Abstract:

The Mesoproterozoic Gaoyuzhuang Fm. at the Qiangou section in Yanqing County of the northwestern suburb of Beijing is a sedimentary succession of stromatolitic-nonstromatolitic carbonates with the thickness of more than 1000 m and with a base of transgressive sandstones. The Gaoyuzhuang Fm. at the Qiangou section can be subdivided into four parts: The first member(or the Guandi Subformation)is made up of a set of transgressive sandstones; The second member(or the Sangshuan Subformation)is constituted by a set of manganese dolomites and lime dolomites with stromatolites; The third member(or the Zhangjiayu Subformation)is marked by a succession of the leiolite limestones richening with matground structures and lacking of stromatolites; The fourth member is a particular sedimentary succession of the stromatolitic reef. Comparing to the Gaoyuzhuang Fm. at the Jixian section, there are three characteristics for the Gaoyuzhuang Fm. at the Qiangou section. First, a set of transgressive sandstones of the first member with the thickness of more than 100 m, not only indicates an obvious regional unconformity between the Gaoyuzhuang Fm. and the underlying Dahongyu Fm. but also makes a clear contrasting of the stromatolitic dolomite succession with the thickness of several hundreds meters at the Jixian section. Second, in a set of leiolite limestones lacking stromatolites and making up the third member, there are no molar-tooth structures that widely develop in leiolite limestones of the upper part of the third member at the Jixian section but there are plentiful of matground structures. Third, a particular sedimentary succession of stromatolite reef with the larger thickness of more than 500 m constitutes the fourth member. These features show that the Gaoyuzhuang Fm. at the Qiangou section forms a complex but regular sedimentary succession of stromatolitic   nonstromatolitic carbonates, which is a representative of both the division of the third-order sequence and the reasonable grouping of the second-order sequence of the Precambrian in terms of the sedimentaryfacies succession. More importantly, the stromatolitic reef of the fourth member and the succession of leiolite limestones of the third member express some meaningful problems that need to further research, such as the depositional model of the stromatolitic reef of the Precambrian, a stromatolitic decline event reflected by the nonstromatolitic succession of leiolite limestones and so on.

Key words: sequence-stratigraphic succession, Gaoyuzhuang Formation, Mesoproterozoic, Qiangou section in Yanqing County