Orbitally Forced Sea-Level Changes in the Upper Turonian-Lower

Cretaceous Research 56 (2015) 691e701

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Cretaceous Research

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Orbitally forced sea-level changes in the upper Turonianelower Coniacian of the Tethyan Himalaya, southern Tibet

* Xi Chen a, , Chengshan Wang a, Huaichun Wu a, Wolfgang Kuhnt b, Jianzhong Jia c, Ann Holbourn b, Laiming Zhang a, Chao Ma d a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China b Institute of Geosciences, Christian-Albrechts-University, Kiel D-24118, Germany c China National Offshore Oil Corporation, Beijing, 100010, China d Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA article info abstract

Article history: Although the mid-Cretaceous is considered to be a typical interval of greenhouse climate and high sea Received 24 April 2014 level, cooling events associated with regressions were inferred in recent years. We conducted a Accepted in revised form 19 July 2014 biostratigraphic, chemostratigraphic, sequence stratigraphic and cyclostratigraphic investigation of up- Available online 16 September 2014 per Turonianelower Coniacian marine strata in the Tethyan Himalaya zone, to retrace the sea-level variations and to clarify their global correlations. According to the planktonic foraminiferal zonation, Keywords: the studied interval is part of the late Turonianeearly Coniacian Marginoruncana sigali and D. concavata Late Turonian Zones. The carbon isotope curve shows a good correlation to reference curves in the Boreal and western Sea-level change d13 ﬁ Tethyan Himalaya Tethys realms with all major and minor late Turonian C events identi ed, indicating that the C-isotope Continental ice curve provides an excellent tool for global stratigraphic correlation in the Turonian. Based on the lithological variations of clastic input and physical and chemical proxies, the succession is divided into two third order and eight fourth order sequences. Spectral analysis indicates that fourth order sea-level changes were linked to the astronomically stable 405-kyr eccentricity cycle. Comparison with classic global sea-level curves, we suggest that late Turonianeearly Coniacian sea-level changes along the southeastern Tethyan margin were controlled by eustasy. The signiﬁcant regressions during ~90e89.8 Ma and ~92e91.4 Ma, which are recorded in different continents, may be interpreted as the result of continental ice expansion, giving some support to the notion that ephemeral polar ice sheets existed even in the super-greenhouse world. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction upper ocean waters as warm as ~30 C at ~60S paleolatitude in the late Turonian (Bice et al., 2003). Previous publications also sug- The mid-Cretaceous, especially the Cenomanian-Turonian in- gested that average SSTs were ~10 C higher during the Turonian terval, is considered to be a typical interval of greenhouse climate than today (Huber et al., 1995; Wilson et al., 2002). These extremely with high sea-surface temperatures (SSTs) (Huber et al., 2002; high temperatures were probably linked to CO2 concentrations Wilson et al., 2002; Bice et al., 2006; Forster et al., 2007a,b), high 4e10 times higher than the pre-industrial level (Berner, 1994; sea level as well as a much flatter equator-to-pole thermal gradient Berner and Kothovala, 2001). than today (Barron, 1983; Huber et al., 1995). Mid-Cretaceous upper The sea level during this interval was estimated as tens of me- ocean temperature estimates from oxygen stable isotope (d18O) ters (Sahagian et al., 1996; Miller et al., 2003, 2004, 2005a,b)to analyses of planktonic foraminifera typically indicate SSTs that are more than 100 m (Haq et al., 1987; Haq, 2014) higher than the 8e20 C warmer than modern values (Bice and Norris, 2002). In the present day. Despite the generally high temperature and sea level Turonian-Coniacian interval, the mean annual temperature was in the Turonian, some short intervals of cooling (Bornemann et al., over 14 C(Tarduno et al., 1998). In addition d18O values indicate 2008; Forster et al., 2007a) and rapid sea-level changes were inferred, which may have been related to cyclic polar glacier growth and decay during intervals of reduced insolation in south- * Corresponding author. ern hemisphere high latitudes (Miller et al., 2004, 2005a, b). E-mail address: [email protected] (X. Chen). Although it is widely accepted that the warm Mesozoic climate was http://dx.doi.org/10.1016/j.cretres.2014.07.010 0195-6671/© 2014 Elsevier Ltd. All rights reserved. 692 X. Chen et al. / Cretaceous Research 56 (2015) 691e701 undoubtedly punctuated by cooling events (Price, 1999; Price and et al., 2006). The Cenomanian-Turonian boundary is at 62 m in the Hart, 2002), it has been intensely debated whether these were section according to planktonic foraminifers and the end of the sufficient to lead to the formation of polar ice (e.g. Barron, 1983; positive d13C excursion. The top of Turonian was placed at 88 m in Kemper, 1987; Crowley and North, 1991; Frakes et al., 1992; the section by Li et al. (2006), whereas Wendler et al. (2009) argued Frakes and Krassay, 1992; Hallam, 1993; Chumakov, 1995; Bennett that the Turonian-Coniacian boundary might be at ~137 m by et al., 1996; Frakes, 1999; Oyarzun et al., 1999; Moriya et al., correlating the d13C curve with the neighboring Zhepure section 2007; Ando et al., 2009). (Willems and Zhang, 1993), which is named Tingri section by In order to test the hypothesis of Cretaceous glacial-eustasy, we Wendler et al. (2009), and the Eastbourne section of southern En- need to evaluate whether regional sea-level changes are globally gland (Jarvis et al., 2006). In the Gongzha section, the lower Turo- coeval. The Cretaceous marine sequences, which are well exposed nian is characterized by thin to medium bedded calcareous in the Tethyan Himalaya zone of southern Tibet, provide a unique wackestone layers that form a relief peak. From 85 to 90 m, the opportunity to investigate the sea level record of the eastern lithology is composed of marls or calcareous shales with thin Tethyan realm. We re-examined the planktonic foraminiferal bedded wackestone layers. From 90 to 145 m, it mainly consists of biostratigraphy in the late Turonianeearly Coniacian record of the marl and marly limestone couplets except for two intervals of ~5 m Gongzha section in southern Tibet (Li et al., 2006) to determine the thick thin bedded limestones (8e10 cm thickness for each single biozonal boundaries. Based on the biozones, the carbon isotope layer). The couplets are commonly a few cm to 20 cm in thickness. curve is correlated to the reference curve of Europe (Jarvis et al., In a single couplet, the marl and limestone layers do not exhibit 2006) to identify the major and minor isotopic events in the sharp contacts. eastern Tethyan realm. In addition, we studied the lithology as well as chemical and physical proxies to establish relative sea-level 3. Materials and methods fluctuations in the Tethyan Himalaya. Previous work showed that 13 d C, CaCO3, Mn, MS (magnetic susceptibility) and GR (gamma ray We undertook detailed sedimentary logging, sequence strati- logging) were helpful proxies to study the effect of sea-level graphic analysis, geochemical sampling and Gamma Ray (GR) changes in marl/limestone alternation successions (e.g. Wendler measurements in the upper Turonianelower Coniacian part of the et al., 2013; Boulila et al., 2008; Jarvis et al., 2001; de Rafelis Gongzha section. High resolution (10e15 cm) GR data sets were 13 et al., 2001; Miller et al., 2004). In this study we used d C, CaCO3 directly measured at the outcrop face with a portable natural and GR profiles as well as lithological descriptions to reconstruct a radioisotope analyzer CIT-2000F (Xinxianda co. ltd., China) with a sea-level curve for the late Turonianeearly Coniacian interval in the detector of 5*5 cm square. Counts per seconds (cps) in selected Tethyan Himalaya. Spectral analysis was used to detect orbital scale energy windows were directly converted to concentrations of po- sea-level changes and to determine the age and duration of sea- tassium, K (%), uranium, U (ppm) and thorium, Th (ppm). A 180 s level cycles. Finally, we correlated the regional sea-level changes time interval was applied at each measuring point with a vertical to previously established sea-level curves to identify global signals sampling step of 10e15 cm. In total 600 measurements were and to assess possible mechanisms of sea-level change in the late collected from the studied succession. Turonian-early Coniacian greenhouse world. Thirty one samples from marls were selected between 85 and 142 m for micropaleontological analysis. About 100 g from each 2. Geological settings sample were submerged in water for several days and sieved over a 63 mm sieve. Generally, 10e20 g of sediment residue were initially Cretaceous marine deposits in southern Tibet are mainly examined for foraminifers and then treated with anionic tensides exposed in the Tethyan Himalaya tectonic zone (Fig. 1), located (REWOQUAT), which helped to completely break down the sample. between the Higher Himalayan Crystalline Belts and the Indus- In most cases tens to hundreds of moderate to well preserved Yarlung Zangbo Suture Zone (Burchfiel et al., 1992). The Mesozoic planktonic foraminifers were obtained in the 63 mm to 1 mm size strata in this area belong to two different tectonic domains: the fraction of each sample. The planktonic foraminifers were identi- passive continental margin of the Indian continental plate and the fied under a binocular microscope and typical specimens were adjacent deep oceanic basin (Yu and Wang, 1990; Liu and Einsele, documented using a scanning electronic microscope. Approxi- 1994). The Gyirong-Kangmar intracrustal thrust subdivides the mately 20 thin sections of limestones or marly limestones were Tethyan Himalayas into the northern and southern subzones of examined to determine the microfacies and lithologic changes of different lithological compositions (Liu and Einsele, 1994). During the interval. the mid-Cretaceous, the area was at a latitude of 21S(Patzelt et al., Six hundred samples were collected for carbonate analysis from 1996) and surrounded by an ocean connected eastward to the Pa- the 80e142 m interval in the section. The carbonate contents were cific Ocean and westward to the Mediterranean Tethys (e.g. Scotese, estimated by using a carbonate bomb. About 0.3 g of dried sample 1991). were was ground to powder and dissolved in 5 ml hydrochloric acid The studied area is located in the southern Tethyan Himalaya of 10% concentration. According to the volume of CO2, we can subzone, which formed the north margin of the Indian continent calculate the weight percentage of carbonate in the samples. Every after it finally separated from Eastern Gondwana in the Early 1 h, the same process was carried on 0.3 g pure CaCO3 powder to Cretaceous (Hu et al., 2010). During the mid-Cretaceous (late estimate the temperature and pressure changes effect on the vol- AlbianeSantonian), the Tethyan Himalaya tectonic zone as passive ume of CO2. margin drifted northward with the Indian continent (Garzanti and In order to evaluate the cycle characteristics of the studied Hu, 2014). The studied section at Gongzha is situated ~50 km west section, a multitaper method (MTM) of spectral analysis (Thomson, of the city of Tingri on the north slope of the Zhepure Mountain (Li 1982) was conducted on the carbonate content and gamma ray et al., 2006). The Cretaceous strata mainly consist of marly lime- data. The cycle length ratio method (Weedon, 2003) is applied to stone couplets with a few limestones and calcareous shales. The investigate the links between detected sedimentary cycles and the positive carbon isotope excursion event at the Cenomanian- theoretical astronomical parameters. MTM power spectra analysis Turonian boundary (OAE2) is well developed in the section, was performed using the SSA-MTM toolkit (Ghil et al., 2002). although it is characterized by thin to medium bedded limestones Robust estimates of background red noise with confidence limits at instead of black shales as in other marine Cretaceous successions (Li the 90%, 95% and 99% level were determined following Mann and X. Chen et al. / Cretaceous Research 56 (2015) 691e701 693

Fig. 1. Geological setting and locality of studied section (modiﬁed from Hu et al., 2012).

Lees (1996). Prior to analysis, the time series was normalized, and as the positive excursions of OAE2 and OAE 3 can be correlated with pre-whitened in Kaleidagraph™ by subtracting 35% weighted reference curves in Europe (Jarvis et al., 2006). In a previous study average (Cleveland, 1979). of the Gongzha section (Li et al., 2006) the Turonian/Coniacian boundary was formerly placed at the boundary of the Margin- 4. Results otruncana sigali zone and Dicarinella primitiva zone, which is poorly defined due to the rare occurrence and the morphologic variability 4.1. Biostratigraphy and carbon isotope stratigraphy of D. primitiva. Based on detailed correlation of the plankton foraminiferal biostratigraphy and carbon isotope stratigraphy in the In southern Tibet, the biostratigraphy and carbon isotopic stra- neighboring Zhepure section, Wendler et al. (2009) argued that the tigraphy of the Upper Cretaceous were studied in detail in two boundary should be placed higher, at the lower part of the sections, the Gongzha section (Li et al., 2006) and the neighboring D. concavata zone. Zhepure section (Wendler et al., 2009), which is located about 2 km To resolve this issue, we re-evaluated the stratigraphic position east of Gongzha. The major carbon isotope excursion events, such of planktonic foraminifers of for the Gongzha section to clarify the 694 X. Chen et al. / Cretaceous Research 56 (2015) 691e701 placement of the Turonian/Coniacian boundary. Planktonic fora- event, which is also referred to as the Hitchwood Event, is identified minifers are relatively abundant in this interval and yield Dicar- at ~91 m in the Gonghza section. Although only characterized by a inella algerina, D. hagni, D. imbricata, Marginotruncana renzi, modest short-term excursion of þ0.3‰, it also represents both the M. schneegansi, M. sigali, M. sinuosa and others. These are known as maximum value of 2.5‰ and the inflection point of the late Turo- typical species of the M. sigali and D. concavata zones. Our nian C-isotope curve, as it rises from the minimum of the Bridge- biostratigraphic results are consistent with the previous work of Li wick Event below, then falls to the minimum of the Navigation et al. (2006). However, we suggest that the Turonian/Coniacian Event above. Between the Bridgewick Event and the Hitchwood boundary should be placed at ~115 m according to GTS 2012 Event, the d13C values exhibit a stepwise increase from ~1.5‰ to (Gradstein et al., 2012) to correspond with the d13C minimum ~2.5‰. Four small positive d13C excursions of ~0.3‰ occur between (Navigation Event), which is about 28 m higher than the base of the the Bridgewick and Hitchwood events. These minor events are D. primitiva Zone, identified by Li et al. (2006). clearly developed in the inter-correlated three sections (Fig. 2). Carbon-isotope variations determined from bulk pelagic and Following the Hitchwood event, the d13C values decline stepwise hemipelagic Upper Cretaceous carbonate rocks (Li et al., 2006) from 2.5‰ to 1.5‰ in the Navigation Event. Three small excursions show consistent stratigraphic trends and identical d13C values in of þ0.2‰ to þ0.4‰ occur between the Hitchwood and Navigation the Boreal and western Tethys realms, thus providing a reliable events. These minor peaks are well developed in European sections, basis for high-resolution correlation (e.g. Scholle & Arthur, 1980; where they are labeled i1-3. Above the Navigation Event, there are Tsikos et al., 2004; Jarvis et al., 2006). We correlated the carbon- two prominent positive excursions, which could be correlated with isotope curve to the expanded successions in the Boreal realm the early Coniacian Beeding Event and Light Point Event (Jarvis and western Tethys, such as the Turonian GSSP candidate et al., 2006). However, more detailed stratigraphic investigations Salzgitter-Salder in Germany (Jarvis et al., 2006) and Liencres in are needed to correlate these excursions. Spain (Fig. 2)(Stoll and Schrag, 2000), which have been already inter-correlated by Jarvis et al. (2006). The values in the Gongzha 4.2. Sequence stratigraphy and relative sea-level change section are on average ~0.5e1.0‰ lower than in the other sections, although the minimum values are similar. Therefore, the ampli- In the Tethyan Himalaya, the Upper Cretaceous is mainly tudes of changes are lower in the Gongzha section. Two significant composed of limestone and marlstone alternations, except for negative excursion events were identified, which we correlate to some limestone units (Willems and Zhang, 1993). The limestones the Bridgewick and Navigation Events in the English chalk refer- deposited during the periods of higher global sea level; for ence section (Jarvis et al., 2006). The Bridgewick Event is charac- example, the lithology spanning the Cenomanian-Turonian terized by a sharp negative excursion of ~0.5‰ in the lower part of boundary in the studied area is characterized by thin to medium M. sigali zone at 75 m in the studied section, which is consistent bedded limestones (Li et al., 2006; Wendler et al., 2009). Owing to with the curve of the Gubbio section in Italy. The Navigation Event the monotonous lithology of marl and limestone alternation, it is characterized by a negative excursion of ~0.3‰ in the middle part initially appeared difficult to develop a sequence stratigraphic of the studied succession and reaches the lowest d13C values framework for the studied succession (Wang et al., 2004). In recent recorded in the uppermost Turonian. The late Turonian maximum years, however, these cycles in carbonate deposits were interpreted

Fig. 2. Correlation of the carbon isotope stratigraphy in the Gongzha section to European reference curves (Li et al., 2006; Jarvis et al., 2006; Stoll and Schrag, 2000). X. Chen et al. / Cretaceous Research 56 (2015) 691e701 695 as a result of orbitally forced climate and sea-level changes (Boulila Based on microscope observations, the lithologies generally consist et al., 2008; Boulila et al., 2010). Chemical and physical proxies, of mudstones and bioclastic wackstones (Fig. 3 A, B) following the 13 such as CaCO3, magnetic susceptibility, d C, gamma ray logging classification of carbonates of Folk (1962) and Dunham (1962). The and some elements which are sensitive to sea-level change proved microfacies types, when correlated to standard microfacies zones to be useful to identify sequences in cyclic sedimentary deposits (Wilson, 1975; Flügel, 2004), indicate that they were deposited in (e.g. Jarvis et al., 2001; Boulila et al., 2010; Miller et al., 2005a,b; outer ramp or pelagic basin environments in a water depth of a few Wendler et al., 2013). Moreover, these proxies allow to study the hundred meters. Very rare occurrences of benthic foraminifers in relationship between sedimentation pattern, sea level and climate relation to planktic foraminifers also indicate a relatively deep fluctuations, based on spectral analysis. In this work, we use CaCO3, depositional environment. Therefore, erosion or exposure surfaces gamma ray logging and microfacies to investigate the late hardly exist in the succession. The sequence stratigraphy was Turonian-early Coniacian sequence stratigraphy and sea-level his- established using the lithological assemblages as well as physical tory of the Tethyan Himalaya. and chemical proxies within the different units. The deposits mainly consist of fine-grained terrigeneous mate- 4.2.1. Lithological assemblage and sequence stratigraphy rial and pelagic carbonates. In each cycle, beds and couplets show The 60-m-thick upper Turonian-lower Coniacian sedimentary fining and become more calcareous from the base to the middle of succession is well exposed along the ridge on the north slope of the the cycle. From the middle of the cycle, the rocks show a trend of Zhepure Mountain (Li et al., 2006). Based on detailed bed-by-bed coarsening and becoming less calcareous toward the top. This re- logging of the section, the succession was subdivided into eight flects a change in terrigeneous input upward within a single cycle. major lithological sedimentary cycles (Fig. 3). Each cycle starts and Therefore, each cycle represents a period of relative sea-level rise ends as ‘soft’ layers which are commonly composed of marls or and fall (transgression/regression cycle, T/R cycle) and is inter- marl/limestone couplets thinner than 10 cm and form troughs in preted as a fourth order sequence. Furthermore, based on the relief. In contrast, the middle parts of the cycles are composed of lithological assemblage, the succession is divided into two third ‘hard’ layers, which consist of limestones or marl/limestone cou- order sequences (Fig. 4). The thin bedded limestone units in the plets thicker than 10 cm. The cyclic sequence displays the typical middle and uppermost part of the succession represent the deepest character of upper Turonianelower Coniacian deposits in this area. water depth and correspond to maximum flooding surfaces

Fig. 3. Typical lithologies in the upper Turonianelower Coniacian of the Gongzha succession viewed under a microscope and at the outcrop. A-mudstone, B-bioclastic wackstone, C- and D-microfacies and outcrop view of current transported sediment occurring at 118 m. The orientated texture of the skeletal fragments, such as foraminifers and echinoderms (C) and the horizontal lamination (D) indicate that the sediments were reworked by weak bottom currents. The sample in ﬁgure C was collected from the marked area in D. 696 X. Chen et al. / Cretaceous Research 56 (2015) 691e701

Fig. 4. Correlation of the sequence stratigraphy with the relative abundances of terrigeneous clasts in different sizes. Transgressions correspond to an increase in fine components (<0.0039 mm) and regressions to an increase in coarse clasts (0.063e0.1 mm). A-Marls intercalated with calcareous shales; B-Marls intercalated with thin bedded limestones; C- Marl and marly limestone couplets of <10 cm thick, commonly the ratio of marls/limestones is higher than 1; D-Marls and marly limestone couplets of >10 cm thick, commonly the ratio of marls/limestones is lower than 1; E-thin bedded (marly) limestones. The dashed lines show that the finest terrigeneous inputs are consistent with the highstand system tracts. Solid arrows show increasing (decreasing) content of finer (coarser) grains. Blank arrows show decreasing (increasing) content of finer (coarser) grains. because of the minimum terrigeneous input flux and the minimum 4.2.2. Carbonate content, natural gamma ray and sequence grain size. The intervals of calcareous shale layers exposed around stratigraphy 88 m and the marls intercalated with thin bedded marly limestones The carbonates in marine sedimentary rocks mainly originate exposed around 115 m were deposited in the shallowest water from the skeletal materials in the ocean, while the natural gamma depth and are inferred to be sequence boundaries (SB). A few ray signals are related to the siliciclastic and organic carbon con- yellowish layers were identified in the interval of 115e118 m, tent (Myers and Wignall, 1987; Bristow and Meijers, 1989). Pure indicating a relative higher content of clays in this interval carbonate contains negligible amounts of K, Th and U except for compared to in the remaning part of the succession. The limestone small amounts of U and Th present in the carbonate lattice (Myers, intercalations just above the sequence boundaries were reworked 1989). Both U and Th are sensitive to climate and sea-level by bottom currents as shown by the preferred orientation of grains changes (e.g. Jarvis et al., 2001; Johan and Postma, 1996). We (Fig. 5C). These features indicate a proximal outer ramp deposi- measured the carbonate contents and natural gamma ray signals tional environment, since storm waves were able to influence the to assess the marine and/or terrigeneous influence on the sedi- sediments (Flügel, 2004). These intervals correspond to the shal- ments (Fig. 5). lowest water depths of each sequence. Therefore, we suggest that The CaCO3 content mainly varies from 50 to 70% in the section the lithological changes in the succession were mainly controlled with an average value of 61%. The lower values of about 50e60% by sea-level fluctuations rather than by climate. occur in the marl or thinner marl and limestone couplet intervals, The sequence stratigraphy inferred from the lithology agrees whereas the higher values of 60e70% occur in the limestone beds well with the relative abundances of terrigeneous components of or thicker marl and limestone couplets. The Th and K natural different sizes (Li et al., unpublished data). The terrigeneous input gamma ray plots show directly opposite trends with the CaCO3 mainly consists of silty-size grains. However, the terrigeneous curve (Fig. 5). The curves contain two semi cycles at the lowermost grains of clay size (<0.0039 mm) within the highstand system and uppermost of the succession respectively. Eight cycles are tracts (HST) are significantly more abundant than in the sequence found in the remaining part of the succession. The cycles 2 to 8 boundary and transgressive system tracts, whereas the fine sandy (Fig. 5) match well with the T/R cycles. However, cycle 1 is not well and silty clasts (0.0039e0.1 mm in size) are more abundant around recorded in the T/R cycle. The cycle 1 is mainly composed of the SB than in HST. The relative abundance of terrigeneous com- calcareous shales, which leads to difficulty in identifying the T/R ponents thus also indicates coarsening upwards in a T/R cycle. cycle by lithology. .Ce ta./Ceaeu eerh5 21)691 (2015) 56 Research Cretaceous / al. et Chen X. e 701

Fig. 5. Sequence stratigraphy and plots of carbonate content (weight percentage), natural gamma (counts per second), Th (ppm) and K contents (weight percentage). Cyclically changing trends are identiﬁed from the plots. HW e Hitchwood Event. 697 698 X. Chen et al. / Cretaceous Research 56 (2015) 691e701

Fig. 6. Spectral analysis of carbonate and Th content series. The two sets of analyses show comparable peaks.

4.2.3. Relative sea-level change The lithology and terrigeneous grain size as well as the chemical Table 1A Spectral frequency ratios calculated from peaks in the spectrum of CaCO3 and Th and physical proxy data indicate consistent water depth changes. series. The carbonate content cycles show an antiphase relationship to the terrigeneous input flux (indicated by gamma ray log) and terrige- 37 40 47 55 73 170 730 37 1.000 neous grain sizes, which can be related to transgressive/regressive 40 0.925 1.000 cycles (Fig. 5). We, therefore, suggest that the carbonate content 47 0.787 0.851 1.000 and/or gamma ray log are reliable proxies to estimate relative sea- 55 0.673 0.727 0.855 1.000 level changes in the Gongzha succession. We propose that higher 73 0.507 0.548 0.644 0.753 1.000 170 0.218 0.235 0.276 0.324 0.429 1.000 carbonate content (minima in gamma radiation/clay content) re- 730 0.051 0.055 0.064 0.075 0.100 0.233 1.000 flects higher sea levels, whereas lower carbonate content (maxima in gamma radiation/clay content) corresponds to intervals of lower sea levels. It was also demonstrated by previous case studies on wavelength corresponds to the thickness of a few couplets. The pelagic successions in the western Tethys, e.g. the upper 7.3 m wavelength corresponds to the T/R cycles. To interpret these Cenomanian-Turonian in Jordanian Levant Platform (Wendler et al., wavelengths in terms of possible orbital periods, we calculated 2013) and Kimmerigian in SE France (Boulila et al., 2010), that the their ratios and compared them to typical ratios for the mid- carbonate content reflects the overall sequence stratigraphic Cretaceous (Tables 1A and 1B)(Laskar et al., 2004; Laskar et al., development. Thus, the plot of carbonate content can be approxi- 2011). This comparison revealed a high correlation coefficient of mated to a curve of relative sea-level change. The sea-level fluc- more than 0.9. Thus, we interpret the 0.37 m, 0.40 and 0.47 m tuations are also consistent with the change in d13C values (Fig. 5). wavelengths as precessional cycles (18.5, 21.5 and 22.7 kyr Minimum values occur during the lowstands owing to the respectively), the 0.73 m, 1.71 m and 7.30 m wavelengths as increased flux of lighter 12C from the oxygenated organic matter obliquity (37.9 kyr), short eccentricity (95 kyr) and long eccentricity and increased release of gas hydrates (Jarvis et al., 2001; Jarvis et al., (405 kyr) cycles. The average sedimentation accumulation rate 2002). On the other hand, positive excursions occur when the sea (without correction for compaction) of the succession, calculated as level rises. The relative sea-level curve shows significant falls at the ~1.8 cm/kyr, agrees well with the rate calculated for the whole sequence boundaries of third order sequences. In addition, the Turonian succession, ~1.4 cm/kyr. microfacies and lithological assemblage change significantly across This result indicates that the relative sea level changed period- the sequence boundaries. Although the magnitude of sea-level fall ically following orbital frequency bands. We suggest that the fourth cannot be estimated precisely, the major changes in the microfacies order sequences (sedimentary cycles) were controlled by 405 kyr and lithological assemblage indicate that the water depth changed eccentricity. The succession was calibrated to the geologic time dramatically across the sequence boundaries. We speculate that the magnitude of these sea-level falls was in an order of tens of meters Table 1B to account for the observed lithological changes, because less than Orbital frequency ratios calculated assuming Cretaceous orbital periods (Laskar 10 m sea-level change could not change the sedimentation process et al., 2004; Laskar et al., 2011). and resulting lithological assemblage in such deep environments. POeE

18.5 21.4 22.6 28.2 37.9 95 405 5. Discussion 18.5 1.000 21.4 0.864 1.000 5.1. Cyclostratigraphy and chronology 22.6 0.819 0.947 1.000 28.2 0.656 0.759 0.801 1.000 The spectra of CaCO3 and Th show numerous peaks with high F- 37.9 0.488 0.565 0.596 0.744 1.000 test values often exceeding 0.95 (Fig. 6). The 37 cm, 73 cm, 170 cm 95 0.195 0.225 0.238 0.297 0.399 1.000 405 0.046 0.053 0.056 0.070 0.094 0.235 1.000 and 730 cm wavelengths are prominent in both spectra. The 0.37 m X. Chen et al. / Cretaceous Research 56 (2015) 691e701 699

Fig. 7. Correlation of the inferred relative sea-level changes during the late Turonianeearly Coniacian in the Tethyan Himalaya with the global sea level curves (from Sahagian et al., 1996 modified by Van Sickel et al., 2004; Haq, 2014) and the composite sequence of New Jersey coastal plain (Miller et al., 2004). scale by establishing a floating astronomical time scale (ATS), also shows a sea-level fall at 91.8 Ma (KTu 4). The sea-level fall at because each T/R cycle spans 405 kyr. This ATS provides solid basis ~90e89.8 Ma is consistent with the late Turonian significant for the correlation of sea-level changes estimated in different lowering (KTu 5) in the curves of Haq (2014) and Sahagian et al. continents. Van Sickel et al. (2004) derived a Late Cretaceous to (1996). In the composite sequence of the New Jersey costal plain, Cenozoic eustasy curve in the New Jersey coastal plain and corre- a hiatus (sequence boundary) between Magothy Ⅰ and MagothyⅡ is lated it to the curve of Sahagian et al. (1996) and Haq (2014).In also dated at 90e89.8 Ma (Miller et al., 2004). Because the classic order to correlate the relative sea-level change in the Tethyan eustasy curves are of relative lower resolution, the fourth order sea- Himalaya to eustasy curves, we estimated the age of beginning and level fluctuations cannot be precisely correlated. However, the third end of each sedimentary cycles (405-kyr eccentricity cycles) by order sea-level curve from the Tethyan Himalaya shows the same anchoring the Turonian/Coniacian boundary to that in GTSs 2012 at trend as the eustasy curves, thus indicating that it represents a 89.8 Ma (Gradstein et al., 2012). The boundaries of third order se- regional responses to the global sea-level fluctuations. The study quences or the significant sea-level falls are then calculated at area was located along the Indian passive margin and was not 89.8 Ma and 91.4 Ma respectively. Furthermore, the duration of affected by regional tectonic events during the late Albian- these two sea-level falls in an order of 10 m are about 200 kyr (half Santonian (Hu et al., 2010, 2012; Garzanti and Hu, 2014), further the 405 kyr eccentricity cycle) and 600 kyr (one and a half the supporting that the water depth changes were resulted from 405 kyr eccentricity cycle) respectively. According to the falling eustasy fluctuations. We suggest that the sea-level falls at magnitude and the duration, the falling rate is then estimated as ~90e89.8 Ma and ~92e91.4 Ma were global. tens to a hundred meters per million years. 5.3. Controls of sea-level changes of late Turonianeearly Coniacian 5.2. Correlation of sea-level changes Fluctuations in global sea level result from changes in water The sea-level curve in the Tethyan Himalaya was correlated to volume controlled by the growth and decay of continental ice the classic eustasy curves (Fig. 7) established by Haq (2014), Van sheets (glacioeustasy), desiccation and inundation of marginal seas, Sickel et al. (2004) and Sahagian et al. (1996). The later two thermal expansion and contraction of seawater (thermo-eusta- eustasy curves were correlated and calibrated into a common time tism), and variations in groundwater and lake storage. Changes in scale by Van Sickel et al. (2004). The late Turonian-early Coniacian ocean basin volume are dominated by slow variations in sea-floor eustasy curves show covariant trends, although the estimates of the spreading rates or ocean ridge lengths, variations in sedimenta- magnitude of sea-level change vary considerably. The sea-level tion, emplacement of oceanic plateaus and thermal subsidence. changes in the Tethyan Himalaya are consistent with the eustatic Changes in sea-floor spreading rates can lead to sea-level fluctua- estimates, at least for the lowstands at ~90e89.8 Ma and tions at a rate of 10 m/Myr over more than 1 Myr (Miller et al., ~92e91.4 Ma. The most significant late Turonian regression at 2005b and references therein), therefore this cannot be the cause ~92e91.4 Ma is coeval with the boundary of the Bass River Ⅱ and of rapid sea-level changes in the late Turonianeearly Coniacian. The Magothy Ⅰ sequences in the New Jersey coastal plain (Miller et al., sedimentation accumulation rate is lower than 2 m/myr, implying 2004) and with the sea-level estimates from the Russian Platform that it led to only a slight water depth change within 1 million (Sahagian et al., 1996). The third order sea-level curve in Haq (2014) years. The thermal subsidence rate of the basin during the 700 X. Chen et al. / Cretaceous Research 56 (2015) 691e701

Cretaceous was a few meters to ~33 m per million years (Garzanti, boundaries correspond to some tens of meters sea-level fall, 2012; Li and Hu, 2013), which could cause a water depth increase of lasting ~200 kyr and ~600 kyr, respectively. several meters within an eccentricity cycle. The presence of (3) Correlation to global eustasy curves shows that the third extensive epicontinental seas during the mid-Cretaceous would order sea-level changes in the Tethyan Himalaya at have significantly reduced the potential for groundwater storage ~90e89.8 Ma and ~92e91.4 Ma are consistent with global (Gale et al., 2002). In the case of thermo-eustatism, a rise of ~1 C sea-level changes. The only known mechanism for these throughout the world oceans is required to raise sea level by 1 m large magnitude and rapid sea-level decreases is the growth (Gale et al., 2002). Fluctuations of 4e8 C and <2 C are recorded of polar ice. Thus, our study supports the formation of from Late Cretaceous subtropical oxygen isotopes (Stoll and Schrag, ephemeral polar ice sheets in the super greenhouse world. 2000) and from late Turonian deep-sea oxygen isotopes (Huber et al., 2002). Although a single factor cannot account for the late Acknowledgments Turonianeearly Coniacian sea-level change in the Tethyan Hima- laya, the combination of sedimentation, thermal subsidence rate The data set of terrigeneous components was kindly provided by and groundwater storage could explain a sea-level change of Professor Xianghui Li from Nanjing University. This work was several meters within an eccentricity cycle. However, the remark- jointly supported by the National Key Basic Research Development able falling of several tens to a hundred of meters per million years Program of China (2012CB822005), the National Science Founda- at ~90e89.8 Ma and ~92e91.4 Ma cannot be attributed to these tion of China (41002035) and open fund from Key Laboratory of factors. The only known mechanism to account for such a magni- Sedimentary Basin and Paleogeography & Lithofacies, MLR tude and rate of sea-level fall is the growth of continental ice sheets. (zdsys2014001). It contributes to IGCP 609. The growth and melt of ice could induce sea-level change of a magnitude of tens meters in less than 1 myr. The present volume of ice cap in Antarctic is equivalent to ~66 m sea level (Barrett, 1999). References A large number of sea-level change studies in the greenhouse world have suggested that lower insolation possibly leads to the Ando, A., Huber, B.T., MacLeod, K.G., Ohta, T., Khin, B.-K., 2009. 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