TY - JOUR
T1 - Taphonomic and diagenetic implications of reduction spot formation in Cretaceous red beds from the Jiaolai Basin, Eastern China
AU - Fu, Wenzhao
AU - Turner, Peter
AU - Clements, Thomas
AU - Spencer, Alan RT
AU - Yu, Jifeng
AU - Yang, Yu
AU - Guo, Bangjie
AU - Ning, Zhenguo
AU - Zhuo, Zishun
AU - Riley, Michael
AU - Hilton, Jason
PY - 2023/3
Y1 - 2023/3
N2 - 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 northern 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 (Va), 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 Fe3+ and the eventual depletion or removal of the resulting Fe2+ (altering the colour of the reduction spot). Simultaneously, the reduction of V4+ and the consequent lowering of the concentration of V as V2+ minerals occur in the reduction spot, explaining their lower density than the host sediment.
AB - 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 northern 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 (Va), 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 Fe3+ and the eventual depletion or removal of the resulting Fe2+ (altering the colour of the reduction spot). Simultaneously, the reduction of V4+ and the consequent lowering of the concentration of V as V2+ minerals occur in the reduction spot, explaining their lower density than the host sediment.
KW - Continental red beds
KW - Diagenesis
KW - Vanadium
KW - Redox
KW - Taphonomy
KW - 3D X-ray Computed Tomography analysis
U2 - 10.1016/j.jseaes.2022.105533
DO - 10.1016/j.jseaes.2022.105533
M3 - Article
SN - 1367-9120
VL - 243
JO - Journal of Asian Earth Sciences
JF - Journal of Asian Earth Sciences
M1 - 105533
ER -