Depositional trends and sequence-stratigraphic successions under the Cambrian second-order transgressive setting in the North China Platform: a case study of the Xiaweidian section in the western suburb of Beijing

In order to conduct the division and correlation of sedimentary rocks within stratigraphic frameworks,the author holds that the core for sequence-stratigraphic studies should be described as follows: the unconformity is used as the main stratigraphic surface,the stratigraphic unit bounded by the unconformity and its relative conformity is defined as the depositional sequence,and the depositional trend within the depositional sequence is regarded as the elementary content.During the Cambrian period,the North China Platform accepted sedimentation relatively late,and hence only half-mature stratigraphic succession was developed.In this Cambrian stratigraphic succession,a sedimentary succession from the mixed tidal-flat to the ramp carbonate deposits made up a retrogressive succession under the second-order transgressive process.It is necessary to resurvey the sequence-stratigraphic features for the Cambrian in the North China Platform according to the new chronostratigraphic system,with the Xiaweidian section in the western suburb of Beijing as an example.Therefore,this study will provide insight for further understanding the depositional pattern of the skeleton-poor stormy sea during the Cambrian marked by the increase of microbial carbonate together with the radiation of metazoan.