Paleoenvironmental factors of Late Ordovician carbonate platforms extinction in northeastern Yunnan province
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Abstract: [Objective] The shrinkage and demise of the Late Ordovician carbonate platform in northeastern Yunnan have been subject to various interpretations. To delve deeper into its origins, this paper intends to investigate the Late Ordovician paleoenvironmental evolution in northeastern Yunnan and analyze the influence of paleoenvironmental factors on the demise of the Late Ordovician carbonate platform in this region. [Methods] The Wanhe section in the northeast of Yunnan Province was selected as the research object. Using methods such as indoor mineralogy, petrology, and sedimentary geochemistry analysis, a systematic study was conducted on the microfacies characteristics of the Ordovician Linxiang-Silurian Longmaxi formations in northeastern Yunnan Province. During this period, an analysis of paleoenvironmental changes was conducted to explore the paleoenvironmental impacts during the contraction and extinction of carbonate platforms in the Late Ordovician in northeastern Yunnan Province. [Results] Through the analysis of lithological characteristics, microscopic mineral composition, and paleontological features within the Linxiang-Longmaxi formations at the Wanhe section, six sedimentary microfacies were identified: nodular limestone, laminated microcrystalline limestone, microcrystalline fossiliferous limestone, laminated calcareous mudstone, sandy mudstone and silty sandstone, and calcareous shale and calcareous carbonaceous mudstone. These were further divided into three sedimentary subfacies: shallow carbonate ramp, deep carbonate ramp, and restricted retention shelf. By utilizing sedimentary geochemical methods, we analyzed the paleoceanographic geochemical characteristics and established a geochemical profile. This analysis revealed frequent and significant changes in the paleoceanographic indicators, including paleoproductivity, paleoredox conditions, terrestrial detrital input, and paleosea level fluctuations, between the Ordovician Daduhe and Silurian Longmaxi formations. [Conclusions] Through the analysis of paleoceanographic changes around the Late Ordovician carbonate platform in Northeastern Yunnan Province, the extinction of the Late Ordovician carbonate platform in this region can be attributed to oceanic hypoxia. This hypoxia led to the decline of photosynthetic organisms and subsequently inhibited carbonate production, resulting in the contraction and eventual extinction of the carbonate platform. Additionally, sea-level changes, climate fluctuations, increased input of terrigenous debris, and the Late Ordovician mass extinction event played crucial roles in the process of the carbonate platform disappearance.