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.
Highlights
6 Conclusions Based on the studies of pyrite framboids and major and trace elements concentrations in core samples collected from the two wells penetrating the ironstone deposits in western Siberia, we can draw the following conclusions: 1) Pyrite of different sizes occurs in various morphological forms such as framboids, irregular and subhedral masses, and euhedral crystals
Narym and Kolpashevo ironstones encompassing the hypoxic intervals reflect to different mineral assemblages and geochemistry
The formation of ooidal ironstone is closely linked to the hypoxic shelf water
Summary
Ooidal ironstones represent non-siliceous sandy and clayey sediments consisting of at least 5% iron oolith/ooid and 15% iron (Young 1989; Petranek and Van Houten 1997), and form abundantly during the Ordovician-Devonian, the Jurassic-Cretaceous and the Paleogene (Van Houten and Bhattacharyya 1982; Van Houten and Arthur 1989; Young 1989) but are rare in modern sediments (Kimberley 1994; Heikoop et al 1996; Sturesson et al 2000). Several studies indicated that mineralogical and chemical compositions of ironstones implicate the mode of origin as well as depositional conditions influence (Van Houten and Purucker 1984; Maynard 1986; Kimberley 1989; Siehl and Thein 1989; Sturesson et al 2000). Recent investigations explore the relationship between periods of intense volcanism (Large Igneous Provinces, LIPs), global ocean anoxic events and the formation of ooidal ironstone deposits (Percival et al 2015; Ernst and Youbi 2017; Scaife et al 2017; Keller et al 2018). Van Houten (1986) and Van Houten and Arthur (1989) indicated a relationship between ooidal ironstones and anoxic events, this is yet to be investigated thoroughly (Turgeon and Creaser 2008; Jenkyns 2010; Raven et al 2018). The origin of the giant ironstone in the Meso-Cenozoic Bakchar deposit has been debated, but no study attempts to relate hypoxia, global volcanism and ironstone deposits, because the integration of data related to mineralogical and chemical composition of ironstones with proxies for redox and volcanism is still lacking
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