Discovery of Siliceous Molar Tooth Structure and Its Genesis in Gaoyuzhuang Formation at Nankou, Beijing

The molar tooth structure(MTS), a special kind of microstructure consisting of pure equigranular micritic calcite or micritic dolomite, mainly developed in Meso-Neoproterozoic strata. During the field geologic investigation, the molar tooth structures were observed in the micritic limestone in the third member of Gaoyuzhuang Formation (Changcheng System) in Nankou, Beijing. Through detailed field observations and microscopic studies, a variety of MTSs were found. The boundaries between host rock and MTS show obvious distinction with a circular arc shape or a gulf-like shape. It is found by scanning electron microscopy (SEM) and energy disperse X-ray microanalysis(EDX) that the MTS consists of siliceous component, such as chalcedony and authigenic quartz. According to the morphology and the filling sequence, the MTS's formation is characterized by multi-genesis and multi-stage, and may be subdivided into four periods, such as crack forming period, crack filling period, crack transforming period, and crack refilling period. It's a key factor of the MTS's formation that the fissure systems are produced by the tectonic movement in semi-consolidated condition.