RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6003, doi:10.2205/2005ES000183, 2005
[73] The world-wide records deposited during 50 years of comprehensive study of phytoliths (stromatolites and microphytoliths) and microfossils showed (1) complexity and diversity of their forms in the Precambrian seas; (2) their promising application to stratigraphy; and, however, (3) certain limitations on correlation by means of phytoliths and microfossils. From our standpoint the major limitation is in consideration of only appearance levels of empirically important taxa [Stanevich and Faizulina, 1992]. The conclusion is based on three axioms. Firstly, from a great deal of schemes of microfossil distribution in the Precambrian sections [Jankauskas, 1982; Jankauskaset al., 1989; Pyatiletov, 1988; Schopf, 1983; Stanevich and Faizulina, 1992; Timofejev, 1979; Veis, 1988; Vidal, 1976] it is seen that once "appearing", many forms are encountered in the younger fragments of the sections. Secondly, the microfossil remains that are well preserved on transportation in surface waters can be most likely redeposited in the younger sequences. Importance of this statement is especially great in studies of folded regions where the intense inversion tectonic transformations occurred. The third reason for use of the appearance principle lies in the conservative character of cyanophytes accepted as stromatolite producers, and likely of some other microorganisms, nowadays examined in acritarchs. This inference is not contradictory to the known successive alteration of phytolith forms in type sections, however, it emphasizes the necessary exclusion of upper boundary of their range on correlations. The lack of such limitation for phytoliths and microfossils is one of the causes of contradictory conclusions on the age of sequences estimated by different methods and, for instance, of the discussion on the Middle-Upper Riphean boundary in the Siberian reference sections [Dol'nik, 2000; Khomentovsky et al., 1985, 1998; Stanevich and Faizulina, 1992].
[74] An important but almost non-touched problem of the Precambrian biostratigraphy are the limits of use of different organic remains for correlation. A preliminary inference follows from answers to two related questions. The first one - whether a certain fossil group is represented by biological morphotypes with established or potentially established temporal transformation of their characters? In this case, if we are oriented to the original state of remains, they should refer to a taxonomically restricted group of flora or fauna. The latter thesis follows from the analysis of the use of paleontological records in stratigraphy [Stepanov and Mesezhnikov, 1979]. It indicates that a correlative reflection of the morphotypes' evolutionary changes is possible only within such groups. The second question concerns the intercorrection of results of the biostratigraphic and other methods - whether the statistically significant alteration of features, ultimately used for correlation, agrees with geological reconstructions and reliable isotope records?
[75] The Precambrian sediments yield rich assemblages of filamentous and other microfossil forms - stromatolite producers that are assigned to the modern cyanobacteria (blue-green algae) taxa based on their extreme morphological similarity. This indicates the occurrence of an invariant organismal and likely cellular structures of certain cyanophyte taxa from at least Middle Riphean to the Cenozoic. That conclusion does not contradict the empirically established temporal alteration of phytolith forms in the sections, however, it emphasizes the groundlessness of absolutization in biostratigraphy of an upper boundary of taxon distribution. Previously the position of considering only a first occurrence level was accepted by the Resolution of the All-Union Colloquium on Microphytoliths, Stromatolites, and Microfossils, 1975, as one of three methods of the stromatolite use in stratigraphy and was applied by Shenfil' [1991] in studies of their distribution in the Riphean of Siberia. The conservatism of cyanophyte forms during the Phanerozoic and Late Proterozoic [Golubic and Gofmann, 1976] does not give grounds to assign an evolutionary meaning to morphological changes of phytolith constructions. On the other hand, the association of some types of constructions with certain Late Precambrian intervals is proved. This regularity can hardly be explained only by changes of sedimentation environment and most likely is of biological, though still unclear nature. Thus as regards stromatolites and microphytoliths, we infer that their cyanophytic nature, the same as for microfossils, dictates an application of the appearance principle. This reasonably decreases the stratigraphic potentials of phytolith forms compared to the traditional notion. However, at the same time this approach reduces possible mistakes in controversial cases on correlations.
[76] Most of morphological innovations used for tracing the beds of regional horizons in the SBFS sections [Nemerov and Stanevich, 2001] are referred to types that we assign to green algae. Part of them is also known from Vendian sediments of the Late Precambrian reference sections. Most of forms of this assemblage was previously included in the 4th SBFS assemblage or in the Subassemblage IIIa of the Siberian platform [Stanevich and Faizulina, 1992]. The occurrence of these forms in both uppermost Late Riphean and Vendian-Lower Cambrian sediments of the SBFS and Siberian platform was among the causes of a discussion on stratigraphic position of the "Moty Series" (Resolutions of the 4th Interdepartmental Regional Stratigraphic Conference on the Refinement of Vendian and Cambrian Stratigraphic Schemes of the Siberian Platform Inner Regions, 1989). It was shown [Stanevich and Faizulina, 1992] that a lot of characters of these acritarchs first occurred in the Dal'netaiga Horizon but became taxonomically clear only in the pre-Vendian Zhuya time. This confirms that evolutionary innovations in the Precambrian biotas can be revealed most likely within the biologically related morphotypes. Their tracing upward from the pre-Vendian beds confirms the principle of considering only a first occurrence of typical microfossil forms and infers the general radiation of green algae during the Late and probably Middle Riphean.
[77] Along with the green algae the microfossil assemblage from the Urin Formation is the most significant for correlation of Neoproterozoic sediments [Faizullin, 1998; Pyatiletov, 1983]. These microfossils owing to a complicated morphology, occurrence in reference sections of distant regions, and appearance in the Neoproterozoic can be considered as index genera for this Precambrian interval. However, it is not improbable that their more simple "ancestors" with similar characters can be encountered in the older Late Riphean beds and still lower. Most of the rest microfossils from SBFS, judging from their simple structure and distribution of the analogues in other regions, are known or likely can be found in the older than the Upper Riphean sequences.
[78] The peculiar Bavlinella Schep. forms that on the basis of their wide distribution in the Vendian of the East European platform were dated strictly in this interval [Jankauskas et al., 1989; Semikhatov et al., 1990; Vidal, 1976], can serve as an example. However, in the SBFS the Bavlinella forms were encountered almost throughout the interval recovered by the reference sections, up to the sediments of the Ballaganakh Horizon. On the other hand, we use the microfossils, which relative chronological significance can acquire an archstratigraphic meaning with further studies, though nowadays they are not formally included in stratigraphic schemes because of the lack of their findings in the reference Proterozoic sections. These are, for instance, the above-reported species of Dictyotidium Eis., emend. Stapl. that are known and used for correlation in the Paleozoic sections [Kir'yanov, 1978; Pashkyavichene, 1980]. In the Baikal zone the oldest forms similar to Dictyotidium were encountered in the upper subformation of the Goloustnaya Formation (Figure 2). This fact confirms a common for the Precambrian microphytology tendency of downward extension of the previously accepted range of characteristic microfossil forms. Among prominent examples are the Late Riphean and Vendian acanthomorphytes [Jankauskas et al., 1989; Moczydlowska et al., 1993] many of which were previously considered as especially Paleozoic remains.
Citation: 2005), Precambrian microfossil-characterized biotopes from the southern margin of the Siberian craton, Russ. J. Earth Sci., 7, ES6003, doi:10.2205/2005ES000183.
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