RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6003, doi:10.2205/2005ES000183, 2005

Introduction

[2]  The discovery of hundreds of microfossil localities in the Precambrian sediments demonstrated a complicated differentiation of life that included bacteria forms and groups of algae, fungi, and animal organisms [Martin, 1993; Semikhatov et al., 1999; Jankauskas et al., 1989]. At the same time the major problem of Precambrian microfossil studies is the elucidation of biological affinity of forms from certain localities. The establishment of natural taxonomic affiliation of microfossils is the most topical issue for Neoproterozoic biotas characterized by the greatest diversity of organic remains. Most of forms derived from syngenetic flints of shallow-water biochemogenic carbonates, according to their morphology and environmental conditions, are correlated with confidence with modern cyanobacteria [Sergejev, 1992, 2003; see reference to Knoll et al., 1991]. Organic-walled microfossils extracted using acid from terrigenous rocks are commonly referred to acritarchs (Acritarcha Evitt, 1963), a group of uncertain taxonomic position [Jankauskas et al., 1989]. However, both silicified and most of organic-walled microfossils were mainly studied from shallow-water beds of paleoshelves' littoral and upper sublittoral zones. Microbiotas from deep-water sediments, let alone bathyal zones, are incomparably less known [Akhmedov et al., 2000; Horodyski, 1993; Moorman, 1974]. But even the available scarce records indicate that forms of deep-water biotas differ greatly in morphology and lesser dimensions from well-known shallow-water microfossils. The knowledge of microfossil assemblages in geodynamic aspect is closely similar. Most of the Proterozoic microbiotas were found in the sediments of paleoshelves that represent relics of passive continental margins. Microfossil findings from sedimentary volcanogenic rocks of Proterozoic island-arc environments are scarce. The major reason for poor knowledge of these forms is a scarce preservation of corresponding sediments. Deep-water sediments and geologic bodies of active continental margins experience more or less intense transformation in collision zones and have a low probability to be preserved [Reding, 1990]. These sediments are commonly represented by tectonic fragments of intensely deformed and more or less metamorphized rocks.

[3]  Among major lines of microfossil studies the microphytologic correlation is of particular interest. However, on numerous occasions, especially in the initial period of Precambrian microphytology, the chronological inferences from microfossils, lacking a paleontological base, followed the adopted stratigraphic schemes. As new records became available, the microfossils with a sufficiently complicated morphology were recorded and were used for correlation of large stratigraphic units. However, eventually the application of these microscopic and relatively poorly studied remains for correlation implies a comprehensive research permitting the separation of original groups of microorganisms. Having regard to convergent features typical of protozoans and possible affiliation of acritarch forms to different biological groups (up to kingdoms) inhabiting various environments, the use of microfossils for correlation should include records of ecologic and natural taxonomic investigations. The latter results are eventually the components of actuopaleontological correlations that are basic to evolutionary conclusions and therefore to biostratigraphic inferences. This methodical model of classic paleontology is still by far little applicable for heterogeneous assemblages of the Precambrian microremains. However, the available records already permit the attempts to separate microfossil morphotypes in hypothetically related groups.

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Figure 1
[4]  Microfossils from the late Precambrian sediments of the Sayany-Baikal fold system (SBFS) (Figure 1) have been studied for over 40 years [Faizullin, 1998; Golovenok and Belova, 1983 Pyatiletov, 1983; Stanevich and Faizulina, 1992; Timofejev, 1966; Treshchetenkova et al., 1982; Jankauskas et al., 1989]. The greater attention was focused on the elucidation of their vertical and lateral distribution in order to use them for correlation. Now it is evident that among all biostratigraphic inferences based on microfossils, the conclusions on interregional distribution of microbiota of the Urin Formation in the Late Riphean [Faizullin, 1998] and on a comparatively pronounced first occurrence level of the forms previously included in Subcomplex IIIa from the Siberian platform (Methodical Recommendations for Processing of Sedimentary Rocks on Spore-Pollen Analysis, VSEGEI, unpublished report, 1986) or in Complex 4 of its southern folded surroundings [Stanevich and Faizulina, 1992], are worthy of notice. The first occurrence of this assemblage in the uppermost Late Riphean was recorded in tens of sections with different sedimentary facies in the SBFS area [Stanevich and Faizulina, 1992].

2005ES000183-fig02
Figure 2
[5]  The morphology and inhabited environments are still the actual criteria for comparison of modern microorganisms and Proterozoic microfossils. From this standpoint we began the ecobiological interpretation of microfossil assemblages from the SBFS sediments. The revealed relationships between geologic bodies from different areas of the outer and inner belts of the SBFS resulted in the reconstruction of Neoproterozoic geodynamic environments in the region [Nemerov and Stanevich, 2001; Stanevich and Perelyaev, 1997]. It was found out that when moving from the north to the south of the SBFS one can observe fragments of shelf, slopes, and depression of the backarc basin and remains of contrast-facies sediments of the island-arc depressions. Results of the studies of organic remains from these sediments are commonly not associated with the reconstruction of sedimentogenesis. However, as the analysis of numerous records showed, both lateral and temporal distribution of organic remains most likely resulted from the alteration of depositional conditions and their evolution in time. The available notion of microorganism habitats and burial conditions, along with the actuomorphological analysis, permitted the inference of different biological affiliation of the previously studied forms from the SBFS assemblages (Figure 2) [Nemerov and Stanevich, 2001]. The morphological features of microfossils allow the comparison with possible modern types of algae and bacteria, which metabolic characters better correspond to the reconstructed biotopes. The SBFS sequences have the advantage over sections from other regions that they represent environments of shelf, slope, and depression of a near-continental basin at different stages of its Late Proterozoic evolution. This paper suggests the reconstruction of sedimentogenesis environments in three SBFS areas with Neoproterozoic sediments and the ecobiological interpretation of the derived microfossil assemblages.


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Citation: Stanevich, A. M., V. K. Nemerov, Yu. K. Sovetov, E. N. Chatta, A. M. Mazukabzov, V. I.  Perelyaev, and T. A. Kornilova (2005), Precambrian microfossil-characterized biotopes from the southern margin of the Siberian craton, Russ. J. Earth Sci., 7, ES6003, doi:10.2205/2005ES000183.

Copyright 2005 by the Russian Journal of Earth Sciences

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