Abstract: Reefal (biohermal) carbonate rocks are important petroleum reservoirs. In Tarim Basin, the occurrences of reefal facies in the Ordovician sequences have been well documented, whereas that in the Cambrian strata have been rarely reported. In this study, the occurrences of stromatolite dolomite and reefal dolomite in the Cambrian Qiulitage Formation at Keping area, Tarim Basin were reported. Based on careful field observations, the micro-structures of these microbial dolomite were investigated by using conventional microscope and scanning electron microscope (SEM). It is suggested that the microbial reefs formed in shallow subtidal environment where the water was turbulent, the deposition rate was relatively high and the platform was relatively open. In contrast, the stromatolite dolomites were developed in tidal flat-intertidal facies. Nanoscale spherical dolomite formed nanocrystal aggregats including dumbbell and chain-like structures. In addition, tubular and sheet-like aggregates were suggested to be in close association with the mineralization of extracellular polymeric substances (EPS). All the above nanostructures resemble the morphology observed in microbial culture experiments. These evidences support the conclusion that microbial mediation might have played an important role in the formation of the Cambrian dolomite. During the formation of microbially mediated dolomite, sulfate reducing bacteria did not play an important role because no pyrite was observed, then SO₄^2- may not serve as an important kinetic inhibitor for the precipitation of dolomite. Thus it is proposed that the hydration of Mg²⁺ should be emphasized in the future study on the origin of dolomite.

Key words: stromatolite dolomite, reefal dolomite, microbial dolomite, origin of dolomite, Cambrian, Tarim Basin