MICROFOSSILS OF THE LATE PROTEROZOIC DEBENGDINSKAYA FORMATION OF THE OLENEKSKIY UPLIFT

silicon of stromatholitic dolomites of Kyutingdinskaya [1] and Debengdinskaya [2] formations were made from the Riphean cut of the Olenekskiy uplift (Figure). Few kinds of stenoorganic acritarchs are described from the well core sample of Debengdinskaya, Khapayskaya and overlying formations [3]. We have extracted a rich mic rophytologic material from aleuroagrillites of this series. A significant part of taxons of this material is known in the world only from the level of Neoproterozoic. A vari ety of the obtained association of forms has enabled to isolate few morphological groups of microfossils and to plan their most probable position in systematization of modern plants. The establishment of biological accesso ry of microfossils is one of the main problems of mic rophytology of Precambrian. To the greatest degree it is related to the stenoorganic forms considered in the clas sification of acritarchs – a group uniting vegetative mic roremains with uncertain systematic position [4, 5]. Questions regarding their nature and natural variability of morphological rows in time are still at the initial sta ge of studying, which causes the known uncertainty of the majority of results of microphytologic correlation of Precambrian stratons. Riphean deposits of the Olenekskiy uplift compose the basal part of the Siberian platform cover (see Figu re). Their capacity is unsteady and changes from 1200 up to 2150 m [6–8]. Basal layers of Riphean deposits with angular unconformity lie on granitoids and broken through by them metamorphic slates of the Lower Pro terozoic [6]. Deposits of the Khorbusuonskaya series, overlapping carbonates and slates of the Khapakhskaya formation with a washout, contain prints of medusoids of the Ediakarskiy type, fine shell faunae and are rela ted to Vendian [8]. The structure and the sequence of layers of the Deb engdinskaya formation have been fully studied by V.Yu. Shenfil, and others [9]. The formation has been di vided by them into five sub formations based on cuts of the river Ochchuguy Sololi and the watershed with the river Khorbusuonka. The pack of the lower sub formati on, from which the association of microfossils is obtai ned, is presented by aleurolites and argillites, less often by sandstones, with low capacity sgtromatolithic hori zon (lens). Higher deposits of the formation are charac terized by different parity of aleuroargillithic, arenace ous layers and bodies of stromatolithic carbonates which replace each other upwards along the cut and, possibly, in the lateral direction. Despite of bad exposure of sedi mentary rocks of the whole Riphean cut, sharp facies va riability is predicted as a result of observable changes in structure of stromatolithic constructions and replace ment of sandstones by aleuroargillites along the strike [7, 9]. In the lower sub formation there are structures of wave influence and resedimentation of the deposit. The combination of these attributes specifies existence in Debengdinskiy time of relatively superficial and hydro dynamic restless condition of the proximal shelf. Judging by pinching out capacities of the whole cut of the Olen ekskiy uplift to the West and its presence of formational analogues in cuts of the Udzhinskiy and Anabarskiy ar eas [6, 8], the whole in Anabaro Olenekskaya area re presented a system of intercommunicating and changing configuration shallow intracratonnal pools. Age position of the Debengdinskaya formation tra ditionally leans on two groups of data. Comparison of sequence of form change of stromatolites upwards along the cut with similar trends in the cut of the Anabarskaya area was used for comparison with the age change of their taxons in the stratotypical cut of the Riphean of Ural Mountains and Siberian regions [6, 7]. At stroma tolithic correlation the Debengdinskaya formation was deduced by the majority of researchers on the level of the average Riphean or less often concerned to bottoms of the upper Riphean [7]. The radiochronological dating, obtained in the 60 of the last century by the K Ar by a method by glauconites, makes for the Debengdinskiy formation a consistently increasing number from 1140 up to 1020 million years [7]. The latest isotope res earches of clay fractions [10] have shown high probabili ty of formation of Debengdinskiy deposits in the interval 1200.1300 million years. At the same time, it is impos sible to exclude the influences on the last result of terri genous impurity, which make the age of sedimentation older [11]. Then known data on microfossils does not come to an agreement with generally accepted view on middle Riphean age of the Debengdinskaya formation. Most of all it is discoveries of arcitarchs Chuaria Walc. , Tawuia Hofm. based on which the conclusion on Neo proterozoic age of the formation [3] has been made. Morphological variety of the forms taken from the same layers predetermines the allochthonous character Historical geology