Siliceous ooze is a siliceous pelagic sediment that covers large areas of the deep ocean floor. Siliceous oozes consist predominantly of the remains of microscopic sea creatures, mostly those of diatoms and radiolarians. Sometimes siliceous oozes also contain silicoflagellates and the spicules of sponges.
- Diatoms are golden-brown algae that construct an opaline silica microscopic shell that is known as a "frustule".
- Silicoflagellates are a minor group of marine algae that construct microscopic shells composed of opaline silica.
- Radiolarians are marine protists that also construct microscopic shells composed of opaline silica and are distant relatives of the foraminifera.
Siliceous ooze accumulates on the ocean floor where the bottom waters are close to saturation with respect to silica, and the opaline remains of either diatoms, radiolarians, silicoflagellates, and sponge spicules, or combinations of these are rapidly buried. These conditions exist within areas of high biological productivity associated with volcanic islands and nutrient-rich upwelling zones. As the least common type of pelagic sediment, it covers only 15% of the ocean floor. It accumulates at a slower rate than calcareous ooze: 0.2–1 cm / 1000 yr.
After burial, most siliceous oozes remain unconsolidated. However, a fraction of siliceous oozes dissolve and reprecipitate as a result of diagenesis to form chert beds or nodules. When siliceous oozes are incorporated into orogenic belts associated with subduction zones, they are also altered by diagenesis, and lithified to form chert, i.e. radiolarian chert.
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