Sedimentary rocks tell Earth's story through their composition and structure. From sandy beaches to ancient swamps, these rocks form in diverse environments. Understanding their classification helps geologists decipher past landscapes and climates.

Clastic, chemical, and organic-rich sedimentary rocks each have unique characteristics. By examining grain size, mineral content, and textures, we can unravel the conditions under which these rocks formed, revealing Earth's dynamic history.

Classification of Sedimentary Rocks

Classification of clastic sedimentary rocks

• Grain size
  • Gravel (> 2 mm)
    • Conglomerate formed from rounded gravel (river rocks, beach pebbles)
    • Breccia composed of angular gravel (talus slopes, fault zones)
  • Sand (1/16 mm to 2 mm)
    • Sandstone common in beaches, rivers, and deserts (Navajo Sandstone)
  • Silt (1/256 mm to 1/16 mm)
    • Siltstone forms from compacted silt (loess deposits, river floodplains)
  • Clay (< 1/256 mm)
    • Claystone made of compacted clay particles (mud flats, deep marine)
    • Shale is a fissile claystone that splits into thin layers (offshore marine)
• Composition
  • Quartz-rich rocks like quartz arenite and quartzite contain over 90% quartz (beach sands)
  • Feldspar-rich arkose forms from rapidly eroded granitic terranes (fault-bounded mountains)
  • Lithic fragments in lithic arenite and graywacke reflect nearby volcanic sources
• Texture
  • Sorting describes grain size uniformity from well-sorted to poorly-sorted (beach vs. glacial till)
  • Rounding of grains ranges from angular to rounded, indicating transport distance
  • Matrix-supported rocks have more fine-grained matrix than larger clasts (mudflow deposits)
  • Clast-supported rocks contain more gravel-sized clasts that support each other (river channel)

Types of chemical sedimentary rocks

• Limestone
  • Composed of calcite (CaCO₃) from precipitated minerals or marine organisms (corals, shells)
  • Reacts readily with dilute hydrochloric acid (HCl) to produce bubbles (carbon dioxide gas)
  • Common in shallow tropical seas (Bahamas) and as cave formations (stalactites, stalagmites)
• Dolostone
  • Composed of dolomite (CaMg(CO₃)₂) formed when magnesium replaces some calcium in limestone
  • Slightly harder than limestone and reacts more slowly with dilute HCl
  • Often associated with evaporative tidal flat environments (sabkhas of Persian Gulf)
• Evaporites
  • Form in restricted basins by evaporation of saline waters in arid climates (Great Salt Lake)
  • Gypsum (CaSO₄·2H₂O) is a soft mineral that forms swallow-tail twins and desert roses
  • Halite (NaCl) is rock salt that tastes salty and dissolves in water
  • Sylvite (KCl) is an important potassium source often mined from evaporite deposits

Organic-rich sedimentary rock formation

• Coal
  1. Plant material accumulates in swamps or marshes and is rapidly buried (Carboniferous coal forests)
  2. With increasing burial depth and temperature, peat is converted to lignite (brown coal)
  3. Further burial and heating transforms lignite into bituminous coal (most common rank)
  4. At the highest temperatures and pressures, bituminous coal becomes anthracite (highest carbon)
  • Coal ranks reflect carbon content and energy value, which increase with burial
• Oil shale
  • Fine-grained sedimentary rock rich in solid, insoluble organic matter called kerogen
  • Deposited in anoxic environments with abundant organic input (algal blooms, marine upwelling)
  • When heated sufficiently, kerogen can generate oil and gas (Green River Formation)
  • Extracted by mining and heating the shale (retorting) to release hydrocarbons

Identification of sedimentary rocks

• Grain size
  • Use a grain size card or comparator to determine average grain diameters
  • Gravel (> 2 mm), sand (1/16 to 2 mm), silt (1/256 to 1/16 mm), or clay (< 1/256 mm)
• Composition
  • Identify primary mineral components like quartz, feldspar, and rock fragments
  • Note any fossils (shells, coral), organic matter (plant debris), or unusual minerals (evaporites)
• Texture
  • Well-sorted rocks have uniform grain sizes, while poorly-sorted contain a mixture
  • Angular grains indicate little transport, rounded grains suggest significant abrasion
  • Matrix-supported if fine sediment is more abundant than larger clasts, clast-supported if the opposite
• Chemical and biochemical rocks
  • Limestone fizzes vigorously with dilute HCl and may contain visible fossils
  • Dolostone reacts more slowly with acid and tends to be harder and more resistant than limestone
  • Evaporites like gypsum are soft (easily scratched) and halite tastes salty
• Organic-rich rocks
  • Coal is black, low-density, and combustible, often breaking along cleats (fractures)
  • Oil shale is fine-grained, dark colored, and may emit a petroliferous odor when broken

Sedimentary Rock Formation and Environments

Depositional environment indicators

• Clastic sedimentary rocks
  • Grain size and sorting
    • Coarse-grained, poorly-sorted sediments indicate proximity to source and high transport energy (alluvial fans, braided rivers)
    • Fine-grained, well-sorted sediments suggest deposition far from source in low-energy settings (offshore marine, deep lakes)
  • Composition
    • Quartz-rich sands form on stable cratons with intense chemical weathering or from recycled sediments (passive margins, beaches)
    • Feldspar-rich arkoses derive from rapid erosion of uplifted metamorphic and igneous terranes (fault-block mountains)
    • Lithic-rich sediments occur in active tectonic settings with nearby volcanic sources (island arcs, continental rifts)
• Chemical and biochemical sedimentary rocks
  • Limestone forms in shallow, warm marine environments (coral reefs, carbonate platforms like the Bahamas)
  • Dolostone is associated with evaporative tidal flats and restricted lagoons (sabkhas of Persian Gulf)
  • Evaporites accumulate in restricted basins with arid climates and high evaporation rates (salt flats of Death Valley)
• Organic-rich sedimentary rocks
  • Coal originates in swamps and marshes with humid climates and rapid burial of plant material (Carboniferous coal forests)
  • Oil shale forms in anoxic marine environments with high organic productivity (algal blooms in upwelling zones, stagnant lagoons)