Characteristics and Genesis of Ironic Oolitic Siltstone of the Benxi Formation in the Liujiang Basin
A distinct layer of Ironic oolitic siltstone stratum is observed at the lower part of the Benxi Formation of the Middle Car-boniferous in Liujiang Basin,exhibiting prominent lithological characteristics and consistent thickness within the region.According to the observation of rock thin sections and scanning electron microscopy,ironic oolites exhibit a diameter ranging from 0.1 to 1.0 mm,characterized by a ring structure with indistinct boundaries consisting of 3 to 5 layers enclosing fine quartz silt particles.According to the results of energy spectrum analysis,the oolite exhibits varying elemental compositions including Fe,Si,Al,O,K,C,and Ti in distinct regions.Through comprehensive analysis,it is determined that the predominant mineral constituents comprise limonite,hema-tite,quartz,and clay minerals.Through the analysis of trace elements,it is inferred that during the initial sedimentation of the Benxi Formation,the region was characterized by a relatively warm and semi-humid climate,a partially enclosed reducing environment bay,and brackish water conditions.After a prolonged period of weathering and erosion,the surface underwent deposition,resulting in a relatively enriched residual Fe element.Subsequently,through sedimentation and runoff processes,it was transported into the bay.The participation of organic carbon facilitated the formation of siderite(FeCO3)through its combination with Fe2O3.This resulted in the gradual deposition of siderite layers around suspended small quartz particles in the water body,which eventually settled on the seabed as particle concentration increased.Siderite undergoes diagenesis,leading to its instability and subsequent decomposition into Fe3O4.This process is further accompanied by oxidation,ultimately resulting in the transformation of siderite into limonite[FeO(OH)·H2O].Additionally,under extremely arid and hot conditions during later stages,siderite readily oxidizes into hematite,thereby giving rise to the coexistence of limonite and hematite within irony ooids.
iron oolitesideritelimonitehematiteBenxi FormationLiujiang Basin
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