Biogenic sediment

Biogenic sediments are more or less solidified deposits , consisting of dead plants share, protozoa and mineral residues of animal skeletons emerge.

Biogenic sediments

Vegetable origin

By overlaying them with layers of sand or clay , algae or remnants of land plants can be cut off from the oxygen in the air and subjected to chemical and mechanical changes by increasing pressure and temperature (see also coalification ).

The sediment with the least age and density is peat . It is formed under the surface of high- or fens while it continues to grow it and its level is raised. Due to a lack of nutrients, only ombotrophic plants that can get by with the nutrients of the rain can thrive here . The peat is u. a. Used in horticulture to improve soil (loosening, increasing porosity ) - see peat cutting .

If a ripe (dark) peat is covered by a thick layer of rock, lignite can develop after millions of years due to the load pressure and increased temperatures . In economic terms, it is the most important biogenic raw material in the earth's crust - next to the even older coal , which was formed at lower crust levels .

Limestones

Biogenic limestones arise from small creatures or certain reef-building organisms such as corals , brachiopods , stromatopores or sponges . In terms of quantity, the calcium deposits from calcite- forming plankton - especially from calcareous algae - are of the greatest importance . Limestone formations with clearly recognizable fossils are often named after them, e.g. B. the crinoid and trochitic limestone from the skeletal remains of echinoderms .

While reef limestone often forms unhistorical rock bodies several hundred meters thick, limestones, which consist of microorganisms, are often well layered, which is related to cyclical climatic fluctuations or changes in the nutrient supply during their formation.

Today, calcareous microorganisms are sedimented in the oceans to a depth of around 4,500 m. At greater depths of the sea, calcareous components are dissolved by the carbon dioxide contained in the seawater .

Pebbles

In the oceans, large areas are covered with fine-grained sediments, which consist of microscopic skeletons of pebbly unicellular organisms. The skeletons belong to the group of diatoms and radiolarians and are made of opal . As the deposits age, the metastable opal turns into chalcedony . Even in lakes that do not have a large amount of clastic sediments , almost pure layers of diatom skeletons can arise; these deposits are called diatomaceous earth or kieselguhr .

Phosphorites

Phosphorites are sedimentary rocks, the main component of which consists of varieties of the mineral apatite . They occur on the edges of the oceans at depths between 60 and 300 m. The mineral substance of the phosphorite arises from the conversion of organic substance containing phosphorus , mostly from phytoplankton . In some cases, identifiable parts of larger living beings, e.g. B. fish scales, bone remnants or coprolites are so enriched that they form phosphoritic sediment layers. In geological history , phosphorites occur particularly in the Ordovician and Lower Carboniferous (North America), as well as in the Upper Cretaceous and in the Eocene (North Africa and the Middle East).

literature

• Maurice E. Tucker: Introduction to Sedimentary Petrology . Enke, Stuttgart 1985, ISBN 3-432-94781-X .