Diagenesis and Fossilization in Sedimentary Rocks

Diagenesis: Transforming Sediments into Rocks

Diagenesis encompasses the transformation of sediments into sedimentary rocks. It occurs within the Earth's crust and involves physical and chemical changes. There are three main phases:

1. Early Diagenesis (Syndiagenesis)

This phase occurs in the same sedimentary environment where sediment accumulates. It's characterized by intense biological activity, where detritivorous bacteria and organisms consume organic matter, using CO2 and producing oxygen. Key parameters change with increasing depth:

• Oxygen decreases.
• Dissolved CO2 increases.
• Pressure increases.
• Conditions become increasingly reductive and acidic.

2. Deep Diagenesis (Anadiagenesis)

This phase occurs when sediments are buried hundreds or thousands of meters deep. It's a more intense phase, completing the transformation of sediment into rock. Key processes include:

• Compaction: Reduction in volume due to pressure.
• Pore collapse: Reduction of spaces between grains.
• Fossilization: Mineralization of organic remains.
• Cementation: Precipitation of minerals in pore spaces, binding grains together.
• Changes in crystal structure: Minerals may recrystallize into more stable forms.

3. Late Diagenesis (Epidiagenesis)

This phase takes place after folding and uplift have created relief. Parameters that increase towards the surface include:

• Presence of oxidizing water.
• Porosity.

Fossilization: Mineralizing Organic Remains

Fossilization is the process of mineralizing organic remains that were included within the sediment. Changes during fossilization include:

1. Mineralization of soft tissues: This includes materials like wood and animal soft tissues.
2. Molecular restructuring: Organic molecules are transformed into more stable forms under pressure and temperature.
3. Mineral substitution: Changes in pH and redox conditions during diagenesis can cause some minerals to be replaced by others.
4. Dissolution and mold formation: Mineralized organic remains (shells, bones, etc.) may dissolve or decay, leaving behind a void that forms internal and external molds.

Sedimentary Rocks

Sedimentary rocks are broadly classified into two categories:

1. Detrital Rocks

These are formed by the accumulation of clasts (fragments of pre-existing rocks). They are classified according to clast size:

• Conglomerates: >2mm (includes breccias and conglomerates)
• Sandstone: 2mm - 0.06mm
• Siltstone: 0.06mm - 0.003mm
• Claystone/Shale: <0.003mm

2. Non-Detrital Rocks

These are formed by various processes other than clast accumulation:

• Carbonate rocks: Limestone, marl, dolomite.
• Evaporitic rocks: Formed by the precipitation of soluble salts from evaporating seawater (e.g., gypsum and salts).
• Organic rocks: Formed from accumulated organic matter (e.g., coal (peat, coal), oil).

Sedimentary Structures

These structures provide clues about the depositional environment:

1. Stratification: Layering of sedimentary materials.
2. Lamination: Arrangement of sediments in thin layers called laminae.
3. Cross-bedding and cross-lamination: Inclined strata that can cut across older layers, indicating currents.
4. Ripples: Undulations on the stratification plane caused by waves or wind.
5. Graded bedding: A sequence of clastic deposits sorted by size, resulting from a settling process.