Sedimentology and geochemistry of the Uk Formation, Upper Riphean, South Urals

Research subject. Carbonates, phosphorites and banded iron formations have been increasingly treated as unique archives of information on the composition of ancient marine water and biomarkers. Materials and methods. This article presents new data on carbonate facies of the upper Uk subformation, Upper Riphean of the South Urals. A particular attention is paid to the distribution of rare-earth elements (REE) and yttrium (Y) in stromatolitic and intraclastic limestones and calcareous shales (bulk samples and their acetic acid leachates). Results. The most representative section of the Upper Uk Subformation located along the eastern edge of the town Ust-Katav can be subdivided into several members of different lithology and thickness: (1) bioherm-dominated member comprising large microbialitic build-ups and inter-bioherm sediments (intraclastic limestones, calcareous biolaminites); (2) transitional member characterised by small bioherms alternating with other carbonates; (3) interbedded coarse- and fine-grained limestones. The presence of molartooth structures in the carbonate rocks of Uk Formation made it possible to constrain the age of this Upper Riphean formation to pre-Cryogenian Conclusions. There are similarities in REE and Y distribution in both clean (devoid of siliciclastic component) bulk limestone samples and in their acetic acid leachates. PAAS-normalised REE + Y patterns demonstrate positive La, Gd, Y anomalies and negative Eu, Ce anomalies. The latter suggest marine depositional environments for the Upper Uk stromatolitic limestones. The results of the pioneering research into the composition of bitumens and biomarkers from the Upper Uk Subformation have shown that carbonates and shales are depleted in the organic matter. The source material for the organic matter was provided by two types of communities comprising both eukaryotes and prokaryotes and inhabiting well aerated environment, perhaps with lowered salinity of marine water.