The Study On The Formation Of Some Particular Sedimentary Structures In Precambrian From Degasification And Dehydration

In addition to the stromatolite, microbial related primary sedimentary structures developed in mat are also belonged to the primary sedimentary structure caused by microbe. The wide spread distribution in the Precambrian and the limitation distribution in the Phanerozoic, make this type of sedimentary structures become meaningful to study the evolutionary change of earth.There are some special sedimentary structures in the Precambrian strata, such as the sand dyke structure in Chuanlinggou formation and the molar-tooth structure which is regarded as the puzzle of the Precambrian. There are many interpretation and controversy on their genesis, for example, liquefaction caused by earthquake, algae, biogeochemistry, dehydration and degasification and so on. Until now, there hasn’t get a compatible opinion, the controversy is going on. For the time limitation of the molar-tooth carbonates, which means they developed during Mesoproterozoic and Neoproterozic(1 700~650 Ma) and their global distribution, they act a significant effect on the correlation of strata in Precambrian strata. Moreover, it may related with the early evolution of life for it widely developed before the explosion of living in Cambrian Era. Therefore, Molar-tooth structure and sand dyke structure have become the hot spot of sedimentary.Though there are other interpretation of this kind of sedimentary structures, for example, the sand dyke structure in Chuanlinggou formation has ever been judged as trace fossils or“sandy liquefaction dyke”caused by the earthquake. According to the complexity and diversity of the structures, they should be the primary sedimentary structure related with microbe.In the thesis, the author do some study on the macroscopic and microscopic features and the sequence location of sand dyke structure containing in silty mudstone or shale and the molar-tooth structures developed in the micrite carbonates. The author thinks that these sedimentary structures occurred during the period of maximum flooding and high water level. The extensive ingression of sea creates the multiplication of microbe group, which results in the rich organic substance in the sediments. When the sediments are buried under microbe mat, anaerobic bacteria began to resolve the organic materials.Different kinds of anaerobic bacteria distribute vertically, at different depth of sediments, there will produce distinct minerals and structures. The occur of the pyrite demonstrates that sulfate reducing bacteria resolve the sediments and the reducibility environment. The methane made the gas pressure in the sediments increase and when it exceeded the interface pressure, the sediments will dehydrate, then will cause the fast concretion of the microsparite and quartz in the sand dyke structure along the decline orientation of the pressure.The calcite saturation level, oxidizing reduction condition, subbottom structure setting and so on will play different effect during the formation of sand dyke structure and molar-tooth structure, the appropriate collocation between them reached the condition of their development in Proterozoic and Neoproterozoic. After the snowball earth event, the environment changed and this kinds of structures disappeared in the Phanerozoic. Therefore, the study on these particular structures have great significance on the reconstruction of paleonclimate, paleoocean and the evolution of the earth in Precambrian.