Environmental Magnetism Of The Limestone-dolostone Ribbon Carbonates In Upper Doushantuo Formation In Yichang, South China

A series ribbon carbonates distributed in the upper part of the third lithostratigraphic member of Doushantuo Formation, Yichang. Previous study showed that the remanence parameter of silty dolostone was less than the adjacent micritic limestone. But the work didn't give good results on the magnetic interpretation, nor the geological processes and environmental implications involved. In this paper, detailed measurements of rock magnetism, CL and SEM have been carried out to discuss the environmental implications of remanence parameter and the formation of ribbon carbonates.Rock magnetic experiments reveal the predominant magnetic minerals in the dolostone and limestone are magnetite and geigite/pyrrhotite, and therefore the larger remanences of limestone indicate the larger content of magnetic minerals and finer grain size than adjacent dolostone. However, XRD results prove that the inputs of terrigenous detrital iron bearing minerals are abundant in the dolostone layer during the sedimentary or early diagenetic period. The combination of rock magnetism and XRD analysis suggest a strong effect of reductive diagenesis on dolostone and the selective dissolution of large number of fine grained ferromagnetic minerals results in the coarser grain size. Remanence parameters play very good response to the degree of reductive diagenesis, that is, the redox state during the deposition. The silty dolostone form in a partial reduction-bit environment and micritic limestone is relatively oxidation.Cathodoluminescence and scanning electron microscopy analysis show that the dolomite is the production of limestone replacement and dolomite has concentric zonation, which means that dissolved Fe²⁺ comes into dolomite lattice during dolomitization. Ribbon carbonates indicate that they form in early diagenetic period and the orgin of Fe²⁺ is dissolution of terrigenous detrital iron bearing minerals. Dolostones which subject to strong reductive diagenesis, the characteristic of pyrite and barite and carbon isotope anomalies are indicative of sulfate-reducing organic origin of dolomite. Larger inputs of organic matter result in strong reduction diagenesis, the energy barrier of dolomitization then was overcame and magnetic signal was weakened. Otherwise, less inputs of organic matter in limestone read to weak reduction diagenesis and high concentrations of sulfate inhibited dolomitization. The original remanence signal was preserved. The lithology of dolostone and limestone also reflect differences in the environment of its formation. The silty dolostone form in a partial reduction-bit environment and micritic limestone is relatively oxidation.The remanence parameters and lithologic features reflect the degree of reduction diagenesis. Both of them can be used as redox alternative indicators of the environment.