A New Model for Genesis of Molar-Tooth Structure：Case Study for the Third Member of Gaoyuzhuang Formation in Jixian Section of Tianjin

Molar-tooth structure (MT structure) is a kind of sedimentary structures mainly developed in non-stromatolitic carbonate succession of the precambrian. The diversity of morphological features and the absence of modern analogies lead to a lively debate on the origin of the MT structure. Lots of models have been built to explain the formation of the MT structure, which can be grouped into two categories: the model of seismic-induced liquation and that of biogenetic origin. Recently a new model, gas-bubble expansion and migration model, has been widely accepted by geologists. For those MT structures existing in the non-stromatol it iccarbonate succession within the third member of Gaoyuzhuang Form ation in Jixian section, we can use the model of the gas-bubble expansion and migration to explain their form ation mechanism. How ever, several features indicate that the formation of MT structures is genetically related to the microbial activities: (1) the sealing to the depositional surface by microbial mat is favorable to induce the gas-bubble expansion and migration, and (2) the gas-bubble is quite possible resulted from the splitting of organic substance provided by microbes. Therefore, MT structures can be grouped into one of the mat ground structures (the fifth category of primary sedimentary structures).