Sedimentary Rocks and Processes: Formation, Types, and Environments

Posted on Nov 25, 2024 in Geology

Sedimentary Rocks and Processes

Sediment Types

1. Terrigenous (Clastic) Sediments: Formed from mineral or rock fragments (clasts). Characterized by grain size, shape, and sorting.

2. Chemical Sediments: Formed from dissolved materials transported and deposited by chemical precipitation or biochemical processes.

Sedimentation

1. Mechanical/Physical Sedimentation: Occurs when materials are transported in a solid state.

2. Chemical/Biochemical Sedimentation: Occurs when dissolved materials undergo chemical precipitation, often involving living organisms (biochemistry).

Types of Weathering

1. Mechanical/Physical Weathering: Disintegration of rocks into smaller pieces without chemical change. Common in desert and cold climates.

  • Decompression: Rock breakage due to reduced pressure from erosion and material removal.
  • Freeze-Thaw (Ice Wedging/Cryoclastia): Fragmentation caused by repeated freezing and expansion of water in cracks.
  • Temperature Changes: Expansion and contraction of rocks in desert environments due to temperature fluctuations, leading to weakening and fracturing.

2. Chemical Weathering: Alteration of rock composition due to chemical reactions, often facilitated by temperature and humidity.

  • Dissolution: Minerals dissolve in water (e.g., halite, gypsum, limestone). Limestone is particularly susceptible to dissolution by carbonic acid (CO2 in rainwater), leading to karst topography.
  • Oxidation: Combination of oxygen with metals (e.g., iron) in rocks.
  • Hydrolysis: Destruction of mineral crystal lattices by water, transforming silicates (especially feldspars) into clay.
  • Hydration: Absorption of water by minerals, leading to expansion and weakening. Clay minerals can form through hydration.

3. Biological Weathering: Weathering caused by living organisms.

  • Physical Biological Weathering: Examples include plant roots growing in rock crevices and burrowing animals.
  • Biochemical Weathering: Organisms secrete acids that chemically alter rocks (e.g., lichens, fungi, bacteria).

Soil Composition

1. Inorganic Part: Solid fraction (rock fragments), liquid fraction (mineral salts and water), and gas fraction (carbon dioxide and oxygen).

2. Organic Part: Living organisms and their decaying remains (humification), producing humic acids that contribute to soil humus.

Soil Formation and Maturation Processes

  • Physical and chemical weathering of bedrock.
  • Biological weathering and colonization by living organisms.
  • Humification and mineralization processes.
  • Translocation of substances: movement, separation, mixing, and concentration of soil minerals by water and organisms (includes leaching and precipitation/accumulation of salts).

Soil Horizons

  • O Horizon: Surface layer of decaying organic matter (leaves, branches).
  • A Horizon (Leaching Horizon): Rich in humus, poor in minerals and clay (washed down). Dark color due to humus.
  • B Horizon (Accumulation/Precipitation Horizon): Accumulates salts leached from A horizon. Lacks humus. Color varies (light if calcium carbonate precipitates, red if iron and aluminum oxides precipitate).
  • C Horizon (Altered Bedrock): Contains fragments of parent rock, sand, and clay.
  • D Horizon (Parent Rock): Unaltered bedrock.

Desert Encroachment and Desertification

1. Desertification: Natural process of soil degradation leading to loss of fertility.

2. Desertification (Human-Induced): Accelerated or caused by human activities (e.g., deforestation, overgrazing, fires, soil contamination).

Sedimentary Environments

Areas where sediments accumulate under low pressure and temperature conditions. Can be continental, coastal, or marine.

Continental Environments

  • Fluvial: Rivers and floodplains. Detrital sedimentation (conglomerates and sandstones in channels, shale in floodplains).
  • Alluvial Fans: Formed at the base of mountains. Detrital sedimentation (conglomerates, clays, sandstones, shale).
  • Lacustrine: Lakes. Detrital sedimentation at edges, chemical sedimentation in the center. Coal formation can occur.
  • Aeolian/Desert: Wind-related detrital sedimentation.
  • Glacial: Glacier-related deposits (moraines). Coarse-grained detrital rocks.

Coastal (Transitional) Environments

  • Deltaic: Detrital sedimentation (sandstones, siltstones). Plant debris can accumulate and form coal. Plankton can accumulate and form oil.
  • Beach: Detrital sedimentation (sandstones, conglomerates).
  • Barrier Island: Detrital sedimentation in the bar, detrital silt and carbonate chemistry in the lagoon.

Marine Environments

  • Continental Shelf: High sediment deposition. Detrital platforms near the coast, carbonate platforms further offshore.
  • Continental Slope: Turbidity currents generate alternating sand and shale deposits.
  • Deep Sea: Low sedimentation. Chemical precipitation of siliceous material and fine-grained detrital sediments.

Characteristics of a Stratum (Layer)

  • Roof: Upper boundary.
  • Wall/Base: Lower boundary.
  • Thickness: Vertical extent.
  • Lithology: Rock type.
  • Dip: Angle of inclination from horizontal.
  • Strike: Direction of the intersection of the stratum with a horizontal plane.
  • Sedimentary Structures: Marks on the roof formed by geological agents (e.g., ripple marks, animal tracks, mud cracks).

Stratigraphy

1. Stratigraphy: Branch of geology that studies strata.

  • Principle of Superposition: Younger strata are above older strata (unless overturned).
  • Principle of Lateral Continuity: A stratum has the same age throughout its extent.

2. Stratigraphic Series: Ordered succession of strata.

Fossils

1. Fossils: Traces or remains of past organisms. Studied by paleontology.

2. Utility of Fossils: Dating and correlating sedimentary rocks. Index fossils are particularly useful.

Diagenesis

1. Diagenesis: Transformations of sediments during and after burial and lithification, resulting in rock formation.

  • Compaction (Physical Diagenesis): Reduction of sediment volume due to particle packing and water expulsion.
  • Cementation (Chemical Diagenesis): Binding of sediments by a cement (commonly calcium carbonate).

Types of Sedimentary Rocks

Detrital Sedimentary Rocks: Classified by fragment size.

  • Rudites: Conglomerates (rounded fragments) and breccias (angular fragments).
  • Arenites (Sandstones):
    • Quartz arenites (mostly quartz).
    • Arkose (quartz, feldspar, mica).
    • Calcarenites (calcium carbonate fragments).
  • Shales/Mudrocks: Fine-grained.
    • Siltstone (silt-sized grains).
    • Claystone/Mudstone (clay-sized particles).
    • Shale (very compact, at the limit of metamorphism).

Sedimentary Deposits

  • Residual Deposits (Weathering): Concentration of valuable minerals through weathering and removal of unwanted material (e.g., laterites, bauxites).
  • Placer Deposits: Secondary accumulations of resistant minerals (e.g., gold, platinum, diamonds) in beaches, rivers, and streams.