Biotic evolution and its relation with geological events in the Proterozoic Yanshan Basin, North China

The Yanshan Basin, located in northern North China, underwent three primary stages in the Mesoproterozoic, and corresponding biotas have been summarized for each stage in the present study. The three stages are stage A (the lower part of the Changcheng Group which dominated by clastic sediments), stage B (the upper part of the Changcheng Group to the Jixian Group which characterized by carbonate sediments), and stage C (the upper part of the Jixian Group which dominated by clastic sediments). This paper will focus on the evolution of biota during stage B. Microbiota of the stage B exhibits a high level of diversity and abundance. Planktonic microfossils have various forms and complex decorations. Most microfossils are generally small. In stage B, there was great variation in the biotic abundance and diversity, the average diameter of spherical benthic microfossils, and the maximum diameter of the spherical microfossils. We analyzed these parameters and identified two typical biotic events, and both biotic events were corresponded with geological events. The first event occurred in the third member of the Dahongyu Formation. In this era, the maximum diameter of the silicified microfossils significantly increased, and large, spherical planktonic specimens are abundant. The flourishing of large organisms in the strata is presumably related to volcanic activity. The second event occurred in the third member of the Gaoyuzhuang Formation. During this era, the microfossil assemblage changed significantly. The changes correspond with a decline in stromatolites as well as the transformations of the carbon isotope ratios and the elemental geochemistry.