Age constraints and stratigraphic significance of the microbiotes of the kaltasa formation of the lower riphean of the Volgo-Ural region

Based on previously obtained and new materials, the association of organ-walled microfossils represented by morphologically complex and large forms from the Kaltasa Formation is considered. In general, the Kaltasa microbiota resembles many associations of Neoproterozoic microfossils (late Riphean and Vendian). It was mistakenly interpreted as Late Riphean in its initial description, although the late Riphean index taxons were absent, and geological data indicated its early Riphean age. The early Riphean age of the Kaltasinskaya Formation has been confirmed in a series of studies in recent years using Re–Os, U–Pb and other modern methods of isotopic geochronology. Additionally, the low ratios of 87 Sr/ 86 Sr and almost invariant ō13C values in the surrounding sediments are comparable with the isotopic characteristics of Mesoproterozoic strata that are older than 1300 million years. The use of geochemical methods using high-reagent iron (FeHR), as well as carbon and sulfur isotopes showed the presence of aerobic conditions during the deposition of relatively deep-water sediments of the Arlan Member of the Kaltasinskaya Formation (FeHR/FeT ratio <0.38). This contradicts the general concept of the existence of an anoxic ocean in the Mesoproterozoic, but explains the presence in the microbiota composition of morphologically complex and large eukaryotic forms that are widely distributed in oxygenic environments of the Neoproterozoic.