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The Orgin of the Algal Dolomite in the Dengying Formation (Neoproterozoic, Southwestern Sichuan Basin, China)


The Neoproterozoic Dengying Formation, as the oldest gas-bearing reservoir in southern China, is dominated by a thick sequence of algal build-ups, which is composed entirely of dolomite. In this study, we have examined 13 core samples of algal dolomites from the Dengying Formation in the southwestern Sichuan basin in order to characterize the petrographical and geochemical features of this dolomite and further constrain its origin. In vertical cross section, the core samples have a laminated, cloddy or pseudo-peloidal structures typical of microbial features. In thin section, the samples display varying proportions of nearly opaque micritie and translucent microspar, together with fenestral pores filled with fibrous rim and sparry dolomite cement. Under cathodeluminescence, the fibrous cements are mostly non-luminescent, whereas micrite and microspar have a dull or non-luminescent, and sparry cements have a bright luminescence. Spot analyses show that the micritic dolomite crystals have a significantly higher amount of Ca (average of 52.4 mol%) than sparry dolomite (average of 48.6 mol%). According to the diffraction analysis of the X-ray, the degree of order in this dolomite is relatively low (ranging from 0.325 to 0.518). The average d18O values for the micritic-microspar host rock and fibrous rim dolomite are −4.3‰ and −5.7‰ VPDB, respectively. Relative to these parts, pore-filling sparry cements are depleted in d18O with an average value of −9.4‰. The dC of Fabric components among individual samples exhibit no substantive difference and the average values are −0.8‰ for host rock, 2.1‰ for fibrous rim cement and 1.4‰ for sparry cements. All these characteristics suggest that the algal dolomite is mainly a result of synsedimentary marine precipitation. Observations using scanning electron microscopy (SEM) reveal the presence of abundant microbial remnants preserved well in micritic dolomite. These microbial remnants include submicron-sized fragments of filamentous, coccoid bacteria and sheet-like bacterial forms, occurring within the primary intercrystalline space and in some cases being embedded in dolomite crystals. Former microbial activity, therefore, appear to be a powerful force driving dolomite precipitation. Hence, there is strong evidence that these algal dolomites formation was mainly microbially-mediated, seawater-derived and penecontemporaneous with deposition.