P. Yu. Petrov

Paleobiology of the Mesoproterozoic-Neoproterozoic transition: the Sukhaya Tunguska Formation, Turukhansk Uplift, Siberia

Silicified carbonates of the latest Mesoproterozoic Sukhaya Tunguska Formation, northwestern Siberia, contain abundant and diverse permineralized microfossils. Peritidal environments are dominated by microbial mats built by filamentous cyanobacteria comparable to modern species of Lyngbya and Phormidium. In subtidal to lower intertidal settings, mat-dwelling microbenthos and possible coastal microplankton are abundant. In contrast, densely woven mat populations with few associated taxa characterize more restricted parts of tidal flats; the preservation of vertically oriented sheath bundles and primary fenestrae indicates that in these mats carbonate cementation was commonly penecontemporaneous with mat growth. Eoentophysalis mats are limited to restricted environments where microlaminated carbonate precipitates formed on or just beneath the sediment surface. Most microbenthic populations are cyanobacterial, although eukaryotic microfossils may occur among the simple spheroidal cells interpreted as coastal plankton. Protists are more securely represented by large (up to 320 micrometers in diameter) but poorly preserved acritarchs in basinal facies. The Sukhaya Tunguska assemblage contains 27 species in 18 genera. By virtue of their stratigraphic longevity and their close and predictable association with specific paleoenvironmental conditions, including substrates, Proterozoic cyanobacteria support a model of bacterial evolution in which populations adapt rapidly to novel environments and, thereafter, resist competitive replacement. The resulting evolutionary pattern is one of accumulation and stasis rather than the turnover and replacement characteristic of Phanerozoic plants and animals.

New paleomagnetic data on the Anabar Uplift and the Uchur-Maya region and their implications for the paleogeography and geological correlation of the Riphean of the Siberian Platform

The results of paleomagnetic studies of the reference sections of the Riphean and Late Proterozoic intrusive bodies of two remote areas of the Siberian Platform are presented. Within the limits of the Uchur-Maya region the sedimentary rocks of the Gonam, Omakhta, Ennin and Konder formations were studied; and the Riphean sedimentary rocks of the Burdur and Kotuykan formations on the northern and western slopes of the Anabar Uplift and, also, the Late Proterozoic intrusive complexes, located in the basins of the Fomich, Magan, Dzhogdzho and Kotuykan Rivers were studied. The paleomagnetic poles obtained in the course of this work and the present-day geochronological data give grounds to assume that: (1) the accumulation of the Riphean of the Anabar Uplift occurred after the formation of the Uchurskaya series of the Uchur-Maya region and was completed in approximately 1.5 Ga; (2) the Konder layers, compared according to the correlation pattern accepted at the present time [Semikhatov and Serebryakov, 1983] with the bottoms of the Totta formation, can be related to the appreciably more ancient stratigraphic level; (3) the intrusion of the studied intrusive bodies of the northern and western slopes of the Anabar Uplift occurred nonsimultaneously, although within close time intervals of approximately 1.5 Ga. The estimates of the kinematic parameters of the drift of the Siberian Platform within an interval of 1.7–1.0 Ga is carried out.

New Riphean microbiotas of the Billyakh Group, the north Anabar region (Fomich River basin): To Riphean biostratigraphy of the Siberian platform

Excellently preserved organic-walled and silicified microfossils are first found in the Lower Riphean Ust-Il’ya and Kotuikan formations of the Billyakh Group in the northern slope of the Anabar Uplift (the Fomich River basin). Similar assemblages were previously known only from sections located southward in the Kotuikan River basin, and taxonomic composition of organic-walled microbiotas from the Ust-Il’ya and Kotuikan formations became a corner stone in competitive microphytological models that are based on different approaches. In their composition and general appearance, microbiotas from the Kotuikan and Ust-Il’ya formations in the Fomich River basin are similar to microbiotas reported from the Kotuikan River basin, although northern sections of the above formations characterize deeper sedimentation settings than in localities known before. The Ust-Il’ya and Kotuikan assemblages of organic-walled microfossils include sphaeromorphic Chuaria circularis and Leiosphaeridia, two-layer vesicles the genus Simia, filamentous Plicatidium and Taenitrichoides, and some others. The silicified microbiota from the lower Kotuikan Subformation is largely composed of akinetes of Anabaena-like cyanobacteria Archaeoellipsoides, spherical Myxococcoides grandis, and short trichomes Filiconstrictosus and Orculiphycus representing initial germination stages of Anabaena-like cyanobacterial spores. Acanthomorphic acritarchs known from lithology-similar Lower and Middle Riphean (Mesoproterozoic) formations of Australia and China have not been observed in the Ust-Il’ya and Kotuikan microbiotas, which are probably of older age. The found microbiotas outline substantially wider distribution area of organic-walled and silicified microfossils, supplement microphytological characteristics of Riphean sediments in the Anabar Uplift, provide information on taxonomic composition of microbiotas from a wider spectrum of facies, and specify relationships between Early and Middle Riphean assemblages of microorganisms from different continents.

The Mukun basin: Settings, paleoenvironmental parameters, and factors controlling the early mesoproterozoic terrestrial sedimentation (Lower Riphean section of the Anabar uplift, Siberia)

Quartz arenites of the Lower Riphean Mukun Group were formed 1.58 Ga ago exclusively in terrestrial settings of the alluvial and eolian-fluvial sedimentation. The subordinate fluvial-sabkha sediments were deposited in intracontinental lacustrine basins. Analysis of paleohydrological and granulometric data, distribution of sedimentary fluxes, and influence of the biotic factor on sedimentogenesis served as a basis for the facies-paleogeographic and basinal-stratigraphic reconstructions. These materials provided grounds for defining a peculiar eolian-fluvial type of microbial sedimentation and revealing the scale and mechanisms of the influence of terrestrial microbial ecosystems on the formation of the sedimentation basins and their sedimentary successions. It is shown that total microbial fixation and biostabilization of sediments resulted in the suppression of fluvial transport, the removal of a significant share of sedimentary material from them, and the formation of peculiar eolian-fluvial landscape, i.e., a system of very shallow ponds connected by intermittent water streams and separated by changeable eolian fields. Development of such landscapes was responsible for the accumulation of thick terrestrial quartz arenite sequences. The microbial type of eolian-fluvial sedimentation characteristic of the Early Proterozoic and initial Riphean basins determined proportions of sandy sediments on continents and shelves.

Microbial Mats As a Source of Carbonate Sediments in the Late Precambrian: Evidence from the Linok Formation, the Middle Riphean of the Turukhansk Uplift, Siberia

The Linok Formation is made up of clayey and carbonate strata, 180–300 m thick, formed at the terminal Middle Riphean on the northwestern margin of the Siberian Platform. In the modern structure, it is exposed in the lower part of the Turukhansk Uplift section. The sediments accumulated in the distal part of the epiplatformal basin as a symmetrical transgressive–regressive cycle. Its lower part represents a deep-water basin environment with the mixed carbonate–clayey sedimentation, whereas the upper part reflects the origination and evolution of a carbonate platform. Microstructures discussed in this work suggest not only the ancient existence of benthic microbial assemblages (mats) but their active influence upon the facies pattern of sediments as well. The influence was determined by the ability of mat-forming communities to produce carbonate sediments under certain environmental conditions. The analysis of the facies succession suggested the absence of an appreciable influx of carbonate material to the basin from other sources. Based on the carbonate generation ability, one can distinguish three (carbonate-free, low-productive, and high-productive) groups of microbial communities. Groups 1 and 2 represent deep-water basin mats, whereas group 3 represents relatively shallow-water platformal microbial–mineral systems. The carbonate productivity of communities is inversely proportional to the depth of their dwelling and the relative rate of clayey sedimentation. The morphological reconstruction of microbiolite structures showed that the structures in basins and platforms greatly differed in terms of the size of elements. The ability of microbial communities to generate carbonate could be realized only within large ecosystems.

Facies characteristics and terrigenous sedimentation features of the lower Riphean Mukun Group (Anabar Uplift, Siberia)

The Mukun Group represents a thick (>750 m) complex of terrigenous features sediments in the lower part of the Riphean sedimentary cover of the Anabar region was formed on the spacious cratonic block. The complex is remarkable for its pure quartzarenitic composition. The facies analysis with emphasis on the genetic interpretation of various sedimentary structures revealed three facies associations in its sedimentary succession: alluvial, eolian-fluvial, and fluvial-sabkha. It is shown that sedimentary material was transported into the terminal basin by a braided system of permanent rivers. Relationships between the eolian and fluvial components in the single sedimentation system of the Mukun paleobasin is considered in line with the proposed model, according to which alternation of long periods with the eolian and fluvial sedimentation was controlled by tectonics.

Microbial factor in the formation of sedimentary continental systems of the Proterozoic (Mukun Basin, Lower Riphean of the Anabar Uplift of Siberia)

Microbially induced sedimentary structures of quartz arenites of the Early Meso-Proterozoic (Lower Riphean, 1.58 Ga) Mukun Basin of the Anabar Uplift of the Siberian Platform are considered. The effect of microbial mats and biofilms on the formation of fluvial landscapes and architecture of the sedimentary basin is shown. It is concluded that continental sedimentary–microbial systems influence the distribution of sandy deposits between continent and shelf in large transgressive sequences of Proterozoic deposits.

Paleomagnetism of the upper Riphean deposits in the Turukhansk and Olenek uplifts and Uda Pre-Sayan region and the neoproterozoic drift of the Siberian Platform

A series of new paleomagnetic results have been recently obtained for the Neoproterozoic of Siberia. Nevertheless, the Neoproterozoic segment of the Apparent Polar Wonder Path (APWP) for this craton and its paleogeography are still unclear. A definite solution of these tasks will certainly take many years of dedicated efforts. However, even now we may take an important step in this direction by establishing the general displacement trend of the Neoproterozoic paleomagnetic poles of the Siberian Platform. For doing this, we need to obtain several high-quality paleomagnetic results whose age corresponds to the least complete (the sparsest) part of the Neoproterozoic paleomagnetic record. In the scope of this task, we carried out paleomagnetic studies of the Late Riphean sedimentary and intrusive rocks from the key Late Riphean sections of the Turukhansk region (Sukhaya Tunguska, Derevnya, and Miroedikha formations), Olenek Uplift (Upper Khaipakh subformation), and Uda Pre-Sayan region (Tagul Formation of the Karagas Group, Nersa intrusion). Based on the results of these investigations, together with the previous data, we suggest two new alternative models of the Neoproterozoic segment for the Siberian APWP and analyze some tectonic implications of these models.

Molar tooth structures: Formation and specificity of carbonate diagenesis in the Late Precambrian, Middle Riphean Sukhaya Tunguska Formation of the Turukhansk Uplift, Siberia

A new way of formation of the problematic Molar Tooth (MT) structures, which, along with stromatolites, could be considered as a “visiting card” of the Riphean, was examined on the example of carbonate sediments of the Riphean Sukhaya Tunguska Formation from the Turukhansk Uplift, Siberia. These structures were formed due to the consequent replacement of oozy constituent of carbonate sediments with calcitic microsparite in the course of diagenesis with substantial shift of the dynamic equilibrium toward the solution in the sediment—porous solution—microsparite system. An excess of soluble magnesium significantly hampering precipitation of crystal cores of the future calcitic microsparite could be one of the possible causes of the shift. It is suggested that magnesium mole fraction of the porous solutions was related to the early dissolution of the metastable high-Mg calcite of the silty sediment brought from the areas of active stromatolite formation. The facies occurrence of the MT-structures indicates that they occurred in descending trails of the most productive zones of carbonate platforms tapering toward the basin. The synchronism of stratigraphic trends of the MT-structures and stromatolites in the Precambrian resulted from the evolution of carbonate-productive microbe-mineral systems which were the immediate suppliers of fine-grained carbonate sediment into external zones of carbonate platforms.

The genus Vendomyces Burzin and facies-ecological specificity of the Staraya Rechka microbiota of the Late Vendian of the Anabar Uplift of Siberia and its stratigraphic analogues

An oligotaxon association of organic-walled microfossils from the Upper Vendian Staraya Rechka Formation of the Anabar Uplift of Siberia is analyzed. Taxonomic and paleobiological interpretation of microfossils, most of which are the fungiform organisms Vendomyces major Burzin. It is shown that Vendomyces was the main destructor of necromass of the primary producers of benthic cyanobacterial communities. It is proposed that, in the Kotlin Time, the distribution of oligotaxon cyanobacterial Vendomyces benthic association resulted from continentalization of paleoenvironments in the epiplatform basins.