van der Meer a,⁎, John Menzies b, 1 a b

Department of Geography (Centre for Micromorphology UoL), Queen Mary, University of London, Mile End Road, London E1 4NS, UK Department of Earth Sciences, Brock University, St. Catharines, Ontario, Canada L2S 3A1

a r t i c l e

i n f o

Article history: Received 10 February 2011 Received in revised form 19 April 2011 Accepted 20 April 2011 Available online 30 April 2011 Editor: B. Jones Keywords: Thin sections Micromorphology Sedimentology

a b s t r a c t This paper aims to describe the use of thin sections/micromorphology in the sedimentology of unconsolidated sediments. It provides examples of the use of thin sections in a variety of sedimentary environments, from fault gouge, through caves and volcanics to aeolian, fluviatile, marine, periglacial and glacial. It demonstrates that in the latter three fields the use of micromorphology is relatively widespread and that in glacial sedimentology it has revolutionised our way of thinking about subglacial sediments. Although micromorphology has been mainly descriptive so far, methods of quantification observations are demonstrated. Some of the important aspects of micromorphology are its use for microstratigraphy and the possibility of relating observations to documented processes thereby allowing a more robust sedimentological interpretation of modern and ancient sediments. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Micromorphology permits the arrangement of particles, matrix and other contained components of unconsolidated sediments to be fully examined (van der Meer, 1996). Through the examination of thin sections of sediment at low magnifications (20 to 50×), micromorphology provides an insight into the architecture of sediments permitting the internal arrangement of all components to be observed in a large number of samples allowing general assumptions and statements about sediment architecture to be made, and from those generalisations an understanding of the processes involved in sedimentary environments and processes of formation (deposition, deformation and/or emplacement) can be developed (van der Meer, 1993, 1996; Menzies, 2000). The advantages of micromorphology are: i) It is an in situ method. Because the material is not disaggregated as is done in laboratory sediment analyses, thin sectioning permits the study of particles in their original relation to each other. Although a sample is taken from the field, which could be considered disruptive, it is an undeformed sample, the material within the sample retains its original position. ii) It allows precise compositional and positional analyses of the sample. It is not only possible to know that a particular constituent (for instance a specific mineralogy, or a microfossil) is present, but also where it is present, thus allowing detailed ⁎ Corresponding author. Tel.: + 44 20 7882 8416; fax: + 44 20 8981 6278. E-mail addresses: [email protected] (J.J.M. van der Meer), [email protected] (J. Menzies). 1 Tel.: + 1 905 688 5550; fax; + 1 905 688 6369. 0037-0738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2011.04.013

microstratigraphy. In many core studies, subsamples are taken at regular intervals, which only allows the positioning of for instance microfossils, at a vaguely defined position. In thin sections it is possible to precisely locate the microfossils, for instance whether they occur in particular laminae, or only in macroscopically non-detectable intraclasts (see below). Certainly the latter would shine a completely new light on the stratigraphy. iii) It can be related to process. Observing a process in the field, be it reworking of silts by wind, or debris melting out of ice, micromorphology allows microstructures to be related to these processes. If these processes can be mimicked in the laboratory under controlled conditions (of e.g. temperature and moisture content) diagnostic microstructures can be established. For a sedimentologist studying a bedrock outcrop it is quite common to take thin section samples of the studied lithified sediments. On the other hand, when studying unlithified sediments, it is quite uncommon to take samples for thin sectioning, notwithstanding the fact that the technique to do so has been available for decades (it has been at least in use in sedimentological studies since 1940, cf. Lundqvist, 1940). The inference is that there is only limited knowledge from the active sedimentary environment relating observed processes to small scale sedimentary structures. Establishing the link between particular suites of microstructures and observed processes allows the recognition of such processes in outcrop and especially in cores. This lack of established suites of microstructures makes one wonder about the basis for the interpretation of thin sections of ancient, lithified sediments. When there is no available data on recent sediments at the appropriate scale, the interpretation of small scale structures in ancient rocks can only be based on educated guesses. The emphasis is here on ‘appropriate scale’;

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of course there is a large body of observations at the meso- or macroscale in present day sedimentary environments, simply not on the microscale. We note with interest an appeal made by Kuenen(1958), in which he urged a soft-rock approach to sedimentological problems and we think that this appeal is as valid today when it comes to the microscale. Evidence for this deficiency in microscale data can, for example, be found in O`Brien and Slatt (1990) and Maltman (1994) which both show images of thin sections of lithified rock and offer interpretations of depositional processes. However, these processes are nowhere demonstrated to produce such microstructures as should be evidenced by present day observations on the same scale (O'Brien, 1996). This does not mean that the interpretations are wrong by definition, only that they have not been tested. In practice these interpretations are taken further, in the sense that they are then cited by others as supporting evidence for the invoked relations between processes and microstructures. It should be pointed out that in the sedimentological literature the term ‘micromorphology’ is not common, instead, reference is given to ‘thin sections’, ‘thin section studies’ or (sedimentary) petrology/ petrography. In sedimentology, thin sections are studied mainly from the objective of composition, whereas in pedology micromorphology is used to ‘describe, measure and interpret the formation and function of (soil) materials’ (Stoops, 2003). The latter approach is still rare in sedimentology. However, it is amazing that the technique is used so little on unconsolidated sediments, since it is the only one that allows the detailed study of a sediment in situ. We are well aware of the large body of literature on the use of thin sections on consolidated, lithified sediments. However, as stated before, observations on such thin sections are inherently difficult to link to processes and thus we do not give reference to that body of literature in this paper. The aim of this paper is to i) provide an overview of the use of micromorphology in the study of unconsolidated sediments from all sedimentary environments where applicable, ii) highlight successful and potential applications of this technique for understanding and recognising depositional and deformational processes, and iii) thereby encourage its use by sedimentologists, glaciologists and applied scientists. Partly our demonstration will be based upon existing publications, and partly on examples taken from our own studies.

2. Sampling and impregnation methods Sediment samples for the purposes of thin section manufacture can be obtained either from field sites or from cores taken from boreholes. In order to obtain in situ, intact samples it is essential that ‘undisturbed’ samples are acquired. Typically, field samples are taken using Kubiena or mammoth tins gently inserted into an exposed face (Fig. 1A, B). The site and sample number as well as its orientation in terms of Cartesian coordinates are marked on the tin. The decisions as to sampling strategy must take into account the rationale behind sampling, and the number of samples required to satisfy this objective. Likewise the cost of taking and preparing samples and the time involved predicates the number and location of samples obtained. In the case of core samples typically an oriented interval is obtained from the working half of a core (Fig. 1C, D) and again orientation (Cartesian, if known), position of the top of the core, and sample number are marked. The number of core type samples taken must be considered in relation to the objectives of the study and the, often limited, supply of core available. In principle all sediments can be sampled: saturated samples can be gravitationally drained before being wrapped up, we have applied this to permafrost samples which we left in the field to melt and drain before further handling. Very hard materials (for instance most tills in summer; semi-lithified sediments) can be sampled with an angle grinder (Fig. 1E, F). Only loose gravels and boulders still provide problems, although interstitial fines can often be sampled.

Once the samples are acquired typically they are wrapped in plastic or some other form of water-proof material, again marked with orientation and sample number, and transported to the lab for standard soft sediment impregnation and thin sectioning (Kemp, 1985; Palmer et al., 2008). Impregnation begins once the sample has been permitted to slowly dry out. A problem with air drying can be that clay-rich samples will fracture. However, such fractures tend to be recognisable as they are more jigsaw-like than ‘normal’ fractures, secondly one can wonder why the fracture develops at that particular locality and it is not actually portraying an existing zone of weakness. Furthermore such fractures aid in bringing the resin inside the sample (Fig. 1G, H), enabling full impregnation, which can be a problem in clay-rich samples (Pusch, 1999). Where necessary a saturated sample may be placed within an acetone bath allowing the acetone to replace the pore water. Subsequently, an epoxy resin is forced into the sample by placing the sample in a low vacuum oven (b15 mm Hg) and soaking the sample in the epoxy resin-acetone mixture. Over a 4–6 week period additional ‘mixture’ is added until full impregnation is achieved. With this method the sample does not crack as much. The use of vacuum does not cause any disruption to the fabric of the sample, even the most delicate structures like acicular calcite (lublinite) are preserved intact (Cailleau et al., 2009 and references therein) as evidenced by the vast body of micromorphological literature on soils. In thin sections of lithified sediments it is common to stain the resin in order to make either pores stand out, or to discriminate between various carbonate minerals, however, we have not come across any publications where this has been applied to unconsolidated sediments. Once fully impregnated, a sample may take a further 2–4 weeks to fully cure into rock-like hardness and to a state where actual thin sectioning can begin. In some cases impregnation and curing can be speeded up by a few days to one or two weeks with exposure to low temperature heating (b60 °C) or to just a few days using gamma radiation exposure. Thin sectioning is then done utilising the normal (Fig. 1G, H) rock thin section techniques (Kemp, 1985; Murphy, 1986; Lee and Kemp, 1992; Camuti and McGuire, 1999). Thin sections are ground to an approximate thickness of 25– 30 μm. In examining thin sections for micromorphological analyses a standard petrographic microscope with a rotating stage is used with magnification typically in the 20–50× range (at QMUL we use Leica© microscopes with a stepless zoom lense at the range 6.3 to 32×). Thin sections can be made in 3 sizes viz. petrologic 2.5 × 4.5 cm, Kubiena 5 × 7 cm, or mammoth thin sections 14 × 8 cm in size, Kubiena size being the most common. 3. Terminology Much of the terminology used in micromorphology stems from original work done in soil science and since then supplemented by terms common to structural geology (Brewer, 1976; Bullock et al., 1985; van der Meer, 1993, 1996; Menzies, 2000; Stoops, 2003; FitzPatrick, 2005). In examining a sediment under a petrological microscope, and using this soil science based terminology, a sediment can be subdivided into two major components: i) those particles b25– 30 μm in size (or less than the thickness of the thin section, i.e. they cannot be seen individually) termed plasma (what has in the past been termed groundmass or matrix) and ii) those particles N35 μm in size, termed skeleton grains (S-matrix), typically individually visible mineral or organic particles. Because of the visibility criterion we find this thin section thickness-related subdivision a more useful analytical tool than the subdivision (matrix — clast/detrital grains) used in sedimentary petrology. A third component can also be recognised where combinations or arrangements of plasma and skeleton grains, or skeleton grains alone, or discontinuities between plasma and skeletal components form recognisable sets or microstructures. As illustrated in Table 1 microstructures and microfabrics, representative of a sediment's internal architecture can be subdivided into i) those

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Fig. 1. A. Sampling till boundary by gradually cutting in a mammoth-sized tin. Photo Martin Rappol. B. Sampling till boundary with tin in place, ready for labelling, removal and wrapping. C. Sampling marine sediments from core. The sample has been cut and a sturdy plastic sheet (as wide as the length of the sample) is to be squeezed in following the coreliner. Plastic sheet can then be used to move sample to another container. D. Sampling from core. The use of a plastic sheet allows for clean removal of a coherent sample. E. Hard, (semi-)lithified and clast-rich sediments can be sampled using an industrial angle -grinder. F. Block prepared by angle-grinder. The block can now partly be wrapped and labelled before removal. Irish punt coin for scale. G. Impregnated and labelled mammoth-sized sample. The actual thin section is made from the central cut to avoid any sampling disturbance. H. Surface of impregnated sample. Note the resin-filled crack. Notches on the right indicate the top of the sample.

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Table 1 Overview of microstructures recognised in glacial sediments. Note that the Brewer, classification of plasmic fabrics is used. From Menzies, 2000.

Microfabrics and Microstructures within the Plasma and S-Matrix of Glacial Sediments Plasma

Skeleton Grains

Plasmic Microfabric

S-Matrix

Ductile

Brittle

Polyphase (Ductile/Brittle)

Porewater Influenced or Induced

VARIETIES OF MASEPIC PLASMIC FABRIC

STRAIN CAPS & SHADOWS

FAULTED DOMAINS

MULTIPLE DIAMICTON DOMAINS

"CUTANS" (ARGILLANS)

OMNISEPIC PLASMIC FABRIC

FOLD STRUCTURE

DISCRETE SHEAR LINES AND LINEATIONS

"COMET" STRUCTURE

WATER ESCAPE STRUCTURES

UNISTRIAL PLASMIC FABRIC

LAYERING & FOLIATION

SHEAR ZONES

SILL & DIKE STRUCTURES

SILT CAPS

INSEPIC PLASMIC FABRIC

"NECKING" STRUCTURES

REVERSE FAULT

TILED UNITS OF LAMINATED CLAYS & SILT

"POLYGONAL" STRUCTURES

"BANDED" PLASMA

"ROTATIONAL" STRUCTURE

KINK BANDS

INTRACLASTS

SILT & CLAY COATINGS

KINKING PLASMIC FABRIC

SECONDARY FOLIATION

CRUSHED GRAINS

SKELSEPIC PLASMIC FABRIC

CRENULATION FOLIATION

BOUDINS AND OTHER ATTENUATED STRUCTURES

(modified from van der Meer, 1993)

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related to plasma (plasmic fabric), ii) those related to the S-matrix and iii) those that combine plasma and skeleton grains. i) plasmic fabrics, first elucidated by Brewer (1976; now termed b-fabrics in soil science, see Stoops, 2003), indicate the presence or otherwise of oriented (bundles of) clay particles that exhibit birefringence on a petrographic microscope stage (cf. Zaniewski, 2001). Of the four dominant types of plasmic fabric (masepic, omnisepic, unistrial and insepic) excluding mixing of plasma with skeleton grains (skelsepic plasmic fabric, see below) each type is illustrative of variations in clay particle orientations from strong random orientation in omnisepic plasmic fabric to strongly, single, preferred orientation in unistrial plasmic fabric. In general, plasmic fabrics are symptomatic of ductile deformation of claysized particles that compose a sediment's matrix. In sharp contrast, unistrial plasmic fabric appears to result in very strongly preferred but very narrow oriented domains of deformable clay-sized particles within a sediment unit, possibly illustrative of continuous and high stress (cf. van der Meer, 1987; Menzies, 1998). Banded and mottled distribution of plasmic fabrics occur within sediments where mixing (deformation) of different sediment facies has occurred. In these cases mixing has not completely homogenised the sediment. Studying plasmic fabric in sedimentary settings is important because its presence and patterns are indicative of stresses imparted on the sediment. A plasmic fabric parallel to bedding indicates that the main stress is caused by loading, while a plasmic fabric at an angle to the bedding indicates shearing (Bordonau and van der Meer, 1994). Much remains to be learned of the significance of plasmic fabrics and their interpretation within the processes of sediment deposition and deformation. The emerging Metripol system, which allows the measurement of the strength of birefringence as well as the mapping of birefringence distribution within a thin section will be an important tool in our understanding of plasmic fabrics. First results on the strength of birefringence of shear structures in lacustrine sediments affected by iceberg scouring (Linch, 2010; Linch and van der Meer, in prep) are very promising. There are no structural geological terms for the different types of plasmic fabric, with the exception of unistrial plasmic fabric which equates with discrete shears. Because in micromorphological interpretation plasmic fabrics play an important role, the lack of discriminating structural geological terms prescribes the pedological terminology. ii) S-matrix microstructures (Table 1) can be divided into those forms indicative of ductile, brittle, polyphase (ductile/brittle), and porewater induced forms (cf. Maltman, 1994; Passchier and Trouw, 1996; Menzies, 1998). All of these S-matrix structures tell of complex formation illustrative of pre-depositional, syndepositional and immediately post-depositional processes. iii) Finally, a combination of plasma and skeletal grains produces a skelsepic plasmic fabric (Table 1) in which oriented plasma can be detected surrounding individual skeletal grains. The causation of this special fabric remains enigmatic. On the one hand it is possible that this fabric may develop as clay permeate, under a localised stress gradient, an initially clast-rich sediment in which there appears limited bulk deformation. On the other hand it is also possible that such a fabric may form following intensive deformation involving rotation (van der Meer, 1993, 1996). However, the former process tends to produce a distinct microlamination which can be differentiated from the more massive structure produced by the latter. 4. Micromorphological description and analysis In glacial micromorphological description and analysis we have developed a routine, which is nowadays also applied to all other

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sedimentary environments. The description starts with a macroscopic characterisation of the thin section to draw attention to any intermediate sized structure easily overseen under the microscope. This is followed by a textural description, similar to that used in sedimentary petrology: grain size and shape, composition and distribution of skeleton grains and of the plasma in as far as the latter can be observed. This is not considered to be the most important part of the description, but is mainly used to describe the sediment and to check on the homogeneity of a set of samples. The emphasis of the description is on the structure (pores, their size, shape and distribution; sedimentary structures like grading; deformational structures; relations between plasma and skeleton as well as between skeleton grains of different size) and on the plasmic fabric. Together these should inform about the (post-)depositional, deformational and stress history of the sample. Interpretation is based on the analysis of all samples in a study, not on individual thin sections. 5. Application of micromorphology to the study of unconsolidated sediments, an overview As said before, in principle it is possible to apply micromorphology to all sedimentary environments and to almost all textures. By itself there is no reason why micromorphology should not be applied in all sedimentary environments and on all (sub)facies. However, when we look at the grand division in sedimentary environments and the application so far of micromorphology across these, we get a widely divergent picture (Table 2). The literature database developed (up to mid-2010) for the Centre for Micromorphology, UoL, consists of over 850 references (there is no claim to be complete), which in one way or another deal with thin sections of unlithified (mainly Quaternary) sediments (Table 2; full database in the supplementary material). Lithification is a rather broad term ranging from very high levels of lithification to limited lithification to completely unlithified states (the majority) and it will come as no surprise that some papers (5%) deal with diagenesis. As outlined above there appears to be a lack of knowledge within the sedimentological/geological community on the production of thin sections of unlithified sediments, but strangely enough 16% of our references deal with technical aspects of, firstly, thin section production (e.g. Catt and Robinson, 1961; Ashley, 1973; Camuti and McGuire, 1999; Josephs and Bettis, 2003; Boës and Fagel, 2005) (the number of publications on this subject appears to have diminished over the years); and secondly there are technical papers on methodology, for instance of counting constituants in any thin section (Clark, 1982) and, more recent, image analysis (Zaniewski, 2001; Francus, 2004). There is also a relatively large number of publications (11%) dealing with general sedimentological aspects, such as grain size (Johnson, 1994), sorting (Harrell, 1984) and orientation (Stroeven et al., Table 2 Percentage of references covering different sedimentary environments and technical aspects. Note that 12% of the publications have been listed under two headings. Organic Volcanic Slopes Tectonic Caves Evaporites, incl. carbonate crusts Fluviatile Aeolian and deserts Marine Periglacial General sedimentology Lacustrine Technical Glacial Diagenesis Experiments

1 1 2 2 3 3 4 5 7 7 11 12 16 30 5 2

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2005). In this paper we will not pay further attention to these technical aspects and general sedimentological papers, the details of which can be found in the supporting material to this paper. In the following we will present the use of micromorphology in different sedimentary environments on the basis of their frequency in our database, starting with the lowest. This format of treating different sedimentary environments in individual chapters is common to textbooks (e.g. Nichols, 2009) and thus familiar to readers. 5.1. Organic Of all the variety of organic sediments very few have been studied micromorphologically for sedimentological purposes. Thin sections of peat have been studied by pedologists, but mainly to study (de-) composition and the formation of Histosols. However, a study by de Vleeschouwer et al. (2007) for which they developed specific impregnation techniques, demonstrated that thin sections will also reveal the state of preservation of plant fragments, the vegetation type as well as the presence of mineral particles and charcoal. The latter is especially relevant to the study of (crypto-)tephras in peat. Similar to peats, organic sediments like dy or gyttja have rarely been studied under the microscope, and then mainly to study their composition (Hahne et al., 1994; Stolt and Lindbo, 2010). Liu et al. (2008) studied guano related sediments on Dongdao Island in the South Chinese Sea. Their microscopy study concentrated on the phosphatic cements derived from the guano. Their multi-proxy study revealed that there had been periods of accumulation related to the presence of seabirds and consequently sea level. Guano derived sediments have also been encountered in caves (Stephens et al., 2005). Although few organic sediments have been studied micromorphologically there is still much sedimentologically relevant information to be found in the micromorphological soil literature. In a recent paper Kooistra and Pulleman (2010) analyse features related to faunal activity because bioturbation is an important factor in initial soil formation. They not only describe sedimentological traces of bioturbation like burrows, but also demonstrate that faecal pellets can be ascribed to particular animal groups. Blazejewski et al. (2005) studied subsurface C in riparian zone soils. Although the title of their paper is not likely to attract the attention of sedimentologists, it actually develops a classification system of separating Soil Organic Matter (SOM) – which here stands for plant remains – by morphology into functionally meaningful fractions, like roots, fragmental organic matter, infillings, etc. Not all wetlands develop into soils, and such classification schemes should be helpful in interpreting ancient sediments. The role of organic matter in marine sedimentary processes has been studied by Watling (1988) and Noffke et al. (1997). The latter demonstrated the role that microbial mats play in the build-up of siliclastic tidal flats. 5.2. Volcanic Thin section studies related to unlithified volcanic rocks mainly deal with tephras, although Pirrung et al. (2008) studied composition and structure of sediments filling in a recently formed maar. Weathering of volcanic particles, as studied by for instance Bishop et al. (2002, and other papers in same volume), illustrate the shape, structure and alteration of volcanic particles. This work is especially interesting since today tephra layers in peat profiles (Fig. 2A, B) or in lake or marine cores are used as geochronologic markers. Since most tephra layers have a unique fingerprint, identifying these layers has become a reliable dating tool. However, because distal tephra layers are usually very thin, they are difficult to detect. Enache and Cumming (2006) recently published a note on the recognition of tephra particles and on their migration within a lacustrine sediment sequence. Based on tephra counts in diatom slides, Enache and Cumming (2006) discussed the downward displacement of the main

Fig. 2. A. Bruarjökull, Iceland, black tephra particles inserted on surface of moss pillow. Sample JM3 (courtesy of Kurt Kjaer, Copenhagen); plane light;hvof (horizontal field of view) 3.9 mm. B. Bruarjökull, Iceland, black tephra particles in more compacted zone of moss pillow, following 2004 eruption Note how tephra particles get concentrated between plant material. Sample JM4 (courtesy of Kurt Kjaer, Copenhagen); plane light; hvof 4.3 mm. C. Bruarjökull, Iceland, black tephra layer as part of tephra-peat-loess sequence underlying push moraine. Lack of sorting suggests that this is not a free airfall layer, but that it has been reworked and mixed. Sample 3971; plane light; hvof 2.6 mm.

tephra body giving it ca 2000 years older apparent age. They explain this by settling through density differences although the bedding above the tephra appears to be intact. De Vleeschouwer et al. (2007) highlight the use of micromorphology of impregnated peat in the detection of tephra. Because the sedimentology and ecology of thin ashfalls on living vegetation are not clear, recent work has begun in

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taking thin sections from bogs where experimental distribution of tephra particles has been carried out (Payne et al., 2006) and from natural ashfalls in Iceland (unpublished; Fig. 2C). Volcanic deposits like lahars tend to be diamictic and as the clasts they carry can be striated (Atkins, 2004), it may be difficult to separate from glacial diamicts. Micromorphological studies of lahars are unknown and for that reason we have started to sample them (van der Meer and Atkins, unpublished). 5.3. Slopes Depending on for instance water content, slope angle or slope material and its availability, numerous slope processes, ranging from dry rockfalls to very liquid mudflows, can be distinguished. Because every process is characterised by its own stress field and stress heterogeneity we can expect resulting slope deposits to be different, macroscopically as well as microscopically. Mücher (1973) recognised the need for micromorphological classification of slope deposits. Since then different types of contemporary slope deposits have systematically been studied (e.g. Bertran, 1993; Bertran and Texier, 1999 and references therein). Their conclusion at the time was that different slope processes can actually generate similar microfacies. As an example they state that massive poorly sorted material with a plasmic fabric determined by the presence of small birefringent mineral grains like calcite may typify debris flow deposits, rock avalanches, earth flows and the basal matrix-rich part of grain-flow deposits (Bertran and Texier, 1999, p. 118). They also point at the similarity between deformation structures observed in for instance earth slides and those in tills. Texier and Meireles (2003) applied micromorphology to slope deposits in northern Portugal and could not find evidence to support alleged periglacial conditions during their emplacement. A similar approach was taken by Harris (1998) in order to distinguish between different types of soliflucted (‘head’) deposits on the Welsh coast. He found that whereas most units exhibit periglacially induced microstructures (Fig. 3; see also Section 5.10), one of the units did not. However this was caused by rapid deposition under paraglacial conditions not because of a lack of cold conditions. Mücher et al. (2010) prepared an up-to-date overview of the micromorphology of colluvial and mass wasting deposits. Because flowtills can slide down ice slopes and this slope disappears over time, unlike other slope deposits where this relation is retained, flowtills are usually treated under glacial headings and thus we refer to Section 5.12 for examples. 5.4. Tectonic Fault gouges pertain to small groups of sediments which are the result of tectonic rather than sedimentary processes. And although they have been studied frequently in lithified form, the study of unlithified fault gouges in thin section is relatively new (for instance Jeong and Cheong, 2005; Mizogouchi et al., 2008) but expanding. The latter paper uses thin sections extensively to describe the structure of the gouge. It shows clear examples of plasmic fabric described in structural geological terms, i.e. foliation and shears. Together the two papers give a good impression of what can be achieved using large thin sections: Mizogouchi et al. (2008) use micromorphology to characterise different types of gouge while assessing the permeability of the fault and its surroundings. Jeong and Cheong (2005) combined pedological and structural micromorphology enabling them to recognise numerous faulting events alternating with infilling, thereby demonstrating that the Korean Peninsula has not been as tectonically stable as previously thought. As with lahars, fault gouge tends to be diamictic and may contain striated clasts (Atkins, 2004) and consequently can be difficult to distinguish from glacial diamicts. Because both are cataclastic sediments or tectomicts (van der Meer et al., 2003) studying them microscopically

Fig. 3. A. Pebbly structure in which quartz grains appear encased in a shell of finegrained material, typical for gelifluction. Sample Mi.312 from.Mt Provender, Antarctica; plane light; hfov 18.0 mm. B. Pebbly structure with strong plasmic fabric (high birefringence) and carbonate caps, for instance around large particle to the right. Sample Mi.312 from Mt Provender, Antarctica; Xpol; hfov 6.4 mm.

is of great interest as to what separates them from non-tectonic diamicts (van der Meer and Atkins, unpublished). 5.5. Caves There are several different settings in which thin sections have been used in cave environments. Archaeologists use micromorphology extensively (Courty et al., 1989), including in excavations in caves. However, we will not deal, in this paper, with such studies as they primarily deal with human occupation, not with sedimentology. Secondly, thin sections have been used to study speleothems, usually in conjunction with palaeoclimatic, isotopic studies of the carbonates involved and finally, thin sections have been used to study cave sediments, derived from solution residues or washed into the cave from outside. A good example of the second type of thin section studies is Paulsen et al. (2003) in which a single picture of a ~0.5 mm thick thin section of a stalagmite is shown to demonstrate very fine laminae. The thin section is only used to establish that couplets are annual layers, though no further information on the couplets is given. In a study by Gradziński et al. (2003) thin sections and SEM of speleothems are used to establish the nature of black laminae. Pictures demonstrate that coloration is caused by charcoal particles and organic compounds which stem from fires during human occupation. The description is about composition and structure only, and no micromorphological terminology is used. In a paper by Pissart et al. (1988) micromorphology is used to demonstrate the former presence of segregation ice in cave sediments,

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as an indication of the presence of permafrost. The paper discusses the microstructure in terms of freezing related fractures and voids, but does not mention other micromorphological features. Goldberg (1979) and Goldberg et al. (2003), on the other hand, micromorphologically analysed cave sediments in great detail, discussing palaeoclimatological information such as the former presence of ice on the basis of sediment structure, while variable moisture conditions are discussed on the basis of the distribution of cements. As (calcite) cements are only present in some archaeologically datable layers, the authors discuss whether this represents wetter conditions with more percolating moisture or whether this represents dryer conditions with formation of an incipient calcic (soil) horizon. Given the high micrite content of cave sediments, there is no information on plasmic fabric. A paper by Forbes and Bestland (2007) is more typical of the general use of thin sections in cave studies as it only discusses the composition of different sediment types mainly in terms of the amount and properties of quartz grains. Very little is said about the matrix and nothing about relations between the skeleton grains and the matrix. Tropical cave sediments were studied by Stephens et al. (2005) and they found that their micromorphological data corroborated field interpretation but also provided important additional information on both depositional (alternating wet and dry condition; colluviation; mudflow) and post-depositional, diagenetic processes. 5.6. Evaporites, including carbonate crusts Arguably, structures seen in diagenesis and in evaporites are largely overlapping. It is their setting which is different, with diagenesis occurring at any depth and in any sediment from the surface downwards, while evaporites mainly form at or near the surface. A number of different chemical sediments have been described and illustrated in Goudie and Pye (1983). Arakel (1980) described the formation of lagoonal sediments from west Australia and used thin sections to study their composition (mainly gypsum). Not surprisingly he found both sedimented gypsum particles as well as in situ grown gypsum crystals. Here we would like to draw attention to the use of thin sections in the study of calcretes and carbonate crusts. Also studied from a pedological point of view, it is obvious that calcretes can be formed by precipitation in situ, aided by biological activity, while erosion and reworking are common processes (Alonso-Zarza et al., 1998). The development of calcretes emerges as a complicated process where vegetation (root development in response to calcareous deposition) plays a more important role than direct sedimentation in the final architecture. The terminology in these studies is mainly sedimentological and biological, but as in diagenesis we suggest that the approach of carbonate petrologists should be at least considered for adoption. Recent work by Menzies and Brand (2007) used thin sections to study carbonate cements within glaciolacustrine ice marginal deltaic environments. The study shows that these sediments underwent repeated incursions of glacial meltwater and demonstrates that brittle fracture occurred within the carbonate cements due to ice overriding. The latter clearly demonstrates that such carbonate cements are not just postglacial/Holocene as perceived wisdom will have it, but can form subglacially (see also Lacelle, 2007). 5.7. Fluviatile In this section we incorporate all papers dealing with sedimentation by running water, from shallow surficial runoff to deep river channel flow. Within aeolian environments reference will be made to a number of papers dealing with reworking of sediments by running water. The source of the running water can be either (snow) meltwater or rain, but each process leaves distinctive microstructures that can be used as differentiating criteria. In a study of sediment transport by overland flow in rills Wilkinson and Bunting (1975)

demonstrated that the process led to sediment accumulation on the valley floor and claimed that sediments would be bioturbated and thus homogenised very quickly. A thin section shows nevertheless that after 12 years layers are still recognisable, although a large number of vughs-type pores had formed. Active riverbed and heavily polluted sediments of the Rhine were studied micromorphologically by van der Meer et al. (1994a, 1996). The aim of those studies was first to establish the nature of the sedimentation: continuous, episodic or net accumulation by alternating deposition and erosion (Fig. 4A); and secondly whether the present sediment package was in a stable position. Micromorphological analyses of composition, structure, aggregates and plasmic fabric demonstrated that sedimentation had been continuous, but that the sediments were chemically highly unstable with widespread neoformations of different composition. Most remarkable was the observation of carbonate precipitation (Fig. 4B, C) and clay illuvation (Fig. 4D). The first occurring both as coating and as quasi/hypocoating. This is remarkable given the environmental setting of the sediments being continuously submerged with a water depth of up to seven metres. The thin sections furthermore showed the widespread presence of up to 2 mm diameter plastic spheres (Fig. 4E), which demonstrates that rivers actively contribute microplastics to the overall plastic load in the oceans (Mato et al., 2001). Johnson (1982) studied the sediments of an arid zone delta, also using thin sections. These were only used to describe the composition of the sediments, not even bedding structures are mentioned. The compositional mix of the delta sediments is strongly reminiscent of the Rhine sediments mentioned above. Blazejewski et al. (2005) extensively used thin sections to classify organic matter in riparian zone soils as a step in developing functionally different morphologic classes of soil carbon.

5.8. Aeolian and deserts In Table 1 only 5% of papers deal with aeolian and desert thin sections/micromorphology. This sounds surprising as there is such a vast literature on loess. However, most of this literature deals with palaeosols in loess, not with the sedimentology of loess. Occasionally papers deal with both (Kemp, 1999; Mestdagh et al., 1999), but in most papers the ‘unaltered’ loess between palaeosols is discussed in soil C-horizon terms only. Kemp (1999) clearly states that in order to unravel the history of a loess sequence one has to adopt a pedosedimentary approach, in which alternating pedogenic and sedimentary processes are taken into account. These papers make full use of the pedologic micromorphologic terminology. One of the earliest papers on a sedimentological aspect of loess is by Matalucci et al. (1969). It deals exclusively with grain orientation, which is used to establish palaeowind direction. Given the number of later papers dealing with the different processes that rework loess under natural conditions, the confidence shown in the paper by Matalucci et al. (1969), may have been misplaced. Reworking of loess happens by running water, which can itself be of different origin, and subsequent drying under different conditions. Micromorphological evidence and differentiation is reported in a number of papers, either dealing with experiments (see above) or with field conditions (for instance Mücher and Vreeken, 1981; Vreeken, 1984). Micromorphological features discussed in these papers mainly deal with the structure of distinct laminae, although aggregates and translocation of silt, clays and carbonates are mentioned. Under desert conditions the formation of a near surface vesicular layer is widespread. Volk and Geyger (1970) and Evenari et al. (1974) discussed the formation of the vesicles (Fig. 5) and whether these influenced the establishment of vegetation, which makes vesicles stand out as the only type of pores warranted dedicated micromorphological studies.

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Fig. 4. Thin sections from River Rhine bed, The Netherlands. Sediments are permanently submerged at c. 7 m waterdepth. A. Mammoth-sized thin section Mi874 from 405 to 420 cm depth. Notice cross-bedding in centre and loadcasting near the top as well as overall lack of erosion. B. Carbonate coating of void at 150 cm sediment depth. Sample Mi.858; X pol; hfov 4.5 mm. C. Carbonate quasi/hypocoating (i.e. precipitation following, but away from pore wall) at 260 cm sediment depth. Sample Mi.861; X pol; hfov 5.6 mm. D. Clay coating as a result of clay translocation at 310 cm sediment depth. Sample Mi.871; plane light; hfov 3.5 mm. E. Translucent plastic spheres (note small one lower right). Sample Mi.857 from Rhinebed at 119–134 cm depth; plane light; hfov 5.6 mm.

5.9. Marine The marine environment reaches from the beach or the river mouth to the deep abyssal plains and in principle thin sections can be made from sediments in all settings. In reality this has not happened yet as we have not been able to find thin section papers on all sedimentary sub-environments. One reason may be that thin sections have to be made from material collected by diving, by grab or by core. With the practise of slicing cores and archiving one half there is often a reluctance to cede complete parts of working half-core for impregnation (Fig. 1D). Although one could argue that everything that can be studied in a core, can be seen and analysed in a thin

section, the general sentiment is that impregnation makes the material inaccessible to other researchers. Even the argument that core studies of microfossils are most usually performed on taking out ‘bulk’ samples at fixed intervals and thus not strongly related to sedimentology, has not swayed opinion even although in thin sections one can actually see the position of the microfossils (Fig. 6). Since tidal flats are accessible at low tide and thus relatively easy to sample there are a number of studies related to this environment. Gerdes et al. (1985) and Noffke et al. (1997) studied tidal flat sediments, including algal mats from North Germany. Thin sections were used to describe the position and characteristics of the algal mats and the results and differentiation of bioturbation by different organisms. A most

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Fig. 5. Vesicles, rounded or ovate voids can form under a multitude of conditions, either cold or hot. This is an example from a recent supraglacial melt-out till. Note pieces of vesicular basalt. Sample C.277 from Kotlujökull, Iceland; plane light; hfov 3.4 mm.

extraordinary study of marsh soils was performed by Borchert (1968) who found that for his structural studies 30 cm2was not enough and produced 50 cm long and 50–80 μm thick ‘thin’ sections. The author established differential behaviour of clay particles depending on cationadsorption and clay content, for instance Na-clays are said to move more easily into a vertical position upon ripening and bioturbation than Caclays. A systematic study of the change from sediment to soil was conducted by Kooistra (1978) who studied all the micromorphological aspects of a substantial number of thin sections from different settings, included a comparison with adjoining sediments and soils after land reclamation. The author demonstrates that each sedimentary subenvironment produces characteristic microstructures, which are preserved for a considerable period. Kilfeather et al. (2007) used micromorphology to assess its potential as a diagnostic tool to distinguish tsunamigenic sediments. The study revealed that micro-rip-up clasts, millimetre-scale banding, organic entrainment, fining-up sequences and erosive contacts were all detectable microscopically but not macroscopically, thereby showing that the technique has great potential to distinguish such sediments. A paper published by Watling, 1988 is interesting because the author claims that techniques used at the time to study (from an ecological point of view) marine sediment microfabric were of limited use and that thin sections were much better suited. This is clearly demonstrated in his paper by providing examples of microstructures, especially the distribution of organic matter. The paper also provides good examples of the use of epi-illumination to show the organic matter distribution. 5.10. Periglacial Periglacial studies can be separated into three distinct groups. In the first place there are those related to present day periglacial areas in the Arctic, Antarctic and in high altitudes. Secondly there are those related to areas formerly (Pleistocene or older) subjected to periglacial conditions; and, finally, studies related to experiment, for instance in climate cabinets or rooms. Examples of the latter have been provided in the section on aeolian environments. In all three groups, thin sections have been used from an early stage and there is by now a substantial literature on their use. Some studies attempt to provide an overview of periglacial microstructures, while others look at particular periglacial landforms, while using thin sections as one of the research methods. Examples of the former are Koniščev et al. (1973) in which turbate structures are described, while also paying attention to the presence of the plasmic fabric. Harris (1985) systematically treated a number of microstructures, such as platy structures and grain coatings, by reviewing the literature. In an

influential paper van Vliet-Lanoë et al. (1984) combined information gained from active, fossil, and experimental data to analyse structures caused by repeated freezing and thawing of various loamy sediments. The microstructures described therein – sorted platy structure, which is more commonly referred to as a silt droplet structure (Fig. 7A); vesicles (Fig. 5; see also Bunting, 1977); and grain coatings – are the most common microstructures found in periglacial settings. Combinations of these microstructures can be used to reconstruct the presence of segregation ice in the active layer and the process of gelifluction (Fig. 3). These microstructures were used in reconstruction by Oh et al. (1987) on samples from northern France, and by van der Meer et al. (1992a) on Antarctic samples. In the latter study it was established for the first time that in the Antarctic clay illuviation related to snow meltwater is common (Fig. 7B–E), where commonly this process is thought to be exclusive to soil formation. The silt droplet structure (Fig. 7A) described independently by van Vliet-Lanoë (1976) and created artificially by Coutard and Mücher (1985) shows-up in almost all periglacial studies: for instance van Vliet-Lanoë (1988) on cryoturbation; van Vliet-Lanoë (1991) on differential frost heave; and Bertran et al. (2003) on permafrost age and degradation. Several other microstructures developed under periglacial conditions have been reported, for example, Elliott and Worsley (1999) pointed at the well developed orientation of clean sand grains in a solifluction lobe from Norway, while Mol et al. (1993) described microjointing in clay-poor fluvio-aeolian sediments as a result of contraction cracking following a sudden drop in temperature. Likewise, Von Buch (1964) described the occurrence of concretions and pseudoconcretions in a periglacial slope deposit in Spain and concluded that they were typical for such a sediment. An overview on a larger scale has been provided by Huijzer (1993). In the first place he discusses all known periglacial microstructures, followed by analyses of specific sites and sequences and how these microstructures are distributed within the site/sequence and how they relate to macrostructures. Finally, there is a valid attempt to relate these observations to frost regime and to come to a regional reconstruction in which conditions are shown to change between permafrost-seasonal frost-steppe-temperate conditions over the last glacial/interglacial cycle. The most recent overview has been presented by van Vliet-Lanoë (2010). In this overview microscopic features are treated systematically according to structure (platy, lenticular, granular), structure stability, groundmass and pedofeatures as well as the implications for pedostratigraphy. 5.11. Lacustrine Ever since it was understood that lake sediments can store palaeoclimatic information on an annual basis, lacustrine deposits have attracted attention. This notion started in glaciolacustrine settings (De Geer, 1912; Sauramo, 1923) but has now expanded to all non-glacial settings, at least where an annual climate signal can be expected, i.e. in non-tropical environments. By now there is a large body of literature on laminated lake sediments (see Zolitschka, 1998; Brauer, 2004; and references therein). Most of these studies deal with the question of whether observed lamination is indeed annual or not, and, for obvious reasons, laminae are studied for their biological components in order to detect seasonal or non-seasonal events (Francus and Karabanov, 2000; Lücke and Brauer, 2004). Interestingly one can also use non-biologic, chemical precipitation of carbonates to detect a seasonal signal (Brauer, 2004). For all of these studies one needs thin sections to establish the true nature of laminations. The most detailed classification to date is by Rein et al. (2007) who recognised a multitude of different combinations of chemical precipitates and biological sediments in an attempt to classify non-glacial varve type sediments. Given the variable nature of lacustrine sedimentation, it does not come as a surprise that the grain size of layers is also used as proxy for

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Fig. 6. Thin section from core ‘Amsterdam Terminal’ The Netherlands from a depth of 39.5 m below surface; laminated sediment from a shallowing marine basin of Eemian age. A, B. Distribution of microfossils (red) and mollusc fragments (blue) in A, faecal pellets in B. Note alternating levels of either. C. Microfossils (note uniformity of species) and mollusc fragments. In most cases microfossils are spread at random through sediments, this is a rare case of concentration in single layers, pointing at particular events. This would not be revealed by sampling the core for microfossils at regular intervals. Plane light, hfov 7.0 mm. D. Concentration of faecal pellets. Plane light, hfov 3.5 mm.

seasonality or even weather patterns (Mangili et al., 2005). Recognising weather patterns enables detailed analysis of long, continuous laminated sequences (e.g. the Scottish Lateglacial, MacLeod, 2010) and effectively study the weather during that period. Studies such as those by Mangili et al. (2005) automatically lead to more sedimentologically oriented studies (Fig. 8), in which not the composition, but the sedimentary structure is the focus of attention (van der Meer et al., 1992b; van der Meer and Warren, 1997; Lücke and Brauer, 2004; Mahaney et al., 2004). Most of these studies report on structure, but in some studies the presence and strength of the plasmic fabric is used to ascertain subsequent deformation (van der Meer et al., 1992b; Bordonau and van der Meer, 1994). Because of the waterlogged nature of lacustrine sediments, deformation may happen in an almost unconfined setting and over longer periods of time, which would not

necessarily lead to the development of a strong birefringent pattern. In a study of lacustrine sediments from central Ireland, van der Meer and Warren (1997) established how simple loadcasting can lead to the loss of lamination by complete mixing and hence the loss of palaeoclimatic signals. It is a relatively recent development to recognise former earthquakes by the reorganisation of lacustrine and fine-grained outwash sediments, and Menzies and Taylor (2003) demonstrated that such strong tremors leave distinct traces in fine grained sediments. 5.12. Glacial Papers on the micromorphology of glacial sediments are with N30% best represented in our overview (Table 2). The main reason is that micromorphology is by now an established technique in studies of

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Fig. 7. Thin sections of Last Glacial (Weichselian) periglacial microstructures in Penultimate Glacial (Saalian) till. A. Silt droplet structure. Note curved surfaces highlighted by (darker coloured) silt concentrations. This is the most common structure found in periglacially overprinted sediments. Sample Mi.826 from ter Idzard, the Netherlands; plane light; hfov 18.0 mm. B. Heavy, but disrupted clay cutan, partly filling up void. Disruption is most likely the result of frost. Clay fragments are very consistent and have a high survival potential. Thin section O.825 from ter Idzard, the Netherlands; plane light; hfov 7.0 mm. Clay coatings (cutans) on and in Tertiary Sirius tillite, Allan Hills, Trans Antarctic Mountains. C. Shiny clay coating on tillite face, such coatings are very common on near vertical. faces. Icepick handle for scale. D. Thin section of surficial clay coating, which consists of multiple laminae, suggesting multiple washing events. Washing is a current process, despite the air temperature never coming above zero (°C) and related to the peculiar properties of snow melt water. Height of sample 6 cm. E. Transition between surficial clay coating and tillite, showing that clay has penetrated in voids in the tillite, while forming the thick surface coat. Plane light, hfov 18.0mm.

glacial sediments, with overviews in handbooks (van der Meer, 1996; Carr, 2004) and the Encyclopaedia of Quaternary Science (Hiemstra, 2007). And although the oldest paper in which a thin section of till was shown dates from 1940 (Lundqvist, 1940), by 1983 no more than a handful of papers in which thin sections were used, was known (van der Meer, 1987). Because of a background in soil micromorphology, the use of thin sections in glacial sedimentology closely follows pedological practise, although the terminology has drifted apart (see Zaniewski and van der Meer, 2005). By now a large number of microstructures from glacial sediments are known (Table 1) and their terminology is generally used as the standard (e.g. Phillips and Auton, 2000; Larsen et al., 2004; Thomason and Iverson, 2006; Lee and Phillips, 2008).

As indicated in the section on terminology, microstructures recognised so far are grouped under several headings (Table 2). In the first place there is the ‘plasmic fabric’ in which all types of arrangement of plasma domains as listed by Brewer (1976) are represented, with the addition of the kinking plasmic fabric (Fig. 9), first recognised in tills (van der Meer, 1982). Other microstructures are related to deformation, either ductile or brittle or polyphase, or related to porewater (Fig. 10A). Because tills are immature sediment mixes (Fig. 10B) different grain sizes occur side by side, which means that even under the same moisture conditions, there are different responses to stress. From an early stage it was recognised that the plasmic fabric can be used to reconstruct the kind of stresses that had

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Fig. 8. Lacustrine sediments. A. Thin section from Lateglacial lake deposits near Clara, Ireland. Note rhythmic bedding with clear, graded couplets (varves?) at base and repetition of coarse layer in second couplet. The central bed shows scouring of the basal clay layer. The bed itself shows cross lamination, which together with the scouring indicates current activity. The center of the scour shows a minor offset indicating microfaulting. The top layer shows a diffuse wavy boundary suggesting load casting. Top right cut and fill structure. Images like this show the great potential of thin sections for analysing sediments and sedimentary processes. Sample Mi.786; plane light; hfov 18 mm. B. Deformation of rhythmites caused by glacial overriding and rafting. Rhythmites have been dragged along and put in a steeply dipping position. This process results in faulting but does not destroy the original sedimentary evidence. Sample R.659 from Lunteren, the Netherlands; plane light; hfov 18.0 mm. C. Bioturbation with filled-in burrows in Lateglacial lacustrine rhythmites. Sample 4545 from Kap James Hill, northernmost Greenland; plane light; hfov 5.3 mm.

been effective (Fig. 10C), whether these were caused by static loading, by sliding of the sediment itself (Menzies and Zaniewski, 2003; Hiemstra et al., 2004) or by glacier overriding (Menzies and Maltman, 1992; van der Meer, 1993) and in this respect micromorphological analyses are further developed for the glacial environment than for most other sedimentary environments. The exception being micromorphology of periglacial sediments as such sediments have been studied for quite some time and by researchers with a similar pedological background. Although it is not assumed that all possible microstructures are known, till micromorphology is now moving into the next phases of scientific development, i.e. interpretation and quantification instead of sole description. It can be argued that so far, till micromorphology has mainly dealt with inventorying, and comprehending what microstructures exist and where they occur and in what sub-environments of the glacial system. Recently a statistical quantification method has been developed that collates the number of differing types of individual microstructures on a statistically large enough sample of thin sections derived from individual tills and other diamictic sediments. This technique permits subglacial tills, for example, to be objectively distinguished from other till-like sediments such as terrestrial debris flow and submarine turbidites (cf. Menzies et al., 2006, see below; Neudorf, 2008). On the other hand a new method of till microstructural analysis based on the methods employed by metamorphic petrologists has been developed (Phillips et al., in press). This is a promising method that allows the detailed separation and stratigraphy of different phases of deformation.

The application of micromorphology to glacial sediments started on the assumption that it might be possible to separate the different types of tills (van der Meer, 1987). However, instead of enabling differentiation it led to the recognition that the different till varieties do not exist (van der Meer et al., 2003) and the classification scheme has to be abandoned. By now the outcome of till micromorphology is starting to advance to the analyses of ice dynamics, what the till microstructures tell us about subglacial conditions and how this affects glacier or ice sheet behaviour (van der Meer et al., 2003; Menzies et al., 2006). In a paper on clastic dykes in subglacial settings, van der Meer et al. (2009) demonstrate not only that their characteristics are based on pressure gradients (Fig. 11), but also the settings in which they form and how this immediately affects glacier dynamics. 5.13. Diagenesis Almost all unconsolidated sediments show evidence of diagenesis, although the term is not often used by Quaternary scientists. However, iron translocation, carbonate cementation, etc., which they do refer to, are all signs of post-depositional alteration of the original sediment and thus diagenetic. In studies of diagenesis the use of thin sections is very common, as these show composition as well as the position and the microstratigraphy of (neo-)formations (Ledésert et al., 2003). Most studies deal with the presence of carbonate cementations in both terrestrial (James, 1985) and marine sediments (Vrolijk and Sheppard,

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the use of staining carbonate minerals as such an approach would reveal more detail about the diagenetic history. But carbonates are certainly not the only cements/neoformations encountered in non-lithified sediments. In a study of Tertiary tillites from Antarctica, Dickinson and Grapes (1997) demonstrated that chabazite (a zeolite) was a common pore-lining mineral. On the other end of the temperature scale, diagenetic zeolites from a tropical setting were described by Renaut (1993). Other cements/neoformations encountered in unlithified sediments are phosphatic in nature. These include calcium phosphate coatings on Antarctic islands (Arocena and Hall, 2003) and widespread vivianites: in Lake Baikal sediments (Fagel et al., 2005; Sapota et al., 2006), in till (Riezebos and Rappol, 1987) and in fluviatile sediments (Figs. 4, 12; van der Meer et al., 1996). 6. Experiments

Fig. 9. A. Crenulated lacustrine silts, as a result of glaciotectonism; coin is 2 cm. Tirvia, Pyrenees, Spain; photo Jaume Bordonau, Barcelona. B. Wavy, lacustrine sediments. Sample Mi.586, derived from sample in A; plane light, scale in cm. C. Alternating bands of highlighted and extinguished birefringence, making up kinking plasmic fabric, related to the wavy bedding as outlined by white quartz grains. Note subhorizontal shearing depicted by unistrial plasmic fabric. Sample Mi.586 from Tirvia, Spain; X pol, hfov 18.0 mm.

1991; Aghib et al., 2003). As in (neo-)formations (minerals formed from solutions in interstitial water) in soils, multiple events of (dis-)solution and precipitation can often be discerned. It should be noted that the geological terminology used to describe the structure of carbonate cements is completely different from that used to describe secondary carbonates in pedology. It could be interesting for Quaternary workers to adopt the approach of carbonate sedimentologists/petrologists to diagenesis and carbonate-rich rocks. Not only to the description, but also

Experimentation has a long tradition in earth sciences, either in order to understand how processes operate, or to understand structures resulting from processes operating under controlled conditions. The scale of experiments ranges from large flumes, shear boxes to wind tunnels. In a number of cases the sediments resulting from experiments are thin sectioned, for instance to study crystal structure of cementations (Badiozamani et al., 1977). However, in most experiments thin sections are a means of studying the results of specific processes, with the objective of understanding these processes. Only occasionally is it the aim of the experiments to directly understand a known microstructure. Examples of the group of process-oriented studies are the papers by Mücher and de Ploey (1977, 1984), Mücher et al. (1981) and Coutard and Mücher (1985). In all of these papers the examined sediment packages are produced by either aeolian sedimentation or reworking of such sediments by running water. Thin sections demonstrate the formation of laminae of particular structure or grain size, depending on process. Some of the artificially produced sediment packages have been subjected to different temperature regimes in order to produce macroand microstructures that can be compared to fossil sequences. Typical microstructures are cracks and vesicles. In an experiment by Dijkmans and Mücher (1989) ‘packages’ of sand or loess were sedimented alternating with natural snowfalls under open (winter) field conditions in the Ardennes (Belgium) in order to study the development of microstructures of niveo-aeolian deposits upon melting of the snow. The results show a spongy structure plus the effects of freezing and of running water and are – not surprisingly – reminiscent of melt-out structures in the glacial environment (see Section 5.12). None of these papers refers to the presence or development of plasmic fabrics. The development of shears has been the subject of a number of experiments and this is an example where microfabrics have been produced specifically for microscopic examination. In these experiments clays were subjected to deformation, mainly through compression tests, after which the samples were thin sectioned. Because of different factors influencing the shearing process (e.g. water content, strain rate, primary fabric, clay composition) there is a whole range of experiments, including incremental sampling (Maltman, 1977, 1987; Arch et al., 1988; Will and Wilson, 1989). The results are described in patterns of shears, their complexity and orientations. Because we are dealing with shears in clays, it would be possible to describe these in terms of plasmic fabrics. However, this is seldom done, as there is a wellestablished geological nomenclature for shears. Exceptions to this are papers by Dalrymple and Jim (1984) on the development of microfabrics as a result of unconfined, alternating wetting and drying which leads to the formation of for instance masepic and skelsepic plasmic fabrics in dependence of sand-silt-clay ratios. Hiemstra and Rijsdijk (2003) performed uniaxial compression tests of clays mixed with coarse sand grains as analogues for subglacial deformation. They discuss the gradual development of microstructures resulting from small-scale slip or shear displacements as well as rotational movements,

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Fig. 10. Glacial sediments. A. Thin section of till from Oakville, Ontario, Canada. Note the porewater escape structures, contorted laminations within the till unit sand ‘floating'microintraclasts. Xpol; bar for scale. B. Intimate mixture of non-homogeneous diamict ground mass with multiple clay/silt and diamict pebbles, which demonstrates how immature many supposedly massive tills are in reality. Saalian till in Lunteren, The Netherlands. Sample R669; plane light; hfov 18.0 mm. C. Thin section from LGM till in Moneydie, Scotland. Unistrial plasmic fabric shows that shears in diamicts split up into numerous strands. Sample R.756; X pol; hfov 4.5 mm.

made visible by skelsepic, masepic and unistrial plasmic fabrics and the relations between them. Their study clearly demonstrated how combinations of planar and rotational microstructures help in distinguishing subglacial tills from mass movement deposits. In a recent paper Thomason and Iverson (2006) discuss the development of shears in remoulded till subjected to deformation in a ring-shear box. Although discussed in terms of shear, the results clearly demonstrate that the microstructures can also be discussed in terms of bimasepic plasmic fabrics.

7. Differentiation and correlation of sedimentary environments Micromorphology, by permitting detailed examination of sediments at the microscopic scale, enables the differentiation and linked association of various sedimentary environments (cf. O`Brien and Slatt, 1990; Maltman, 1994; Carr, 2001, 2004; Lachniet et al., 2001; van der Meer et al., 2003; Menzies et al., 2006). As the range of sediments examined using micromorphology has expanded, so the ability to compare and separate sediments derived and/or deposited in

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Fig. 12. Diagenesis. A. Thin section of river Rhine sediments, showing the actual surface of the river bed. Formation of vivianite is happening throughout the sediment right down from the surface. Sample C.161 from the Biesbosch, the Netherlands; plane light; hfov 3.5 mm. B. Widespread formation of vivianite in plant material. Sample Mi.870 river Rhine bed at 210–225 cm depth; plane light; hfov 3.5 mm.

Fig. 11. A. Intricate bedding in Water Escape Structure. Note cross-cutting relations, grading and clay linings. Sample O.929 from. San Martin de los Andes, Argentina; plane light; hfov 18.0 mm. B. Intricate bedding in Water Escape Structure with water moving up. Note different directions of grading, directed by pressure gradients. Sample R.168 from Donatyre, Switzerland; plane light; hfov 18.0 mm.

differing environments has become possible. Within the plethora of individual microstructures that occur within sediments it is rare, if unlikely, that any one microstructure type is diagnostic of a particular sedimentary environment (cf. Menzies et al., 2006). However, ‘tile’ structures noted within debris flow sediments (e.g., Bertran and Texier, 1999; Menzies and Zaniewski, 2003; Theler, 2004) and ‘silt droplets’ within sediments affected by periglacial activity (e.g. van Vliet-Lanoë et al., 1984) do seem specific to these environments and/or processes. In general it is not one single microstructure but rather an assemblage of differing microstructures that are more or less indicative of a specific sedimentary environment.

In studying a range of diamictons, for example, from glacial tills to glacial marine diamictons to marine debris flow diamictons a statistical test of a range of microstructures using a simple Chi-square test permitted statistically significant differentiation between these remarkably visually similar diamictons (Table 3). In carrying out this statistical analysis, a series of individual thin sections from each diamicton type was examined on the basis of the number of microstructures present in each thin section. First, the thin sections were subdivided into individual frames (25 × 25 mm) and within each frame all recognisable microstructures were counted; then all frames from each thin section were summed. The resultant microstructure Table 3 Frequency of microstructures and other sedimentary attributes within examined diamictite/diamicton thin-section frames from Brora–Helmsdale, Scotland and Ontario, Canada. n = 81

ls

lmd

sdl

rt

ee

sa

sr

ne

im

gmx

cmx

wsx

gls

KBB HBB QDmm

19 22 20

3 4 11

17 21 23

5 5 8

20 27 19

38 39 34

3 2 11

5 6 8

8 7 10

35 36 13

4 5 24

– 3 8

5 4 9

KBB—Kintradwell Boulder Beds, NE. Scotland; HBB—Helmsdale Boulder Beds, NE. Scotland; QDmm—Quaternary Diamicton, Oakville, Ontario, Canada; n—number of thin sections. ls—single lineations; lmd—multiple directional lineations; sdl—short distance lineations; rt—rotational structures; ee—edge-to-edge grain crushing; sa—subangular clast fragment (b15 mm); sr—subrounded clast fragment (b 15 mm); ne—necking structures; im—imbricate structures; gmx—grain-dominated matrix; cmx—clay dominated matrix; wsx—well sorted, evenly distributed clast fragments within matrix; gls—grain line stacking.

J.J.M. van der Meer, J. Menzies / Sedimentary Geology 238 (2011) 213–232

type numbers for each diamicton were then statistically compared, testing whether the thin sections were from one large population or from statistically significantly different populations. The results of this exercise permitted differentiation of one diamicton from another. The converse also applies such that by making statistical comparison, the ability to correlate similar sediments based on statistically ‘common’ microstructure elements permits the identification of sediments from similar environments to be identified. As a result, sediments, for example, taken from a core can be compared to a master sediment list of ‘known’ sedimentary environments allowing less speculation and clearer specific identification of sediment sources. In addition to statistic testing, visual comparison of ‘sets’ of microstructures permit differentiation or co-association of sediments from the same or different sedimentary environments (cf. van der Meer et al., 1994b; Carr, 2001; Lachniet et al., 2001; Larsen et al., 2004). This non-statistical, subjective, method can be successful when clear and marked differences between microstructure ‘sets’ are clearly discernable, especially when coupled with other data such as the geomorphic context or the presence of discriminating evidence such as specific clast provenance or the presence of features diagnostic of specific environments. Considerable research work in the development of large data sets of thin sections, ideally aided by image analyses, should in the near future allow discrimination and co-association links to be developed. These data sets will then allow an innovative method of rapid and objective comparison to be made between known and unknown sediments. 8. Geogenetic microstructures in soils When sedimentation and deformation cease or when subaqueous or subglacial sediments become subaerial the top of the sediment pile will be subjected to soil formation. As we have seen all of these sediments will have their own suite of microstructures; soil formation does not start on a blank sheet. Thus the question is whether all sedimentary microstructures will be overprinted or obliterated by soil microstructures, or whether some are preserved and still present in mature soils. The second question is whether all sedimentary microstructures are equally affected by pedogenesis or whether particular microstructures are more resistant than others. For instance clay illuviation and clay coating under periglacial conditions are well established (see above), and it has been demonstrated that the coatings produced by this process easily break up after a limited number of frost cycles (Fig. 7B). The resulting ‘papules’ could easily become part of an emerging soil and survive pedogenesis (van Vliet-Lanoë, 2010). 9. Discussion The above overview demonstrates that thin sections have been and are used in almost every sedimentary environment. In this overview the emphasis is on unconsolidated to weakly lithified sediments as these best provide an unimpeded idea of the relationship to active sedimentary processes. What is clear is that thin sections are not often used to their full potential. There are several reasons for this. First, in many cases only petrographic thin sections are used, which are so small that relationships between the constituent particles, microstructures and fabrics are difficult to assess. But even if larger sized thin sections, up to Kubiena size, are produced, the emphasis, in the past, has too often been solely on grainsize and composition. Especially in laminated lacustrine sediments, the organic composition of certain layers is emphasised. A good example is the Antarctic Cape Roberts Project (Davey et al., 2001). In this project each aspect of the cores was studied and described, including thin sections. Stratigraphically a large number of sedimentary cycles were recognised, many cycles starting with a diamicton overlying a hiatus. Thin sections of the diamictons were used to establish the glacial history of the drillsite (van der Meer and

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Hiemstra, 1998; van der Meer, 2000) but it was also noticed that some of the diamicts contained microfossils, sometimes in intraclasts. Since the diamicts consist of locally reworked (glaci-)marine sediments, they represent the hiatus. In reality the sediments that made up the part of the sequence that is now the hiatus are still present but in a different, thoroughly mixed shape and displaced position. The ‘missing’ microfossils can be studied by studying them in the diamict thin sections, but this has still to be done. The thin sections of the diamicts have also shown the presence of intraclasts which themselves contain microfossils, allowing further detail in the study of the microfossils. This example demonstrates the microstratigraphic detail that thin sections allow, a potential that has hardly been tapped into. Secondly, micromorphology permits an in situ view into a sediments ‘undisturbed’ structural geology such that the specific grain by grain, lineations, and essential rheology of the sediment as it was being deposited or emplaced can be witnessed. For example, in studying a set of thin section samples from a Jurassic diamictite from northeast Scotland that could, in hand view, be regarded as a sediment deposited by glacial transport, river flood deposits, subaerial mass movement, subaerial mass movement into the sea, or a subaquatic debris flow from a submarine fault scarp. It becomes only too apparent in thin section that the diamictite is a subaqueous debris deposit emplaced within a tropical oceanic environment (Menzies and Whiteman, 2009). Thirdly, in examining experimentally produced thin sections Piotrowski et al. (2006) have demonstrated the ability to derive not only sense of shear and rheology but stress levels involved in the deformation processes ongoing with the sediment being transported and emplaced. As a further example of the intrinsic value of micromorphological analyses it is possible to determine from thin section analyses the impact of seismic activity on sediments and in areas where known seismic activity has apparently long ceased to be regarded as significant but may constitute considerable future environmental risks where, for example, a nuclear plant or high-pressure buried pipelines are located. As a final example of the value of micromorphology, recent work on the depth of pervasive deformation on sea-bed sediments off the east coast of Canada in areas of iceberg scouring where oil and gas pipelines come on shore and are therefore in danger of rupture has shown that the depth of penetration of stress levels within these sediments in much greater detail and to much greater depths than visual examination has led past investigators to assume. At this stage in the science, micromorphology has, for example, fundamentally changed our views on glacial sediments and of subglacial dynamics (van der Meer et al., 2003; Menzies et al., 2006). Likewise other areas of sedimentology and geoarchaeology show enormous potential. There is still a lot to discover and it is our conviction that systematic application of thin sections or micromorphology in all aspects of all sedimentary environments will be very rewarding. This will be aided by moving into 3D studies of microstructures by the use of X-ray μCT (Kilfeather and van der Meer, 2008; Tarplee et al., in press) in combination with traditional thin sections.

10. Conclusions From the above we can reach the following conclusions: • micromorphology is slowly becoming a recognised research tool in sedimentology, • micromorphology has clearly established itself as such in aeolian, lacustrine, periglacial and glacial sedimentology, but is slow to catch up in other sedimentary environments, • in many studies thin sections are mainly used to study composition, not structure, diagenesis or plasmic fabric development, • plasmic fabric development should be one of the most interesting aspects of micromorphological studies of sediments as it strongly related to imparted stress fields.

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Acknowledgements We would like to thank the technicians that over the years have provided us with thin sections of good quality, despite the difficult nature of the material: Frans Backer, Candy Kramer, Jerry Lee, Marty Ouellette, Adrian Palmer, John Taylor, and Cees Zeegers. We would also like to thank Ed Oliver and Mike Lozon for their help with the figures. Furthermore we would like to acknowledge Georges Stoops, Emrys Phillips, John Hiemstra, Simon Carr and Jim Rose for improving the quality of this paper by their comments to different drafts.

Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.sedgeo.2011.04.013.

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