Easc116 Ch. 6 - Sedimentary Rocks

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Easc116 Ch. 6 - Sedimentary Rocks

  1. 1. CH. 6 – SEDIMENTARY RX
  2. 2. Sediment: - weathering products (gravel, sand, clay minerals) - chemical precipitates - organic remains
  3. 3. Sedimentary Rocks “sedimentum” = “settling” Sediment settles out of wind or water - forms layers at the surface
  4. 4. Importance of Sedimentary Rx 1) Form 75% of exposed rx at surface (outcrops) - compose only 5% of total crust 2) Contain clues to reconstruct past environments on Earth
  5. 5. Importance of Sedimentary Rx 3) Fossil record 4) Economic value - coal, petroleum, construction
  6. 6. Sedimentary Rx Classification 1) Detrital = weathering products (mechanical or chemical) Named based on particle size
  7. 7. Detrital Sedimentary Rocks Particle Size Gravel Name Breccia (angular) Conglomerate (rounded)
  8. 8. Detrital Sedimentary Rocks Particle Size Sand Name Quartz Sandstone (quartz sand) Arkose (K-spar)
  9. 9. Detrital Sedimentary Rocks Particle Size Sand Name Graywacke (rock frags, mafic minerals)
  10. 10. Detrital Sedimentary Rocks Particle Size Clay-size Name *Shale (clay minerals) *Shale is the most abundant detrital sedimentary rock
  11. 11. Detrital Rock Process 1) Weathering 2) Erosion (transportation) 3) Deposition (sediment settles) 4) Lithification – turning sediment into rock (“lithos” = rock) (after sediment is buried)
  12. 12. Types of Lithification Processes a) Compaction - most effective on fine-grained rx (ex: shale) Exception: St. Peters Sandstone Starved Rock State Park, IL
  13. 13. Lithification Processes b) Cementation Minerals dissolved in groundwater precipitate around sediment Ex: silica, calcite, hematite
  14. 14. Sedimentary Rx Classification 2) Chemical – named based on composition Ex: calcite = limestone halite = rock salt plant remains = coal
  15. 15. Chemical Sedimentary Rocks a) Inorganic – formed by chemical reactions in environment Ex: evaporites – rock gypsum, rock salt Ex: Travertine (limestone)
  16. 16. Inorganic Chemical Sed. Rx Calcite: *Limestone = mostly marine origin Travertine = speleothems Oolitic Limestone = tidal flats * Most abundant chemical sedimentary rock
  17. 17. Inorganic Chemical Sed. Rx Silica (microcrystalline quartz): Varieties: chert agate
  18. 18. Silica (microcrystalline quartz): Varieties: Jasper Petrified Wood
  19. 19. Chemical Sedimentary Rx b) Organic (“biochemical”) Plant remains = coal Microscopic sea shells = chalk Broken sea shells = coquina limestone
  20. 20. Detective Analogy Geologists are solving a mystery Scene of the Crime Clues Mystery Solved
  21. 21. Mystery Solved (Sed. Rx) Depositional environment - any area on surface where sediment accumulates Ex:
  22. 22. Principle of Uniformitarianism “Present is the key to the past” Study modern depositional environments for clues Ex: coquina limestone Ex: ripple marks
  23. 23. Sedimentary Facies Cumulative physical characteristics of a sedimentary rock which indicates its depositional environment “Clues” in detective analogy
  24. 24. Principle of Lateral Continuity Nicolaus Steno (1669) Sediment is accumulated to edge of its depositional environment
  25. 25. Sedimentary Facies Note: At the same time, each environment (facies) is accumulating its own sediment and characteristics!
  26. 26. Sedimentary Facies
  27. 27. Clues 1) Sediment size Distance deposited from source area Large = (closer to, further from) Small = (closer to, further from)
  28. 28. Clues (sediment size) High vs. low energy environment Large particles = (high, low) Ex: ___________ Small particles = (high, low) Ex: ___________
  29. 29. Clues 2) Sediment shape Distance deposited from source area: close to or far from? angular = ______ rounded = ______
  30. 30. Clues 3) Composition – minerals present Two most common minerals found in sedimentary rocks are _____ and ______ .
  31. 31. Variety of minerals: Ex: quartz, K-spar, plagioclase feldspar, mafic minerals, rock fragments - very little chemical weathering - indicates rapid deposition close to source area Ex: arkose in alluvial fan
  32. 32. One mineral composition: Well-sorted - sediment traveled far from source area Ex: Quartz sandstone, shale
  33. 33. Clues 4) Sedimentary Structures - features produced in sediment BEFORE it’s lithified
  34. 34. Sedimentary Structures a) Bedding – layering w/in rock Bedding plane – flat surfaces along which rx tend to break or separate - separates different episodes of deposition
  35. 35. Bedding i) Laminar bedding – horizontal layers - usually deposited in calm energy environment Ex: shale, some sandstones
  36. 36. Bedding ii) Graded bedding – sorted by size - occurs w/sudden decrease in velocity Ex: turbidity currents, alluvial fans
  37. 37. Bedding iii) Cross-bedding (Figure 6.22) - layers are angled in sets - occurs with change of current direction (either wind or water)
  38. 38. Sedimentary Structures b) Ripple marks - wave action moves sediment - shallow water
  39. 39. Types of Ripple Marks i) Asymmetrical - also called “current ripple marks” - used to determine paleocurrent directions
  40. 40. Types of Ripple Marks ii) Oscillation (symmetrical) - wave action is back and forth - usually indicates shallow lagoon environment
  41. 41. Sedimentary Structures c) Mudcracks Environmental conditions: a) b) Ex:
  42. 42. Sedimentary Structures d) Trace Fossils Any evidence an organism was in the environment but no physical remains of the organism exist
  43. 43. Trace Fossils Examples: i) Footprints ii) Coprolites iii) Bioturbation
  44. 44. Clues 5) Fossils – evidence of prehistoric life Fossil record is incomplete - shows remarkable pattern of change from simple to complex life forms
  45. 45. Steps to becoming a fossil Step 1: Death Step 2: Hard parts – bones, teeth, shells (Who you are!) Step 3: Rapid burial (Where you die!) Step 4: Time (prehistoric)
  46. 46. Fossils Fossil record biased towards marine environment - abundant life in oceans - many critters have hard parts - lots of sediment for quick burial
  47. 47. Fossils Good evidence for reconstructing past environments Ex: Beach sandstone vs. desert sandstone
  48. 48. Clues 6) Color Black or green = unoxidized iron - lack of oxygen in environment Ex: deep ocean, deep lake, swamp
  49. 49. Color Red = oxidized iron Ex: river floodplain, tidal flats, desert deposits Green/purple = volcanic ash mixed in with sediment
  50. 50. Clues 7) Geometry of rock unit - shape & thickness of rock unit Ex: beach sandstone vs. desert sandstone
  51. 51. Depositional Environments Handout
  52. 52. Lithofacies map For a particular moment in geologic time, rocks indicate the landscape 1) Map out the areas where each rock type is found & draw boundaries
  53. 53. Lithofacies map 2) Look at the lithologies (rock types) & compare them to: - nearby rx - geometry of rock unit 3) Determine the original depositional environment
  54. 54. Cyclothems Cyclical alternations of lithologies deposited as shorelines shift
  55. 55. Sedimentary Facies
  56. 56. Stratigraphic Sequences 1) Transgressions - sea level rises Strat column changes vertically from coarser sediment to finer sediment “Fining-upward” sequence
  57. 57. Stratigraphic Sequences 2) Regressions – sea level drops Strat column changes vertically from finer sediment to coarser sediment “Coarsening-upward” sequence
  58. 58. Color Red = oxidized iron Ex: river floodplain, tidal flats, desert deposits Green/purple = volcanic ash mixed in with sediment

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