Sedimentary Geology 235 (2011) Contents lists available at ScienceDirect. Sedimentary Geology

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1 Sedimentary Geology 235 (2011) Contents lists available at ScienceDirect Sedimentary Geology journal homepage: 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 , China b Institute of Geosciences, Christian-Albrechts-University, Kiel D-24118, Germany article info abstract 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 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 geochemical 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 Corresponding author. Tel.: address: chnchx@126.com (X. Chen). 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 depositional environments of red beds deposited under oxygenated bottom waterconditionsonthebasisoffaciesassemblagesandmicrofacies variations. The interpretation of depositional environments in CORBs is based on correlation to Wilson's standard microfacies types (Wilson, 1975) /$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.sedgeo

2 X. Chen et al. / Sedimentary Geology 235 (2011) Fig. 1. Sketch geological map of studied area showing geological setting and location of studied sections. 2. Geological setting Mesozoic marine sediments in southern Tibet are mainly outcropping in the Tethyan Himalaya, located between the Higher Himalayan Crystalline Belts and the Indus-Yarlung Zangbo Suture zone (Gansser, 1964; Burchfiel et al., 1992). The Mesozoic strata in this area belong to two different tectonic domains: the passive continental margin of the Indian continental plate and the adjacent deep oceanic basin that separated the former from the Asian Plate (Yu and Wang, 1990; Liu and Einsele, 1994). The Gyirong-Kangmar intracrustal thrust subdivides the Tethyan Himalayas into the northern and southern subzones of different lithological compositions (Yin, 1988; Liu, 1992; Liu and Einsele, 1994). During the mid Cretaceous, the area was at a latitude of 21 S (Patzelt et al., 1996; Hay and DeConto, 1999) and surrounded by an ocean connected eastward to the Pacific Ocean and westward to the Mediterranean Tethys (Scotese, 1991). The Gyangze basin lies in the east of the northern Tethyan Himalaya subzone and is characterized by widespread Lower Cretaceous to Paleogene marine sediments (Chen et al., 2008). The Lower to Middle Cretaceous sedimentary deposits consist of black shales and calcareous shales intercalated with marls deposited on a lower slope to pelagic basin environment (Chen et al., 2008). The black shales were overlain by Late Cretaceous red mudstones and variegated limestones. Based on the geological survey of the eastern Gyangze basin, Li et al. (1999) and Wang et al. (2000) subdivided the Cretaceous marine succession into three formations: Gyabula, Chuangde, and Zongzhuo. The Gyabula Formation ( m thick) is composed of black shale with pyrite nodules intercalated with sandstones deposited by turbidity currents during the Berriasian Coniacian (Wang et al., 2001). The overlying Chuangde Formation is about 30 m thick and consists of violet-red shales intercalated with thin marlstone beds (CORBs). Foraminifers and nannofossils indicate a Santonian Campanian age for the formation (Wan et al., 2005). The overlying Zongzhuo Formation is more than 200 m thick and of late Campanian Palaeocene age (Liu and Aitchison, 2002). It is predominantly composed of dark grey to black shales enclosing various olistoliths of sandstone, limestone, and bedded chert that correspond to the Beijia Olistostrome of Liu and Einsele (1996). 3. Lithostratigraphy, biostratigraphy and microfacies of Chuangde Formation 3.1. Lithostratigraphy and biostratigraphy This study is based on five sections (Cailangba, Chuangde, Daba, Tianba A and B) (Fig. 1). Although CORBs are generally composed of red shales and limestones, the ratios of shales to limestones vary significantly in the different sections (Fig. 2). Biostratigraphic correlation

3 Fig. 2. Lithological logs of studied CORB sections in the Gyangze Basin, southern Tibet. Lithology, thickness and position of samples are shown. 102 X. Chen et al. / Sedimentary Geology 235 (2011)

4 X. Chen et al. / Sedimentary Geology 235 (2011) of the section is based on determination of planktonic foraminifers in thin sections. (Fig. 3). Age assignment is based on the zonal scheme for planktonic foraminifers in thin sections of Sliter (1989), with age assignments following the revised Late Cretaceous zonation of Robaszynski and Caron (1995) Chuangde Section (28:58:00 N, 89:44:05 E) This section is close to the village of Chuangde, which is located about 10 km to the east of Gyangze town. CORBs are mainly composed of ca. 30 m thick red siliceous shales intercalated with thin bedded marls, resulting from turbidites (Hu et al., 2006b). Medium to thick bedded variegated limestones were found at the base and in the upper part of the CORB succession. The limestone beds are commonly deformed and in sharp contact with the shales. At the base of the succession, a single limestone bed (limestone unit of Chuangde 1, Lc1) is out-cropping with an average thickness of 0.5 m. The thickness of the bed varies laterally from tens of centimeters to about 1.5 m. In contrast, the medium bedded limestones are interbedded with shales and are discrete laterally in the middle part of the succession (Lc2). Mudstone fragments paralleling the dip of the sequence are distributed widely in the limestone beds. The upper 10 m of the formation is characterized by the occurrence of lenticular and round limestone olistoliths enclosed by red shales (Lc3). The limestones are mainly gray in color. The limestone unit Lc1 yielded a few moderate to well preserved planktonic foraminiferal species including Globotruncana sp., Globotruncanita cf. atlantica, Globotruncana linneiana, Globotruncanita ex gr. stuarti, Globotruncana arca, Globotruncana cf. bulloides, Globotruncana orientalis, Globotruncanita stuartiformis, Contusotruncana fornicata, and Heterohelix sp. This assemblage is indicative of early Campanian age. This assignment indicates that the onset of the Chuangde CORB succession may be slightly younger than previously documented: mid Santonian to early Campanian (Li et al., 2005) or late Santonian to middle Campanian (Wan et al., 2005) Cailangba section (28:55:33 N, 89:13:11 E) The Cailangba section lies in the western part of the Gyangze area. The CORB succession is about 26 m thick and mainly consists of purple medium bedded limestones intercalated with conglomerate beds. It overlies and is overlain by gray limestones. The unit can be divided into two members by the first occurrence of a 1.5 m thick breccia layer at 14 m of the section. Three thick clasts supported conglomerate layers, which are m thick, occur in the upper part of the section at 14, 19.5 and 26 m, respectively. A 3 m thick succession of red limestone alternating with reddish chert was found between the first and second conglomerate layers. Slumps are quite common in the section, especially in the upper part. In the lower member, conglomerate beds are generally of centimeter to decimeter scale. Pebbles are common and consist of subangular to subrounded cherts, limestones, and conglomerates, which vary from a few millimeters to decimeters in size. Burrows are found at the base of red limestones in the lower member. Rare foraminifers are preserved in the limestones and in the carbonate pebbles of the conglomerate beds. The assemblages are dominated by planktonic species, although a deep water benthic species, Aragonia velacoensis, was found in the lower part of the CORB succession (06CL04). This species is considered to live in bathyal to abyssal depths (below 200 m down to 3000 m according to Van Morkhoven et al. 1986). The planktonic foraminiferal species recorded in the lower member belong to the Globotruncanita calcarata zone, and include Globotruncanita. ex gr. stuarti (3 m, 06CL05), Globotruncana cf. linneiana (10.5 m, 06CL09), Globotruncana arca (13 m), Contusotruncana fornicata and Globotruncanita stuartiformis (13.5 m). This assemblage indicates a middle Campanian age for the lower and middle part of the section. In the upper member, planktonic foraminifers are extremely rare Tianba A section (28:43: N, 89:56: E) The remaining three sections lie in the southeastern part of the Gyangze area. In the Tianba A section, CORBs consist of 115 m thick red shales and 3 variegated limestone units. This formation is continuously overlying and is overlain by dark siliceous shales. Three limestone units arepresentinthelower(lt1),middle(lt2)andupperpart(lt3)ofthe succession. The limestone beds are deformed and in sharp contact with the red shales. The surfaces of single beds within the units can be hardly identified due to strong deformation. A few carbonate concretions within the variegated limestones are out-cropping in the middle and upper parts of the section. The concretions are ellipsoidal with a maximum size over 1m and parallel the dip of the stratigraphy. Slumps are also observed in the limestone units. Between Lt1 and Lt2, the lithology is characterized by marls alternating with shales. The marls thicken from 4 to 5 cm at the base to more than 15cm at the top. Above the last limestone unit, three meter thick brick red shales were deposited, which were continuously overlain by dark gray siliceous shales. Planktonic foraminifers occur sporadically within the limestones. In limestone unit Lt1, extremely rare foraminifers are poorly preserved. Unit Lt2 yields Globotruncanita subspinosa, Globotruncana arca, Globotruncana bulloides, Contusotruncana fornicata, Globotruncana sp., Globotruncana bulloides and Globotruncanita stuartiformis. In the base and mid part of Lt3, a few Globotruncana bulloides, Globotruncana sp., Globotruncana ventricosa and Globotruncanita stuartiformis are sporadically distributed. In the upper part of Lt3, the assemblage contains Globotruncanita stuartiformis, Globotruncana linneiana and Globotruncanita elevata-stuartiformis, indicating that the entire succession is of middle Campanian age (Globotruncana ventricosa Zone) Tianba B section (28:43: N, 89: 57: E) The Tianba B section lies about 1 km east of the Tianba A section. CORBs are about 210 m thick and mainly consist of variegated limestones besides the red shales intercalated with marls, which outcrop in the lower 14 m of the section. A 12 m thick unit between the red shales and limestones is not exposed. The limestone unit is about 185 m thick and has similar sedimentary characteristics as the ones of Tianba A, except that it lacks slumped carbonate concretions. In this section, CORBs are also overlain by dark gray siliceous shales. In the lower part of the section, rare Globotruncana orientalis occur within the intercalated marls. A foraminiferal assemblage characteristic of the Globotruncana ventricosa zone (middle Campanian) is found in the lower part of limestone unit, including Globotruncana ventricosa, Contusotruncana fornicata and Globotruncana bulloides. The assemblage in the upper part of the unit contains Globotruncana bulloides, Globotruncana arca and Globotruncana orientalis Daba section (28: 43: N, 89:56: E) In the Daba section, located 1 km west of the Tianba A section, CORBs are mainly characterized by a 270-m-thick succession of red shales and variegated limestones. Four slump limestone units out-crop in the section: Ld1 from 3 to 50 m, Ld2 from 66 to 73 m, Ld3 from 117 to 137 m, and Ld4 from 159 to the top of CORB succession. The limestone units are variegated except for Ld2, which is light gray. Interestingly, three rhythmic red and yellowish green shales are deposited between Ld1 and Ld2. Each red-green rhythm is about 3 m thick. Planktonic foraminifers are more abundant here than in the other sections. At the base of Ld1, the assemblage consists of Globotruncana bulloides, Contusotruncana fornicata, Globotruncana arca, Globotruncanita subspinosa, Globotruncana sp., and Globotruncana ventricosa. The upper part of this unit yields Globotruncana bulloides and Globotruncana arca, as well as a benthic species, Reussella szajnochae. Foraminifers in Ld2 are relatively rare and poorly preserved. However, in the thin bedded limestone intercalations immediately below the unit, a few planktonic foraminifers including Globotruncana sp. and Globotruncanita stuartiformis were found. Ld3 contains Globotruncana sp., Globotruncana arca and Globotruncana ventricosa. In the lower part

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6 X. Chen et al. / Sedimentary Geology 235 (2011) of Ld4, Rosita fornicata, Globotruncana sp., Globotruncana arca and Globotruncana bulloides were identified. Globotruncana sp., Globotruncana linneiana, Contusotruncana fornicata, Globotruncana cf. bulloides, Globotruncana ventricosa, Globotruncana orientalis, Globotruncanita stuartiformis and Globotruncana cf. falsostuarti (including G. esnehensis) occur in the middle part of Ld4, whereas the upper part of Ld4 contains Globotruncana sp., Globotruncana orientalis, Globotruncana arca, Globotruncana cf. falsostuarti, Globotruncanita ex gr. stuarti and Globotruncana bulloides. The planktonic foraminiferal assemblage in Ld4 was assigned to the Globotruncana ventricosa Globotruncana falsostuarti Zone of middle to late Campanian age (Robaszynski and Caron, 1995) Microfacies types Eight microfacies types (Fig. 4) were identified according to microscopic characteristics of carbonate rocks including red marls, variegated limestones and gray limestones etc. The classification of the MFTs was based on Dunham (1962), Folk (1962) and Zeng and Xia (1986) MFT1 micrite (mudstone) This type has mud-supported lithologies with very rare (b5%) and small allochems scattered in a cryptocrystalline matrix. Although the matrix is generally red colored, it can also be gray or red with elliptic to round gray intraclasts. Extremely rare foraminifers are well preserved in this microfacies. The low ratio of grains to matrix suggests that this MFT was deposited in a low energy facies zone. Commonly, foraminifers are filled with sparite, which indicates a strong alteration after deposition. Dolomitization was observed in samples collected from the Cailangba section. About 20% of the samples are categorized in this microfacies type. The marl intercalations in the CORB successions belong mainly to this type MFT2 foraminifer-bearing micrite (wackestone with foraminifers) MFT2 is the most widespread microfacies type in the CORB succession of the Gyangze basin (representing about 40% of the samples). It is mud-supported with foraminifers (10 to 25%) scattered within the cryptocrystalline matrix. The matrix is commonly red colored except for gray colored carbonate rocks occasionally occurring in the Chuangde, Tianba A and B, and Daba sections. Allochems are mainly planktonic foraminifers, which are moderate to well preserved and filled with sparite. A single benthic foraminifer, Aragonia velacoensis, was found in the lower part of the Cailangba section MFT3 foraminifer- and intraclast-bearing micrite (wackestone with foraminifers and intraclasts) This mud-supported microfacies type is identified in the Chuangde, Tianba A and B sections. The matrix is red or gray in color, and the grains are mainly composed of scattered planktonic foraminifers and intraclasts. The content of grains is not higher than 25% and can be lower than 10%. The planktonic foraminifers are moderately to well preserved and filled with sparite. Intraclasts are generally rounded and about 1 mm in size, although some are larger than 2 mm. Light gray color differentiates the intraclasts from the red or gray matrix. Smaller foraminifers occur occasionally in the intraclasts. Dolomitization was observed in the samples from limestone unit Lt3 of the Tianba A section MFT4 foraminiferal micrite (wackestone with foraminifers) Carbonate rocks of this type, which has a mud-supported structure, out-crop in the Cailangba, Daba and Tianba A sections. The matrix is generally red colored with planktonic foraminifers as allochems, which are mainly filled with sparite. These features are relative similar to MFT2, but the abundance of grains reaches 25 50%, which is significantly higher than in MFT2. In some samples of the Cailangba section, the foraminifers are distributed unevenly in thin sections. Oriented foraminifers are concentrated in some areas, indicating a possible origin as gravity flow MTF5 foraminiferal packstone (packstone with foraminifers) This microfacies type with grain-supported structure is identified in the Cailangba, Daba and Tianba A sections. The matrix is red cryptocrystalline, and allochems are characterized by moderate to well preserved planktonic foraminifers, mainly filled by sparite. The abundance of allochems is higher than 50%. In the samples from the uppermost part of the Daba section, microsparitic subangular to subrounded intraclasts, which are a little larger than 1 mm, were found. As in MFT4, the foraminifers are unevenly distributed in some thin sections of the Cailangba section, and commonly occur in mm-scale bands within a grain supported structure without mud. In contrast rare fossils are scattered within the matrix in other areas. We suggest that the foraminifer grains originated from grain flows MFT6 intraclastic packstone (packstone with intraclasts) This microfacies type is identified in only one sample from the upper Cailangba section: 07CL10. The allochems consist mainly of subrounded to round intraclasts, which are mostly mm in size and represent more than 50% of the sediment. The intraclasts are micro sparitic, and differ from those in MFT MFT7 foraminiferal limestone (grainstone with foraminifers) This microfacies type is identified in two samples collected from the limestone unit Lc3 in the Chuangde section and in carbonate concretions of the mid Daba section (07CDR07 and 06DB48). The allochems are mainly planktonic foraminifers, which cover more than 90% of the thin sections. The sediment is gray colored, but the foraminifers are occasionally filled with red mud MFT8 breccia This microfacies type is identified only in the Cailangba section. Lithologically, the breccias are characterized by a mixture of sandstones, carbonate rocks and cherts within a black matrix. Few foraminifers were found in the carbonate pebbles. In general, the foraminifers are well preserved and are scattered in a gray or black matrix. The carbonate pebbles are therefore named foraminiferal micrites. 4. Discussion 4.1. Stratigraphic distribution and bottom water oxygenation of CORB Planktonic foraminiferal assemblages indicate that the Chuangde Formation mainly belongs to the G. elevata to G. ventricosa zones, characteristic of early to middle Campanian age (Robaszynski and Caron, 1995). This age range differs from previous estimates by Wan Fig. 3. Foraminiferal species found in carbonate rocks of CORBs in Gyangze. Bars are 0.2 mm long. 1 Globotruncana cf. falsostuarti, from 06DB056, Daba section; 2 Globotruncana cf. arca, 07CDR01, Chuangde section; 3 Globotruncana orientalis, 07CDR01, Chuangde section; 4 Globotruncana ventricosa, 06TBa35, Tianba A section; 5 Globotruncanita ex gr. stuarti, CD082, Chuangde section; 6 Globotruncanita ex gr. stuarti, CD080, Chuangde section; 7 Contusotruncana fornicata, 06CL12, Cailangba section; 8 Globotruncana bulloides, 06TBa16, Tianba A section; 9 Globotruncanita stuartiformis, 06TBa22, Tianba A section; 10 Globotruncanita atlantica, 07CDR02, Chuangde section; 11 Globotruncana. linneiana, 06TBa36, Tianba A section; 12 Globotruncanita ex gr. stuarti, 06DB58, Daba section; 13 intermediate morphotypes between Globotruncana arca and Globotruncana orientalis, 07CDR02, Chuangde section; 14 Globotruncana ventricosa, 06TBb7, Tianba B section; 15 Globotruncana cf. bulloides, CD083, Chuangde section; 16 Globotruncanita. ex gr. stuarti, 07CL05, Cailangba section; 17 Globotruncanita elevata-stuartiformis, 06TBa36, Tianba A section; 18 Contusotruncana fornicata, intermediate to Globotruncana. arca, 07CDR04, Chuangde section; 19 Globotruncanita cf. atlantica, CD080, Chuangde section; 20 Heterohelix sp., 07CDR04, Chuangde section; 21 Globotuncanita calcarata, 06CL11, Cailangba section; 22 Globotruncana cf. linneiana, 06CL09, Cailangba section; 23 Reussella szajnochae, 06DB32, Daba section; 24 Aragonia velascoensis, 06CL04, Cailangba section.

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8 X. Chen et al. / Sedimentary Geology 235 (2011) et al. (2005), who suggested that the lowermost part of Chuangde Formation is of late Santonian age. In contrast, we found calcareous rocks containing globotruncanids of at least early Campanian age at the base of the Late Cretaceous red beds of the Chuangde section. The sedimentological evidence in the Chuangde section indicates a significant amount of gravitational displacement of sediments in this unit including slumps and slides. Thus, the late Santonian foraminifers in the lower part of the formation may be redeposited. The red colour of CORBs is the result of the pigmentation of finely disseminated ferric oxides (Hu et al., 2005), normally in the form of haematite (Turner, 1980), formed in an oxic condition during the early diagenesis (Channell et al., 1982; Eren and Kadir, 1999, 2001; Hu, 2002). The oxic condition can be caused by the highly oxygenated bottom water, low flux of organic matter to reduce biomineralization in pore water, or low accumulation rate to increase the oxygen exposure time. According to the age range, the Chuangde Formation was deposited within about 5 million years. The accumulation rate of the formation varies from a few (Chuangde section) to more than 20 mm/kyr (e.g. 23 mm/kyr in Tianba A section) without compaction correction. The rate is similar to the overlain black unit (Berriasian to Albian) of the Gyabala Formation (Li et al., 1999) in the area, which is about 60 m thick. This result indicates that the accumulation rate is not a major controlling factor to form marine red beds in the Gyangze basin. A less productive ocean during CORB formation was supported by the extremely low TOC content (Wang et al., 2005) and phosphorous species (Huang et al., 2007a,b). Phosphorus, a limiting nutrient for marine production, was mainly trapped in the sediments in the form of FeP as the reactive phosphorus species during deposition of the red sediments within the Chuangde section, which led to less phosphorus feeding back to the water and consequent low productivity. However, some samples consist of red packstones (MFT 5) with abundant foraminiferal tests, which may indicate a normal biogenic flux at these levels. Therefore, neither sediment accumulation rate nor carbonate productivity appear causally linked to CORB deposition. It is likely that the bottom water was sufficiently oxygenated in the Gyangze basin during the early-middle Campanian to facilitate red bed formation. A similar conclusion was drawn by Wendler et al. (2009), who argued that relatively high oxygen concentration can lead the CORB deposition in places, where organic carbon flux and sedimentation rates remained below a certain threshold. The enhanced oxygenation of bottom water may result from increased concentration of O 2 in the ocean. Firstly, as the concentration of CO 2 decreased following OAE2, the concentration of O 2 probably increased within the ocean climate system (Arthur et al., 1988; Kuypers et al., 1999), which led to an enhanced oxidizing capacity of oceanic deep waters (Arthur et al., 1988). This was suggested as at least a partial cause for CORB deposition (Arthur et al., 1988; Wang and Hu, 2005; Chen et al., 2007). Secondly, oxygen isotope data record a global cooling in the late Santonian Campanian (Huber et al., 2002), which broadly coincided with CORB deposition. Indeed, a significant Turonian Maastrichtian cooling trend was reported from the Exmouth Plateau, which was located close to the Gyangze area in the Campanian (Clarke and Jenkyns, 1999). As the solubility of oxygen is substantially lower in warm deep water than in cool water (Hay, 2009), dissolved oxygen may have increased significantly in the cooler deep water during deposition of the Chuangde Formation Lithofacies assemblages and depositional environment Sedimentary characteristics such as deformation and slump structures suggest that the limestone or conglomerate units in the CORB successions of the Chuangde Formation are gravity transported, and can be recognized as carbonate slope, apron, and basin plain environments. A carbonate apron model (Mullins and Cook, 1986) is frequently applied to interpret the depositional environments of re-sedimented carbonates (Tucker and Wright, 1990; Flügel, 2004). Two types of carbonate aprons develop: carbonate slope aprons and carbonate base-of-slope aprons, which can be differentiated by vertical variations in lithology. In slope apron, carbonate turbidites do not systematically develop vertical cycles; on the other hand, in base-of-slope aprons thickening-upward cycles occur occasionally (Mullins and Cook, 1986). Both apron types were further subdivided into inner and outer facies belts. The different facies belts can be classified according to facies assemblages (Mullins and Neumann, 1979; Mullins and Cook, 1986). We classified the facies assemblages of Chuangde Formation using the terminology of Mutti and Ricci Lucchi (1972) for gravity flow facies. Four facies of mass-transported sediments were identified in the CORBs of the Gyangze basin; however, facies assemblages differ from section to section. In the Chuangde section, limestone units Lc1 and Lc2 and mudstone fragments within the limestones are deformed, and we interpret these deformation limestone units as slump sediments (Fig. 5). By contrast, limestone unit Lc3 exhibits relatively little internal deformation and may represent a submarine slide mass. We assign these units of submarine slumps and slides to Facies F of Mutti and Ricci Lucchi, 1972 and Mullins and Cook (1986). The unit between Lc1 and Lc2 is composed of red shales (hemipelagic and pelagic sediments, Facies G) with minor thin bedded marls which are turbidites (Fig. 5). No obvious graded sequences or bedding structures were found in the marl layers. We thus suggest they represent Td-e in the Bouma sequence, referred to as Facies D (base-cut-out turbidites), which indicates that this unit was deposited in a more basinal environment. The facies of re-sedimented carbonates in the Tianba A, B and Daba sections are comparable to the ones in the Chuangde section. Except for the limestone units of the Tianba B section and Ld2 in the Daba section, which show intraformational truncation surfaces with little internal deformations, the other units have similar sedimentary features as Lc1 and Lc2 in the Chuangde section. Therefore, we include limestone units with slides or slumps into Facies F. Facies G and D are also common in these sections. In the unit of red shales intercalated with turbiditic marls between Lt3 and Lt4 in the Tianba A section, the turbidites thicken upward from a few centimeters to about 15 cm, and can be interpreted as base-of-slope deposits. The facies in the Cailangba section are quite different. The background sediments are red micrites (Facies G) besides red shales with marl intercalations. Clasts-supported conglomerates (Facies A, Fig. 5) out-crop only in this section indicating that the succession was deposited within the outer apron. The conglomerate layers also thicken upward from centimeter scale to about 1.5 m, and inverse grading sequences were additionally found in the lower part of the section. In the upper part, slumps (Facies F) are well developed. Facies assemblages suggest that CORBs in the Gyangze basin were deposited in outer base-of-slope apron to basin environments, since Facies D is much less frequent than Facies G and upward thickening of turbidites and conglomerate layers occurs. The paleoenvironments are comparable to those of CORBs in different ocean basins (Chen et al., 2007). However, they are deeper than many CORB settings in the western Tethys, which were interpreted as outer shelf or marginal slope basin settings at the northern margin of the Alpine Carpathian basins (Hu et al., 2005). It is likely that depositional environment is one of the controlling factors of the age ranges of CORB Fig. 4. Carbonate microfacies types in CORBs of the Gyangze basin; bars are 1 mm long. 1 Micrite, 06TBa25, Tianba A section; 2 Foraminifer-bearing micrite, 06DB31, Daba section; 3 Foraminifer- and intraclast-bearing micrite, 06TBb16, Tianba B section; 4 Foraminiferal micrite, 06TBa33, Tianba A section; 5 Foraminiferal packstone, 06TBa34, Tianba A section; 6 Intraclastic packstone, 07CL10, Cailangba section; 7 Foraminiferal limestone, 06DB48, Daba section; 8 Breccia, 07CL05, Cailangba section.

9 108 X. Chen et al. / Sedimentary Geology 235 (2011) Fig. 5. Lithofacies assemblages in CORBs of the Gyangze basin. 1 Limestone units and facies assemblages in the Chuangde Section; 2 Tianba A section; 3 Upward thickening of marls between Lt3 and Lt4 in the Tianba A section; 4 Carbonate concretion in the Tianba A section; 5 Clasts supported conglomerate layer in lower member of the Cailangba section; 6 Slumps in upper member of the Cailangba section. sediments, since they are rarely deposited on shallow platforms receiving increased organic carbon flux the sea floor. CORBs are commonly of Turonian Maastrichtian age in the North Atlantic Ocean and Tethyan realm and of Campanian Maastrichtian age in other ocean basins, including southern Tibet (Hu et al., 2005; Chen et al., 2007). We suggest, that the immediate effect of a decrease in pco 2 after OAE2 was much stronger and faster in relatively enclosed or shallower areas such as the North Atlantic Ocean and western Tethys than in the open and deep ocean basin such as the Gyangze basin Microfacies types and depositional environments The carbonates in the CORB successions of the Gyangze basin consist of eight distinct microfacies, dominated by MFT1 and MFT2, i.e. micrites and foraminiferal micrites. According to the classification of Dunham (1962), they are mudstones and wackestones, which are mudsupported. The low grain/mud ratios reveal that they were deposited in low energy environments. Extremely rare benthic foraminifers, such as Aragonia velascoensis and Reussella szajnochae, indicate that they were deposited in bathyal to abyssal depths (from 200 m to thousands of meters following van Morkhoven et al., 1986). The MFTs of CORBs in the Gyangze basin can be mainly compared with standard microfacies type 3(SMF3, Wilson, 1975), which consists of pelagic lime mudstones and wackestones with abundant pelagic microfossils (Wilson, 1975; Chaper 14.3, 681p. Flügel, 2004). SMF3 occurs in the basin (FZ1) and open deep shelf margin or toe-of-slope (FZ3) facies zones (Wilson, 1975). MFT of foraminiferal packstones can be compared with SMF4, which corresponds to bioclastic lithoclastic packstones in basinal, deep shelf to toeof-slope setting (FZ 1-3). Most of the intraclasts in the carbonates are foraminiferal wackestones, indicating that they were deposited in the same settings as the carbonates. Therefore, the original depositional environments of intraclasts, carbonates and background sediments of the CORBs are relatively uniform in the Gyangze basin, mainly comprising basinal, deep shelf to toe-of-slope settings. As these facies zones are continuous, we suggest that the widespread and rapid mass transportation of carbonates or breccias resulted from steepening of the

10 X. Chen et al. / Sedimentary Geology 235 (2011) base of the slope. In the Campanian, the outer continental crust of the Indian passive margin was bent and upwarped after subduction started and the accretionary prism to the north of the Gyangze basin was formed (Liu and Einsele, 1994). This tectonic setting led to the development of numerous fault-bounded subbasins along the continental rise and to steepening of the sea floor in the Gyangze basin (Liu and Einsele, 1994; Chen et al., 2008). The color of the Chuangde Formation suggests that relatively oxic bottom water conditions prevailed below the toe-of-slope environment in the Gyangze basin during the middle Campanian. The deposition of gray interclasts within a multicoloured limestone matrix indicates less oxic conditions at shallower water depths within the basin. 5. Conclusions CORBs in the Gyangze basin mainly consist of shales, thin bedded marls, gravity transported limestones, and clast supported conglomerates. Eight microfacies types dominated by micrites and foraminiferal micrites were identified in the carbonates of the CORB succession. Facies assemblages suggest depositional environments of CORBs at the outer base-of-slope apron to basin. The MFTs of carbonates are comparable with standard microfacies 3 or 4, which occur in basin (FZ1), open deep shelf margin (FZ2) or toe-of-slope (FZ3) settings. Mass transport of carbonates resulted from steepening of the sea floor in the Gyangze basin. Relatively oxic conditions prevailed below the toe-of-slope environment although the occurrence of gray interclasts within a multicoloured limestone matrix indicates less oxic conditions at shallower water depths within the basin. Planktonic foraminifers within CORB carbonates indicate an early to middle Campanian age. Highly oxygenated bottom water appears to have been a more important factor to facilitate the formation of CORBs in the Gyangze basin than low sediment accumulation rates or low carbonate/ organic carbon productivity. The oxygenation of bottom waters was probably enhanced by atmospheric CO 2 drawdown following OAE2 and OAE3 and by deep water cooling during the Campanian, which increased the concentration and solubility of O 2 in the ocean. The long-term decrease in CO 2 concentrations following oceanic anoxic events appeared to have a lesser impact in deep oceanic basins than in shallower and restricted marginal areas. Acknowledgements This work is part of the research programs supported by the specialty Program of the National Basic Research Program of China (Grant No. 2006CB701400). It is also a contribution to IGCP 555. This paper benefited from the thoughtful comments and suggestions of Ines Wendler and Marcos A. Lamolda. 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