Marine sediment
As marine sediments or marine sediments indicating geology those sediments that are deposited in ocean basins or have deposited. They make up the largest proportion of both recent sediments and sedimentary rocks worldwide . The mass under the sea sediments are in turn the shelf sediments . Opposite the marine sediments are the terrestrial sediments (also called continental sediments), which comprise all deposits of the mainland, including freshwater.
Marine sediments and sedimentary rocks are important bedrock and storage rocks for crude oil and natural gas .
Rough breakdown
There are two primary types of marine sediments: terrigenous sediments and aquagenic sediments. Terrigenic sediments (also called lithogenic sediments) result from the entry of mostly fine and finely grained eroded (→ detritic ) rock material from the mainland into the oceans by rivers, and to a lesser extent by wind or glaciers. One therefore speaks of allochthonous sediments. These include marine sand, silt and clay sediments . Aquagenic sediments, on the other hand, arise in situ (on site) through passive precipitation from seawater or as a result of active separation from the tissue of marine life. One therefore speaks of autochthonous sediments. They include u. a. the marine carbonate and silicate sludge and reef limestone .
Differentiation from terrestrial sediments
Some types of sediment, such as sand, silt and clay or the resulting sedimentary rocks, may also have been deposited on the mainland, e.g. B. in plains . An important criterion for differentiating between marine and continental sediments is the fossil record. Marine sediments contain fossils of typical marine life. Among the macro fossils, these include, above all, the cephalopods , arm pods , echinoderms , trilobites (only in Paleozoic sedimentary rocks) and corals . Among the meso- and microfossils, foraminifera , coccolithophores and conodonts (ieL in the Upper Palaeozoic) are typical indicators of the marine environment . Certain trace fossils , such as thalassinoides , also characterize marine deposits.
Facial types of marine sedimentary rocks
The large sea basin storage room is divided internally into numerous smaller storage rooms. Each of these deposition areas has special deposition conditions (water depth, distance to the coast, rate of terrigenous input, light and nutrient supply, chemical environment including oxygen content, salinity, water temperature, etc.), which leads to sediments that are in a certain deposition area accumulate, possess typical characteristics. For example, B. the mean grain size of terrigenous sediments in most cases with increasing distance of the deposit area from the coast. The fossil record can also provide clues about the depository space. Cephalopod fossils are more typical of offshore sediments.
The totality of all characteristics that a marine sedimentary rock exhibits depending on its deposit conditions is called marine sedimentary facies and is specified in more detail depending on the deposit area. For example, a coastal facies , which includes all of the sediments deposited directly near the coast , becomes a neritic facies , which includes all of the shallow sea sediments at a certain distance from the coast, and this in turn is a pelagic facies , which includes all of the deep sea sediments of the open ocean , distinguished. Within this rough facies division, further subdivisions are made.
The world's highest diversity of deposit areas / environments and thus the greatest variety of types of facies can be found in the coastal areas and the Flachsee (shelf). So there u. a. Watt facies Sabcha facies, reef facies, several lagoons facies, delta facies (as Übergangsfazies of fluvial to marine sedimentation), Prodeltafazies and open shelf (at relatively deep water and coastal distance and accordingly relatively monotonous fine-grained sediment as hemipelagic facies called) distinction . All these deposition areas or facies can in turn be divided internally into individual deposition areas or sub-facies.
Deep-sea sediments are far less diverse and primarily represented by sludge or very fine-grained rocks of various compositions. A special deep sea facies is the turbidite facies , which u. a. is characterized by a special transport mechanism of the sediment, so-called turbidity flows , and therefore has relatively coarse-grained deposits. A special form of the turbidite facies is the flysch facies , which represents a deposit area on an active continental margin .
Geological tradition
Today there are large deposits of marine sedimentary rocks on the mainland. There are two main reasons for this: sea level fluctuations and mountain formation .
Sea level fluctuations
Viewed in geological time, the eustatic sea level falls and rises and areas of the continental crust rise and fall in relatively short succession for various reasons. A rise in the eustatic sea level or the lowering of a crustal clod at the edge of a continental block leads to so-called marine transgressions , i.e. In other words, the sea penetrates the continent, causing deposits to accumulate there. Since the eustatic sea level is currently relatively low compared to the Phanerozoic mean, large parts of the continental blocks are now mainland and the marine deposits of past geological epochs are literally "on dry land".
Examples of such former sea areas on today's mainlands (so-called epicontinental seas ) are the Zechstein Sea of the Permian and the Muschelkalkmeer of the Triassic of Central Europe as well as the Western Interior Seaway in the Chalk of North America.
Mountain formations
The second mechanism by which marine deposits can get onto the mainland is the plate-tectonically caused mountain formation as a result of the collision of two continental blocks. During the collision, the sediments that had accumulated in the sea basin that existed between these continental blocks before the collision are folded, pushed one on top of the other and finally lifted out over a large area in the form of a chain of mountains (outdated folds of mountains , geol .: collision orogen ). There are examples of marine sediments in chain mountains all over the world, in Europe especially in the form of the alpine chains , which are largely characterized by marine limestones , such as the Alps (especially Eastern and Southern Alps ), the Apennines or the Dinaric Mountains .
literature
- Angela L. Coe (Ed.): The Sedimentary Record of Sea-Level Change. Cambridge University Press, Cambridge 2003, 288 pp., ISBN 978-0-5215-3842-8 .
- Wolf von Engelhardt, Hans Füchtbauer, German Müller: Sediment Petrology. Volume 2: Hans Füchtbauer (Ed.): Sediments and sedimentary rocks. 4th, completely revised edition. Schweizerbart, Stuttgart 1988, ISBN 3-510-65138-3 .
- Douglas A. Segar, Elaine Stamman Segar: Chapter 6: Ocean Sediments. In: Introduction to Ocean Science. 3rd edition (electronic version 3.2), ISBN 978-0-9857859-0-1 ( PDF 7.1 MB)
- Reed Wicander, James S. Monroe: Historical Geology: Evolution of Earth and Life Through Time. Sixth Edition, Brooks / Cole, Belmont (CA) 2010, ISBN 978-0-495-56007-4 .