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Stratigraphy and Palaeoenvironment of the Dinosaur-Bearing Upper Cretaceous Iren Dabasu Formation, Inner Mongolia, People’S Republic of China

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Stratigraphy and Palaeoenvironment of the Dinosaur-Bearing Upper Cretaceous Iren Dabasu Formation, Inner Mongolia, People’S Republic of China Cretaceous Research 26 (2005) 699e725 www.elsevier.com/locate/CretRes Stratigraphy and palaeoenvironment of the dinosaur-bearing Upper Cretaceous Iren Dabasu Formation, Inner Mongolia, People’s Republic of China Jimmy Van Itterbeeck a,*, David J. Horne b, Pierre Bultynck a,c, Noe¨l Vandenberghe a a Afdeling Historische Geologie, Laboratorium voor Stratografie, Katholieke Universiteit Leuven, Redingenstraat 16, B-3000 Leuven, Belgium b Department of Geography, Queen Mary, University of London, Mile End Road, London E1 4NS, UK and Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK c Departement Paleontologie, Koninklijk Belgisch Instituut voor Natuurwetenschappen, Vautierstraat 29, B-1000 Brussel, Belgium Received 26 March 2004; accepted in revised form 24 March 2005 Available online 9 September 2005 Abstract New field observations and sedimentological analyses of the dinosaur-bearing Upper Cretaceous Iren Dabasu Formation in the Iren Nor region of Inner Mongolia (People’s Republic of China) have led to a better understanding of its palaeoenvironment. The fluvial deposits represent a braided river that, due to the large amount of fines, does not fit the classical model for braided rivers. The study area is divided into two parts: in the northern part, sediments of the main channel belt of the ancient braided river system are exposed along a dry river valley on the northern edge of the Iren Nor salt lake, while in the southern part, comprising all other studied exposures, different facies of the ancient floodplain are represented, including minor channels, temporary ponds, and palaeosols. The difference between the northern and southern parts is also reflected in the fossil content; only the southern exposures have yielded dinosaur remains. The ancient braided river had a broad, vegetated floodplain populated by a diverse dinosaur fauna. Four species of charophytes are described and illustrated from the Iren Nor site, together with eight species of ostracods, one of which (Cypridea irennorensis sp. nov.) is new. Contrary to the vertebrate data, both groups of microfossils indicate a latest Cretaceous age (CampanianeMaastrichtian) for the Iren Dabasu Formation, and suggest a possible correlation with the Nemegt Formation, which would allow the age estimation to be refined to latest CampanianeEarly Maastrichtian. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Inner Mongolia; Iren Dabasu Formation; Late Cretaceous; Charophyta; Ostracoda 1. Introduction Cretaceous hadrosaurids. Accordingly, the SBDE has conducted three field campaigns on the Iren Dabasu The Sino-Belgian Dinosaur Expedition (SBDE, Formation at the Iren Nor site in the vicinity of Erenhot 1995e2002), a collaboration between the Inner Mongo- (Fig. 1) to collect more material of the primitive lia Museum, Hohhot and the Royal Belgian Institute Hadrosauroidea Bactrosaurus johnsoni and Gilmoreo- of Natural Sciences, Brussels, aimed to elucidate the saurus mongoliensis. replacement of Early Cretaceous iguanodontids by Late The first dinosaur discoveries at the Iren Nor site, the first known dinosaur locality in Central Asia, were * Corresponding author. made in 1922 during the Central Asiatic Expeditions of E-mail address: [email protected] (J. Van the American Museum of Natural History. Since Itterbeeck). then, numerous expeditions have visited the area and 0195-6671/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2005.03.004 700 J. Van Itterbeeck et al. / Cretaceous Research 26 (2005) 699e725 KANGHA Arvayheer 0 100 200 km MONG I OL ALT MONGOLIA AI Ondai Sayr ULAN NUUR BAGA BODG GOB Sayn Shand SH I ALTAI I GRABEN REG HAILAR U N G GOBI BASIN BASIN G ASH TRANS- RABE UN Bayn Dzak ALTAI N Bayn Shire Iren Nor GOBI GURVAN SAYH Khulsan NEM Erenhot Nemegt EGT AN EASTERN GOBI Khermeen GRABEN Tsav SOUTHERN GOBI ERLIAN BASIN CHINA major faults Bayan Mandahu GRABEN AN SH G LAN YIN SHAN BAYANMANDAHU Yellow Ri ver HOHHOT Beijing Baotou Dashuiguo 40°N Dongsheng ORDOS 105°E BASIN HELAN SHAN LUYA SHAN Fig. 1. Generalised map of the Gobi Basins, showing the location of Erenhot and Iren Nor (modified after Jerzykiewicz, 1995). Inset shows the general location of the area within the P.R. China. extracted dinosaur fossils (see Currie and Eberth, 1993 Erlian and the Mongolian Gobi Basins (Meng et al., for an overview). In spite of this long tradition of 2003). The latter is sometimes divided into smaller palaeontological expeditions, however, it was not until basins (e.g., Graham et al., 2001). These extensional the Sino-Canadian expedition in the early 1990s that basins consist of a series of subbasins separated by studies of the sedimentology and palaeoecology of the intrabasinal highs, and all have a similar tectonic history site were initiated (Currie and Eberth, 1993). During the (Meng et al., 2003). Two regional unconformities divide different SBDE field campaigns, sedimentological, the sedimentary record into three megasequences: stratigraphical and palaeontological data were gathered prerift, synrift and postrift deposits (Graham et al., to complement the earlier work. Here we present the 2001). The Late Jurassic and Early Cretaceous synrift results of these studies, including the first list of sediments are relatively thick and contain abundant micropalaeontological taxa, thus facilitating a more volcanic intercalations, while the Upper Cretaceous detailed interpretation of the palaeoecology of the Iren postrift strata are relatively thin and devoid of volcanics Nor site and providing an opportunity for discussion of (Shuvalov, 2000). the age of the Iren Dabasu Formation. The Iren Nor site is situated in the Erlian Basin but it has often been compared to the Mongolian dinosaur sites of the Gobi Basin. Therefore, a short review of the Upper Cretaceous stratigraphy of the Gobi Basin is 2. Stratigraphical and geological setting provided, followed by an overview of the geological setting of the Iren Nor site and previous estimations of The China-Mongolia border region contains many the stratigraphical position and age of the Iren Dabasu Late Mesozoic extensional basins, including the Chinese Formation. J. Van Itterbeeck et al. / Cretaceous Research 26 (2005) 699e725 701 2.1. The Gobi Basin four formations: the Nemegt, Barun Goyot, Djadokhta and Bayn Shire formations. The Nemegt and Barun Due to repeated intervals of block faulting, the Goyot formations have been defined by Gradzinski and sedimentary sequences are discontinuous and contain Jerzykiewicz (1974) and formally described by Gradzin- several hiatuses and unconformities; nowhere in the ski et al. (1977). Based on the presence of the Gobi Basin is a complete stratigraphical column pre- hadrosaurine dinosaur Saurolophus, the Nemegt For- served, but composite sections have been compiled mation is correlated with the Early Maastrichtian (Jerzykiewicz and Russell, 1991; Shuvalov, 2000). Un- Edmontonian Stage in North America (Jerzykiewicz fortunately there are no standard rules for the correct and Russell, 1991). The mid-Campanian in North transcription of the Mongolian names, so that a large America is characterised by an increase in aridity and variety of different names is used: Hu¨hteeg and a marked change of dinosaur fauna. According to Bayanshiree (Shuvalov, 2000) are the equivalents of, Jerzykiewicz and Russell (1991), this change can also be respectively, Khukhtek and Bayn Shire (Jerzykiewicz observed at the boundary between the Bayn Shire and and Russell, 1991). Another and more important Djadokhta formations. Based on this hypothesis, the complication is the different stratigraphical concepts Djadokhta and Barun Goyot formations are considered used by different workers. Russian and Mongolian the equivalents of the Judithian Stage of North scientists use ‘‘svita’’ as the fundamental stratigraphic America, in which case they are respectively mid- and unit, combining both lithostratigraphical and chrono- Late Campanian in age. This age estimation is sup- stratigraphical concepts; the boundaries of a svita are ported by the radiometric ages of the volcanic layers considered to be isochronous. European and American within the Barun Goyot Formation (Fig. 2). The type scientists, on the other hand, use ‘formations’ to group locality of the Djadokhta Formation (defined by Berkey rocks, clearly a lithostratigraphical concept without time and Morris, 1927 and redefined by Gradzinski et al., implications. Care must therefore be taken when 1977) is situated at the Bayn Dzak locality in Mongolia comparing svitas and formations (Barsbold, 1972; (Fig. 1), although a more complete and better-studied Gradzinski et al., 1977; Hicks et al., 1999). section is known from the Bayan Mandahu locality in All the Mesozoic deposits in the Gobi Basin are China (Eberth, 1993; Jerzykiewicz et al., 1993). entirely terrestrial/continental, since the last major The Bayn Shire Formation (defined by Vasiliev et al., marine incursion in Central Asia occurred during the 1959) is more difficult to date. The freshwater molluscs Permian. The lack of marine fossils precludes direct can be compared with similar species in Cretaceous biostratigraphical correlation of the deposits with inter- deposits, interbedded with CenomanianeSantonian national marine stages. As a consequence, non-marine marine strata in the Fergana and Aral Sea regions fossils have been used to correlate the lithostrati- (Martinson and Kolesnikov, 1974; Makulekov and graphical units of the Gobi Basin with the international
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