Digenesis and Lithification

4.1 Digenesis vs. Lithification

4.2 Diagenesis

Introduction

  • Diagenesis consists of the physical, chemical, and biological changes that alter sediments during lithification.
  • Two most common chemical processes in diagenesis are cementation and dissolution.
  • Physical conditions: Maximum T = 250 degree C. maximum pressure= 5 kilobar. maximum depth = 15 km.
    • Beyond this metamorphism takes place.
    • However, there is no sharp boundary between diagenesis and metamorphism.
  • It operates for million of years, and occurs in

Processes of Diagenesis

  • Diagenesis generally takes place via three process stages: Compaction, Cementation, and Recrystallisation.
  • Physical process like Compaction results due to the application of pressure.
  • Biological Processes like bioturbation, bioirrigation, and ingestion are controlled by biological organisms and burrowing organisms.
  • Chemical processes are controlled by physical changes in temperature and pressure. The solubility of minerals increases with an increase in temperature and pressure, resulting in fluid in the pore space that is more heavily saturated with chemical ions. This increased ionization will ultimately lead to a greater chance of chemical reactivity.
  • Two of the most common chemical processes found in diagenesis are cementation and dissolution. While cementation adds new material in the pore spaces, dissolution removes the material from these spaces.
  • Most diagenetic processes will tend to decrease porosity during lithification, but in dissolution the overall pore space is increased.

1. Compaction

  • One of the most common physical forms of diagenesis comes from the application of pressure.
  • Compaction is the process of packing of the grains closer, through the expulsion of air and interstitial water from the pores.
  • The amount of compaction is dependent on the type and shape of the grainy material.
  • Water residing in the pore spaces comes out of the existing sediment.
  • As younger sediments are deposited on top of older sediments, the overlying pressure compacts the existing sediments. This creates less pore space between the existing grains, which can be compressed and are often re-arranged into a tighter configuration.
  • There is considerable reduction in porosity and volume of rock. g. A saturated mud may contain as much as 80% water, and can be very highly compressed simply due to this overburden pressure.
  • Example of compaction: Peat converting into Coal.
  • Extreme compaction may result into formation of Stylolites are serrated (jagged or saw-like) surfaces within a rock.

Factors

  • More compaction if the super-incumbent load is high.
  • Finer the grain size, more is the compaction.
    • Higher compaction if Clay content is more.
    • More in Sandstone.
  • Rate of compaction decreases with time.
  • Lesser compaction if angularity of grains is high.

2. Cementation

  • It is the process of precipitation of dissolved mineral matter in pore spaces of coarse grain sediments. (between framework grains and matrix particles)
  • It results into hardening of rocks.
  • It is also known as Authigenetic mineralisation (in-situ precipitation).
  • It includes oxidation, reduction, hydration, hydrolysis, carbonation and ionic reactions.
  • It is used to reconstruct the burial history, because different minerals are stable at different T and P.

Cementing materials:

  • These are relatively simple in chemical composition.
  • Silica, Quartz, calcium carbonate (CaCO3), iron oxides, Siderite (FeCO3), clay minerals (Kaolinite, illite), carbonate cement (calcite, dolomite)
  • Identification of Cementing materials:
    • Calcite cement effervesce with dilute HCl.
    • Iron oxide gives red, orange or yellow colour.
    • Silica is present: if harder sedimentary rocks.

3. Recrystallisation or Neomorphism

  • Molecules reorganise in orientation in a continuous sinking basin.
  • Unlike first two, it is not a major process of diagenesis.
  • It is a gradual process, occurring in millions of years.

Biological Processes

  • Organisms such as worms and crabs mix the loose sediments, and leave evidence of this behaviour in the texture of the rock in a process called
  • in the process of bioirrigation, organisms flush overlying water through their burrows, leading to an exchange of minerals in the irrigated zone.
  • Burrowing organisms also ingest the organic matter contained in the loose material, which alter the chemical composition.

Phases of Diagenesis

  • Halmyrolysis is the early diagenesis, modification, or decomposition of sediments on the sea floor. Sediments are modified on the sea floor before burial. For example, the breakdown of ferromagnesian minerals, and the growth of glauconite aggregates in seafloor sediments, are types of halmyrolytic process.
  • Bulk of diagenesis is post burial in three phases.

Phase 1: Syndiagenesis

  • It takes place early in the digenetic cycle.
  • Interstitial water content is high, and it is expelled slowly.
  • The porosity is high.
  • The oxidation potential is high.

Phase 2: Anadiagenesis

  • It is the deep burial phase. Late in diagenetic cycle.
  • It takes place from approx. 100 metres to 15 km.
  • It is the phase of compaction and maturation of sedimentary rocks.
  • It is characterised by expulsion of interstitial fluids.
  • There is substantial reduction of porosity.
  • It is characterised by condition of reduction.

Phase 3: Epidiagenesis (phase of upliftment)

  • Interstitial water is replaced by meteoric water.
  • There is reversal to condition of oxidation.

4.3 Lithification or Consolidation

Introduction

  • Lithification is the process of conversion of freshly deposited loose grains of sediment into solid sedimentary rocks.

  • The sediments are compacted to gradually become solid rock under pressure, reducing pore space and driving out interstitial liquid or connate fluids.
  • Cementation is one of the main processes involved, particularly for sandstones and conglomerates.
  • In addition, reactions take place within a sediment between various minerals and the fluids trapped in the pores. These reactions are These may form new minerals or add to others already present in the sediment.
  • Minerals may be dissolved and redistributed into nodules and other concretions. Minerals in solution entering the sediment from another area may be deposited or may react with minerals already present.
  • Physical and chemical changes occur during Lithification. These are known as
  • Compaction and cementation takes place simultaneously.
  • Recrystallisation may also take place in some cases.
  • Note: Connate fluids are the liquids that were trapped in the pores of sedimentary rocks as they were deposited.

Processes of Lithification

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Phases of Lithification

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