Ooidal ironstones in the Meso-Cenozoic sequences in western Siberia: assessment of formation processes and relationship with regional and global earth processes

Abstract

This study investigates the process of formation of ooidal ironstones in the Upper Cretaceous-Paleogene succession in western Siberia. The formation of such carbonate-based ironstones is a continuing problem in sedimentary geology, and in this study, we use a variety of data and proxies assembled from core samples to develop a model to explain how the ooidal ironstones formed. Research on pyrite framboids and geochemical redox proxies reveals three intervals of oceanic hypoxia during the deposition of marine ooidal ironstones in the Late Cretaceous to the Early Paleogene Bakchar ironstone deposit in western Siberia; the absence of pyrite indicates oxic conditions for the remaining sequence. While goethite formed in oxic depositional condition, chamosite, pyrite and siderite represented hypoxic seawater. Euhedral pyrite crystals form through a series of transition originating from massive aggregate followed by normal and polygonal framboid. Sediments associated with goethite-chamosite ironstones, encompassing hypoxic intervals exhibit positive cerium, negative europium, and negative yttrium anomalies. Mercury anomalies, associated with the initial stages of hypoxia, correlate with global volcanic events. Redox sensitive proxies and ore mineral assemblages of deposits reflect hydrothermal activation. Rifting and global volcanism possibly induced hydrothermal convection in the sedimentary cover of western Siberia, and released iron-rich fluid and methane in coastal and shallow marine environments. This investigation, therefore, reveals a potential geological connection between Large Igneous Provinces (LIPs), marine hypoxia, rifting and the formation of ooidal ironstones in ancient West Siberian Sea.

Depositional Conditions of Cretaceous Ironstones Deposit in the Chulym-Yenisey Basin (Western Siberia)

“…The sedimentary record of the Ilek and Kiya Formations [34,90] includes known oceanic anoxic events [91][92][93], such as the Early Aptian anoxia (OAE Ia) and Cenomanian-Turonian anoxia (OAE 2). A relationship was established between the hypoxia of the bottom water and global oceanic anoxia during the Cretaceous period in coeval marine ooidal ironstone deposits in Western Siberia [47]. The global geological events possibly influenced regional geological processes within the Chulym-Yenisey basin.…”

“…The absence of the Ce* and the posit Eu* are also characteristic of the studied sediments. [47]. Normalization factors are from published data [66].…”

“…Upper Continental Crust (UCC)-normalized multi-element spider diagram (areas represent the minimum and maximum values) for ironstones and wall rocks of the Chulym-Yenisei basin[62,66]. Data of the Bakchar marine ironstone deposit are plotted for comparison[47].…”

“…Figure 9. Post-Archean Australian shale (PAAS)-normalized REE-Y (REY) patterns of Upper Cretaceous iron-bearing sediments within the Chulym-Yenisei basin (areas represent the minimum and maximum values) compared with marine ironstones from the Bakchar deposit[47]. Normalization factors are from published data[66].…”

The formation of authigenic deposits during Paleogene warm climatic intervals: a review

“…Therefore, an increase in productivity and related oxygen deficiency provides abundant ferrous iron in shelf waters, thus facilitating massive ironstone deposits during the warm climatic intervals (Homoky 2017;Konhauser et al 2017). While the pyrite can be formed in hypoxic and anoxic seawater, chamosite and/or berthierine formation is favoured in hypoxic seawater conditions (Berner 1981;Taylor and Macquaker 2011;Todd et al 2019;Rudmin et al 2020). Rudmin et al (op.…”