Kuang Hongwei1, Liu Yongqing1, Peng Nan2, Liu Yanxue1, Li Jiahua3
1 Institute of Geology，Chinese Academy of Geological Sciences，Beijing 100037 2 School of the Earth Sciences and Resources, China University of Geosciences(Beijing), Beijing 100083 3 Shengli Oilfield Company, SINOPEC, Dongying 257051，Shandong
Abstract：Molar Tooth (abbreviated MT) carbonate rocks，i.e.，the Proterozoic carbonate rocks with particular texture，are characterized by the enigmatic micro-spar fabric，limitation of temporal and spatial distribution or diverse origin interpretations since the discovery of more than a hundred years，and attract the attention of numerous geologists worldwide.Based on the previous researches and our own new studies on MT microscopic fabric in the recent decade，some new results were proposed in this paper.MT is a stacked equant calcite sparry constructed by aggradated accretion of calcite microspar surrounding an initial nucleus of calcite.The Proterozoic ocean was a deoxidation or suboxidation environment with a high carbonate saturation.As a result，the crystallization and consolidation of lime mud on the bottom of the shallow sea became more slowly.Due to continuous decomposition of organic matter in lime mud，gas was generated.And then，within a closed system，the concentration of the gas was increased to a higher value，resulting in the formation of cracks and voids in the plastic lime mud.Cracks were perhaps generated by some unknown mechanisms and enlarged in size or deformed，destructed as well.By the catalyzed reaction of organic matter，calcites(mainly CaCO3)were precipitated quickly from liquids within voids or cracks，and then the initial nucleus of calcite appeared.Afterwards，a stacked equant calcite sparry was constructed by the aggradated accretion of calcite microspar surrounding an initial nucleus of calcite.Consequently，calcite sparry was solidified and deposited within the cracks.The proposed model for the MT formation indicates that the void-forming and crack-filling represents microscale gas-sediment interactions and the geophysical and geochemical condition of the Proterozoic paleo-ocean plays an important role，which is not only the geological setting constraint but also provides sediment materials and dynamic conditions，and temporally and spatially restricted the development and distribution of MT.
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