Subglacial sediments are subject to erosion, transport, and deposition in active, ephemeral, and spatially localized glacial environments. It is critical to determine how these mobilized sediments become immobilized in a time-transgressive process and can be frequently remobilized and reimmobilized. Microscopic sedimentary structural signatures provide invaluable information on subglacial processes and contribute to understanding till formation. Data were obtained from a series of field sites in Canada and Austria investigating the microsedimentological aspects of both alpine and continental glaciation tills to construct a conceptual model of subglacial deformation. Microstructures in these tills indicate rheological behaviors that can be summarized into a potential model for soft deforming subglacial sediments. Most microstructures noted in these subglacial till examples highlight the development of subglacial interface kinematics providing clues to till deposition mechanics, subglacial bedform development, and the processes involved in till provenance distributions. A conceptual process model of subglacial interface conditions in soft mobile sediments is developed that uses microsedimentological evidence and highlights how an active ice mass integrates with ongoing substrate deformation. In the model, interaction occurs between the ice and its sediment bed with internal sediment microstructures evolving where multiple transient shear deformation processes cause localized deformation linked to pervasive and nonpervasive sediment deformation.
The deformation of subglacial tills arises largely at “moderately” low effective stress that occurs at all scales in saturated sediments under actively flowing ice masses leading to sediment failure and mobilization. Deformation is commonly localized in regions of relatively high strain with intervening areas of moderately low strain. It is generally agreed that such regions are often only millimeters in thickness, are laterally variable, and can quickly be removed or replaced as remobilization occurs (e.g.,Hart and Boulton 1991; Benn and Evans 1996; Hart and Rose 2001; Iverson and Zoet 2015; Halberstadt et al. 2018; Zoet and Iverson 2020). Subglacial sediment mobilization beneath ice masses occurs largely in response transmission of basal ice stress. Tills are heterogeneous in composition, and, in the evolving behavior of till deformation, it is likely that rheometric measurements of yield strength and rate-dependent shear resistance have limited relevance to subglacial till mechanics (Iverson 2003; Nagl et al. 2020; Zoet et al. 2023). Evidence from slurry experiments on intergranular mixes suggests that variability in pore pressure, liquefaction, and sediment strength are key aspects of till-like sediment-flow behavior. Deformation stresses can be quickly diffused in subglacial tills containing coarse clasts by the transmission of applied stresses at depth through grain-to-grain contact (Iverson et al. 1996; Tulaczyk 1999; Iverson 2003; Zhang and Kamrin 2017) or vertical porewater pressure fluctuations (Iverson et al. 1998; Tulaczyk et al. 2000; Damsgaard et al. 2015, 2016, 2020).
It is now acknowledged that extensive areas beneath the Quaternary ice sheets were underlain by deforming sediment beds (inter aliaBoulton et al. 2001; Rose and Hart 2009; Leysinger et al. 2010; Stokes 2018). Additionally, considerable work has been carried out on modern ice masses where the presence of active deforming sediments is well known (Alley 1989a, 1989b, 1991; Boulton 1996; Tulaczyk et al. 2001; Graham et al. 2009; Smith and Murray 2009; Spagnolo et al. 2016; Davies et al. 2018; Fernández et al. 2018). As subglacial sediment becomes mobilized there is a substantial sediment flux towards the ice-mass margins (Nygård et al. 2007; Melanson et al. 2013; Batchelor and Dowdeswell 2014; Bradwell and Stoker 2015; Dowdeswell et al. 2015; Livingstone et al. 2016; Clayton 2017; Halberstadt et al. 2018; Stokes 2018; Brouard and Lajeunesse 2019; Hart et al. 2019; Damsgaard et al. 2020; Franke et al. 2020a, 2020b; Hogan et al. 2020). As subglacial sediment is entrained towards the ice margin, the deforming sediment moving over a relatively low gradient beneath an ice mass also contributes to movement or flow of the ice (Alley and MacAyeal 1994; Dowdeswell and Ó Cofaigh 2002; Dowdeswell et al. 2004; Bingham et al. 2010; Franke et al. 2020a, 2020b). For example, based on modeling, flow rates and net subglacial sediment fluxes predict that ice streams transporting sediments to the margin of the northern part of the Barents–Kara Sea ice sheet delivered sediment at a rate of up to 4 cm a−1 (0.13 cm a−1 averaged over the fan) to the 200-km-wide mouth of the Bear Island trough (Dowdeswell and Siegert 1999; Ottesen et al. 2005; Dowdeswell et al. 2010). Based on field observations, Anandakrishnan et al. (2007) suggested a vastly larger net sediment flux rate in the order of 150 to 105 cm3 a−1 occurring beneath the present-day fast-moving Whillans Ice Stream in Antarctica. These findings suggest that an understanding of how sediments deform beneath an ice mass will provide valuable insights into ice mass stability and how ice-mass flows and transports sediments over time.
Sediment Structures in Tills
Sediment structures diagnostic of deformation processes in subglacial tills have long been described and interpreted at the macroscopic level (e.g., Slater 1926; Boulton 1979; Alley et al. 1986; Blankenship et al. 1986; Truffer and Harrison 2006; Atkinson 2009; Bougamont et al. 2019; Knight 2019; Damsgaard et al. 2020), and, over the past decades, at the microscopic level (e.g., van der Meer 1993; Kjær et al. 2006; Cowan et al. 2012; Phillips et al. 2018; Menzies and van der Meer 2018; Gehrmann et al. 2019; Mehlhorn et al. 2019; Narloch et al. 2020; Menzies 2022a, 2022b). The analyses of microshear orientations and fabrics, for example, have been shown to be illustrative of the presence of high and low shear zones in tills (Narloch et al. 2012, 2015, 2020; Piotrowski et al. 2014; Menzies and Reitner 2019; Menzies et al. 2019; Rice et al. 2019a; Reitner and Menzies 2020; Zoet et al. 2023). Based on the orientation of microshears, in relation to shear direction and thickness of deforming sediment, levels of shear stress can be estimated (Tchalenko 1968; Logan et al. 1992; Thomason and Iverson 2006; Narloch et al. 2012, 2020). It can be shown that, as shear-stress levels increase, the angles of the microshear to the horizontal decrease and that a value of approximately 25° can be set to differentiate between high (< 25°) and low (> 25°) stress levels (Logan et al. 1979; Menzies and Reitner 2016; Menzies et al. 2019). In the thin sections presented here, where microshear-angle data have been documented, variation in shear stress can be deduced. Unquestionably, microshears in till matrix are often subtle and lack the more obvious signs of displacement and shear direction often attributed to other shears in deposited sediments that have not undergone repeated deformation in both pervasive and nonpervasive modes (Stroeven et al. 2002; Passchier and Trouw 2005; Vernon 2018, Chapters 2 and 5). It is likely that shear localization in the deforming till is transient, and therefore evidence of deformation partitioning on a grain-to-grain basis may be reset or obliterated, owing to progressive deformation. Secondly, the microtexture of the thin sections reveals clear indications of subtle changes in particle orientations indicative of very localized stress changes related to the presence of décollement surfaces, previously defined as deformation front/bands (cf. Menzies et al. 2019) and shear-zone formation. Décollement surfaces develop at the junction between immobilized and mobile till units during active deformation that may be transitory and quickly redeformed or removed (cf. Evans et al. 2006 at the macroscale). In a sense, these shear surfaces are possibly only ephemeral in their existence such that only fragments may persist in thin section. In many instances, décollement surfaces are no longer horizontal, even though they likely first developed horizontally (parallel to shear stress of active ice (τi) and they show a wide variation in angle along the horizontal plane probably due to subsequent postdepositional rotation, as is common in deforming sediments, or because they conform to the local bedform (Roberts 1989; Müller and Schlüchter 2000; Hart and Rose 2001; Mandal et al. 2005; Thomason and Iverson 2006; McClay 2013; Iverson 2017; Zoet et al. 2023; Phillips, personal communication 2023).
Sediment mobilization is a complex system of processes related to sediment thermal and rheological states that is also influenced by overlying ice conditions. The sediments are rheologically highly variable in terms of their grain size, clast morphology, porewater content (pressure), and overall and internal structural geometry. Likewise, it must be recognized that these sediments, at various times and stages in the deformation process, may have undergone pervasive or nonpervasive deformation (Evans et al. 2006; Evans 2017; Le Heron et al. 2018; Menzies et al. 2018; Damsgaard et al. 2020; Zhang et al. 2020; Zoet et al. 2023). In the past, these sediments have often been termed lodgement tills, comminution tills, glacial tectonites, or more recently deformation tills, tectomicts, or subglacial traction tills (cf. Elson 1989; Hart and Rose 2001; van der Meer et al. 2003; Ruszczynska-Szenajch et al. 2003; Clarke 2005; Menzies et al. 2006; van der Meer and Menzies 2011; Menzies 2012; Evans 2017). An added complexity of the subglacial environment is the need to fundamentally understand critical changes in thermal regimes, sediment rheology, basal ice stress levels, sediment stratigraphy, basal topography, and geomorphological fluctuations. One important aspect is the thermal and geotechnical subdivision of subglacial into either being in part frozen or overconsolidated as hard (stiff) (H) or being unfrozen and soft (M) beds (e.g., Menzies 1989; Shipp et al. 2002; Cowan et al. 2012; Greenwood et al. 2016; Halberstadt et al. 2016; Brisbourne et al. 2017; Bougamont et al. 2019; Davies et al. 2018; Prothro et al. 2018). In many instances, the distinction between hard and soft beds remains difficult to determine and model (Ó Cofaigh et al. 2005, 2007; Reinardy et al. 2011; Campo et al. 2017; Larter et al. 2019; Robinson et al. 2021).
Several case studies are selected here to demonstrate the complexity of subglacial soft-bed sedimentology at the microscale and to better understand the overall deformation processes in subglacial sediments. Only a few studies can be discussed in any detail here, however references to many other studies demonstrates the commonality in terms of sedimentological signatures found in subglacial tills in many parts of the world in Quaternary, pre-Quaternary, and modern glacial sediments (e.g., Ferguson et al. 2011; Phillips et al. 2013, 2021; Skolasińska et al. 2016; Hart 2017; Menzies and van der Meer 2018; Gehrmann et al. 2019; Hermanowski et al. 2020; Le Heron et al. 2020; Narloch et al. 2020; Linnemann et al. 2021; Menzies 2022c).
Case Examples: Microscale Deformation in Subglacial Tills
The analysis and interpretation of deformed subglacial sediments are illustrated using six case studies on till thin sections from locations in Canada and Austria. These examples provide a spectrum of subglacial till types formed during the Pleistocene and Pliocene glaciations. The tills from these case studies were deposited by both mountain (alpine) glaciers and lowland (continental) ice sheets, containing a range of bedrock clast types and matrices composed of metamorphic, igneous, and sedimentary rock types with high and low clay contents under varying subglacial conditions. The glacial deposits, from these case examples, are subglacial tills from: 1) Aberdeen Lake, Nunavut, deposited by an ice sheet flowing out of the Keewatin sector or center of the Laurentide Ice Sheet (LIS), 2) Pine Point Mine, near Hay River, Northwest Territories, accumulated along the western margin of the LIS likely under fast ice-streaming conditions, 3) Attawapiskat, James Bay Lowlands, northern Ontario, formed in the lowland by an early ice sheet in the Pliocene spreading southwest from James Bay, 4) Aschbach, Austria, emplaced in a side tributary valley (Wildschönauer) by the Inn Glacier Ice Stream (IGIS), 5) Wegscheidalm, northern Austria, emplaced to the south of the Wilder Kaiser mountains in a side valley entered by a diffluent lobe of the IGIS, and 6) Weissbach, northern Austria, also deposited by the IGIS in the Wildschönauer valley some 4 km downvalley from the Aschbach site.
The choice of samples exhibits a range of microstructure examples found in subglacial tills (the figures of the samples are illustrated by showing the original thin-section image followed by subsequent mapped sets of individual microstructures (grain stacks, domains, microshears, rotation structures, and décollement surfaces) and finally an aggregate of all the microstructures mapped per thin section). The tills contain penecontemporaneous and postemplacement sets of microstructures. All the microstructures discussed here have been defined and discussed using examples in detail in Menzies and van der Meer (2018, Chapter 21). After these case studies, a glacial sedimentological conceptual model is developed to comprehend and integrate these microstructures as a whole and to place them within a contextual model framework of subglacial soft-sediment deformation processes. The glacial histories and settings of the individual sites are briefly described to provide an overall perspective of the thin sections. For details, readers can refer to the original publications as cited in the main text. Note that in this paper all thin-section images are in plane polarized light with the top of image directed toward the ground surface. In all figures the microstructures are annotated using the same color scheme and style.
Aberdeen Lake, Nunavut, Canada
The drillcore is from the central lowland region of the Keewatin sector of the LIS (Fig. 1). The extensive and thick till sampled in the Aberdeen Lake area was deposited by an ice stream of the LIS which flowed to the north from an ice divide in the Keewatin dome that migrated across the study area (Hodder et al. 2016). A detailed analysis and correlation of ice-flow indicators, till fabric, and till provenance from riverbank and drillcore sections provide evidence for at least five ice-flow phases and associated till production phases (Klassen 1995; McMartin and Henderson 2004; Hodder et al. 2016).
The till from the sampled site (sample H, blue circle) was likely deposited/emplaced below an ice stream and consists of a matrix-supported massive diamicton (Figs. 1, 2A). The drillcore sample was sampled close to parallel with ice flow direction. The till sample presented here contains numerous subrounded and subangular clasts within a clay-rich banded matrix. Domains (purple) obliquely bisect the sample (Fig. 2B). Numerous grain stacks are dominantly in a vertical orientation at right angles to the banded domains, and most grain stacks overprint the domains, indicating that the domains pre-date the beginning of pervasive deformation, as evidenced by the grain stacks (Menzies 2022b) (Fig. 2B). Microshears (blue) occur having two predominant orientations nearly perpendicular to one another. Earlier formed shears will be progressively rotated or be overprinted as deformation continues and new shears develop, so there approximates a complex array of shears which can be developed within a till (Narloch et al. 2012, 2020) or mass-flow diamicton (Phillips and Kearsey 2020). One set of microshears appear to roughly follow the clay-rich bands, while the other set are offset typical of Reidel-style shears at an approximate angle of 22°. The microshears indicative of nonpervasive deformation (cf. Iverson 2003) are all relatively short length and cross only a very few of the clay-rich bands, indicating that sequentially they were largely confined to the non-clay rich matrix and formed within those matrices (Fig. 2C). The microshear fabrics in Figure 2D are bimodal with a stronger principal orientation parallel to the distinct shear band through the center of the thin section. Low angles (< 25°) account for 51.4% of the microshears, indicating a relatively high stress environment during emplacement of the till (see Logan et al. 1979, 1992). Two distinctive shear bands exist acting as décollement surfaces trending parallel with the clay-rich domains; likely the domains acted a locus of lubrication and temporally immobilization surfaces during emplacement (Fig. 2E). There are a few rotation structures (Fig. 2E) that indicate nonpervasive deformation within the non-clay-rich matrix occurring usually just below a décollement surface. The rotations are all strictly confined to rotating larger clasts within the deforming matrix. Rotations, based on overprinting of all other microstructures, indicate the late stage of development of these microstructures. The composite image of all the microstructures with yellow background matrix illustrates the very complex nature of this till from Aberdeen Lake (Fig. 2F).
Pine Point, Northwest Territories, Canada
A sample collected from subglacial till in the Pine Point Pb‐Zn Mining District is located along the southern shore of Great Slave Lake in northern Canada (Rice et al. 2019a; Menzies et al. 2019) (Fig. 3). The LIS advanced over the entire region, depositing a ubiquitous till cover of varying thickness subsequently overlain by glaciolacustrine sediments deposited by glacial Lake McConnell (Lemmen et al. 1994). The sample (blue dot) was taken from open pit O-28, where three tills, named in ascending order unit A, unit B, and unit C, were identified in a vertical section at the north end of the pit based on color, clast lithology and clast fabrics, and erosional bounding surfaces or contacts. The differences between the three tills in color and clast composition and orientation probably reflect variations in glacial provenance during differing glacial episodes or may simply, as seems more probable, the change in provenance of the till materials in a single accumulating till package during the last Wisconsinan glaciation, as the Keewatin dispersal center migrated considerably and caused ice flow directions to change significantly over time (McMartin and Henderson 2004; Oviatt et al. 2015; McClenaghan et al. 2018), therefore changing the provenance of the tills.
The sample is from till unit B, taken from the northeast section of the pit exposure (Fig. 4A) (Rice et al. 2019a; Menzies et al. 2019). The thin-section sample is a fine-grained grayish brown till with occasional larger subangular to subrounded clasts with subtle crosshatch (almost lattisepic in terms of S-matrix) banding. Thin-section mapping (Fig. 4B) reveals grain stacks with a random set of orientations and no obvious domains. Microshears show a strong near-horizontal orientation paralleling the dominant ice flow direction. As with the subtle banding on this thin section, the microshears are usually at two dominant angles (Fig. 4C, D) typical of pure shear and progressive deformation and associated rotation as R and P shears (Passchier and Trouw 2005, p. 157). Microshear fabrics (Fig. 4D) indicate that 62% are low-angle (< 25°) shears indicative of a relatively high strain environment. It is however essential that consideration is given to possible reorientation of shears so there is always a degree of uncertainty with these results that need to temper interpretations. The décollement surfaces are all short length and match the general microshear fabric trend (Fig. 4E). Rotation structures are scattered throughout this till sample and largely related to larger clast rotation. It must be pointed out that many of the rotation structures may have been totally or partially truncated by shearing before shear initiation leading to the lateral propagation of these microscale detachment surfaces. Most rotations occur below the décollement surfaces, likely indicative that the rotation occurred just before the formation of the décollement surface and the surfaces were subsequently partially destroyed and rotated themselves (Fig. 4E). These features together indicate that this till unit has undergone some form of shear partitioning (Johnson et al. 2004; Fossen 2010) probably because of local occurrences of rapid immobilization in the till during its emplacement (Fig. 4F). Of all the case examples presented here, the till at this site appears to possibly have the highest strain percentage, as evidenced by the high percentage of low-angle microshears, probably resulting from the topographic position of this site in an unimpeded relatively flat terrain permitting relatively rapid basal ice flow (cf. Attawapiskat site, Fig. 5).
Attawapiskat, James Bay Lowlands, northern Ontario, Canada (Fig. 5A)
The Pliocene diamicton at Attawapiskat is up to 50 m thick and consists of a compact, noncalcareous, clayey to sandy subglacial till (Gao et al. 2012; Menzies et al. 2013). The till is light to dark brown in color and has a relatively high clay content (> 20%) (Fig. 5B, C), containing many subrounded to dominantly subangular and faceted clasts, many of exotic provenance (e.g., quartzite pebbles). It also includes occasional small units of thin but spatially extensive organics and organic-rich muds, with some silt and clay layers. In thin section (Fig. 6A) the till has a high concentration of subangular clasts within a high clay-rich matrix that can be subdivided into zones of clast-free clay matrix. Strong evidence can be observed of flow within the matrix in the form of movement around large clasts within a subtle banding across the thin-section sample. Distinctive domains appear to be flowing from left to right across the image (Fig. 6B) that were in place before grain stacks developed in the pervasively deforming matrix. There are relatively few microshears in this till and it exhibits two distinct subsets of dipping shears indicative of ice-flow shear effects (cf. Zoet et al. 2023) (Fig. 6D). The microshear fabrics show only 23.1% of shears less than 25° to the horizontal, indicative of a possibly relatively low strain effect. Several prominent décollement surfaces (that are consistent with development at the late stage of localized deformation) can be noted that parallel the general left-to-right flow pattern seen in the image, and again rotation structures appear to mirror the surfaces likely developed just below the immobilization surface before final development of the surfaces that were subsequently partially deformed and, in places, likely destroyed. The image in Figure 6F illustrates the relationship of all the microstructures in this till and the prominent role played in deformation partitioning by the presence of the large clasts and domains in the subsequent development of this till during emplacement.
The site at Aschbach is situated in a small valley off the main Wildschönauer Valley south-southwest of Wörgl, Austria, in typical Alpine topography (AB in Fig. 7). The site lies in a 6-meter-thick section of various till units underlain and overlain by proglacial deltaic to glaciolacustrine sediments (blue dot) (Fig. 8). The till, which is the dominant deposit in the section, is an approximately 1.5-meter-thick unit of gray to dark brown, matrix-supported diamicton, deposited in a side valley of the larger IGIS system where ice firstly flowed up valley from the Inn Valley to the south and finally from south-southwest to north-northeast down the small valley (Menzies and Reitner 2019). In thin section (Fig. 9A), the sample is densely compact fine-grained till with occasional subrounded to subangular clast fragments. The clay-rich matrix is heavily deformed with no obvious banding or stratigraphy. No domains can be discerned in this till sample and grain stacks are randomly distributed with a slightly dominant vertical orientation (Fig. 9B). Microshears are typically approximately parallel to the dominant ice flow direction (Fig. 9C). Microshear fabrics (Fig. 9D) are bipolar with 43.9% < 25° suggesting emplacement in a relatively high-strain environment, as is likely indicated by the evidence of strong matrix deformation (Fig. 9A). There are a series of post-emplacement contorted décollement surfaces (Fig. 9E), and, as in almost all other thin sections here, rotation structures are typically related to the to rotation of larger clasts and occur below the décollement surfaces, a pre-immobilization artefact of deformation. The composite images in Figure 9F illustrate the complexity of this till and the issues present in deciphering the interplay and crossover of microstructures present in this and all tills.
The sample site Wegscheidalm is located along the southern flank of the Wilder Kaiser Mountains in a W–E trending depression (Ellmau-Scheffau) (S in Fig. 7). Samples of a pale reddish-brown till (Munsell 4.5YR) (blue dot) were taken from this site 100 m southeast of the Wegscheid–Niederalm hut, some 2 km north-northeast of the community of Scheffau in the Wegscheid Creek valley (Fig. 10). Reitner (2007) established that the tills are derived from the IGIS in contact with the ice-dammed lakes filled with glaciolacustrine sediments. Wegscheidalm till was deposited by a local glacier (Menzies and Reitner 2016). The sample thin section (Fig. 11A) is dense clast-supported diamicton with occasional larger subangular and subrounded clasts. Evidence of deformation can be seen in the image. Two domains of slightly denser units with higher clay content, possibly modified due to subsequent deformation, thus can no longer be seen (light green and pink) in the image ((Fig. 11B). Grain stacks can be observed crossing the domains evidence of them post-dating the domains and developing before pervasive deformation began. There are only a few microshears evident in this thin section, all closely parallel to the décollement surfaces (Fig. 11C, E). With only 14 microshears the fabric results are rather limited, showing only 35.7% < 25° which indicates a moderate strain effect during shear development. Décollement surfaces are often warped indicative of post-deformation after formation (Fig. 11E). The surfaces seem less related to the presence of rotation structures than in other thin-section examples mapped here. The rotation structures in many places appear related to complex deformation of small areas of matrix (Fig. 11E). The composite of the mapped microstructures (Fig. 11F) perhaps best displays the spatial variation in décollement surfaces.
The sample site at Weissbach (WB, Fig. 7) sits in the same Wildschönau Valley as Aschbach (AB) but is located ∼ 4 km down valley toward the main IGIS system in the Inn valley. The till from Weissbach is a reddish to gray, brown diamicton (blue dot) (Fig. 12) which is derived from the IGIS in contact with the ice-dammed lakes filled with glaciolacustrine sediments. In thin section (Fig. 13A), this till is marked by a distinct oblique partition of sediments in what appears as an oblique possible shear zone in a variably scattered clast-supported matrix. A few subrounded to subangular clasts occur on either side of the central possible shear zone. A domain (light green) can be observed (Fig. 13B) that essentially separates the till on either side of the shear zone, and a small domain cap that surrounds the large subrounded clast in the left-hand side of the image. Grain stacks occur randomly across the sample and appear unaffected by the central zone, which suggests that the zone is not of a shear origin but was already emplaced before pervasive deformation occurred. The microshears in this sample show typical R and Y or P formation (Fig. 13C). The microshear fabrics exhibit a 39.5% < 25°, indicating a moderate-strain environment on emplacement (Fig. 13D). A large décollement surface occurs through the central (shear) zone and others all more or less at least subparallel to the doming ice flow direction. Rotation structures are confined to surrounding the large clasts in the till and appear related only to localized nonpervasive deformation events (Fig. 13E). The composite mapped thin section shows the central décollement surface as separating the till in two distinct halves unrelated to other domains (Fig. 13F).
Mapped Thin Sections: Interpretation
All the thin sections mapped in this study exhibit typical microstructures found in subglacial tills (e.g., Phillips et al. 2011; Menzies and van der Meer 2018). Each example differs from one another, but nevertheless, commonalities exist with all subglacial tills at the microscale. The tills have all been advected beneath variable ice-flow regimes, as shown by extensive evidence of various forms of deformation and microstructures. It is clear that considerable variations in strain occurred in the tills, varying over time as well as space. Local “patches” of fluctuating strain may be linked to areas close to larger clasts or between décollement surfaces (note variability in low-angle microshears as shown in the fabric diagrams). Also, patches occur where local pockets of higher or lower porewater content occur, possibly even temporarily, leading to greater or lesser mobile deformation. It is likewise apparent that generations of microstructures were typically overprinted such that complex mosaics of microstructures were developed, and at times and places, partially removed and others extant. The question of chronology of microstructure formation is a fraught aspect in any till. Much can be made of overprinting and crossover formative microstructures, but the chronosequence of such structures remain, a very tangled and complex issue difficult but essential to resolve through further research.
Most of the tills described here have extensive and, in places, contorted domains, some relatively rounded in appearance while others almost boudinaged such that in both instances later transported in the deforming till matrix. The domains can be perceived as intercalations in the tills but, in general, domains form part of the original matrix and occur by contemporaneous emplacement. The domains likely represent relicts of pre-existing sediments which have been incorporated into actively accreting and deforming till, and/or are the remnants of laminations in originally bedded sequences which have been largely overprinted because of subglacial deformation (Phillips, personal communication 2023). The presence of the domains is part of what tills represent as a byproduct of sediment incorporation due to source availability (cf. McMartin and Paulen 2009).
Grain stacks are likely one of the first microstructures developed as the till matrix begins to initially deform, probably under pervasive flow (cf. Hooke and Iverson 1995). Grain stacks are ubiquitous, with lines of small clasts (grains) “flowing” around larger clasts and typically overprinting domains. In many instances, grain stacks quickly become distorted with subsequent deformation. Grain-to-grain interaction occurs, producing even smaller grain fragments.
Microshears are indicative of localized shear or deformation partitioning and are largely viewed as distinct discontinuities of localized fracture planes within nonpervasive deforming subglacial tills. Microshears, in most of the tills here, exhibit Riedel shear-style relationships with marked crosscutting correlation (Passchier and Trouw 2005). Microshears indicate that the sediment has been differentially “adjusted” or brittle sheared through strain partitioning. They indicate that a local readjustment due to shear-stress application has occurred at some stage in their evolution. In most instances, this local movement is the result of external stress of overlying basal ice movement that has overcome the local sediment matrix yield strength in these tills. The evidence in the till images presented here are of microshears at differing orientations and declinations in the form of repeated crosscutting microstructures indicative of repeated phases of deformation at different levels of shear in what must have been a soft-sediment subglacial mobile zone (Fig. 14B).
The microshear fabrics presented here reflect stress-failure orientations typically partly aligned with orientation of the principal shear stress due to ice-flow direction (the principal axis of stress) and the general nonpervasive flow direction of local deformation mechanics (cf. Hart and Rose 2001; Larsen et al. 2006; Thomason and Iverson 2006; Hart 2017). Where the matrix shearing process abuts against larger clasts or existing previous décollement surfaces, microshears may be terminated or change direction in relation to the plane of the thin section presented here. In terms of strain in the tills sampled here, the samples show different local strain levels, with the highest being Pine Point (62.0%), followed by Aberdeen Lake (51.4%), Aschbach (43.9%), Weissbach (39.5%), Wegscheidalm (35.7%), and Attawapiskat (23.1%). These data must be carefully examined and considered in terms of likely other causes of microfabric strengths given that deformation (loading/compaction) may occur once the ice had stopped moving, and that dewatering effects on tills and/or partial remobilization of the till by mass-flow processes after the ice retreat. However, any relationships between the locations of the tills under varying styles of ice mass and differing topography (unconfined at Pine Point, Attawapiskat, and Aberdeen Lake as opposed to more confined-valley settings at Aschbach, Wegscheidalm, and Weissbach) is hard to distinguish. It is evident from previous work that large fluctuations in strain are typically found in tills (e.g., Menzies et al. 2019, their Fig. 8) to the extent that such data are of only limited overall value when investigating bulk till samples and broader basal till units. The microshears reflect conditions of nonpervasive flow deformation after initial pervasive grain-stack alignments (Iverson et al. 1996; Hooke et al. 1997; Thomason and Iverson 2006). This response indicates that in each deformation cycle following grain-stack development, under pervasive deformation, microshear development happens under conditions of nonpervasive deformation.
The thin sections all carry evidence of décollement surfaces, although in some instances only contorted fragments exist. Décollement surfaces are, at times, referred to as shear bands and, in the past, as deformation fronts. As mobile sediment units become immobilized, shear surfaces develop ephemerally at the mobile–immobile interface. Likewise, where multiple microshears coalesce, such surfaces are likely to develop. Often microshears crosscut the décollement surfaces, indicating that two or more different deformation stages occurred in a subglacial till as expected and is repeatedly shown in the mapped thin sections. Décollement surfaces tend to occur in subglacial tills that have a > 20% clay content. Also, in the tills thin sections examined in this paper, décollement surfaces occur or “spread across” zones of relatively fine-grained subglacial tills. The presence of these décollement surfaces, and likely associated plasmic unistrial fabrics, are both indicative of relatively high strain conditions, as can be seen in the values of microshear fabrics. As rheological conditions change, both external and internal, in the basal deforming sediment body, the position of the “boundary surface” will fluctuate in the sediment package as porewater fluctuates, either mobilizing or immobilizing sediments (as shown in the model in Fig. 14).
Rotation structures occur throughout the thin sections but appear to have little relationship to specific sets of microshears and grain stacks, which likely reflect their later development (ten Grotenhuis et al. 2002; Mandal et al. 2005; Phillips 2006; Griera et al. 2013). The rotation structures, therefore, are developed during nonpervasive or partitioned deformation around larger clasts and/or “clots” of possibly more cohesive matrix units. In those thin sections with high clay content matrices and larger numbers of clasts, it is possible that more rotation structures developed as a function of sediment rheological processes (cf. Menzies 2000). This variability in the number and size of rotation structures is apparent in the thin sections examined in this paper. It must be recalled that often rotation structures only appear as partial micro-forms as the plane of the thin section may “cut off” parts of the rotation. From the thin sections mapped here it is perhaps correlative that rotation structures appear at the “underside” of many décollement surfaces. Such juxtaposition may indicate the effect of late strain conditions across the mobile sediment as it immobilizes, and the surface develops soon thereafter, or it could be that décollement surfaces may also represent the boundaries of ductile shear zones with rotational structures developing in response to simple shear (rotational deformation) occurring between these shear surfaces (Phillips, personal communication 2023).
The mapped thin sections carry the signature of multiple microstructure generations where some survive whilst others are presumably destroyed during subsequent deformation (cf. Phillips et al. 2011, 2018, 2021; Gehrmann et al. 2019; Cartelle et al. 2021). The use of multiple thin-section images for each original thin section attempts to increase our ability to unravel the complex nature of till sedimentology (Figs. 2, 4, 6, 9, 11, 13).
These examined subglacial till units, as is the case with all subglacial tills worldwide, show widespread evidence of great complexity in the deformation processes. This is exemplified by the various microstructures mapped here that reveal that multiple transfers of small units of mobile sediments often parallel with deformation shearing while others, due to rotation and fragmentation, become detached and autonomous (Bloomfield and Covey-Crump 1993; Ji and Xia 2002; Treagus 2002; Gardner et al. 2017). To comprehend the myriad of microstructures and their various forms a conceptual model is needed that encompasses all these aspects related to till microsedimentology. The locations of microstructures in the till matrix, their relationship to each other and other structures, coupled with mixed rheology conditions during deformation and likely redeformation from remobilization after initial emplacement is necessary in such a model (Fig. 14A, B).
A Conceptual Model Based on Subglacial Microsedimentological Data
In any subglacial deforming sediment package, there is an upper interface at the ice-sediment contact where the ice above slides and, below it within the sediment, one or more lower interfaces (décollement surfaces) along which mobilized till material moves within the sediment matrix in the down-ice direction (Fig. 14A, B) (Evans et al. 2006; Kjær et al. 2006; Damsgaard et al. 2013; Phillips et al. 2013; Evans 2017). The development of these interfaces is poorly understood. Depending on basal meltwater availability and drainage style (e.g., channelized or distributed), thermal states, and basal ice and sediment velocities, a shear boundary may be established, or, in other instances, a gradational ephemeral freeze-on or inertial contact may develop (cf. Mueth et al. 2000; Nitsche et al. 2013). The lower interface(s) where mobile sediment is in contact with the underlying immobile substrate that may be bedrock or deeper sediment substrate (Fig. 14B) is poorly recognized. For instance, in the underlying substrate, several transient deformation shear surfaces (décollement surfaces) may be established that can potentially lead to multiple local and more widespread sediment deformations that may even evolve into bedforms (e.g., Evans et al. 2006; Menzies et al. 2016; Rice et al. 2019a; Ely et al. 2023). The model proposed here is of a simple uniform till with little variation in till sediment stratigraphies or rheological changes, to permit a first exposition of conditions of soft deforming sediment beneath a moving ice mass.
As soft-sediment deformation is taking place below an ice mass, there are likely to be multiple deformed packages or layers stacked on top of each other in a subglacial sediment sequence (Fig. 14B). As subglacial sediment is deformed, due to transient localized high porewater pressures and low effective normal stress, transport of the sediment down ice can be rapid at shallow depths (Virkkala 1952; Boulton et al. 1974; Hart and Rose 2001; Damsgaard et al. 2016, 2020; Evan et al. 2016; Evans 2017). As each or part of a package ceases to be deformed, a new deforming unit may be advected over it. In examining an exposure of soft deforming till, multiple packages, although essentially identical in grain size, clast provenance, microfabric, and other sedimentary characteristics at the macro-scale, contain minute subtle differences in sedimentary structures indicative of the effect of deformation and such deformation microstructures are discernible at the microscale in each package. Many of the packages are likely to be of limited areal extent and depth (e.g., Hart and Rose 2001; Spagnolo et al. 2016; Phillips et al. 2018; Damsgaard et al. 2020). These packages, formed at depth in the deforming sediment under subglacial conditions—far from the ice margins when created—overlie, intersect, and intercalate with each other, and in many places may have been erased or overprinted by subsequent deformation events. Therefore, a mosaic of deformation units and patterns occurs and can be detected with microsedimentological mapping. Based on the analyses of multiple thin sections mapped and examined during this study, a hypothetical model is put forward to explain many of the findings observed.
A Model of Basal Ice: Subglacial Sediment Interaction
For the proposed conceptual the model (Fig. 14A, B), it is important to understand that shear stress due to basal ice flow can be transmitted at depth within the sediment pile (cf. Boulton et al. 1974, 2001; Hart 2013, 2017). However, the form of stress diminution with depth is difficult to quantify but any diffusion of stress within the sediment is likely exponential, at least at the bulk scale (Zhang and Kamrin 2017, their Fig. 1; Zoet et al. 2023). In the sediment pile, where applied stresses become less than the internal yield strength of the sediment, immobilization starts, and incipient deposition (emplacement/accretion) occurs (cf. Menzies 1989, Evans et al. 2006). Even with the myriad of field observations of basal ice-sediment deformation, our understanding of these subglacial processes remains incomplete (Damsgaard et al. 2020). On the other hand, microsedimentological studies provide immense value in deciphering the processes as the “prime forensic tools to unravel the history of a [sediment] that allows us to deduce the succession of strain rates, stresses, diagenetic conditions and the [sediment’s] rheology during deformation” (Piazolo et al. 2019, p. 112; Hanáček et al. 2021).
The general conceptual model proposed here demonstrates the likely interactions between the basal ice movement, soft subglacial sediments, and the underlying bedrock under active temperate ice (Fig. 14). In this setting the ice mass is “coupled to” the soft substrate bed, and shear (décollement) surfaces lie within the deformable substrate. Evidence of subglacial sediment deformation in the form of multiple generations of microstructures formed, partially destroyed, and rotated or contorted, can be expected, and is observed (Figs. 2, 4, 6, 8, 11, 13) (cf. van der Meer 1993; McCarroll and Rijsdijk 2003; Meer and Menzies 2011 Phillips et al. 2013, 2018; Narloch et al. 2020). Wherever sediment bulk shear strength is overcome by extrinsic applied shear stress, i.e., the overlying basal ice stress, mobilization takes place, usually resulting from meltwater assimilation into the underlying sediment pile followed by sediment failure and corresponding movement (Iverson 2010; Bougamont et al. 2019). Under these conditions rates of internal till mobility will likely vary locally as a function of mixed rheological conditions (cf. Hooke et al. 1997; Hart and Rose 2001; Iverson 2003; Rathbun et al. 2008; Austin 2011; Caballero et al. 2014), varying beneath individual ice masses (e.g., Bougamont et al. 2019; the Pine Island Glacier, West Antarctica).
As shown in Figure 14, at the base of a temperate ice mass where a soft-deforming bed occurs, there are likely to be a series of widespread and complex interfaces. In the model, there is active stress-related interaction between the overlying ice and the subglacial sediment in the bed with transient shear zones and décollement surfaces forming in the underlying substrate, leading to erosion, entrainment, and subsequent deposition at the bed interface or “scavenging” across underlying substrate sediments, and sediment deformation (e.g., Stanley 2009; Hooke et al. 2013; Menzies et al. 2016b; Le Heron et al. 2020; Schaetzl et al. 2020). These deformation processes within the substrate sediment led to localized erosion and accretion of the sediments at various sub-interfaces linked to pervasive and non-pervasive deformation.
In addition to the upper interface immediately below basal ice and the underlying sediment contact, there are likely lower interfaces at various depths between mobile and immobile sediments (Fig. 14A–D) (cf. Menzies 1989; Lee and Phillips 2008). These in-sediment interfaces are complex in nature and, depending on thermal and mixed rheological states and the depth at which they develop, may be sharply defined shear surfaces or somewhat diffuse contacts of a very ephemeral nature. In terms of where and to what depth deformation occurs in the accreted sediment pile, it can be expected that beneath any part of an ice mass, where thawed subglacial sediment with sufficient thickness may permit infiltration of porewater (as evidence by the presence of argillans) to deeper subjacent units, a low level of effective stress will develop leading to potential sediment failure and mobilization due to increased porewater pressure (Boulton 1979; Engelhardt and Kamb 1998; Alley 1989b; Hart and Rose 2011; Tulaczyk et al. 2001a, 2001b; Iverson 2010; Stokes 2018; Bougamont et al. 2019). In the subglacial sediment, it is the upper parts of the sediments that are subjected to the greatest influence by applied overlying stress from the ice mass and consequent elevated porewater pressures and hence are most likely to experience sediment mobilization. A proviso needs to be added in complicated till sedimentologies, where aquitards, clay, and/or sand lenses and aquifers exist, as the model described here is simplified in terms of mobilized and immobilized sediment conditions regarding downward dissipation of the elevated porewater pressures to a depth in sediment of mixed stratigraphies. In simpler terms, where internal sediment strength overcomes the applied stress, sediment mobilization ceases (Fig. 14B).
The critical questions are at what depth does stress penetration reach and at what depth does sediment deformation no longer occur? As shear-stress levels and sediment rheology are likely to vary over the subglacial bed and fluctuate over time, the location where deformation ceases to occur is highly variable, both across the subglacial bed and in any given sediment (e.g., Houssais et al. 2019). There has been considerable debate about the possible depths where sediment immobilization occurs with estimates ranging from a few centimetres (Hooke et al. 1997; Hart and Rose 2001; Mair et al. 2003; Truffer and Harrison 2006) to several meters (Alley et al. 1986, 1987; Boulton 1996; Hindmarsh 1998; Evans et al. 2006; Piotrowski et al. 2006; Stokes et al. 2013; Davison et al. 2019; Damsgaard et al. 2020). Interestingly, thick, and seemingly homogeneous Quaternary till sheets have been interpreted to reflect much thicker deforming layers for decades (e.g.,Piotrowski et al. 2001, 2006), but whether it is physically plausible is still debated because the possibility that such till sheets from episodic differing of distinct glacial episodes leading to accretion cannot be excluded (e.g.,Damsgaard et al. 2020; Zoet and Iverson 2020).
Subglacial sediments are subject to repetitive erosion, transport, and deposition (emplacement) in very active, ephemeral, and often spatially localized, environments (e.g., Stanley 2009, their Fig. 8; Hooke et al. 2013; Paulen 2013). In attempts to understand the depths of deforming substrate the following variables need to be considered: 1) the physical properties, i.e., texture, particle density, internal angle of friction, and variable rheological properties of the substrate, whether the sediments are already deposited and immobile, the stratigraphic layering, if any, and the contribution of other sediment sources, e.g., basal melt rate of debris-rich ice and any bedrock erosion supplying new material to the deforming layer, 2) the overlying ice velocity and basal shear stresses, and 3) the presence and distribution of “weak” or “strong” (deformable or undeformable) layers in the substrate (Alves and Lourenço 2010; Rice et al. 2019b). Under dynamic subglacial conditions, it is perhaps more relevant to observe how these mobilized sediments change from mobile deformable sediments to immobile sediments and vice versa over time (Hicock and Dreimanis 1992; Iverson 2003; Menzies et al. 2006; Stanley 2009; Menzies 2012; Melanson et al. 2013; Phillips et al. 2018; Zoet and Iverson 2020). As such, it is crucial to understand the mechanism on how such a “transformative switch” occurs from mobile to immobile sediments.
The various interface interactions relating to different stress distributions in the mobile layer beneath an active ice mass is illustrated in Figure 14. Figure 14B illustrates a scenario adopted to indicate zero deformation in the substrate; the ice–bed interface is either completely decoupled and basal ice is sliding onto soft sediment, or the sediment yield strength exceeds basal shear stress through its entire thickness. In Figure 14B the sediment yield strength is greater than the applied shear stress down to a certain depth where a strain (deformation) décollement surface develops along the boundary below which yield strength exceeds shear stress. In Figure 14B, multiple strain fronts (décollement surfaces) can be observed at various depths within the sediment substrate, probably due to subsurface stratigraphy and related changes in material rheology. In Figure 14B, the décollement surface ascends in the sediment package as illustrated by “steps” in stress distribution with multiple deformed zones episodically developing. Signatures of these strain décollement surfaces are detectable at the microscale, as shown in the thin section images mapped here In Figure 14B, décollement surfaces (shown in orange) were illustrated in the inset which curve upward in the down-ice direction as more sediment is emplaced (cf. Lee and Phillips 2008). As stress levels vary, décollement surfaces will likely penetrate farther into the substrate if stress levels increase (Lee and Phillips 2008; Goren et al. 2011; Damsgaard et al. 2016 2020; Le Heron et al. 2020). Where scavenging of already-emplaced sediment occurs, it can be expected that overall sediment thicknesses will decrease locally, and décollement surface signatures may be destroyed or contorted.
Décollement surfaces develop at a location in the sediment unit where a potential plane of discontinuity is likely to form. In the thin sections examined here the microscale décollement surface is often a thin zone or area of microshears that coalesce into a broader zone of discontinuity. It needs to be stated that since this is a two-dimensional plane that is being viewed, often displacement is not readily apparent; thus, collecting oriented samples and taking a thin section cut parallel to the main ice movement direction identified at the site is critical to understanding the deformation history recorded by the till. The location of this discontinuity or slip zone marks the interface between mobile and immobile sediment where a décollement surface develops (Fig. 14A, B). The plane is likely to be an agglomeration of microplanes that may coalesce into a décollement (cf. Torabi and Fossen 2009; Fossen et al. 2019). As stress levels change, or as porewater content increases or decreases, or thermal fluctuations between thawed and frozen patches occur, the plane of décollement, under progressive deformation, is likely to change angles and migrate between deeper or shallower levels across an approximately horizontal area of the mobile/immobile till interface or occur parallel to the axis of maximum shear stress (Fig. 14B). If, for example, stress levels rapidly decrease, the position of the décollement surface will migrate towards the upper surface of the deforming unit, and under increasing stress levels, the surface will propagate deeper into the sediment, thereby mobilizing more sediment. The migration of the décollement surface results (as seen in the thin sections examined above) in differentiating between mobile and immobile sediments and thus two distinct rheological groups may be either relatively small in area (a few centimeters to several square meters) or possibly occur over wide expanses of the subglacial bed (cf. Piotrowski et al. 2004; Trommelen et al. 2014; Fossen and Cavalcante 2017; Pennacchioni and Mancktelow 2018; Gauthier et al. 2019; Fossen et al. 2019). Depending on sediment supply and the down-ice sediment flux and outflow rates, the thickness of any deforming part of the subglacial bed may thicken or thin, consistent with the general observations and previous studies (Goren et al. 2011; Melanson et al. 2013; Damsgaard et al. 2016, 2017, 2020; Bougamont et al. 2019; Hogan et al. 2020). It is therefore plausible that local thickening, caused by accumulating immobilized sediment, may act as “growing” sticky spots or nuclei around which deforming sediment may accrete and begin to form drumlins and other bedforms (Eyles et al. 2018; Hermanowski et al. 2020; Sookhan et al. 2021); of course, in contrast, erosion sediment around a “sticky” spot may likewise take place.
One of the ongoing goals of till microsedimentology is to examine the microstructures in thin section and understand how they relate to till sediment deposition (emplacement) (inter aliaEvans 2017; Menzies and van der Meer 2018; Phillips et al. 2018; Hermanowski et al. 2020; Menzies 2023; Zoet et al. 2023). An understanding of the processes responsible for soft-sediment microstructures in tills is essential for making reliable interpretations and inferences as to how tills are deposited (cf. Piazolo et al. 2019). From the thin sections examined here it is clear that multiple microstructures are symptomatic of repeated deformation phases before, during, and after emplacement of subglacial till. Therefore, interrelationships that exist between all these structures are critical to understanding the development of microstructures in subglacial tills. In analyzing these subglacial tills, extensive kinematic deformation relationships can be observed between microstructures (Davies et al. 2009; Menzies et al. 2016,a, 2016b; Robinson et al. 2021). As subglacial till is deformed or strained during its transport and emplacement under varying conditions of porewater content, rheology, percentage of clays present, and changing thermal regimen, a series of different microstructures, as demonstrated here, sequentially evolve. Previous research suggests that, before deformation, domains are already mostly present. In most cases, domains form because of different sediment sources being incorporated into the mobile layer before emplacement and/or the effects of porewater elutriation of clay particles within the till units post emplacement due to changes in overlying porewater flow pressure (cf. Hiemstra and van der Meer 1997; Major and Iverson 1999; Ravier et al. 2014; Menzies et al. 2016b; Thomson et al. 2019). Initially, under conditions of pervasive deformation, grain stacking occurs, and as sediment deformation becomes increasingly nonpervasive, microshear structures and rotation structures develop (e.g.,Menzies et al. 2016a; Phillips and Hiemstra 2022; Menzies 2022b). This sequence is repeated multiple times as conditions evolve. During recurring deformation events, décollement surfaces and larger shear zones will form. As localized till units become immobilized during the movement of a mobilized subglacial sediment unit, décollement surfaces form at the upper surfaces of these immobilized units. In many cases, as manifested in the thin sections (Figs. 2, 4, 6, 9, 11, 13), the signatures of décollement surfaces can become disturbed and partially destroyed because of the consequent remobilization. This sequence of microstructural development is indicative of subglacial till deformation under wet-bed temperate ice masses, and the model proposed provides important insights into the complex subglacial sedimentary conditions under which tills develop (Fig. 14A, B) (Leighton et al. 2013; Vernon 2018).
Based on microsedimentological observations and interpretations from Quaternary and pre-Quaternary till deposits, a model of subglacial bed processes postulated here to better understand the sedimentary conditions where an active ice mass moves across a soft deforming sediment bed. Data from the mapped thin sections provide supporting evidence of the graphic hypothesis illustrated in Figure 14. The main conclusions of this work, with a focus on the model presented here, are based on the microsedimentological evidence that can be summarized as:
Mapped microstructures illustrate the complexity and interrelated nature subglacial till sedimentology.
The microstructures show the impact of various forms of deformation mechanics at various stages of till emplacement and subsequent redeformation.
A crude chronology of microstructure formation can be derived in the mapped thin sections that indicates a process of microstructure evolution related to conditions of overlying stress and rheology affecting the mobile sediment units.
The impact of overlying stress, whether from overlying moving ice or deforming sediment, transfers into the subjacent sediment imparting strain signatures in the form of microshears that can be assessed in terms of overall stress levels on formation.
A consequence of this formative timeline is that a time-transgressive repetition of microsedimentological processes occurs subglacially in the sediment and can be traced in these tills.
A significant “marker” are those microstructures left by a décollement surface in the sediment. The surfaces may be scavenged, partially to completely removed, or overprinted with the preceding ones due to remobilization of previously immobilized sediment. Additionally, in many cases, parts of the features of the previous décollement will be rotated and repositioned within the sediment pile.
The development and microstructural evolution of décollement surfaces is a complex process of changing stress levels and thermal and rheological conditions (Menzies et al. 2019).
The model predicts the development of microstructure types including décollement surfaces that, typically with abundant sediment supply, might be expected to ascend through an immobilizing till package.
Much remains to be explored both in laboratory and field settings in terms of the type and nature of microstructure development in subglacial tills (cf. Menzies 2022b; Zoet et al. 2023). For example, a quantitative re-examination of microclast fabrics, grain stack orientations, and microshear fabrics may reveal a clearer interrelationship and a connection with formation of décollement surfaces and their locations in till units. Further investigations are crucial as to whether microstructures can be shown quantitatively and spatially in subglacial till chronological sequences. Likewise, investigation of shear fabrics should allow a closer differentiation of décollement surface locations (Gehrmann et al. 2017). At a more fundamental level, whether there are likely intrinsic or causal relationships between discrete sets of microstructures and décollement surfaces and the mixed rheology of subglacial tills needs further study (e.g., Menzies et al. 2016a; Menzies 2022a; Zoet et al. 2023). All these questions remain to be further investigated if the mechanics of the creation and emplacement of subglacial till in a soft wet-based subglacial environment is to be fully understood.
We wish to thank many individuals for their support and advice in the field and lab, namely Jessey Rice, Beth McClenaghan, and Natasha Oviatt. Useful past insights and advice from Emrys Phillips and Jaap van der Meer are much appreciated. Our especial thanks to Jessey Rice who gave an insightful and very useful early review of drafts of the manuscript. Our thanks to both Marty Ouellette for thin-section production and Mike Lozon, of Brock University, for superb drafting skills. Part of this research was funded by the Geological Survey of Canada’s Geo-Mapping for Energy and Minerals (GEM) Program (NRCan contribution #20200679), by the Geologische Bundesanstalt, Geological Survey of Austria, and the Commission for Quaternary Research of the Austrian Academy of Science. Support also came from the Ontario Geological Survey, De Beers Canada, GeoSphere Austria, Cameco Corporation, the Canadian Polar Commission, and NERSC Canada.