Taphonomic and diagenetic implications of reduction spot formation in Cretaceous red beds from the Jiaolai Basin, Eastern China. (March 2023)

• Record Type: Journal Article
• Authors: Fu, Wenzhao; Turner, Peter; Clements, Thomas; Spencer, Alan R.T.; Yu, Jifeng; Yang, Yu; Guo, Bangjie; Ning, Zhenguo; Zhuo, Xizhun; Riley, Michael S.; Hilton, Jason
• Is Part Of: Journal of Asian earth sciences. Volume 243(2023)
• Publication Date: 2023-03
• Subjects: Continental red beds -- Diagenesis -- Vanadium -- Redox -- Taphonomy -- 3D X-ray Computed Tomography analysis; Earth sciences -- Asia -- Periodicals; Sciences de la terre -- Asie -- Périodiques; Earth sciences; Asia; Periodicals; 555.05
• DOI: 10.1016/j.jseaes.2022.105533
• Languages: English
• ISSNs: 1367-9120

Abstract

Highlights: First reduction spot X-CT analysis shows lower density than surrounding sediment. Microbially decay of organic matter in water needed to form reduction spots. Model proposed for formation of reduction spots in continental red beds. Red alluvial sediment and in-situ intrastratal diagenetic alteration colours red beds. Abstract: Green-grey coloured reduction spots are common in continental red beds through geological history and occur in a range of different lithologies and depositional environments, but their timing and mode of formation remain controversial. We investigate the Late Cretaceous to earliest Paleogene Jiaozhou Formation using borehole data from the Jiaolai Basin in Shandong province of eastern China, and consider the distribution, morphology, and geochemistry of reduction spots in these continental red beds to evaluate how the reduction spots formed. Here, we report a novel application of three-dimensional X-ray Computed Tomography (XCT) to analyse reduction spot morphology, composition and density. Our data show that individual reduction spots are spheroidal, tubular or irregular shaped, and often contain small, grey, dark brown or black organic cores, referred to as loci. Typically, reduction spots have a similar chemical composition to the host red beds, but with elevated levels of vanadium (V), lower levels of iron (Fe), and lower density. Isolated, small refractory fossils (e.g., charcoal) in the sediment alongside reduction spots but not within them indicates that microbial decay of organic labile (reactive) tissues in early diagenesis is an important control in reduction spot formation. We propose a new taphonomic model of reduction spot formation: post burial, during the primary sedimentary cycle in the groundwater zone, vanadium is released by intrastratal oxidation of titanomagnetite. Decay of organic matter creates localised reducing conditions resulting in the reduction of Fe 3+ and the eventual depletion or removal of the resulting Fe 2+ (altering the colour of the reduction spot). Simultaneously, the reduction of V 4+ and the consequent lowering of the concentration of V as V 2+ minerals occur in the reduction spot, explaining their lower density than the host sediment.