| As the first period of the Late Paleozoic,the Devonian has received extensive attention in recent decades due to its complex climate change,frequent global sea level rise and fall,and multiple biological catastrophe events.The Late Devonian-Early Carboniferous in northeastern Guangxi is a complex paleogeographic pattern of “ platform-trench-basin facies ”.The stratigraphic sedimentation from platform basin facies to open platform facies area is relatively continuous,and a kilometer thick carbonate rock is developed,on which a world-famous karst geomorphic landscape is formed.On this basis,the world-famous karst landform landscape has been formed,and the international Devonian-Carboniferous boundary auxiliary layer profile has been established.Therefore,The study of Devonian system in this area has good research basis and significance.Most of the previous studies in this area focus on the sedimentary facies,paleontology and key organisms and event stratum(“ F-F ”biological catastrophe event layer and D/C boundary layer)of the Late Devonian,and pay little attention to some important events(Ka(?)ak-otomari event)in the Late Middle Devonian,and lack of global scale comparative study.ased on the relatively complete sedimentary profile of the Middle Devonian Eiffelian Givertian boundary,this paper analyzes and reconstructs the paleomarine sedimentary environment near the Eiffelian-Givertian boundary by analyzing the microscopic characteristics of carbonate rocks,the content of element components,the ratio characteristics of related elements and the data of carbon and oxygen isotopes.Research shows that :(1)Referring to Wilson’s standard microfacies sedimentary model,combined with lithologic characteristics,four microfacies types,such as laminated carbonate limestone mudstone and bioclastic-intrinsic clastic mudstone limestone,are recognized,and then three sedimentary subfacies,restricted platform facies,slope facies and open platform facies,are divided;(2)Based on the results of petrology,element geochemistry and carbon and oxygen isotope analysis,the horizon where samples YS1 and YS2 are located was formed in a coastal sedimentary environment with relatively shallow water body and certain provenance supply.From the dolomite of Tangjiawan Formation to the lamellar chert bearing microcrystalline limestone of Mintang Formation,it reflects the rapid transformation process of the marine environment from limited semi limited platform to slope facies in the early stage of marine invasion;The lamellar microcrystalline limestone in YS3 to YS4 stage and the low content of terrigenous components reflect the far shore sedimentary environment with deep sedimentary water body and relatively high sea level.The marine environment in this stage is characterized by hypoxia,which is globally consistent;Each index of YS5 to YS10 samples has a small fluctuation,in which the content of terrigenous components gradually increases,and the sparite limestone representing higher water kinetic energy appears in the rocks,which indicates that the sea level in the sedimentary area shows a downward trend in fluctuation,and several progressive sequences thickened upward in the profile are good sedimentological responses.(3)According to the oxygen isotope,the paleo seawater temperature evolution curve is reconstructed.The study shows that the average seawater temperature in this period is 21 ℃,reflecting the warm subtropical climate,which is consistent with the paleogeographic environment near the equator;(4)The sample YS3 is located in the Ka(?)k-otomari event horizon,and the δ13C value of this point shows a significant negative drift,which is consistent with the global boundary stratigraphic profile of Morocco and the δ13C value curve of the same horizon in Canada,reflecting the transgression event under the background of global scale transgression. |
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