Mesoproterozoic silicified microbiotas of Russia and India—Characteristics and Contrasts

Abstract

The paper analyses eight silicified Mesoproterozoic microbiotas of peritidal and shallow subtidal settings from Siberia, Ural and India. These microbiotas, subdivided into three main types - Kotuikan, Satka and Kataskin-are characterized by different taxonomic composition of microfossils. Mat-building entophysalidacean algae Eoentophysalis, ellipsoidal akinetes of nostocalean cyanobacteria genus Archaeoellipsoides and spherical large planktic microfossils Myxococcoides grandis of uncertain affinities dominate the Kotuikan-type microbiotas, the short trichomes are a rare but a distinctive element of these assemblages. The Satka type microbiotas are dominated by mat-building hormogonian cyanobacteria of genus Siphonophycus and chroococcacean dwellers genera Gloeodiniopsis, Eosynechococcus, Sphaerophycus, whereas entophysalidacean cyanobacteria are conspicuously missing and akinetes of genus Archaeoellipsoides occur but never abundant. Besides, microbiotas of Satka type include morphologically simple remains of phytoplanktic eukaryotic microorganisms–sphaeromorphic acritarchs genera Satka, Pterospermopsimorpha, Granomarginata? and Leiosphaeridia. The late Mesoproterozoic Kataskin-type microbiotas contain mat-forming entophysalidacean, oscillatoriacean and nostocalean as well as mat-dwelling and planktic chroococcacean cyanobacteria, but the most typical feature of these microfossil assemblages is the presence of a stalked cyanobacterium, Polybessurus bipartitus. Almost all-available data on relevant silicified Mesoproterozoic microbiotas from China, Greenland and North America have been analysed. Further different types of Mesoproterozoic silicified microbiotas have been compared with Palaeo- and Neoproterozoic microbiotas in cherts as well as with the assemblages of organic-walled microfossils throughout the world and explained differences and similarities in their composition. The analysis indicate that the Mesoproterozoic microbiotas have their own specific taxonomic composition and differ from the Palaeo- and Neoproterozoic microfossils occurring in the same and different palaeoenvironmental setting. The presence of newly evolved type of cyanobacteria, red algae and acanthomorphic acritarchs in the Kataskin-type microbiotas and contemporaneous open-shelf facies suggest that the terminal Mesoproterozoic can be separated as an independent biostratigraphical unit.