Sedimentary Rocks Formation and Classification

Formation of Sedimentary Rocks

  • Sedimentary rocks are formed at or near the Earth's surface.
  • They are composed of sediments, which can be commonly observed around us (e.g., mud, sand, gravel).

Definition of Sediments

  • Sediment: A naturally occurring unconsolidated material consisting of loose fragments of bedrock, minerals, shells, and crystals that precipitate from water.
  • Examples include:
    • Mud
    • Sand
    • Gravel

Processes Leading to Sedimentary Rock Formation

  • Sediments can be deposited in various environments.
  • After deposition, sediments consolidate into sedimentary rock via diagenesis, which involves:
    • Compaction: Pressure squeezes sediments closer together to expel water.
    • Cementation: Formation or precipitation of minerals binding sediments together, leading to lithification, the process of turning sediments into rock.

Influence of Depositional Environment

  • The environment of deposition significantly affects the resulting sedimentary rock type.
    • Different environments yield different characteristics in sedimentary rock.

Categories of Sedimentary Rocks

Two Main Categories

  1. Detrital (Clastic) Rocks
  2. Chemical/Biochemical Rocks

Detrital (Clastic) Rocks

  • Composed of sediments derived from weathered rocks.
  • Types are classified by grain size:
    • Conglomerate:
    • Composition: Rounded pebbles.
    • Breccia:
    • Composition: Angular rubble fragments.
    • Sandstone:
    • Grains size between 0.06 mm and 2 mm.
    • Mudstone:
    • Grains size smaller than 0.06 mm.
    • Siltstone:
    • Similar to mudstone but silt sized sediments may be visible under magnification.
    • Shale:
    • Mudstone with well-defined thin layers that split easily.

Further Classification of Sandstones

  • Arco Sandstone:
    • Contains > 25% potassium feldspar or orthoclase.
    • Indicates less weathering and proximity to source rock.
  • Lithic Sandstones:
    • Composed of rock fragments; poorly sorted and rounded, indicating rapid deposition.
  • Quartz Sandstones (Quartz Arenite):
    • Mainly made of quartz, indicating high weathering stability.
  • Graywacke:
    • Composed of > 15% fine-grained matrix (silt/mud).
    • Characteristics include poor sorting and high mud content, often found offshore.

Chemical/Biochemical Sedimentary Rocks

  • Formed through the precipitation of minerals from solutions; further categorized into four types:
    • Carbonates:
    • Rocks that contain the carbonate anion ext{CO}_3.
    • Composition: Usually calcite (limestones) or dolomite (dolostones).
    • Limestones can have both chemical and biochemical origins.
    • Evaporates:
    • Formed from precipitation of salts from water, often seawater.
    • Common minerals include:
      • Gypsum
      • Anhydrite
      • Halite (rock salt).
    • Cherts:
    • Composed of microcrystalline quartz.
    • Can be chemical (precipitated from seawater) or biochemical (formed from tiny plankton skeletons).
    • Coal:
    • Formed from organic plant remains, predominantly composed of carbon.
    • Forms in anoxic environments, usually swamps.
    • Affected by compression and heating:
      • Low compression: Coal ball with lower carbon, preserving plant remains.
      • Higher compression: Higher carbon percentage, less preservation of plant remains, more suited for heating purposes.

Summary

  • Understanding sedimentary rock types involves recognizing the role of sediment deposition environments and the classification of rocks based on composition and characteristics of sediments.