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Introduction to Sedimentary Structures - Part 1

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Geology 370: Sedimentology and Stratigraphy
Topic 3 Pt. 1: Sedimentary Structures

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Introduction to Sedimentary Structures - Part 1

  1. 1. Sedimentary Bedding and Structures Sedimentary structures are features found within or on the surface of a sedimentary bed that formed during or following deposition and provide information pertaining to depositional environment or burial history.
  2. 2. Photo by W. W. Little Bedding The most significant feature of sedimentary rocks is that they form layers. These layers are like the pages of a book and provide the history of the earth’s surface (stratigraphy).
  3. 3. Bedding Scale Scan from Stow Bed thickness is typically tied to depositional process and is, therefore, an indicator of depositional environment. To assist in describing sedimentary rock units, a formal classification of bed thickness has been established.
  4. 4. Photo by W. W Bed Nesting Often, thinner beds are nested within thicker beds, which, in turn, are nested within yet thicker beds, indicating multiple levels of depositional patterns.
  5. 5. Photo by W. W. Little Vertical changes in bed thickness are indicators of changes in depositional environments and can be indicative of the depositional system or of variations in base-level. Bedding Patterns
  6. 6. Photo by W. W. Little Massive Bedding
  7. 7. Bed Shapes Bed shape is an indicator of the depositional conditions under which the bed formed.
  8. 8. Photo by W. W. Little Tabular Beds
  9. 9. Photo by W. W. Little Wedge-shaped Beds
  10. 10. Lenticular Beds
  11. 11. Photo by W. W. Little Bedsets Typically, beds occur in sets in which the beds are genetically related to one another.
  12. 12. Heterolithic Bedding Heterolithic bedding refers to finely-interbedded grain sizes, such as sand and mud and can occur at a variety of scales.
  13. 13. Photo by W. W. Little Types of Sedimentary Structures • Bedforms • Surface markings • Sole marks • Biological structures • Soft-sediment deformation • Diagenetic structures • Unconformities
  14. 14. Photo by W. W. Little Bedforms Bedforms are produced as sediment actively accumulates during and following fluid flow and are characterized internally by a variety of sedimentary structures, such as cross-bedding.
  15. 15. Lower Flow Regime Bedforms Flow-regime bedforms are those that are produced by a moving, non- viscous fluid, such as water or wind. Bedform size tends to increase with increasing flow rate. Flow regime is also influenced by grain size.
  16. 16. Ripple Classification (size) Terminology is not completely fixed, but ripples give way to dunes with increasing flow velocity. Some schemes have an transitional form, referred to as sand waves. Ripples are 0.5 to 3.0 cm in height with wavelengths of 5 to 40 cm. They are typically found under low to moderate flow velocities in sand that is less than 0.7 mm in diameter. Dunes are over 3.0 cm in height with wavelengths of at least 40 cm. They typically form under moderate to high flow velocities in relatively deep water and sand that is more than 0.2 mm in diameter. Dune height and spacing is related to water depth. Both ripples and dunes tend to be straight- crested under lower flow velocities and sinuous under higher velocities. Another classification uses the terms microforms (e.g. ripples), mesoforms (e.g. dunes), and macroforms (e.g. bars).
  17. 17. Ripple Classification (morphology) Morphological ripple classification is based on plan geometry and increases in complexity with shallower depths and higher flow velocities.
  18. 18. Wave-generated RipplesCurrent-generated Ripples Ripple Classification (process) Ripple classification is based on plan morphology as related to interpreted process of formation.
  19. 19. Photo by W. W. Little Asymmetrical (Current) Ripples
  20. 20. Photo by W. W. Little
  21. 21. Photo by W. W. Little
  22. 22. Photo by W. W. Little Symmetrical (Oscillation) Ripples Symmetrical ripples are typically produced by oscillatory motion of waves. In addition to their symmetry, they can often be distinguished from current-formed ripples by bi-directionally-dipping cross-laminae.
  23. 23. Photo by W. W. Little
  24. 24. Photo by W. W. Little
  25. 25. Photo by W. W. Little
  26. 26. Photo by W. W. Little Interference Ripples Surface ponding can lead to the development of interference ripples.
  27. 27. Photo by W. W. Little Megaripples
  28. 28. Hummocky (Cross) Bedding (HCS) Hummocky cross-bedded sand is produced mostly on the shallow sea floor during storms by a combination of current and oscillatory flow, resulting in aggradation of mounds and swales, mostly from vertical accretion.
  29. 29. Photo by W. W. Little
  30. 30. Photo by W. W. Little
  31. 31. Hummocky Cross-bedding Animation http://walrus.wr.usgs.gov/seds/bedforms/
  32. 32. Photo by W. W. Little Cross-bedding Crossbedding is layering that dips between the upper and lower boundaries of a sedimentary bed and is formed by moving water or wind. It can be used to determine water depth, fluid velocity, and flow direction.
  33. 33. Cross-bed Formation Ripples are characterized internally by cross-bedding. Cross-beds are formed in fluid flow as sediment is eroded from and transported up the relatively gentle stoss side of a ripple and deposited as avalanches on the steeper lee side. Cross-beds form in both aqueous and eolian environments.
  34. 34. Flow Separation In eolian deposits, as sediment reaches the dune crest, courser grains avalanche down the lee face; whereas, finer particles blow across to the top of the stoss slope of a leading dune.
  35. 35. Cross-bed Sets Cross-bed sets form as one ripple migrates over another. A single layer of cross-bedding is a “set.” Multiple layers are “co-sets.”
  36. 36. Cross-bed Set Animation http://walrus.wr.usgs.gov/seds/bedforms/
  37. 37. Photo by W. W. Little Trough Cross-bedding Trough cross-bedding clearly flattens toward the base in longitudinal profile and forms “trough” shapes in transverse sections. Trough cross-stratification is produced by sinuous-crested ripples.
  38. 38. Photo by W. W. Little
  39. 39. Photo by W. W. Little
  40. 40. Trough Cross-bedding Animation 1 http://walrus.wr.usgs.gov/seds/bedforms/
  41. 41. Trough Cross-bedding Animation 2 http://walrus.wr.usgs.gov/seds/bedforms/
  42. 42. Photo by W. W. Little Planar (tabular) Cross-bedding Planar cross-bedding flattens little or none toward the base in longitudinal profile and forms apparent planar bedding in transverse sections. Planar cross-stratification is produced by straight-crested ripples.
  43. 43. Photo by W. W. Little
  44. 44. Photo by W. W. Little
  45. 45. Planar Cross-bedding Animation http://walrus.wr.usgs.gov/seds/bedforms/
  46. 46. Flaser-bedding
  47. 47. Photo by W. W. Little
  48. 48. Photo by W. W. Little
  49. 49. Climbing Ripples The angle of climb between ripple sets increases with the rate of deposition. Very high depositional rates result in climbing ripples.
  50. 50. Climbing Ripple Animation http://walrus.wr.usgs.gov/seds/bedforms/
  51. 51. Photo by W. W. Little Herring Bone Cross-bedding
  52. 52. Photo by W. W. Little
  53. 53. Photo by W. W. Little Transitional Flow-Regime Planar Beds At the boundary between lower and upper flow regime (Fr = 1), “washed-out ripples” or planar beds are produced. Sand greater than 0.7 mm in diameter can form similar structures under lower flow regime conditions.
  54. 54. Photo by W. W. Little
  55. 55. Photo by W. W. Little Parallel Laminations in Sandstone
  56. 56. Photo by Clark Little
  57. 57. Photo by Clark Little
  58. 58. Photo by Clark Little
  59. 59. Photo by W. W. Little Swash Deposits
  60. 60. Photo by W. W. Little
  61. 61. Photo by W. W. Little Antidunes Under upper flow regime conditions bedforms accrete on the upstream side and erode at the downstream end, forming antidunes.
  62. 62. Antidune Cross-bedding Animation http://walrus.wr.usgs.gov/seds/bedforms/
  63. 63. Antidune – Standing Wave
  64. 64. Antidune – Breaking Wave
  65. 65. Photo by W. W. Little
  66. 66. Photo by W. W. Little Channel Forms
  67. 67. Abandoned Channel Fill
  68. 68. Photo by W. W. Little
  69. 69. Photo by W. W. Little Lateral Accretion Surfaces
  70. 70. Photo by W. W. Little Scroll Bars
  71. 71. Photo by W. W. Little
  72. 72. Cut-and-Fill Structures
  73. 73. Photo by W. W. Little Parallel Laminations in Mudstone
  74. 74. Suspended Sediment
  75. 75. Photo by NASA

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