Early Cretaceous continental sedimentatary environments, palaeogeography and biota of southeastern Western Siberia

Abstract

Abstract The Ilek Formation was distinguished by Ragozin in 1936 in the SE of Western Siberia, as a sequence of continental deposits accumulated in the alluvial and lacustrine environments, dating Barremian–Early Albian. At the end of the twentieth century a series of sites, comprising rich and diverse Early Cretaceous vertebrates, was discovered, which forced an intensive comprehensive research of the Ilek Formation. The stratotype section of the Bolshoi Ilek represents an abnormal section of the Ilek Formation, comprising the deposits of prodelta, delta front and mouth bars. Three regions within the area of the Ilek Formation reflect the evolution of the fluvial system. The eastern region represents the oldest part of the section, preliminarily dated to the Barremian based on spore and pollen analysis in addition to lithological characteristics. The western region was probably deposited during ocean anoxic event (OAE) 1a. The central region represents the final stage of the tectonic cycle, deposited within the environments of the calm plain river and fluvial delta. The vertebrate assemblages distinguished for the Kiya and Bolshoi Kemchug basins have specific features, which can support the heterochrony of the Ilekian deposits.