Diagenesis and Lithification | Geology Optional for UPSC PDF Download

Sediment to Sedimentary Rocks

Overview

In this section, we delve into the transformation of sediment into sedimentary rock through processes like lithification and diagenesis.

Sedimentary Petrology

Each individual grain in a sedimentary rock has a unique history. For instance, a quartz grain may have originated from igneous, metamorphic, or other sedimentary rocks. These grains are weathered, transported, and eventually deposited in basins to form sedimentary layers.

Lithification and Diagenesis

The conversion of loose sediments into solid sedimentary rocks is termed lithification. Diagenesis encompasses the processes leading to lithification, such as compaction, cementation, and mineral alteration.

Example: Sandstone Formation

Consider the formation of sandstone. Loose sand grains accumulate in basins and undergo compaction and cementation processes, binding the grains together to form solid sandstone over time.

Lithification

Definition of Lithification

Lithification is the process through which loose sediments are transformed into sedimentary rocks. The term "lithification" is derived from "lithos," meaning 'stone,' and "fication," meaning 'making.'

Key characteristics include:

• Cementing of sandstones.
• Recrystallization of limestones.
• Compaction of shales.
• Reduction of original porosity during transformation.

Processes Involved in Lithification

1. Burial

• Occurs when additional sediments are deposited on top of existing sediment layers.

• Continued deposition increases pressure at the basin floor, causing subsidence.

• Pressure varies based on the basin's structure and tectonic settings.

2. Compaction

• Weight of overlying materials exerts pressure, reducing sediment volume.

• Pore spaces between grains decrease as water is expelled.

• Tighter packing results in:

  • Mineral recrystallization.

  • Formation of cementing minerals.

3. Cementation

• Sediments become saturated with water, leading to cementation.
• Minerals in water precipitate between sediment grains, binding them together.
• Cementation helps compact sediments and form sedimentary rocks.

Common cementing materials:

• Silica
• Calcium carbonate
• Iron oxide

Formation of Sedimentary Rocks

Key Processes

• Detrital Grains: Undergo compaction and cementation to form sedimentary rocks.
• Chemical Processes: Precipitation of minerals like calcite and hematite in pore spaces.
• Matrix Formation:
  • Sediments harden through lithification.
  • Matrix can include finer-grained materials like clay or silt embedding larger clasts.
• Packing:
  • Clast-supported: Larger grains in contact with each other.
  • Matrix-supported: Finer grains fill spaces between clasts.

Diagenesis

Diagenesis includes physical, chemical, and biological changes occurring post-deposition but before metamorphism. These changes occur at low temperatures and pressures, typically below 250°C and 2 kilobars.

Examples of Diagenesis

• Alteration of feldspar to clay minerals.

Characteristics

• Diagenetic processes exclude erosion or metamorphism.

Types of Diagenesis

• Syndiagenesis: Early diagenetic processes during deposition and shallow burial.
• Anadiagenesis: Late diagenesis during deeper burial stages.
• Epidiagenesis: Further diagenetic processes in sedimentary rocks.