Soft-sediment deformation structures or microbial mat structures: The description and possible origin of the “loopites” in the Mesoproterozoic Wumishan Formation, North China

Abstract

Within the lower Wumishan Formation at the eastern edge of the Tai-hang Mountains in North China, a ~ 10 m stratigraphic interval contains of alternately “bright and dark” laminites, with enigmatic, cross-sectional preserved loop structures (2.5 ~ 27.5 cm in length and 0.6 ~ 12 cm in height), named as “loopites” by this study. Based on the different morphologies and formations, the loopites, composed of the cores and annulate laminations, can be divided into three different types which are type Ⅰ, Ⅱ and Ⅲ. Despite the loopites are similar to the loop beddings of soft-sediment deformation structures, we suggest that they are previously undescribed microbial mat structures (MMS). The formation of type Ⅰ is interpreted as the initial microbial mat, grows on the micro-highland of carbonate deposits, are wrapped by the subsequent microbial mats. The core is the initial microbial mat, but the micro-highland, which are formed by increased carbonate sedimentation rate, are not wrapped together. In contrast, the formation of type Ⅱ and Ⅲ is interpreted that the micro-highlands, which also can be treated as the cores and may be formed by the thrombolites, rock debris and the fragments of microbial mats lie on the flat microbial mats, are wrapped by the top-covered and underlaid microbial mats. In consequence, differing from the earthquake-induced loop beddings, the formation of loopites is due to the growth, wrapping and deposition of the microbial mats. Furthermore, the discovery and the possible formation of the loopites may provide a new type of MMS and indicate a stable, anoxic and carbonate supersaturated environment with a relatively weak hydrodynamics for microbial mats to form the annulate structures which is controlled by illumination, microtopography and hydrodynamics.