Radiolarite
Radiolarite is a siliceous, comparatively hard, fine-grained, chert-like, and homogeneous sedimentary rock that is composed predominantly of the microscopic remains of radiolarians.
According to Takahashi (1983) radiolarians stay for 2 to 6 weeks in the euphotic zone (productive surface layer to 200 meters water depth) before they start sinking.
The dissolution of silica in the oceans parallels the temperature/depth curve and is most effective in the uppermost 750 meters of the water column, farther below it rapidly diminishes.
According to Dunbar & Berger (1981)[4] even this minimal preservation of one percent is merely due to the fact that radiolarians form colonies and that they are occasionally embedded in fecal pellets and other organic aggregates.
The organic wrappings act as a protection for the tests (Casey et al. 1979)[full citation needed] and spare them from dissolution, but of course speed up the sinking time by a factor of 10.
[5] According to Iljima et al. 1978 the Triassic radiolarites of central Japan reveal an exceptionally high sedimentation rate of 27 to 34 meters/million years.
What seems to be important for the preservation of radiolarian oozes is that they are deposited well below the storm wave base and below the jets of erosive surface currents.
[8] The characteristic banding and ribbon-like layering often observed in radiolarites is primarily due to changing sediment influx, which is secondarily enhanced by diagenetic effects.
In the simple two component system clay/silica with constant clay supply the rhythmically changing radiolarian blooms are responsible for creating a clay-chert interlayering.
These purely sedimentary differences become enhanced during diagenesis as the silica leaves the clayey layers and migrates towards the opal-rich horizons.
Two situations occur: with high silica input and constant clay background sedimentation thick chert layers form.
Resulting are complex layering relationships that depend on the initial clay/silica/carbonate ratio and the temporal variations of the single components during sedimentation.
At the Silurian/Devonian boundary black cherts (locally called lydites or flinty slates) developed from radiolarians mainly in the Franconian Forest region and in the Vogtland in Germany.
The novaculites are milky-white, thinly-bedded cherts of great hardness; they underwent a low-grade metamorphism during the Ouachita orogeny.
[15] During the Triassic (Upper Norian and Rhaetian) cherty, platy limestones are deposited in the Tethyan region, an example being the Hornsteinplattenkalk of the Frauenkogel Formation in the southern Karawanks of Austria.
On local horsts and farther upslope these sediments undergo a facies change to red, radiolarian-rich, ammonite-bearing limestones.
From the Middle Jurassic onwards radiolarites also formed in the Pacific domain along the West Coast of North America, an example being the Franciscan complex.
The accumulation of radiolarian ooze on Jurassic oceanic crust was continuous here from the Callovian onward and lasted till the end of the Valanginian.
The formation contains abundant foraminifera, radiolaria and calcareous nanoplankton fossils[19] Locally the varicolored opaline to chalcedonic radiolarite is mined and used as an ornamental stone termed mookaite.
Red radiolarian clays associated with manganese nodules are reported from Borneo, Roti, Seram and Western Timor.
The group was deposited in the time range Early Eocene till Middle Miocene on oceanic crust which is subducting now under the island arc of the Lesser Antilles.
