Radiolarite
Radiolarite is a sedimentary rock, which is also known as chert or chert and composed mainly of quartz mass. The English word Chert (including chert ) is dense and micro-to cryptocrystalline quartz rocks. Black radiolarite is called Lydit. The term chert is not valid because the rock is not a real slate, but is widely used in German-speaking countries for historical reasons.
- 8.1 Literature
- 8.2 Notes and references
- 8.3 External links
Mineralogy, petrography
Radiolarites are biogenic, marine sedimentary rocks with schichtweisem construction. Documents to go in them with clastic mica, occasional carbonate mineral, radiolarian skeletal material and organic pigment from. The proportion of clay minerals is low. Radiolarites may occur in association with Karbonatgesteinsschichten if they originated in lower water depth. Usually, they are but a greater sediment depths.
The rock is brittle and can not be split easily, but breaks with sharp edges and conchoidal. Typical of radiolarites is the decay into sharp, small-sized fragments of rectangular shape in the course of weathering. The colors range from pale whitish to reddish, greenish and brownish to black.
Radiolarites consist primarily of radiolarian skeletons and its ruins, that of silicon dioxide. Radiolarians ( radiolarians ) are marine, planktonic, unicellular animals that possess an internal skeleton of amorphous silica and their size between 0.1 and 0.5 millimeters varied. In its wealth of forms can be in addition to the Albaillellaria and Ectinaria the two major systems of spherical Spumellaria and the cap -shaped Nassellaria retire. Otherwise radiolarites are largely fossil -free.
Sedimentation
According to Takahashi (1983 ) remain radiolarians after its death still 2 to 6 weeks floating in the euphotic zone (productive surface layer up to about 200 m water depth), and only then begins its gradual decline. This Sinkprozess by 5000 meters of water can take between 2 weeks and 14 months to complete, according to Takahashi (1981).
With the decomposition of the organic matter begins the dissolution of the silica skeleton. The solution processes are most effective in the ocean in the upper 750 meters, including very little SiO2 is dissolved. The boundary layer sediment / water solution is again very active ( with a penetration depth into the sediment of a few centimeters ), about three times as high as in the underlying sediment. But even in the sediment go the solution and conversion processes further (see diagenesis ).
Actually, it's amazing that even Radiolarienskelette remain. The survival rate is low, it is estimated to be about 1 percent! According to Dunbar & Berger (1981 ) Radiolaria owe this one percent their colony-forming ability and the fact that they are sometimes embedded in faecal pellets and other organic aggregates. The organic shell protects the pebble skeletons before the dissolution ( Casey et al, 1979), also increased, according to Vinogradov & Tsitlin the rate of descent through the water column to ten times.
Diagenesis, compaction and sedimentation
After discontinuation of Radiolarienschlammes start diagenetic processes acting on the sediment. It depends on the silicious skeletons to purchase and resolutions, and the gradual transformation of the amorphous starting material from opal -A to opal - CT ( opal with crystal nuclei of cristobalite and tridymite ) and with increasing temperature and increasing pressure to chalcedony and finally to stable, cryptocrystalline quartz. With these phase changes to the progressive reduction of the porosity is accompanied by the manifests itself as compaction.
The compaction of radiolarites made depending on their chemistry and generally correlates positively with SiO2 content. It usually varies between 3.2 and 5 factors, that is 1 meter solidified radiolarite represents 3.2 to 5 feet unconsolidated Radiolarienschlamm. In the alpine radiolarites Upper Jurassic were taking into account the compaction sedimentation rates from 7 to 15.5 meters per 1 million years (or from 0.007 to 0.0155 mm / a) and in the solidified state achieved 2.2 to 3 meters / million years. In comparison, for the radiolarites of Pindos was closed to a comparable value of 1.8-2.0 meters per 1 million years for the Eastern Alps arose, according to Garrison & Fischer (1969 ), however, only 0.71 meters per 1 million years. Extremely high sedimentation rates were measured in the Triassic in central Japan with 27-34 meters / million years ( Iljima et al 1978).
According to De Wever & Origlia - Devos (1982 ) usually have recent, unconsolidated Radiolarienschlämme sedimentation rates 1-5 meters / million years. For Radiolarienschlämme of the equatorial East Atlantic 11.5 meters / one million years were measured. The upwelling area off Peru has, according to Schrader (1992 ) in comparison with extremely high values of 100 meters / million years!
Deposition depth
The previously held view radiolarians and consequently radiolarites would be deposited only in pelagic deep-water conditions, can now no longer be maintained. Radiolarienreiche layers occur, for example in the Solnhofen limestone and Werkkalk Middle Franconia, both shallow-water sediments. Most important condition for the deposition depth of radiolarites is below the storm wave base and the avoidance of erosive surface currents. Kalklose radiolarites are obviously below the Kalzitkompensationstiefe (English calcite compensation depth or CCD) has been discontinued. This is important to remember that the CCD in the course of the earth was subject to strong fluctuations and low- current depending on the latitude is - it reaches its maximum depth of around 5,000 meters at the equator.
Banding
The characteristic radiolarites banding is primarily driven by the change in sediment supply, which is overlaid by secondary diagenetic effects. In the simple clay / silica system is created for the same clay feed due to rhythmic differences in Radiolarienproduktion a mudstone / chert interbedded. The sedimentary differences are further amplified during diagenesis, as the silica migrates from the clay-rich layers in the direction Opallagen. It can be distinguished here two cases: In case of heavy Radiolarienzufuhr and constant Tonsedimentation quite thick chert bands form. Conversely, can lead to big pitches with only thin chert layers separating at constant Opalzufuhr high, periodically changing clay feed. This simple two-component system, however, further complicated by the addition of carbonate. Between the components carbonate and silica that is also a chemical incompatibility that causes aggregation of the silica component to tubers within the calcareous parts during diagenesis. It can therefore develop complex storage conditions, the clay, silica and carbonate, and depend on the temporal variations of the individual components in the course of sedimentation ( is crucial here, which occur the components in phase and which provide the components, the Hintergrundsedimentation ) from the respective output ratio of the components.
The long-period rhythmicity in the Radiolarienproduktion could be partly associated with Milankovitch cycles, short-period fluctuations can be attributed to El Nino cycles and / or comparable traced with varves on an annual rhythms.
Geographical and temporal occurrence
Paleozoic
The oldest known radiolarites come from the Upper Cambrian of Kazakhstan. Here Radiolarienschlick was sedimented to the Ordovician in Over a period of 15 million years. The deposited near the equator, then deep-water sediment is associated with remnants of oceanic crust and could be dated on the basis of conodonts. In calcareous parts of four radiolarian faunal could be eliminated, the oldest, relatively species-poor fauna comes from the second stage of the Ordovician (formerly Lower Arenig ). The youngest, with 15 taxa relatively species-rich Faunengemeinschaft dated to the fifth stage (former Lower Caradoc ).
In the Middle Ordovician of Scotland (Upper Darriwilium or Upper Llanvirn ) radiolarites were also formed, so at Ballantrae. Cherts are stored here on Spiliten and volcanics. Radiolarites are also found in the adjacent Southern Uplands and here are associated with pillow lava.
Next Next Radiolaritvorkommen is the Strong Iceland Chert from the Middle to Upper Ordovician of Newfoundland. The radiolarite is designed as a red chert and overlying ophiolites.
Im Vogtland, in Frankish -Thuringian Slate Mountains and in the Franconian Forest and north Saxony Slate Mountains deposits of dark gray to blackish Pebble shale ( Lyditen ) from the Silurian are widespread. You use the Silurian at the transition from the Ordovician. Chert occur here mostly in connection with Alaunschiefern. Your deposits often form elongated lenses and hardly traceable over large distances benches.
Of great importance are the North American Novaculite (Arkansas, Oklahoma and Texas) from the uppermost Devonian. The Novaculite are dünnbankige, milky - white colored pebble rocks that were metamorphosed weak during the Ouachita orogeny. They consist mainly of micro- quartz ( 5-35 μ ), which has emerged from Schwammskleren and radiolarians.
In Mississippian also emerged in the Rhenish Slate Mountains lydite. At the output of the Paleozoic were radiolarites on the southern edge Laurasias around Mashad in Iran during the Permian in Sicily in Kalkolistolithen and in the northwest of Turkey ( belonging to the Pontides Karakaya complex of the Sakarya zone), in Mittelperm in Phyllitserie Crete. and deposited in the Upper Permian in the Hawasina ceiling Oman
Mesozoic
During the so-called triad Hornsteinplattenkalke were sedimented in Tethysraum, for example, in the upper Norian and Rhaetian in the Southern Karawanken ( Frauenkogel Formation). They are made of uneven bank limits exchange overlapping Chertlagen and micrites, the Chertlagen silicified representing radiolarienreiche Kalklagen. In Greece, the Hornsteinplattenkalke also contain Kalkturbidite. In deep sleepers or uphill they can go into red, radiolarienführende ammonite. In central Japan tonlagige radiolarites arrived in the Upper Triassic in a shallow marginal sea to the deposit, which have 30 years mm/1000 very high accumulation rates. In addition to radiolarians sponge spicules are very common.
In the Alps, formed during the Jurassic ( period Oberbajoc to Untertithon ) radiolarites in the Northern Limestone Alps (collectively Ruhpolding radiolarite group) and in the Penninic France and Graubünden. In the same context, the radiolarites Corsica are mentioned. The radiolarites of the Ligurian Apennines arrived a little later towards the end of the Jura.
On the west coast of the United States originated radiolarites from the Middle Jurassic, for example, in the Franciscan Complex. The radiolarites in the Great Valley Sequence are a bit younger, they come from the Upper Jurassic.
To this end, parallel to the east of the Mariana Trench have been continuously sedimented radiolarites from Obercallov until the end of Valanginiums about mitteljurassischer, oceanic crust in the western Pacific.
In the Lower Cretaceous ( Aptian ) of Windalia - radiolarite Western Australia and the radiolarite from the Franciscan formation of the Marin Headlands will be presented here in San Francisco; from the Upper Cretaceous the radiolarites come in the Zagros and from the Troodos in Cyprus ( Campanian ) and the red, associated with manganese nodules Radiolarientone of Borneo, Rotti, Seram and West Timor. The radiolarites northwestern Syria are similar to the deposits in the Troodos Mountains of Cyprus.
Cenozoic
An example of Cenozoic Radiolarientone and radiolarites, the Oceanic Group of Barbados represents the group was discontinued in the period early Eocene to Middle Miocene on oceanic crust, which now subducted beneath the island arc.
Applications, occurrence
Radiolarite or Lydit is a very hard stone and is considered the " iron of the Stone Age ". It was made into stone tools such as axes, blades, drills and scrapers. However, its broken edges are not as sharp as that of flint. Spreads are hatchets Lydit in western Lower Saxony and northern Westphalia. The material occurs in Bramscher Massif ( in Bramsche ) to the surface. The color varies, depending on the pigment content between reddish, greenish, brownish and black. Lydit, a black Palaeozoic radiolarite, is used as a touchstone.
Other occurrences that have been used commercially, there are in Thuringia Schleiz and Long Green (both Silurian ), in Saxony at Schönfels, Altmannsgrün, Oelsnitz and Nossen in the resin at St. Andrew Berg and Wernigerode and in wall near Vienna. The purpose of mining in quarries was mainly the gravel extraction. There are also used, among other occurrences in the Czech Republic, Austria, the United Kingdom (Scotland ), United States (Utah, Nevada, Idaho, Wyoming) as well as the world's most important deposits in Australia ( Australian Eastern Tasmansynklinale ).