Microbially induced sedimentary structures from the 1.64 Ga Chuanlinggou Formation, Jixian, North China

Abstract

The present study documents abundant exceptionally preserved microbially induced sedimentary structures (MISS) from the 1.64Ga Chuanlinggou Formation of the Jixian area, North China Craton. The Chuanlinggou Formation is dominated by black silty shale/mudstone with fine sandstone of the lower part and was likely deposited in a relatively restricted shallow marine environment varying from fair-weather wave action zone to below storm wave action zone. Both wrinkle structures and sand veins are very abundant in the lower and middle parts of the formation. Wrinkled layers are dark colored, densely laminated, and contain floating quartz grains and abundant mica grains, both of which are aligned parallel to bedding plane. Such an arrangement pattern is attributed to the microbe's binding and trapping of MISS. SEM imaging also shows that sticky networks interwoven by filamentous extracellular polymer substances (EPS) and biofilms, with distinct pores, are very pronounced in clay matrix of wrinkled layers, providing direct evidence of microbial activities during the formation of wrinkle structures. In addition, the circularly arranged quartz grains, forming hollow cavities, and vertical channels of quartz grains beneath the laminated layers are unique to the wrinkle structures showing initial forming stage of the sand veins. Sand veins, similar to fills of sand cracks, are usually preserved in silty shales and occasionally occur beneath the winkled surfaces. They were formed due to methane degassing of decay of microbial mats and biofilms. Abundant nano-particles of various EPSs (isolated nanoglobules, paired nanoglobules, dumbbell-shaped nanoglobules, and polyhedron aggregates) are detected in clay matrix close to sand veins, indicating microbial participation in the formation of sand veins. Wide spatial distribution, broadened environmental inhabitation, and extremely high abundance and bed surface density of the MISSs indicate proliferation of MISS-forming microbial mats in Mesoproterozoic times.