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Mar Simonet Roda,Jeong-Hyun Lee 대한지질학회 2021 대한지질학회 학술대회 Vol.2021 No.10
Cretaceous lacustrine stromatolites developed in the Banyawol Formation, Hayang Group, Gyeongsang Basin, south-eastern Korea. The Banyawol Formation comprises dark gray mudstones with thin limestone beds that were deposited in a semi-arid lacustrine environment. Stromatolites of the Hayang Group are characterized by decimetre-scale mounds with millimetre-thick alternation of micrite and crystalline layers. In this study, we observed the Banyawol stromatolites using Electron Backscatter Diffraction (EBSD) and Energy-dispersive X-ray spectroscopy (EDS) to understand sedimentary processes in ultra-high-resolution records. Combining both techniques, we could distinguish three types of layers according to their texture, crystal morphologies and elemental composition. EBSD measurements showed that the crystalline layers are crystallographically well co-oriented, with the c-axes of the calcite always perpendicular to the stromatolite layers. The crystal morphologies are dendritic or fan-like, with highly misorientated internal structures. These crystals are morphologically similar to those observed in some modern freshwater stromatolites, which are interpreted to have imprints of cyanobacterial fascicules. In addition, very thin layers of detrital sediments (sometimes single-grain thickness) that separate calcite fibre crystals below and above are observed within the crystalline layers. The thin detritus layers are interpreted as occasional storm events that would have transported detritus on the stromatolite and sometimes interrupted the growth of the calcite fibre crystals. The micritic layers are characterized by the poorly co-oriented amalgamation of microscale grains (1–20 microns) that either shows depletion in magnesium with enriched silicon and aluminum, or enriched magnesium without a significant amount of silicon and aluminum. The former is interpreted to have resulted from the inclusion of a significant amount of detrital sediments in microbial mats, whereas the latter would have been more influenced by microbial precipitation that would have formed either aragonite or high-Mg calcite. We observe highly misoriented grains, where the misorientation is concentrated in specific regions inside the individual grains, indicating the incorporation of organic matters into the crystal structure. The differences in elemental composition likely reflect the original conditions of deposition as no clear sign of selective diagenesis is observed. This study demonstrates that EBSD combined with EDS is proven to be a good tool to (1) unravel the genesis and formation of the lacustrine stromatolites and (2) to have a better comprehension of the diagenetic processes in the stromatolite.
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