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Abstract:
Samples of the Cambrian microbial dolomites were collected from Penglaiba section, a well-exposed stratigraphic section in the northwestern area of the Tarim Basin. This study provides an analogue for mediated dolomites that can precipitate in microbial mats and biofilms. The Cambrian stromatolitic dolomites were studied using high-resolution scanning electron microscopy. The results are as follows: (1) dolomites with 50 nm to 100 nm spherical nanostructures are aggregated into minerals of larger sphericities; (2) nanospherical dolomites of 50 nm to 170 nm diameter are densely arranged as dumbbell-shaped or chained aggregates; (3) silicified filaments, as well as dumbbell-shaped and chain arrangements, are preserved as important microstructures. On the basis of sedimentological, compositional, geochemical, and petrographic data, the microstructures were interpreted as nanoglobules that function as bacteria in the nucleation and filament mineralization stages. The microstructures function as such because they are wrapped in extracellular polymeric substance (EPS) or mucus and mineralized fossils. Silicification accounts for the exceptional preservation of microbial mat structures, including biofilms, as well as filamentous and coccoid microbes. In addition, EPS process is capable of binding different elements, with preference for Si, Mg, and Ca. Such suitable composition favors microbe mineralization and dolomite nucleation on organic substrates. These microscopic structures suggest bacterial mineralization and provide visual evidence for the origin of microbial dolomites.
