The effect of grain texture on the occurrence of microstructural properties in subglacial till

Hart, Jane K.; Khatwa, Anjana; Sammonds, Peter

doi:10.1016/j.quascirev.2004.06.006

Cited by 25 publications

(28 citation statements)

References 37 publications

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“…In the studied tills, there is a juxtaposition of rotational and shear line features. Hiemstra and Rijsdijk (2003) and Hart et al (2004) showed that where turbates occur throughout a sediment, without any apparent relation to shear planes, the sediment might represent a mass movement deposit in which the flow was the predominant deformation mode. Turbates may be considered diagnostic for a subglacial deformation where they are closely related to planar shear structures, although shearing may take place also during mass flows.…”

Section: Discussionmentioning

confidence: 99%

Micromorphology of modern tills in southwestern Spitsbergen – insights into depositional and post-depositional processes

Skolasińska¹,

Rachlewicz

Szczuciński³

2016

Polish Polar Research

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Textural properties and microstructures are commonly used properties in the analysis of Pleistocene and older glacial deposits. However, contemporary glacial deposits are seldom studied, particularly in the context of post-depositional changes. This paper presents the results of a micromorphological study of recently deposited tills in the marginal zones of Hansbreen and Torellbreen, glaciers in southwestern Spitsbergen. The main objectives of this study were to compare modern tills deposited in subglacial and supraglacial conditions, as well as tills that were freshly released from ice with those laid down several decades ago. The investigated tills are primarily composed of large clasts of metamorphic rocks and represent coarse-grained, matrix-supported diamictons. The tills reveal several characteristic features for ductile (e.g. turbate structures) and brittle (e.g. lineations, microshears) deformations, which have been considered to be indicative of subglacial conditions. In supraglacial tills, the same structures are common as in the subglacial deposits, which points to the preservation of the primary features, though the sediment was transferred up to the glacier surface due to basal ice layer deformation and redeposited as slumps, or to formation of similar structures due to short-distance sediment re-deposition by mass flows. This study revealed that it might not be possible to distinguish subglacial and supraglacial tills on the basis of micromorphology if the latter are derived from a subglacial position. The only noted difference was the presence of iron oxide cementation zones and carbonate dissolution features in supraglacial tills. These features were found in tills that were deposited at least a few years ago and are interpreted to be induced by early post-depositional processes involving porewater/ sediment interactions.

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Section: Discussionmentioning

confidence: 99%

Micromorphology of modern tills in southwestern Spitsbergen – insights into depositional and post-depositional processes

Skolasińska¹,

Rachlewicz

Szczuciński³

2016

Polish Polar Research

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“…Micro-morphological analyses of the Langj äokull sediment carried out by Hart et al (2004) clearly showed that the soil suŠered breakage of its clasts, with the grain size distribution ranging from large clasts to clasts that have been highly altered into clay-size aggregates during shear- Fig. 1.…”

Section: Langj äOkull Sedimentmentioning

confidence: 99%

“…Langj äokull sediment is a subglacial till of basaltic origin (Hart et al, 2004), which is usually made of detritus carried in the basal or debris layer part, at the bed-glacier interface, where severe particle interaction causes mutual attrition to particles is severe due to the high frequency of clast-to-clast contacts (Shilts, 1976). Shearing between clasts is dominant in this zone and drastically alters the physical characteristic of the debris during transport (Benn, 1995;Hart et al, 2004;Hooke and Iverson, 1995), resulting in a remoulded and homogeneous glacier bed with a wide range of particle sizes (Alley, 1991;Boulton and Hindmarsh, 1987;Hart and Boulton, 1991).…”

Section: Langj äOkull Sedimentmentioning

confidence: 99%

“…Shearing between clasts is dominant in this zone and drastically alters the physical characteristic of the debris during transport (Benn, 1995;Hart et al, 2004;Hooke and Iverson, 1995), resulting in a remoulded and homogeneous glacier bed with a wide range of particle sizes (Alley, 1991;Boulton and Hindmarsh, 1987;Hart and Boulton, 1991). The grain size distribution of the Langj äokull sediment is shown in Fig.…”

Section: Langj äOkull Sedimentmentioning

confidence: 99%

“…The soil samples were collected from the area exposed by the 1980's retreat at the Southern margin of the Vestari Hagafells glacier in Western Iceland (Theod áorsson, 1980), a temperate ice cap resting upon unfrozen deformable bed (Hart, 1995;Hart et al, 2004). A map showing the main features of the location as reported by Hart et al (2004) is given in Fig. 2.…”

Section: Samplingmentioning

confidence: 99%

See 2 more Smart Citations

On the Particle Size Distribution of a Basaltic Till

Altuhafi

Baudet

Sammonds

2011

Soils and Foundations

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Stress and strain history play a major role in deˆning theˆnal grading of a soil, and by studying how the particle size distribution of a soil evolves with stress and strain it may be possible to highlight some important aspects of the sediment past history. In this paper, the characteristics and evolution of the grading of a till from Iceland, so-called Langj äokull sediment, are examined. Specimens of the till were tested in compression (triaxial, oedometer tests) to very high pressures, and in shearing (triaxial, ring shear tests) to very high strains. The change in grading corresponding to particle breakage during compression and shearing was monitored using sieve analysis and scanning electron microscopy. The results show that the particle size distribution of the till, which is well graded, does not evolve when the till is subjected to high stresses or strains. Additional tests on specimens made of the larger-sized till particles did however show signs of breakage during testing. This suggests that the current natural grading of the till was achieved by incessant shearing during its past history, and corresponds to an ultimate,``critical'' grading. This is conˆrmed by the mineralogical analysis of the sediment, which highlights a single parent rock and weathering of the particles by mechanical processes only.

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

Larsen

Piotrowski

Menzies

2007

J Quaternary Science

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Microstructural analyses were used to investigate the formation of a macroscalemassive 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. 10 1 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.

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The effect of grain texture on the occurrence of microstructural properties in subglacial till

Cited by 25 publications

References 37 publications

Micromorphology of modern tills in southwestern Spitsbergen – insights into depositional and post-depositional processes

Micromorphology of modern tills in southwestern Spitsbergen – insights into depositional and post-depositional processes

On the Particle Size Distribution of a Basaltic Till

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

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