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NEW RESULTS OF INTEGRATED LITHOFACIES AND BIOSTRATIGRAPHICAL STUDY OF THE LOWER CAMBRIAN OSA HORIZON IN THE SOUTH OF THE SIBERIAN PLATFORM (BOLSHETIRSKAYA 7 WELL)

D. A. Tokarev 1,2, A. V. Plyusnin 3, A. A. Terleev 1, N. A. Ivanova 4, I. V. Varaksina 1, A. V. Lipyanina 4

1 А.А.Тrofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia; 2 Novosibirsk State University, Novosibirsk, Russia; 3 OOO INK, Irkutsk, Russia; 4 Siberian Research Institute of Geology, Geophysics and Mineral Resources, Novosibirsk, Russia

The section of the Osa Subformation of the Usolka Formation of the Lower Cambrian Tommotian stage penetrated by the Bolshetirskaya 7 well in the south of the Nepa-Botuoba anteclise has been studied. As a result of lithological-facies investigations, seven lithological types of rocks were identified, they are regularly replaced by each other along the section. Facies environments of the reef complex were dеtected: bar bank, rear part of the reef, organogenic bioherm bildup, reef rear bars, reef buildup, core of reef buildup. In the section of the subformation, three fourth-order sequences are identified. The sequence boundaries are fixed by subaerial hiatuses represented in the core by sedimentation breccias, karst zones. The largest lowering of sea level, in terms of amplitude and duration, is confined to the border of the second and third sequences. The conducted biostratigraphic analysis confirms the Lower Cambrian age. Remains of calcareous algae Renalcis polymorphum, R. gelatinosum, R. granosum, Botomaella zelenovi were found. Small shell fauna of the genus of Namacalathus Grotzinger, Watt ers et Knoll. also was found. It has global distribution in the Lower Vendian (Ediacaran) deposits in Namibia, Brazil, Canada, Oman, Spain, China, Russia, etc.

Keywords: Cambrian, Osa Horizon, East Siberia, archaeocyatha, cyanobacteriae, organogenic buildups, sequence-stratigraphic analysis.

DOI 10.20403/2078-0575-2021-2-56-66

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