Литосфера (Aug 2018)
Upper Vendian in the east, northeast and north of East European Platform: Depositional processes and biotic evolution
Abstract
Subject. Analysis of lithogeochemical proxies in the Upper Vendian mudstones reveals little if any variation in depositional environment for the Redkinian, Belomorian and Kotlinian regional stages in the east, northeast and north of East European Platform. The coeval macrobiota, in contrast, demonstrates significant macroevolutionary and macroecological transformations. Thus, the Avalon-type ecological association consisting of frondomorphs and vendobionts evolved in low-energy inner shelf during the Redkinian, the Belomorian Stage is characterised by diversification of frondomorphs, migration of vendobionts into relatively high-energy depositional settings (shoreface and prodelta), and emergence of tribrachiomorphs and bilateralomorphs, where as the Kotlinian Stage is marked by a sharp decline in taxonomic diversity of soft-bodied organisms (the Kotlinian Crisis). We don’t know to what degree, if at all, depositional parameters as palaeogeodynamics, palaeoclimate, sediment composition, volcanic activity influenced the Ediacaran biota, but these agents were not responsible for the above mentioned biotic transformations. Materials and methods. We suggest that intrinsic factors such as ecological interactions could be the primary trigger of the Kotlinian crisis. This conclusion has been reached based on the study of composition of major rock-forming oxides, rare- and trace elements in fine-grained aluminosiliciclastic rocks (argillites, shales and silt-rich mudstones). Geological samples were collected in outcrops of the Asha Group of South Urals and Sylvitsa Group of Central Urals, as well as from the drill core of the Keltma-1 (Vychegda Trough) and Tuchkino-1000 (Southeast White Sea area) boreholes. We also used the data on chemical composition of mudstones from the Staraya Russa and Vasil’evsky Ostrov formations form the southern slope of the Baltic Shield. Results. With this information in hand we could assess, with varying degree of confidence, such parameters as a degree of recycling of the material supplied into the late Vendian Mezen Basin; sediment provenance; composition of the substrate that microbial mats and soft-bodied organisms lived on in different parts of the basin; and palaeogeodynamic environment at the time when different groups of soft-bodies organisms were emerging.
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