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Sea-level changes recorded by cerium anomalies in the Late Jurassic (Tithonian) black rock series of Qiangtang basin, north-central 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: [Ce.sup.3+] and [Ce.sup.4+]. In oxic conditions, Ce is less readily dissolved in seawater, which shows negative Ce anomaly (<-0.1) of log[3[Ce.sub.n]/(2[La.sub.n] + [Nd.sub.n])] [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), north-central 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[degrees]47'W, lat. 32[degrees]26'N (Fig. 1). Jiang [36] named it Qiangmuleiqu Formation (J3q). The strata are of west-east strike and dipping 60[degrees] to 70[degrees] 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.

[FIGURE 1 OMITTED]

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.

[FIGURE 2 OMITTED]

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 ([gamma]) 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 [SIGMA]LREE / [SIGMA]HREE ([SIGMA]light rare earth elements / [SIGMA]heavy 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 ([gamma] = 0.7). In addition, the other REEs show good positive correlations. Cerium and the trivalent REEs represented by La indicate similar correlation coefficient value ([gamma] = 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 sea-level 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.sup.*] values using the equation Pr/[Pr.sup.*] = 2[Pr.sub.n]/([Ce.sub.n] + [Nd.sub.n]). The existence of a true Ce anomalies should lead to Pr/[Pr.sup.*] [greater than or equal to] 1. If, however, samples show Pr/[Pr.sup.*] = 1, this would imply that anomalous La enrichment must be the sole cause of any [Ce.sub.anom]. As shown in Table 1, the average Pr/[Pr.sup.*] 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 [Ce.sub.anom] values vary from -0.238 to -0.015, whereas Eu/[Eu.sup.*] values (using the equation Eu/[Eu.sup.*] = [Eu.sub.n] / [([Sm.sub.n] x [Gd.sub.n]).sup.0.5] [49]), vary from 0.972 to 1.133.

[FIGURE 3 OMITTED]

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 sea-level. 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 Dobs 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 [Ce.sub.anom] value (-0.015 of H05b1) is in the lower part and gradually decreases in the middle part. The [Ce.sub.anom] 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 [H14.sup.2]) 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 [Ce.sub.anom] value indicates that sea-level 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 [Ce.sub.anom.] At 370 m, the [Ce.sub.anom] 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 [Ce.sub.anom] 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 [Ce.sub.anom] 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 sea-level 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 sea-level 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 north-western 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 marginal-marine 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 sea-level 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 [Ce.sub.anom] 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.

doi: 10.3176/oil.2012.1.03

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.

Presented by V. Kalm

Received March 16, 2011

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LAN CHEN (a,b,d) *, ANDREW TIEN-SHUN LIN (b), XUEJUAN DA (a), HAISHENG YI (c), LOUIS LOUNG-YIE TSAI (d), GUIWEN XU (a)

(a) College of Petroleum Engineering

Chongqing University of Science and Technology

Chongqing 401331, China

(b) Institute of Geophysics

National Central University

Jhongli 32001, Taiwan, R.O.C

(c) Institute of Sedimentary Geology

Chengdu University of Technology

Chengdu 610059, China

(d) Institute of Applied Geology

National Central University

Jhongli 32001, Taiwan, R.O.C

* Corresponding author: e-mail cllc-10@163.com
Table 1. REE concentrations (ppm) with Ce and Eu anomaly values
of the Upper Jurassic black rock series in the Amdo 114 station,
northern Tibet

Sample Lithology La Ce Pr

[H24.sup.3] argillaceous limestone 3.586 4.717 0.663
[H24.sup.1] argillaceous limestone 7.056 11.977 1.418
H23b argillaceous limestone 7.511 12.465 1.530
[H21.sup.2] marl 7.877 12.350 1.584
[H20.sup.2] marl 9.273 14.352 1.735
[H20.sup.1] marl 9.278 14.602 1.835
[H19.sup.2] marl 9.861 14.783 1.872
[H18.sup.3] calcareous mudstone 11.256 17.932 2.301
[H18.sup.1] calcareous mudstone 9.831 14.777 1.842
H17b1 marl 8.466 12.152 1.577
H16b1 silty mudstone 9.514 12.233 1.618
[H15.sup.1] marl 11.460 18.197 2.205
[H14.sup.2] mudstone 9.109 14.819 1.825
[H13.sup.1] marl 9.171 12.469 1.653
[H12.sup.2] limestone 9.883 11.183 1.617
H11B marl 6.529 6.993 1.064
H10B calcareous sandstone 6.275 7.314 1.046
H09B siltstone 6.368 7.436 1.045
H08b calcareous siltstone 4.533 7.056 0.865
H07b1 cryptite 0.800 1.471 0.171
H06b1 biosparite 2.214 4.150 0.458
H05b1 andesite 23.926 45.318 4.957
Average 8.353 12.670 1.585
Average shale 92.000 59.000 5.600
 OU-6 True 33 74.42 7.8
 Measured 32.162 73.31 7.69
 AHM-1 True 15.87 33.03 4.21
 Measured 15.217 31.545 4.065

Sample Lithology Nd Sm Eu

[H24.sup.3] argillaceous limestone 2.608 0.508 0.114
[H24.sup.1] argillaceous limestone 5.363 1.066 0.235
H23b argillaceous limestone 5.612 1.014 0.228
[H21.sup.2] marl 5.766 1.047 0.239
[H20.sup.2] marl 6.521 1.318 0.284
[H20.sup.1] marl 6.955 1.429 0.285
[H19.sup.2] marl 6.912 1.324 0.289
[H18.sup.3] calcareous mudstone 8.199 1.633 0.330
[H18.sup.1] calcareous mudstone 6.922 1.272 0.301
H17b1 marl 5.772 1.148 0.253
H16b1 silty mudstone 6.172 1.140 0.259
[H15.sup.1] marl 8.021 1.515 0.339
[H14.sup.2] mudstone 6.624 1.374 0.313
[H13.sup.1] marl 6.091 1.150 0.241
[H12.sup.2] limestone 6.212 1.106 0.235
H11B marl 3.880 0.709 0.163
H10B calcareous sandstone 3.941 0.730 0.167
H09B siltstone 4.171 0.717 0.168
H08b calcareous siltstone 3.337 0.708 0.159
H07b1 cryptite 0.672 0.135 0.030
H06b1 biosparite 1.716 0.318 0.076
H05b1 andesite 17.431 2.958 0.707
Average 5.859 1.105 0.246
Average shale 24.000 6.400 1.000
 OU-6 True 29.01 5.92 1.36
 Measured 28.58 5.763 1.316
 AHM-1 True 17.69 3.68 1.16
 Measured 15.817 3.404 1.079

Sample Lithology Gd Tb Dy

[H24.sup.3] argillaceous limestone 0.542 0.080 0.473
[H24.sup.1] argillaceous limestone 1.049 0.155 0.870
H23b argillaceous limestone 0.966 0.142 0.832
[H21.sup.2] marl 1.044 0.163 0.947
[H20.sup.2] marl 1.221 0.190 1.152
[H20.sup.1] marl 1.322 0.207 1.144
[H19.sup.2] marl 1.199 0.202 1.062
[H18.sup.3] calcareous mudstone 1.444 0.240 1.329
[H18.sup.1] calcareous mudstone 1.272 0.222 1.182
H17b1 marl 1.088 0.168 0.997
H16b1 silty mudstone 1.144 0.173 0.995
[H15.sup.1] marl 1.414 0.224 1.350
[H14.sup.2] mudstone 1.329 0.209 1.148
[H13.sup.1] marl 1.189 0.182 0.958
[H12.sup.2] limestone 1.135 0.189 1.032
H11B marl 0.745 0.125 0.639
H10B calcareous sandstone 0.823 0.125 0.646
H09B siltstone 0.729 0.123 0.681
H08b calcareous siltstone 0.677 0.101 0.530
H07b1 cryptite 0.131 0.020 0.126
H06b1 biosparite 0.316 0.049 0.225
H05b1 andesite 2.191 0.297 1.330
Average 1.044 0.163 0.893
Average shale 6.400 1.000 4.600
 OU-6 True 5.27 0.85 4.99
 Measured 5.13 0.836 4.913
 AHM-1 True 3.34 0.51 2.84
 Measured 3.184 0.513 2.646

Sample Lithology Ho Er Tm

[H24.sup.3] argillaceous limestone 0.114 0.289 0.041
[H24.sup.1] argillaceous limestone 0.200 0.520 0.074
H23b argillaceous limestone 0.197 0.522 0.071
[H21.sup.2] marl 0.222 0.661 0.077
[H20.sup.2] marl 0.255 0.677 0.090
[H20.sup.1] marl 0.248 0.692 0.098
[H19.sup.2] marl 0.253 0.712 0.087
[H18.sup.3] calcareous mudstone 0.309 0.845 0.121
[H18.sup.1] calcareous mudstone 0.271 0.723 0.088
H17b1 marl 0.221 0.573 0.077
H16b1 silty mudstone 0.228 0.599 0.084
[H15.sup.1] marl 0.283 0.786 0.115
[H14.sup.2] mudstone 0.257 0.712 0.098
[H13.sup.1] marl 0.227 0.582 0.081
[H12.sup.2] limestone 0.226 0.593 0.085
H11B marl 0.175 0.459 0.059
H10B calcareous sandstone 0.164 0.436 0.055
H09B siltstone 0.175 0.451 0.059
H08b calcareous siltstone 0.138 0.331 0.043
H07b1 cryptite 0.025 0.080 0.007
H06b1 biosparite 0.047 0.134 0.023
H05b1 andesite 0.269 0.694 0.083
Average 0.205 0.549 0.073
Average shale 1.200 2.500 0.200
 OU-6 True 1.01 2.98 0.44
 Measured 1.074 3.072 0.448
 AHM-1 True 0.57 1.52 0.21
 Measured 0.574 1.479 0.209

Sample Lithology Yb Lu [SIGMA]REE

[H24.sup.3] argillaceous limestone 0.243 0.039 14.016
[H24.sup.1] argillaceous limestone 0.488 0.075 30.544
H23b argillaceous limestone 0.474 0.069 31.632
[H21.sup.2] marl 0.500 0.072 32.549
[H20.sup.2] marl 0.540 0.088 37.696
[H20.sup.1] marl 0.618 0.095 38.808
[H19.sup.2] marl 0.613 0.080 39.247
[H18.sup.3] calcareous mudstone 0.774 0.105 46.818
[H18.sup.1] calcareous mudstone 0.632 0.082 39.417
H17b1 marl 0.522 0.067 33.081
H16b1 silty mudstone 0.526 0.069 34.752
[H15.sup.1] marl 0.688 0.096 46.694
[H14.sup.2] mudstone 0.636 0.083 38.536
[H13.sup.1] marl 0.504 0.076 34.574
[H12.sup.2] limestone 0.495 0.063 34.055
H11B marl 0.370 0.050 21.959
H10B calcareous sandstone 0.363 0.050 22.134
H09B siltstone 0.339 0.046 22.506
H08b calcareous siltstone 0.294 0.042 18.814
H07b1 cryptite 0.063 0.009 3.741
H06b1 biosparite 0.140 0.018 9.883
H05b1 andesite 0.598 0.074 100.832
Average 0.474 0.066 33.286
Average shale 2.600 0.700
 OU-6 True 3 0.45
 Measured 3.196 0.473
 AHM-1 True 1.37 0.21
 Measured 1.393 0.2

Sample Lithology [SIGMA]LREE/ [La.sub.n]/
 [SIGMA]HREE [Sm.sub.n]

[H24.sup.3] argillaceous limestone 6.699 1.026
[H24.sup.1] argillaceous limestone 7.906 0.962
H23b argillaceous limestone 8.664 1.077
[H21.sup.2] marl 7.831 1.093
[H20.sup.2] marl 7.948 1.022
[H20.sup.1] marl 7.773 0.943
[H19.sup.2] marl 8.330 1.082
[H18.sup.3] calcareous mudstone 8.061 1.001
[H18.sup.1] calcareous mudstone 7.816 1.123
H17b1 marl 7.911 1.071
H16b1 silty mudstone 8.102 1.213
[H15.sup.1] marl 8.421 1.099
[H14.sup.2] mudstone 7.617 0.963
[H13.sup.1] marl 8.101 1.159
[H12.sup.2] limestone 7.919 1.298
H11B marl 7.374 1.339
H10B calcareous sandstone 7.315 1.250
H09B siltstone 7.648 1.291
H08b calcareous siltstone 7.727 0.931
H07b1 cryptite 7.105 0.858
H06b1 biosparite 9.393 1.011
H05b1 andesite 17.214 1.175
Average 8.313 1.090
Average shale
 OU-6 True
 Measured
 AHM-1 True
 Measured

Sample Lithology Pr/ [Ce.sub.anom]
 Pr *

[H24.sup.3] argillaceous limestone 1.103 -0.173
[H24.sup.1] argillaceous limestone 1.041 -0.068
H23b argillaceous limestone 1.075 -0.075
[H21.sup.2] marl 1.103 -0.097
[H20.sup.2] marl 1.055 -0.098
[H20.sup.1] marl 1.069 -0.099
[H19.sup.2] marl 1.088 -0.111
[H18.sup.3] calcareous mudstone 1.116 -0.090
[H18.sup.1] calcareous mudstone 1.070 -0.111
H17b1 marl 1.106 -0.127
H16b1 silty mudstone 1.091 -0.169
[H15.sup.1] marl 1.074 -0.086
[H14.sup.2] mudstone 1.083 -0.081
[H13.sup.1] marl 1.113 -0.147
[H12.sup.2] limestone 1.131 -0.221
H11B marl 1.191 -0.238
H10B calcareous sandstone 1.138 -0.208
H09B siltstone 1.093 -0.212
H08b calcareous siltstone 1.047 -0.101
H07b1 cryptite 1.013 -0.047
H06b1 biosparite 1.010 -0.027
H05b1 andesite 1.036 -0.015
Average -0.118
Average shale
 OU-6 True
 Measured
 AHM-1 True
 Measured

Sample Lithology Eu/
 Eu *

[H24.sup.3] argillaceous limestone 1.026
[H24.sup.1] argillaceous limestone 1.047
H23b argillaceous limestone 1.084
[H21.sup.2] marl 1.078
[H20.sup.2] marl 1.055
[H20.sup.1] marl 0.978
[H19.sup.2] marl 1.079
[H18.sup.3] calcareous mudstone 1.013
[H18.sup.1] calcareous mudstone 1.113
H17b1 marl 1.067
H16b1 silty mudstone 1.066
[H15.sup.1] marl 1.092
[H14.sup.2] mudstone 1.090
[H13.sup.1] marl 0.972
[H12.sup.2] limestone 0.989
H11B marl 1.054
H10B calcareous sandstone 1.013
H09B siltstone 1.093
H08b calcareous siltstone 1.080
H07b1 cryptite 1.063
H06b1 biosparite 1.133
H05b1 andesite 1.307
Average 1.068
Average shale
 OU-6 True
 Measured
 AHM-1 True
 Measured

Table 2. Pearson's correlation coefficients of REEs for all
samples in the Amdo 114 station, Qiangtang basin in northern
Tibet

 La Ce Pr Nd Sm Eu Gd

 La 1
 Ce 0.974 1
 Pr 0.991 0.994 1
 Nd 0.994 0.990 0.999 1
 Sm 0.981 0.974 0.988 0.990 1
 Eu 0.986 0.989 0.995 0.995 0.993 1
 Gd 0.955 0.913 0.945 0.954 0.977 0.959 1
 Tb 0.905 0.839 0.885 0.897 0.932 0.903 0.981
 Dy 0.810 0.730 0.785 0.801 0.854 0.812 0.933
 Ho 0.756 0.657 0.723 0.741 0.797 0.751 0.894
 Er 0.727 0.634 0.700 0.716 0.775 0.727 0.873
 Tm 0.668 0.570 0.639 0.656 0.727 0.666 0.836
 Yb 0.718 0.637 0.699 0.714 0.781 0.728 0.876
 Lu 0.644 0.569 0.629 0.646 0.725 0.661 0.829

 Tb Dy Ho Er Tm Yb Lu

 La
 Ce
 Pr
 Nd
 Sm
 Eu
 Gd
 Tb 1
 Dy 0.973 1
 Ho 0.951 0.984 1
 Er 0.938 0.979 0.993 1
 Tm 0.903 0.966 0.973 0.977 1
 Yb 0.937 0.976 0.984 0.987 0.983 1
 Lu 0.887 0.951 0.961 0.966 0.975 0.974 1
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