Radiolarite is a siliceous, comparatively hard, fine-grained, chert-like, and homogeneous sedimentary rock that is composed predominantly of the microscopic remains of radiolarians. This term is also used for indurated radiolarian oozes and sometimes as a synonym of radiolarian earth. However, radiolarian earth is typically regarded by Earth scientists to be the unconsolidated equivalent of a radiolarite. A radiolarian chert is well-bedded, microcrystalline radiolarite that has a well-developed siliceous cement or groundmass.
Mineralogy and petrology Radiolarites are biogenic, marine, finely layered sedimentary rocks. The layers reveal an interchange of clastic mica grains, radiolarian tests, carbonates and organic pigments. Clay minerals are usually not abundant. Radiolarites deposited in relatively shallow depths can interleave with carbonate layers. Yet most often radiolarites are pelagic, deep water sediments. Radiolarites are very brittle rocks and hard to split. They break conchoidally with sharp edges. During weathering they decompose into small, rectangular pieces. The colors range from light (whitish) to dark (black) via red, green and brown hues. Radiolarites are composed mainly of radiolarian tests and their fragments. The skeletal material consists of amorphous silica (opal A). Radiolarians are marine, planktonic protists with an inner skeleton. Their sizes range from 0.1 to 0.5 millimeters. Amongst their major orders albaillellaria, ectinaria, the spherical spumellaria and the hood-shaped nassellaria can be distinguished.
Sedimentation 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. Their descent through 5000 meters of ocean water can take from two weeks to as long as 14 months. As soon as the protist dies and starts decaying, silica dissolution affects the skeleton. 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. Upon reaching the sediment/water interface the dissolution drastically increases again. Several centimeters below this interface the dissolution continues also within the sediment, but at a much reduced rate. It is in fact astonishing that any radiolarian tests survive at all. It is estimated that only as little as one percent of the original skeletal material is preserved in radiolarian oozes. According to Dunbar & Berger (1981) 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) and spare them from dissolution, but of course speed up the sinking time by a factor of 10.
Diagenesis, compaction and sedimentation rates
After deposition diagenetic processes start affecting the freshly laid down sediment. The silica skeletons are etched and the original opal A slowly commences to transform into opal CT (opal with crystallites of cristobalite and tridymite). With increasing temperature and pressure the transformation proceeds to chalcedony and finally to stable, cryptocrystalline quartz. These phase changes are accompanied by a decrease in porosity of the ooze which becomes manifest as a compaction of the sediment. The compaction of radiolarites is dependent on their chemical composition and correlates positively with the original SiO2-content. The compaction factor varies generally between 3.2 and 5, which means that 1 meter of consolidated sediment is equivalent to 3.2 to 5 meters of ooze. The alpine radiolarites of the Upper Jurassic for instance show sedimentation rates of 7 to 15.5 meters/million years (or 0.007 to 0.0155 millimeters/year), which after compaction is equivalent to 2.2 to 3.1 meters/million years. As a comparison the radiolarites of the Pindos Mountains in Greece yield a comparable value of 1.8 to 2.0 meters/million years, whereas the radiolarites of the Eastern Alps have a rather small sedimentation rate of 0.71 meters/million years. 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. Recent non-consolidated radiolarian oozes have sedimentation rates of 1 to 5 meters/million years. In radiolarian oozes deposited in the equatorial Eastern Atlantic 11.5 meters/million years have been measured. In upwelling areas like off the Peruvian coast extremely high values of 100 meters/million years were reported.
Depth of deposition The view that radiolarites are strictly deposited under pelagic (deep water) conditions cannot be asserted any longer. Layers enriched in radiolarians have been found in shallow water limestones, for example the Solnhofen limestone and the Werkkalk Formation of Bavaria. 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. Radiolarites without any carbonates have most likely been sedimented below the carbonate compensation depth (CCD). Note that due to changing atmospheric CO2 concentrations the CCD has not been stationary in the geological past and is also a function of latitude. At present, the CCD reaches a maximum depth of about 5000 meters near the equator while being as shallow as 4200 meters in the north Pacific.
Banding and ribbons
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