Diagenesis and Lithification | Geology Optional Notes for UPSC PDF Download

8.5 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.

Unit 8: 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'.
• It involves the cementing of sandstones, recrystallization of limestones, and compaction of shales.
• During lithification, the original porosity of the rock decreases.

Processes Involved in Lithification

Burial

• Burial occurs when additional sediments are deposited on top of existing sediment layers.
• Continued deposition leads to increased pressure at the basin floor, causing subsidence.
• This pressure allows for substantial burial depending on the basin's structure and tectonic settings.

Compaction

• As sediments are buried, the weight of overlying materials exerts pressure, leading to compaction.
• Compaction reduces the volume of sediments by squeezing them from all directions.
• Pore spaces between grains decrease as original water is expelled, resulting in tighter packing.
• Compaction may induce mineral recrystallization and the formation of cementing minerals.

Cementation

• During lithification, sediments become saturated with water, leading to cementation.
• Minerals carried in the water can precipitate between sediment grains, binding them together.
• Cementation helps in compacting sediments and forming sedimentary rocks.
• Common cementing materials include silica, calcium carbonate, and iron oxide.

Fig. 8.6 illustrates the processes of compaction and cementation in forming sedimentary rocks.

Formation of Sedimentary Rocks

• Detrital grains undergo compaction and cementation to form sedimentary rocks.
• Chemical processes involve the precipitation of minerals like calcite and hematite in pore spaces.
• Matrix formation occurs when sediments harden through lithification.
• Matrix can be finer-grained material like clay or silt embedding larger clasts.
• Packing of grains can be clast-supported or matrix-supported.

Diagenesis

• Diagenesis encompasses physical, chemical, and biological changes post-deposition but pre-metamorphism.
• Occurs at low temperatures and pressures, typically below 250°C and 2 kilobars.
• Examples include feldspar alteration to clay minerals.
• Diagenetic processes do not involve 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.

Studying diagenesis aids in understanding geological history and predicting mineral deposits.