Upper Cretaceous oceanic red beds in southern Tibet: a major change from anoxic to oxic, deep-sea...

Science in China: Series D Earth Sciences 2006 Vol.49 No.8 785—795 785

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DOI: 10.1007/s11430-006-0785-7

Upper Cretaceous oceanic red beds in southern Tibet: Lithofacies, environments and colour origin

HU Xiumian1, WANG Chengshan2, LI Xianghui3 & Jansa Luba4 1. State Key Laboratory of Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093,

China; 2. Geological Centre of the Qinghai-Tibetan Plateau, China University of Geosciences, Beijing 100083, China; 3. School of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China; 4. Department of Earth Sciences, Dalhousie University, Halifax B3H 3J5, Canada Correspondence should be addressed to Hu Xiumian (email: [email protected])

Received May 26, 2005; accepted February 27, 2006

Abstract Application of mineralogy, geochemistry, sedimentary petrology, and sedimentology methods result in better understanding of the genesis and paleoenvironmens of the Upper Cretaceous oceanic red beds exposed in southern Tibet. The red beds comprise the Chungde Formation. Nine lithofacies recognized within this formation are: red foraminiferal packstone/grainstone, red microfos-sils wackestone, red marlstone with microfossils, red marlstone, red to variegated floatstone and rud-stone (debris flow), red shale, red radiolarite, red chert with radiolaria, and red chert. Sedimentary structures and textures, microfossils, and carbonate content show that the Chuangde Fm was depos-ited near the base of a continental slope in a deep oceanic basin environment, with the basin floor below the carbonate compensation depth (CCD). Red marlstones and limestones intercalated within red shales represent slides and slumps from the upper part of the continental margin. Debris flow and turbidity deposits consist of volcaniclastic, fossilliferous rudstone and floatstone, and very thin cal-careous mudstone, intercalated with red shales.

The Upper Cretaceous oceanic red beds in southern Tibet are characterized by high Fe2O3, low FeO, which indicates an oxic diagenetic environment, resulting in precipitation of hematite. The latter occurs as finely, disseminated ferric oxide giving the red color to the rocks. It is concluded that the red beds in southern Tibet were deposited under highly oxygenated bottom conditions in the deep ocean basin. Such conditions not only occurred in a deep ocean basin as indicated by the occurrence of pelagic red shale deposited below the CCD, but also extended up the continental margin as indicated by the presence of red colored marlstones and limestones embedded in the Chuangde Fm. The latter were deposited above CCD, most probably on the continental slope. The oxic bottom conditions are interpreted to be a result of a combination of climate cooling, active bottom ocean circulation, and change in the ocean-atmosphere oxygen budget.

Keywords: oceanic red beds, Upper Cretaceous, lithofacies, sedimentary environment, red colour, southern Tibet.

786 Science in China: Series D Earth Sciences

1 Introduction

“Red beds” are a term used for description of sedi-mentary sequences which are predominantly red in colour[1]. Based on the sedimentary environments, these can be grouped into: continental red beds and oceanic (or marine) red beds[2]. The oceanic red beds receive less attention due to scarcity of the occurrence, when compared with studies of continental red beds[1,3]. The occurrence of Upper Cretaceous oceanic red beds (CORBs) has been known for at least 140 years, since Stur[4] first described them from the Puchov beds in the Carpathians. Since then, mainly stratigraphic and paleotologic studies were conducted predominatly in Europe[5]. Recently, two IGCP pro-jects (IGCP 463 and IGCP 494), initiated in 2002, and 2003 respectively, improved our understanding of the CORBs origin[6] and their significance for paleocli-mate, organic carbon burial and paleoceanograpahy. Hu et al.[5] documented that the CORBs are distributed in a wide geographic belt extending from the North Atlantic, Spain, Italy, Alps, Carpathians, Black Sea,

Caucasus, Himalayas, to West Pacific. Melinte et al.[7] investigated the Campanian-Maastrichtian oceanic red beds in Romania. In the Northern Calcareous Alps, Wagreich et al.’s[8] study shows that basin topography and local clastic input strongly influenced the occur-rence and facies of the oceanic red beds developed in the Gosau Group. Statistical analysis of deep-water benthic foraminiferal assemblages from the Cam-panian-Maastrichtian of the Western Tethys reveals three biofacies of CORB: the abyssal red claystones with dwarfed abyssal assemblages, red deep-sea marls with calcareous agglutinated assemblages, and deep-

water pelagic limestones with fragile Rhizammina as-semblage[9].

Multidisciplinary studies conducted at the Chuangde Fm in southern Tibet (Figs. 1 and 2), in-cluded biostratigraphy[10,11], lithostratigraphy[12 ― 14], inorganic geochemistry[15] and organic geochemis-try[16]. In this paper we concentrate on the iron miner-alogy, petrology, and sedimentology, to improve un-derstanding of the CORB’s environments and origin in southern Tibet.

Fig. 1. a, The road map in the Lhasa-Gyangze area, Tibet, showing the locations of the Chuangde section near Gyangze and the Baergang section near Yamzho Yum; b, the outcrops of the CORBs and the locations of the studied sections in the Gyangze area. CD, Chuangde section; BN, Banggang section; CX, Caixiong section; YL, Yongla section; WM, Weimei section.

Upper Cretaceous oceanic red beds in southern Tibet: Lithofacies environments and colour origin 787

Fig. 2. Schematic distribution of the Chuangde Fm. near the Yamzho Yum Lake showing the location of the Baergang section.

2 Geologic setting

The studied areas are located at Gyangze and Na-garze regions, which tectonically belong to the north-ern subzone of Tethys Himalayas (Figs. 1 and 2)[17,18]. During the Cretaceous, this region was part of the northern continental margin of the Indian Plate[17,18]. Wang et al.[13] modified the Cretaceous stratigraphy in the Gyangze area and subdivided the succession into three formations: the Gyabula Fm, the Chuangde Fm, and the Zongzhuo Fm. The Berriasian-Coniacian Gy-abula Fm (150―300 m thick) is composed of black shales with pyrite nodules intercalated with sandstone beds. The overlying Chuangde Fm consists of vio-let-red shale intercalated with thin-bedded marlstone. The overlying Late Campanian-Palaeocene Zongzhuo Fm is predominantly composed of dark grey to black shales enclosing various olistoliths of sandstones, limestones, and bedded chert[19].

Due to easy-to-recognize red color, the Chuangde Fm was used as a mapping marker in the field[12]. It is widely distributed in the Gyangze area, but mostly as tectonic slivers, (Fig. 1b). In the type-section at Chungde, it is about 28.9 m thick. Abundant plank-tonic foraminifera and nannofossils in the Chuangde section correspond to the planktonic foraminifer zones of Dicarinella asymetrica, Globotrancanita elevata, Globotruncana calcarata, G. ventricosa[10―11], indi-cating a Santonian-Campanian age.

Recent field investigation shows that the red beds

also are present in the Yamzho Yum Lake area (Fig. 2)[20]. At Baergang locality red shales, red cherts with radiolaria, and red limestones with abundant plank-tonic foraminifera[20] are infracalated. Microfauana including planktonic foraminiferal Globotruncana linneiana, G. stuarti, G. carinata, Pseudotextularia sp., Heterohelix sp. and benthic foraminifera[9] including Paratrochamminoides spp., Nothia spp., Arenobu-limina dorbignyi, Goesella rugosa, Remesella varians, Dorothia oxycona, Dorothia retusa, Gaudryina pyra-midata, Clavulina subparisiensis indicate Cam-panian-Maastrichtian age of the strata.

3 Methods

Samples were taken from the Chuangde, Banggang, Caixiong, Yongla, and Weimei sections near Gyangze (Fig. 1) as well as from the Baergang section south to the Yamzho Yum (Fig. 2). More stratigraphic infor-mation can be found in Hu1).

To study inorganic mineral composition the sam-ples were crushed to 200 mesh in agate bowl. 1mol acetic acid was added for 40 hours with powered sam-ples to remove CaCO3. H2O2 were added to remove organic matter. Samples were washed using distilled water, separated by centrifuge, and dried at 65℃. Dried samples were analysed at the Max-3A X-ray power diffraction in the Department of Earth Sciences, Nanjing University. Scanning conditions: copper butt, electric current 20 nnJL, voltage 40 kV, scanning an-gle 2°―70°.

Samples from the Chuangde section were ana-lyzed for the iron minerals using diffuse reflectance spectroscopy (DRS). Samples preparation and analy-sis followed procedures described in Balsam and Deaton[21]. Samples were grounded to <38 μm to thoroughly disperse the minerals. Ground samples were made into a slurry on a glassy microslide with distilled water, smoothed, dried slowly at low tem-perature (<40℃), and analyzed in a Perkin-Elmer Lambda 6 spectrophotometer, with a diffuse reflec-tance attachment (reflectance sphere) from 320―2480 nm, in the State Key Laboratory of Mineral Deposits

1) Hu X M. Sedimentary geology of Cretaceous in southern Tibet and the Upper Cretaceous oceanic red beds. Doctoral Dissertation of Chengdu

University of Technology, 2002, 216


Research, Nanjing University. Data processing was restricted to the VIS (400―700 nm), the region of the spectrum most sensitive to the iron oxide minerals responsible for color differences[21]. First-derivative values (percent per nanometer) were calculated at 10-nm intervals to enhance the variability of the re-flectance data.

Major elements were analyzed by volumetric analysis in the Laboratory Centre of the Yichang In-stitute of Geology and Mineral Resources, China. Analysis error is lower than 5%.

4 Lithofacies

4.1 Gyangze area

The red beds lithofacies in the Gyangze area consist of :

(1) Red foraminiferal packstone/grainstone: with up to 90% of foraminifera (Fig. 3a), bright violet-red color, but if it is less calcareous, then the color is light violet. The planktonic foraminifera are rede-posited and transported from nearby shallower slope/shelf area. Angular quartz and crinoid debris occasionally occur.

(2) Red microfossils wackestone: there are two types according to different microfossils. One is fo-raminifera wackestone having 30%―50% planktonic foraminifera tests with mainly planktonic, few benthic foraminifers and radiolarians (Fig. 3b). Bioturbation is common. Few rounded carbonate intraclasts occur. Another variety is radiolarian wackestone with 30% radiolarian. Radiolarians are mainly replaced by cal-cite. Quartz grains are rare (3%). The matrix is highly oxygenated reddish-brown carbonate mud.

(3) Red marlstone with microfossils: microfossils are mainly foraminifers (15%―25%). Foraminifera appear in laminated layers. Recrystallization is common.

(4) Red marlstone: it is composed of a micrite mixed with argillaceous component. Very few fo-raminifers occur. Biotrubation and silty-size quartz are common.

(5) Red to variegated floatstone and rudstone (de-bris flow): intraclasts mostly larger than 2 mm, up to 8 mm, poorly sorted, subangular to subrounded (Fig. 3c). They are composed of foraminifera packstone, for-

aminiferal wackestone, micritic limestone, foraminif-era-bearing marlstone, and marlstone. Red calcareous matrix infill space between intraclasts indicates that the foraminifera were redeposited. Crinoid debris (up to 6 mm) and radiolaria occasionally occur.

(6) Red shales: dark-brown reddish, paper-sheet-

like shale (Fig. 3d) variably siliceous and completely devoid of any carbonate. Mirco-stylolites and silt-size quartz (5%) are common.

4.2 Yamzho Yum area

Red beds in the Yamzho Yum are of similar lithologic composition to those in the Gyangze area. They include red foraminiferal packstones/grainstones, red wackestones with foraminifera, and red shales. In addition, there occur several varieties of chert, such as:

(1) Red radiolarite: dark reddish, with 70%―80% radiolarian tests 70―210 μm in diameter (Fig. 3e). Most radiolarians are replaced by chalcedony. The matrix is stained by iron oxide. Bioturbation is com-mon. Few siliceous spicules occur. Radiolarite is de-void of carbonate.

(2) Red chert with radiolarians: the radiolarians, 10% to 50%, are mostly smaller than 70 μm and most of them are replaced by chalcedony. The matrix is stained by iron oxide. Bioturbation are common. Ra-diolaria concentrate into thin laminae, suggesting that they were redeposited.

(3) Red chert: the content of radiolarian is less than 10%. Present is silt-size quartz. Few of the radiolaria were replaced by calcite, or glauconite. Bioturbation is common.

(4) Yellow-gray volcaniclastic and bioclastic float-stone and rudstone (debris flow): this lithofacies is embedded within the red facies. The bioclastic grains (60%―90%) are angular or subangular, poorly sorted, with size up to 10 mm (Fig. 3f). The bioclasts are mainly sponges and echinoderms, with less frequently occurring nullipore, bivalve debris. The volcaniclastic grains (10%―30%) are mainly basaltic composition. The latter are well-sorted, rounded, with average size 0.2 mm. This lithofacies is interpreted as debris flow deposits generated from a shallower carbonate envi-ronment sitting on a volcanic basement.

There is an interesting texture: within one strati-


Fig. 3. Sedimentary structures and microscopic textures of the CORBs in southern Tibet. a, Red foraminiferal packstone, sample CD36b1, Chuangde section; b, red foraminiferal wackestone, sample YL12, Yongla section; c, red intraclasts floatstone, debris flow, sample 0602Q, Bed 35, Chuangde section; d, red siliceous shale, intercalated with thin bedded turbiditic marlstone, Bed 34, Chuangde section; e, red radiolarite, sample BG05, Baergang section; f, yellow-gray volcaniclastic and bioclastic floatstone, debris flow, sample BR9A1, Baergang section; g, the middle part of reddish chert with radiolarian sharp contacts with the underlying and overlying reddish pelagic wackestone with foraminifera, Bed No. 23, Baergang section; h, gray rounded carbonate-intraclasts in the limestone, sample CD35.1, Chuangde section.


graphic horizon, the top and bottom of the bed is red-dish wackestones with foraminifera, whilst the middle part is reddish chert with radiolaria. Chert has sharp contacts with the underlying and overlying wack-estones (Fig. 3g). Between the wackestones and chert is a 0.04-mm-thick laminae of dark reddish mudstones, providing evidence that deposition was occurring near the CCD.

5 Sedimentary environment

sit.

In the Gyangze area, brick red shales are siliceous, intercalated with centimetre-thick, pink to red marl-stones. The contact of the marlstones with the shales is sharp (Fig. 3d). Foraminifera in the marlstones con-centrate into laminae. The thin-bedded marlstone within the red shales is therefore interpreted as turbid-ite. There are few benthic foraminifera in the red shales, with the latter representing a background basinal depo

The red colored carbonates of the Chuangde Fm in the Gyangze area are rich in planktonic foraminifera, mainly Globotruncana and crinoid debris indicating a mixed pelagic/hemipelagic source. The contorted tex-ture of some of marl beds is clear evidence for em-placement by slides and slumps from the upper part of the continental margin (Fig. 3c). In an addition are present gray, gray-green intraclasts of 0.1―10 mm size. They are rounded, elliptical in shape and com-posed of marlstone and or foraminiferal wackestone (Fig. 3h).

In the uppermost part of the formation are interca-lated olistoliths, with the largest one more than 5 m in diameter. The olistolith is composed of gray to whitish foraminiferal packstone. Near the Baisa Village along the road from Nagarze to Gyangze, red shales are at-tached to a block of basalt, as a whole outcrop is a big olistolith (Fig. 1). In the Yongla section, the bedded reddish limestones sequence is a huge olistolith, or a tectonic sliver embedded within the Zongzhuo Fm. (Fig. 1).

The sediment composition and texture of the Chuangde Fm red beds in the Gyangze area document deposition near the base of “continental” slope and in the deep oceanic basin. Some of the sediments were redeposited from a shallower upper slope, or even

from a deep outer shelf areas. There is a sharp, sudden change in color from the underlying (Fig. 4), gray si-liceous shales of the Gyabula Fm. into the red sili-ceous shales of the Chuangde Fm. This color change indicates the redox change from dysaerobic to aerobic depositional or change in the early diagenetic condi-tion. The top of red beds at Chungde locality is in a tectonic contact with the Zongzhuo Fm (Figs. 4 and 5).

The environment interpretation of the red beds at the Baergang locality is more difficult, due to extensive tectonics as beds are intensively folded and faulted (Figs. 2 and 6). Many yellow-gray volcaniclas-tic and bioclastic floatstone and rudstone beds occur within the red beds sequences. We consider the former to be debris flow deposits (Fig. 6). The red beds are composed of microfossil marlstones radiolarian cherts, and shales and are devoid of any terrigenous particles,

Fig. 4. Stratigraphic column of the CORBs in the Chuangde section, showing the evolution of sedimentary environments.


Fig. 5. Generalised model of sedimentary environments for the CORBs in southern Tibet.

indicating that the deposition was occurring far from the continental margin, near CCD. Occurrence of de-bris flow sourced from a shallower area (Fig. 5) sug-gests deposition mostly probably from carbonate en-vironment sitting on volcanic basement.

6 Colour

The red color is due to the presence of finely dis-seminated hematite. The scanning electron microscopy did not find any crystal form of a hematite. The lime-stone with more impurities has darker red color, than impurities richer-pinkish limestone. The XRD patterns of acid residues show that quartz, albite, hematite are the main impurities. Hematite was detected in all red samples and identified by its characteristic XRD re-flections at 3.68, 2.70, 2.52, 2.20, 1.84, 1.69, and 1.486 Ǻ.

As documented by Deaton and Balsam[22] in the study of deep-sea deposits by diffuse reflectance spec-trometry (DRS), hematite and goethite can be easily identified in the first derivative curves. The hematite is associated with a single prominent peak, either at 565 or 575 nm, while goethite has two first derivative peaks, which the primary peak is at 535 nm with a secondary peak at 435 nm[21,22]. DRS data from twenty red samples in the Chuangde section whatever it is limestone, marlstone, or shale and color dark or light, red (-5R5/4, 5YR6/1, 5R6/2, 5YR8/1, 10R4/2, 10R5/4, 10R6/2)[23] show only a single prominent peak be-tween 560―570 nm of hematite in the first derivative curves (Fig. 7a). Other, gray, gray-green colored sam-ples in the Chuangde section (N5, N6, 5Y7/2, 5Y8/1) have neither hematite’s peak, nor two peaks of

Fig. 6. Stratigraphic column of the CORBs in the Baergang section, showing the evolution of sedimentary environments.

goethite, thus these rocks are devoid of both hematite and goethite (Fig. 7b).

Major element abundances in the red beds of the


Fig. 7. First derivative peaks of diffuse reflectance spectrometry for the red and gray samples from the Chuangde section, Gyangze. a, Red samples; b, gray, gray-green samples. Chuangde Fm. differ from the underlying and overlying grey shales of the Gyabula and Zongzhuo Fms. mainly by contents of iron oxide (Table 1). In red shales Fe2O3 values are up to 12.6%, averaging 8.4% in weight, and FeO is lower than that in gray shales. Other elements such as MnO and total iron show no significant changes, except that K2O content of red shales is little higher than that in gray shales (Table 1). The red shales in the Baergang section are also characterized by high Fe2O3, with an average of 8.57% and low FeO, an average of 0.38% (Table 1). By comparing

the Chungde red shales with the Global Standard Shales[24] and modern red clays in Pacific[25], Fe2O3 values in red shales are mostly higher than that in the GSS, but similar to that in modern red clays. The FeO in red shales is lower than that in the GSS.

The red carbonates of the Chuangde Fm. have a high content of Fe2O3, varying between 3.43%―5.62%, with an average of 4.27% in the Chuangde section, and 1.2% in the Baergang section. The gray carbonates in Chuangde Fm. have only 0.57% of Fe2O3.

7 Discussion

7.1 Sedimentary and tectonic control on the forma-tion of CORBs

The red noncalcareous shales are autochthonous deposits of the deep oceanic basin, while thin-bedded marlstones which have sharp contacts with shales were emplaced by turbidity currents (Fig. 3d). The color of marlstones is predominantly reddish-gray and or gray. In some beds, the top and bottom of the marlstone bed are reddish, while the middle part of the bed is gray, which indicates that the rapid deposition prevented complete oxidation of deposited sediments. Therefore the sediment color remains gray, or white. Similar condition can be found in Cretaceous red beds se-quences, for example in Italy, in the Fornaci Quarry near Ancona. Here, the white limestones beds interca-lated within red limestones were found to be calcare-ous turbidites[5]. In the Gosau Basin, Austria, the red limestones occur cyclically with gray limestones where the red limestones are pelagic, while gray lime-stones are the turbidite deposits[8].

Table 1 Major elements (%) of the Upper Cretaceous shales, limestones in southern Tibet and their comparison

Samples Number SiO2 TiO2 Al2O3 Fe2O3 FeO MgO CaO Na2O K2O MnO P2O5

Gray shales in Zongzuo Fm. 3 40.80 ― 17.37 3.15 4.26 ― 6.01 ― ― 0.12 0.16Gray shales in Chuangde Fm. 2 37.89 ― 15.97 1.77 6.68 ― 11.55 ― ― 0.28 0.21Red shales in Chuangde Fm. 14 50.27 1.20 15.82 8.39 1.77 2.14 5.90 0.88 4.04 0.22 0.30

Chuangde Section

Gray shales in Gyabula Fm. 10 59.14 0.87 13.04 4.07 2.52 2.02 5.32 0.95 2.56 0.25 0.62Red shales in the Baergang section 4 56.88 1.08 15.34 8.12 0.59 3.12 2.97 1.08 3.27 0.34 0.16

Global Standard Shales[24] 58.10 0.65 15.40 4.02 2.45 2.44 3.11 1.30 3.24 ― 0.17

Shal

es

Modern Pacific red clay[25] 54.9 0.78 16.6 7.7 — 3.4 0.7 1.3 2.7 0.56 0.25

Red limestones 4 31.43 ― 9.67 4.27 1.80 ― 25.78 ― ― 0.24 0.17Chuangde Section

Gray limestones 4 21.73 ― 3.85 0.57 0.85 ― 38.65 ― ― 0.97 0.10Red limestones in the Baergang section 1 25.30 ― 2.83 1.20 0.11 ― 37.56 ― ― 0.59 0.14

Lim

esto

nes

Upper Cretaceous red limestones in Turkey [26] 17.70 ― 2.80 2.00 0.40 ― 41.90 ― ― 0.20 0.20


In southern Tibet, most of the Chuangde Fm. red beds occur as laterally discontinuous lens-shape out-crops. They represent tectonic slivers within tectonic melange, such as the red beds in the Yongla, Caixiong sections. Only in the Gyangze area at Chungde locality are the red beds part of undisturbed Cretaceous se-quence. Tectonic melanges which contain pelagic red beds can be also found in other western areas in Tethys Himalaya, such as Saga, Syaga, Zanda areas[11].

7.2 Red colour and its origin

Field observations indicate that the Upper Creta-ceous pelagic red sediments in southern Tibet are bedded and overlying gray colored strata. Within sin-gle red bed, the color is uniform, which indicates that the color is a result of syn-sedimentary or early dige-netic processes, rather than the result of weathering. Colour variations from red to white are usually tied to bedding, but sometimes occur within single beds; from bottom to top the color changes from gray-white, gray to pink, red. The distribution of red colouration strongly indicates that color is neither the product of weathering, nor to be of detrital origin. For the latter, the evidence is that the clay minerals in the gray beds are the same as in red beds. Clay fraction is dominated by illite (65%―90%) and chlorite (~30%)1). Also, detrital-sensitive elements such as Al, Si do not show major difference between gray and red beds, indicating that the source was the same for gray and red beds.

Based on data from iron minerals, XRD, DRS, and major elements we concluded that the red color of the red beds is due to the presence of invisible finely dis-persed hematite. The hematite could be syn- sedimen-tary or early diagenetic in origin, but in the deep sea sediments it is clearly not of detrital origin. Eren et al.[26] investigated the Upper Cretaceous pelagic red sediments in Turkey and found that hematite content ranges from 0.5 to 3.0 wt.%. Their electron micro-scope observations suggest that the hematite platelets are hexagonal shapes and grow epitaxially on the grains, strongly indicating a diagenetic product. Channell et al.[27] studied the magnetic properties of the Upper Cretaceous Scalia Rossa red pelagic lime-

stones in Italy. They found that the various magnetiza-tion components do not record the polarity reversal at the same stratigraphic level. Few tens of centimetres below a recorded geomagnetic reversal defined by the magnetite magnetization, some of the hematite grains were magnetized in the post-reversal field. They sug-gested that the growth of hematite crystals occurred at ~105 years during early diagenesis.

7.3 Paleoceanographic implication of CORB

It is cumulative effect of organic carbon “rain” rate and dissolved oxygen in bottom waters which deter-mines the redox state at the sediment-water interface. Changes in either of these properties result in a change of redox state at the sediment/ocean water inter-face[28,29]. As evidenced by the occurrence of fo-raminifera, radiolarian and other fossils in the Chuangde Fm red beds, the organic matter input dur-ing red beds sedimentation would not be so low. Thus an increase in dissolved oxygen in bottom waters would be the main cause for the CORBs origin. Such oxic conditions may have been extensive in deep oce-anic basin and extended up to the continental slopes, where the red limestones were deposited. During the Late Cretaceous, the Gyangze-Yamzho Yum areas were situated near the northern continental margin of the Indian Plate, approximately near the equator[30]. Thus the bottom waters with high dissolved oxygen content could have been widely distributed during the Late Cretaceous in the Eastern Tethys.

We speculate that earth processes which could have contributed to the CORBs’ origin are: global climate cooling, intensification of bottom circulation related to the change in global paleogeography, and/or the change in the oxygen budget as a consequence of ex-tensive organic carbon burial during mid-Cretaceous.

(1) A significant climate cooling after the Cenoma-nian-Turonian boundary is indicated by oxygen iso-topes[31,32] from carbonates, planktonic foraminifera, plant fossils, as well as by GCM study. The cooling trend extended up into latest Cretaceous[33,34]. As cold waters can hold higher content of dissolved oxygen, this may have resulted in development of oxic condi-

1) See foodnote 1) on page 787


tions at the ocean floor. (2) Global cooling may have resulted in intensified

formation of deep waters thus more oxygen would be transported by bottom waters. Another possibility is significant change in the mode of the ocean circulation as a result of salt variation in the low latitude during the Cretaceous[35]. During post-Turonian time deep ocean circulation changed from sluggish in the mid-

Cretaceous to well ventilated open ocean as a result of increased dispersal of southern continents and of the opening of high latitude, Southern Hemispheric con-nection[36,37].

(3) Excess organic carbon burial during Early- mid-dle Cretaceous OAEs’[38] may have significantly af-fected CO2 and O2 concentrations in the oceans and atmosphere[39]. Significant reduction in pCO2 as a re-sult of large organic carbon burial during mid- Creta-ceous could induce significant global climate cooling and as a consequence increase the vigor of ocean bot-tom circulation. On the other hand, Arthur et al.[39] considered that increased organic carbon burial would have resulted in an increase in O2 in the atmosphere. Therefore, an increase in pO2 resulting from cooling would have increased the oxidizing capacity of oce-anic deep waters.

8 Conclusions

Nine lithofacies recognized in the Chuangde Fm. red beds include red foraminiferal packstone/ grain-stone, red microfossils wackestone, red marlstone with microfossils, red marlstone, red to variegated float-stone and rudstone (debris flow), red shale, red radio-larite, red chert with radiolaria, and red chert.

The red shales in the Chuangde Fm were deposited below CCD at the base of the continental slope-basin floor transition, with deposition frequently affected by turbidity currents, slumps and slides. Red marlstones and limestones were emplaced into noncalcareous red shales by reoccurring slides and slumps from a shal-lower continental margin. The red beds at the Baer-gang locality indicate that deposition was occurring in a (hemi)pelagic environment near CCD, frequently affected by debris flows sourced from a shallow-water carbonate environment sitting on a volcanic basement.

Mineralogic study, XRD, and DRS show that the

red color of the CORBs in southern Tibet is due to the presence of finely dispersed hematite, which is either syn-sedimentary, or early diagenetic in origin. Both red shales and red limestones are characterized by high Fe2O3, low FeO, which indicates an oxic deposi-tional environment where hematite is formed.

We conclude that deposition of red colored pelagic facies in southern Tibet occurred under highly oxy-genated bottom conditions in the deep ocean basin, when dissolved oxygen in the bottom waters was high. Such conditions not only occurred below the CCD, but also extended up the continental slope, as indicated by the red marlstones and limestones in the Chuangde Fm. We conclude that the change in the deep sea sediments to oxidized red beds is a result of a combination of climate cooling, increased vigour of ocean bottom circulation and of a change in the ocean-atmosphere oxygen budget.

Acknowledgements We thank Prof. Liu Zhifei, Prof. Ji Junfeng, and Dr. Cai Yuanfeng for help in SEM, DRS, and XRD, respectively, and to Chen Lei for his preparing Fig. 5. This work was supported by the National Natural Science Foundation of China (Grant No. 40332020) and the MOST 973 Project (Grant No. 2006CB701402). This is a contribu-tion to the IGCP 463/494.

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