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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Mesozoic-Cenozoic pelagic sedimentary cherts exposed in mobile belts or recovered from the deep sea are diagenetic alteration products of a variety of primary sediments rich in opaline skeletons.
The abundance of skeletal opal in fresh sediments depends on its rate of production, losses to solution during transport and before burial, and dilution by other sediment. The chief effect of volcanism on silica deposition is probably one of inhibiting solution. Maximal quantities of silica are buried in the shallower pelagic sediments, but the percentage of organic silica is normally highest near the carbonate compensation depth, where dilution by carbonate is slight. These normally deep sediments are most susceptible to wholesale silicification (radiolarites). Widespread cherts in otherwise noncherty sequences (Reflectors A and B in the North Atlantic) record chemical changes from a regime of silica solution to one of silica retention and back.
Predilection of chert for permeable beds indicates localization along zones of water movement. Paragenesis may be complicated. In normal abyssal sediments, complete conversion of skeletal opal requires 30-60 m.y., but in areas of rapid burial, high heat flow, and faster connate water flow, the rate must be more rapid.
Two organic events have greatly affected patterns of chert sedimentation. First was the rise, in the early Paleozoic, of organisms with siliceous skeletons; prior to that time silica had been precipitated inorganically. Second was the rise of planktonic carbonate producers during the Jurassic, resulting in restriction of highly opaline sediments to great depths (radiolarian oozes) or to areas of unusual circulation (diatom oozes).
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