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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Serie: The Cryosphere Science Series
- Sprache: Englisch
Provides the first comprehensive review of the current state of the science on tills
It is critical that glacial scientists continue to refine their interpretations of ancient archives of subglacial processes, specifically those represented by tills and associated deposits, as they form the most widespread and accessible record of processes at the ice-bed interface. Unfortunately, despite a long history of investigation and a lexicon of process-based nomenclature, glacial sedimentologists have yet to reach a consensus on diagnostic criteria for identifying till genesis in the geological record. What should be called till? Based on the author’s extensive field research, as well as the latest literature on the subject, this book attempts to provide a definitive answer to that question. It critically reviews the global till literature and experimental and laboratory-based assessments of subglacial processes, as well as the theoretical constructs that have emerged from process sedimentology over the past century. Drawing on a wide range of knowledge bases, David Evans develops a more precise, contemporary till nomenclature and new investigatory strategies for understanding a critical aspect of glacial process sedimentology.
- Provides an in-depth discussion of subglacial sedimentary processes, with an emphasis on the origins of till matrix and terminal grade and the latest observations on till evolution
- Describes contemporary laboratory and modelling experiments on till evolution and techniques for measuring strain signatures in glacial deposits
- Develops an updated till nomenclature based on an array of knowledge bases and describes new strategies for field description and analysis of glacial diamictons
Written by an internationally recognised expert in the field, this book represents an important step forward in the modern understanding of glacial process sedimentology. As such, Till: A Glacial Process Sedimentology is an indispensable resource for advanced undergraduates and researchers in sedimentology, glacier science and related areas.
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Veröffentlichungsjahr: 2017
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Table of Contents
Cover
Title Page
Copyright
Acknowledgements
Chapter 1: Glacigenic Diamictons – A Rationale for Study
Chapter 2: A Brief History of Till Research and Developing Nomenclature
Chapter 3: Till – When is it an Inappropriate Term?
Chapter 4: Glacigenic Diamictons: A Strategy for Field Description and Analysis
4.1 Diamicton
4.2 (Glacigenic) Melange
4.3 Physics of Material Behaviour
4.4 Typical Structures
4.5 Clast Macrofabrics and Microfabrics
Chapter 5: Subglacial Sedimentary Processes: Origins of Till Matrix and Terminal Grade
Chapter 6: Subglacial Sedimentary Processes: Modern Observations on Till Evolution
6.1 Lodgement, Lee-Side Cavity Filling and Ploughing
6.2 Deformation
6.3 Soft-Bed Sliding (Ice Keel Ploughing), Meltwater Drainage and Ice–Bed Decoupling
6.4 Melt-Out
6.5 Glacitectonite Production, Rafting and Cannibalisation
Chapter 7: Subglacial Sedimentary Processes: Laboratory and Modelling Experiments on Till Evolution
Chapter 8: Measuring Strain Signatures in Glacial Deposits
Chapter 9: The Geological Record: Products of Lodgement, Cavity Fill and the Boulder Pavement Problem
9.1 Introduction – Repositioning Field Studies and Experimental Reductionism
9.2 Lodgement
9.3 Clast (Boulder) Pavements
9.4 Lee-side Cavity Fills
Chapter 10: The Geological Record: Deforming Bed Deposits
Chapter 11: The Geological Record: Sliding Bed Deposits
Chapter 12: The Geological Record: Impacts of Pressurised Water (Clastic Dykes)
Chapter 13: The Geological Record: Melt-out Till
Chapter 14: The Geological Record: Glacitectonite
Chapter 15: Glacial Diamictons Unrelated to Subglacial Processes
Chapter 16: Till Spatial Mosaics, Temporal Variability and Architecture
Chapter 17: Concluding Remarks: The Case for a Simplified Nomenclature
References
Index
End User License Agreement
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Guide
Cover
Table of Contents
Begin Reading
List of Illustrations
Chapter 1: Glacigenic Diamictons – A Rationale for Study
Figure 1.1 Flow diagram to illustrate the inter-relationships between the main glaciological and sedimentological processes associated with the subglacial environment (modified from Menzies and Shilts (1996) to acknowledge glacitectonic processes as deformational rather than depositional).
Figure 1.2 The glacial debris cascade and transport pathways: (a) the glacial debris cascade (from Benn and Evans, 2010); (b) simplified diagram to show the main debris transport pathways through a simple valley glacier, indicating that some debris may bypass the subglacial traction zone and follow a passive transport route (after Boulton, 1978). The impacts of various transport routes on clast form signatures are illustrated using: (1) glacifluvial outwash, (2) subglacial till, (3) supraglacial debris and (4) scree. Clast form data is depicted in the commonly used graphics of ternary diagrams (depicting clast shape based upon principle A – long, B – intermediate and C – short axes), histograms (depicting roundness, VA-WR or 0–5) and co-variance graph (plotting RA roundness or VA+A% against C40 form or % clasts below 0.4 c:a axial ratio). Other statistics are RWR = R+WR% and R = average roundness.
Figure 1.3 Examples of the range of deposits generally referred to as
glacigenic diamictons
(including tills) and sediments that are stratigraphically and genetically related to them: (a) stratified diamicton, Filey Bay, eastern England; (b) pseudo-laminated diamicton with gravel clot/intraclast, Red Deer Lake, Alberta, Canada; (c) stratified diamicton and horizontally bedded interbeds, Drayton Valley, Alberta, Canada; (d) fissile, clast-rich diamicton, Glen Varragill, Isle of Skye, Scotland; (e) discontinuous boulder pavement beneath massive, matrix-rich diamicton, Whitburn, northeast England; (f) heterogeneous, tectonically laminated and shale-rich diamicton, near Kinsella, Alberta, Canada; (g) mélange of stratified sands and gravels, pseudo stratified diamictons and sand and gravel intraclasts (rafts), West Runton, East Anglia, England; and (h) heavily deformed and attenuated stratified diamicton, Sheringham, East Anglia, England.
Figure 1.4 Summary schematic diagram showing the range of subglacial sediment transport mechanisms that can operate near the sub-marginal zone of an ice sheet or glacier (from Alley
et al
., 1997). The debris flux and its typical relationships with ice and meltwater flux are also depicted.
Chapter 2: A Brief History of Till Research and Developing Nomenclature
Figure 2.1 Symposium volumes compiled on the subject of tills during the 1970s to the early 1990s.
Figure 2.2 Researchers involved in the historical development of till sedimentology up to, and in some cases beyond, the late 1970s.
Figure 2.3 A variety of till and till process classification schemes compiled by the Till Work Group of the INQUA Commission on Genesis and Lithology of Glacial Quaternary Deposits (from Dreimanis, 1989). The upper diagram is the groups' genetic classification of tills in 1979 (after Dreimanis, 1969, 1976) which attempted to summarise both the position and migration of debris during glacial transport on the left and the final deposits, firstly in relation to position of deposition (middle column) and secondly in terms of facies nomenclature on the right. Note that deformed materials (not including ‘deformation till’) occur along the base of a diagram that mimics the vertical stratigraphic succession related to any one glacial phase (Hambrey and Harland, 1981). The middle diagram is the summary in 1982 of the groups' deliberations on process–form relationships, with factors that influence till production in the top half and the till genetic classifications at the base (Dreimanis, 1982). The lower diagram is the ‘depositional genetic classification of till’ compiled by Dreimanis (1989), with debris release and deposition on the left and till type on the right (no horizontal correlation is implied).
Figure 2.4 Examples of section sketches of the coastal exposures through the East Yorkshire coast tills at Bridlington, England by Lamplugh (1881a). The extent of stratified sediments and their deformation are well illustrated and were influential in Lamplugh's (1911) use of surging Svalbard glacier snouts and their proglacial deformation of foreland deposits as a modern analogue in his interpretations of till genesis.
Figure 2.5 Diagrams produced by Carruthers (1947–1948, 1953) to explain his undermelt theory.
Figure 2.6 Results of field observations on till production on Svalbard glacier snouts by Geoffrey Boulton: (a) diagrammatic sequence of depositional events related to the downwasting of debris-charged ice. A-D
1
depicts the development of hummocky terrain due to the continuous topographic inversions created by flowage of debris once melted out from discrete debris-rich ice folia. A-D
2
depicts the alternative scenario of till plain production due to more fluid ‘flow till’. E shows the process of topographic inversion and till flowage due to uneven surface melting (from Boulton, 1972a); (b) the classic supraglacial process–form (landsystem) model of Boulton (1972a), showing the spatial relationships between subglacial, melt-out and flow tills and associated glacifluvial sediments due to the downwasting of a polythermal, debris-charged glacier snout; (c) field sketch and macrofabric data of ‘flow till’ observed to be accumulating on the surface of a debris-charged polythermal glacier on Svalbard by Boulton (1971); (d) simplified diagram to show the development of melt-out till as observed on Svalbard polythermal glaciers (from Boulton, 1971).
Figure 2.7 Schematic diagram interpreting the complex glacigenic sediments at Glanllynnau, North Wales (lower panel), guided by the process–form relationships observed in Svalbard (upper panel) and conveying the principle of tripartite (till-stratified sediments-till) sequences relating to one glacial advance (from Boulton, 1977).
Figure 2.8 A classification scheme for glacitectonite compiled by Pedersen (1989) using the previous proposals of Banham (1977) and Berthelsen (1978).
Figure 2.9 Photograph of the typical features of the Sveg till (J. Lundqvist).
Figure 2.10 Till type tetrahedron (from Dreimanis, 1989).
Chapter 3: Till – When is it an Inappropriate Term?
Figure 3.1 The sedimentary characteristics and process based classification schemes for supraglacial diamictons: (a) Lawson (1979a, b) classification scheme, showing four sediment flow types identified at the Matanuska Glacier, Aslaska. (b) the processes (upper panel) and resulting deposits (lower panel) of the ‘supraglacial morainic till complex’ of Eyles (1979).
Figure 3.2 Modern-day examples of remobilisation and resedimentation of glacigenic deposits, including tills and their sedimentological products: (a) retrogressive flow sides in supraglacial debris on the ice-cored Little Ice Age lateral moraines of Horbyebreen, Svalbard; b) Late Wisconsinan debris-rich buried glacier ice creating debris flows in englacial and supraglacial materials, northern Banks Island, Arctic Canada; (c) crudely stratified diamicton created by paraglacial debris flows in former glacigenic deposits, Leirdalen, Norway (photo by A.M. Curry); (d) debris flows emanating from the distal face of a push moraine during its construction by the advancing snout of Fláajökull, Iceland; (e) debris flows on the proximal slope of the Little Ice Age lateral moraines of Kvíárjökull, Iceland.
Chapter 4: Glacigenic Diamictons: A Strategy for Field Description and Analysis
Figure 4.1 Classification schemes and facies coding systems for diamictons and related materials. Upper panel shows a facies coding scheme modified from Eyles
et al.
(1983a) and Evans and Benn (2004). Lower panel shows the Moncrieff (1989) scheme for classifying poorly sorted materials emphasising specific grain-size characteristics.
Figure 4.2 Various types of intraclasts and intrabeds or lenses.
Figure 4.3 The fourfold classification scheme for melanges proposed by Cowan (1985).
Figure 4.4 Schematic diagram showing the process of dilation during shear, whereby grains climb over one another and increase the sediment volume (from Benn and Evans, 1998).
Figure 4.5 Graphs to show the stress–strain relationships of materials common to glacial environments: a) examples of typical stress–strain relationships. Line ‘a’ is a perfect plastic which remains rigid until the shear stress reaches the yield stress. This example shows that at 100 kPa the material deforms at whatever rate is required to prevent the shear stress exceeding the yield stress. Line ‘b’ is a Newtonian, linear-viscous material, for which strain rate is linearly proportional to shear stress. Line ‘c’ is a non-linearly viscous material, like ice, for which the strain rate is non-linearly proportional to shear stress (from Benn and Evans, 2010, after Paterson, 1994); b) graphs depicting the concept of ‘critical state’ and attainment of ultimate strength (from Iverson, 2010).
Figure 4.6 Comprehensive classification scheme for the deformation structures typically found in glacial materials (from McCarroll and Rijsdijk, 2003), showing the different forces on strain ellipses (
F
p,
F
s,
F
c and
F
g) as pure shear, simple shear, compressional and gravitational.
Figure 4.7 Styles of deformation fabrics and structures observed in glacially deformed materials (from van der Wateren
et al.
, 2000).
Figure 4.8 Glacitectonic signatures compiled by Berthelsen (1978): (1) boundary between an upper kineto-stratigraphic unit with domainal deformation and subjacent strata with extra-domainal deformation; (2) base of subglacial till/deforming layer; (3) way-up in glacifluvial sediments; (4) zone of structural investigation into glacitectonite; (5) overthrusts; (6) conjugate thrusts; (7) sub-sole drag; (8) lineations/tectonic laminae; (9) torpedo (boudin) structure; (10) intrafolial folds; (11) macrofabric; (12) macroscale glaciodynamic structures, visible only in pseudo-laminated till.
Figure 4.9 Typical deformation structures that can be identified at microscale (from Menzies, 2000, after van der Meer, 1993).
Figure 4.10 Typical directional elements that can be identified in glacial deposits (from Berthelsen, 1978): (1) striae on bedrock; (2) striae on clast pavement; (3) glacitectonic folds and faults; (4) clast macrofabric displaying ice flow-parallel (a) and transverse (b) and mixed (c) orientations; (5) lineations or tectonic laminae in sheared materials.
Figure 4.11 The principle of preferred clast macrofabric orientations in subglacial tills, whereby either A axes or A/B planes (defined in top-left sketch) orientate themselves parallel with the principal stress direction. Photographs show typically lodged boulders with striated upper facets in a multiple till sequence such as that depicted in the vertical profile log (from Evans
et al.
2016). The macrofabrics measured using the orientations of the clast A axes and A/B planes of 50 clasts in each till are represented in contoured stereonets alongside the profile log. The macrofabrics and surface striae of the lodged boulders only are depicted in contoured stereonets and rose diagrams on the right, a procedure that isolates the lodged component from the deformation component of the tills.
Figure 4.12 Schematic diagrams to compare Jeffery and March type models of particle orientation in a deforming medium. March type rotation involves particles rotating passively so that the fabric ellipsoid reflects the deformation ellipsoid. Jeffery type rotation involves particles continuously rolling so that the fabric ellipsoid may be more or less elongate than the deformation ellipsoid (from Benn and Evans, 1996).
Figure 4.13 Conceptual diagram of the subglacial shear zone with typical strain ellipses and strain trajectories (from van der Wateren
et al.
, 2000).
Figure 4.14 Examples of the quantification of fabric ‘shape’ using the relative magnitudes of the three derived eigenvalues
S
1
,
S
2
and
S
3
in ternary plots: (a) key to fabric shape triangles, in which sample populations are plotted according to their isotropy and elongation (from Benn, 1994b); (b) clast A-axis samples from two Iceland glaciers (after Benn and Evans, 1996); (c) clast A/B plane samples from glacitectonites, Icelandic subglacial tills, ice-rafted sediments and subglacially lodged clasts (from Evans
et al.
, 2007); (d) clast A axis samples from melt-out till and debris-rich ice (data from Lawson, 1979b).
Figure 4.15 Quantification of fabric ‘shape’ using the modality/isotropy plot. Upper diagrams show stereonets and plot as first employed by Hicock
et al.
(1996). Lower diagram shows refined plot as proposed by Evans
et al.
(2007) in order to isolate clast lodgement from matrix deformation. Data plotted on these examples are A axes (left) and A/B planes (right) from Icelandic multiple till sequence of Evans
et al.
(2016), with comparative data for A/B planes from till, lodged clasts and glacitectonite from Evans
et al.
(2007).
Chapter 5: Subglacial Sedimentary Processes: Origins of Till Matrix and Terminal Grade
Figure 5.1 Diagnostic bimodal or polymodal grain-size distributions for tills; (a) debris-rich ice samples from the Matanuska Glacier, Alaska (Lawson, 1979a); (b) mean grain-size distribution of 150 till samples with areas of the graph classified as resistant crushed fraction (A), abrasion component (B) and residual component (C) by Haldorsen (1981); (c) frequency distribution of dolomite in till samples over transport distance (from Dreimanis and Vagners, 1971); (d) illustration of variation of terminal grades according to bedrock type (from Elson, 1961).
Figure 5.2 Details of quartz sand grain microscopy from glacial deposits: (i) schematic diagram showing styles of grain fracture (from Hiemstra and van der Meer, 1997, after Brzesowsky, 1995), where dashed lines define the centre lines of the grains in contact, arrows represent contact forces, and shaded areas are deformed contact regions. The styles include (a) diametrical loading at low compression rates with ring cracks passing into divergent cone cracks, (b) diametrical loading at high compression rates with radial fractures propagating as meridional cracks, (c) tangential loading at high angles of incidence with frictional sliding leading to the chipping of flakes and (d) tangential loading at low angles of incidence with attrition leading to crushing of all or part of a grain; (ii) photomicrograph illustrating crushing of a grain due to tangential loading at the centre of the image (from Hiemstra and van der Meer, 1997); (iii) typical glacially diagnostic quartz grain morphological features (from Sharp and Gomez, 1986) including (A) angular outline, (B) high-relief surface, (C) conchoidal breakage patterns, (D) stepped surface, (E) breakage blocks, (F) edge abrasion; (iv) typical grain surface textures from subglacial till (from Hart, 2006), including (A) pre-erosional surface (P) and smoothing and rounding, (B) pre-erosional surface (P) on one face with medium impacts, (C) pre-erosional surface (P) at base of ‘hand-axe’ form; (D) ‘jagged’ form, (E) smoothed conchoidal form, (F) conchoidal form. Mechanical features are numbered as: (1) large conchoidal fracture, (2) small conchoidal fracture, (3) arcuate steps, (4) straight steps, (5) crescentic gouges, (6) large breakage blocks, (7) fractured plates, (8) sub-parallel linear fractures, (9) curved grooves, (10) straight grooves.
Chapter 6: Subglacial Sedimentary Processes: Modern Observations on Till Evolution
Figure 6.1 Boulton's (1982) depiction of the lodgement process: (a) particles lodging by (i) frictional retardation against a rigid bed and (ii) against obstacles ploughing on a soft bed; (b) lodgement of debris-rich ice masses, with a whole assemblage lodging against the bed and melting out in situ.
Figure 6.2 The relationship between shear traction and effective normal stress over time as measured during experiments in the Svartisen subglacial laboratory by Cohen
et al
. (2005).
Figure 6.3 Mechanisms of debris release and accumulation observed in subglacial cavities by Boulton (1982).
Figure 6.4 Observations on the till infilling of a subglacial cavity on hard bedrock beneath the Salieckna Glacier, Sweden by Boulton (1975).
Figure 6.5 Models of preferential till sedimentation in low areas of a glacier bed by the processes of: (a) basal debris-rich ice thickening (Boulton, 1975); and (b) concentrated heat flow and elevated basal ice melt rates in depressions (Nobles and Weertman, 1971).
Figure 6.6 Shearing in subglacial till indicative of its operation as a fault gouge: top left – recent photograph and top right – Boulton's (1970a) sketch of cross-cutting thrust faults superimposed on ‘lodged’ till beneath Nordenskjoldbreen, Svalbard; lower left and right – depictions of the fault gouge as it pertains to subglacial materials (from Eyles and Boyce, 1998). Numbers in lower left are as follows: (1) flute ridge in lee of large clasts, (2) abraded clast, (3) crescentic groove on stoss side of striated clast, (4) crescentic fractures; (5) striated clast, (6) ridge-in-groove structure, (7) nail head striation, (8) gouge diamict with slip planes, (9) undisturbed footwall strata. The schematic comparison in lower right shows diamict production between fault blocks in (a) and between a glacier and its substrate in (b).
Figure 6.7 The relationships between a ploughed or lodged clast and the till matrix that propagates down flow to form a fluting. Upper diagram is Boulton's (1976) theoretical analysis of the process and lower diagram is from Benn (1994b).
Figure 6.8 Observations from experiments on ploughing clasts: (a) the pattern of compressional impact of a ploughmeter on subglacial till (from Fischer
et al.,
2001); (b) theoretical and experimental results from the assessment of clast ploughing. Box A shows Tulaczyk's (1999) application of Baligh's (1972) experiments on wedges dragged through soils, depicted here as an angular clast ploughing through perfectly plastic till. Symbols α and β are two slip lines, ϕ is the leading angle of the ploughing clast, and ϒ is the trailing angle of the cavity developed behind the clast. In B, the deformation is measured by the distortions in an initially square grid, which is depicted in the right box from a photograph of the outcome by an experiment in which a wedge was ploughed through a homogenous clay.
Figure 6.9 The subglacial till at Breiðamerkurjökull: (a) two-tiered till structure, comprising upper A horizon above pen knife overlying lower B horizon, with clast macrofabrics plotted on stereonets with rose plots (from Benn, 1995); (b) the pattern of strain as shown by the displacement of strain markers at three of the excavated locations (
A–C
) from the subglacial experiments (from Boulton and Dobbie, 1998). The sketch depicted in (
D
) shows how horizontally bedded sediments would have been deformed based upon the displacement patterns from the strain markers.
Figure 6.10 The location (upper panel) and simplified sketch of the results (lower panel) of the first Breiðamerkurjökull subglacial till experiment (from Boulton and Hindmarsh, 1987). The strain markers are depicted by broken lines and change from their original vertical alignment to a down flow displaced curve after 136 hours. The cross in circle symbols 1–3 are pore pressure gauge locations.
Figure 6.11 Data from the second Breiðamerkurjökull experiment of 1988/89, reported by Boulton and Dobbie (1998), Boulton
et al
. (2001) and Boulton (2006): (a) relationships between till shear strength at depths of 0.6 m (T1), 1.75 m (T2) and 2.5 m (T3) and shear stress; (b) time-dependent pattern of till strain inferred from the drag spool records; (c) proportion of movement of the glacier sole due to sliding versus till deformation.
Figure 6.12 Stick–slip cycles of sliding and deformation during till generation: (a) the model of Boulton
et al
. (2001), emphasising the role of water pressure cycles in till deformation; (b) the model of Phillips
et al
. (in press) depicting the role of seismic activity in liquefaction and soft-bed sliding. Upper panel shows the conceptual model of the potential effects of seismic waves generated during an ice quake on unconsolidated subglacial sediments. The pulse of energy passes into the underlying saturated sediments and triggers transient liquefaction and soft-bed sliding. The short duration energy pulse causes individual clasts to vibrate, modifying the packing of the grains and leading to the pressurisation of the intergranular porewater. This then reduces the number of grain to grain contacts, allowing the individual clasts to move (slide or rotate) past one another, reducing sediment cohesion and leading to liquefaction and soft-bed sliding. The vibrating effect propagates away from the focus of the ice-quake as a pulse or series of pulses, so that areas of the bed initially undergo localised liquefaction and soft-bed sliding, followed by stabilisation outwards away from the ice quake focus. Liquefaction and soft-bed sliding probably occurs within discrete, laterally discontinuous patches or narrow zones in the order of only a few centimetres or even millimetres thick. Once the ice-quake energy has been dissipated (likely in a few minutes), the fall in intergranular porewater pressure and increase in sediment cohesive strength results in cessation of flow deformation. Lower panel shows a conceptual diagram of the microscale processes occurring during bed deformation, soft-bed sliding and basal sliding as a result of increasing porewater content within a soft glacier bed.
Figure 6.13 Sketches showing possible configurations of subglacial drainage networks (from Benn and Evans, 1996): (1) bulk water movement in deforming till; (2) Darcian porewater flow; (3) pipe flow; (4) dendritic channel network; (5) linked cavity system; (6) braided canal network; (7) thin film at ice–bed interface.
Figure 6.14 Shaw's (1977) genetic model for the production of sublimation till in arid, polar environments, where the englacial debris structures created by the folding of basal, debris-rich ice facies are preserved after passive removal of interstitial ice.
Figure 6.15 Idealised vertical profile of the typical features observed in the melt-out till at the Matanuska Glacier, Alaska (from Lawson, 1981a). Zones include (a) structureless, pebbly, sandy silt, (b) discontinuous laminae, stratified lenses and pods of texturally distinct sediment in massive pebbly silt, and (c) layers of texturally, compositionally or colour-contrasted sediment. Laminae or layers may drape over large clasts.
Figure 6.16 Details of the Matanuska Glacier melt-out sequence (from Larson
et al
., 2016): (a) cross-sectional sketch through the glacier margin, showing the stratigraphy of the basal debris-rich ice sequence and its accumulating supraglacial debris cover; (b) exposure and (c) annotated sketch of exposure through the debris-rich basal ice and its overlying melt-out till. Light grey bands are debris-rich layers and dark bands are debris-poor layers and upper part of exposure is pseudo-stratified diamicton. Area marked with red diagonal lines is the approximate transition zone between ice and melt-out till; (d) basal debris-rich ice facies, showing discontinuous debris-rich (grey) and debris-poor (dark) laminations with a small fold and clear imbrication of elongate clasts; (e) detail of the pseudo-stratified diamicton created by melt-out, showing laminae of silt deformed around clasts.
Figure 6.17 Microscale details of the Matanuska Glacier melt-out till (from Larson
et al
., 2016): (a) thin section (left) and interpreted structures (right), showing silt-rich laminated diamicton with laminations (black lines), deformed aggregates of clayey silt (yellow fill) and porewater pathways (magenta lines). Aggregates occur in bands with their long axes generally subparallel to lamina; (b) thin section (left) and interpreted structures (right), showing silt-rich laminated diamicton with laminations (black lines), deformed aggregates of clayey silt (yellow fill), porewater pathways (magenta lines) and fine to coarse sand at base (pink fill). Aggregate long axes are generally subparallel to laminae.
Figure 6.18 The stratigraphy and fabric characteristics of subglacial melt-out till and basal-debris-rich ice at the Matanuska Glacier, Alaska, showing virtually identical clast fabrics in the till and glacier ice (from Lawson, 1979b).
Figure 6.19 The production of a glacitectonite carapace during the second phase of glacitectonic disturbance of pre-existing deposits by an advancing glacier, when proglacially thrust ridges have been streamlined into a cupola hill (after Evans and Benn, 2001). The structural details of the carapace (inset diagram from van der Wateren, 1995aa, b) displays the vertical continuum from to to (see Figure 6.21).
Figure 6.21 Schematic diagram to display the typical vertical continuum of structures in a subglacial shear zone (from van der Wateren, 1995aa).
Figure 6.20 A model for the production of glacitectonite in glacier sub-marginal locations based on the margin of Holmströmbreen, Svalbard by van der Wateren (1995a, b, 2003). Upper panel: reconstruction of the late-nineteenth-century surge margin and thrust moraine of Holmströmbreen. Middle panel: glacitectonic styles as they relate to the ice margin (dashed line). A = undeformed foreland, B = steeply inclined structures, C = overturned and recumbent structures, D = nappes, E = extensional structures, including Ec (internally compressive structures), Ee (strong extension). Lower panel: examples of overprinted glacitectonic styles in (A) advance sequence and (B) readvance sequence during retreat.
Figure 6.22 Modes of raft detachment or substrate cannibalisation in subglacial deforming materials: (a) Boulton
et al
s (2001) model of ‘tectonic/depositional slices’; (b) fault propagation folding (from Brandes and Le Heron, 2010); (c) generalised sketch of detachment of large slabs of soft bedrock where decollement zones are dictated by horizontal strata (from Evans and Benn, 2001).
Figure 6.23 Examples of deformed ice masses entombed in permafrost: (a) section sketch of exposure in coastal bluff on the Yamal west coast, Siberia (from Astakhov
et al.,
1996). M = clayey silt of the Marresale Formation, which is folded and occasionally faulted and displays diapirs, L = brecciated Labsuyaha sand, Kl = local Kara till, Kd = foreign diamict facies, B = Baydarata sandy silt with large ice wedges and minor syngenetic ice veins, H = Holocene limnic sediments with collapse structures, A = active layer. Thin lines are sedimentary and deformation structures and thick lines and blackened areas are ice. Dotted pattern represents sand. The Kl and Kd facies are heavily glacitectonised; (b)–(d) structures reported by Waller
et al
. (2009) as indicative of glacier–permafrost interactions at the former northwest margin of the Laurentide Ice Sheet in the Western Canadian Arctic; (b) frozen glacitectonite up to 8 m thick overlying massive ice; (c) raft of massive ice approximately 15 m long within frozen glacitectonite; (d) sand lenses and fold noses between silty clay indicative of glacier sub-marginal ductile deformation of ice-rich materials; (e) relict basal ice of the former Laurentide Ice Sheet, showing tight folds highlighted by layers of debris-poor ice.
Chapter 7: Subglacial Sedimentary Processes: Laboratory and Modelling Experiments on Till Evolution
Figure 7.1 Micromorphological evidence of artificially induced strain developed during a shearing experiment on potter's clay (from Hiemstra and Rijsdijk, 2003): (i) grain alignments and plasmic fabrics that develop sequentially (from top to bottom) of grain lineaments and unidirectional plasmic fabrics. Dashed line in upper sketch represents the position of a developing shear plane. Elongate grains near the shear plane rotate until they are aligned plane-parallel. They also move towards the shear plane due to contraction of the sediment (see strain boxes); (ii) sketch to illustrate the relationship between unidirectional plasmic fabrics, skelsepic plasmic fabrics and turbate structures; (iii) schematic diagram to show the development of branching and merging characteristics in unistrial plasmic fabrics. Short, discontinuous unistrials in (a) grow to form continuous features in (b), and where unistrials meet they split or bifurcate.
Figure 7.2 Numerical modelling results for a particle–fluid mixture undergoing shear deformation (from Damsgaard
et al
., 2013, 2015).
Chapter 8: Measuring Strain Signatures in Glacial Deposits
Figure 8.1 Results of shearing experiments on the Douglas and Batestown tills of the Laurentide Ice Sheet, showing microfabric strength related to strains up to 108. The rose diagrams record the alignments of sand grain long axes in two dimensions and therefore eigenvalues will not fall below 0.5 (Thomason and Iverson, 2006).
Figure 8.2 The relationship between fabric shape development and consolidation and shear strain, plotted on the standard clast fabric shape ternary plot, based on AMS fabrics produced during shearing experiments (form Iverson
et al
., 2008).
Figure 8.3 Example of consistently strongly orientated clast macrofabrics in a Polish stacked till sequence (from Larsen and Piotrowski, 2003): (i) section sketch showing main facies and locations of fabric samples. Macrofabrics are displayed below as stereonets; (ii) the structureless appearance of Till C; (iii) till unit B showing the characteristic millimetre-thick sand stringers which give the till a stratified appearance (A), and a view on to the top of a decollement surface within the till (B); (iv) transition zone between outwash sands and till, showing gradational contacts between component units which relate to the upwards-increasing strain rate; (v) clast macrofabrics plotted on clast fabric shape ternary diagram.
Figure 8.4 Macro- and microfabric data from a homogenous, ≥1.4-m-thick, single massive silty–sandy diamicton within megascale glacial lineations (from Spagnolo
et al
., 2016): (top left) composite stereonet (a) and clast fabric shape ternary plot (b); (lower left) X-ray tomography image and microfabric subdivided according to grain size; (right) example of thin section mapping, showing: (a) summary of different sets of Riedel shears developed within the diamicton; (b) microstructural map with colours representing different generations of microfabrics which define the Riedel shears, sub-horizontal shear fabric and up-ice dipping foliation; (c) image showing massive, fine-grained sandy nature of the diamicton.
Figure 8.6 Clast macrofabrics from a case study of multiple, stacked tills at þorisjökull, Iceland (from Evans
et al
. 2016): (a) annotated photographic log showing one exposure through the stacked till units and discontinuous clast lines comprising lodged clasts; (b) macrofabric and striae data for lodged boulders from the till sequences; (c) clast macrofabric shape ternary diagrams, depicting the positioning of glacial deposits of known origin as envelopes and the samples from the stacked till units at þorisjökull. The indications of fabric shape development in relation to consolidation and shear strain, after Iverson
et al
. (2008), are also shown.
Figure 8.5 Examples of the nature of clast macrofabrics in flutings: (a) herringbone fabric pattern from fluting at Slettmarkbreen, Norway (from Benn, 1994a); (b) selected flutings and their clast macrofabrics from Sandfellsjökull, Iceland (from Evans
et al
., 2010); (c) stratigraphy, morphology and fabrics from flutings at Austre Okstindbreen, Norway, displaying herringbone patterns (from Rose, 1989).
Figure 8.7 Clast macrofabrics and subglacial bedforms on the foreland of the Saskatchewan Glacier, Canada (from Eyles
et al
., 2015): Upper panel shows macrofabrics measured in boulder-initiated, crevasse-related and ice-pushed flute ridges (ice flow direction from left to right). Lower panel is a schematic reconstruction of the process–form regime in which three principal flute types are created, including (1) boulder-initiated flutes, (2) flute ridge formed by till squeezing into crevasses, and (3) ice-pushed flutes and ploughed grooves.
Figure 8.8 Stratigraphy and clast macrofabric data from a 1.5-m-thick till sequence beneath a fluting field on the Sandfellsjökull foreland, Iceland (from Evans
et al
., 2010).
Figure 8.9 Clast macrofabric data from the Dubawnt Lake palaeo-ice stream MSGL tills, plotted on modality-isotropy and fabric shape ternary diagrams (from Ó Cofaigh
et al
., 2013).
Figure 8.10 Clast macrofabrics from various depths within a fluting on the Breiðamerkurjökull foreland (from Boulton, 1976). Arrows on stereonets show ice-flow direction.
Figure 8.11 Clast macrofabric data from the surge flutings on the Bruarjokull foreland, Iceland; (a) data from parallel-sided flutings. Left panel shows data from the flanks and centre of a single fluting through two separate cross-sections. Right panel shows clast macrofabrics from both flanks and the centre of a 100-m-long fluting (from Kjær
et al
., 2006); (b) data from two separate examples of prows or incipient flutings developed in front of ploughing boulders.
Figure 8.12 Stratigraphy and clast macrofabric and microstructural signatures within sub-marginal till wedges or the ice-proximal ramps of push moraines: (a) multiple till wedges/moraines constructed at the margin of Fláajökull, Iceland in the early- to mid-1990s (after Evans and Hiemstra, 2005); (b) multiple till units emplaced on the ice-proximal side of a glacially overridden ice-contact subaqueous outwash fan and delta sequence at south Loch Lomond, Scotland (after Benn and Evans, 1996; Phillips
et al
., 2002; Benn
et al
., 2004; Evans and Hiemstra, 2005). Upper panel shows the main lithofacies associations, of which LFA 3 includes the stacked till units. Middle photograph and sketch shows the till details and the locations of macrofabrics (data at lower right) and thin section samples (images at lower left). The thin sections reveal: (A) silt-rich till with linear arrangement of silt and sand grains (arrow) and diffusely bounded intraclast (circle); (B) turbate structure with associated lineament ‘tail’ to the right; (C) anastomosing silt and clay laminae with many silt grains aligned parallel to the lamination; (D) muddy intraclasts (arrows) and casings around individual grains (circles); (E) irregularly shaped mammilated void, composed of intergrown more rounded pore spaces; (F) water escape structure (arrows); (c) data from overprinted till sequences within the proximal ramps of sub-marginal till wedges, including independently collected lodged surface clast data, from Breiðarlon on the Breiðamerkurjökull foreland and Skalafellsjökull, Iceland.
Figure 8.13 Glacitectonite and till carapace over the Brennhola-alda overridden thrust moraine at Breiðamerkurjökull, Iceland (after Evans and Twigg, 2002; Evans and Hiemstra, 2005). Photograph shows stratified sands, silts, clays and fine gravels with peat layers which have been glacitectonically thrust and cross-cut by a sub-vertical clastic dyke (hydrofracture fill) and sheared into an amalgamation zone (clast fabric BA-11) at the base of an overlying till sequence (clast fabrics BA8–10). Top left section sketch shows the details of the upper glacitectonically deformed part of the stratified deposits and the overlying amalgamation zone (Dmm with sheared rafts) and capping till. Inset photograph shows a vertically descending till-filled dyke injected into the stratified deposits during subglacial shearing. Top right section sketch shows the details in the box on the top left sketch and the location of clast macrofabric samples BA1–7.
Figure 8.14 Clasts macrofabrics from a selection of glacitectonites (from Evans
et al
., 1998), displaying an apparent strain signature continuum. Note that the glacitectonite maturity was assessed independently based upon their structural appearance and degree of mixing/homogenisation compared to their parent materials.
Figure 8.15 Parameters and data relating to the identification and quantification of microscale shearing indicators (from Larsen
et al
., 2006; Thomason and Iverson, 2006): (a) summary diagram to show how most strain is accommodated by Riedel shears aligned at low [R1] and high [R2] angles to the shearing direction; (b) graphic plot that demonstrates a progressive
I
L
index increase during shearing.
Chapter 9: The Geological Record: Products of Lodgement, Cavity Fill and the Boulder Pavement Problem
Figure 9.1 Boulton's (1996a) model of regional till architecture using a time–distance diagram (a) to portray the shifting patterns of erosional and depositional zones. The curves A-I represent the changing trends of erosion and deposition at selected locations through time, with advance and retreat tills identified. The lower cross-section (b) displays the pattern of till deposition at the end of a glacial cycle, emphasising the net ice sheet marginal-thickening of such deposits.
Figure 9.2 Products of the lodgement and ploughing processes: (a) photographs of lodged clasts exposed on till surfaces and displaying prominent upper striated facets, stoss-and-lee forms, clast clustering and ice flow-parallel A-axis alignments; (b) schematic diagrams to explain the development of asymmetric clast wear patterns associated with sliding ice and deforming till, showing principal locations of abrasion (A) and fracture (F). Arrow lengths show relative velocities. (a) lodged clast with stoss-lee form due to stoss-side abrasion and lee-side fracture. (b, c) double stoss-lee morphology due to a two-stage process of ploughing and lodgement. (d) double stoss-lee clast due to a single-stage process within a deforming layer. Low-pressure zones are shaded. (e, f) flat, polished facets eroded on the upper and lower surfaces of clasts, where there is significant slip between the clast and the adjacent shear plane. (Modified from Krüger, 1984; Benn and Evans, 1996); (c) sketches of cross-sections through sediment slab containing a ploughed clast and prow (from Clark and Hansel, 1989). The long axes of elongate grains are depicted as lines; (d) left panel shows sketches of examples of plough marks on the basal contacts of tills (from Ehlers and Stephan, 1979): (1) ribs (with one clast in place), (2) wedge, (3) edge, (4) slickenside, and (5) undulation. Right photograph shows slickensides, at end of pencil, developed at the boundary between laminated clays and till (former ice flow from left to right).
Figure 9.4 Boulton's (1996a) model for clast pavement development in relation to the rising and descending interface between A and B horizons in tills, whereby clasts concentrate at the interface and can be isolated at depth when tills thicken in the depositional zone beneath ice sheets.
Figure 9.3 Examples of clast lines or pavements. Upper panel shows clast pavement developed between tills at Milk River, Alberta (from Evans
et al
., 2012a). A macrofabric from the lower clast pavement is numbered MR2, which can be compared with those from the matrix of the tills. Lower panel shows a discontinuous clast pavement (clast line) between tills at Whitburn, northeast England, together with macrofabric (from Davies
et al
., 2009). Inset photograph shows striated nature of upper facets.
Figure 9.5 Lee-side cavity fills: (a) summary sketch of cavity infill stratigraphy developed on a hard rock bed with roches moutonnées (from Hillefors, 1973); (b) stratigraphy and clast form and macrofabric data from a lee-side cavity fill on the floor of a Norwegian mountain valley (after Evans
et al
., 1998). Valley axis and downstream trend is marked by blue arrow.
Chapter 10: The Geological Record: Deforming Bed Deposits
Figure 10.1 Idealised reconstruction of the subglacial deforming till layer and the development of porewater migration pathways, till–matrix framework and till pebbles and their relationship to potential A and B horizon development and geotechnical properties relevant to dilation and solid state deformation (after Evans
et al
., 2006b).
Figure 10.2 Two scenarios for subglacial till formation where either an A and B horizon stratigraphy arises (e.g. Breiðamerkurjökull) or only a B type horizon (e.g. Slettmarkbreen, Norway; after Benn and Evans, 1996). TRS = total relative strain or differential horizontal displacement. Photographs show structures typical of A (upper) and B (lower) horizons.
Figure 10.3 Sketches of sections exposed in a fluting at Isfallsglaciaren, Sweden, by Eklund and Hart (1996), showing a shear zone created by the amalgamation of the till base and the underlying stratified sediments.
Figure 10.5 Diagram to illustrate the progressive development of brittle to ductile microscale structures in tills when subject to shear (from Menzies, 2000).
Figure 10.6 The concepts of constructional deformation or till accretion (a) and excavational deformation (b), where deforming till sheets <1 m thick either vertically build composite till sheets or erode and streamline the underlying substrate, respectively (from Eyles
et al
., 2016, after Hart (1997).
Figure 10.7 Example of an amalgamation zone (gravelly diamicton) at the boundary between preglacial (Empress Group) gravels and sands and till in the Lethbridge area, Alberta, Canada (from Evans
et al
., 2012a). Note also the subglacial fluvially infilled scours and clast pavement separating lower and upper tills.
Figure 10.8 The origins of shear zones and intraclasts by substrate cannibalisation or the creation of ‘tectonic/depositional slices’: (a) left panel shows the various ways in which simple shear can liberate rafts of substrate into a subglacial deforming layer (from Hart and Roberts, 1994). Right panel shows examples of intraclasts in diamictons at increasing levels of attenuation during deformation, including a gravelly sand pod (upper), a sand boudin (middle) and a highly attenuated shale raft (lower); (b) an example from the island of Rügen, north Germany, of folding and attenuation of sub-till stratified sands and gravels and their ingestion and attenuation into the base of the deforming layer to produce glacitectonic lamination. The box in the right hand photograph shows the area of the enlarged view of glacitectonic lamination at bottom left.
Figure 10.9 Examples of the patchy nature of amalgamation zone development at till–substrate contacts: (a) details of the contact between till (SU3) and stratified sediment (SU2) in the former sub-marginal zone of the Scandinavian Ice Sheet in northern Poland (from Tylmann
et al
., 2013) showing: (A) sharp, erosive contact between undeformed outwash deposits and massive basal till, with no evidence of deformation; (B) deformed lens (DF1) of gravel and sand mixed to varying degrees with diamicton; (C) deformed lens (DF2) of partly homogenised gravel, sand and diamicton; (D) upward-convex and flat-based, deformed lens (DF3) of heavily deformed sand, gravel and till with admixture of till in the form of tongues, lenses and stringers being most prominent at the top of DF3. Open circles indicate macrofabric samples (1–3 and 6–11), crosses are AMS fabrics (1–3), and black dots mark samples for grain size and petrographic analysis (4–7). (b) detailed images identified in (a) and showing: (E) sharp contact between the massive basal till and undeformed and truncated trough crossbedded outwash deposits; (F) base of massive till with embedded pebble overlying undeformed, sub-horizontally-bedded outwash sand; (G) middle part of lens DF3 showing heavily deformed gravel, sand and diamicton with contrasting top (gradual) and bottom (sharp) boundaries; (H) flank of lens DF3 consisting of 30-cm-thick diamicton mixed with outwash deposits; (I) cut-and-fill structure interpreted as a subglacial N-type channel at the contact between outwash sand and basal till. (c) clast macrofabric, AMS fabric, grain size and petrographic data from located sampling points (from Tylmann
et al
., 2013). (d) glacilacustrine sediment capped by till in the Leipzig area of eastern Germany, showing: (A) largely undisturbed varved clay with sharp upper contact (photo by Prof. L. Eissmann); and (B) the same varved clay with heavily deformed upper half, grading into till through a zone of intensive mixing; (e) glacilacustrine sediment capped by till at Knud Strand, northwest Jutland, Denmark, showing (A) largely intact stratified sand, silt and clay with sharp upper contact and no apparent diffusive mixing with the till; (B) the same outcrop but located ca. 150 m distant to (A) is an area of heavily disturbed glacilacustrine sediment below the till contact (from Piotrowski
et al
., 2004).
Figure 10.10 Till and intervening stratified sediments in the Lethbridge area of Alberta, Canada, showing details of basal sequence of diamictons with deformed bedrock rafts or smudges separated by discontinuous sand and gravel lenses and stringers or distinct partings (from Evans
et al
., 2012a). An amalgamation zone occurs at the contact between the till and underlying Empress Group sands and gravels. The clast macrofabric strengths in the basal till sequence weaken immediately above the amalgamation zone.
Figure 10.11 Examples of deformed inclusions or intraclasts: (a) stratified sands and gravels folded and attenuated into the base of a matrix-rich till, Filey Bay, eastern England; (b) small rafts of rippled sand in the base of a matrix-rich till, liberated from underlying climbing ripple drift deposits, Whitburn, northeast England; (c) preglacial sands rafted into basal shear zone of till sequence at Drayton Valley, Alberta, Canada. Sand intraclasts vary in shape from elongate blocks to boudins to attenuated lenses/laminae; (d) attenuated chalk rafts (glacitectonic laminae) in the Skipsea Till of eastern England; (e) deformed sand pods in clay-rich diamicton, Elk Point, Alberta, Canada.
Figure 10.12 Dispersion tails extending from soft-sediment intraclasts (from Piotrowski and Kraus, 1997). Sketch shows two possibilities: (a) a deforming bed where the vertical distribution of velocity causes the clast to rotate and dispersion tails develop on its up-ice lower side and down-ice upper side (photograph shows example from Funen, Denmark); (b) a stable bed subject to ice sliding where the dispersion tail is at the sliding interface only (photograph shows sand lens with truncated upper contact with overlying till, Elk Point, Alberta, Canada).
Figure 10.13 Examples of banded or (pseudo) laminated diamictons: (a) crude banding in the Skipsea Till at Barmston, Yorkshire, England. This deposit appears massive when first exposed but develops a banded appearance after wave erosion; (b) strong lamination likely derived from stratified (glacilacustrine) deposits in the Bacton Green Member of East Anglia, England; (c) folded laminations in the base of the Filey Bay diamicton (‘till’), Yorkshire, England; (d) strongly laminated Bacton Green Member, East Anglia, England, with boudin created by deformation of coherent (initially frozen?) sand body.
Figure 10.14 Fissility development in diamictons: Upper photographs show strongly developed, densely spaced fissility with slickensides in compact diamicton (left) and crudely developed, widely spaced fissile structures in clast-rich diamicton (right). Lower panel shows conceptual diagrams to explain how dilatancy impacts on framework development and hence potential fissility in tills (from Evans
et al
., 2006b): (a) compression/loading leads to an increase in density of the till-matrix framework with depth and hence stronger density of fractures; (b) unloading also leads to the increase in intensity of bedding-parallel structures with depth.
Figure 10.15 Microscale features related by van der Meer (1993) to processes operating in the vertical deformation profile compiled by Alley (1991).
Figure 10.16 Schematic diagram to illustrate the development of a typical marble bed structure by the production of till pebbles or rounded, soft-sediment inclusions/intraclasts during shear (from Hiemstra and van der Meer, 1997): (a) shear zone development after dissipation of water with displacement along discrete planes with strain hardening; (b) brecciation of dry till with aggregation and progressive shear creating marble-bed appearance; (c) additional strain causes till to dilate with water entering voids; (d) collapse of dilated structure due to reduction in strain rate and expulsion of water and stiffening of till. This may trigger further shear stress and a return to marble bed.
Figure 10.17 Pore space modification in shearing till (from Kilfeather and van der Meer, 2008): (i) and (ii) thin section examples of structures that effect the development of pores; (iii) schematic sketches to show the initial state of sediment prior to subglacial deformation; (iv) schematic sketches to show interpretations of the forms of deformation that result in the development and destruction of pores and other microstructures. This includes: (Aiv) long axes of elongate small grains align along the sides of a rotating pebble. Particles find the paths of least resistance and infill pores; (Biv) till shearing to form grain lineations or plasma. Porosity may decrease along these shear zones as particles migrate towards them and find the paths of least resistance to infill pores; (Civ) brittle break-up during shearing, possibly associated with water-escape, forming fissile partings that increase the porosity; (Div) break-up of a dense and dry till bed during high shear stress and rotation of individual aggregates to form marble-bed with increasing porosity.
Figure 10.18 The collection and display of clast microfabric data (from Phillips
et al
., 2011). The five stages are: (1) import high-resolution scans of thin sections into the graphics package; (2) measure the orientation of the clast long axes; (3) plot orientation data on a rose diagram; (4) identify main clast microfabrics; and (5) identify clast microfabric domains and make final interpretation.
Figure 10.19 Illustration of the proposed non-genetic terminology for the morphological description of clast microfabrics in glacial sediments (from Phillips
et al
., 2011), based upon the system used for the description and classification of cleavage and/or schistosity in metamorphic rocks (cf. Passchier and Trouw, 1996).
Figure 10.20 Examples of the employment of the microstructural mapping protocol proposed by Phillips
et al
. (2011): (a) schematic 3D block diagram showing the relationships between the various microfabrics developed within a fluting. There is a highly irregular boundary between the two lithologically distinct areas of diamicton, across which the main clast microfabrics cut; (b) schematic diagram to show the possible microfabric relationships developed in response to polyphase deformation associated with the formation of a subglacial till.
Figure 10.21 Examples of microstructures associated with till deformation: (a) cannibalisation of stratified rafts and development of water escape conduits at a till–glacitectonite contact (from Phillips
et al
., 2007); (b) a microstructural map of polydeformed, thinly laminated sand silt and clay overlain by subglacial till from Raitts Burn, Strathspey, Scotland (from Phillips and Auton, 2000; Phillips
et al
., 2011).
Figure 10.22 Micromorphological evidence compiled by Roberts and Hart (2005) for their: (a) ‘Type 1 laminae’ (glacitectonic lamination) and (b) ‘Type 2 laminae’ (glacitectonite).
Chapter 11: The Geological Record: Sliding Bed Deposits
Figure 11.1 Conceptual diagram to illustrate the perceived variable rates of basal sliding due to changing ice–bed coupling (from Fischer and Clarke, 2001). Ice flow direction and magnitude are depicted by arrows during periods of: (a) low subglacial water pressures, and (b) high subglacial water pressures in the connected region of the bed (outlined). The locations of ploughmeters (PL1 and 2) and sliding sensor (SL), upon which this reconstruction is based, are marked.
Figure 11.2 The stratigraphy of subglacial tills and associated sediments at Sandy Bay, northeast England (from Eyles
et al
., 1982). Bedrock cliffline at north end of bay comprises Carboniferous sandstone (S) with a thin coal and shale band (C). Other features are: (1) striated rockhead; (2) bedrock rafts; (3) coarse rubbly till (lee-side cavity fill); (5) intrusion of till into bedrock joints; (8) glacifluvial channel fills or till interbeds (canal fills); (11) glacitectonised upper surfaces of canal fills with rafts of fill material in base of overlying till; (14) vertical jointing in till.
Figure 11.3 Sedimentary structures associated with the Skipsea Till of Yorkshire, eastern England (after Evans
et al
., 1995), indicative of meltwater canal fill activity between till emplacement events: (A) overview of section face showing Skipsea Till and associated intrabeds of stratified sediments; (B–F) details of structures outlined by boxes in A with labels locating the following structures: (a) deformed stratified, poorly sorted, coarse and pebbly sands; (b) concentrations of chalk clasts; (c) concentrations of rounded pebbles; (d) deformed sand lenses; (e) crude lamination in the till due to subtle grain size variations; (f) major discontinuity in the till, marking the top of the zone of deformed sand lenses; (g) strongly developed vertical joints; (h) interdigitisation of till and sand; (i) crudely planar stratified sands; (j) laminated till comprising red, grey-brown and buff lamina derived from cannibalised soft bedrock; (k) chalk stringers; (l) laminated till; (m) laminated till with chalk stringers; (n) chevron fold; (o) stratified sand with interstratified minor diamicton beds <10 mm thick; (p) till with weak stratification at base and chalk stringers subparallel to lower contact; (q) dark brown till with chalk stringers; (r) sand with stratification parallel to base of overlying till; (s) cross-stratified sands; (t) normal faulted sands; (u) gentle folds; (v) light brown stratified diamicton with sandy intercalations; (w) folded stratified diamicton; (x) concentrations of surrounded chalk pebbles, some of which are deformed into stringers within the surrounding till; (y) sand stingers in till; (z) sandy diamicton interstratified with cm-thick beds of clayey sand folded into a recumbent fold; (aa) deformed stratified clayey sands; (ab) stratified sand lenses (dm wide) with convex tops and flat bases; (ac) shears in till lined with sand; (ad) laminated till grading into massive till; (ae) moderately-to-well-sorted, coarse-to-fine sands; (af) rippled medium sands; (ag) deformed lenses of rippled sands; (ah) fine to very coarse, poorly sorted clayey sands containing pebbles towards the top; (ai) stratified diamicton interdigitated with sandy clay; (aj) chalk stringer deformed by a diapir; (ak) coarse to fine sands with low-angled cross-stratification deformed by small normal faults; (al) smooth base of laminated till parallel to the laminations of the underlying sands; (am) massive to moderately well-sorted coarse sands; (an) faulted, stratified, poor to moderately sorted, medium to fine sands.
Figure 11.4 Details of the subglacial canal fills of eastern England: (a) series of partially connected canal fills with heavily glacitectonised tops in the Skipsea Till at Skipsea, Yorkshire; (b) laminated fines in a series of canal fills in the Horden Till at Whitburn, Durham, where the subglacial tunnel locally widened into a slack water cavity near the glacier grounding line in Glacial Lake Wear (cf. Davies
et al
., 2009); (c–e) details of the glacitectonic structures developed at the tops and margins of the Skipsea Till canal fills.
Figure 11.5 A conceptual model for the evolution of tills and their stratified interbeds (from Boyce and Eyles, 2000): (a) erosion and deformation of pre-existing unlithified sediments to form drumlinised surface; (b) conformable aggradation of deformation tills on drumlinised surface; (c) subglacial fluvial reworking of diamicton to form sheet-like interbeds and boulder pavements; (d) continued aggradation of deformation tills and stratified interbeds.
Figure 11.6 Examples of proposed ice–bed separation features (from Piotrowski and Tulaczyk, 1999), showing a lack of significant deformation and with sand layers thought to be the product of thin water films at the ice–bed interface: (a) sub-horizontal, often slickensided fissures, in places filled with syn-depositional sorted sediments; (b) sub-horizontal fissures (upper part) and thin stringers of sorted sediments interbedded with till matrix (lower part); (c) horizontally stratified till consisting of mm-thick sand layers intercalated with till matrix; (d) single horizontal stringer of stratified sand in till matrix.
Figure 11.7 Example of a sub-till stratified lens lying beneath a thin ice stream bed till and overlying relatively impermeable substrate of shale in Alberta, Canada.
Figure 11.8 Schematic diagrams summarising the various modes of basal motion associated with ice streams and surging glaciers (from Kjær
et al
., 2006): (a) decoupling sustained by enhanced basal sliding across the glacier–till interface with limited or no subglacial deformation; (b) glacier–bed coupling and fast ice flow sustained through subglacial deformation of water-saturated sediment and the development of a fast-deforming sediment (H
A1
) over a more slowly deforming sediment (H
A2
) that in turn is superimposed on a stable horizon (H
B
); (c) a dual-coupled model where the glacier is coupled to its bed to create slow subglacial deformation, while the substrate is decoupled from the bedrock by a water film leading to fast ice flow.
Figure 11.9 Sedimentological evidence for till squirting into canal fills, demonstrated by interdigitisation of till and stratified sands around the margins of the Skipsea Till canal fills, Yorkshire, England.
Chapter 12: The Geological Record: Impacts of Pressurised Water (Clastic Dykes)
Figure 12.1 Examples of clastic dykes: (a) burst-out structure of gravel dyke and its branches in the Horden Till at Whitburn, Durham, North East England; (b) vertical gravel-filled dykes in crudely stratified diamictons, Clifden, Connemara, western Ireland; (c) sand and silt-filled dykes in locally liquefied and subglacially sheared rhythmites beneath till at Swarthy Hill, northwest Cumbria, England; (d) and (e) hydrofracture infills containing stratified clay, silt, sand and granule gravel and cross-cutting granule gravel glacifluvial outwash capped by till, Slettjökull foreland, Iceland.
Figure 12.2 Models of clastic dyke infills due to downward injection of remobilised subglacial till into sand and gravel (from Le Heron and Etienne, 2005).
Figure 12.3 Microscale characteristics of the lamination in finer-grained clastic dykes (from van der Meer
et al.
, 2009). Left panel shows: (a) thin section of constricted part of clastic dyke from San Martin de los Andes, Argentina; (b) sketch of laminae in the thin section with single barb arrows pointing to displacement along microfaults and normal arrows pointing to related intraclasts; (c) sketch of the distribution of drip-like structures in the thin section; (d) grading (sense indicated by arrows) in individual laminae. Right panel shows finer details of thin section: (a) small fault (arrowed) in top right corner and micro-WES in centre; (b) broken up intraclasts with small intraclasts in left hand lower corner; (c) image and sketch (right) of downwards oriented drip-like structures along boundaries of laminae.
Figure 12.4 Sedimentological evidence for the interaction of water escape pathways with subglacial till deformation at Slettjökull, Iceland (from van der Meer
et al.
, 1999): (a) black WES and till structure at sites 1 and 2; (b) reconstruction of the events responsible for the development of the water-escape structures and till structures.
Figure 12.5 Clastic dykes developed in tills based upon examples from Killiney Bay, Ireland (from Rijsdijk
et al.
, 1999): (a) conceptual model for dyke development; (b) and (c) sketched examples of plumes or burst-out structures; (d) proposed stages of the hydrofracturing process at Killiney (for the
P/t
graph,
P
= water pressure,
t
= time, τ = maximal tensile shear strength of capping aquifer,
Fls
= fluidisation velocity of sand,
Flg
= fluidisation velocity of gravels). (A) Stage 1 – water pressures within confined aquifer increase when meltwater supply exceeds capacity of the aquifer. (B) Stage 2 – water pressures exceed the overburden pressures and a water blister may form and lifts the till layer. (C) Stage 3 – water pressures exceed the tensile strength (τ) of capping till layer and a component of the total normal stress and a hydrofracture forms. A high-pressure potential is generated across the fracture, leading to upward water flow. Convergence of flow at crack base leads to flow velocities exceeding the minimal fluidisation velocity of coarse gravels (Flg), causing them and anything smaller to fluidise. The ejected fluid supersaturates the overlying diamict, allowing ejected materials to settle through the matrix to form a crudely stratified diamict. When flow through the fractures ceases, fluidised sediments settle; (e) schematic diagrams showing stages of branching of clastic dykes. Left (a) shows that during burst-out through a vertical hydrofracture, hydraulic pressure builds up locally along the dyke walls, where pre-existing joints (broken lines) may dilate and begin to fill. Right (b) shows that during filling of the joints, the tensile strength of the capping till is locally exceeded, small-scale hydrofractures may form and water bursts out to form smaller-scale vertical offshoots; (f) broad stratigraphic architecture of the Killiney Bay site showing the post-injection shearing of the dyke tops by a capping till layer.
Figure 12.6 Complex water escape and associated structures viewed at microscale in stratified fine-grained glacimarine sediments occurring as thrust bound rafts at Clava, North East Scotland (from Phillips and Merritt, 2008).
Figure 12.7 Case study of multiphase hydrofracture development in sandstone beneath till at Meads of St John, Inverness, North East Scotland (from Phillips
et al.
, 2013): (a) compilation of numerous thin section images and their locations in the Meads of St John hydrofracture system developed in sandstone bedrock; (b) set of schematic diagrams to show the evolutionary stages of the hydrofracture complex; (c) interpretive sequence of events linking the recession of the local Findhorn Glacier with the development of the hydrofracture complex.
Figure 12.8 Example of clay/silt intraclast concentrations or swarms. The clay/silt bodies have been injected into deltaic sands in a glacially overridden ice-contact delta at south Loch Lomond, Scotland, and connect up to form a vein-like network.
Chapter 13: The Geological Record: Melt-out Till
Figure 13.1 The classification scheme proposed by Paul and Eyles (1990) for soft-sediment deformation zones defined by the occurrence of shear instability and/or hydraulic instability during the melt-out process.
Figure 13.2 Supraglacial debris undergoing failure and debris flow mobilisation on the surface of Longyearbreen, Svalbard.
Figure 13.3 Characteristics of the melt-out till sequence at the margin of the Matanuska Glacier, Alaska (from Larson
et al
., 2016): (a) detail of the transition zone between debris-rich ice and overlying melt-out till sequence, showing accreted silt aggregates at the base of the diamicton; (b) the pseudo-stratified diamicton, showing laminae, bedding and scattered clasts; (c) details of laminations, showing coarse sand bed at tip of pen with pebbly silt laminae above and below; (d) bed of coarse sand between pebbly silt beds and aligned clasts at base of image.
