Discussion on formation mechanism and age of Beijing Wali conglomerate

The Wali conglomerate is a kind of special conglomerate which was found beneath the Quaternary in Wali area of Haidian district of Beijing during the geothermal exploration process in the 1960s, with the features of shallow burial, large thickness, well-developed karst and good water-bearing capacity. The lithology of it is mainly the dolomite and is quite different from the surrounding Jurassic volcanic sedimentary rocks; and it seems to be much abrupt and abnormal in the occurrence of the conglomerate. The characteristics of the Wali conglomerate mainly include four aspects. Firstly, the composition is mainly chert strip dolomite mixing with volcanic sedimentary rocks and other components, which is more complex. Secondly, the gravel sorting is poor, mainly in the form of sub-angular. Thirdly, the cementation mode belongs to porous cementation, and the cements are calcareous, siliceous and sandy substances. For the fourth, dissolution process in the conglomerate is relatively well developed, and the distribution of karst caves has a feature of stratification. Wali conglomerate is located between the Babaoshan fault and the Huangzhuang-Gaoliying fault, which are two important faults in Beijing area. The study of its formation mechanism will be helpful to deepen the understanding of the characteristics of the two major faults. Also the study has important theoretical and practical significance for the study of basic geology, regional geological structure, seismic monitoring, groundwater and geothermal development, engineering construction and so on. Because of the special structure and lithology, there is a long dispute about the formation mechanism of Beijing Wali conglomerate. There are two main different views of tectonic origin and sedimentary origins. Based on the data derived from previous and new geothermal drillings in recent years, a geological profile was constructed, from which stratigraphic alignment and structural analysis were made. It is realized that there are several reverse faults with various sizes that occurred at different stages and intercept one another; and these faults have greatly disturbed the normal sequence of rock formations in the area. This reflects that the thrusting effect of the Babaoshan fault of this section is very strong and is multiphase. On the basis of the analyses of the geological structures, the lithologic composition and structure, the development and occurrence level of karst, as well as the formation process of Beijing depression, the formation mechanism of Wali conglomerate was analyzed. Result shows that the conglomerate belongs to the fault breccia of Babaoshan fault zone that developed among the Wumishan formation of Mesoproterozoic Jixian System. The age of formation of the conglomerate is approximately the middle and late Mesozoic era.