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首页> 外文期刊>Sedimentary geology >Dolomitized cells within chert of the Permian Assistencia Formation, Parana Basin, Brazil
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Dolomitized cells within chert of the Permian Assistencia Formation, Parana Basin, Brazil

机译：巴西巴拉那盆地二叠纪Assistencia组的硅质岩中的白云石化细胞

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Dolomitic microscopic structures in the form of microspheres, "horseshoe- shaped" objects, and thin botryoidal crusts found within microfossiliferous chert within stromatolites of the Evaporite Bed (EB) of the Permian Assistencia Formation, Irati Subgroup, Parana Basin, Brazil, have been investigated by means of optical microscopy, X-ray fluorescence, scanning electron microscopy, Raman spectrometry and energy-dispersive X-ray spectrometry. The microspheres were identified as dolomitized coccoidal cyanobacteria based on similarity in size, spheroidal and paired hemispheroidal morphologies and colonial habit to co-occurring silicified organic walled cyanobacteria embedded within the same microfabric and rock samples. The co-occurrence of dolomite, pyrite framboids, and abundant dispersed carbonaceous material and silicified cells is consistent with a hypersaline depositional environment with abundant cyanobacterial mats and elevated Mg2+/Ca2+ ratios and reducing conditions with active anoxic microbial processes near the water-(bio)sediment interface. The abundance of extracellular polymeric substances facilitated anoxic microbial processes (sulfate reduction), providing essential conditions for possible primary microbially induced dolomitization. In most of the dolomitized cells dolomite occurs only as an external layer; in fully dolomitized cells magnesium is richest in the outermost layer. Presumably, the dolomitization process was favored by the presence of anoxic microbial degraders and negatively charged functional groups at the surface of the cyanobacterial cells. Botryoidal dolomite rims of silica-filled fenestrae formed by a similar process and inherited the botryoidal morphology of the cell as originally lining the fenestrae. Silicification interrupted the dolomitization of the largely organic biosediment, mostly by permineralization, but locally by substitution, thereby preserving not only dolomitic microspheres, but also huge numbers of structurally well-preserved organic-walled cyanobacteria and portions of microbial mat. Clearly, dolomitization began very early in the microbial mats, prior to compaction of the sediment or full obliteration of cellular remains, followed very closely by silicification thereby impeding continued degradation and providing a window onto very well-preserved Permian microbial mats. (C) 2016 Elsevier B.V. All rights reserved.
机译：研究了巴西巴拉那盆地伊拉蒂亚组二叠纪Assistencia组蒸发岩层（EB）的微化石硅质岩在微化石硅质within石中发现的以微球，“马蹄形”物体和薄的类葡萄状壳形式的白云岩微观结构。借助光学显微镜，X射线荧光，扫描电子显微镜，拉曼光谱和能量色散X射线光谱。根据大小，球状和成对的半球状形态的相似性以及与在同一微结构和岩石样品中共生的硅化有机壁蓝细菌的殖民地习性，将微球鉴定为白云石化球状蓝细菌。白云石，黄铁矿黄铁矿，大量分散的碳质物质和硅化细胞的共存与高盐度沉积环境，丰富的蓝藻垫和提高的Mg2 + / Ca2 +比率以及在水-（生物）附近具有活跃的缺氧微生物过程的还原条件相一致。沉积物界面。大量的细胞外聚合物促进了缺氧的微生物过程（硫酸盐还原），为可能的初级微生物诱导的白云石化提供了必要条件。在大多数白云石化的细胞中，白云石仅作为外层存在。在完全白云石化的细胞中，镁在最外层最丰富。据推测，由于在蓝细菌细胞表面存在缺氧微生物降解物和带负电的官能团，白云化过程受到了青睐。二氧化硅填充的窗饰的葡萄状白云岩边缘通过类似的过程形成，并继承了原本排列在窗饰中的细胞的晶状体形态。硅化作用主要是通过矿化作用，但大部分是通过取代作用，从而中断了大部分有机生物沉积物的白云石化作用，从而不仅保留了白云质微球，而且还保留了大量结构良好保存的有机壁蓝细菌和部分微生物垫。显然，白云石化作用是在微生物垫很早就开始的，然后沉积物被压实或细胞残渣被完全消灭，随后硅化作用非常紧密，从而阻止了继续降解，并为保存良好的二叠纪微生物垫提供了一个窗口。 （C）2016 Elsevier B.V.保留所有权利。

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