SEA-LEVEL CHANGES RECORDED BY CERIUM ANOMALIES IN THE LATE JURASSIC (TITHONIAN) BLACK ROCK SERIES OF QIANGTANG BASIN, NORTH-CENTRAL TIBET

Headnote

The Upper Jurassic black rock series of the Amdo area of north-central Tibet have attracted attention because of its organic-rich matter and oil seepage in the rock series. Abundant ammonites of Aulacosphinctes and Virgatosphinctes as well as bivalves including Buchia and Chlamys, are well preserved in the Qiangmuleiqu Formation of the Middle to Late Tithonian (Late Jurassic). The total rare earth elements (ΣREE) abundance of the formation varies from 18.814 to 46.818 ppm and is lower than that of an average in marine shales. Inter-element correlations suggest that the shale-normalized REE patterns provide the information about the origin of sedimentary rocks (i.e., not affected by diagenesis), and can be used as a potential indicator for eustatic sea-level changes. The Ceanom values gradually increased from -0.015 to -0.238 from the lower to middle part of the series, indicating a rise in sea level. The Ceanom then dropped to -0.081, suggesting a lowering of sea level. From the middle to upper part of the section, there are two more sea-level fluctuations indicated by variations of cerium anomaly values. Thus, sea-level fluctuations occurred three times in this Late Jurassic section, with major episodes of eustatic rise took place in the lower-middle part of this section. These episodic cycles show that the sediments in the Amdo area of north-central Qiangtang basin were deposited in continental shelf or deep-water continental slope environments, which is not in agreement with the previous views that there are no deep-water sediments in Qiangtang basin. Our results call for further studies to be undertaken on Jurassic stratigraphic framework and tectono-sedimentary evolution in Qiangtang basin, northern Tibet.

Introduction

Sea-level fluctuations have important implications for organic productivity of the oceans and sediment distribution patterns along the continental margins and in the interior basins [1]. Therefore, the study of these fluctuations is of prime interest to hydrocarbon exploration. Sea-level changes are also thought to control hydrographic-climatic patterns and, indirectly, biotic distribution patterns as well [1-5]. Understanding these changes is of considerable value in deciphering past oceanographic (palaeoceanographic) conditions [1]. Advances in seismic stratigraphy, sequence stratigraphy and the development of depositional models have provided the basis for the recognition of sea-level events in subsurface data and in outcrops of marine sediments around the world [6-13]. However, these traditional methods cannot provide quantitative and continuous curves of sea-level changes in the Phanerozoic. A possible independent method for determining relative sea level, for specific geologic situations, is suggested by the use of the cerium anomaly as part of a chemostratigraphic study [14-16].

In recent years, much attention has been paid to the behaviour of cerium (Ce), and particularly the Ce-anomaly in the marine environment, for understanding the palaeoceanographic conditions [17-23]. Ce anomaly has also been used as a potential indicator of eustatic sea-level changes [15, 24] (except for the redox conditions [15, 19, 25-27]), tectonic reconstruction [28] and diagenetic process [18]. Ce has two possible oxidation states: Ce3+ and Ce4+. In oxic conditions, Ce is less readily dissolved in seawater, which shows negative Ce anomaly (<-0.1) of log[3Cen/(2Lan+Ndn)] [29]. Oxic sediments on the other hand are more enhanced with respect to Ce and show less negative to positive Ce anomaly (>-0.1) [14]. Conversely, anoxic water would have positive Ce anomalies and anoxic sediments would show negative anomalies. Therefore, changes in the value of the anomaly could be related to redox conditions predicted by the ventilation model of Wilde [30], with more negative values found during warmer climates and transgressive conditions and more positive values found during cooler to glacial climates and regressive conditions. Thus, whole-rock cerium anomaly is a geochemical parameter that characterizes chemical palaeoceanographic conditions related to relative sea-level changes independent of sedimentological or seismic considerations [15]. Furthermore, the use of a chemical parameter offers the possibility of the extension of its interpretation to quantitative palaeoenvironmental studies beyond that of sea-level curves.

Although the Qiangtang Basin of north Tibet has arguably the most complete and extensive marine sedimentary strata of the Jurassic period [31], its ammonites zones or subzones of Early to Late Jurassic have not been well established. Previous studies have shown that only a few ammonites have been found in the Shuanghu-Sewa-Amdo area and the geographic distribution of ammonite fauna can be potentially associated with sea-level rise [32-33]. In this study we present Ce anomaly determinations from late Jurassic ammonites found in black shales of the Amdo Highway 114 station, southern Qiangtang basin. Combined with the published palaeontological data [32-35], we discuss the Late Jurassic sea-level changes and geological significance of the black rock series in the Amdo area, northern Tibet.

Geologic setting

The black rock series for this study come from exposures in Gangni village (Township) and Amdo 114 station (of the Qinghai-Tibet highway), northcentral Tibet (Fig. 1). Rocks mainly consist of black shales, dark grey calcareous mudstones and marls with abundant ammonites of Jurassic, which are significantly different from the Yanshiping Group biological assemblage and/or sedimentary characteristics. The Gangni Township section yields ammonites of Sonninidae, mainly including Sonninia, Dorsetensia, Witchellia, etc., and belonging to the Early to Middle Bajocian (Middle Jurassic) [34].

The section near the Amdo 114 station has been noticed by many geologists because of oil seepage. This section is located at long. 91°47' W, lat. 32°26' N (Fig. 1). Jiang [36] named it Qiangmuleiqu Formation (J3q). The strata are of west-east strike and dipping 60° to 70°southward. The whole succession is divided into three intervals characterized by different lithological types. The lower part of this section is about 134 m thick and outcrops in the core of an anticline. Rocks in the lower part consist in general of mauve sandstones, siltstones and greyly green siltstones interbedded with a few intercalated mud boulder horizons. Continuous black sedimentation of limestones, bioclastic and silty limestones occurred in the middle interval. The upper part of the section is divided into two members. The lower member, about 75 m thick, developed greyish green siltstones and calcareous fine sandstones with intercalated argillaceous siltstones and silty mudstones, whereas, the upper member, about 296 m thick, is of abundant ammonite fauna and mainly deposited as grey marls and greyish black calcareous mudstones with cycles. Its horizon is indicated by oil seepage. The top of this section is covered by Quaternary sediments.

Because of its excellent exposure and easy access, the Amdo 114 station section is well known and probably the best studied locality of Late Jurassic strata in the northern Qiangtang basin, Tibet. Additionally, the section mainly consists of abundant fossils, such as ammonites, bivalves and brachiopods. Recent results show this section mainly consists of 7 genus and 7 species in ammonites, 3 genera and 1 species in bivalves, and 1 genus and 1 species in brachiopods. Most of the ammonites belong to Virgatosphinctinae subfamily, mainly including Aulacosphinctes, Virgatosphinctes, the Berriasellinae subfamily, with blanfordiceras, and Spiticeratinae subfamily with Spiticeras (Fig. 2). Above observed ammonites are more classic fossils of Middle-Late Tithonian (Late Jurassic). It is well known that the Aulacosphinctes occurred wildly in Himalaya of China, southeast France, Denmark, Madagascar, Kachchh district in Gujarat state, India, and Nepal [33, 37-38] and can be correlated with the Spiti ammonites in the western Himalayan region, while the Virgatosphinctes yielded the whole world [39]. In addition, bivalves were found throughout the section including Buchia and Chlamys. It is difficult to compare these ammonoid assemblages with the European standards because of the absence of respective data. However, it is realistic to make a regional biochronologic correlation with other ammonoid successions of Spiti Shales in western Himalayan region, or Bailongbinghe Formation in the north Qiangtang basin, or Hongqilapu Formation in Karakorum area, or Menkadun Formation in Everest area, south Tibet [32, 40].

Material and methods

We carried out detailed investigations of the Upper Jurassic black shales of the Amdo 114 station section, Qiangtang basin of northern Tibet. This 650 m thick section was studied bed by bed with the vertical sample spacing of 29 m on the average (22 samples in total) (Fig. 2). On each bed, samples are palaeoecological and/or palaeoceanographic interpretations. The analysis of rare earth elements (REE) has been carried out by a Perkin-Elmer Sciex taken less attention. We also collected samples for isotopic dating (Re-Os and Pb-Pb), in order to obtain a comprehensive database for strata ages, ELAN 6000 inductively coupled plasma mass spectrometer (ICP-MS) at the dark to black color, higher argillaceous sediments, so the lower part was State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, using fully quantitative solution analysis methods described earlier [41-43]. The international reference standards OU-6 and AHM-1 were used to check the analytical accuracy. Analytical uncertainties are better than 5% for all elements based on the reproducibility of standards.

Results and discussion

Bulk concentration of REE abundance of sediments from the Amdo 114 station section is presented in Table 1, and the correlation coefficient values (γ) among REE are listed in Table 2 by Pearson's mathematical formula. The Post Archean Australian Shale (PAAS)-normalized REE pattern of the sediments is presented in Fig. 3 to understand their behaviors. The Ce anomaly values vary from -0.238 to -0.015 and exhibit the eustatic sea-level changes (Fig. 2).

REE concentrations

Total concentrations of REE's of the Upper Jurassic black rock series stay between 18.814 and 46.818 ppm (Table 1), except for two samples (H06b1 and H07b1) having REE's concentration <10 ppm and one sample (H05b1) having REE's concentration >100 ppm. Concentrations are lower than those of the average shales from Turekian and Wedepohl [44]. We calculated the ΣLREE/ΣHREE (Σlight rare earth elements / Σheavy rare earth elements) as (La+Ce+...+Eu)/(Gd+Tb+...+Lu) and their ratios vary from 6.6 to 9.4 (except for sample H05b1 having ratio of 17.214) and show a positive correlation with La/Yb (γ = 0.7). In addition, the other REEs show good positive correlations. Cerium and the trivalent REEs represented by La indicate similar correlation coefficient value (γ = 0.974) with Sm whereas Eu is correlated with La (0.986) and Ce (0.989). On the other hand Ce is not better correlated with Lu (0.569) than La (0.644) (Table 2). The stronger positive correlations show that the REE concentrations may represent original sedimentary information not reflected by diagenetic constraints: thus the REE concentrations can be used for a potential indicator as eustatic sealevel changes [16, 19]. The shale-normalized [45] REE patterns are characterized by the flat shale type with unstable Ce anomalies and very weak positive Eu anomalies. Furthermore, the lower part of this section shows distinctly different REE pattern from the middle and upper part. Sample H05b1 is an andesite and has a significant left-toward trend, while the samples H06b1 and H07b1 are limestones and have a lightly flat type (Fig. 3c) with a positive cerium anomaly. The middle and upper part is identified by flat shale type rock with significantly negative cerium anomaly (Fig. 3a, 3b). Major changes in REE composition and abundance can be seen in samples H05b1, H06b1 and H07b1, which show that basal samples have low concentration of REEs, whereas the remaining layers are abundant in REEs compared to the underlying strata. The relatively flat pattern of REE concentrations indicates a detrital origin and good mixing during transportation [46].

The Ce concentration in the bulk shales and phosphorite is generally determined by detrital constituents like clays and organic matter, hence, in most shales or phosphorites, the La/Sm ratio increases and the Ce anomaly diminishes with the increase of detrital fraction [46]. Ce anomalies with flat REE distribution and (La/Sm)n ratios >0.35 could be used as indication of oceanic anoxia [20, 23, 47]. All samples with (La/Sm)n ratios >0.858-1.339 do not have correlation with Ce anomaly, which indicates that diagenesis has no effect on the values of true Ce anomaly. Bau and Dulski [48] discussed the La and Ce anomalies by looking additionally at praseodymium Pr/Pr* values using the equation Pr/Pr* = 2Prn/(Cen+Ndn). The existence of a true Ce anomalies should lead to Pr/Pr*≥1. If, however, samples show Pr/Pr* = 1, this would imply that anomalous La enrichment must be the sole cause of any Ceanom. As shown in Table 1, the average Pr/Pr*ratio is 1.084 in the Amdo 114 station, which indicates that these Ce anomaly differences are subtle and may have been artificially enhanced by different La concentrations [48]. The Ceanom values vary from -0.238 to -0.015, whereas Eu/Eu* values (using the equation Eu/Eu* = Eun/(Smn×Gdn)0.5 [49]), vary from 0.972 to 1.133.

Cerium anomaly as an indicator of sea-level change

Geochemistry of cerium

The use of the cerium anomaly was first proposed by Elderfield and Greaves [29] as a consequence of the change in the ionic state of Ce as a function of oxidation state. The removal of dissolved Ce (+3) as insoluble form of Ce (+4) preferentially occurs in the upper part of the water column, so oxic seawater shows a negative Ce anomaly, whereas oxic sediments have a less negative to a positive value [14]. Conversely, in anoxic sediments, Ce is released and the sediments show a negative anomaly. Most of the black shales, Cambrian chert-phosphorite assemblages, fossil apatite discussed during past years have a positive cerium anomaly associated with anoxic sediments during warmer climate and transgressive conditions [14-15, 46, 50]. In other words, the observed Ce anomaly is not limited to marine carbonate, conodonts and ichthyoliths, while the whole-rock Ce anomaly can be used as an indicator of intensity of anoxia and therefore of eustatic sealevel. A positive-trending whole-rock cerium anomaly indicates more oxic conditions or a sea-level fall. In contrast, negative-trending anomaly indicates more reducing conditions or a sea-level rise [15]. Ce anomaly methods have been applied for the Middle Ordovician to Early Silurian quantitative sea-level curve at Dob's Linn, Scotland [15] and in the study of Chinese seawater from the Sinian to the early Cambrian in the Yangtze Region [51].

Sea-level change

Table 1 and Fig. 2 show the cerium anomaly changes obtained in this study. The highest Ceanom value (-0.015 of H05b1) is in the lower part and gradually decreases in the middle part. The Ceanom values fell down to the lowest (-0.238 of H11B), then rose rapidly to about -0.1. The relatively stable values occurred in the upper part, but the values changed from -0.075 to -0.173 in the argillaceous limestones in the top part. Three cycles of the relative sea-level fall and rise were identified based on Ce anomalies in the Amdo 114 station (Fig. 2).

Cycle I occurred in the lower and middle parts of this section (from samples H05b1 to H142) showing positive to slightly negative Ce anomaly for the whole-rock. During a transgression the bottom waters became more anoxic. The sample H05b1 with the highest Ceanom value indicates that sealevel surface fell to the lowest, then from samples H05b1 to H08b, the relative sea-level began to rise gradually as can be seen from dropping value of Ceanom. At 370 m, the Ceanom value (-0.238) decreased to the lowest, indicating the sea-level changes are maximum flooding surface (Mfs) until at 420 m (Fig. 2). Additionally, during the transgressive process, the abundant ammonites including Blanfordiceras sp. and Aulacosphinctes sp. were found. Yin and Enay [32] discussed the reasons why such Tithonian ammonites of Gondwana facies suddenly occurred in northern Tibet (for example, the palaeoclimate, tectonic framework and palaeogeography changes). As mentioned above, the occurrence of Blanfordiceras sp. and Aulacosphinctes sp. may indicate the extension of the Himalayan marine basin and sea-level rise, so this fauna is interpreted as a marker of the deeper water outer shelf or continental slope [32, 40]. Subsequently, the sea level began to fall slowly into the Cycle II.

After the Cycle I, the relative sea-level began to rise with slightly negative Ceanom values between -0.081 and -0.169, and fall at the beginning of about 465 m. As shown in Fig. 2, the Cycle II mainly occurred in the greyly black, dark grey marls, limestones, mudstones, or calcareous mudstones with abundant ammonites. In particular, the grey marls of layer No. 15 are characterized by ammonites as follows: Aulacosphinctes pachygyrus (Uhlig), Aulacosphinctes hundesianus (Uhlig), Blanfordiceras boehmi (Uhlig), Blanfordiceras sp., Aulacosphinctes Hollandi, Alligaticeras sp., and Ptexolytoceras sp. (Fig. 2). In addition, the Virgatosphinctes sp. occurs for the first time in No. 16 bed. The above-mentioned ammonites of Aulacosphinctes and Virgatosphinctes generally lived in the outer shelf and upper slope [32, 37, 52].

Sea-level stillstand followed by the Cycle II and the Ceanom values very close to -0.10, probably indicate a weakly oxic environment. At the beginning of the Cycle III, there are no distinct cerium anomalies. Ce anomalies then are lower than -0.10 and show a gradual increasing trend at about 600 m, indicating enhanced oxidizing conditions in seawater and sealevel fall. In the upper part of this section, light grey marls, yellow grey marls and calcareous mudstones formed with the ammonite fossils of Spiticeras tobleri, which is regarded as the latest Tithonian. In the Himalayan region, the Late Jurassic is marked by the deep marine Spiti Shales [37, 39], which showed the transition of continental shelf to slope.

The Tithonian part of the eustatic sea-level curve of Haq et al. [1] shows several unusually rapid oscillations. Hallam [53] argued that this was rather a consequence of tensional tectonic activity, producing rotated fault blocks. Both Haq et al. [1] and Hallam [53] agree that the Tithonian marked a sealevel peak and was followed by a fall into the earliest Cretaceous. According to the relationship between the Ce anomaly and the eustatic sea-level curve from the Amdo 114 station in northern Tibet (Fig. 2), we identified the Late Jurassic sea-level changes. Changes in the Ce anomaly that are positive would indicate a lowering of sea-level as the apparent depth. Negative relative changes would indicate a rise in sea level. Thus, we can conclude that the deepest sea-water occurred in the middle part of the section, which is in consistent with the appearance of abundant ammonites. According to the results of analysis we assume that formation of the Upper Jurassic black rock series may be controlled by sea-level fluctuations.

Geological implications

Age of studied section

Upper Jurassic marine strata of northern Tibet were first reported by Fan et al. [54] who found ammonites, bivalves, and corals in Gaze area of northwestern Tibet. Upper Jurassic strata with abundant ammonites only outcropped at the Amdo 114 station in Amdo county of north-central Tibet. However, due to a small sample size, poor or even erroneous stratigraphic control, as well as inadequate identification of the previous collections, the previous studies failed to establish well-defined levels of ammonoid genera or assemblages. Although the Tithonian sediments are mostly fossiliferous in northern Tibet, knowledge on their faunal range and distribution and Jurassic biochronology remains questionable [32]. Jiang [36] named the rocks in Amdo 114 station as the Qiangmuleiqu Formation, which underlies of the Xushan Formation and overlies the Wenquan Formation of Middle Jurassic. Yong et al. [55] suggested that rocks at the station may be correlated with the Suowa Formation (also Upper Jurassic). The Qiangmuleiqu Formation is distinctly different from Yanshiping Group in the Qiantang basin in terms of lithology and fossils assemblages. In general, the Yanshiping Group in the northern Qiangtang is characterized by three sandstones horizons with two intercalated limestones horizons representing the continental or marginalmarine facies. In the light of abundant ammonites and bivalves, we confirm the strata at Amdo 114 station belong to middle to Late Tithonian (Late Jurassic).

Depositional environment

It is well-known that most areas of Qiangtang basin are shallow water environments from Late Triassic to Jurassic with carbonates and siliciclastic rocks showing episodic cycles [34, 56-57]. However, based on ammonites and cerium anomaly, present study shows that there may be stratigraphic sequence in the continental shelf with deep-water continental slope environments, or vice versa, which suggest the occurrence of deep water deposits in southern Qiangtang basin. As shown in Fig. 2, the eustatic sea-level began to rise in the lower part of the section until at about 390 m before reaching its maximum, then gradually fell near the reference sea-level with Ce anomaly of 0.1. As the observed three cycles could provide new insights for the sealevel changes, redox conditions, climatic and palaeogeographic events, further studies should be undertaken on Jurassic stratigraphic framework and tectono-sedimentary evolution in the Qiangtang basin of northern Tibet.

Conclusions

The Upper Jurassic black rock series in the southern Qiangtang basin, northern Tibet contain ammonites of Aulacosphinctes and Virgatosphinctes plus bivalves including Buchia and Chlamys, that are of Middle-Late Tithonian (Late Jurassic). The concentration of total REEs mainly range between 18.814 and 46.818 ppm and are lower than those of average shales. The shale-normalized [45] REE patterns are characterized by the flat-shale type with unstable Ce anomaly values of -0.238 to -0.015. Based on the Ceanom values, three cycles of eustatic sea-level changes were identified from the series. Changes in the positive anomalies would indicate a lowering of sea level as the apparent depth on the redox curve would reflect oxic conditions. Conversely, relative changes negative with time would indicate a rise in sea-level, as the apparent depth reflects anoxic conditions. Depending on the cerium anomalies and ammonites, we would suggest the Upper Jurassic black rock series in the southern Qiangtang basin exhibit alternative facies of the continental shelf with a deep-water continental slope, which is not consistent with the previous results.

Acknowledgements

Thanks are extended to the Qinghai-Tibet research teams at Chengdu University of Technology for their kind help during our field investigation. We would like to thank Xixi Zhao, University of California (Santa Cruz) and two anonymous reviewers for their constructive reviews that improved the quality of the manuscript. This work was supported by National Natural Science Foundation of China (Grant No. 41102066, 40972084), Natural Science Foundation Project of CQ CSTC (Grant No. 2009BB7383), and Opening Foundation of the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences.

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Presented by V. Kalm

Received March 16, 2011