Assessment of the Conditions of Carbonate Rock Formation on the Karelian Craton in the Paleoproterozoic Based on Geochemical Data

A comparative analysis of the carbonate sedimentation conditions in two Paleoproterozoic basins (Onega and Pana-Kuolajarvi), located in the southeastern and northern areas of the Late Archean Karelian Craton is presented. The carbonate accumulation began in both basins during the Late Jatulian. Carbonate rocks in the Onega succession are predominantly dolostones, including stromatolite varieties; in the Pana-Kuolajarvi succession, they comprise both dolostones and limestones. During the Late Jatulian, cyanobacteria thrived in coastal marine settings of the Onega Basin. Some areas of the basin lost connection with the open sea at this time, facilitating the development of evaporation processes. The Pana-Kuolajarvi Basin lacked such diversity of cyanobacterial communities and any evaporitization. The geological and lithological data suggest that shallow, lagoon, playa lake, and sabkha environments existed in the Onega Basin in the Late Jatulian. The Pana-Kuolajarvi Basin was characterized by shallow (with occasional increased water input from the continent) and open marine settings. The geochemical characteristics of carbonate rocks obtained in our work lead to the same facies conclusions. Stromatolites in the Onega Basin were formed mainly in the intertidal zone. Occasionally, the connection of basin with the open sea was reduced and the conditions approached the lagoonal type. The oxic conditions existed for a limited time during the Jatulian only in the Onega Basin. Basically, the oxygen content in both sedimentation basins was close to the disoxic/oxic transition boundary. Fluctuations in the Ce anomaly magnitude in stromatolite laminas reflect changes in the oxygen content in water at immediate contact with the stromatolite buildup, suggesting the existence of oxygen “oases” in the basin with disoxic and oxygen-deficient conditions. Conclusions about redox conditions existing in the basin, based only on geochemical markers, are not sufficient, whereas conclusions about the carbonate sedimentation conditions prevailing in the basin agree maximally with the geological and lithological data based on several geochemical characteristics, such as REE spectra, Ce and Eu anomaly values, and ratios of individual REEs supported by the pair correlations between redox-sensitive elements.