Unit 4: Sedimentary Petrology

Sediments and their Genetic Classes/Types

Sedimentary Petrology is the study of sediments (unconsolidated materials like sand, mud, and gravel) and sedimentary rocks (the solid rocks they become).

Sedimentary rocks are classified by their origin, or genetic class:

• Clastic (or Detrital) Rocks: Made from the broken fragments (clasts) of pre-existing rocks. (e.g., Sandstone, Shale, Conglomerate).
• Chemical Rocks: Formed by chemical precipitation from water (often seawater). (e.g., Rock Salt, Gypsum, some Limestone).
• Biogenic (or Biochemical) Rocks: Formed from the remains of living organisms. (e.g., Coal, Chalk, most Limestone).

Weathering and Sedimentary Flux

The Sedimentary Flux describes the entire life cycle of a sediment:

Weathering (breakdown) → Erosion (removal) → Transportation (by wind, water, ice) → Deposition (settling) → Lithification (turning into rock)

Weathering is the in-situ (in place) physical and chemical breakdown of rock at or near the Earth's surface. This is the source of all clastic sediment.

Physical and Chemical Weathering

Physical Weathering

The mechanical breakdown of rock into smaller pieces (clasts) without changing its chemical composition.

• Frost Wedging: Water seeps into cracks, freezes, expands, and breaks the rock.
• Exfoliation (Pressure Release): Deeply buried rock (like granite) expands and "sheets" off in layers when the overlying rock is eroded away.
• Thermal Expansion/Contraction: Daily heating and cooling (e.g., in deserts) can cause rock to crack.
• Biological Activity: Tree roots growing into cracks and breaking rock.

Chemical Weathering

The chemical decomposition of minerals in a rock, forming new, more stable minerals and dissolved ions.

• Dissolution: Some minerals simply dissolve in water, especially acidic water. (e.g., Calcite in limestone dissolving to form caves).
• Hydrolysis: The most important process. Water (H₂O) reacts with silicate minerals. (e.g., Feldspar + water → Clay Minerals + dissolved ions). This is why clay is so abundant.
• Oxidation: Reaction with oxygen, essentially "rusting." (e.g., Iron-bearing minerals like Pyroxene or Biotite → Hematite or Limonite (iron oxides)).

Lithification and Diagenesis

Lithification is the process of turning loose sediment into solid rock. It is part of Diagenesis, which includes all physical and chemical changes that happen to sediment after deposition but before metamorphism.

Key Diagenetic Processes

• Compaction: The weight of overlying sediment buries the lower layers, squeezing out water and reducing pore space. (Mud → Shale).
• Cementation: The "glue" that holds clastic rocks together. Groundwater percolates through the pores and precipitates new minerals, which bind the grains.
  • Common Cements: Silica (Quartz), Calcite, Hematite (Iron Oxide).
• Dissolution: Unstable minerals (like feldspar or aragonite) dissolve, creating new pore space.
• Precipitation / Authigenesis: Growth of new minerals within the sediment (e.g., authigenic quartz).
• Recrystallization: Small, unstable crystals change into larger, more stable crystals of the same mineral (e.g., fine-grained Aragonite in shells → coarser-grained Calcite in limestone).
• Metasomatism: Replacement of a mineral by another of different composition.

Sedimentary Structures

These are large-scale features in sedimentary rocks that provide powerful clues about the depositional environment (e.g., a river, a desert, a deep sea).

Physical Structures (Primary Structures)

Formed during deposition by physical processes (water/wind currents).

• Bedding / Stratification: The layering seen in sedimentary rocks.
• Graded Bedding: A single bed shows a gradual decrease in grain size from bottom to top. Indicates a high-energy current that slowed down (e.g., a turbidity current).
• Cross-Bedding: Sets of angled layers within a main bed. Formed by migrating ripples or dunes. (Tells you the direction of the current!).
• Ripple Marks: Small ridges preserved on a bedding surface.
  • Asymmetric: (Steep and gentle slopes) = One-way current (river, wind).
  • Symmetric: (Equal slopes) = Oscillating current (waves on a beach).
• Mudcracks: Polygonal cracks that form when wet mud dries out and shrinks (indicates exposure to air).

Chemical Structures

Formed by chemical processes during or after deposition.

• Nodules / Concretions: Hard, spherical or irregular bodies of cement that grow within the host rock (e.g., chert nodules in limestone).

Biogenic Structures

Formed by living organisms.

• Stromatolites: Mounded, layered structures built by mats of cyanobacteria trapping sediment.
• Fossils: Preserved remains (body fossils) or evidence (trace fossils) of past life.
• Bioturbation: The churning and burrowing of sediment by organisms, which can destroy other primary structures.

Mineralogical Composition and Texture

Mineralogical Composition

For clastic rocks, this tells us about the source rock and the degree of weathering. This is described by Maturity.

• Compositionally Immature: Contains unstable minerals (like Feldspar, Pyroxene) and rock fragments (lithics). Implies short transport, steep slopes. (e.g., Arkose).
• Compositionally Mature: Contains only the most stable minerals (almost 100% Quartz). Implies long transport, extensive weathering. (e.g., Quartz Arenite).

Texture

Describes the size, shape, and sorting of the grains.

Size and Shapes of Grain

• Grain Size: (Wentworth Scale)
  • Gravel (> 2 mm) → Rock: Conglomerate (rounded) or Breccia (angular)
  • Sand (2 mm - 1/16 mm) → Rock: Sandstone
  • Silt (1/16 mm - 1/256 mm) → Rock: Siltstone
  • Clay (< 1/256 mm) → Rock: Claystone or Shale (if fissile)
• Grain Shape:
  • Sphericity: How close the grain is to a perfect sphere.
  • Roundness: The sharpness of the corners. (Angular → Well-Rounded). Roundness indicates the amount of transport.

Mode of Origin and Depositional Basis (Texture)

This part of the syllabus refers to textural properties that indicate the depositional process.

• Sorting: The uniformity of grain sizes.
  • Well-Sorted: All grains are roughly the same size (e.g., beach sand, desert dune). Indicates a persistent, steady current.
  • Poorly-Sorted: A mix of all sizes (gravel, sand, mud) (e.g., glacial till, alluvial fan). Indicates a rapid dumping of sediment.

Geological Occurrence of Sedimentary Rocks

This refers to the major depositional environments where sedimentary rocks form.

• Continental: Fluvial (rivers), Lacustrine (lakes), Glacial, Eolian (deserts).
• Transitional (Coastal): Deltaic (river mouths), Littoral (beach), Lagoonal.
• Marine: Neritic (shallow marine, on the continental shelf), Bathyal (continental slope), Abyssal (deep sea floor).