Soft-sediment deformation structures or microbially induced sedimentary structures: the description and possible origin of the "loopites" in the Mesoproterozoic Wumishan Formation, North China

Within the lower Wumishan Formation at the eastern edge of the Tai-hang Mountains in North China, a ~ 10 m stratigraphic interval contains alternately "bright and dark" laminites with enigmatic loop structures (2.5–27.5 cm in length and 0.6–12 cm in height), preserved in cross-sectional and named "loopites" in this study. The loopites are composed of cores and annulate laminations. Based on the different morphologies, they can be divided into three different types: type I, II and III. Although the loopites are similar to the loop beddings, the formation mechanisms are different. The former is possibly microbially induced sedimentary structures (MISS), while the loop beddings preserve evidence of soft-sediment deformation structures (SSDS) such as boudinage or chain structures, joints and small-scale tensional faults. All three types of loopites have cores. The type I core is made up of relicts of previous microbial mat and the microhighlands, while the type II and III loopites have cores defined by debris and rock fragments. The cores are completely wrapped by microbial mats of later generation. Thus, we can conclude that the formation of loopites is due to the growth, wrapping and deposition of microbial mats, while loop beddings are generated by external triggering mechanism such as earthquake. Furthermore, the discovery and possible formation of loopites may provide a new type of MISS and indicate a stable, anoxic and carbonate-supersaturated environment favorable for microbial mats to form annulate structures, which are controlled by illumination, microtopography and hydrodynamics.