What is organic sedimentary rock made of

The formation of sedimentary rocks

Sedimentary rocks are rocks that are formed by the deposition, overlay and solidification of loose material.

The constituents of sedimentary rocks

The starting materials of sedimentary rocks can be mineral-containing solutions, relics or fragments of organisms as well as rock material crushed by previous weathering. The rock material crushed by weathering can be of igneous, metamorphic or sedimentary origin.

The deposition of sediments

Sedimentary rocks are formed on the earth's surface, in the sea, in isolated shallow water and coastal areas or in river mouths - depending on where the various loose sediments were transported by water as a solution or suspension, ice or wind, and continentally and marinely deposited or deposited - therefore also the term sediment. The name sediment is derived from the Latin word sedere = to sit derived.

Frequently, in connection with the place of deposition, or rather the conditions under which loose sediments were deposited in the geological past, the terms fluvial, limnic, marine, aeolian or glacial are used to describe sedimentation and the transport of loose material over flowing waters corresponds to still waters (e.g. lakes), in the sea, by wind and as particles carried along by glaciers.

Chemical, biogenic and clastic sediments

The Origin of the constituent components differ in sediments

  • chemical - Carbonates and sulphates, e.g. plaster of paris and salt
  • biogenic - organic raw materials
  • detritic or clastic - Fragments / fragments of weathered rocks.

From loose sediment to solid rock

The sedimentation of the rock-forming material initially takes place through the deposition in layers.
Sometimes you can see the stratification of sedimentary rocks in the form of differently colored alternating layers, sometimes the layers also differ with regard to the composition of the mineral stock. The causes are manifold. Often it is the admixture of minerals that give the rock a different color in layers. But the grain size of the aggregates of sedimentary rocks also has an influence on the optics. The grain size of the aggregates can vary between fine, medium and coarse, and thus provide an indication of the route and type of transport and also of the distances that a loose sediment had to cover to the location of the sedimentation.

Each sediment layer represents a unit, the material composition of which is differentiated from the overlying and underlying layers and has different thicknesses.
The material is homogeneous within a layer, but the stratification is graded - the grain size increases towards the lower layer surface, an expression of the slowed flow velocity, loose material in bodies of water.

Due to the ongoing process of sedimentation, the original loose masses are exposed to increased pressure and temperature conditions due to the load of the following sediments and are gradually solidified (diagenesis). The water contained is pressed out of the rock, and binding agents or putty ensure the solidification of the deposits (compaction).
The water content of juvenile, freshly deposited layers is around 70 to 90%, even with an overlay of 1000 m of material, up to 60% of liquid is displaced.

As Sedimentary rock binding agent come Lime, silica and clay minerals into consideration.
Allochemical diagenesis is characterized by far-reaching chemical changes in contrast to isochemical diagenesis.

The compaction and water loss can lead to dry cracks, some of which are later filled with other sediments.
Occasionally, crystal impressions appear in sedimentary rocks, which arise when crystals or mineral-containing solutions crystallize out during sedimentation.

Some sedimentary rocks also have organic components such as parts of plants or living beings from a bygone era. Those fossils are primarily of scientific interest, since fossils allow reconstructions of life and the conditions of formation at the time of deposition.

Reading like a book

Sedimentary rocks are exciting not least because you can draw conclusions about the transport medium based on the size of the rock-forming components and the grain shape.
Smaller stones and pebbles that were moved by wind (i.e. aeolian) to the deposition site are angular. On the other hand, stones that were moved by water are comparatively round and smooth, sometimes also of a flat shape, while sand or quartz grains from Aeolian transport are round and angular in water transport.

The Grain size is very variable - can be fine, coarse or medium-grained.
For example, aeolian sediments, i.e. sediments transported by the wind, are fine-grained, while fluviatile sediments are larger and rounded at the edges.

The structure of sedimentary rocks

The structure of sedimentary rocks is as a layer more or less clearly recognizable; As for example with sandstones, which have a kind of fine banding due to the finest mineral admixtures, for example iron-containing minerals such as goethite.

Some of the components are loosely held together, easy to remove or compact and hard connected to one another in the form of mighty banks. Sedimentary rocks break primarily along the strata.

When stratified rocks are weathered, square masses often form, due to crevices in the rock that run perpendicular to the strata.

According to the origin already mentioned above, sedimentary rocks are in clastic sediments, residual rocks, chemical and biogenic sedimentary rocks subdivided.

Clastic, detritic sediments are products of eroded, weathered rocks, both loosely stored and coherently connected to one another. In addition to lime, silicic acid and clay minerals, binding agents can be used, and sometimes a slate-like parallel structure can be seen when exposed to pressure.

Residual rocks consist of weathering material that is solidified at the place of origin and weathering.
Chemical sediments are precipitates of mineral-containing solutions (evaporites).

Biogenic sediments are formed from excretions, skeletons of organisms or plants.

Origin / origin Examples
detritic / clasticConglomerates, breccias, boulder clay, loess, sandstone, greywacke, arkose, mudstone, siltstone, marl.
Residual rocks Bauxite and laterite
chemicallyRock salt, potash salt, limestone, writing chalk, shell limestone, dolomite, travertine, stalactite, iron stone
biogenicPeat, flint, amber, silica rock, pyrite bulbs

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See also:
⇒ cycle of rocks
⇒ The formation of igneous rocks
⇒ The formation of metamorphic rocks

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⇒ Grotzinger, J., Jordan, T. (2016): Press Siever, Allgemeine Geologie, Springer Spektrum
⇒ Hann, H. P. (2015): Basics and practice of rock determination
⇒ Murawski, H. (1992): Geological Dictionary. Ferdinand Enke Verlag Stuttgart
⇒ Maresch, W., Medenbach, O .; Trochim, H.-D. (1987): The colored natural guide rocks. Mosaik Verlag GmbH Munich *
⇒ Yassamanov, N.A. (1991): Small Spectrum of Sciences - Geology: Excursion to Earth
⇒ Georgi, K.-H. (1983): The Rock Cycle - An Introduction to Geology
⇒ Zepp, H. (2011): Grundriß Allgemeine Geographie: Geomorphologie
⇒ Ahnert, F. (2009): Introduction to Geomorphology

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Last updated: April 1, 2019

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