The discovery of large-scale acanthomorphic acritarch assemblage on the southern margin of North China old land and an analysis of its paleogeographic environment

Abstract：Numerous large-scale acanthomorpic acritarchs have been found in Neoproterozoic Ruyang Group on the southern margin of North China old land. They are acritarchs with double layers, complex ornaments on the outer layer, dense hollow or nets on the inner layer and nucleus in the cell. Some geologists and palaeogeologists are doubtful why these acritarchs occur in Mesoproterozoic strata in that they usually occur in Neoproterozoic strata. However, scientists fail to know their origin and evolutional pedigree. In recent years, the Meso- and Neoproterozoic section in Jixian has been regarded as a stratotype section in China; nevertheless, new SHRIMP zircon U-Pb dating (1368±12 Ma) was performed for Xiamaling Formation, which gives the formation a new location in the Meso- and Neoproterozoic column and breaks the old evolutional sequence. The new dating suggests that there is a gap in the geological record in 1200～900 Ma. According to a biostratigraphic analysis, the Neoproterozoic strata with large-scale acanthomorpic acritarchs will fill up the gap of the Jixian section. This paper deals with the appearance of the acritarchs, stratigraphic correlation, and the possible tectonic implication of the acritarchs in the light of biostratigraphy, geochemistry and lithology.