Environment controls on Mesoproterozoic thrombolite morphogenesis: A case study from the North China Platform

Thrombolites are widespread in the Mesoproterozoic Wumishan Formation in the North China Platform. This study shows that they mainly concentrated in subtidal carbonate facies with relatively low hydrodynamic conditions, rather than in intertidal zone as suggested previously. From the deep lower to the shallow upper subtidal facies, the thrombolites show evident changes in morphology from dominantly domal to tabular forms, likely suggestive of environmental controls on their morphogenesis and distribution. As the most important component in thrombolites, mesoclots typically consist of organic-rich micritic nuclei and organic-poor fibrous aragonite rims. Mesoclots may vary considerably in their morphology, but a type of specifically shaped mesoclots tends to concentrate predominantly in a particular group of thrombolites. The proportion of the fibrous aragonite rims in mesoclots decrease as the depositional environments become shallower, likely suggesting that the environmental changes also have controls on the internal fabrics of thrombolites. Putative filamentous bacterial colonies are well preserved in some aragonite fans in the matrix between mesoclots, invoking rapid precipitation and aragonite-supersaturated conditions in the ambient waters. It seems that a suboxic to anoxic environment, highly alkaline seawater and relatively low hydrodynamic conditions were among the important factors that facilitated the development and preservation of thrombolites in the Mesoproterozoic epeiric sea on the North China Platform.