Rock Magnetism of Greigite-bearing Carbonate Concretions in the upper Doushantuo Formation (South China) and Its Environmental Implications
|School||Chinese Geology University (Beijing)|
|Keywords||carbonate concretion greigite inverse magnetic fabric reductive diagenesis Doushantuo Formation|
Abundant carbonate concretions are distributed in the black shale of the upper Ediacaran Doushantuo Formation in South China. Scanning Electron Microscope (SEM) and rock magnetism studies reveal that the predominant magnetic minerals in the concretions are greigites. Concretion grows through authigenic carbonates cementing detrital materials, and greigite usually forms authigenically in anoxic sedimentary environments in association with dissolution of reactive detrital iron-bearing minerals. So the study of growth and magnetic property of concretions are significant in revealing the early burial and diagenetic environment.Mineralogical and textural characteristics reveal the detrital components in concretions are similar to that of the host rocks, which suggest concretions were formed by authigenic carbonate cementation of detrital materials. High organic carbon content and abundance of framboidal pyrites in the hosting shale suggest an anoxic depositional environment. Well-preserved cardhouse clay fabrics in the comcretions suggest their formation at 0-3 m burial depth, likely associated with microbial decomposition of organic matter and anaerobic oxidation of methane. Gases through decomposition of organic matter and/or from methanogenesis created bubbles and cavities, and anaerobic methane oxidation at the sulfate reduction zone resulted in carbonate precipitation, filling in bubbles and cavities to form spherical structures of the concretions. Massive textures and pervasive spherical structures of the Doushantuo concretions suggest the pervasive model of the concretionary growth. Isolated carbonate crystals in the concretions are nucleated and crystallized simultaneously.Observation of SEM and results of rock magnetic study all indicate greigite exists in the concretions and host rocks. Especially in the concretions, the magnetic minerals are dominated by greigite, with trace magnetite. The measurement of anisotropy of magnetic susceptibility (AMS) reveals concretions have different magnetic fabric from host rocks. The host rocks show oblate AMS shape with magnetic foliation parallel to the bedding plane, which is referred to as a normal sedimentary magnetic fabric. The concretions show prolate AMS shape with magnetic lineation perpendicular to the bedding plane, which is referred to as an inverse magnetic fabric. Besides AMS, anisotropy of isothermal magnetic remanence (AIRM) are also measured to the sample of concretions and host rocks. The orientation of the principal axes of the AIRM ellipsoid is very similar to those of the AMS ellipsoid. Rock magnetism studies show that greigite contribute to the inverse magnetic fabric of the concretions. As compared with host rocks, concretions had resisted compaction through concretionary growth and preserved primary sedimentary fabric. This evidences that the inverse magnetic fabric have occurred at early sediment. Preferred crystallographic orientation of the greigite produces inverse magnetic fabric of the concretions, which is possibly related with early sedimentary environment. During early reductive diagenesis, vertical change of geochemistry in the sediment column and fluid flow possibly cause  axis perpendicular to the bedding plane. This primary magnetic fabric was preserved in the concretions during concretionary growth and change into normal sedimentary-compaction magnetic fabric in the host rocks being greigite  axis parallel to the bedding plane by compaction.