B.B. Kochnev

Ediacaria in the Siberian hypostratotype of the Riphean

The study of stratigraphy and paleontology of the Riphean/Vendian boundary strata is fundamental to decoding the transitional (Neoproterozoic) stage in the evolution of the biosphere. During this stage, the Proterozoic-style biota with limited morphological diversity, small size of individual organisms, lack of biogeographic zonation, and low rates of evolutionary turnover was replaced by the diverse Phanerozoic-style biota, with bioprovinciality and high dynamics of macroevolutionary processes [1]. The study of the transitional period invokes the analysis of the most representative Upper Proterozoic sections along the periphery of the Siberian Craton. Here, the uppermost Riphean was separated into the Baikalian Complex [2], which was proposed as a Regional Stage [3] or, later, as an Erathem of the General Stratigraphic Chart for the Precambrian of Northern Eurasia and equivalent to the Cryogenian of the International Stratigraphic Chart for the Precambrian [4]. Ediacaran fossils recently discovered in the Ui Group suggest that the biosphere shift commenced during the Baikalian. In 2005, large isolated outcrops of light- and yellowish gray quartzitic and arkosic sandstones, light gray siltstones and greenish gray mudstones were studied in the middle reaches of the Maya River on a segment between the mouths of the Malyi Kandyk and UlakhanKrestyakh Creeks (eastern slope of the Aldan Shield) (Fig. 1). The sections comprise the stratotype of the Kandyk Formation of the Ui Group, Upper Riphean [5, 6]. The total thickness of the succession exceeds 300 m (Fig. 1). The upper part of the sequence can be traced in large-blocky Felsenmeere, screes, and isolated outcrops at the mouth of the Yudoma River, where it is overlain by gray flaggy dolostones and oncolitic limestones of the Yudoma Group of Vendian age [2, 6, 7]. The studied sedimentary succession of the Kandyk Formation consists of two large depositional systems. The first depositional system, up to 100 m in thickness (Fig. 1, Member 1), has a distinctive complexly rhythmic stratification pattern composed of alternating sheets of wavy-bedded sandstones (0.1‐0.4 m), intervals of siltstone‐mudstone couplets, and sheets of wavy-laminated limestones (up to 0.7 m). The second depositional system, over 200 m thick (Fig. 1, members 2‐4), has a relatively simple structure: thick (up to 70 m) packages of thin- and wavy-laminated and thick-bedded and cross-bedded sandstones are interstratified with thick (up to 65 m) intervals of siltstone‐mudstone couplets.

Sr isotope signatures in the Vendian Khorbusuonka group of the Olenek uplift (Northeastern Siberian Platform)

Owing to the comprehensive knowledge [1–3],unique paleontological characteristic, and availabilityof geochronological dates [4], the Vendian section ofthe Olenek uplift is one of the best reference successions for both regional and global correlation of UpperPrecambrian deposits. At the same time, no data areavailable on the Sr isotope chemostratigraphy, whichprevents adequate assessment of age constraints for theVendian in the Siberian hypostratotype. The publisheddata on the C isotope composition in these rocks [2, 3]provide an ambiguous age interpretation of sedimentatogenesis. In this connection, we studied the distribution of Sr isotopes in Vendian carbonate rocks of theOlenek uplift to assess their age more adequately ascompared with other isotopic–chemostratigraphicmethods.A collection of >200 samples of carbonate rocksfrom the Maastakh, Khatyspyt, and Turkut formationsof the Khorbusuonka Group taken from three sectionsin the middle reaches of the Khorbusuonka Riverbasin served as material for this study (Fig. 1). The carbonate rocks of the Maastakh and Turkut formationsare represented by biolaminitic, stromatolitic, oolitic,pisolitic, and oncolitic dolomites and their massiveand detrital varieties. The Khatyspyt Formation iscomposed of bedded and detrital, frequently, bituminous, and, locally, clayey limestones. According to thesedimentological analysis [1, 3, 5], each formationreflects a complete transgresssive–regressive sedimentation cycle being bounded at the base and top by erosional surfaces and hiatuses. Finds of Late Vendianbiota of the Ediacaran and Miaohen types and ichnofossils in the Khatyspyt Formation and also smallshelled fossils characteristic of the uppermost VendianNemakit–Daldynian Horizon in the Turkut Formation provide very important paleontological data [1, 5](Fig. 1). The oldest age of the overlying KesyusyaGroup, the middle part of which is marked by the firstfinds of Early Cambrian skeletal remains and ichnofossils [1], is estimated by the SHRIMP U–Pbmethod on zircons from tuff breccias of the uppermostKhorbusuonka Group to be 543 ± 0.3 Ma [4] (Fig. 1).At the first stage, the least altered samples wereselected macroscopically for assessing the degree ofpreservation of the

Sr isotopic characteristic of neoproterozoic carbonate sediments from the southern Yenisei Ridge

This paper presents the first Sr isotopic data for the Late Precambrian carbonate rocks of the southern Yenisei Ridge. Their geochemical study allowed estimation of the degree of secondary alterations and gave the possibility to reveal rocks with a less disturbed Rb-Sr isotopic system. The Sr isotopic data indicated Neoproterozoic sedimentation of the rocks about 1070–750 Ma ago. Sr and C isotopic data showed that carbonate rocks of the Sukhoi Pit, Tungusik, and Shirokino groups are Late Riphean and could be comparable with sedimentary sequences of three Precambrian key sections of the Northern Eurasia: the subsequent Derevnino, Burovaya, and Shorikha formations from the Turukhansk Uplift, the Lakhanda Group from the Uchur-Maya region, and the Karatav Group from the South Urals. All studied carbonate rocks are older than 750 Ma and, according to the International Stratigraphic Chart, accumulated prior to global glaciations in the Cryogenian. This is evident from sedimentological study indicating the absence of tillite horizons in the studied sections. δ13C values in the sections vary from +0.4 up to +5.3‰, which testifies to the absence of periods of great cold.

The Upper Neoproterozoic glacial complex in central areas of the Siberian platform

A stratum of glacial diamictites with thickness of up to 160 m at the basement of the sedimentary cover, overlain by a series of cap carbonates up to 40 m thick is described for several boreholes in the central regions of the Siberian platform. The isotope composition of carbon in carbonates is identical to the Ediacaran cap carbonates from the Patom upland. The series of detrital zircons from diamictites (60 grains) contains three young grains aged 740–690 Ma, which restricts the time of formation of the glacial deposits to the late Neoproterozoic. Thus, the glaciations that were synchronous to the Stert and/or Marino glacial stages on the Siberian platform had a cap character.

Early Cambrian magmatism in the northeastern Siberian Craton (Olenek Uplift)

The Vendian–Lower Cambrian tectonomagmatic activation took place in the northeastern Siberian Craton, within the Olenek Uplift and in the Kharaulakh segment of the Verkhoyansk fold-and-thrust belt (the lower reaches of the Lena River). The Early Paleozoic volcanic activity in the Olenek Uplift is expressed in the form of basitic diatremes, small basaltic covers, and doleritic dikes and sills intruding and covering the Upper Vendian carbonate deposits. The material specificity of the Lower Cambrian basites and their mantle sources, jointly with the Vendian–Cambrian sedimentation history, gives reason to consider the Lower Cambrian riftogenesis and the associated magmatism as a consequence of the plume–lithosphere interaction in the northeastern Siberian Craton.