Microfossils in upper riphean stratigraphy of the Siberian platform.

Stratigraphic studies allowed us to compile generalized sections of Riphean deposits of the Baykit anteclise and Katanga saddle and correlate them between each other and with the key sections of the Yenisey Ridge and Chadobets uplift (Fig.). The correlation was revealed between the Dzhelindukon and Delingdeken strata (Katanga and Baykit sections), Semyonov Formation of the Chadobets uplift and Pogoryui Formation of the Yenisey Ridge, which are the basal parts of a first large sedimentological cycle; Dolgokta stratum of the Baykit anteclise and Ayan stratum of the Katanga saddle, correlating with the Krasnaya Gorka (Tungusik Series, Yenisey Ridge) and Chuktukon (Chadobets uplift) Formations opens the second cvcle; the Kopchera-Yukta rhythm correlates with the Shuntara and Seriy Klutch Formations, the Tokur-Iremeken rhythm corresponds to the Dadykta Formation.

Microbiotas of a lower cycle contains two groups of microfossils. The first group includes taxa which have been previously reported only in Upper Riphean (Neoproterozoic) successions: Ohruchevella and Glomovertella filaments, forming a sporangium-like widening at their termination (Palaeovaucheria), and acritarchs of genera Osculosphaera, Tasmanites, Lophosphaeridium, Miroedichia, Cymatiosphaera, Dictiotiditim. Organisms of the second group (Pulvinosphaeridium sp., Tappania plana Yin., and Valeria lophoslriata Jankauskas) have been previously detected in the successions some older then 1 Ga. The wide occurrence ofacanthomorphytae Tappania plana Yin. in the study deposits is indicative of the older than a Lakhandinian age (1000-850 Ma) attributed to a lower cycle of the Kamo Series. On the other hand, the great number of the Upper Riphean taxa does not allow comparison of the study microbiotas to the Lower or Middle Riphean assemblages. A group of microfossils which includes acanthomorphytae Trachyhystrichosphaera sp. indicative of a Lakhandinian age has been recorded in the of upper part of the Kamo Series (begining from the Dolgokta stratum).

The obtained paleontological findings give evidence for a Late Riphean age of the Kamo Series from its base and are of great paleontological importance for reconstructing biotas evolutionary history, which is laid as a basis of stratigraphic schemes. The most prominent events in the overall evolutionary picture of biotas are documented by the enrichment in new prokaryotic and eukaryotic taxa. The first eukaryotic forms were recorded in upper successions of the Lower Riphean. The Middle Riphean biotas are represented mainly by transitional forms. The qualitatively new biotas represented by the rich variable assortment of eukaryotic morphotypes appeared since the beginning of Upper Riphean. Recorded in the Kerpylian sequence were such acanthomorphic arcitarchs as Tappania, Miroedichia, Lophosphaeridium, and other arcitarch s: striate Valeria, complex vesicles of Tasmanites, Osculosphaera. In the Lakhanda sequence, this assortment is enriched in more complex morphotypes, such orders as Siphonocladales, Vaucheriales as well as Ulophyton, Majaphyton, Eosolena, Trachyhystrichosphaera, Cymatiosphaeroides etc. Alone with most above-mentioned forms, some new genera of acanthomorphic arcitarchs (Cavaspina, Tanarium, Oodium, Comasphaeridium, Micrhysfndium, SIdagia, etc.), previously known only in Vendian-Cambrian successions, appear in the Baykalian, thus determining its intermediate position between Lakhandinian and Vendian.

Data on Kerpylian microfossils which have been laid as a base of this study are qualitatively new in principle for the Siberian Platform. Relatively rich biotas of the same stratigraphic level previously identified in the East European Platform are completely lacking in acanthomorphic arcitarchs, the representatives of the most important group of microfossils. There is a sole rich microbiota, containing complex acanthomorphic arcitarchs as well, that has been recorded in a close stratigraphic level in China (Ruyang Group deposits. North Chinese Platform). However, it age attribution remains less exact.

The obtained biostratigraphic data were used for dividing and correlating petroleum-bearing Riphean deposits from the western Siberian Platform, This information may be helpful in constructing the Late Precambrian regional stratigraphic schemes used in any type of geological survey and exploration appraisal.

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