Geologicheskii Vestnik

It is shown that the average Fe_T/Al value in the Upper Precambrian clay rocks of the Volga-Ural region is 0.50 ± 0.16. This is almost exactly equal to the value of this parameter in the “average shale” (0.54) and is comparable with the Fe / Al values obtained for the carbonate-clay and clay rocks of the Arlan Subformation of the Lower Riphean Kaltasa Formation [Sperling et al., 2014]. The latter, together with data on the values of a number of other indicators of the redox-status of bottom waters, served as the basis for the conclusion about the accumulation of sediments original for them in a water mass containing free oxygen. Our data dramatically (by more than an order of magnitude) expand the time frame of the existence of this type of water in the Late Precambrian sedimentation basins in the east of the East European Platform. This calls into question the widely held literature ideas about the specific (euxinic, ferruginous, etc.) oceans of the Precambrian and Meso-Neoproterozoic in particular.

Riphean, Vendian, Kama-Belsk aulacogen, Shkapovo-Shikhan depression, clay rocks, redox-status of the water column

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