Origin of Molar-Tooth Structure Based on Sequence-Stratigraphic Position and Macroscopic Features: Example from Mesoproterozoic Gaoyuzhuang Formation at Jixian Section, Tianjin, North China

Both the macroscopic feature and the sequence-stratigraphic position of the molar-tooth structure developed in the third member of the Gaoyuzhuang (高于庄) Formation at the Jixian (蓟县) Section in Tianjin (天津) can provide some useful information about its origin and can reveal some problems to be further researched in the future. The Mesoproterozoic Gaoyuzhuang Formation is a set of ¹ 600 m thick carbonate strata. This formation can be divided into four members. The first member is mainly made up of stromatolitic dolomites; the second is marked by a set of manganese dolomites; the third is mainly composed of lamina limestones with the development of molar-tooth strcutures; the fourth is a set of stromatolitic-lithoherm dolomites. According to lithofacies and its succession,several types of meter-scale cycles can be discerned in the Gaoyuzhuang Formation: the L-M type,the subtidal type and the peritidal type. There is a regularly vertical stacking pattern for meter-scale cycles in the third-order sequence. Therefore,the Mesoproterozoic Gaoyuzhuang Formation can be divided into 13 third-order sequences (SQ₁ to SQ₁₃) and can further be grouped into 4 second-order sequences. The third member is marked by lamina limestones and can be grouped into three third-order sequences (SQ₉ to SQ₁₁). The molar-tooth structure is developed in the middle part of the third sequence,i.e. SQ₁₁,in the third member. Several features of this kind of molar-tooth structure reflect some features of carbonate sedimentation in the Precambrian,such as the particular configuration,abundant organic matter,and easy silication. Stromatolites are chiefly formed in a shallow tidal-flat environment; lamina are mainly formed in the shallow ramp and molar-tooth structures are mainly generated in a relatively more deep-water environment from the middle to the deep ramp. Therefore,similar to stromatolite and lamina,the molar-tooth structure might also be a kind of bio-sedimentation structure. This suggestion is based on macroscopic observation and the sedimentary-facies analysis of the molar-tooth structures from the sequence-stratigraphic position. These features of Precambrian sedimentation also reveal the problem of Precambrian carbonate sedimentation. With more detailed study,a more practical solution for these problems may be obtained in the future.