Lithofacies, microfacies and depositional environments of Upper Cretaceous Oceanic red beds (Chuangde Formation) in southern Tibet Xi Chen a, ⁎, Chengshan Wang a , Wolfgang Kuhnt b , Ann Holbourn b , Yongjian Huang a , Chao Ma a a State Key Laboratory of Geological Processes and Mineral Resources, and Research Center for Tibetan Plateau Geology of China University of Geosciences, Beijing 100083, China b Institute of Geosciences, Christian-Albrechts-University, Kiel D-24118, Germany abstract article info Article history: Received 15 June 2009 Received in revised form 14 May 2010 Accepted 3 June 2010 Available online 11 June 2010 Keywords: Facies assemblages Microfacies types CORBs Gyangze basin Southern Tibet The Gyangze basin, located in southern Tibet, is one of the representative areas for Cretaceous Oceanic Red Beds (CORBs). We investigated and inter-correlated several new CORB outcrops in the western and southeastern part of the Gyangze area in addition to the previously described Chuangde section. The CORBs in the Gyangze basin mainly consist of shales, thin-bedded marls, re-sedimented limestones and clasts supported conglomerates-breccias. Planktonic foraminifers in the carbonate rocks within CORBs belong to the Globotruncana ventricosa and Globotruncanita calcarata zones, indicative of middle Campanian age. The facies assemblages in the different sections indicate that the depositional environments of CORBs range from outer base-of-slope apron to basin zones. Within carbonates, we observed microfacies types characteristic of basin, open deep shelf margin or toe-of-slope environments. Gravity transport resulted from the steepening of the sea floor in the Gyangze basin owing to subduction during the Campanian. Relatively oxic bottom water conditions prevailed below the toe-of-slope environment in the Gyangze basin during CORB deposition. However, the presence of gray clasts within a multicoloured matrix suggests less oxic conditions at shallower water depths within the basin. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Marine sediments changed dramatically from black shales in the Lower Cretaceous to oceanic red beds (CORBs, Wang et al., 2005; Hu et al., 2005) in the Upper Cretaceous, which are recorded globally both in continental outcrops and deep sea drilling cores (Chen et al., 2007). Based on the increasing number of CORB drilling sites and outcrops occurring immediately above black shale layers and ocean anoxic events (OAEs, Jenkyns, 1980) of various ages, Chen et al. (2007) suggested that the onset of CORBs is potentially linked with black shale deposition. Although the occurrence of CORBs has been known for over 150 years (von Strombeck, 1857; Štur, 1860; Gümbel, 1861), their implications for paleoceanography and paleoclimatology started only to be perceived in the last few years. The biostratigraphic, sedimentological and geochem- ical characteristics of CORBs were studied in Spain, Italy, Slovakia, Poland, Austria, New Zealand and Tibet (e.g., Birkenmajer, 1977; Premoli Silva, 1977; Butt, 1981; Premoli Silva and Sliter, 1994; Bak, 1998; Vera and Molina, 1999; Bak, 2000; Wagreich and Krenmayr, 2005; Wang et al., 2005; Hu et al., 2006a,b; Hikuroa, et al., 2009). Until now, research has been mainly focused on a few areas, e.g. southern Tibet, the North Calcareous Alps and Eastern Alps in Austria, the Umbro-Marche basin in central east Italy, the Subbetic Zone in southeast Spain, and ODP Site 1049 at Blake Nose in the western Atlantic. It has been inferred that the deposition and the wide geographic distribution of CORBs were related to a variety of earth processes such as climatic, paleogeographic, ocean current and nutrient cycle changes in the Late Cretaceous (Hu et al., 2005; Wang et al., 2009). Proxies for active iron and phosphorus indicate relatively well oxygenated bottom waters and enhanced nutrient burial in Upper Cretaceous oceanic red sediments of the Scaglia Rossa Formation in Italy and Chuangde Formation in southern Tibet (Hu, 2002; Huang et al., 2007a,b, 2009). Until recently, the investigation of CORBs in southern Tibet has been focused on the Chuangde section in the northeastern area of the Gyangze region and the depositional environment of CORBS was loosely defined as an upper slope to oceanic basin (Hu et al., 2006a). Over the last two years, we investigated several new CORB outcrops in the western and southeastern part of the Gyangze area in addition to the Chuangde section. Four sections named the Cailangba, Tianba A, and B and Daba sections (Fig. 1) were chosen for their favourable exposure and low tectonic overprint. Ninety carbonate samples were collected from these sections (Fig. 2) to investigate the evolution of microfacies and constrain the biostratigraphy of CORBs. The primary aims of this paper are to provide a biostratigraphical framework and to reconstruct the deposi- tional environments of red beds deposited under oxygenated bottom water conditions on the basis of facies assemblages and microfacies variations. The interpretation of depositional environments in CORBs is based on correlation to Wilson's standard microfacies types (Wilson, 1975). Sedimentary Geology 235 (2011) 100–110 ⁎ Corresponding author. Tel.: +86 10 8232 2171. E-mail address: chnchx@126.com (X. Chen). 0037-0738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2010.06.008 Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo