SEDIMENTARY ROCKS/Chert 53 can be sedimented, and provide a concentration of metastable biogenic opal This opal can then be converted to chert in situ to produce bedded cherts, or be dissolved and transported to a site of diagenetic deposition to produce chert cement and replacement nodules Volcanic There is a strong association between chert and submarine volcanics in the geological record Thus, it has been postulated that silica is derived from the devitrification of volcanic glass, leading to the production of smectite and silica However, it is likely that the higher Si contents of water in volcanically active areas result in population explosions (‘blooms’) of diatoms and radiolaria, and that the silica is fixed by organisms Hydrothermal Silica Hydrothermal systems, developed in the waning phase of volcanicity, produce hot springs and geysers that frequently deposit silica in the form of amorphous siliceous sinter The silica is dissolved from hot rocks at depth and, as water is circulated through the convecting hydrothermal system, hot silica-saturated water is brought to the surface where silica is deposited due to cooling Silica Precipitation in Lakes The sources of silica resulting in cherts in non-marine lakes are various In sediment-starved Tertiary to Holocene lakes, diatoms can accumulate to form a siliceous sediment (diatomite) that can be converted to chert through time and diagenesis Lakes (e.g., Lake Magadi) in volcanically active areas of the African Rift Valley contain sodium carbonate brines with pH > 10 Silica is leached from volcanic rocks and Si concentrations can rise to 2500 ppm Seasonal evaporation and dilution of the brine by river waters causes the deposition of hydrated sodium silicates that are converted to chert during diagenesis In the Coorong region of South Australia, the pH in some Mg-rich carbonate lakes can rise above pH 10 due to algal photosynthesis Silica is derived by the corrosion of detrital minerals, resulting in Si supersaturation of the lake waters Subsequently, the silica is deposited in lake carbonates as a gel, giving the potential for conversion to chert during diagenesis Occurrence of Chert There are a number of modes of occurrence of chert, the most common, and volumetrically the most important, being bedded cherts and nodular cherts in limestone sequences Bedded cherts in ocean basins Nodular cherts in limestone sequences Cherts of hydrothermal origin, both surface and subsurface Cherts in lake basins Silcrete, chert in palaeosols Silicified wood Bedded Cherts Bedded cherts have been formed through the burial and diagenesis of siliceous oozes throughout Phanerozoic time (Figure 2) However, the Palaeozoic is dominated by radiolaria, and diatoms not make a significant contribution until the Late Mesozoic There are also extensive bedded cherts in the Precambrian, at a time from which no silica-secreting organisms are known Thus, it is pertinent to consider the mechanisms and environments of accumulation of siliceous oozes through time At the present time, siliceous oozes are accumulating in deep ocean basins, in areas starved of detrital supply A broad band of siliceous diatom-dominated deposits surrounds Antarctica, and similar deposits are accumulating between North America and Asia in the northern Pacific to the south of the Aleutian Island chain An equatorial belt of radiolariandominated ooze is present in the Pacific and Indian Oceans Drilling by the Deep Sea Drilling Project has shown that, in some oceanic areas, the siliceous oozes are converted at depth to bedded cherts, the chert generally being of Tertiary age The conditions considered to be favourable for the accumulation of siliceous ooze are summarized below (Figure 3) High organic productivity in surface waters due to upwelling of nutrient-rich oceanic currents Lack of significant input of land-derived detritus that would dilute the deposit Such material is carried by the wind from deserts, and by ocean currents from sources of clastic input Limited presence of calcareous plankton Calcareous oozes are accumulating at present at rates of 10–50 mm Ka 1, compared with