867 ISSN 0031-0301, Paleontological Journal, 2019, Vol. 53, No. 8, pp. 867–872. © Pleiades Publishing, Ltd., 2019. Facies-Stratigraphic Distribution of Organic-Walled and Silicified Microfossils in the Early Billiyakh Basin (Lower Riphean, Anabar Uplift, Siberia) P. Yu. Petrov a , M. Sharma b, **, N. G. Vorob’eva a , and V. N. Sergeev a, * a Geological Institute, Russian Academy of Sciences, Moscow, 119017 Russia b Birbal Sahni Institute of Palaeobotany, Lucknow, 226007 India *e-mail: sergeev-micro@rambler.ru **e-mail: sharmamukund1@rediffmail.com Received October 12, 2018 Abstract—The sedimentary succession of the Anabar Uplift (North Siberia) contains the Early Riphean abundant and diverse microfossil assemblages. The Ust-Il’ya and lower Kotuikan organic-walled assem- blages of prokaryotic and eukaryotic microorganisms from open shelf facies reflect the level of the marine biosphere evolution in the early Mesoproterozoic. The silicified microbiotas of the upper subformations of Kotuikan and Yusmastakh formations are represented by prokaryotic cyanobacteria only. Keywords: Anabar Uplift, early Riphean, cyanobacteria, eukaryotes, microfossils DOI: 10.1134/S0031030119080161 The Anabar Uplift is large (about 500 km in diam- eter) gentle-arched structure (Fig. 1), located in the northern part of the Siberian Platform (Khudoley et al., 2007; Komar, 1966). Within this structure the Archean and early Proterozoic crystalline basement is overlain by lower Riphean deposits, the sedimentary succession of which (up to 2 km thick) forms the lower stratigraphic level of the sedimentary cover of the Siberian Craton. This succession deposited in an age range of 1580–1500 Ma and after a long hiatus (>950 Ma) was overlain by late Vendian and lower Paleozoic sediments (Gorokhov et al., 2010; Petrov, 2011; Vorob’eva and Petrov, 2014). The ideas that the upper Riphean deposits occur in the Cis-Anabar sec- tions are not well grounded (Kuptsova et al., 2015). The Riphean succession of the Cis-Anabar is sub- divided into two groups that are contrast in the com- position and formation conditions (Zlobin and Golo- vanov, 1970). The lower Mukun Group (up to 800 m) is composed of continental sandy deposits (Petrov, 2014). The upper Billyakh Group (up to 1200 m) is represented by clayey–carbonate shelf sediments, which contain representative lower Riphean organic- walled and silicified microbiotas (Sergeev et al., 2017; Vorob’eva et al., 2015). The transition between continental and coastal- marine facies in the succession is gradual. Based on the dynamics of the variations in sea level, the strati- graphic interval in the volume of Ust’-Il’ya and Kotu- ikan formations, corresponding to the early Billyakh paleobasin can be clearly distinguished. At the base this interval is constrained by the Ust’-Il’ya transgres- sion concordant surface, and at the top it is corre- sponded to the surface of short-term subaerial exposi- tion, which preceded the accumulation of sediments of the Yusmastakh Formation (Fig. 2). The geological history of this paleobasin includes three types of sedimentation settings and three corre- sponding types of facies-ecological niches (E1–E3), which enclose taxonomically-different microfossil assemblages (Fig. 2). Ust’-Il’ya niche E1 represents the most deep-water settings of terrigenous shelf deposits with an intense storm influences. Organic-walled microfossils from the Ust’-Il’ya deposits are represented by abundant and relatively large microfossil remains, characteristic of the open-sea facies, which contain relicts of diverse eukaryotes. The following taxa of microbiota domi- nate: ?Chuaria circularis, Caudosphaera sp., Syns- phaeridium sp., Heliconema ustilense, Leiosphaeridia jacutica, L. aff. wimanii, Lineaforma aff. elongata, Oscillatoriopsis media, O. breviconvexa, Plicatidium latum, Polysphaeroides filiformis (Fig. 3), as well as Leiosphaeridia tenuissima Eisenack, 1958, L. ternata (Timofeev, 1966), L. minutissima (Naumova, 1949), L. atava (Naumova, 1960), L. crassa (Naumova, 1949), Leiosphaeridia sp. 1, Leiosphaeridia sp. 2, Elat- era aff. minor Vorob’eva et Sergeev, 2015, Navifusa sp., Palaeolyngbya helva Hermann, 1981, P. hebeiensis Zhang et Yan, 1984, Satka favosa Jankauskas, 1979,