Microbially induced sedimentary structures from the Mesoproterozoic Huangqikou Formation, Helan Mountain region, northern China

Seven types of microbially induced sedimentary structures (MISS): wrinkle structures, levelled ripple marks, organic carbonaceous laminae, microsequences, polygonal sand cracks, polygonal sand crack fills, and gas domes are documented from the Mesoproterozoic Huangqikou Formation of the Helan Mountain region, western part of North China. Facies analysis suggests that the MISS-hosting rocks were deposited in a supertidal to intertidal setting. Both wrinkle structures and gas domes are morphologically different from those previously documented from other ancient siliciclastic rocks, but probably formed by the same microbially mediated process as those published elsewhere. Other MISS resemble their ancient and modern counterparts in both morphology and size. The presence of carbonaceous laminae, coupled with laser Raman and carbon isotopic data (-25. ±. 0.5‰), indicates a biogenic origin of the wrinkle structures. The absence of intercalating mudstone layers as well as different lithologic composition between polygonal sand cracks/crack fills and host rock indicate that both the polygonal sand cracks and polygonal sand crack fills likely resulted from dehydration and desiccation of organic rich microbial mats. The biogenicity of gas domes is supported by the different lithologic composition between infillings and wall.