Sedimentary Characteristics And Environmental Significance In The Late Early Triassic Along Lower Yangtze River Of Anhui Province

The middle-southern of Anhui,which is located in the Lower Yangtze depression of the central of the Lower-Yangtze region.It’s separated from the north China plate by the Tan-lu fault in the west. The Pacific plate locates in the eastern part of the middle-southern of Anhui and it’s southwest is adjacent to the Dabie orogenic belt. At the beginning of the Triassic,it basically inherited the sedimentary environment of Late Paleozoic, the entire lower-middle Triassic was carbonate rocks which represented regressive sequence. The lower-middle Triassic is complete and well outcropped in the middle-southern of Anhui. This area is an ideal area for the early-middle Triassic researches.The Nanlinghu Formation of the Zuoshan profile and Majiashan profile, can be sudivided into five parts according to distributional features of depositional types, body fossils, trace fossils and microbialites. According to field profile researches and microscopic analysis, ten depositional types and fourteen sedimentary structures can be identified in the Nanlinghu Formation and the Dongmaanshan Formation,which provide an intuitive field data for biological activity and analysis of sedimentary environments of the Nanlinghu Formation and the Dongmaanshan Formation.Nine microfacies can be divided according to field observation and laboratory micro-analysises. The carbonate rocks were named following Dunham’s Classification of Limestones. Five sedimentary facies belts can be summarized as platform margin slope,winnowed platform edge sands,open platforms,restricted platforms and platform evaporates, according to vertical and horizontal microfacies combination. Sedimentary characteristics and microfacies indicate that the Nanlinghu Formation was deposited in shallow marine and seawater became shallower gradually.The sedimentation of Nanlinghu Formation in this study area includes storm events and storm deposits; biological events and biolith deposits; siliceous hydrothermal events and siliceous hydrothermal deposits. According the combination features of the storm sedimentary sequence, ten storm layers found in the Majiashan profile in Chaohu are divided into four sets of combinations of storm deposits. The early part of Nanlinghu Formation is mainly formed by proximal deposits and the later part is formed by distal deposits.The research of the biological mass mortality events and the microbial algal mats events shows that the biotic recovery experienced a very slow and tortuous process in the Early Triassic.Based on field observations and microscope analysis, we know the siliceous nodules and bands in Nanlinghu Formation result from the siliceous hydrothermal metasomatism in early diagenetic processes.The early deposition of Nanlinghu formation in the study area occurred a mutation event of biological evolution, which may result from early volcanic activity in Olenekian. A large number of trace fossils are found in the Nanlinghu Formation, and they belong to eleven generas identified by researchers. The trace fossils in the profile appear the phenomenon of extinction-appear again-again the demise, and this may also indicate the unstable environment in the Early Triassic. The organisms appear or disappear alternately due to be affected by periodic fluctuation environment.Algae and microbolite are found in the middle of Nanlinghu Formation in Chaohu, but it appeared in the top of same formation in Susong. If microbolite regards as the symbol of the biological recovery during the early Triassic, we may conclude that the general biological recovery trend from east to west is gradually lagging.Geochemical data analysis of upper Nanlinghu Formation in Chaohu shows that the mainly depositional environment is restricted platform environment of shallow and warm water and weak circulation. In the middle of the early Triassic, geological events last a long time, which caused the sea level declined significantly and the main elements and trace elements fluctuated strongly. Then the sea level comes to recovery until the middle Triassic when the depositional environment turns to evaporate platform. The general trend of the depositional environment in the study area is from deep to shallow. The early climate is hot and dry, but the latter mainly is warm and humid. Fluctuations in period may be affected by Milankovitch cycles; Oxygen-depleted may be the cause of this thin section.