Unit 4: Sedimentary Petrology

Sediments and Their Genetic Classes/Types

Sediments are unconsolidated (loose) materials at the Earth's surface, such as sand, gravel, and mud. They are the product of weathering and are transported by water, wind, or ice.

Genetic Classes (Based on Origin)

1. Clastic (or Detrital/Terrigenous) Sediments:
   • Composed of solid fragments (clasts) of pre-existing rocks.
   • They are transported as solid particles.
   • Examples: Gravel, sand, silt, clay.
2. Chemical Sediments:
   • Formed by inorganic precipitation of dissolved ions from water.
   • Example: Evaporite minerals (halite, gypsum) precipitating from a drying lake.
3. Biochemical (or Biogenic) Sediments:
   • Formed from the remains of living organisms.
   • Examples: Shell fragments, coral reefs, peat (from plants).

Weathering and Sedimentary Flux

Weathering is the in-situ breakdown of rocks at the Earth's surface. Sedimentary flux refers to the movement of this weathered material (sediment) from the source (e.g., mountains) to the sink (e.g., ocean basin).

Physical (Mechanical) Weathering

The mechanical breakdown of rocks into smaller pieces (clasts) without changing their chemical composition.

• Frost Wedging: Water freezes and expands in cracks, breaking the rock.
• Thermal Expansion: Rocks expand and contract with daily temperature changes, causing them to crack.
• Unloading (Exfoliation): Buried rocks expand and fracture in onion-like layers as overlying rock is eroded away.

Chemical Weathering

The decomposition of minerals through chemical reactions, which fundamentally changes them.

• Hydrolysis: Water reacts with minerals. This is the most important chemical weathering process. It's how feldspars break down into clay minerals.
• Oxidation: Reaction with oxygen (rusting). Affects iron-bearing minerals (like pyroxene or olivine) and turns rocks reddish.
• Dissolution: Minerals dissolve completely in water (e.g., calcite, halite). This is how caves are formed in limestone.

Lithification and Diagenesis

Lithification is the process of turning loose sediment into solid sedimentary rock.

Diagenesis includes all the physical, chemical, and biological changes that sediment undergoes after deposition and during/after lithification, but before metamorphism. Lithification is part of diagenesis.

Key Diagenetic Processes

• Compaction: The weight of overlying sediment squeezes the sediment together, reducing pore space and expelling water. Clays are highly compactible.
• Cementation: This is the "gluing" process. Dissolved minerals in groundwater precipitate in the pore spaces, binding the grains together.
  • Common Cements: Silica (Quartz), Calcite, and Iron Oxides.
• Dissolution: Unstable minerals (like feldspar or shell fragments) dissolve, creating new pore space (secondary porosity).
• Precipitation: New minerals grow within the pore spaces.
• Recrystallisation: Small crystals of one mineral change into larger crystals of the same mineral (e.g., fine-grained lime mud (micrite) recrystallizes into coarser calcite (sparite)).
• Replacement: One mineral is dissolved and another mineral takes its place, often preserving the original texture (e.g., silicification of wood to form petrified wood).

Classification of Sedimentary Rocks

Sedimentary rocks are classified in several ways, primarily by composition and texture.

Classification Based on Mineralogical Composition

• Siliciclastic: Rich in silica (quartz, feldspar, clay). E.g., Sandstone, Shale.
• Carbonate: Rich in carbonate minerals (calcite, dolomite). E.g., Limestone, Dolostone.
• Evaporite: Formed from evaporated seawater. E.g., Rock Salt, Rock Gypsum.
• Carbonaceous: Rich in organic carbon. E.g., Coal.

Classification Based on Texture (Size and Shapes of Grain)

This is the primary way to classify clastic rocks.

• Grain Shape: Refers to Rounding (smoothness of corners) and Sphericity (how close to a sphere). Angular grains imply short transport; well-rounded grains imply long transport.
• Sorting: Describes the uniformity of grain sizes. "Well-sorted" means all grains are one size (e.g., beach sand). "Poorly-sorted" means a mix of all sizes (e.g., glacial deposit).

Classification Based on Chemical Composition

This is used for non-clastic rocks. E.g., Limestone (CaCO₃), Dolostone (CaMg(CO₃)₂), Chert (SiO₂).

Classification Based on Mode of Origin and Depositional Basin

This classifies rocks by the environment they formed in.

• Fluvial: River environment (e.g., Conglomerate, Sandstone).
• Eolian: Wind/desert environment (e.g., well-sorted Sandstone with large cross-beds).
• Lacustrine: Lake environment (e.g., Shale).
• Marine: Ocean environment (e.g., Limestone, Chalk, deep-sea Shale).

Geological Importance of Sedimentary Rocks

Sedimentary rocks are "Earth's history books." They cover ~75% of the land surface and are vital for several reasons:

• Recorders of Past Life (Fossils): They are the only rocks that contain fossils, providing the complete record of the evolution of life.
• Indicators of Past Environments (Paleogeography): Sedimentary structures (like mud cracks or ripple marks) and rock types tell us exactly what the environment was like (e.g., desert, beach, deep sea).
• Economic Resources: They host the vast majority of our energy and natural resources:
  • Fossil Fuels: Coal, Oil, and Natural Gas.
  • Groundwater: Sandstones and limestones form aquifers, which store our drinking water.
  • Construction: Sand, gravel, building stone (limestone), and cement (from limestone).