Several genetic models of nodular chert hosted in Phanerozoic carbonate

Carbonate-hosted nodular chert is very common in Phanerozoic strata. Siliceous organisms are the main source of silica for the chert. The sedimentary environment of chert evolved from shallow water to deep water with the evolution of main siliceous organism spices during the geological history. Some typical characteristics exist among nodular cherts in different districts and different ages. The nodular chert usually exists in isolation and dispersion. The authors could observe from the thin section that the calcite grains are replaced selectively and the dolomite crystals are still euhedral. The siliceous minerals exhibit regular change from crypto-crystalline to microcrystalline and finally to macrocrystalline form. Based on the typical characteristics and specific phenomena of the research area, four main genetic models are proposed by different researchers, which are Organic Matter Oxidation Model, Semiper-meable Membrane Model, Mixing Zone Model and Force of Crystallization Controlled Replacement Model. However, due to the complexity of the genesis and various possibilities of formation stage during the sedimentary and diagenetic history, those models all have some limitations and can only explain the characteristics partially. Since the nodular chert can indicate the sedimentary environ-ment and diagenetic history well, the research on its origin is meaningful. Although these models have been proposed for decades, they still have reference value in the future because of the careful and profound thinking.