Event Stratigraphy of the Vendian TIRA HORIZON in the Southwest of the Siberian Platform: Lithology, Micropaleontology, Carbon- and O-Isotopic Chemraostigraphy
D.A. Gorshkov1, 2, K.A. Tikhonova3, 4
1Federal State Budgetary Institution "All-Russian Scientific Research Geological Oil Institute", Moscow, Russia
2Geological Institute of the Russian Academy of Sciences, Moscow, Russia
3Krasnoyarsk Branch office of LLC RN-GIR, Krasnoyarsk, Russia
4Federal State Autonomous Educational Institution of Higher Education "Siberian Federal University", Krasnoyarsk, Russia
Keywords: microfossils, lithology, event stratigraphy, carbon isotope composition, Tyra horizon, Siberian platform
Abstract
There is the Tira Horizon, represented by the Chistyakov, Omoro, and Oskoba Formations in different facies zones, widespread in the central and southwestern parts of the Siberian Platform. Deposits of the Tira Horizon in most sections of the Southwestern Siberian Platform are differ significantly in lithology from the underlying Nepa and overlying Danilov Horizons. In this paper considers the most complete section in the eastern part of the Angara Folded Zone, which serves as a key section. There are three main transgressive-regressive cycles, separated by sequence boundaries, are noted in the Tira interval. Three members correspond to these cycles. The first member records an abrupt change in facies from coastal to deep-sea, containing organic-walled microfossils of the first assemblage with Vanavarataenia insolita. The second member, formed in a coastal marine environment, exhibits an increase in carbonate and sulfate content, the appearance of chert lenses and nodules with microfossils, and the second assemblage with Tinnajaphyton amplum (lower submember). The third member is characterized by upper littoral, sabkha, and continental-deltaic deposits. The identified associations of microfossils, along with the features of the lithological composition and isotopic characteristics, allow them to be used as benchmarks in regional constructions on the Siberian platform, and possibly also beyond its borders. A carbon isotope anomaly of up to -11.3‰ (EPDB) was detected in dolomites of the Oskoba Formation at the Kamo Arch of the Baikit Anteclise. This may indicate a restructuring of the ecosystem in the paleobasin, accompanied by the appearance of certain microbiota groups and points to global climate change.
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