Microstructures, subglacial till deposition, and shear band development revealing up-section changes in shear–A study from Weissbach, Austria

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Proceedings of the Geologists’ Association xxx (2018) xxx–xxx

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Microstructures, subglacial till deposition, and shear band development revealing up-section changes in shear–A study from Weissbach, Austria John Menziesa,* , Jürgen M. Reitnerb a b

Department of Earth Sciences, Brock University, St. Catharines, Ontario, L2S 3A1, Canada Geologische Bundesanstalt /Geological Survey of Austria, Neulinggasse 38, A-1030, Wien, Austria

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 April 2018 Received in revised form 7 November 2018 Accepted 12 November 2018 Available online xxx

Tills from an exposure in Wildschönau Valley, northern Austria were examined using microsedimentological techniques. The tills exhibit a range of microstructures indicative of soft sediment deformation within temperate subglacial bed conditions. The tills can be subdivided at the macroscale into a lower grey and upper red till both of which exhibit some sedimentological variations; however, at the micro-level the tills appear essentially identical. The microstructures in the tills are illustrative of structures developed during deformation both during and following their emplacement. Of note are the microshears within these tills that are demonstrative of changes in applied stress. Both low (<25
) and high angle (>25
) microshears were mapped and their fabric data analyzed. The microshears show a change in stress levels ascending through successive till units. The changes in stress are demonstrative of spatially and temporally changing rheological conditions undergone by the subglacial tills during deformation, ongoing deposition/ emplacement and stress localization. These findings indicate that microstructures reveal local deformation conditions in tills and a more detailed micro-history of paleostress. © 2018 The Geologists' Association. Published by Elsevier Ltd. All rights reserved.

Keywords: Sedimentology Stratigraphy Quaternary Geology Geomorphology

1. Introduction The rheological behavior of tills deforming within a subglacial soft sediment layer beneath an ice mass has been the subject of considerable interest over the past decades (Piotrowski et al., 2004, 2006;Kjær et al., 2006; Truffer and Harrison, 2006; Schomacker and Kjær, 2007; Larter et al., 2009; Smith and Murray, 2009; Narloch et al., 2012; McCracken et al., 2016; Spagnolo et al., 2016; Phillips et al., 2018a, b). With the discovery of “deforming soft sediment packages” beneath modern ice masses, it has been commonly acknowledged that similar subglacial conditions more than likely prevailed beneath Quaternary and Pre-Quaternary ice masses. However, the detection of soft beds in the ancient record remains challenging (cf. Busfield and Le Heron, 2018). Comprehension of the mechanics of just how subglacial tills are deposited and/or emplaced remains imperfect. Although macrofabric elements of tills have been well established for over 60 years (cf. Holmes, 1941; Evans et al., 2006; Menzies et al., 2018), the genesis

* Corresponding author. E-mail address: [email protected] (J. Menzies).

and kinematic evolution of microstructures indicative of both brittle and ductile shear within tills formed and forming during shear, and the link to bulk rheological properties remain poorly understood (Phillips et al., 2013a, b, 2018a, b; van der Meer et al., 2003; Roberts and Hart, 2005; Benn and Prave, 2006; Reinardy and Lukas, 2009; van der Meer and Menzies, 2011; Phillips et al., 2011; Denis et al., 2010; Arnaud, 2012; Clerc et al., 2012; Busfield and Le Heron, 2013; Ravier et al., 2014; Le Heron, 2015; Menzies et al., 2016a, b; Cowsill et al., 2016; Phillips et al., 2018a, b). This paper investigates microstructures in subglacial till formation and their development from a site at Weissbach in the Wildschönau Valley, northern Austria with an emphasis on the analyses of microshears that occur within these tills. The development of a method of microshear analyses in tills has been discussed in the past (e.g., Thomason and Iverson, 2006; Narloch et al., 2012) utilizing microshear length rather than geometry. This paper introduces an alternate method of microshear analyses showing, in an experimental manner, the potential value of such studies. The paper sets out to detect the level of localized change in stress within a specific near-vertical till exposure using microshear analyses and provides a better understanding of till deformation and depositional/ emplacement mechanics.

https://doi.org/10.1016/j.pgeola.2018.11.001 0016-7878/© 2018 The Geologists' Association. Published by Elsevier Ltd. All rights reserved.

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2. Geological and palaeogeographical setting The Wildschönau Valley (Fig. 1a), within which the Weissbach site occurs, is a tributary valley of the Inn Valley south of the village of Kundl, Austria. The valley drained by the creek Wildschönauer Ache is incised into bedrock of the Greywacke Zone (GWZ), part of the Austroalpine Superunit (Schuster et al., 2014). The GWZ

comprises Paleozoic slates and phyllites. In its lowermost gorgelike sector (along the southern rim of the Northern Calcareous Alps (NCA)), there occur red siltstones and sandstones overlain by dolostone. Regionally the Wildschönau area is important in being illustrative of the glacial dynamics at the onset of the Last Glacial Maximum (LGM) (van Husen and Reitner, 2011), with the formation of thick proglacial fluvial, deltaic and glaciolacustrine sediments (up to 100 m) overlain by LGM tills (the focus of this paper) (Reitner, 2008) (Fig. 1b). The tills were later covered by glaciolacustrine to deltaic deposits of the Lateglacial phase of icedecay (end of Marine Isotope stage 2) (Reitner, 2007). Based on the sedimentology and clast lithology of the stratigraphic successions along the valley floor of Wildschönau (Reitner, 2008; Menzies and Reitner, 2016), it is evident that the Inn Glacier blocked the Wildschönau Valley at the lowermost gorge at an early stage during ice-build up during the onset of the LGM (Fig. 1b). This blockage resulted in the formation of ice-dammed lakes in front of the advancing Inn Glacier. Fining-upward sequences of glaciolacustrine deposits, with erratic clasts as dropstones, followed documenting the drowning of the landscape before the Inn Glacier advanced and deposited the erratic-rich subglacial till studied at Weissbach. 3. Methods

Fig. 1. (a): Location of the sample site in the Wildschönau (area covered by b) near Kundl, Austria, within the ice-extent of the LGM. (b): Map with sample site (WB = Weissbach) showing the palaeogeographic situation during the formation of the WB-samples (modified from Menzies and Reitner, 2016). The formation of icedammed lakes with variable size and depth started with the closure of the gorge in the Wildschönau Valley by the southward advancing Inn glacier. The dotted lines show the inferred further scenario for the advance of the Inn Glacier (blue) as well as of the northward flowing local glaciers (blue) (shaded relief image from TIRIS online map of the Province of Tyrol: www.tirol.gv.at) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

An undisturbed till exposure that allowed good sampling access was chosen at Weissbach, in the Wildschönauer Valley (47
250 38.80 North, 12
01010.40 East). The site is located on the western side of the Wildschönau Valley 15 km southwest of the village Wildschönau (Fig. 1b). This site had already been mapped, sediments logged, and standard macro-sedimentological data obtained (Reitner, 2008) (Fig. 2a, b) as part of a broad sedimentological study conducted in northern Austria in the Wörgl area (see Menzies and Reitner, 2016). At this location a much more detailed analysis of a single site was carried out. Sampling of the till was done on a near-vertical exposure where samples were obtained at approximately 50 cm to 1 m intervals utilizing Kubiëna cans and bulk samples (Fig. 2c, d). Samples were transported back to the laboratory for preparation and thin section manufacture (Rice et al., 2014; Menzies and van der Meer, 2018). As is often the case in thin section manufacture some samples proved more difficult to impregnate or the thin sections produced were unsatisfactory. Therefore, for example in the case of sample WB6 a larger number of thin sections were produced. In other cases, multiple thin sections showed essentially identical features and, for economies sake, are nor reproduced here. All the sample sites in Fig. 2c & d were as illustrated but in the case of WB6 several local closely sampled as shown in Fig. 2. At all the other sites, samples were taken no more than 10 cm apart, thus the multiple samples for WB3, 4 and 5. Once thin sections were made, standard micromorphological analyses were performed on each sample and all microstructures described and collated (cf. Phillips et al., 2011; Menzies and Reitner, 2016). At Weissbach, microstructures were differentiated into rotation structures, microshears, grain stacks, deformation bands, edge to edge grain events, domains, and shear zones (Table 1) (for a glossary or definition of microstructures see van der Meer and Menzies, 2011, or Menzies, 2012 or Menzies and van der Meer, 2018). 4. Macrosedimentology The sampling site at Weissbach displays a sedimentary sequence typical for the Wildschönau Valley (Figs. 1 and 2). In the uppermost part (Lithofacies III), two visually different tills

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Fig. 2. (a): Sedimentological log of Weissbach (WB) (Co = cobble; F = clay and silt; G = gravel, S = sand; GWZ = rock from the Greywacke Zone (metasiltstone, metasandstone, metabasite) (note different lithofacies units). (b): Grainsize distribution, clast shape, clast lithology of the red and grey diamictons; Equal area projection, lower hemisphere of shear planes with and without kinematics (indicated by arrows) found in the diamictons; Clast shapes (a = angular; r – rounded, sa = subangular; sr = subrounded); Clast lithologies (PSK = Permian to Lower Triassic red siltstone and sandstone; NCA-carbonate = Mesozoic limestone and dolostone from the Northern Calcareous Alps (c): The uppermost 5 m of the Weissbach section showing sample sites and sample numbers. Note details of sample site WB 6. (d): Sketch showing exposure and sampling sites as in (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

(Fig. 2a) were sampled overlying a basal contact of a fining-upward succession (Lithofacies I) consisting of fluvial deposits with clasts of local provenance (GWZ) and glaciolacustrine deposits (lithofacies II) (Fig. 2a), with dropstones of erratic lithologies (orthogneiss, mica schists, NCA carbonates) (Fig. 2a). The lower grey till (Fig. 2d) has a high mud content (60–70% clay & silt in Fig. 2b), not unlike dropstone diamictons described by Benn and Evans (2010). The overlying and, at times, intercalated red till (Fig. 2c) has a grain size distribution of <40% silt and clay (Fig. 2b), typical for subglacial tills of the local area (cf. Reitner et al., 2010).

The clast lithology of both tills is dominated by erratic lithologies especially NCA carbonates, compared to only 4–10% of local bedrock lithologies (GWZ). The abundance of crystalline rocks (orthogneiss and mica schists) differs between the two tills with 12% in the grey till compared to <1% in the red till. The latter has a considerable content of red siltstone and sandstone (12%) from the base of the NCA which accounts for the distinctive matrix colour. The red till has a lower portion of angular clasts and a corresponding higher content of subangular and subrounded clasts (Fig. 2b). Both tills are highly compact with the red till being

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Table 1 Microsedimentological data and microstructures in sampled thin sections from the Weissbach Site, Austria. Figure # Samples WB#

4a 1.1a

4b 2.1a

4c 3.1a

4d 4.4a

5a 5.1a

5b 6.1

5c 6.2

5d 6.3

6a 6.4

6b 6.6

6c 6.8

6d 6.10

6e 6.12

Colour Munsell Clast size Clast Provenance Clast shape Domain (dm) Grain stacks (gls) Rotation (rt) Edge-to edge (ee) Microshears (ms) Plasmic Fabric Deformation bands Remarks

dbg 6/0 sfl le

dbg 6/0 wr le

drg 5/0 wr le

drb 5/2 wrf le

drb 5/2 wrf le

drb 5/0 wrf le

drb 5/0 wrf le

drb 5/0 wr le

drb 5/0 wrf le

drb 5/0 wrf le

drb 5/0 wrf le

drb 5/0 wrf le

drb 5/0 wrf le

sr wr

sr wr

sr wr

sr wr

sr wr

sr wr

sr wr

sr wr

Sr wr

sr wr

sr wr

sr wr













sr wr  









































































ma

om

om

ma

lat

lat

ma

Wu

ma

ma

ma

ma

lat



























Large vertical clay band Grey Till

Sand stringers, necking structures Grey Till

Vertical structures Red / Grey Till transition

Shear zones Red Till

Necking Deformation structures in two strong Red Till directions Red Till

Strong Clast to Matrix interaction Red/ Grey Till transition

Strong Clast to Matrix interaction Red/ Grey Till transition

Fine matrix Red Till

Fine matrix Red Till

Fine Strong matrix deformation Higher Red Till stress Red Till

Strong Clast to Matrix interaction Red Till

Colour: dbg - dark brown gray, drg - dark red gray, drb - dark red brown; Munsell: all 7.5YR; Clast Size: sfl - small, a few large >35 mm; wr – wide range of clast sizes; wide range of clasts sizes but few clasts; Clast provenance: le – local and exotic sources; Clast shape: sr wr – subrounded to well rounded; Plasmic Fabric: ma – masepic; om- omnisepic; lat – lattisepic; W u – weak unistrial.

slightly more consolidated based upon visual examination and hardness during excavation. The contact between both units is sharp and is cut by discrete shear planes developed within the tills (Fig. 3a, b) with lengths of up to 50 cm (cf. Busfield and Le Heron, 2013; Fleming et al., 2016). Additionally, sharp, straight shear planes occur in the tills at the macroscale (Fig. 3c). When identifiable, the shear planes have normal fault kinematics exhibiting extensional stress with the hanging wall dropping relative to the footwall (cf. Fossen, 2010a, chapter 17). At the contact between the tills, the shear planes dip dominantly toward the East (down valley) where they exhibit a conjugate pattern in the tills (Schmidt net in Fig. 2b). The dip of the faults is generally higher than 25
. The data indicate a sub-vertical shortening direction, yielding a sub-vertical maximum principal stress, assuming infinitesimal strain (cf. Fossen, 2010a). In addition, the red till exhibits macroscopic deformation structures, specifically, intraclasts of grey laminated silts (Fig. 3a) where scavenged units of grey till appear to have been ‘up-loaded’ into the overlying red tills (Hiemstra et al., 2007). 5. Microsedimentology 5.1. Weissbach thin sections The locations of the thin section samples are shown on Fig. 2c, d, and the salient descriptive aspects of the thin sections are shown in Table 1. In total over 50 thin sections were made (in many instances several thin sections were produced from a single Kubiëna tin) and examination of all were conducted such that the examples presented here represent the best and most useful thin sections for discussion (see Supplementary Data). Sampling for each sample using both Kubiëna and bulk samples was done over an area of approximately 0.25 m2 of exposed till face. In a few cases, specifically Site WB 6 several samples were taken that spread vertically and laterally over almost 0.2 m2 of the exposure.

As Table 1 shows grey and red tills show little or no significant difference in terms of microstructure sets, types or numbers present. The presence of numerous microstructures is symptomatic of the deformation conditions under which the tills were deposited (emplaced) (Hiemstra and van der Meer, 1997; Menzies et al., 2016a; Menzies and van der Meer, 2018). Of the microstructures noted here at Weissbach, there are no single microstructure that can be readily perceived as diagnostic, rather the combination of various microstructures is indicative of deformation processes (discussed in detail in Menzies et al., 2016a). It should be noted that, at the microscopic scale, till colour is indistinguishable and any distinction made between grey and red till is no longer applicable. It is interesting to not that edge-to-edge grain crushing was only noted in red till samples but whether this is of significance has still be further investigated. The occurrence of sand stringers is occasionally observed in the tills, for example, in Fig. 4b (Sample WB2.1a) shows structureless sand stringers running roughly sub-horizontally and sub-parallel to each other that are indicative of very high shear stress, often resulting in being boudinaged (nb. Kessler et al., 2012, Table 1). The identification of microshear planes in these tills is founded upon several attributes. First, recognition is based upon the presence of a microscale S-C fabric within the tills (cf. Phillips et al., 2007). It is apparent that the amount of off-set in most of these shear planes is a matter of a few microns. Secondly, the direction of shear, as can be seen from shear plane orientation, is shown to vary as a function of the direction of the shear. Finally, direction of downthrow in the two-dimensions of a thin section, is almost, if not impossible, to estimate (nb. Passchier and Trouw, 1996; Fossen, 2010a). As a component of anisotropic deformation, microshears are indicative of localized high stress zones (nb. Larsen et al., 2006, 2007; Shanmugam, 2017). In a classification scheme proposed by Logan et al. (1979), it can be shown that as the amount of stress increases, and sheared sediment thickness reduces, the angle between conjugate microshears reduces and a value of  25
can

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Fig. 3. Details of outcrop Weissbach (sites indicated in Fig. 2d). (a): Red diamicton with a white quartzite clast, a former dropstone, which is in contact with remnants of grey, laminated glaciolacustrine mud. Thin layers of this mud are evident to the left of the clast. Just above is another orthogneiss dropstone again in contact with grey, laminated glaciolacustrine mud. (b): Unconformable contact between the grey mud-rich till and red till. Some shear planes are evident. (c): Shear planes within the intercalation of red and grey diamictons (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

5

be used to differentiate between low stress levels and high stress levels (see extended discussion in the section on Microshear fabrics below). In Fig. 4c (Sample WB3.1a), many of the microshears and grain stacks present appear curved which may be a function of postdepositional/emplacement deformation caused during or immediately after emplacement, or the fracture planes were, in the first instance, curved. In Fig. 5a (Sample WB5.1a), a small deformation band was noted with microshears on both sides in the upper right-hand side of the image (in yellow). This and other deformation bands within these tills exhibit some cataclasis within the band but also possibly some clay infilling (Antonellini et al., 1994; Mair et al., 2000; Fossen, 2010b). The question is, are these deformation bands of synsedimentary or glaciotectonic origin? Deformation bands are defined here as mm-thick roughly tabular zones or units of localized deformation. In some instances, such bands are often described as shear bands. Following Fossen’s classification (Fossen, 2010a, Fig. 7.40) these deformation bands may best be described as cataclastic bands. However, most deformation bands described in the literature refer to highly porous sediments of high permeability unlike most tills. Włodarski (2005) noted similar deformation band in tills in Poland where a change in localized microporosity clearly changed in relation to the host matrix material, evidence of discrete localized shearing. The deformation bands described here at Weissbach would appear to be of glaciotectonic origin evolving during till emplacement. In most of the thin sections there is evidence of sediment deformation in the form of strong clast and clay plasmic fabric alignment. All the microstructures present indicate ongoing sediment deformation processes. Deformation is often nonpervasive and would indicate, based upon multiple crosscutting events of differing microstructures, that the tills at Weissbach have been repeatedly deformed (cf. Boulton, 2006; Fossen et al., 2018). A range of plasmic fabrics can be noted, in active crosspolarized light, from omnisepic and masepic to lattisepic and unistrial. In the case of Figure 5d (Sample WB6.3), a weak unistrial fabric was detected and, in several instances, lattisepic fabrics are observed in Figures. 4d, 5 b, 6 c and e (Samples WB4.4a, 6.1, 6.8, 6.12) (cf. for plasmic fabric definitions see Brewer, 1976; van der Meer, 1987, 1993; Menzies and van der Meer, 2018). Grain stacks are present in all thin sections but are particularly prominent in Figure 6,a and c (Samples WB6.4 & 6.8). Grain stacks, in general, crosscut all other microstructures in the sense that they appear to have been amongst the first microstructures developed in these tills such that all other microstructures over print them (cf. Menzies et al., 2016a). Rotation structures are prevalent throughout the thin section samples (Figs. 4c, d, 5 b, 6 a, b) (Samples WB3.1a, 4.4a, 6.1, 6.4, 6.6). In many cases, the rotations crosscut each other and, in some instances, are, at least in 2D, ellipsoidal or ovoid. Edge-to-edge grain crushing events (e-e) are not commonly noted but do occur in Figures 4d, 5 a, and 6 b (Samples WB 4.4a, 5.1a, 6.6). Unlike all the other thin sections from the Weissbach site, at least two edge-to-edge grain crushing events can be noted (yellow circles) in Figure 4d (Sample WB4.4a). Distinct domains within thin section samples are relatively few, other than in Figure 5d (Sample WB6.3). Also, in Figure 5d microstructures are noted crossing over between and into separate domains. As might be expected with subglacial sediment deformation, there are many instances of ‘necking’ structures where clasts and plasma interact with finer grained material largely composed of plasma being squeezed through between larger clasts, indicative of plastic deformation – see Figures 4b, 5 a, c, d, 6 b (Samples WB2.1a, 4.4a, 5.1a, 6.2, 6.3, 6.6). The orientation of each microshear in the samples obtained at Weissbach was measured and plotted on a stereonet diagram

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Fig. 4. a–d Photomicrographs of thin sections (WB1.1a, 2.1a, 3.1a, 4.4a). In 4a a clay band is highlighted in orange, in 4b a necking structure is highlighted in green, and in 4d two edge-to-edge grain crushing events are highlighted in yellow. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). Note all the images in Figures. 4–6 from Weissbach are in plane light and have an annotated duplicate. Note scale on each image. Top of each thin section is at the top of the image. Note microshears are solid blacklines, grain stacks are purple dotted lines, rotation structures are thin black lines with an arrow.

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Fig. 5. a–c, d Photomicrographs of thin sections (WB5.1a, 6.1, 6.2, 6.3). Note in 5a necking structures are shown in green, and a shear band in blue, and in 5d two domains are shown – the right-hand domain in pink. Also note necking structures highlighted in green and edge to edge grain events in yellow (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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5.1.1. Interpretation of Weissbach thin sections On an individual basis, each of the thin sections exhibit characteristics that are symptomatic of subglacial conditions related to temperature, grain size, porewater content and varying deformation states. The tills at Weissbach have undergone considerable change from first inception to final deposition / emplacement. These thin section samples are ‘windows’ into the tills at Weissbach and great care must be exercised in not making too many broad assumptions or ‘gross’ interpretations (nb. Larsen et al., 2006, p. 892). For example, in Figure 4a (WB1.1a), a clay band (orange) under cross-polarized light exhibits moiré banding illustrative of likely post-till emplacement deformation, i.e., the banding possibly was formed after the till was initially deposited (Menzies and Ellwanger, 2011). The moiré banding of clay indicated that after initial emplacement the clay band was subsequently deformed. Examination of the clay plasma in this band suggests that it has been likely emplaced within a fracture formed or intruded around the large clast in the centre of the image subsequent to earlier sediment deformation. In Figure 4c (WB3.1a), several parallel to subparallel low angle microshears are present indicative of localized high stress levels resulting in attenuation. The shear zones appear partially intact and are sufficiently subtle to be indicative of incomplete destruction from later progressive deformation. In contrast in Figure 5a (WB5.1a), localized deformation bands are easily recognized by the presence of a micron thick zone of localized compaction of very fine-grained clasts. Such a band is typically indicative of brittle fracture leading to small localized shear displacements (cf. Fossen, 2010a, Fig. 7.37). The deformation band in this till unit may be the result of cataclasis or granular flow or clay particle intercalation. Insufficient information could be gleaned from this thin section to make a stronger conclusion. In polarized light, this thin section has a lattisepic plasmic fabric (note discussion of microshears and stress levels below) (van der Meer, 1993; Menzies, 2012). In samples where domains are present (e.g., Fig. 5d (WB6.3) they indicate the incorporation of ‘extraneous’ units within these tills. 5.2. Plasmic fabric and stress levels

Fig. 6. a–e Photomicrographs of thin sections (WB6.4., 6.6, 6.8, 6.10, 6.12). In 6a large rotation structure can be noted around the large clast in the centre of the image and above it a ‘necking’ structure in green. In 6c and 6e note shear lines (blue). In 6d good evidence of lattisepic plasmic fabric. Note 6b edge to edge grain events in yellow. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

(Fig. 7a, b) (see Microshear fabrics below). The data are plotted with microshears <25
(low angle, high stress) shown in red and microshears > 25
in black (high angle, low stress). The percentage of microshears above and below 25
was used, rather than the actual number, to measure the change, as a percentage, in low and high angle microshears across the till exposure (cf. Larsen et al., 2006; Ballas et al., 2013) (Fig. 7a, b)

The Weissbach thin sections all exhibit varying types of plasmic fabrics that reveal strain level signatures upon clay particles in these tills (see references in Menzies and van der Meer, 2018) (Table 1). Plasmic fabrics develop in sediments with grain sizes of <2 mm (for plasmic definitions see Hiemstra and Rijsdijk, 2003, appendix, p.383). The fabrics are detected as birefringent on the petrological microscope in which oriented domains can be seen. For example, an omnisepic fabric indicates that the clay particle fabric is scattered over a wide range of orientations with no dominant direction apparent. In masepic and unistrial fabrics the domains are relatively elongate indicative of formation under stress; in the latter plasma appear as thin, almost continuous, stringers of clays. It is interesting that Hiemstra and Rijsdijk (2003) pointed out, following experimental work, that often unistrial fabrics developed at the expense of other fabric types by destroying or overriding the previous fabrics possibly indicative of higher stress values leading to re-deformation of pre-existing stress level signatures in tills (Piotrowski et al., 2006; Menzies, 2012; Linch and van der Meer, 2013). A similar conclusion on a discussion of masepic fabrics of tills from Co. Durham was suggested by Davies et al. (2009) as indicative of relatively high compressive stress (cf. Hooyer and Iverson, 2000). Finally, lattisepic fabrics occur where a fabric indicative of deformation of clays from two distinct directions, almost at 90
to each other, can be detected (Jim, 1990).

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Fig. 7. Stereonet fabric diagrams of microshear orientations at Weissbach. Note ‘n’ = the number of microshears per sample. The angles of the microshears in red are <25% and those in black > 25% to the horizontal, as measured on respective thin sections (Figs. 4–6) where in Figure 7a: thin Sections 1.1 A, 2.1 A, 3.1 A, 4.4 A, 5.1, 6.1, & 6.3 are Figures 4a, b,c,d, 5 a,b & S11. In Fig. 7b: thin sections 6.4, 6.6, 6.7,6.8, 6.10 & 6.12 are Figures 6a, b, S10, 6 c–e (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

In the samples from Weissbach plasma fabrics were identified as omnisepic, masepic, unistrial and lattisepic fabric (Table 1). Figure 4b and c (Samples WB2.1a & WB3.1a), both, exhibit omnisepic fabrics indicative of a very broad range of clay particle orientations with no dominant direction apparent (Zaniewski and van der Meer, 2005). In Figure 5d (Sample WB6.3) a unistrial fabric [indicative of high internal sediment stress levels (Brewer, 1976; van der Meer, 1993)] is found symptomatic of localized higher stress levels. These stress levels are borne out by the stress level diagram which indicates the top and bottom cross-over point of a sliding bed between mobile and immobile units in the till sequence (Fig. 9) (see section below on “Mobile till layers and shear ‘bands’ within a subglacial soft deforming layer”). Since plasmic fabrics often occur as ‘patches’ within till units and, at times, cannot be detected (Lagerlund and van der Meer, 1990) the intrinsic value of plasmic fabric data must be assessed with great caution. In many instances, where patches of limited plasmic fabric appear it may be that this lack of evidence can be attributed to high porewater pressures causing particle to particle separation (dilatancy) or simply higher destructive levels of internal shear stress causing microstructures formed at lower stress levels to be destroyed (cf. Lade, 2002; Reinardy et al., 2011; Menzies et al., 2013, 2016a, b). 5.3. Microshear fabrics Recent work in tills (Larsen et al., 2006; Kaproth et al., 2010; Haines et al., 2013; Narloch et al., 2012; Iverson and Zoet, 2015; Menzies et al., 2016b), based upon experimental research by Tchalenko (1968); Logan et al. (1992); Hooyer and Iverson (2000); Larsen et al. (2006) and Thomason and Iverson (2006), demonstrates that the orientation of microshears in relation to shear direction and deforming sediment thickness is indicative of stress

levels or possibly increased porewater content. It is possible that as porewater increased locally so effective stress would be reduced and greater shearing would occur resulting in low angle microshears (pers. comm. J. Lee, November 2018). In response to this second idea, of raised porewater content (cf. Lee and Phillips, 2013), it might be expected that evidence of increased porewater would be visible at the micro-scale. So far, in the samples from Austria in this paper, no such evidence of increased porewater was detected. Following Logan et al. (1979), it has been shown that as stress increases, and sheared sediment thickness reduces, the angle between conjugate microshears reduces and a value of  25
can be set to differentiate between low stress levels (>25
) and high stress levels (<25
) (Tchalenko, 1968; Larsen et al., 2006, 2007; Thomason and Iverson, 2006; Narloch et al., 2012; Iverson and Zoet, 2015) (Fig. 7a, b). Microshears, observed in the sediments, are likely of several generations, due to the evidence of multiple crosscutting microstructures, where some formed at the time of immediate emplacement while others repeatedly were likely formed due to later shear stress leading to localized compaction processes, possibly even due to settling of the sediment after porewaters have been largely dissipated. Past work by Larsen et al. (2006); Thomason and Iverson (2006) and Narloch et al. (2012) have used the ‘IL index of cumulative length of low-angle microshears’. This dimensionless index calculates the degree to which low angle shears accommodate strain using the lengths of low and high angle shears. Since lengths are not reliable values in the two-dimensional geometry of thin sections, the use of this index has not been adopted. However, in dealing with 2 dimensional microshears in thin sections it is thought by the present authors that there is potential error in such a value. Instead only the number of microshears, and their angle to the horizontal, as a percentile, is considered acceptable data in the present

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investigation. Larsen et al. (2006) established a log-linear relationship between the IL Index and strain, however microshear lengths can only be estimates drawn from a 2-dimensional image and therefore any relationship drawn must be uncertain. However, even with this ‘investigational flaw’, Larsen et al. (2006) suggest that once a quasi-steady state is established at relatively low strains, microstructure change, and ‘evolution’ ceased. If this is the case much more experimental 3-dimensional work needs to be carried out. Microshear fabrics, when plotted on the vertical, show the change in stress levels ascending within the till exposure as illustrated in Figure 8 (see below for discussion of the implications of these stress changes). What emerges is a change vertically in stress levels through the till swinging from high levels to lower levels, and back and forward. It is estimated that high stress levels must have occurred at Sites WB1.1a to 2.1a (Fig. 4a, b) followed by a reduction in stress levels at Sites WB6.1 to 6.6 (Figs. 5b, 6 b), subsequently followed by again higher stress levels at Sites WB6.7 to 6.8 (Figs. S10, 6c), again followed by a low stress level ‘zone’ at Sites WB6.10 to 3.1a (Figs. 6d, 4 c), and finally, followed by higher stress levels at WB4.4a to 5.1a (Figs. 4d, 5 a) at the upper part of the till exposure. Sharp fluctuations in stress levels within the tills can be expected and are consistent with a soft deforming sediment being emplaced as stress levels on the deforming sediment reduces leading to immobilization and once emplaced followed with cumulative compaction under dissipating porewater associated with high stressing of the now deposited till. Such changes in stress history can be expected to appear in the paleo-signatures of microstructure types, numbers and interrelationship geometries. The data are challenging to interpret since like any subglacial till, the tills at Weissbach, as can be seen both in hand specimen in the field and in thin section, have likely undergone repeated deformation, re-entrainment and transport such that stress level signatures are blurred and those formed under lower stress are partially or completely obliterated under certain higher stress subglacial conditions (Menzies, 2012). The fact that in many of these tills low stress microshears can be detected, likely formed just prior to emplacement lends credence to evidence that points to rapid emplacement, and the subsequent

effect of porewater dissipation due to compaction leading to a rapid increase in intrinsic yield strength of these till once deposited and likely new high stress microstructures formed at this stage (Ingólfsson et al., 2016; Phillips et al., 2018a,b, Fig. 1). As the till is deforming in the mobile package at the ice-till upper boundary layer – it follows that stress levels within the mobile units must be comparatively low with intrinsic high porewater pressures but almost immediately on emplacement caused by lower applied stress levels or porewater dissipation, stress levels must ‘creep up;’ and porewater pressure reduce from the mobile to immobile phase within the till rheology. In general, tills possess an anisotropic fabric (where the arrangement of the solid particles or aggregates differs significantly in all directions), which is manifested macroscopically as a deviation of the principal direction of the yield surface from the hydrostatic stress axis (cf. Yang et al., 2015). Inferences, therefore, need to be derived from symptomatic signatures that can be seen in macro- and microscale examination of tills (nb. Lade, 2002; Phillips et al., 2018a,b). Based upon the data in Figure 8, several implications need to be considered. First, do spatially variable high stress levels detected in these tills suggest that high stress implies high deformation rates? Secondly, does high stress imply high porewater contents or vice versa? Finally, does high stress imply low effective stress levels or again vice versa? It is proposed here that as the till deformed, then at least two modes of deformation must have occurred. It can be assumed that the strength of these tills is primarily proportional to the effective confining pressure. A reduction in effective stress levels beneath the ice mass and within a deforming sediment layer may be caused by increasing pore pressure resulting in localized till failure and thus local sediment instability. One failure mode of instability occurs in certain localized regions of the stress ‘space’ within the till that potentially results in liquefaction (cf. Lade, 2002; Phillips et al., 2018a,b). An alternate mode, initiated by localization of plastic strains leads to the development of shear bands due to stress localization (Fossen, 2010a, b; Bigoni, 2012). These two modes appear to be mutually exclusive yet likely occur in these tills for different loading and material conditions.

Fig. 8. Sample sites at Weissbach till exposure on the left, with plotted microshear angles shown as <25% plotted on the right. Indicating change in stress vertically within till. Sample Sites: 1.1, 2.1, 6.1, 6.6, 6.7, 6.8, 6.10, 3.1, 4.4, & 5.1 are found as Figures 4a, b, 5b, 6b, S10, 6c, d, 4 c, d and 5 a.

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At the Weissbach site it appears that the evidence points to at least localized shear band formation whilst in other sites in northern Austria evidence of liquefaction appears to have taken place, at least locally, within the tills (cf. Menzies and Reitner, 2016). As the till appears to have undergone considerable stress fluctuations, as seen in Figure 8, microshearing has occurred within the till. As the average angles of microshearing indicate, the level of applied stress where low angles prevail point to evidence of higher stress levels and vice versa when high angles prevail. Where there is predominance of either high or low angle microshears, one can reasonably extrapolate, based on the thin section evidence, suggest that bands of varying stress levels do occur within the till at Weissbach. Subsequently, shear bands have formed within the tills. The appearance of these shear band lends credence to the style of deformation at Weissbach within a deforming soft sediment bed. To the argument that such tills at Weissbach may exhibit evidence of passive melt-out from basal ice layers, as has been suggested in review; the evidence from investigated melt-out tills where passive deposition and limited deformation occurs and strong evidence of porewater escape abounds (cf. Larson et al., 2016 and references therein) undoubtedly negates such a conclusion. As the till was being emplaced as either an immobilizing subglacial traction till (cf. Evans et al., 2006) or an emplaced stacked sequence of deforming units, stress localization, porewater fluctuations, and freezing/thawing conditions must have ephemerally occurred as is evidence from the microscopic evidence provided here. 6. Mobile till layers and shear ‘bands’ within a subglacial soft deforming layer From the above data, it is apparent that till shear bands have formed spatially and temporally within a soft sliding subglacial bed on top of the underlying immobile basement, whether bedrock or pre-deposited till (Fig. 9). In many cases these bands will be totally or only partially remobilized and subsequently deposited downice whilst, in other instances, the till within the layer will be redeformed and “tectonized” again before being finally ‘covered’ by enough later till that no further glacially-enhanced stress effects will be encountered (Menzies, 2012; Menzies et al., 2016b). The tills from Weissbach reveal that the complex process of till deposition / emplacement can be ‘read’ within the microsedimentology of the till. The sequence of stages of microstructure formation are shown in Figure 9 show this process of immobilization and subsequent build-up of till units within a till sequence formed under soft sliding subglacial bed conditions. The record of paleo-stress manifest from evidence of high and low angle microshears and stress localization demonstrates this ‘sequence’ of emplaced layers and the overlying, still active, mobile till phase. 7. Discussion and conclusion

Fig. 9. Schematic diagram of sliding bed conditions as deforming beds become progressively immobilized and overrun. (A) a block diagram to illustrate the similarity between fault gouge and the ‘situation’ in the basal zone of an ice mass (cf. Eyles & Boyce, 1998, Fig. 7), (B) a chronosquence of subglacial sediment deposition from Time 1 (T1) to Time 2 and 3 (T2, T3). T1 illustrates, in block format, the conditions beneath a deforming ice sheet within deforming subglacial sediment layers, (T2) as till is deposited /emplaced a sequence of till units both mobile and immobile form, and in (T3) a sequence of now immobile deposited till units can be noted with a still active mobile till unit at the ice/basal sediment interface.

It is evident that the tills emplaced/deposited at Weissbach are sedimentologically similar at the macroscale and exhibit significant features in thin section. From investigations at the microscale the tills appear to have been formed under soft sediment, temperate, subglacial conditions where variations in stress levels could be expected both during emplacement and subsequent diagenesis. In exploring the differences at the microscale some assessment can be made of stress level signatures as evidenced by the number and orientation of microshears. Microshears developed as the till was differentially ‘adjusted’ or brittle sheared through strain partitioning during basal layer deformation such that local ‘tears’ occurred probably in response to the application of differential load (inter alia Møller et al., 2008; Fossen, 2010a, b; Tembe et al., 2010; Haines et al., 2013). In most instances, this local

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movement is the result of external stress overcoming local sediment yield strength thus developing a slip surface (shear plane, or plane of décollement). Within a granular material, such as till, where the globally imposed shear rate is not distributed homogeneously, a consequence is that stress heterogeneities develop, such that, where the stress is above the yield stress till deforms and vice versa. The effect is to cause shear ‘banding’ illustrated by the presence and disposition of microshears (cf. Lade, 2002; Gudehus and Nübel, 2004; Møller et al., 2008; Kaproth et al., 2010; Yang et al., 2015). In many instances within thin section, microshear would appear to be symptomatic of short localizes shear differential movement while elsewhere an agglomeration of microshears leads to local ‘banding’ and shear or deformation bands develop (cf. Passchier and Trouw, 1996; Włodarski, 2005; Fossen, 2010a, b) In viewing the many microstructures in these tills in terms of a chronosequence of microstructure formation, it is evident that most of the microstructures overlap and overprint each other, however the sequence of events remains difficult to differentiate and much more work is required in future research endeavors (cf. Phillips et al., 2011, 2018a, b). In analyzing the samples, it is assumed that where one microstructure overlaps another that the overlapping structure is the latest event. However, at Weissbach there is no consistent evidence of one type of microstructure overlapping another, thus meaningful chronosequences of structures cannot, yet, be easily attained. In those till samples where evidence of high and low stress events and strong deformation occurs, many structures are often absent possibly having been removed under high stress conditions (cf. Menzies, 2012). To the question as to how many crosscutting ‘events’ can be recognized at different points in the till succession? It must be stated that it is virtually impossible to account for the multiple events that have occurred as each crosscut does not ‘leave’ any distinctive specific clue as to timing or spatial relationship with previous or subsequent events. Of the overlapping microstructures, it is not possible at this stage to venture as to whether they are more numerous in high or low shear zones. The microsedimentological evidence from the Weissbach till exposure indicates the presence of a deforming, stress-driven, subglacial layer and the intrinsic formation of sets of shear bands. This proxy evidence of paleo-stress introduces the concept that the microstructures within tills, as might be expected, are formed due to repeated transport, emplacement, reworking and, probably further deformation events, an emplaced ‘till’ develops that carries most, if not all, of the microstructures, in varying percentages and at varying degrees of development or eradication as observed at Weissbach. These preliminary findings from this case example suggest that microstructures be utilized to assess local deformation conditions within subglacial tills and a more detailed microhistory of paleo-stress than hitherto acknowledged. Acknowledgements The authors’ wishes to thank Marty Ouellette for thin section production, Mikhail Minin for rose diagram construction, and Mike Lozon for his superb draftsmanship. JMz is most grateful to the Geological Survey of Austria and the Commission for Quaternary Research of the Austrian Academy of Science for supporting field work. JMR thanks Mathias Bichler, Christoph Iglseder and Benjamin Huet for their help in the field and fruitful discussion. In addition, JMR is grateful to Benjamin Huet for his support regarding structural data. Our thanks to Emrys Phillips for his very useful and insightful review of an earlier version of this paper. Finally, a major thanks to both anonymous reviewers and Jon Lee who provided truly invaluable ideas and thoughts to, hopefully, greatly improve this paper.

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Please cite this article in press as: J. Menzies, J.M. Reitner, Microstructures, subglacial till deposition, and shear band development revealing upsection changes in shear–A study from Weissbach, Austria, Proc. Geol. Assoc. (2018), https://doi.org/10.1016/j.pgeola.2018.11.001