Elsevier

Cretaceous Research

Volume 115, November 2020, 104482
Cretaceous Research

Discussion
Comment on “Relative bathymetric position of opoka and chalk in the Late Cretaceous European Basin” by M. Machalski and O. Malchyk [Cretaceous Res. 102 (2019) 30–36]

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Abstract

In their recent paper, Machalski and Malchyk, 2019, present a paleobathymetric model proposed relations among main facies types, including opoka and chalk, along a perpendicular to the shore. The model is seemed to be a pioneering one based on paleoecological and taphonomical data from the successions from Poland and western Ukraine. However, speaking about lithofacies, the authors do not take into account such crucial factors as mineral composition of rocks and origin of silica in the basin. “Camouflaged” pyroclastic material which is evidence of volcanic ash falls, could be among major contributors to mineral composition of opokas and among minors of chalks. Thus, considering this factor could be essential to sediment accumulation the paleobathymetric model would not face mismatches revealed in different parts of the basin. More research (including quantitative XRD and electron microscopy analysis) is necessary to study precise mineral composition and peculiarities of rock microfabrics in order to understand the proper origin of opokas and chalks, making a basis for more reliable paleobathymetric model of distribution of chalks and opokas in the Late Cretaceous European Basin.

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Acknowledgements

I thank all the reviewers for their critical comments and useful suggestions to improve an earlier version of the manuscript. The work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. The work was supported by the Ministry of Science and High Education of the Russian Federation contract No. 14.Y26.31.0029 in the framework of the Resolution No.220 of the Government of the Russian Federation.

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    However, a hydrothermal/volcanic origin of DSi for the Late Cretaceous siliceous nodules and their abiotic precipitation has not been widely accepted due to a lack of direct evidence (e.g. glass shards, ash, etc.) and geochemical signals enabling the identification of the volcanic/hydrothermal origin. With the increasing distance of magmatic sources, the products of volcanic eruptions are completely dissolved, and since the associations of opal-CT, clinoptilolite, and authigenic clays are no longer indicators (Jurkowska et al., 2019b) of ‘camouflaged pyroclastics’ (but see Zorina et al., 2018; Zorina, 2020 for a contrasting opinion; see also Machalski and Machlyk, 2020) only detailed microtextural studies of silica polymorphs can yet reveal the origin of DSi (Jurkowska et al., 2019b). These are similar to geochemical signals, which can be weakened during long periods of transport (Vogt, 1989).

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