Turning Sediments into Rocks - Diagenesis

Page 1

Diagenesis is all the changes that take place in sediments at low temperature and pressure, at or near to the Earth's surface.

Lithification is the process of changing unconsolidated sediment into rock.

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 can be reduced by as much as 80% in mudrocks. The compacted sediment becomes mudstone, or if the minerals show preferred alignment, it becomes shale. The diagram shows how porosity varies with depth of burial and therefore compaction in sand and mud.

Plants and Coals

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 (lignin), resins and waxes and preserved. If peat is buried beneath other sediments it is subjected to increased pressure and temperature, which expels water and volatiles such as CH4 and CO2. This reduces volume and increases the proportion of carbon it contains. As diagenesis continues, the peat gradually takes on 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 muds. Groundwater containing minerals in solution flows through the pore spaces and where conditions are right the minerals are precipitated forming a cement, which binds grains together to form sandstones and limestones. The most common cementing minerals are:

quartz - from pressure solution e.g. orthoquartzites

calcite - from solution of carbonate shells e.g. in fossilifeous limestone

iron minerals - often hematite or limonite

Limestones

Modern limestones are composed of aragonite, an unstable form of CaCO3. It changes to become the more stable form, calcite. Some ancient corals had hard parts composed of calcite. The details in these fossils are better preserved than in fossils of creatures that had hard parts composed of aragonite that converted to calcite during diagenesis. Pore spaces between shell fragments may be filled by coarse grained sparite. Spaces left when the soft parts of organisms decay are also filled by calcite crystals that grow from the internal surfaces of the fossil fragments. Part of crinoid fossils, made from large single crystals of calcite, develop large overgrowths of calcite.

New Page