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Geological Techniques in Site Investigation Lecture 5

Geological Techniques in Site Investigation Lecture 5. CHARACTER OF GLACIAL TILL. Summary CHARACTER AND DESCRIPTION OF GLACIAL SEDIMENTS. Glacial debris Subglacial lodgement till Subglacial deformation products Flow tills and related deposits Systematic description of diamicton facies.

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Geological Techniques in Site Investigation Lecture 5

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  1. Geological Techniques in Site InvestigationLecture 5 CHARACTER OF GLACIAL TILL

  2. SummaryCHARACTER AND DESCRIPTION OF GLACIAL SEDIMENTS Glacial debris Subglacial lodgement till Subglacial deformation products Flow tills and related deposits Systematic description of diamicton facies

  3. Glacial debris

  4. Character of glacial debris • Derived from bedrock sources along the flow path by the processes of ‘quarrying’ and abrasion. • Ideal particle size curve is fractal (law of crushing) in a homogeneous, isotropic material. • In practice, the fabric and composition of the parent rock usually imposes significant deviations from this ideal.

  5. Tills from Aavatsmarkbreen, Spitsbergen Boulton & Paul 1976

  6. Benn & Evans 1998

  7. Character of glacial debris • In many cases the till has a polymodal size distribution that arises from the mineralogy of the substrate. • The sand sized distribution is usually based on lithic fragments and the silt sized distribution on characteristic (?terminal) grades for the individual minerals.

  8. Haldorsen 1981

  9. Benn & Evans 1998

  10. Character of glacial debris • Those clasts that have contacted the glacier bed have multiple facets formed by subglacial abrasion. Lodged clasts also have a characteristic truncated shape due to the distribution of internal stress during lodgement. • Sand- and silt-sized grains have characteristic surface textures that provide evidence of fracture and abrasion at small scales.

  11. Benn & Evans 1998

  12. Breidamerkurjökull Photo: M.A.Paul

  13. Buchananisen Spitsbergen Photo: M.A.Paul

  14. Subglacial lodgement till

  15. Sedimentology of subglacial till • Typically contains durable, exotic clasts and locally derived matrix. • Clast-matrix relation gives matrix-dominant vs clast-dominant classification. • This is widely used in engineering applications - there is a significant change in several properties at around 30% fines.

  16. Till from Cumbria Photo: J.W.Merritt

  17. Northumberland coast Photo: M.A.Paul

  18. Athabasca Glacier Canada Photo: M.A.Paul

  19. Sveg till Sweden Photo: M.A.Paul

  20. Sedimentology of subglacial till • Flowage and sorting by secondary processes leads to a wide variety of modifications. • Clasts may have a statistically preferred orientation (to be discussed later).

  21. Subglacial deformation products

  22. Deformation till • Deformation till (deforming bed till) is now argued to be an important genetic type • In the UK it occurs in several areas, notably Holderness, Norfolk and the Irish Sea margin. • This reflects the presence of rapid ice streams (possibly surge lobes) and poor subglacial drainage in these marine basins.

  23. Boulton et al. 1991

  24. Deformation till • Usually classified into glacitectonite and true deformation till, based on the original usage of Banham (1977) and modified by Benn and Evans (1995). • Evidence of deformation is provided by shear lamination, folding and rotation of deformed inclusions.

  25. Based on: Benn & Evans 1995

  26. Shear lamination indeformation till West Runton, Norfolk Photo: M.A.Paul

  27. Probable deformation till Holderness Photo: M.A.Paul

  28. Probable deformation till Holderness Photo: M.A.Paul

  29. Probable deformation till Holderness Photo: B.F.Barras

  30. Probable deformation till Cumbria Photo: J.W.Merritt

  31. Probable deformation till Cheshire Photo: M.A.Paul

  32. Probable deformation till Norfolk Photo: M.A.Paul

  33. Shear structures in deformation till West Runton, Norfolk Photo: M.A.Paul

  34. Probable deformation till Norfolk Photo: M.A.Paul

  35. Probable subglacial deformation Inverness area Photo: J.W.Merritt

  36. Probable subglacial deformation Inverness area Photo: J.W.Merritt

  37. Deformation till • Argued to be associated with high basal pore pressures in areas of low bed permeability and/or rapid sliding on low-competence subglacial layers. • This explains its presence in marine basins, which accumulate fine grained sediments during periods of high sea-level (interglacials).

  38. Flow tills and related deposits

  39. Flow tills and related deposits • Flow tills occur as successions of superimposed flows separated by sorted horizons and gravel lags. • The location and thickness of the successions is controlled by ice-cored ridges and the local drainage systems that form between them. • Often flow till successions are found as relatively thin (~2m) cappings to more substantial accumulations of glacifluvial deposits

  40. Aavatsmarkbreen Spitsbergen Photo: M.A.Paul

  41. Aavatsmarkbreen Spitsbergen Photo: M.A.Paul

  42. Flow tills and related deposits • The character of flow till is based on englacial debris, modified by flowage and other surface activity such as washing or ponding. • As a result, flow tills usually contain sorted horizons, pods and lenses and show a variety of basal relationships with underlying deposits. • They occur in complex facies associations due to the proximity of these differing processes.

  43. Flow till Kongsvegen, Spitsbergen Photo: M.A.Paul

  44. Aavatsmarkbreen Spitsbergen Photo: M.A.Paul

  45. Flow tills and related deposits • Flow till is of generally low density and may contain air bubbles that have been entrained during flow • Sorted inclusions often show considerable internal deformation • This is due to secondary flowage, often following from collapse during the melting of underlying ice.

  46. Probable flow till Cheshire Photo: M.A.Paul

  47. Probable flow till Inverness area Photo: J.W.Merritt

  48. Probable flow till Inverness area Photo: J.W.Merritt

  49. Flow till Kongsvegen, Spitsbergen Photo: M.A.Paul

  50. Systematic description of diamicton facies

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