Geology 101 - Lecture 12

(Brief Outline Only)

I. Sedimentary rocks - two types

• A. Clastic or detrital sedimentary rocks
  • Particle size depends on energy of environment
    Coarsest gravels - found closest to source
    
    
  • These sediments are the products of physical (mechanical) weathering
    
    
  • Clastic Sediments can be described by
    • Degree of sorting
    • Degree of rounding
    
    
  • Classification of sediments and sedimentary rocks
  
  
• B. Chemical Sedimentary rocks: precipitation from a solution
  
  
  • Derived from products of chemical weathering (materials in solution)
    
    
  • Two types - inorganic and organic chemical sedimentary rocks
    
    
    1. Inorganic chemical sedimentary rock
    a. Evaporites such as rock salt, formed by evaporation of water
    
    b. Direct precipitates of calcite (CaCO3) from water
    
    i. stalactites in caves
    ii. travertine in thermal hot springs
    iii. Tufa towers in Mono Lake
    
    c. Direct precipitates of SiO2 from water (i.e., chert)
    
    2. Organic chemical sedimentary rock
    a. Limestone
    
    • Shellfish precipitate calcite shells
    • Coral secretes calcareous exoskeleton
    • Chalk (microscopic marine organisms)
    
    b. Chert
    • Diatoms and radiolaria precipitate SiO2 shells
      
      
    c. Other organic sediment
    • Coal

II. Sedimentary Environments

• Terrestrial and marine - see text

III. Sedimentary Structures

• strata or bedding
• bedding planes
• cross bedding
• graded bedding As sediments move from high energy to low energy environment, the heaviest (largest) particles settle out first, and the lighter ones (smaller) settle out later
• ripple marks
• muccracks

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