Stratigraphy, sedimentology and palaeoecology of the dinosaur-bearing Kundur section (Zeya-Bureya Basin, Amur Region, Far Eastern Russia)

Abstract

Since 1990, the Kundur locality (Amur Region, Far Eastern Russia) has yielded a rich dinosaur fauna. The main fossil site occurs along a road section with a nearly continuous exposure of continental sediments of the Kundur Formation and the Tsagayan Group (Udurchukan and Bureya formations). The sedimentary environment of the Kundur Formation evolves from lacustrine to wetland settings. The succession of megafloras discovered in this formation confirms the sedimentological data. The Tsagayan Group beds were deposited in an alluvial environment of the ‘gravel-meandering’ type. The dinosaur fossils are restricted to the Udurchukan Formation. Scarce and eroded bones can be found within channel deposits, whereas abundant and well-preserved specimens, including sub-complete skeletons, have been discovered in diamicts. These massive, unsorted strata represent the deposits of ancient sediment gravity flows that originated from the uplifted areas at the borders of the Zeya-Bureya Basin. These gravity flows assured the concentration of dinosaur bones and carcasses as well as their quick burial. Such taphonomic conditions allowed the preservation of sub-complete hadrosaurid skeletons unearthed at the Kundur site. Palaeobotanical data indicate a subtropical climate during the deposition of the Kundur and Udurchukan formations. Several elements in the composition of the Kundur vertebrate fauna suggest a strong influence of the North American late Cretaceous vertebrate communities: the abundance of corythosaur-like lambeosaurines, the probable presence of a nodosaurid dinosaur and of a eucosmodontid or microcosmodontid multituberculate. A late Maastrichtian age is tentatively proposed for the dinosaur-bearing sediments in Amur Region, by comparison with the information collected in the Western Interior Basin of North America. As it is also observed in the latter area, important floristic changes (diminution of angiosperm pollens and predominance of modern families) and the disappearance of dinosaurs mark the end of the Maastrichtian age in the Amur Region. Late Maastrichtian dinosaur localities from Amur Region are dominated by lambeosaurines, whereas these dinosaurs apparently disappeared from western North America long before the iridium horizon that defines the K/P boundary. This local disappearance is therefore probably due to ecological factors rather than indicating a gradual extinction of the dinosaurs long before the K/P boundary.