臼齿状构造成因新解——以天津蓟县高于庄组第三段为例

臼齿状构造（Molar—tooth structure）是一种主要发育在前寒武纪非叠层石碳酸盐岩中的沉积构造。由于形态多样且缺乏现代的类比物，国内外地质学家曾先后建立多种模式来解释其成因，这些模式大致可以归为两类：地震液化模式和生物成因模式。近年来，一种新的模式：气泡膨胀与迁移模式得到了地质学家的广泛引用。产在天津蓟县剖面高于庄组第三段非叠层石碳酸盐岩沉积序列上部的臼齿状构造，虽然可以应用气泡膨胀与迁移模式来解释其形成机制，但是，考虑到臼齿状构造可能是微生物席封闭沉积面所激发的气泡膨胀与迁移作用的产物，并且气泡可能是由于细粒沉积中微生物提供的有机质的裂解作用的结果，表明了臼齿状构造的形成间接地与微生物生命活动存在关联。因此，可以将臼齿状构造归为席底构造（第五类原生沉积构造）的一种类型。

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-stromatolitie 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-stromatolitic carbonate succession within the third member of Gaoyuzhuang Formation in Jixian section, we can use the model of the gas-bubble expansion and migration to explain their formation mechanism. However, 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）.