A Research On The Depositional Environments Changing And Its' Biological Responsing During The Sedimentation Of The Neoproterozoic Jiuliqiao Formation, Huainan Region, Anhui Province, China

The Neoproterozoic Jiuliqiao Formation in Huainan region is 40-70m thick, consists of sandy, silty and dolomitic limestone. It has been taken as one perfect object for the research on the life evolution, the relationship between organisms and environments during the Neoproterozoic, because of the well outcropped strata, various depositional types, abundant stromatolite reefs, macroscopic alga, acritarchs and macroscopic worm-like organisms in the Formation.The event deposits, outcrop sequence stratigraphy, paleontology, stromatolite reefs and depositional geochemistry of the carbonatites have been studied, and the environment changing process and corresponding biological responsing characters were well discussed in this paper, some new evidences and information were appended to the Neoproterozoic biology, sedimentology and environmental sciences.From bottom to top, three parts of the Formation was subvided as: the lower sandy and silty limestone part (with molar-tooth-like veins, is 13-18m thick), the mid stromatolite limestone part (is 19-62m thick) and the upper sandy limestone part(with macroscopic fossils, is 7-10m thick), based on the characters of the lithology, event deposits, paleontology and correlation of different outcrops of the Formation.Different events, such as violent earthquakes during the early period, frequent strong storms and the maximum flooding events during the middle period, the abrupt environment changes and regression during the later period of the Jiuliqiao Time, had different influences on organisms. The violent earthquakes made few organisms survive during the early period; the storm events comprised the occasional stage during the early period, the frequent stage and the decreasing stage during the middle period and the quiet stage during the later period of the Jiuliqiao Time. Five typical storm deposition cycles were identified during the Jiuliqiao Time. The process from puissant to recessionary of the storm events could be correlated with such process: the stromatolite reefs becoming bigger, the micro and macro fossil become manifolding, the phenomena indicate that frequent storm events had negative influence on the organisms in Huainan region, while the stromatolite-made cyanobacteria had better adaptability than others during the Jiuliqiao Time.Abundant acritarchs were obtained from the Jiuliqiao Formation for the first time. The acritarch assemblage, which comprises Acanthomorphitae, Herkomorphitae, Pteromorphitae, Nematomorphitae, Sphaeromorphitae and many multicellular algal fragments, has seventeen form genus, fifteen species and thirteen species indeterminate. The assemblage has importantbiostratigraphy significance, because of it's transitional characters from the Qingbaikou Period to the Sinian Period. The characters of acritarchs indicate that a new developing stage of the microplankton had come forth during the Jiuliqiao Time.The distribution characters of the macroscopic fossils, acritarchs and the stromatolite reefs in the Jiuliqiao Formation indicate that, the complicated biocommunity had been formed by abundant macro-organisms, micro-plankton and stromatolite-made cyanobacteria during this Time. The struggles for existence between different organisms had become obvious, and might resulted in the interbedded outcropping characters of the macroscopic fossils and acritarchs.The stromatolites in the Formation had such changing dendency: tuberosity, broken and forked shortprismatic in the lower part, complete and nonforked long-column in the mid part, tuberosity, complete forked shortprismatic in the upper part. The stromatolite reefs had such changing dendency: small scale lenticular and moundy in the lower part, large scale moundy in the mid part, and small scale lenticular in the upper part. The stromatolite reefs in the Formation were devided as the storm environment type, the transgression environment type and the regression environment type, eight reef-building cycles, which indicated the stromatolite-made cyanobacteria's adapting process to different environments, we...