Mesoproterozoic Micobialites From North China Patform And Their Paleoceaographic Significance

The Wumishan Formation is one of the most widely distributed lithostratigraphic units in the Mesoproterozoic of North China Platform and dominated by various dolomites of tidal flat origin, showing clearly rhythmic deposition. The succession is made up of numerous parasequences which are typically composed of thrombolitic dolomite, biolaminitic dolomite with cherty bands and micritic dolomite, showing a clear tendency of shallowing upward. Abundant microbilites have been recognized from this formation, including thrombolite of most characteristic, followed by stratiformed, mound-shaped, and conical stromatolites. In the subtidal lagoon or deep subtidal zone there outputs black biolaminite and microdigital stromatolites.The presence of abundant well-preserved microbial mats and various microbialites suggests strong activity of microbial communities in the Mesoproterozoic epicontinental sea of North China. Their interaction with sedimentary environments resulted in the microbialites with morphology highly variable. In addition, some fabrics which may derive from microorganism etabolisms have also been identified, such as biolaminitic, oolitic, micritic components, as well as microbially coated slender spins and MISS (microbilly induced Sedimentary structures). The study also shows that different microbialites in the formation are in good coincident to the water depth of deposition. The black biolaminite and microdigital stromatolite developed mainly in subtidal lagoon or deep subtidal zone, thrombolite largely in shallow subtidal, a variety forms of stromatolite more often in shallow subtidal to lower intertidal zone, laminated dolomite with chert bands largely in intertidal zone. While dolomicrite with rare mats is commonly restrictic to upper intertidal to supraltidal zone.Abundant fibrous aragonites have been identified from the microbialites. Due to the high CO₂ in atmosphere and low oxygen in ocean during Mesoproterozoic, bacterial sulfate reduction(BSR) mihght have resulted in oversaturated CaCO3 in the water, thus induced the precipitation of authigenic carbonate on sea-floor directly. The present study of microstructures of various carbonates in this formation shows that the black biolaminite and microdigital stromatolites in deep subtidal or subtidal lagoon are typically composed of alternated microbial mats and isopachous acicular aragonites. Subtidal thrombolitic dolomite is mainly composed of organic clots encrusted by fan-shaped or ring-shaped acicular aragonites, but rare micritics. Laminated dolomite and stratiform stromatolite in shallow subtidal to the lower intertidal zone are mainly composed of micritic carbonate or cements precipitated naturally, with a few isopachous, fan-shaped, or botryoidal aggregate of aragonites that might be generated as pore-fillings. However, dolomite in upper intertidal to supratidal zone is almost completely composed of carbonate micritics, with no acicular aragonites.In general, from deep subtidal to supratidal, fibrous aragonites show a clear tendency of decline, suggesting that BSR was active in deep subtidal, locally in upper subtidal and lower intertidal in the sediments underlying microbial mats, but absent from supratidal zone. This gradient may reflect the variation of redox in environments of various water depth. Thus aragonite sea-floor precipitation may have a potential to be used an indicator for redox condition in carbonates.