Formation and Classification of Sedimentary Rocks

Introduction to Sedimentary Rocks

  • Transition from igneous rock formation to sedimentary rock formation.
  • Sedimentary rocks form at or near the Earth's surface.
  • Definition of sediment:
    • "A naturally occurring unconsolidated material consisting of loose fragments of bedrock, minerals, shells, and crystals that precipitate from water."
    • Common examples: mud, sand, gravel.

Formation of Sedimentary Rocks

  • Process of Formation:
    • Sediment deposition occurs in various environments.
    • Consolidation into sedimentary rock occurs through:
    • Diagenesis: The process that transforms sediments into solid rock.
    • Compaction: Pressure squeezes sediments closer together, eliminating water.
    • Cementation: Minerals precipitate and bind the sediments together.
    • Lithification: The entire process of forming rock from sediments.

Types of Sedimentary Rocks

  • Main Categories:
    • Detrital (Clastic) Rocks
    • Chemical (Biochemical) Rocks

Detrital (Clastic) Rocks

  • Composed of sediments derived from weathered rocks, such as gravel, sand, and mud.
  • Classification Based on Grain Size:
    • Conglomerate: > 2 mm (rounded pebbles).
    • Breccia: > 2 mm (angular rubble fragments).
    • Sandstone: 0.06 mm - 2 mm (smaller grains).
    • Mudstones and Siltstones: < 0.06 mm (finest grains).
    • Siltstone: May be visible under a hand lens or microscope.
    • Mudstone or Claystone: Even finer particles; shales defined as having thin layers that split easily.
Sandstone Classifications
  • Arco Sandstone: > 25% potassium feldspar or orthoclase.
    • Indicates less weathering (feldspars weather more than quartz).
    • Found close to source rock or in dry areas.
  • Lithic Sandstone: Contains rock fragments; generally poorly sorted and rounded, indicating rapid deposition.
  • Quartz Sandstone: Primarily quartz, signifies extensive weathering (quartz is stable and weathers slowly).
  • Graywacke: > 15% fine-grained matrix (often silt or mud); indicates high mud content and poor sorting, typically found offshore.

Chemical and Biochemical Rocks

  • Formed by the precipitation of minerals from solutions.
  • Categories of Chemical and Biochemical Rocks:
    • Carbonates: Contain the carbonate anion ($CO_3^{2−}$).
    • Limestones: Composed mainly of calcite; can have both chemical and biochemical origins.
    • Dolostones: Composed mainly of dolomite.
    • Evaporates: Form from the precipitation of salts from water, especially seawater.
    • Common minerals: gypsum, anhydrite, halite (rock salt).
    • Cherts: Composed of microcrystalline quartz.
    • Can form chemically from ocean water or biochemically from silica skeletons of plankton.
    • Varieties of chert: black, red, grayish white; used for making tools like arrowheads.
    • Coal: Formed from organic remains of plants; almost entirely carbon.
    • Requires anoxic environments (low to no oxygen).
    • Commonly forms in swamps with high sedimentation rates.
    • Grades of coal depend on compression and heating:
      • Coal Ball: Low carbon content, can contain preserved plant remains.
      • Higher compression leads to purer coal used for energy and heating (no preserved plant remains).

Closing Summary

  • Overview of the different types of sedimentary rocks:
    • Detrital (clastic) vs