Unit 4: Classification and Petrography of Sedimentary Rocks

Classification of Sedimentary Rocks

Sedimentary rocks are broadly classified into two main groups based on their texture and origin:

1. Clastic (or Detrital) Rocks: Composed of fragments (clasts) of other rocks. They are classified primarily based on grain size.
2. Non-Clastic (Chemical/Biochemical) Rocks: Formed from precipitation or biological activity. They are classified primarily based on mineralogical composition.

Classification Based on Grain Size (Wentworth Scale)

This is the primary way to classify clastic rocks.

Classification Based on Texture (Grain Shape)

• Rounding: Describes the smoothness of the grain corners. Ranges from Angular (sharp edges) to Well-Rounded (smooth). Rounding indicates the duration and distance of transport.
• Sphericity: Describes how close the grain shape is to a perfect sphere.

Example: A Breccia has angular clasts, indicating minimal transport from the source. A Conglomerate has rounded clasts, indicating transport by a river or waves.

Classification Based on Mineralogical/Chemical Composition

This is used to sub-divide clastic rocks (like sandstone) and to classify non-clastic rocks.

• Siliciclastic: Rich in silica (quartz, feldspar, clay). This includes all the clastic rocks in the table above.
• Carbonate: Rich in carbonate minerals (calcite, aragonite, dolomite). E.g., Limestone, Dolostone.
• Evaporite: Formed from evaporated seawater. E.g., Rock Salt (Halite), Rock Gypsum.
• Carbonaceous: Rich in organic carbon. E.g., Coal.

Other Classification Bases

• Mode of Origin / Depositional Basin: Classifying by environment.
  • Fluvial: River deposits (e.g., conglomerate, sandstone).
  • Eolian: Wind deposits (e.g., well-sorted desert sandstone).
  • Lacustrine: Lake deposits (e.g., shale).
  • Marine: Ocean deposits (e.g., limestone, shale).

Textures of Sedimentary Rocks

Clastic Textures

These describe the grains and their relationships in clastic rocks.

• Grain Size: (See table above). A key indicator of transport energy.
• Sorting: Describes the uniformity of grain sizes in the rock.
  • Well-Sorted: All grains are roughly the same size (e.g., beach sand, dune sand). Indicates consistent energy.
  • Poorly-Sorted: A wide range of grain sizes (e.g., glacial till, alluvial fan). Indicates variable or rapid deposition.
• Rounding & Sphericity: (See above). Indicators of transport history and maturity.
• Fabric: The orientation of grains. Grains can be imbricated (stacked) by water flow, showing the current direction.
• Matrix vs. Cement:
  • Matrix: The fine-grained material (silt and clay) deposited at the same time as the larger grains, filling the gaps. A "dirty" rock.
  • Cement: The "glue" (calcite, silica, etc.) that precipitates later in the pore spaces. A "clean" rock.

Non-Clastic (Crystalline) Textures

These rocks are composed of interlocking crystals.

• Crystalline: Composed of crystals precipitated from solution (e.g., Rock Salt).
• Microcrystalline: Composed of crystals too small to see (e.g., Chert).
• Skeletal/Bioclastic: Composed of visible fossils and shell fragments (e.g., Coquina).

Nomenclature and Petrographic Descriptions

Siliciclastic Rocks

Conglomerate and Breccia (Rudaceous)

• Conglomerate: A poorly-sorted rock with rounded gravel-sized (>2 mm) clasts. Protolith is typically deposited by a high-energy river or beach.
• Breccia: A poorly-sorted rock with angular gravel-sized (>2 mm) clasts. Protolith is deposited very close to the source (e.g., fault zone, alluvial fan).

Sandstones (Arenaceous)

Classified based on the "Pettijohn" or "Folk" classification, which uses a triangle diagram.

• Quartz Arenite: > 90-95% Quartz grains. Very mature. Usually formed in a stable, high-energy environment (beach, desert).
• Arkose (or Arkosic Sandstone): > 25% Feldspar grains. Immature. The feldspar grains are angular, implying rapid erosion (often mechanical weathering) of a granitic source rock and short transport, often in a cold/dry climate.
• Lithic Arenite (or Litharenite): Contains abundant unstable rock fragments (e.g., volcanic, metamorphic, or fine-grained sedimentary). Immature.
• Greywacke: A "dirty" sandstone with more than 15% clay/silt matrix. Poorly-sorted, angular grains. Typically formed in deep-sea environments (turbidites).

Mudrocks (Argillaceous)

• Mudstone: A massive rock composed of a mix of silt and clay. Breaks into blocky pieces.
• Shale: A mudrock that is fissile (splits easily into thin layers). This alignment of clay minerals indicates deposition in quiet water (e.g., deep sea, lake).
• Siltstone: Composed mostly of silt-sized grains. Feels slightly gritty.

Carbonate Rocks

Primarily classified using the Dunham Classification (based on texture) or Folk Classification (based on components).

Limestone

Rock composed primarily of Calcite (CaCO₃). Most limestones are biogenic or biochemical, formed in warm, shallow, clear marine waters.

• Micrite (Lime Mudstone): A dense, fine-grained limestone made of calcite mud. Indicates a quiet water setting. (Dunham: Mudstone)
• Fossiliferous Limestone: Contains abundant visible fossils.
• Coquina: A limestone composed almost entirely of large, poorly-cemented shell fragments. (Dunham: Grainstone)
• Oolitic Limestone: Composed of ooids, which are small, spherical, concentrically-layered grains of calcite formed by precipitation in high-energy, wave-agitated water. (Dunham: Grainstone)
• Chalk: A soft, white limestone composed of the microscopic skeletons of plankton (coccolithophores).

Dolomite (or Dolostone)

A rock composed primarily of the mineral Dolomite (CaMg(CO₃)₂). Most dolomite forms from the diagenetic replacement of calcite in limestone by magnesium-rich fluids.

Biogenic Rocks

Coal

A combustible black rock composed almost entirely of organic matter (plant remains) that has been compacted and altered. Forms in swamp environments with low oxygen.

Rank of Coal (Increasing burial, heat, carbon content):

Peat (sediment) → Lignite (rock) → Bituminous Coal → Anthracite (metamorphic)

Chert

A dense, hard, microcrystalline or cryptocrystalline rock made of silica (SiO₂). It can be chemical (precipitated) or biogenic (from the accumulation of silica-based skeletons of organisms like radiolaria or diatoms).