This section is taken from Bethan Davies’ PhD thesis.
Multiproxy analysis of glacigenic sediments? | Micromorphological interpretation of glacigenic sediments | References | Comments |
Multiproxy analysis of glacigenic sediments
Lodgement and Deformation tills
Thin-section analysis can be used in conjunction with macroscale sedimentological analysis to identify subglacial processes. Traditionally, tills have been subdivided based on the typical processes assumed to have been dominant in their formation. These were thought principally to be sliding1, lodgement2 and deformation3. Lodgement till has a long history of research, being originally defined by Chamberlin (1895) as,
Sediment deposited by plastering of glacial debris from a sliding glacier sole due to the combined effects of pressure melting and frictional drag4.
This process resulted in massive or fissile tills, with slickensides resulting from shearing (Boulton, 1970). Clasts are lodged into the substrate and have typical bullet-shaped ends and clear stoss-and-lee ends. Alternatively, deformation till5,6 refers to a,
Rock or sediment that has largely been homogenised by shearing in the subglacial layer.
Subglacial deformation of soft sediments is considered to account for much of the forward motion by glaciers3,7. Massive tills are thought to record evidence of high cumulative strains8. Others have argued that massive tills are simply the product of melt-out9. Larsen et al. (2004) argued that a melt-out / deformation continuum was responsible for thick sequences of massive tills, with vertical accretion of subglacial sediments being melted out at the ice-bed interface, and then deformed10. However, if deformation of soft beds is widespread, then deformation tills should be more prevalent (cf.11), and macroscopically massive ‘deformation’ tills often overly undeformed sediments12.
Glacier motion by sliding and lodgement over soft beds (cf.13) was thought to occur by the decoupling of the glacier from its bed due to increased basal water pressures, which would prevent the transmission of stress to the substrate1. This theory evolved into an proposal of ‘slip-stick’ sliding at the ice-bed interface, with areas of high water pressure inducing decoupling14.
Clay-rich tills are less permeable (cf. sandy tills), encouraging the development of high water pressures; stick-slip behaviour and decoupling may therefore be at least partially lithologically controlled15,16. Piotrowski and Kraus (1997) were among the first to propose a mosaic of sliding bed conditions and deforming conditions, where the ice is coupled to the bed. This explains the heterogeneity in tills in Germany17.
A continuum with lodgement and deformation end-members will lead to progressive changes in bed properties at a particular location18-20. This viewpoint highlights the spatial and temporal variability of glacier beds, with ice-bed coupling variability brought about by changes in pore-water pressure. Piotrowski et al. (2004) argued that the spatial variability in sliding intensity resulted in a mosaic with sliding conditions and deforming spots. During sliding, ploughing of clasts may take place21. ‘Glaciotectonite’ (as originally defined by Banham, 197722, and Pedersen, 198823) refers to sheared rocks and sediments, which still retains some of structural characteristics of its parent material6. They can display both brittle and ductile deformation, or a combination of the two processes.
Mass flow diamictons (or flow tills) originate from water and sediment released by ablation from debris-rich basal ice. These deposits are typically macroscopically massive. They may have microscopic near-horizontal laminations. Rotational structures, kinking plasmic fabrics, and tile structures form as primary depositional features on a micromorphological scale24. Debris flows share many characteristics in common with subglacial sediments, such as pressure shadows, folds, laminations, shears, faults, water-escape structures, and rotational structures25. Mass flow diamictons, however, can be distinguished by the presence of ‘tile structures’ in close association with rotational structures26. This long and extensive history of thin-section analysis of glacigenic sediments has given rise to a set of well-defined criteria, summarised in Table 1.
A combination of processes
Recently, several researchers have argued that processes at the ice-bed interface are a result of a continuum of processes, including melt-out, lodgement, deformation, and sliding16,20,27-29. It is therefore difficult to pin an exact genetic name onto a specific outcrop of diamicton. Evans et al. (2006) argued that the subglacial processes of deformation, flow, sliding, lodgement and ploughing all exist contemporaneously at the base of temperate ice.
These processes result in the mobilisation, transportation and deposition of sediment. This results in stratified or folded to texturally homogenous diamictons. Evans et al. (2006) argued that, while specific processes can and should be recognised in the sedimentary record, genetic ‘finger-printing’ of subglacial tills should be less process-specific.
Subglacial tills are polygenetic, and till classification must recognise the range of processes involved by the subglacial till ‘production continuum’16. Evans et al. (2006) proposed the use of the terms, ‘glaciotectonite’, as defined above, and ‘traction till’, which includes sediments deposited by sliding or deforming at the glacier bed, sediment released by pressure melting, and sediment homogenised by shearing. While ‘traction till’ is a generic term for all these processes, they can still be individually recognised in the geological record. Evans et al. (2006) also formally recognise ‘melt-out till’, as a sediment released by melting or sublimation at stagnant or slowly-moving, debris-rich ice, without subsequent transport or deformation.
Glaciotectonic deformation of subglacial sediments can result in tectonic laminations, which are distinct from glaciomarine or subaqueous laminations. Roberts and Hart (2005) identified two types of lamination. Type 1 laminations / stringers typically emanate from soft sediment clasts (e.g. chalk), are discontinuous, subhorizontal, and ungraded. In thin section, Type 1 laminations have sharp, undulatory contacts with silty or sandy stringers. Isoclinal folds are common30. Type 2 laminations are laterally continuous, subhorizontal, and poorly sorted with dropstone-like structures and often exhibit reworked soft sediment clasts. Contact boundaries are sharp and unconformable, and dropstone structures are evident. Microfabric birefringence is low, but there are some areas of high birefringence sub-parallel to silty lamination contacts30. Type 1 laminations are the result of subglacial deformation by ductile, inter-granular, pervasive shear. Hart and Roberts (1994) argued that this type of lamination occurs as a result of high extensional shear, leading to boudinage. Smaller, less competent perturbations, such as chalk clasts, can become stretched out into a lamination under this high shear strain. The laminations can deform and rotate within the deforming layer, producing tails to appear as sedimentary augens31.
Type 2 laminations have a subaqueous signal, despite often containing a number of syntectonic ductile deformation structures30. At West Runton, Norfolk, the subhorizontal lateral continuity and dropstone structures with down-warped lower contacts and draped upper contacts are indicative of primary subaqueous origin followed by secondary subglacial deformation. The planar, bedded nature of the strata, with sharp contact boundaries, are characteristic of sediments deposited by underflows, overflows, suspension fallout, ice-rafted debris processes and subaqueous debris flows, with each lamination representing a clear separate depositional event30. The clear characteristics of these laminations enable easy discrimination of glaciomarine and glaciolacustrine diamictons.
Glaciomarine and Glaciolacustrine diamictons
Several additional criteria can distinguish massive and laminated glaciomarine diamictons from subglacial diamictons (Table 1). Glaciomarine diamictons usually have a coarse, winnowed structure with dropstones; common, in situ marine microfossils; a lack of deformation structures; laminations, banding, or graded bedding structures; and a lack of plasmic fabric development30,32,33. Distal glaciomarine sediments are characterised by their bedding and lamination; a medium to fine matrix; uniform grain shapes; few deformation structures; dropstone features and no plasmic fabric development; and the presence of bioturbation32.
Glaciolacustrine sediments share many characteristics of glaciomarine diamictons; however, they lack in situ marine microfossils and are mostly geographically more limited in extent34. Menzies et al. (2006) argued that plasmic fabrics indicate the presence of orientated clay particles by strong birefringence. The type of plasmic fabric is indicative of a suite of orientations induced by ductile deformation.
Micromorphological interpretation of glacigenic sediments
Lodgement tills, formed in a high-strain environment with considerable shear and deformation, undergo complete homogenisation and demonstrate unistrial plasmic fabrics35. Subglacial traction tills can therefore have ductile, brittle, polyphase or intermediate structures (see Thin-Section Analysis). Ductile deformation structures include soft sediment pebbles (Type II and III), banding and flow of matrix material, rotational structures with associated skelsepic / masepic plasmic fabrics, and strain caps and pressure shadows. Planar features such as grain lineations are commonly associated with rotational structures, and occur in plastically deforming sediments36. Brittle deformation structures include edge-to-edge grain contacts and grain crushing, grain stacking, and brittle faulting and discrete shear zones. Grain stacks form to support stresses developing in a sediment, and form perpendicular to the stress field36.
Glaciomarine deposits are characterised by graded laminations, iceberg-dump and dropstone structures, and in situ marine microfossils. Porewater-induced soft-sediment deformation structures include liquefaction and homogenisation of sands, rafting, and water-escape structures.
Davies, B.J., 2009. British and Fennoscandian Ice-Sheet Interactions during the Quaternary, Unpubl. PhD Thesis. Department of Geography, Durham University, Durham, 502 pp.
Bethan Davies Thesis (Zipped PDFs – 70MB)
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