Glacial deposits of the Bokson Group, East Sayan Mountains, Buryatian Republic, Russian Federation
Nickolay M. Chumakov, 2011. "Glacial deposits of the Bokson Group, East Sayan Mountains, Buryatian Republic, Russian Federation", The Geological Record of Neoproterozoic Glaciations, Emmanuelle Arnaud, Galen P. Halverson, Graham Shields-Zhou
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The Bokson Group (Gr.) forms the platform cover in northern part of the Tuva-Mongolian Massif, south of the East Sayan Mountains. Widespread diamictites occur in the lower part of the Bokson Gr., in the Zabit Formation (Fm.). These contain erratic, faceted and striated stones along with dropstones. They have disconformable or gradual lower boundaries and conformable or transitional upper boundaries with overlying deposits. The isotopic δ13Ccrb curve records some maximum and minimum values as a background of insignificantly varying values (from –3 to 2‰). Maximum values (up to 5.5‰), are recorded in deposits underlying diamictites, whereas the overlying beds of the Zabit Formation (Fm.) yielded three peaks or minima down to −4‰. A fourth negative anomaly derived from the middle part of the Bokson Gr. in the lower part (close to base) of the Tabinzurt Fm. is as low as –5.7‰. The deposits underlying the diamictites contain Ediacaran (Vendian) microfossils. Cloudina sp. was discovered in the granule of diamictites, whereas beds just above include small shelly fossils (SSF) of the Nemakit-Daldynian type (Cambrotubulus decurvatus Miss., Anabarites trisulcatus Miss.), which is overlain by a member with abundant fossils of the Tommotian assemblage. The diamictites represent glaciomarine deposits formed in the uppermost Ediacaran or lowermost Cambrian stage (Upper Vendian).
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In recent years, interest in Neoproterozoic glaciations has grown as their pivotal role in Earth system evolution has become increasingly clear. One of the main goals of the IGCP Project No. 512 was to produce a synthesis of newly available information on Neoproterozoic successions worldwide similar in format to Hambrey & Harland’s (1981) Earth’s pre-Pleistocene Glacial Record. This Memoir therefore consists of a series of overview chapters followed by site-specific chapters. The overview chapters cover key topics including the history of research on Neoproterozoic glaciations, identification of glacial deposits, chemostratigraphic techniques and datasets, palaeomagnetism, biostratigraphy, geochronology and climate modelling. The site specific chapters for 60 successions worldwide include reviews of the history of research on these rocks and up-to-date syntheses of the structural framework, tectonic setting, palaeomagnetic and geochronological constraints, physical, biological, and chemical stratigraphy, and descriptions of the glaciogenic and associated strata, including economic deposits.