Unit 3: Introduction to Sedimentology

Sediments and Their Genetic Classes/Types

Sediments are naturally occurring materials that are broken down by processes of weathering and erosion, and are subsequently transported by wind, water, or ice, and/or the force of gravity. They are the raw materials for sedimentary rocks.

Genetic Classes (Based on Origin)

1. Clastic (or Detrital/Terrigenous) Sediments:
   • Composed of solid fragments (clasts) of pre-existing rocks (igneous, metamorphic, or older sedimentary).
   • They are transported as solid particles.
   • Examples: Gravel, sand, silt, clay.
2. Chemical Sediments:
   • Formed by precipitation of dissolved ions from water (e.g., seawater).
   • This precipitation can be inorganic.
   • Examples: Ooids (forming oolitic limestone), evaporite minerals (halite, gypsum).
3. Biochemical (or Biogenic) Sediments:
   • Formed from the remains of living organisms.
   • The organisms (like corals, mollusks, or plankton) extract dissolved ions from water to build their shells or skeletons. When they die, these hard parts accumulate.
   • Examples: Shell fragments, coral reefs, chalk (from coccolithophores), peat (from plants).

Sedimentary Rock Forming Processes

This is the sequence of events that transforms a source rock into a sedimentary rock.

1. Weathering (Disintegrating Rock into Clasts)

Weathering is the in-situ breakdown of rocks at the Earth's surface.

• Physical Weathering: Mechanical breakdown into smaller pieces (clasts) without chemical change.
  • Frost Wedging: Water freezes in cracks and expands.
  • Thermal Expansion: Rocks expand and contract with daily temperature changes.
  • Unloading (Exfoliation): Buried rocks expand as overlying rock is eroded away.
• Chemical Weathering Reactions: Decomposition of minerals through chemical reactions.
  • Hydrolysis: Water reacts with minerals (e.g., feldspar) to form new minerals (e.g., clay). This is the most common weathering process.
  • Oxidation: Reaction with oxygen (rusting). Affects iron-bearing minerals (e.g., pyroxene, olivine) and turns rocks reddish.
  • Dissolution: Minerals dissolve completely in water (e.g., calcite, halite). This is key to forming caves in limestone.

2. Transport and Deposition

• Transport: Sediments are moved from their source by agents like rivers (fluvial), wind (aeolian), glaciers (glacial), or gravity (mass wasting).
• Deposition: Occurs when the transport agent loses energy and can no longer carry the sediment. Sediments are deposited in layers (beds or strata) in basins (e.g., lakes, deltas, ocean basins).

3. Chemical and Biochemical Sedimentation

This happens concurrently with deposition. Dissolved ions in water become concentrated and precipitate out, either inorganically (chemical) or through the action of organisms (biochemical), forming new sedimentary layers.

4. Lithification and Diagenesis

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

Diagenesis: Refers to all the physical, chemical, and biological changes that sediment undergoes after deposition and during/after lithification, but before metamorphism.

Key diagenetic processes that lead to lithification include:

• Compaction: The weight of overlying sediment squeezes the sediment together, reducing pore space and expelling water. Clays are highly compactible.
• Cementation: Dissolved minerals in groundwater (acting like a glue) precipitate in the pore spaces, binding the grains together. Common cements are:
  • Silica (Quartz, SiO₂): Very strong, durable cement.
  • Calcite (CaCO₃): Common, but weaker; fizzes with acid.
  • Iron Oxides (Hematite): Gives rocks a red or yellow color.

Other diagenetic processes include:

• Dissolution: Unstable minerals (like feldspar or shell fragments) dissolve, creating secondary porosity (new pore space).
• 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).

Soils and Paleosols

Soils

Soil is the layer of unconsolidated material at the Earth's surface produced by the interaction of weathering and biological activity on the parent rock. It is a mixture of mineral matter, organic matter, water, and air. A mature soil has distinct layers called horizons (e.g., O-organic, A-topsoil, B-subsoil, C-weathered parent rock).

Paleosols

A Paleosol (from Greek paleo = "old soil") is an ancient soil that has been preserved by burial under younger sediments or volcanic deposits. They are effectively fossilized soils found within the rock record.

Importance of Paleosols:

• They indicate a period of non-deposition and subaerial exposure (exposure to the atmosphere).
• They are powerful indicators of past climatic conditions (e.g., red-colored paleosols with iron oxides suggest a warm, oxidizing environment).
• They help geologists identify unconformities (gaps in the geological record).

Mineralogical Composition of Sedimentary Rocks

The minerals found in sedimentary rocks are often those that are most resistant to chemical and physical weathering.

• Quartz: Extremely stable and durable. The most common mineral in sandstones.
• Feldspars: Less stable than quartz; chemically weather to form clay minerals. Their presence (e.g., in Arkose) implies rapid erosion and burial in a cold/dry climate.
• Clay Minerals: (e.g., Kaolinite, Illite) The most abundant product of chemical weathering (especially hydrolysis of feldspars). They form the bulk of shales and mudrocks.
• Carbonate Minerals: (e.g., Calcite, Aragonite, Dolomite) Form the bulk of limestones and dolostones. They are chemically precipitated, usually by organisms.
• Other Materials: Iron oxides, evaporite minerals (halite, gypsum), organic matter (forming coal).

Mineralogical Maturity: A "mature" sediment is one that has been weathered and transported extensively, leaving only the most stable minerals (i.e., pure quartz sand). An "immature" sediment contains many unstable minerals (like feldspars and pyroxenes).

Geological Importance of Sedimentary Rocks

Although they make up only a small fraction of the Earth's crust by volume, sedimentary rocks cover ~75% of the Earth's land surface. They are vital for understanding Earth's history and for our economy.

• Recorders of Earth History: They are the only rocks that preserve fossils, providing the record of the evolution of life.
• Indicators of Past Environments: Sedimentary structures (like ripple marks or mud cracks) and rock types tell us about the environment of deposition (e.g., beach, deep sea, desert).
• Economic Resources: They host the majority of our energy and natural resources:
  • Fossil Fuels: Coal, Oil, and Natural Gas.
  • Water: Sandstones and limestones are major aquifers (underground water reservoirs).
  • Construction Materials: Sand, gravel, building stone (limestone, sandstone), and cement (from limestone).
  • Industrial Minerals: Gypsum (for plaster), Halite (salt), Phosphate (for fertilizer).