Sedimentary rocks and sediments

• Sediment


  Loose and unconsolidated material that forms at or near the Earth's surface

  
• Lithification


  The processes by which sediment is converted into sedimentary rock

  
• Sedimentology


  Study of sediments/sedimentary rocks and the processes that form them

  
• Sedimentary petrology


  The branch of geology concerned with the study of sedimentary rocks
• Sedimentary basins


  • Subsiding areas of the Earth's crust in which sediment accumulates
  • Examples: Rift basins (e.g. East African Rift), Continental basins (e.g. Congo Basin), Continental margin basins (passive margins)

  
• Sedimentary rocks constitute ~7% by volume of the Earth's crust but cover 75% of the surface area

  
• Importance of sedimentary rocks

  • Derived from material deposited on or near the Earth's surface under "everyday" conditions
  • They contain the entire fossil record; preserve evidence for biological evolution
  • Preserve evidence for the past climates and for the physical, chemical and biological conditions of ancient terrestrial and marine environments
• Sedimentary rocks are arranged in layers (beds) and thus provide a geochronological record (relative age) of the events and processes that formed the layers

  
• Fossil fuels (oil, natural gas and coal) are found only in sedimentary rocks
• Sedimentary rock types


  • Clastic sedimentary rocks
  • Biogenic (Biochemical) sedimentary rock
  • Chemical sedimentary rock
  • Organic sedimentary rock

  
• Clastic sedimentary rocks


  Detritus held together by natural cement
• Clastic sedimentary rock types

  • Siliciclastic sedimentary rock (made up of grains of silicate minerals, mainly quartz)
  • Volcaniclastic sedimentary rock (dominated by detritus of volcanic origin; ash and lapilli)
  • Bioclastic sedimentary rock (detritus is broken and fragmented skeletons of animals)

  
• Biogenic (Biochemical) sedimentary rock


  Produced directly by the activity of organisms (e.g. limestone and chert)

  
• Chemical sedimentary rock


  Produced by precipitation of minerals directly from water (e.g. evaporite deposits, travertine CaC03 at hot springs and in caves)

  
• Organic sedimentary rock


  Consisting of carbon rich material; the fossil fuels (plant remains in coal, organic chemicals (lipids) in organic shale)
• Characteristics of clastic sediment

  • Texture (grain size, sorting, roundness, grain fabric)
  • Grain size is a function of current strength: the stronger the current the larger are the particles deposited

  
• Sorting


  • The spread of grain size distribution
  • Poorly sorted sediments consist of grains with a large variation in size
  • Well sorted sediments have grains that tend to be more or less the same size

  
• Sorting and depositional mechanism

  • Sediments deposited from glaciers are poorly sorted
  • Sediments deposited by fluvial processes (rivers/streams) are reasonably well sorted
  • Sediments deposited by aeolian (wind) processes are the best sorted

  
• Roundness


  • A function of the number of corners a grain possesses
  • Degree of roundness increases with transport distance due to abrasion of particles during transport
• Fabric


  The spatial arrangement of grains, i.e. the position of grains relative to one another (e.g. preferred orientation, imbrication)
• Formation of clastic sedimentary rocks


  1. Weathering

  2. Erosion

  3. Transportation

  4. Deposition

  5. Lithification (compaction and cementation)

  
• Diamictite


  • Matrix supported conglomerate with muddy matrix
• Tillite


  • A diamictite of glacial origin
• Sandstones


  • Sedimentary rocks with a grain size between 0.063 - 2 mm (i.e. sand-sized)
  • The dominant component is quartz, but feldspar and lithic grains (rock fragments) are also important

  
• Mudrocks


  • Fine-grained sedimentary rocks made up of silt and clay (i.e. <0.063 mm in diameter)
  • Siltstone - made up of silt-sized grains
  • Mudrock - made up of clay-sized particles
  • Mudstone - massive mudrock
  • Shale - mudrock with well-developed lamination (fissility)

  
• Mudrocks - colours


  Linked to the amount of organic carbon present and the oxidation state of iron
• Carbonates


  • Biogenic/biochemical rocks composed of calcite (which makes up limestone) and dolomite
  • Limestone is deposited in clean, warm, shallow-water marine environments (tropical climate)
  • Dolomite is not formed by marine organisms, but forms during diagenetic replacement of calcite

  
• Evaporite deposits


  • Anhydrite, gypsum and halite