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Sediments and Sedimentary Rocks: Classification and Formation Processes in Geology

This chapter provides an overview of sediments and sedimentary rocks, focusing on their formation, classification, and significance in geology. Sediments are loose, solid particles produced by weathering and erosion and categorized by size into gravel, sand, silt, and clay. Understanding the transition from sediments to sedimentary rocks involves concepts of transportation, deposition, lithification, and various processes like compaction and cementation. Additionally, the chapter covers sedimentary environments and the economic importance of these rocks, including fossil fuel sources and mineral deposits.

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Sediments and Sedimentary Rocks: Classification and Formation Processes in Geology

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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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