Quantitative Study on the Composition and Evolution of the Late Triassic Carnian Shallow-Water Carbonate Factories in Northwestern Sichuan
[Objective]The precipitation and accumulation of massive carbonates are called the"carbonate factory".Environmental and biological factors,such as water depth,light intensity,nutrients,hydrodynamics,climatic condi-tions,and sea-water chemistry control the composition of different carbonate factories.To properly understand the evolution of the carbonate sedimentary system,it is essential to accurately identify different types of carbonate facto-ries and the factors controlling their development.To achieve this,the quantitative evaluation of the environmental and biological parameters in the transition of sedimentary systems is key.[Methods]In this study,the composition,characteristics,type transitions,and driving forces of tropical shallow-water carbonate factories within the Ma'antang Formation were studied in detail.We applied a systematic microfacies analysis in the deposition of the Ma'antang For-mation at the Guanyinya section,Hanwang town,Mianzhu city,northwest Sichuan.Microfacies characteristics,in-cluding types and percentages of carbonate grains,matrix,and fossils,combined with quantitative analysis of the size and morphological parameters of 20,391 carbonate grains,including circularity,roundness,solidity,and as-pect ratio,were analyzed from the thin sections.The size data were corrected and then used to calculate the average particle sizes,degree of sorting,skewness,and kurtosis following the Folk and Ward method.The analyses of the above-mentioned morphological parameters were accomplished in ImageJ using high-resolution scanning photos of the studied thin sections.[Results]Eleven carbonate microfacies are recognized in the study area.Based on the micro-facies characteristics,the 11 microfacies types are further classified into five lithofacies associations(LA):LA-peloids,LA-ooids,LA-skeletal grain,LA-sponge reef,and LA-shale,correlating to four specific carbonate factories,including peloids,ooids,skeletal grains,and sponge reef.The statistical data reveal that the LA-peloids exhibit the smallest average grain size,characterized by regular shapes and good roundness.However,the sedimentation was a carbonate sediment mixture with relatively poor sorting,and the shape coefficient had significant fluctuations,sug-gesting a medium to high-energy environment.Carbonate grains are relatively pure and slightly larger in the LA-ooids than the LA-peloids.Multiple samples demonstrate a concentrated average grain size distribution with predominantly near-circular grains.The ooids are well cemented with sparry calcite,indicating the strongest hydrodynamic condition over the studied sedimentary succession.The LA-skeletal grain contains various carbonate grains,displaying poor sorting and roundness in the statistical results.Furthermore,part of the carbonate grains of the LA-skeletal grains is deposited in the micritic matrix,indicating relatively deep and quiet water with weak hydrodynamic energy.[Conclu-sions]The primary carbonate grain types in the different specific factories indicated different depositional mecha-nisms that drive different carbonate production pathways.By analyzing the composition,depositional environment,hydrodynamic,and nutrient level of tropical shallow water factories in the study area,each specific factory has strong"habitat"characteristics.The rapid transition of different specific carbonate factories in the sedimentary succession indicates that they coexisted at the horizontal level.Frequent sea level changes can lead to rapid changes in hydrodynamic level,carbonate saturation,and other factors,driving the transition of factory types.Additionally,the nutrient level is crucial in regulating carbonate production and may control the formation of different specific types of carbonate factories in shallow-water environments.Therefore,the rapid transformation of subdivided specific carbonate factories on a short time scale is primarily related to the horizontal migration of factory types controlled by regional sea level and may not be related to extreme climate events and the transformation of sedimentary systems.
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