Palaeoproterozoic magnesite-stromatolite-dolostone-'red bed' association, Russian Karelia: Palaeoenvironmental constraints on the 2.0 Ga-positive carbon isotope shift

The ca. 2000 Ma Tulomozerskaya Formation, Russian Karelia, is composed of an 800 m-thick magnesite-stromatolite-dolostone-'red bed' succession with the most 13 C-rich dolostones (up to +18‰ V-PDB) that have ever been reported. Terrigenous 'red beds' are developed throughout the sequence and represent three main depositional settings: (1) a braided fluvial system over a lower energy, river-dominated coastal plain, (2) a low-energy, barred lagoon or bight, and (3) a non-marine, playa lake. A significant component of the sequence consists of biostromal and biohermal columnar stromatolites accreted in shallow-water, low-energy, intertidal zones, barred evaporitic lagoons and peritidal evaporitic environments. Only a small portion of stromatolites might have been accreted in relatively 'open' marine environments. The red, flat-laminated, dolomitic and magnesite stromatolites formed in evaporative ephemeral ponds, coastal sabkhas and playa lakes. Tepees, mudcracks, pseudomorphs after calcium sulphate, halite ca...