Mesoproterozoic biogenic thrombolites from the North China platform

Thrombolites are abundant in the subtidal dolostones of the Mesoproterozoic Wumishan Formation (ca 1.50–1.45 Ga) in the North China platform. Three major components are identified within the thrombolites: irregular mesoclots, micritic matrix and spar-filled voids. The mesoclot generally comprises a relatively organic-rich micritic core and a microsparitic outer layer that consists of fibrous aragonite (pseudocrystals) with less organic matter. In the core of mesoclots, abundant fossilized organic remnants, such as putative coccoidal and filamentous bacteria and mucus- to film-like extracellular polymeric substance (EPS), are closely associated with organominerals including nanoglobules and submicron-scale polyhedrons. In exceptionally well-preserved mesoclots, their outer layers commonly contain micropores displaying as bacterial molds and filamentous bacteria fossils. The matrix of mesoclots consists mainly of micropeloids (20–30 μm in diameter) and minor terrigenous detritus. Some mesoclots have denticulate edges and their matrix shows growth laminations that envelope the outlines of mesoclots. These features indicate that the mesoclots are primary and they were mineralized earlier than the surrounding matrix. The mineralization of mesoclots may have proceeded in two stages: (1) organomineralization of the cores through replacement of organic matter by minute organominerals resulting from anaerobic degradation of bacteria and EPS and (2) inorganic precipitation of the outer layers fostered by an increase in carbonate alkalinity in micro-environment due to organic matter decomposition. The thrombolites from the Mesoproterozoic Wumishan Formation may have formed through complex interactions between microbes and environments and represent the earliest known Precambrian biogenic thrombolites.