chapter 4

• Sedimentology

  The study of sediments and sedimentary rocks
• Sediments


  Accumulations of detrital (loose) material derived from the decomposition and weathering of rocks exposed at the earth's surface
• Sediment transportation and deposition

  Detrital material transported by the agents of erosion (wind, water, gravity, glaciers) from topographically high lying regions to lower lying regions (sedimentary basins) where it accumulates in sequences of layers (beds) called sedimentary successions
• Geomorphology


  The shape of the earth's surface, which is controlled by tectonism and the processes of erosion and deposition
• Lithification


  The process by which sediments become hardened to form sedimentary rock
• Diagenesis


  The process by which sediments become hardened to form sedimentary rock
• This chapter deals with the processes of weathering, erosion, deposition, lithification, as well as the description and classification of sedimentary rocks
• Weathering


  The process by which rocks decompose
• Chemical weathering


  1. Hydrolysis

  2. Leaching

  3. Oxidation and hydration
• Weathering begins on the exposed surfaces of rocks, so it forms a rind of weathered products which coat the rock
• Mechanical weathering


  1. Action of running water

  2. Gravity

  3. Glacial movement

  4. Expansion and contraction associated with heating and cooling

  5. Freezing and thawing

  6. Growth of plant roots

  7. Burrowing action of animals

  8. Farming and civil engineering efforts of man
• Regolith


  The zone or layer of loose weathered material near the surface
• Soil


  Consists of the products of weathering including clay minerals, hydroxide minerals, and minerals that survive the weathering process
• Minerals resistant to weathering

  • Quartz
  • Ilmenite
  • Garnet
  • Rutile
  • Monazite
  • Zircon
  • Gold
  • Diamonds
• Bowen's reaction series


  Lists minerals in terms of their temperatures of crystallisation from a magma, which is related to their atomic structures and bonding and thus their weathering resistance
• Mineral stability during weathering

  • Olivine - very unstable
  • Pyroxene - moderately stable
  • Amphibole - moderately stable
  • Muscovite - moderately stable
  • Microcline - very stable
  • Quartz - very stable
  • Fe and Al oxides and hydroxides - very stable
• Quartz


  • Hard (7.0)
  • Resilient
  • Low density (2.65)
• Microcline


  • Hard (6.0-6.5)
  • Resilient
  • Low density (2.56-2.63)
• Muscovite


  • Soft (2.5-3.0)
  • Friable
  • Low density (2.76-3.00)
• Ilmenite


  • Hard (5.0-6.0)
  • Resilient
  • High density (4.50-5.00)
• Soil horizons


  • A-horizon (well developed clay-rich soil and organic material)
  • B-horizon (clay-rich soil material without organic material)
  • C-horizon (partly weathered rock material)
  • D-horizon (solid bedrock)

  
• Soil profiles can be seen in excavations, gulleys etc. and their distribution should cover all possible variations within the site area

  
• Soil profile


  A vertical section through the soil that records the top and bottom of each significant horizon

  
• Properties used to describe soil horizons

  • Moisture content
  • Colour
  • Consistency
  • Structure
  • Soil texture
  • Origin of the soil

  
• Moisture content


  Dry, slightly moist, moist, very moist, wet
• Colour


  Indicator of chemical and mineralogical processes associated with iron compounds
• Consistency categories for cohesive soils

  • Very soft
  • Soft
  • Firm
  • Stiff
  • Very stiff
• Consistency categories for cohesionless soils

  • Very loose
  • Loose
  • Medium dense
  • Dense
  • Very dense
• Soil structure


  • Intact (no joints)
  • Fissured (closed joints)
  • Slickensided (polished joint surfaces implying past movement)
  • Shattered (open, air-filled joints)
  • Micro-shattered (soil breaks away in crumbs)
• Soil texture


  • Gravel (>2mm)
  • Sand (0.06-2mm)
  • Silt (0.002-0.06mm)
  • Clay (<0.002mm)

  
• Genetic categories of soil

  • Rock
  • Weathered rock
  • Residual soil
  • Transported soil
  • Pedocrete
• Transported soils may overly unweathered rock or residual soils, with the contact normally marked by a gravel layer called the pebble marker
• Residual soil categories


  • Residual felsic igneous rock
  • Residual mafic igneous rock
  • Residual calcareous rock
  • Residual argillaceous rock
  • Residual arenaceous rock
• Residual felsic igneous rock

  Residual granite

  
• Residual felsic igneous rock

  • Clayey sand
  • Collapsable grain structure, high erodibility

  
• Residual mafic igneous rock


  Residual dolerite
• Residual mafic igneous rock

  • Clay
  • Heave compressibility

  
• Residual calcareous rock


  Residual dolomite
• Residual calcareous rock


  • Chert rubble
  • Sinkholes
• Residual argillaceous rock


  Residual shale