Sedimentary Petrology: Transport and Lithification
Recap of Sedimentary Rock Formation
UNIT I (b): Transportation, Deposition & Lithification (Sedimentary Petrology – CC VI)
Background Concepts
Before understanding transportation and deposition, we must recall what happens before sediments move:
Sedimentary Rocks
- Formed by the accumulation, deposition, and lithification of sediments.
- They cover approximately 75% of the Earth’s surface.
Weathering
Weathering is the breakdown of rocks in situ.
- Physical weathering: Mechanical disintegration with no chemical change; produces clastic sediments.
- Chemical weathering: Chemical decomposition; produces ions in solution; important for chemical load.
Erosion
Erosion involves the removal of weathered material. Agents include water, wind, glaciers, and gravity. After erosion, sediments are transported.
Transportation of Sediments
Defining Transportation
Transportation is the movement of sediments from the source area to the depositional site.
State of Sediments
Sediments may be transported in two forms:
- Solution: Chemical form.
- Solid form: Particles.
Critical Shear Stress
Sediment movement begins only when the shear stress exerted by a fluid exceeds a critical value. This minimum value is called critical shear stress. It depends on:
- Grain size
- Grain density
- Flow velocity
Modes of Transportation
Transportation occurs in two major modes:
Chemical (Solution) Load
Sediments are transported as dissolved ions produced by chemical weathering. Examples include Ca2+, Mg2+, and HCO3–. These later precipitate as limestone or evaporites.
Solid Load
Transported as solid particles, divided into:
- Suspended Load: Fine particles like clay, silt, and fine sand. They remain suspended due to turbulence, do not touch the river bed, and give rivers a muddy appearance.
- Bed Load: Coarser particles like sand, gravel, and pebbles that move along the river bed.
River Transport Dynamics
Types of River Flow
- Laminar Flow: Water moves in parallel layers with no mixing between layers. It has low velocity and is rare in natural rivers.
- Turbulent Flow: Water molecules move in irregular, zig-zag paths. This high-energy flow is dominant in rivers and is responsible for sediment transport.
Reynolds Number (Re)
Definition and Formula
The Reynolds Number is a dimensionless number that determines whether flow is laminar or turbulent.
Formula: Re = (ρVD) / μ
Where:
- ρ = density of fluid
- V = velocity
- D = diameter
- μ = viscosity
Interpretation of Re
- Re < 2000 → Laminar flow
- Re > 4000 → Turbulent flow
- Between 2000–4000 → Transitional
Lithification and Diagenesis
Importance and Main Processes
Lithification is essential for turning sediment into rock. The main processes include compaction and cementation.
Compaction
Compaction involves the reduction of pore spaces caused by overburden pressure. Water is expelled, a process common in clay-rich sediments.
Cementation
Cementation is the precipitation of minerals in pore spaces. This binds grains together and makes the rock hard.
Authigenic Minerals
Definition and Formation
Authigenic minerals are minerals formed in situ during diagenesis. They form during deposition, during burial, or by chemical reactions.
Examples and Significance
Examples include calcite, silica, hematite, siderite, and clay minerals. Their presence is indicative of the depositional environment.
Lithification Sequence
Sediments contain grains and pores. Compaction reduces pores, and the remaining pores are filled by:
- Matrix: Fine sediment.
- Cement: Authigenic minerals.
Through this sequence, the rock becomes lithified.
Summary of Key Concepts
In this unit, you have learned about:
- Sediment transport by water, wind, and glaciers.
- Reynolds number and the Hjulström diagram.
- Graded bedding.
- Diagenesis and lithification.
- Authigenic minerals.