Sequence stratigraphy, sea-level changes and depositional systems in the cambrian of the north China platform: A case study of kouquan section, Shanxi province, China

This study is an attempt to reconsider the sequence stratigraphy of the Cambrian succession through field investigation of the deposited strata in the central part of the North China Platform. Contrary to the other parts of the North China Platform, the Kouquan section of Datong city, Shanxi province received a relatively late sedimentation i.e. during Middle Terreneuvian, resulting in the development of two-third of the total Cambrian succession in the study area. The strata outcropping in the section comprise of Xuzhuang, Zhangxia, Gushan, Changshan and Fengshan formations, having an angular unconformable contact with the underlying Archean metamorphic rocks and parallel unconformable contact with the overlying Ordovician Yeli Formation. Based on the facies stacking patterns and cyclicity in the chronostratigraphic context, the strata are subdivided into six third-order depositional sequences (DS1-DS6). The shift in sedimentary strata from the mixed tidal flat facies at the base of Xuzhuang Formation to the overlying ramp carbonate deposits shows retrogradation under the second-order transgressive process. The change from the depositional sequence “transgressive system tract → condensed section → high stand system tract” in Xuzhuang Formation to the drowning unconformity type in the overlying strata controlled by “condensed section → high stand system tract” sequence defines the evolution of carbonate platform from a developing to a mature stage in the north China during Cambrian time. The lithological change from oolitic grain banks during Cambrian Series 3 to bioherm bearing massive micrites and calcareous mudstone with lenses of edgewise conglomerates in Changshan and Fengshan formations represents the depositional pattern of the skeleton-poor stormy sea during the Furongian marked by an increase in microbial carbonate buildups.