中国科学院南京地质古生物研究所机构知识库

Red colouring in marine red beds (MRB) is commonly attributed to deposition and early diagenesis under specific redox conditions. Therefore, the MRB can be considered time-specific facies. However, since red colouring is a sub-jective criterion, it is difficult to establish a colour limit for the MRB in the scale from grey to yellow, orange, pink to red. Using spectral reflectance, carbonate petrology, bulk-rock and in-situ geochemistry data from three sections of Ordovician orthoceratite carbonates of South China, we addressed the question whether the incipient reddening in the pink carbonates was associated with similar redox changes and palaeoceanographic conditions like in the MRB. The yellowish grey to greyish orange pink (Munsell Rock Colour Chart) carbonates with low concentrations of he-matite (< 0.01 %) are transitional from goethite-bearing grey to hematite-enriched true MRB. The red-coloured skel-etal interiors, microstromatolites, nodules and filamentous microborings suggest an extensive microbial activity which was accompanied by precipitation of authigenic aluminosilicates (clays). We hypothesize that the microbial clay precipitation is an important intermediate step in Fe transformation from its primary sources to hematite in the MRB. The carbonate deposition was followed by early diagenetic, shallow-subsurface REE fractionation, and Fe\\Mn (+Mo, U and V) redox cycling along microbially controlled redox microgradients. The geochemical redox signature of the pink carbonates is very similar to the MRBs of Devonian and Ordovician age. They were deposited under sim-ilar palaeoenvironmental conditions on a deeper shelf inhabited by skeletal heterotrophs, with reduced rates of or-ganic matter burial and slow sedimentation rates. The sedimentation of the pink carbonates and MRBs seem to randomly coincide with the coeval global sea-level changes and delta 13Ccarb fluctuations suggesting that the local controls of sediment colour override the global ones.(c) 2022 Elsevier B.V. All rights reserved.