Nature of the Neoproterozoic oncolites of the Urinsk Uplift (the southern part of the Siberian Platform)

Abstract

Oncolites are widely distributed in the Neoproterozoic (Riphean and Vendian) deposits of Eastern Siberia. Their nature remained insufficiently substantiated by the factual material. Oncolites are also widely distributed in the Neoproterozoic sedimentary carbonate rocks of the Urinsk Uplift; they have been proved suitable for the identification of their nature. Many of these rocks (such as osagia tenuilamellata) are similar to oolites and other carbonate grains of chemogenic origin and are often difficult to distinguish without detailed studies of their nature. In the Neoproterozoic oncolites of the Urinsk Uplift, spherical and filamentous unquestionable microorganisms were found, which are presumably photosynthesizing iron bacteria, cyanobacteria and algae. They are represented by calcified cells, colonies and filamentous forms, and provide the first indication of the organogenic-sedimentary nature of the Neoproterozoic oncolites, rather than chemogenic, as accepted in some publications. The results obtained can be used to clarify the environmental conditions of sedimentation, which are associated with the prediction of potential carbonate reservoirs of oil and gas in the Neoproterozoic section of the Predpatomsk depression.