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Silicification of continental carbonates

AutorBustillo, Mª Ángeles
Palabras claveChert
Fecha de publicación2010
CitaciónCarbonates in Continental Settings: processes, facies and applications Developments in Sedimentology Series (Ed: Van Loon A.J). Volume 62, pages 153-174
ResumenThe silicification of carbonate rocks is a diagenetic process that involves the major replacement of carbonate minerals by silica minerals (opaline phases, quartz and moganite), plus minor process of silica cementation in voids. When silicification is volumetrically significant, it generates silica rocks (cherts and opaline rocks) that stand out in carbonate outcrops as nodules, beds or lenticular beds, and other irregular accumulations. The silica required for this process may be obtained locally from siliceous components included within the carbonate rock (mainly siliceous microfossils, but also siliciclastic grains, clays and volcanic ash), or it may be transported from remote sites, generally by phreatic or hydrothermal water. During diagenesis, the opaline minerals of the siliceous microfossils (diatoms, spicules, phytholiths, etc.) are dissolved or recrystallized to quartz (ageing). The well-known opal A→opal CT→quartz transition releases silica and influences the diagenesis of the carbonates, producing generally silica-carbonate replacements, silica cementations or clay neoformations. Silicification generally involves pseudomorphic replacement since the volumetric rate of silica precipitation is equal to the calcite dissolution rate. Thus, it tends to preserve information regarding the sedimentology, biota and diagenesis of the carbonate host-rock. Most silicifications encountered in continental carbonates are interpreted as having occurred during the early burial diagenesis of lacustrine or palustrine sediments, or during meteoric diagenesis (the formation of silcretes and the silicification of palaeosols, calcretes and dolocretes). Currently, it is generally accepted that fluctuations in pH around a value of about 9 are a major controlling factor of silicification, since an inverse solubility relationship exists between calcite and silica. Changes in the salinity of pore fluids also induce silica precipitation. Other factors that affect silicification include the porosity of the carbonate rock and groundwater flow. The time of silicification is difficult to establish, but can be defined in relation to: (1) the carbonate cementation and diagenesis of the carbonate host-rocks (physical and chemical compaction, dolomitization, dedolomitization, etc.), (2) the incorporation of fragments of silica rocks in younger formations, and (3) the characteristics of the silica rocks themselves (mainly the time of dewatering). The oxygen and, to a lesser extent, the hydrogen isotope compositions of chert have been used to interpret the environmental conditions associated with quartz/carbonate replacement. If opaline phases are formed before quartz, however, no deductions regarding replacement can be made. Since most of the silicification of continental carbonates occurs in surface environments, the δ18O and δD of the quartz formed usually record surface-water compositions, which are strongly influenced by evaporation and changes in salinity. When the silicification of the carbonates is synsedimentary or very early diagenetic, the isotopic composition of chert nodules can record the conditions of the sedimentary basin. In burial diagenesis, the chert isotopic composition can mark the thermal history of the carbonate host rock. The study of the silica rocks (especially chert) formed by the replacement of carbonate rocks is an excellent tool for deducing many of the circumstances surrounding continental carbonate deposits and their diagenetic history.
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