Paleoproterozoic microbially induced sedimentary structures from lagoonal depositional settings in northern China

Microbially induced sand cracks/crack-fills occur extensively on the top surface of fine sandstone beds of the Paleoproterozoic Zhaojiazhuang Formation, Changcheng Group (> 1.7 Ga) around the Cangyan Mountain, Hebei Province, northern China. Detailed field and microscopic petrographical evidence reveal that these sand cracks/crack-fills possibly resulted from dehydration and desiccation of microbial mats. The age and peculiar morphology of these microbially induced sedimentary structures do not allow comparison with trace fossils or purely physical desiccation cracks. The fine sandstone beds on the surfaces of which these microbially induced sedimentary structures formed were deposited in a lagoon/brackish depositional setting (marine to non-marine transitional setting) with episodic injection of marine water. As such, these microbially induced sedimentary structures suggest the colonization of the marine to non-marine transitional settings in the Paleoproterozoic period, and that the search for evidence of early microbial life should be sought in the marine to non-marine transitional settings. These broad habitats suggest these microbes could have been eurytopic organisms capable of adapting to varying extents of salt and oxygen content variations, like their modern counterparts.