56 SEDIMENTARY ROCKS/Chert Figure Photomicrographs of Gunflint Chert in thin section showing chert replaced intraclasts and ooliths within a crack between stromatolites (A) Viewed in PPL (plane polarized light) (B) Viewed in XPL (cross polarized light) showing replacement mainly in the form of microcrystalline quartz Precambrian, Ontario, Canada Scale bars 500 mm and occurs to a lesser extent in mudstones and evaporites The apparently bedded cherts in this situation are the result of the replacement of original sedimentary beds by chert (Figure 6) The source of the silica is considered to be biogenic, with the dissolution products of biogenic opal being redistributed in solution and precipitated as cement and replacement during diagenesis The replacement nodules show a great variety in form, ranging from irregular forms with smooth curved margins (Figure 7), to more tabular and diffuse cherts seen in Carboniferous limestones, and the generally spherical nodules (geodes) representing the replacement of original anhydrite nodules Both chalcedony and microquartz are present (Figure 6), and there is frequently evidence for the direct precipitation of quartz in the rock in the form of isolated bipyramidal crystals The general process of formation of the nodules involves the dissolution of biogenic opal, present in low abundance in the deposited sediment (ca 1%) The mobilized silica is then deposited at suitable nucleation sites, probably as opal-CT Such sites are controlled by rock texture and biogenic content; hence, silica deposition may favour specific beds The opal-CT fills the pore space and replaces carbonate, and, with burial, is converted to chert The silicification appears to be a relatively early diagenetic event, taking place during shallow burial In marine phreatic conditions, silica precipitation and replacement of carbonate tend to occur along redox boundaries between aerobic surface sediments and underlying sediments dominated by sulphatereducing bacteria The degradation of organic material by sulphate-reducing bacteria releases carboxyl and sulphide ions Many carbonate sediments contain very little iron; therefore, very little sulphide is precipitated as pyrite The rest is hydrolysed to hydrogen sulphide which then diffuses to more oxic conditions Oxidation produces sulphate and hydrogen ions; the former diffuse back into the sulphate reduction zone, whilst the increased acidity causes carbonate dissolution at the redox boundary The high concentration of carbonate ions, organic matter, and the reduced pH promote silicification Early silicification may also take place in emergent areas where marine pore waters in carbonate sediments mix with meteoric pore waters In these ‘mixing zones’, the mixing of waters with suitable differences in PCO2 provides ideal conditions for carbonate dissolution with contemporaneous silica replacement and precipitation The concentration of chert replacement nodules at specific horizons, often on a basin-wide scale, can be