Coarse and medium grained clastic sedimentary rocks

Breccia, conglomorate, sandstone, orthoquartzite, desert sandstone, arkose, greywacke

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  • Coarse and medium grained clastic sedimentary rocks
    • Breccia
      • Coarse grained clastic, angular rock, poorly sorted
      • Coarse clasts often mixed with finer grains and set in matrix
      • Commonly form as scree, alluvial fans and wadi deposits
    • Conglomerate
      • Coarse grained clastic, rounded rock
      • Poorly sorted, Large clasts surrounded by a finer grained matrix
      • Commonly form as beach and river channel deposits
    • Sandstones
      • May be well cemented meaning low porosity or poorly cemented meaning high porosity
      • Main component is quartz but also minerals low on Bowen's R S
      • Found in virtually all sedimentary environments
    • Orthoquartzite
      • A Sandstone, consists of only quartz grains held by quartz cement
      • Grains are well sorted and well rounded
      • Commonly form in beach and shallow marine deposits
    • Desert sandstone
      • Red in colour due to iron oxide coating the quartz grains
      • Very well sorted, very well rounded, medium sand-sized grains having high sphericity
      • Commonly form in arid environments as wind blown sands
      • Clasts cemented together by silica or iron minerals
    • Arkose
      • Medium to coarse grained sandstone containing at least 25% K feldspar
      • Other main mineral is quartz though fragments of rock and mica may be present
      • Grains are sub-angular to sub-rounded
      • Commonly formed in alluvial fan environments in arid areas
    • Greywacke
      • Fine to coarse in grain size
      • Dark coloured and poorly sorted with angular to sub-angular clasts
      • Clasts usually consist of rock fragments with some quartz and K feldspar
      • More than 15% of it is clay matrix
      • Commonly form as turbidite deposits and often show graded bedding


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