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Chapter 6 Sediments and Sedimentary Rocks

GEOL 101 Introductory Geology. Chapter 6 Sediments and Sedimentary Rocks. Sediments. Collective name for loose, solid particles originated from weathering and erosion, thus Unconsolidated. Classification of sediments based on sizes Gravel > 2mm Sand between 1/16 and 2 mm

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Chapter 6 Sediments and Sedimentary Rocks

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  1. GEOL 101 Introductory Geology Chapter 6 Sediments and Sedimentary Rocks

  2. Sediments • Collective name for loose, solid particles originated from weathering and erosion, thus Unconsolidated. • Classification of sediments based on sizes • Gravel > 2mm • Sand between 1/16 and 2 mm • Silt between 1/256 mm and 1/16 mm • Clay < 1/256 mm

  3. Change from Sediments to Sed Rxs • Transportation • Roundness: round – distance from source • Sorting: well sorted - distance from source • Deposition: settle of particles • Preservation

  4. Change from Sediments to Sed Rxs • Lithification • Compaction – reduction in pore space • Cementation – seal out the pores • Crystallization – growth of crystals from solution.

  5. Sedimentation Process

  6. What is Sedimentary Rock? • Sedimentary rocks are products of mechanical and chemical weathering • Account for about 5 % (by volume) of Earth’s upper crust • Contain evidence of past environments • Provide information about sediment transport • Often contain fossils • Economic importance • Coal • Petroleum and natural gas • Sources of iron, aluminum, and manganese

  7. Sedimentary Rocks • Rock types • Classification • Sedimentary Environments • Fossils

  8. Sedimentary Rock Classification • Classified according to material type • Clastic • Chemical • Two major textures are used in the classification of sedimentary rocks • Clastic • Discrete fragments and particles • All detrital rocks have a clastic texture • Nonclastic • Pattern of interlocking crystals • May resemble an igneous rock

  9. Sedimentary RocksTypes • Sediment originates from mechanical and/or chemical weathering • Rock types are based on the source of the material • Clastic rocks – transported sediment as solid particles • Chemical rocks – sediment that was once in solution

  10. Detrital Sedimentary Rocks • The chief constituents of detrital rocks include: • Clay minerals • Quartz • Feldspars • Micas • Particle size is used to distinguish among the various types of detrital rocks • Particle shape and sorting give indication of transport distance (maturity)

  11. Particle Size Classification

  12. Sorting, Shape, Setting Sorting Sphericity Roundness Depositional Setting

  13. Clastic Sedimentary Rocks • Common Clastic sedimentary rocks (in order of increasing particle size) • Shale • Mud-sized particles in thin layers that are commonly referred to as laminea • Most common sedimentary rock • Sandstone • Composed of sand-sized particles • Forms in a variety of environments • Sorting, shape, and composition of the grains can be used to interpret the rock’s history • Quartz is the predominant mineral

  14. Detrital Sedimentary Rocks • Conglomerate and breccia • Both are composed of particles greater than 2mm in diameter • Conglomerateconsists largely of rounded gravels • Breccia is composed mainly of large angular particles

  15. Shale containing plant remains

  16. Sandstone

  17. Conglomerate

  18. Breccia

  19. Chemical Sedimentary Rocks • Consist of precipitated material that was once in solution • Precipitation of material occurs in two ways • Inorganic processes • Organic processes (biochemical origin) • Common chemical sedimentary rocks

  20. Chemical Sedimentary RocksCommon Types • Limestone • Dolostone • Chert • Evaporites • Coal

  21. Limestone • Most abundant chemical rock • Composed of calcite carbonate CaCO3 • Marine biochemical limestones form as coral reefs,coquina (broken shells), and chalk(microscopic organisms) • Inorganic limestones include travertine and oolitic limestone

  22. Coquina

  23. Fossiliferous Limestone

  24. Chemical Sedimentary Rocks • Dolostone • Composed of calcite, magnesium carbonate CaMgCO3 • Typically formed secondarily from limestone • Chert • Made of microcrystalline quartz • Varieties include flint , jasper and agate (banded form) • Evaporites • Evaporation triggers deposition of chemical precipitates • Examples include rock salt and rock gypsum

  25. Chert Agate – Banded Chert

  26. Coal • Common chemical sedimentary rocks • Different from other rocks because it is composed of organic material • Stages in coal formation (in order) • Plant material • Peat • Lignite • Bituminous

  27. Coal

  28. Coal Formation Stages

  29. Sedimentary Environments • A geographic setting where sediment is accumulating • Determines the nature of the sediments that accumulate (grain size, grain shape, etc.) • Identified by sedimentary facies, fossils, and structures

  30. Sedimentary Environments • Continental • Dominated by stream erosion and deposition • Glacial • Wind (eolian) • Marine • Shallow (< 200 meters) • Deep (seaward of continental shelves) • Transitional (shoreline) • Tidal flats • Lagoons • Deltas

  31. Depositional Environments

  32. Depositional Environments Marine Continental

  33. Turbidity Currents

  34. Turbidity Currents Graded Bedding

  35. Evaporite Formation

  36. Sedimentary Facies • Different sediments often accumulate adjacent to one another at the same time • Facies: possesses a distinctive set of characteristics reflecting the conditions in a particular environment • Merging of adjacent facies tends to be a gradual transition

  37. Sedimentary Facies

  38. Sedimentary Structures • Useful for interpreting sedimentary environment • Strata: or beds, layers of similar sediment (most characteristic of sedimentary rocks) • Lamina: thin layers (< 1cm) • Bedding planes separate strata • Cross-bedding • Graded beds • Ripple marks • Mud cracks

  39. Bedding

  40. Bedding Structures Graded bedding Lamina

  41. Cross Beds

  42. Ripple Marks unconsolidated sediment lithified rock

  43. Mud Cracks

  44. Grand Canyon o

  45. Fossils: Evidence of past life • By definition, fossils are the traces or remains of prehistoric life now preserved in rock • Fossils are generally found in sediment or sedimentary rock (rarely in metamorphic and never in igneous rock)

  46. Dinosaur footprint in limestone

  47. Fossils: Evidence of past life • Geologically fossils are important for several reasons • Aid in interpretation of the geologic past • Serve as important time indicators • Allow for correlation of rocks from different places • Studied in detail in Historical Geology (GEOL 102), offered in Spring 2004

  48. Chalk Cliffs of Dover, England Tiny planckon shells Deposited on sea floor Later uplifted

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