A study of fluted moraines in the Torridon area,NW Scotland

A study of certain small-scale fluted moraines, all less than 4m high but up to 400m long, in the Torridon area has shown that they are subglacial features composed of a clast-rich till, and formed in the Loch Lomond Stadial beneath ice with a maximum depth of 100-200m. The evidence from lithological analysis of till samples from the fluted moraines suggests that they were formed by subglacial deformation of a pre-existing till with little in the way of net down-glacier movement of the material.     

An investigation of subglacial processes at the microscale from Briksdalsbreen, Norway

The subglacial processes at Briksdalsbreen, Norway, are examined by a combination of sedimentology, thin section and scanning electron microscope (SEM) analysis of till samples from an exposed subglacial surface and from beneath the glacier. Studies of a fluted surface indicate that subglacial deformation is occurring on a field scale with flutes forming behind most clasts 0·6 m high. At the thin section scale (0·014–2·0 mm) it is seen that deformation is by rotation and attenuation and is dependent on till texture. At the SEM scale (0·1–0·4 mm) it is seen that erosion is controlled by abrasion and percussion which produces distinct grain ‘styles’ as part of an erosional continuum. Overall it is shown that rotation and attenuation is a dominant process at all scales and that the clast interactions associated with different scale perturbations within the shear zone control erosion and deposition, as well as landform and fabric production.     

Formation of subglacial till under transient bed conditions: deposition, deformation, and basal decoupling under a Weichselian ice sheet lobe, central Poland

A multi-proxy approach involving a study of sediment architecture, grain size, grain roundness and crushing index, petrographic and clay mineral composition, till fabric and till micromorphology was applied to infer processes of till formation and deformation under a Weichselian ice sheet at Kurzetnik, Poland. The succession consists of three superposed till units overlying outwash sediments deformed at the top. The textural characteristics of tills vary little throughout the till thickness, whereas structural appearance is diversified including massive and bedded regions. Indicators of intergranular bed deformation include overturned, attenuated folds, boudinage structures, a sediment-mixing zone, grain crushing, microstructural lineations, grain stacking and high fabric strength. Lodgement proxies are grooved intra-till surfaces, ploughing marks and consistently striated clast surfaces. Basal decoupling by pressurized meltwater is indicated by undisturbed sand stringers, sand-filled meltwater scours under pebbles and partly armoured till pellets. It is suggested that the till experienced multiple transitions between lodgement, deformation and basal decoupling. Cumulative strain was high, but the depth of (time-transgressive) deformation much lower (centimetre range) than the entire till thickness ( ca 2 m) at any point in time, consistent with the deforming bed mosaic model. Throughout most of ice overriding, porewater pressure was high, in the vicinity of glacier floatation pressure indicating that the substratum, consisting of 11 m thick sand, was unable to drain subglacial meltwater sufficiently.     

Microstructural evidence of low‐strain,time‐transgressive subglacial deformation

Microstructural analyses were used to investigate the formation of a macroscale‐massive till at Knud Strand in Denmark. More than 100 thin sections were examined and microstructures mapped and counted for quantitative comparison and interpretation. Microstructures indicative of both brittle (grain lineations, edge‐to‐edge crushed grains) and ductile (turbate structures) deformation are evenly distributed in vertical profiles through the till, suggesting that strain contributed to its formation. Discrete shears (grain lineations and plasmic fabric) probably accommodated most deformation, whereas rotational deformation was less prominent. The microshear geometry fits the predicted Coulomb–Mohr failure criterion, indicating that till behaves as a plastic material. Strain estimate of ca. 10¹ from micromorphological proxies is two–three orders of magnitude lower than expected if the till was subjected to pervasive deformation. A hybrid of lodgement and time‐transgressive deformation is envisaged as the till‐forming processes. Our data suggest that even abundant evidence of microscale deformation at continuing high levels of strain may only record the latest process of deposition and deformation and therefore not fully reflect the complexity of till genesis. Copyright © 2007 John Wiley & Sons, Ltd.     

An anisotropy of magnetic susceptibility (AMS) investigation of the till fabric of drumlins: support for an accretionary origin

This paper describes the results of a spatially dense anisotropy of magnetic susceptibility (AMS) till fabric study of a single drumlin in the Weedsport Drumlin Field, New York State, USA. AMS till fabrics provide a robust, quantitative and unbiased approach to assess subglacial till kinematics and infer ice‐flow dynamics. The drumlin selected for this detailed investigation was systematically sampled at 18 locations to evaluate the patterns of ice flow and associated till kinematics within a drumlin and to test erosional vs. depositional models for its formation. AMS till fabric analysis yielded strong fabrics that increase in strength towards the drumlin crest, indicating that bed deformation occurred during till deposition and that deformation within the drumlin was greater than that in the interdrumlin low. Fabric orientations reveal drumlin convergent, divergent and parallel ice‐flow paths that illustrate a complex interaction between ice flow and the drumlin form; fabric strength and shape reveal systematic differences in bed deformation between the interdrumlin and drumlin regions. These observations are inconsistent with purely erosional models of drumlin genesis; instead, these observations are more consistent with syndepositional streamlining of till transported, probably locally as a deforming bed, from the interdrumlin low towards the drumlin locality.     

Till kinematics in the Stargard drumlin field,NW Poland constrained by microstructural proxies

Deciphering the relationships between streamlined subglacial landforms and deposits therein helps to constrain the dynamics of past ice sheets. Here we present microstructural analyses of till from the largest drumlin field in the North European Lowland generated by a major palaeo-ice stream of the Scandinavian Ice Sheet. Data derived from thin sections and X-ray microtomography (μCT) reveal multiple microstructures including microshears, circular structures, grain stacks and crushed grains. Long axes of micro-clasts measured in thin sections are directionally clustered and have low, down-ice-oriented dip angles. Some samples reveal subordinate clustering oriented at high angles to the main direction. The μCT data show low clustering strengths of clasts and no systematic fabric pattern in the entire set of samples. There is no correspondence between the clast orientations and the directions of drumlin long axes, suggesting that the till is antecedent in relation to the drumlin-shaping process. The observations indicate spatially and temporally variable, shallow-seated brittle and ductile deformation controlled by porewater pressure fluctuations and generated time-transgressively parallel to till accretion. Consistently with previous studies, the data suggest that the Stargard drumlins are erosional remnants of a pre-existing terrain shaped by material removal from the inter-drumlin areas.     

A multiproxy study of a basal till: a time‐transgressive accretion and deformation hypothesis

A two‐part basal till at Knud Strand, Denmark reveals a uniform fabric pattern and strength, petrographical composition and clay mineralogy. The nature of the contact with the underlying sediments, ductile deformation structures, partly intact soft sediment clasts, small meltwater channels and thin horizontal outwash stringers dispersed in the till indicate both bed deformation and basal decoupling by pressurised subglacial water. A time‐transgressive model is suggested to explain the lack of vertical gradation in till properties in which debris released from the active ice sole is sheared in a thin zone moving upward as till accretion proceeds. It is suggested that, although strain indicators occur throughout the entire till thickness, the deformation at any point of time encompassed the uppermost part of the till only, allowing preservation of fragile clasts below. The substantial thickness of the till (up to 6 m) coupled with a much smaller (by more than one order of magnitude) inferred thickness of the deforming bed suggests that the bulk of till material was transported englacially prior to deposition. The lack of petrographical gradation in the till is attributed to effective mixing and homogenisation of material along the ice flow path. Copyright © 2004 John Wiley & Sons, Ltd.     

Lobal interactions and rheologic superposition in subglacial till near Bradtville, Ontario, Canada

Structural, stratigraphic, and lithologic data from a section 69 m long of Catfish Creek drift (north shore of Lake Erie) tell a complex story of two competing glacial lobes. Stone surface features and orientations indicate that stones rotated in viscously deforming, fine-medium textured subglacial till prior to final emplacement. Fractures, shears, and attenuated sediment lenses in tills reveal that they experienced some brittle shear superposed on ductile shear during till dewatering and stiffening. The Huron-Georgian Bay lobe advanced first from the northwest, deforming interstadial sediments and depositing subglacial till. Next, southward confluent flow of the Huron, Georgian Bay, and Erie lobes carved subglacial troughs into sediments and deposited (then deformed) bouldery deformation till by squeeze flow. The northwest flowing Erie lobe then prevailed, depositing deformation till, subglacial aquatic sediments, and mudflows. Finally, a pavement-bearing, hybrid deformation-lodgement till covered the section. Till formation was mainly by subglacial viscous flow with minor lodgement superposed as water content decreased and some fines were probably winnowed. This implies that till deformation probably accounted for much of the glacier movement. Therefore, rapid ice flow could have occurred over the section, along the southern margin of the Laurentide Ice Sheet.     

An investigation of subglacial shear zone processes from Weybourne,Norfolk, UK

The glacial sediments of north Norfolk are a type site for subglacial deforming bed sediments. This investigation focussed on subglacial shear zone process at the field and thin section scale, in order to understand subglacial processes, as well as considering the implications for regional stratigraphies. The sandy and chalky tills from three sites (within 1 km) at Weybourne, Norfolk, showed evidence for subglacial deformation associated with simple shear, producing extension, compression and rotation. It was demonstrated how these processes interacted to cause chalk fragmentation and predictable fabric strengths (dependant on sorting and grain size). It is suggested that the ‘Marly Drift’ is a diachronous unit, and the resultant stratigraphy at Weybourne reflects one deformation till, resulting from a series of ice advances, but with a lithology derived from the local chalk bed rock (with some far travelled erratics), which have undergone different degrees of chalk fragmentation reflecting the nature and duration of the subglacial deformation.     

Magnetic fabrics of the Douglas Till of the Superior lobe: exploring bed-deformation kinematics

The Superior lobe of the Laurentide ice sheet, during its last advance into northwestern Wisconsin, may have moved and transported sediment primarily through deep, widespread shear of its bed to high strains (>100). To study the kinematics of this deformation, we measured the anisotropy of magnetic susceptibility of 3750 intact samples of its basal till—the Douglas member of the Miller Creek formation—collected from eight profiles at 0.2 m depth increments. A benchmark for interpretations was provided by results of ring-shear experiments conducted on the Douglas Till, which provided fabric characteristics for the case of horizontal simple shear. Orientations of maximum susceptibility (k₁) were controlled by preferred orientations of silt-sized magnetite grains and were similar to sand-grain orientations measured in thin sections. Strengths of fabrics formed by orientations of k₁ indicate that most of the till sampled was deformed but to variable strains. Deformation averaged over the 20 km spanned by the study approximated horizontal simple shear. However, large differences in k₁ fabric azimuths (>45°) over lateral distances of meters to tens of meters indicate highly heterogeneous bed deformation, perhaps focused in anastomosing zones with associated divergent and convergent till shear. This interpretation is supported by orientations of principal susceptibilities that imply, in many cases, either that shear planes were steeply dipping, particularly transverse to the shearing direction, or that there were major components of pure shear. Variations in k₁ fabric azimuth with depth indicate that most of the till thickness did not shear simultaneously; rather, till accumulated at the bed as shear direction changed in response to temporally shifting zones of shallow deformation (<1 m). This heterogeneous, temporally variable deformation of the bed differs from many applications of the bed-deformation model but is consistent with subglacial measurements at modern glaciers.     

Texture and fabric studies across the Kishorn Nappe, near Kyle of Lochalsh, Western Scotland

The progressive development of mylonitic fabrics in a series of Torridonian sandstones and shales has been studied along traverses across the Kishorn Nappe. The fabrics developed have been investigated using the following techniques.

1. 1. Optical examination of thin sections.

2. 2. Measurements of the anisotropy of magnetic susceptibility.

3. 3. X-ray texture goniometry.

The results are used in support of a proposed deformation history of the area and the relative advantages of the techniques used are discussed.The early deformation was well lubricated with layer-parallel sliding and little internal deformation of the rocks, except for development, in the east, of a layer-parallel penetrative fabric with an extension direction to the ESE. This deformation produced a westward facing isoclinal anticline and a recumbent syncline in the Torridonian rocks which became at least partly decoupled from the basement.The important phases of fabric development post date this folding. In the west the sandstones developed a spaced, pressure solution cleavage, but in the east the grain shape fabric has been produced by both dislocation and diffusion processes. The shales reveal more details of the deformation episodes than do the sandstones and thus show different fabric intensities and orientations when measured by magnetic and X-ray techniques.The magnetic anisotropy technique of fabric analysis gives a rapid method of mapping the deformation domains formed by different deformation mechanisms and intensities. However, the rocks carry several magnetic components and these have different anisotropy tensors and different responses to deformation, also, measurements made at high fields (5 kOe) give magnitudes and orientations of the magnetic anisotropy tensor which are different from those made at low fields. It is concluded that it is not possible to relate variations in the magnitude and shape of the magnetic anisotropy ellipsoid quantitatively to the deformation.Chlorite and muscovite fabrics measured by X-ray techniques show variations in intensity and orientation similar to those of the magnetic anisotropy ellipsoid due to paramagnetic minerals. However, the data demonstrate the difficulty of correlating this fabric intensity with deformation intensity where there has been a change in deformation mechanisms with time and space.     

Lodgement till deposition and deformation investigated by macroscopic observation, thin-section analysis and electron microscope study at site Dçbe, central Poland

Macroscopic observation, thin-section analysis and electron microscope study were used in a detailed examination of till exposed in a valley escarpment. The till displays two kinds of bedding: (1) beds of different colour (lithology) that are not separated from one another by distinct boundaries, and (2) beds that are separated by distinct subhorizontal planes of discontinuity, commonly consisting of several beds of the first type. Beds showing no clear separation are interpreted as having been deposited through continuous lodgement, while the distinct discontinuities most probably record sliding planes of the glacier sole during breaks in till deposition. These features, along with the qualitative and quantitative microscopic evidence, indicate that the lower and middle parts of the till were deposited by hard lodgement. The process of deposition was interrupted several times by episodes of non-deposition, some of them accompanied by glacial erosion. The upper part of the till was deposited by continuous lodgement that was transitional between hard and soft lodgement processes. Such a succession indicates the prevalence of a cooler subglacial thermal regime which changed upwards into a milder one. The till examined displays mainly brittle deformation - namely Riedel shears that are primarily concentrated between the subhorizontal discontinuities characteristic of the lower and middle parts of the section. A dense set of Riedel shears also cuts thicker portions of the upper part of the till and, in some places, such shears combine along additional subhorizontal (discrete) shear planes. The direction of the fractures, as well as the strong fabric orientation of stone a-axes within the till, indicate an ice flow in the studied area from NW to SE.     

Microtextural analysis of a glacially ‘deformed’ bedrock: implications for inheritance of preferred clast orientations in diamictons

Although analysis of clast macrofabrics has been used to differentiate between different types of glacial diamictons and to determine palaeo‐ice flow directions, no account appears to have been made of preferred clast orientations inherited from the parental source material. Clast macrofabrics in tills are typically interpreted as having developed in response to an imposed subglacial deformation and as such provide a link between the sedimentary record and glacier dynamics. They rely on the assumption that any preferred clast orientation is a result of deformation/flow. The results of the micromorphological study of the Langholm Till exposed at North Corbelly near Dumfries (southwestern Scotland) clearly demonstrate that bedrock structure can influence clast orientation (macrofabric) within diamictons. In the lower part of the till, the orientation of elongate clasts preserves the geometry of the tectonic cleavage present within the underlying bedrock. The intensity of this steeply inclined, ‘inherited’ clast fabric decreases upward through the till, to be replaced by a more complex pattern of successive generations of clast microfabrics developed in response to deformation/flow. These results indicate potential limitations of applying clast macrofabric or microfabric analysis in isolation to establish till genesis or palaeo ice‐flow directions. Consequently, due account should be made of other glacial palaeo‐environmental and ice flow indicators, as well as rockhead depth and morphology in relation to the selection of fabric measurements sites. © British Geological Survey/Natural Environment Research Council copyright 2007. Reproduced with the permission of BGS/NERC. Published by John Wiley & Sons, Ltd.     

Signature of the Baltic Ice Stream on Funen Island, Denmark during the Weichselian glaciation

Ice streams are major dynamic elements of modern ice sheets, and are believed to have significantly influenced the behaviour of past ice sheets. Funen Island exhibits a number of geomorphological and geological features indicative of a Late Weichselian ice stream, a land-based, terminal branch of the major Baltic Ice Stream that drained the Scandinavian Ice Sheet along the Baltic Sea depression. The ice stream in the study area operated during the Young Baltic Advance. Its track on Funen is characterized by a prominent drumlin field with long, attenuated drumlins consisting of till. The field has an arcuate shape indicating ice-flow deflection around the island's interior. Beneath the drumlin-forming till is a major erosional surface with a boulder pavement, the stones of which have heavily faceted and striated upper surfaces. Ploughing marks are found around the boulders. Exact correspondence of striations, till fabric and drumlin orientation indicates a remarkably consistent flow direction during ice streaming. We infer that fast ice flow was facilitated by basal water pressure elevated to the vicinity of the flotation point. The ice movement was by basal sliding and bed deformation under water pressure at the flotation level or slightly below it, respectively. Subglacial channels and eskers post-dating the drumlins mark a drainage phase that terminated the ice-stream activity close to the deglaciation. Identification of other ice streams in the Peribaltic area is essential for better understanding the dynamics of the land-based part of the Scandinavian Ice Sheet during the last glaciation.     

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

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 paleo-stress.     

Common origin for De Geer moraines of variable composition in Raudvassdalen,northern Norway

The study of De Geer moraines in Raudvassdalen shows that most De Geer moraines are likely to have a common origin at the grounding line of glaciers despite variability in composition of the ridges. Pebble fabric, grain‐size analysis and structures within exposures of De Geer moraines in the Raudvassdalen area, with compositions ranging from mostly till to mostly sorted sediment, indicate that the ridges all formed at the grounding line of a tidewater glacier by common processes: deposition of sorted sediments beyond the grounding line followed by deformation of pre‐existing sediments and deposition of till as the glacier overrode the ridges. The compositional variation of the ridges is probably related to the position of the section studied relative to the location of the outlet of subglacial streams. Ridges composed entirely of till form at locations remote from the outlet of subglacial streams, and ridges with a component of sorted sediments form in closer proximity to these streams. This unifying theory of De Geer moraine formation, along with theoretical and geological evidence showing that there are limited physical conditions where basal crevasses can form, suggests that the number of De Geer moraines interpreted to have formed in basal crevasses is probably unrealistic. Copyright © 2000 John Wiley & Sons, Ltd.     

Comparative study of striations and basal till clast fabrics, Malpeque—Bedeque region, Prince Edward Island, Canada

Coastal erosion has resulted in exposure of a succession of basal till deposits and striation localities along the Malpeque and Bedeque Bays, PEI, Canada, permitting repetitive determinations of clast fabrics and investigation of recently exhumed striations. Striations indicate initial eastward ice flow, followed by a second glacial event with flow towards the south and southwest from the Gulf of St. Lawrence. The degree of local inconsistency and the variations with striation assemblages indicate that the orientations of individual striations cannot be regarded as 'absolute' indicators of ice flow directions. Diamictons identified as basal tills using sedimentological criteria have clast fabrics typical of basal till deposition. Local deviations of fabric trend within individual outcrops are common. A comparative analysis of the data obtained from striation and clast fabric orientations indicates that the ice flow directions independently suggested by each are in general agreement. Differences are present between the orientations of striations and clast fabrics at individual sites. The orientations of the fabrics lie within 20° of the modal striation orientations in only 28% of the cases, and the median deviation between the suggested ice flow directions was 29°. In 40%, of the cases, the azimuth deviation between the two methods exceeded 40°, and the maximum deviation recorded was 84°. At many sites, the deviations are so severe as to potentially result in the misinterpretation of the modal ice flow direction, based on the use of either striation orientation or clast fabric in isolation. Variations in glacial flow can readily produce striations and clast fabrics which record different phases of glacial activity associated with the same glacial event at an individual site.     

Till‐forming processes beneath parts of the Cordilleran Ice Sheet,British Columbia,Canada: macroscale and microscale evidence and a new statistical technique for analysing microstructure data

This paper presents the first integrated macroscale and microscale examination of subglacial till associated with the last‐glacial (Fraser Glaciation) Cordilleran Ice Sheet (CIS). A new statistical approach to quantifying till micromorphology (multivariate hierarchical cluster analysis for compositional data) is also described and implemented. Till macrostructures, macrofabrics and microstructures support previous assertions that primary till in this region formed through a combination of lodgement and deformation processes in a temperate subglacial environment. Macroscale observations suggest that subglacial environments below the CIS were probably influenced by topography, whereby poor drainage of the substrate in topographically constricted areas, or on slopes adverse to the ice‐flow direction at glacial maximum, facilitated ductile deformation of the glacier bed. Microscale observations suggest that subglacial till below the CIS experienced both ductile and brittle deformation, including grain rotation and squeeze flow of sediment between grains under moist conditions, and microshearing, grain stacking and grain fracturing under well‐drained conditions. Macroscale observations suggest that ductile deformation events were probably followed by brittle deformation events as the substrate subsequently drained. The prevalence of ductile‐type microstructures in most till exposures investigated in this study suggests that ductile deformation signatures can be preserved at the microscale after brittle deformation events that result in larger‐scale fractures and shear structures. It is likely that microscale ductile deformation can also occur within distributed shear zones during lodgement processes. Cluster analysis of microstructure data and qualitative observations made from thin sections suggest that the relative frequency of countable microstructures in this till is influenced by topography in relation to ice‐flow direction (bed drainage conditions) as well as by the frequency and distribution of voids in the till matrix and skeletal grain shapes.     

山地冰川沉积相模式与特征

山地冰川沉积研究在中国有特殊重要性。因为欧、美对冰碛沉积研究多限于大陆冰盖,沉积环境比较单纯,冰水沉积比重很大,而中国山地冰川多限于山体内部,冰水沉积极不发育,且沉积环境很复杂,亚相变化也大。此外中国山地冰川多属大陆性,与欧、美海洋性冰川在物理性质和沉积特征上都有不同。如冰川磨蚀作用弱,冰碛中粉砂少,冰上融出碛发育而流碛和滞碛不发育,原生(同生)构造如粗糙斜层很发育等。这些都将丰富冰碛沉积学内容,并对科学地鉴别中国东部各种混杂堆积的成因有重要意义。如据现代冰川沉积指标庐山就没有任何冰川堆积。