Palaeontology, sedimentology and palaeoecology of the Iren Dabasu Formation (Upper Cretaceous), Inner Mongolia, People's Republic of China

Abstract

Since the discovery of dinosaurs in the Iren Dabasu Formation at Iren Nor in 1922, several expeditions have collected fossils from the low-relief exposures in this relatively small area. Articulated dinosaur skeletons, bonebeds comprising dinosaurs and vertebrate microfossils, nests of dinosaur eggs, invertebrate shell beds and trace fossils have been found. Nevertheless, most of the Iren Dabasu collections have not been prepared or described and the geology of this site has remained unstudied. Thus, the fossil assemblages and depositional environments have remained poorly known and somewhat enigmatic. The Sino-Canadian expeditions of 1988 and 1990 rekindled an interest in this formation and new results from these and previous expeditions are presented here. The fossil assemblage is close to that of the Bayn Shire Formation of Mongolia, but the generally accepted Cenomanian designation may be too old. The Iren Dabasu Formation comprises the remains of a low-sinuosity braided fluvial system that flowed toward the north-west and was deposited in a semiarid climate. Channels were broad (up to 1 km wide) and relatively shallow (less than 4 m deep). The floodplain was the site of frequent and energetic overbank flooding and early stages of soil formation. Although ephemeral floodbasin lakes and playas—resulting from overbank flooding—existed from time to time, there is no sedimentological or palaeontological evidence for laterally extensive and long term lacustrine environments as suggested by previous workers.