Biofacies zonation of Ordovician deposits of the southern Siberian Platform

Based on the inventory, revision, and analysis of stratigraphic, paleontological, and lithofacies data on the area of Ordovician deposits in the southern Siberian Platform (Irkut amphitheater), the refined and detailed schemes of biofacies zonation of this paleobasin are substantiated. Schemes of zonation have been compiled for the Nyaian, Ugorian, Kimaian, Mukteian, Volginian, Kirenskian-Kudrinian, Chertovskian, and Baksanian Horizons of the regional Ordovician stratigraphic chart of the Siberian Platform. The schemes present biofacies different in lithologic composition, spread of dominant groups of fauna, and hydrochemical regime (normal-marine salinity, freshwater, or high salinity). It is shown that contrasting changes in paleogeographic environments and the spread of faunal communities under changing environmental conditions were influenced by the transgression-regression cyclicity of the evolution of the Siberian Platform paleobasin and by the proximity of the land. Specific groups of fauna localized in particular facies are described. These groups are regarded as communities that were the first to occupy the littoral zones of epicontinental sea basins in the Ordovician.