CALCIFIED CYANOBACTERIA IN THE LATE MIDDLE ORDOVICIAN BIOHERMS AND PELECYPOD BANKS OF THE MOYERO RIVER SECTION (NORTHERN SIBERIAN PLATFORM)
N.A. Lykov1,2, Yu. V. Rostovtseva3, I.G. Zakiryanov4, A.V. Timokhin4, A.V. Dronov1, T.V. Gonta4, O.V. Postnikova2
1Geological Institute, Russian Academy of Sciences, Moscow, Russia
2Gubkin Russian State University of Oil and Gas, Moscow, Russia
3Geophysical Center of the Russian Academy of Sciences,Moscow, Russia
4Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
Keywords: сalcified cyanobacteria, Ortonella, Garwoodia, Hedstroemia, bioherms, biostroms, pelecypod banks, Siberian platform, uppermost Middle Ordovician, сyanotoxins
Abstract
In the Middle Ordovician (Late Darrivillian) deposits (Kirensk-Kudrinian Regional Stage) of the Moyero River section (Tunguska syneclise, Eastern Siberia), the remains of calcareous cyanobacteria of the genera Garwoodia Wood, 1941, Ortonella Garwood, 1914, and Hedstroemia
Rothpletz, 1913 (family Garwoodiaceae) were identified for the first time in thin sections. Previously, Ortonella and Garwoodia were recorded on the Siberian Platform only in Silurian deposits, which makes the discovery of representatives of these genera the earliest for that region. Ortonella
(center part) and Garwoodia (uppermost part) dominate in stromatoporoid-cyanobacterial bioherms, while Hedstroemia occupies a subordinate position. Only Ortonella was found in the biostromes and shelly interlayers of pelecypod banks. The study of the lithology of Kirensk-Kudrinian deposits allowed us to reconstruct a shallow lagoonal type sedimentation environment for the studied part of the section. Under these conditions despite the global increase in biodiversity during the Great Ordovician Biodiversification Event (GOBE), skeletal metazoan organisms failed to become the main framework of biostromes and bioherms. Their role was taken over by cyanobacterial communities resistant to the conditions of extreme shallow-water carbonate-siliciclastic sedimentation regime with terrigenous domination. The growth and subsequent bloom of the latter apparently provoked an increase in phosphorus content in the bottom sediments, which probably caused a local paleoecological crisis of the benthic fauna by the end of the Kirenian time in this part of the paleobasin. This is also confirmed by the depleted ostracod complex in the Moyero River section (in contrast to the same-age deposits of the Lena, Podkamennaya Tunguska, and Kulymbe River sections). The decrease in biodiversity of benthic fauna and, in particular, of skeletal metazoan reef-builders during microbial ‘blooms’ can be explained by the impact of cyanotoxins, which are destructive to marine life. The research demonstrates that even in the epochs of global biota rise, local conditions could limit the participation of skeletal metazoan organisms in the formation of reef ecosystems, preserving the dominance of microbe communities. This emphasizes the role of microbial communities as key agents of carbonate sedimentation during transitional periods of palaeoecological rearrangements and changes in sedimentation type.
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