Turning Sediments Into Rock – Diagenesis (Geology)

Sandstone is not the same as sand. Diagenesis is the group of processes which change sand from a sediment into sandstone, a sedimentary rock. It takes place at low temperatures and pressures, at or near the Earth’s surface.

The main processes involved in Diagenesis are compaction and cementation. Sediments are lithified, meaning they become sedimentary rocks. Diagenesis may continue after sediment has been converted to a rock, so lithication is part of Diagenesis.

Compaction – Mud and Sand

As layers of sediment accumulate, one on top of another, their mass produces load pressure. This acts vertically and affects the sediments below causing compaction to take place. Grains become more closely packed and this reduces the porosity of the sediment.

Mud and clay are much more affected by compaction than sandstones. The original thickness of sediment becomes mudstone, of if the minerals show preferred alignment, it becomes shale. The diagram shows how porosity varies with depth and burial and therefore compaction in sand and mud.

Plants and corals

Where plant remains fall into swamps the process of decay uses up the available oxygen. Anaerobic bacteria change the plant material into peat. Woody material resins and waxes are preserved. If peat is buried beneath other sediments it is subjected to increase pressure and temperature, which expels water and volatiles such as CH4 and CO2 which reduces volume and increases the properties of coal. These gradual changes result in different types of coal, which are ranked according to the proportion of carbon they contain.

Cementation
Sandstones

Sands and many biologically formed types of sediments have greater permeability than mud. Groundwater containing minerals in solution flows through pore spaces and where conditions are right the minerals are precipitated forming cement, which binds the grains together to form sandstones and limestone. The most common cementing minerals are:

• Quartz, from pressure solution, e.g. in orthoquartzites
• Calcite, from solution of carbonate shells, e.g. in fossiliferous limestone
• Iron minerals, often hematite or limonite