Discovery Of The Inner Exposed Surface Of The Cambrian Yuertusi Formation In The Northwestern Tarim Basin And Its Research Significance

Deep-shallow water disputes have existed for a long time in the forming environment of organic-rich shale in the Cambrian Yuertus Formation in the Tarim Basin.Based on the macro and micro work of outcrop profiles in Akesu-Keping-Wushi area,northwest Tarim Basin,it is found that the Yuertus Formation of each profile has the following petrological characteristics.(1)The Yuertus Formation consists of several upward shallowing sequences consisting of thin-bedded mudstone-rich shale or thin rhythmic interbedding with siliceous rocks,carbonate rocks with antigranular sequence and terrigenous clastic mixed granulite;(2)siliceous rocks occur mainly in three forms:reticulate siliceous veins,layered siliceous rocks and siliceous lenses,in which layered siliceous rocks have typical grain structure;(3)shallow-changing sequence.There are iron crust layers at the top,karst ditches,karst fissures and cystic caves below,near-surface karst(plastic)breccia and breccia and terrigenous clastic fillings in Karst system,etc.Exposure karst marks;(4)Organic shale above the exposed surface has typical overlap sedimentary characteristics;(5)The bottom of Cambrian in seismic profile has typical overburden characteristics toward high geomorphology.Further analysis shows that fine-grained sedimentary rocks enriched with organic matter develop shallowly upward.The lower part of the cycle is thin shale or thin rhythmic interbedded with siliceous rocks,while the upper part is transformed into mixed granular rocks with antigranular sequence.The results show that the organic-rich shale of the Yuertus Formation was formed in a limited shallow water reduction environment under the wave base and has typical characteristics of shallow water overburden deposits.