International Journal of Coal Geology 147–148 (2015) 58–70
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Organic-matter accumulation of the lacustrine Lunpola oil shale, central Tibetan Plateau: Controlled by the paleoclimate, provenance, and drainage system Pengfei Ma a,c, Licheng Wang b,⁎, Chengshan Wang a, Xinhe Wu d, Yushuai Wei a a
State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China c Department of Geoscience, University of Wisconsin, 1215 West Dayton Street, Madison, WI 53706, USA d Oil and Gas Survey, China Geological Survey, Beijing 100029, China b
a r t i c l e
i n f o
Article history: Received 18 April 2015 Received in revised form 21 June 2015 Accepted 22 June 2015 Available online 24 June 2015 Keywords: Lunpola Basin Oil shale Element geochemistry Depositional history Central Tibetan Plateau
a b s t r a c t The Cenozoic lacustrine Lunpola oil shale, located in the central Tibetan Plateau, has received little research interest in its depositional history although it is estimated to be a huge resource reserve. This study investigates the palaeoclimate, provenance, and regional drainage system of the ancient lakes using a combined analysis of element geochemistry and stratigraphy to elucidate the depositional history of the Lunpola oil shale. Most of the samples in the lower part of the section have chemical index of alteration (CIA) values between 65 and 75, reflecting a moderate weathered source, while samples in the upper part the section have CIA values between 60 and 65, which indicate relatively weak chemical weathering in the source area. This stratigraphic grouping suggests a paleoclimate transition from a relatively warm or humid period to a cold or dry period. The Zr/Sc and Th/Sc ratios of the samples reveal that sorting and recycling were minor during deposition of the oil shale. The geochemical results (Al2O3–(CaO⁎ + Na2O)–K2O (A–CN–K) diagrams, high contents of Th, plots of Co/Th–La/Sc and Cr/Th–Sc/Th) demonstrate that the sedimentary sources were felsic rocks. Plots of La–Th–Sc, Th–Sc–Zr/10, Th–Co–Zr/10, and Ti/Zr versus La/Sc illustrate that the source rocks were from continental arc and active continental margin. The Baingoin batholiths to the south of the Lunpola Basin consisting mainly of Cretaceous granodiorite and adamellite are likely responsible for supplying materials to the Lunpola oil shale. Similar special REE patterns further suggest the affiliation of the oil shale with the batholiths. Provenance analyses indicate a main river from the south; moreover, stratigraphic study confirms an upstream ancient Baingoin Lake between the source area and the ancient Lunpola Lake. In this drainage system, paleoclimate clearly restricted the development of rivers around the lakes. Although terrestrial organic matter (higher plants) dominated the drainage area, the main river originating from the south could only bring a certain amount of them to the lakes. Moreover, considering the filtration of the upstream ancient Baingoin Lake, minor terrestrial organic matter could be carried and deposited in the ancient Lunpola Lake. Therefore, lacustrine algae could proliferate as the main organic source and preserve in the stratified hypersaline lake to form the largest oil shale resource in Tibet. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Oil shale, one of the substantial unconventional fossil resources that can be produced and converted to liquid fuels (DOE of U.S., 2006), has received much attention. Oil shale deposits are widespread in many regions of China, with proven reserves of about 7199.37 × 108 t (Liu et al., 2009). Although oil shale reserves in the Tibetan Plateau alone are about 1200 × 108 t (Liu et al., 2009), little information about oil ⁎ Corresponding author at: MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, 26# Baiwanzhuang Street, Xicheng District, Beijing 100037, China. E-mail address:
[email protected] (L. Wang).
http://dx.doi.org/10.1016/j.coal.2015.06.011 0166-5162/© 2015 Elsevier B.V. All rights reserved.
shale in this area has been published compared with the Maoming, Fushun, and Huadian oil shales in south and northeast China (e.g., Brassell et al., 1986, 1988; Sun et al., 2013; Fig. 1A). Recent investigations show that oil shales in the Tibetan Plateau are mainly found in the Mesozoic marine sequences of the Qiangtang Basin (Fu et al., 2009, 2010) and in the Cenozoic lacustrine sequences in the Lunpola Basin (Fu et al., 2012; Wang et al., 2011a,b; Xu and Lee, 1984; Fig. 1). Of which, the Lunpola Basin contains the largest oil shale resource in the Tibetan Plateau, that is estimated to be more than 60 × 108 t (Fu et al., 2012). For the Lunpola oil shale, Wang et al. (2011b) presented detailed sedimentology and organic geochemistry data which indicate that the main organic matter contributions are algae and bacteria. Fu et al. (2012) confirmed that the oil shale has
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Fig. 1. (A) Map of China, dark shadow indicates the Tibetan plateau and surrounding area. (B) Simplified tectonic map of the Tibetan plateau (modified from Wang and Wei, 2013; Wang et al., 2014a). (C) Geologic map showing the distribution of strata in the study area and the location of the Lunpori oil shale section (modified from Geological Survey Academy of Tibet, 2002, 2003; Geological Survey Academy of Jilin, 2003, 2006). ISZ: Indus–Yarlung suture zone; BNSZ: Bangong–Nujiang suture zone; JSZ: Jingsha suture zone; AKSZ: Ayimaqin–Kunlun Suture zone; M: Maoming Basin; F: Fushun Basin; H: Huadian Basin; N: Nima Basin; DC: Dongco Basin; AC: Avengco Basin.
abundant carbonate minerals. These studies, however, shed little light on the depositional mechanisms of the oil shale. A combination of factors controls the deposition of organic matterrich rocks, including tectonics, climate, global anoxic events, watercolumn stratification, and provenance supply (e.g., Carroll and Bohacs, 1999, 2001; Tanavsuu-Milkeviciene and Sarg, 2012). In this study, we used an approach integrating element geochemistry and stratigraphy to constrain the regional palaeoclimate and drainage system as well as sedimentary provenance of the Lunpola oil shale and to further reconstruct its depositional history. 2. Geological setting The Lunpola Basin, located in the Bangong–Nujiang suture zone (BNSZ), central Tibet, covers an east–west trending area of 5000 km2 (Wang et al., 2011b; Fig. 1). Cenozoic lacustrine organic matter-rich shales (DeCelles et al., 2007; Kapp et al., 2007) are widespread here (Fig. 2) as well as in other basins like Nima, Dongco, and Avengco in the BNSZ (Wang et al., 2011a,b; Fig. 1B). The Lunpola Basin has been explored geophysically and through a series of boreholes (Rowley and Currie, 2006; Wang et al., 2008). The
basin can be subdivided into three structural domains including a northern thrust nappe, a central depression, and a southern thrustfaulted uplift (Ma et al., 2013). The sedimentary succession is mainly composed of the Niubao Formation and the overlying Dingqinghu Formation, which together have a thickness of 4000 m (Du et al., 2004; Fig. 3). The underlying basement comprises Jurassic–Cretaceous marine carbonate, siliciclastic, and pyroclastic rocks. Arid condition probably began to dominate the central Tibet from the late Cretaceous according to the sedimentology, isotope (DeCelles et al., 2007), and organic geochemistry (Sun et al., 2014b) studies. However, relatively wet episodes were also visible during this period (Deng et al., 2012; Ma et al., 1996; Sun et al., 2014a). In this paleoclimate condition, the ~2400 m thick Niubao Formation was deposited in fluvial–lacustrine environment (Fig. 3) with purple–gray conglomerate and sandstone in the lower part, and relatively dark sandstone, mudstone, and laminated shale in the middle-upper part (Du et al., 2004; Luo et al., 1996). The lacustrine Dingqinghu Formation without widespread fluvial–deltaic deposits (Fig. 3) has a total thickness of ~1000 m and consists mainly of dark mudstone, marl, laminated shale, and siltstone (Xia and Liu, 1997). Geologic mapping indicates that the Niubao Formation is Paleocene– Eocene and the Dingqinghu Formation is Oligocene in age (Xia and Liu,
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Fig. 2. (A) General scene of the measured section of the Dingqinghu Formation. (B) Marl and mudstone layers. (C) Oil shale layers. The length of both rock hammers are about 33 cm.
Fig. 3. Schematic stratigraphy (modified from Du et al., 2004; Luo et al., 1996) and depositional environment (modified from Du, 2014) of the Lunpola basin including stratigraphic column of the measured Lunpori section, vertical variations of some selected elements (in %), and related parameters. Sample depths are indicated on the columns. The dashed line represent the Oligocene–Miocene boundary (Section 5.1), duration is calculated with the mean sedimentation rate given by Sun et al. (2014a).
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1997). Recently, He et al. (2012) presented a SIMS (secondary ion mass spectrometry) U–Pb zircon age of 23.5 ± 0.2 Ma (2σ, MSWD = 1.1) from a bentonite layer intercalated within the middle Dingqinghu Formation. In the same section, Sun et al. (2014a) gave the age of 19.8 Ma to 25.5 Ma and sedimentation rate of 60 m/Ma to 61.2 m/Ma with paleomagnetic study. Deng et al. (2012) discovered a rhinocerotid humerus from the upper part of the Dingqinghu Formation at the Lunpori outcrop section and suggested that the age is late Early Miocene (18–16 Ma). The oil shale layers in the Dingqinghu Formation are exposed in the central part of the Lunpola Basin (Fig. 1B). In this study, a 290 m thick section was measured in the Lunpori area (Figs. 2, 3). Considering the new age controls discussed above, the section is about late Oligocene to early Miocene in age and has sedimentation duration of about 4.8 Ma (Fig. 3). Oil shale alternates with marl in the lower part of the section, and the upper part mainly consists of thick-bedded oil shale, mudstone, and silty claystone (Fig. 2). The Dingqinghu oil shale is gray–brown and finely laminated (Fig. 2) with high shale oil content (5.0% to 10.3%, Fu et al., 2012), type I organic matter (OM, Liu et al., 2009; Sun et al., 2014b; Wang et al., 2011b), and low maturity (averaging Tmax of 432 °C, Wang et al., 2011b). 3. Samples and analytical methods A total of 34 outcrop channel samples, including oil shale (n = 23), marl (n = 6), and mudstone (n = 5), were collected from the Lunpori section (Figs. 2, 3). To minimize the effects of surface weathering,
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surface material was removed before sampling and all samples are from a prospecting trench with a depth of 0.3 m. All samples were analyzed for major and trace element compositions at the Beijing Research Institute of Uranium Geology. The samples were ground to 74-μm using an agate mortar. To determine the oxides of major elements, samples were prepared according to Chinese National Standard GB/T14506.28-2010 (National Standard of P.R. China) (2011). About 0.7 g powders and 7 g latent solvent (Li2B4O7 + LiF + NH4NO3) were weighed and stirred well in a platinum crucible. Then 1 ml BrLi was added in the crucible at 1200 °C for 20 min. The liquid melt was used to make a matrix for analysis. About 1 g sample was added in a crucible (W1) and weighted (W2). The crucible was placed in a muffle furnace at 1000 °C for 2 h and then dried and weighted (W3). The LOI = (W2 − W3) / (W2 − W1). The major element analyses were performed using a Philips PW2404 X-ray fluorescence spectrometer (XRF) with the procedure described by Long et al. (2008). The Chinese national reference material GBW07107 was used for quality control. The method detection limit (MDL) for each element was calculated as three times the standard deviation of the average from the blank samples (n = 10) (Dai et al., 2015a; Li et al., 2014) (Table 1). The XRF analytical precision was estimated to be better than 5%. The sample preparation for trace elements was conducted using Chinese National Standard GB/T14506.30-2010 (National Standard of P.R. China) (2011). Powder samples of about 50 mg were weighed and then dissolved in HNO3 (0.5 ml)–HF (1 ml) in a Teflon bomb at 185 °C for 24 h, dried, and then treated with HNO3 (0.5 ml). All samples underwent acid digestion twice and were then treated with HNO3
Table 1 Major element concentrations (wt.%) of the Lunpola samples. Sample no.
Lithology
SiO2
Al2O3
TFe2O3
MgO
CaO
Na2O
K2O
MnO
TiO2
P2O5
FeO
LOI
CIA
D-1 D-2 D-3 D-4 D-5 D-6 D-7 D-8 D-9 D-10 D-11 D-12 D-13 D-14 D-15 D-16 D-17 D-18 D-19 D-20 D-21 D-22 D-23 D-24 D-25 D-26 D-27 D-28 D-29 D-30 D-31 D-32 D-33 D-34 UC PAAS
Oil shale Oil shale Oil shale Oil shale Marl Oil shale Oil shale Marl Oil shale Oil shale Marl Oil shale Marl Marl Oil shale Oil shale Oil shale Oil shale Mudstone Mudstone Marl Oil shale Mudstone Oil shale Oil shale Mudstone Mudstone Oil shale Oil shale Oil shale Oil shale Oil shale Oil shale Oil shale
45.8 49.97 40.61 48.47 9.02 40.56 41.42 9.63 51.08 39.86 6.02 44.65 14.46 9.73 46.93 49.7 41.13 47.85 44.2 42.2 9.7 48.16 42.36 36.46 45.23 34.88 43.97 45.42 41.42 30.15 46.48 44.8 41.75 38.4 66.6 62.8
15.6 17.19 14.64 17.5 3.62 14.9 14.96 3.3 13.9 13.78 2.37 15.15 5.41 3.52 15.38 16.51 12.93 16.25 12.84 12.95 3.03 16.17 14.28 12.32 16.51 12.39 16.75 16.35 14.44 10.59 16.14 16.6 14.51 14.09 15.4 18.90
5.1 4.28 4.63 4.11 5.99 3.99 6.13 5.71 5.82 4.82 5.49 5.38 6.54 6.23 4.05 5.01 7.97 5.16 3.7 4.23 4.43 4.31 6.55 6.02 4.03 5.25 6.62 5.46 4.72 5.65 5.36 4.88 4.59 5.74 5.04 7.22
3.04 2.82 4.9 2.3 15.4 4.53 2.03 15.02 2.08 2.89 16.59 2.57 13.48 15.33 2.66 2.1 2.01 2.43 3.27 4.63 14.73 2.55 4.33 5.79 2.61 5.96 2.62 3.16 5.37 7.75 3.69 1.97 3.39 4.86 2.48 2.20
4.68 1.92 10.62 3.31 24.68 6.43 5.16 24.91 0.993 3.55 26 6.39 21.99 24.32 2.26 2.97 4.69 3.44 13.36 9.69 26.49 6.44 9.08 12.64 5.89 14.25 8.1 5.08 7.18 11.66 5.9 2.1 5.35 10.6 3.59 1.30
1.23 1.11 0.813 1.24 0.37 0.966 0.543 0.27 0.969 1.09 0.271 1.43 0.535 0.316 1.16 0.986 0.416 0.801 0.816 1.08 0.494 1.51 1.52 1.5 2.12 1.34 2.11 1.65 1.47 1.27 1.62 1.97 1.39 1.54 3.27 1.20
3.52 3.47 3.09 3.61 0.717 3.54 3.33 0.706 3 3.01 0.496 3.23 1.14 0.725 3.4 3.63 2.79 3.44 2.99 2.9 0.756 4.69 4.11 2.89 3.57 2.76 3.83 3.98 3.44 2.47 3.43 3.7 3.6 2.94 2.8 3.70
0.062 0.033 0.098 0.068 0.065 0.063 0.044 0.147 0.011 0.043 0.087 0.074 0.074 0.087 0.039 0.053 0.051 0.048 0.087 0.088 0.129 0.068 0.086 0.082 0.089 0.079 0.079 0.068 0.099 0.083 0.094 0.062 0.07 0.104 0.10 0.11
0.587 0.606 0.532 0.679 0.134 0.534 0.55 0.147 0.547 0.482 0.092 0.582 0.183 0.136 0.567 0.6 0.492 0.557 0.557 0.522 0.14 0.561 0.501 0.48 0.614 0.416 0.63 0.563 0.559 0.389 0.621 0.609 0.548 0.538 0.64 1.00
0.216 0.068 0.079 0.084 0.037 0.079 0.925 0.066 0.081 0.081 0.047 0.984 0.031 0.031 0.093 0.541 0.172 0.067 0.193 0.075 0.066 0.076 0.174 0.083 0.483 0.054 0.058 0.07 0.054 0.06 0.073 0.059 0.099 0.092 0.15 0.16
1.7 1.55 1.68 1.63 4 2.13 3.58 3.33 3.7 3.18 3.73 2 4.3 4.13 2.23 2.45 2.55 1.53 1.49 2.45 2.1 1.45 1.63 2.48 2.5 1.4 1.25 2.6 2.48 4.58 2.15 2.53 4.08 1.63 – –
20.09 18.26 19.53 18.51 39.93 24.35 24.83 40 21.49 30.4 42.45 19.56 36.12 39.49 23.35 17.87 27.32 19.7 17.41 21.42 39.76 15.3 16.87 21.5 18.81 22.53 15.11 17.98 21.16 29.88 16.36 23.13 24.61 21.04 – 6.00
66.48 69.86 70.83 68.63 64.47 67.98 73.48 66.61 68.33 66.79 62.37 64.85 64.35 65.84 67.20 69.68 74.63 71.84 68.41 65.90 55.33 61.65 60.15 60.42 60.35 62.59 60.15 62.65 62.76 60.69 64.07 61.26 63.11 63.05 52.74 70.36
TFe2O3, total iron; LOI, loss content on ignition; CIA = [Al2O3 / (Al2O3 + CaO⁎ + Na2O + K2O)] × 100 in molecular proportions, CaO⁎ in CIA is CaO in silicate fraction only. Because we don't have CO2 data, the CaO of carbonate couldn't be corrected precisely. The method of McLennan et al. (1993) is used here, after correcting for P2O5 (apatite), if the mole fraction of CaO ≤ Na2O, the value is accepted. On the other hand, if CaO N Na2O, we use mole fraction of Na2O as CaO⁎. The method of detection limit (MDL): Na Mg, Al, Fe, Si, K and Ca (0.01%); P and Ti (0.006%); Mn (0.004%).
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(5 ml) at 130 °C for 3 h. Dissolved samples were diluted to 50 ml in a clean PET bottle prior to the analyses. 103Rh and 185Re were used as the inner standard references. For the method detection limits (MDL) for each trace element, see details in Table 2. The trace elements were measured using a Finnigan MAT high-resolution inductively coupled plasma mass spectrometer (ICP-MS). The details of using the ICP-MS were those described by Cullen et al. (2001). The accuracy of the ICP-MS analyses was better than 5%.
concentrated elements of Si and Ca in fine clastic samples (oil shale and mudstone) and marl (Fig. 3), respectively. This agrees with mineral study indicating that the Lunpori oil shale samples are predominantly composed of clay minerals and quartz, while marl samples are dominated by calcite and dolomite (Fu et al., 2012). Fig. 3 shows the vertical distribution of several representative elements in the Lunpori section. Curves of the lower section with marl intervals exhibit obvious sawtooth forms. Contents of terrestrially derived elements like Al and Ti are much higher in the fine clastic samples than in the marl samples, and the same trend is evident in the distribution of Si, Na, K, and P (Fig. 3). Conversely, Ca, Mg, and Mn are more concentrated in marl samples. The major element enrichment factors (EF) of different lithologies are shown in Fig. 4. Here the EF is defined as the concentrations of samples to those of the UC. Clearly, all Lunpori samples, especially the marl samples, are enriched in Ca and Mg, which corresponds to the abundant content of dolomite and calcite reported by Fu et al. (2012), and indicates the high production of carbonates (Cohen, 2003; Freytet and Verrecchia, 2002) in the Oligocene–Miocene lake (Fu et al., 2012). Fe and Mn contents of marl samples are about the same as UC, but Al and Ti are significantly depleted (Fig. 4A). For the Lunpola oil shale and mudstone samples, Na shows apparent depletion, while others are not far from the UC (Fig. 4C, E).
4. Results
4.2. Trace elements
4.1. Major elements
Twenty-nine trace elements with concentrations lower than 0.1 wt.% (1000 ppm) are listed in Table 3. These data are also used to calculate the EF (Fig. 4). Vertically, the data did not have distinct changing trends along the depth, with the exceptions of Ba and Be, which are more concentrated after D-20 (Table 3). However, the data show apparent diversity between lithologies. Generally, most trace elements have higher concentrations in oil shale and mudstone than in marl (Fig. 4B, D, F), except for Sr, which has a similar atomic radius to Ca and could
Table 2 Method detection limit (MDL, μg/g) of trace elements for ICP-MS analysis. Element
MDL
Element
MDL
Element
MDL
Element
MDL
Li Be Sc V Cr Co Ni Cu Zn Ga Rb
0.2 0.05 0.05 0.2 0.1 0.01 0.01 0.05 0.05 0.05 0.1
Sr Y Nb Mo Cd In Sb Cs Ba La Ce
0.02 0.002 0.005 0.005 0.01 0.002 0.01 0.01 0.2 0.002 0.002
Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb
0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002
Lu Ta W Re Tl Pb Bi Th U Zr Hf
0.002 0.01 0.02 0.002 0.002 0.05 0.002 0.01 0.002 0.01 0.002
Table 1 compares the major element concentrations of the 34 Lunpori samples with the upper continental crust (UC, data from Rudnick and Gao, 2003) and post-Archaean Australian shales (PAAS, data from Taylor and McLennan, 1985). SiO2 ranging from 30.15 to 51.08 wt.%, with median value of 44.09 wt.% and CaO ranging from 21.99 to 26.49 wt.%, with a median of 24.79 wt.% represent the most
Fig. 4. Enrichment factors of major (A, C, E) and selected trace elements (B, D, F).
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Table 3 Trace elements concentrations (ppm) and some relevant parameters. Sample no.
Li
Be
Sc
V
Cr
Co
Ni
Cu
Zn
Ga
Rb
Sr
Nb
Mo
Cd
In
Sb
D-1 D-2 D-3 D-4 D-5 D-6 D-7 D-8 D-9 D-10 D-11 D-12 D-13 D-14 D-15 D-16 D-17 D-18 D-19 D-20 D-21 D-22 D-23 D-24 D-25 D-26 D-27 D-28 D-29 D-30 D-31 D-32 D-33 D-34 UC PAAS
104 139 111 110 33.6 118 101 37.7 128 110 29.5 125 33.1 27.9 103 128 92.4 118 105 130 36.9 118 105 92.9 113 184 117 123 117 91.8 117 113 129 117 24 75
2.54 3.02 2.82 2.95 1.43 2.91 2.8 1.73 2.42 2.47 1.34 2.61 1.34 1.03 2.41 3.6 2.6 3.28 2.5 3.41 2.28 3.09 3.02 2.51 3.42 2.31 2.72 3.01 2.91 2.54 2.94 2.44 2.62 3.43 2.1 –
15.2 15.7 15.7 15.7 4.92 13.9 14.7 5.7 13 11.7 5.86 15.4 5.57 4.75 13.1 16.1 12.3 14.2 11.1 12.6 7.63 14.7 13.4 11.5 20.8 12 15.6 16 16.1 16 15.5 14.6 15.1 15.1 14 16
118 115 102 118 45 107 104 49.7 108 96.3 37.1 96.2 49 35.4 102 134 97 111 86.2 97.3 34 115 110 87.4 111 90.7 118 123 99.6 90.7 114 102 116 110 97 140
101 103 89.1 108 26.6 86.3 81.4 27.8 91.2 74 17.8 79.8 32.5 22.2 89.6 109 76.8 89.6 80 76 23.2 91.1 86.7 72.1 94.3 81.5 103 101 89.9 66.3 94.3 89.4 104 89.3 92 100
18.4 10.2 10.5 12.3 2.22 13.7 15.6 2.06 10.5 13.1 2.43 11.8 2.99 2.26 15.3 15.2 18.8 15 7.51 7.56 2.98 8.58 14.8 9.93 11.7 10.1 12.9 16.6 11.6 12.1 13.4 14.2 20 12.3 17.3 20
105 67.5 50.3 69.6 10.9 55.9 95.4 11.6 55.4 71.3 7.36 64.9 13.2 8.81 93.6 103 109 69.5 51.8 39.3 17.8 58.5 85.6 55.3 58.8 58.3 81.6 96.1 57.7 57.7 74.3 77.3 122 62.4 47 60
70 51.3 25 39.6 6.93 62.7 57 7.44 66.9 91.8 5.92 33.2 11.4 5.4 68.3 56.3 60.2 35.5 29.9 26.7 9.34 38.7 40.6 25.6 41.8 27.4 30.1 58 38.5 55.9 28 39.2 50.7 28 28 50
104 89.5 72.2 96.1 27.3 92.9 95.9 27.3 73.9 112 19.1 83.1 32 21.2 118 110 102 106 74.9 79.1 28.7 94.6 110 82.2 96 80.8 109 105 81.1 80.9 94.8 87.3 97.8 91.9 67 85
20 23.4 19.9 22.4 5.03 21.4 18.4 4.39 18.9 17.5 3.59 16.6 6.78 4.37 19.6 23 17 21.9 16.3 18.2 4.18 21.1 19.6 15.6 22.1 16 22.9 23.3 19.1 14.6 19.6 19.9 21.9 18.9 17.5 20
173 189 163 198 41.5 177 164 38.9 155 147 28.2 160 60.5 39.7 170 198 145 183 154 158 39.6 194 174 140 181 149 194 196 165 123 187 175 166 168 82 160
246 182 350 240 899 324 317 552 191 285 1093 338 655 798 148 225 233 186 496 367 1198 269 389 504 344 468 311 225 306 491 255 147 240 406 320 200
10.7 12.4 11.2 13.3 2.59 11.2 9.55 2.85 11.1 8.42 1.9 9.62 3.24 2.29 10.3 11.5 8.47 10.6 10.3 10.1 2.79 11.4 10.3 8.75 11.9 7.76 13.2 12.2 11.1 7.83 11.8 11.1 11.3 10.9 12 18
27.3 5.57 2.54 2.62 0.657 2.65 4.98 1.04 11.8 5.53 0.898 6.21 0.459 0.389 1.64 2.63 6.35 2.1 0.796 2.71 1.64 4.42 9.84 2.43 1.24 1.47 15.8 6.63 1.41 4.11 1.39 2.56 8.86 1.81 1.1 1
0.363 0.265 0.164 0.265 0.03 0.169 0.381 0.06 0.174 0.385 0.066 0.247 0.066 0.062 0.34 0.408 0.453 0.279 0.127 0.185 0.052 0.192 0.21 0.132 0.207 0.194 0.239 0.368 0.266 0.424 0.213 0.296 0.341 0.181 0.09 –
0.076 0.085 0.076 0.078 0.022 0.088 0.067 0.018 0.068 0.062 0.012 0.059 0.025 0.018 0.077 0.085 0.062 0.074 0.071 0.061 0.016 0.074 0.067 0.056 0.077 0.06 0.083 0.084 0.07 0.059 0.072 0.076 0.084 0.069 0.056 –
1.55 2.18 1.76 2.24 0.414 2.16 2.64 0.681 2.71 2.33 0.337 2.48 0.693 0.384 2.12 3.8 2.37 3.11 1.12 1.93 0.746 2.09 2.94 2.41 2.17 1.77 2.19 2.75 1.75 2.92 2.62 2.96 3.35 2.54 0.4 –
substitute it in minerals (Cohen, 2003). Compared with UC, oil shale and mudstone samples are significantly enriched with Li, Cd, Sb, Cs, Pb, Bi, and U and slightly enriched with Be, V, Mo, Rb, Zn, Ni, In, and Th; but marl samples only have slightly concentrated elements of Li, Sr, Cs, Sb, and Bi (Fig. 4B; Table 3).
presence of heavy minerals in sample D-25 (Bauluz et al., 2000; Ross et al., 1995).
5. Discussion 5.1. Chemical weathering of source area and paleoclimate implications
4.3. Rare earth elements The concentrations of rare earth elements (REE) and some parameters (e.g., Eu and Ce anomalies) are listed in Table 4. Generally, the contents of REE of marl samples, ranging from 47.11 to 72.76 ppm with a median of 52.73 ppm, are lower than that of PAAS (211.76 ppm, Taylor and McLennan, 1985) and those of the fine clastic samples (median content 176.42 ppm, range 135.25–292.41 ppm). The ratio of light rare earth element (LREE) to heavy rare earth element (HREE) (LREE/HREE) and LaN/YbN of the fine clastic samples range from 3.25 to 4.70 and 0.85 to 1.26, respectively, with only one exception of sample D-25. The abnormal sample, D-25, has a LREE/HREE ratio of 1.70 and LaN/YbN ratio of 0.47, similar to the ratios of marl samples (LREE/ HREE: 1.73–3.01; LaN/YbN: 0.39–0.71). All of the samples were normalized to PAAS to show the REE distribution patterns (Fig. 5). Yttrium was also plotted here considering its similar geochemical characteristics with Dy and Ho (Seredin and Dai, 2012). The lithologic variations are apparent. Most of the oil shale (apart from D-25) and mudstone samples are characterized by flat REE profiles and negative Eu and Ce anomalies (Fig. 5B, C; Table 4). But for marl samples and the D-25 oil shale sample, leftward tilted REE profiles (Fig. 5A, B) suggest some kind of HREE enrichment comparison with PAAS, this is probably associated with high pH in the paleohydrological environment in the case of marl, and with the
To quantitatively evaluate source area weathering, a chemical index of alteration (CIA) is widely used and defined as [Al 2O 3 / (Al 2O 3 + CaO⁎ + Na 2 O + K 2 O)] × 100 after Nesbitt and Young (1982). However, for lacustrine deposits, the CIA could probably be influenced by diagenesis resulting in clay mineral conversion during burial (Xiao et al., 2010). In the Lunpola Basin, the significant conversion of clay minerals only occurred in a certain depth mainly below the lower Dingqinghu Formation (Chen et al., 1998). Therefore, for the Lunpori section that corresponds to the middle Dingqinghu Formation (Fig. 3), the CIA values are credible and could reflect the integrated chemical weathering in this drainage area (Xiao et al., 2010). The CaO⁎ in CIA represents Ca in silicates only, here we use the method of McLennan et al. (1993) to calibrate it. Overall, CIA values of marl samples are relatively low compared with those of adjacent oil shale and mudstone samples. Most of the Lunpori CIA values are higher than that of UC, but lower than that of PAAS and can be divided into two groups: samples D-1 to D-20 with values mostly between 65 and 75 reflecting a moderate weathered source; and samples D-21 to D-34, whose source rocks experienced relatively weak chemical weathering indicated by CIA values between 60 and 65 (Fedo et al., 1995; Nesbitt and Young, 1989; Fig. 3; Table 1). This stratigraphic grouping suggests a paleoclimate transition from a relatively warm or humid period to a cold or dry period (Nesbitt and
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Table 3 Trace elements concentrations (ppm) and some relevant parameters. Cs
Ba
Ta
W
Re
Tl
Pb
Bi
Th
U
Zr
Hf
Th/Sc
Zr/Sc
La/Sc
Co/Th
Cr/Th
Ti/Zr
24.3 27.8 24.9 28.1 6.51 28.5 24.6 5.47 22.7 21 4.27 22.9 9.22 6.16 26.9 30.2 21 26.8 21.4 24.1 5.52 25.5 22.7 20 27.6 20.6 28.1 28 25.6 19.4 28.3 27 26.6 28.6 4.9 6
458 345 424 422 231 449 409 330 284 322 405 391 192 249 283 367 386 351 512 425 686 522 510 477 476 522 626 470 443 455 462 415 441 497 628 650
0.918 0.994 0.874 1.03 0.2 0.833 0.709 0.223 0.889 0.613 0.163 0.777 0.298 0.207 0.772 0.89 0.518 0.795 0.877 0.66 0.238 0.897 0.845 0.735 0.865 0.651 1.03 0.936 0.83 0.646 0.954 0.898 0.715 0.885 0.9 1.28
2.71 2.63 2.44 2.72 1.19 2.67 1.97 3.04 2.13 1.76 1.36 1.77 1.08 1.31 2.13 2.6 2.22 2.58 2.6 2.71 1.72 2.52 2.22 2.09 2.63 1.6 2.66 2.25 2.31 1.87 2.64 2.3 2.26 2.68 1.9 2.7
0.008 0.008 0.006 0.006 0.003 0.017 0.006 0.003 0.006 0.014 0.003 0.011 0.003 0.003 0.008 0.007 0.005 0.006 0.006 0.006 0.006 0.017 0.014 0.005 0.014 0.005 0.006 0.006 0.005 0.005 0.009 0.008 0.005 0.005 0.198 –
1.09 1.16 1 1.11 0.251 1.15 1.05 0.244 1.02 0.894 0.195 0.989 0.379 0.236 1.15 1.35 1.03 1.14 0.821 0.873 0.31 1.1 1.12 0.76 0.983 0.857 1.04 1.04 0.897 0.683 0.999 0.934 0.99 0.986 0.9 –
54.2 43.4 33.7 39.3 8.13 32.7 54.1 7.03 69.7 57.4 6.17 34.9 10.8 6.59 56.6 62.2 64.1 45.6 32.9 37 8.43 39.2 46.6 27.8 51.1 35.1 37.1 59.8 39.4 68.7 48.7 50.5 42.8 41.9 17 20
1.49 1.09 0.72 0.84 0.15 0.83 1.23 0.17 1.62 1.35 0.12 0.69 0.31 0.13 1.56 1.31 1.17 1.01 0.66 0.64 0.13 0.96 1.02 0.48 0.76 0.59 0.72 1.23 0.81 1.18 0.88 0.97 0.91 0.73 0.16 0.25
16.7 18.3 18 20.4 3.55 14.2 15.5 4.23 14.7 13.1 6.12 16 5.26 4.36 15 17.7 13.5 14.2 18.1 16.5 13.9 18.1 15.7 13 59.4 16.3 15.5 16.3 14.5 15.3 15.8 13.6 15.5 17.2 10.5 14.6
11.4 10.1 3.87 4.82 1.8 4.84 7.85 2.18 5.91 4.87 3.86 19.7 2.21 1.45 3.55 10 7.38 4.46 6.98 5.3 13.9 4.34 5.77 4.6 46 8.13 4.66 3.41 3.71 5.07 3.7 3.36 6.89 5.04 2.7 3.1
146 168 145 168 40.2 149 139 37.4 169 121 29.1 140 44.1 31.2 146 172 114 159 139 131 35.3 147 134 108 149 104 153 155 132 114 142 130 149 134 193 210
4.37 5 4.4 4.9 1.05 4.46 3.8 1.02 5.05 3.78 0.819 4.04 1.46 0.953 4.17 5 3.48 4.74 4.3 3.94 1.05 4.51 4.07 3.38 4.64 3.3 4.59 4.81 3.99 3.46 4.5 4.06 4.19 4.29 5.3 5
1.10 1.17 1.15 1.30 0.72 1.02 1.05 0.74 1.13 1.12 1.04 1.04 0.94 0.92 1.15 1.10 1.10 1.00 1.63 1.31 1.82 1.23 1.17 1.13 2.86 1.36 0.99 1.02 0.90 0.96 1.02 0.93 1.03 1.14 0.75 0.91
9.61 10.70 9.24 10.70 8.17 10.72 9.46 6.56 13.00 10.34 4.97 9.09 7.92 6.57 11.15 10.68 9.27 11.20 12.52 10.40 4.63 10.00 10.00 9.39 7.16 8.67 9.81 9.69 8.20 7.13 9.16 8.90 9.87 8.87 13.79 13.13
2.41 2.73 2.14 2.53 1.66 2.48 2.39 1.54 2.67 2.56 1.17 2.10 2.03 1.73 2.47 2.53 2.28 2.19 2.73 2.34 1.36 2.31 2.04 2.21 2.00 2.30 2.37 2.29 1.99 1.69 2.36 2.43 2.47 2.23 2.21 2.39
1.10 0.56 0.58 0.60 0.63 0.96 1.01 0.49 0.71 1.00 0.40 0.74 0.57 0.52 1.02 0.86 1.39 1.06 0.41 0.46 0.21 0.47 0.94 0.76 0.20 0.62 0.83 1.02 0.80 0.79 0.85 1.04 1.29 0.72 1.65 1.37
6.05 5.63 4.95 5.29 7.49 6.08 5.25 6.57 6.20 5.65 2.91 4.99 6.18 5.09 5.97 6.16 5.69 6.31 4.42 4.61 1.67 5.03 5.52 5.55 1.59 5.00 6.65 6.20 6.20 4.33 5.97 6.57 6.71 5.19 8.76 6.85
24.10 21.62 21.99 24.22 19.98 21.48 23.72 23.56 19.40 23.87 18.95 24.92 24.87 26.13 23.28 20.91 25.87 21.00 24.02 23.88 23.77 22.87 22.41 26.64 24.70 23.97 24.68 21.77 25.38 20.45 26.21 28.08 22.04 24.06 33.16 47.62
Young, 1982; Wang et al., 2006). The cerium anomaly log (Fig. 3) also shows more negative values in the lower part and more positive values in the upper part, suggesting a shift to more oxic conditions probably caused by aridity (Och et al., 2014; Wright et al., 1987). Even though DeCelles et al. (2007) argued for arid paleoclimate in the Nima area at 26 Ma; Sun et al. (2014b) got the same result at the Lunpola area based on paleolake condition and regional comparison, neither gave the consequent evolution of the paleoclimate. Some other studies (Billups et al., 2002; Dong et al., 2013; Friedrich et al., 2012) conducted in basins of inland Asia indicate that this apparent drying is synchronous with global climate cooling across the Oligocene/Miocene (O/M) boundary. In addition, Sun et al. (2014a) also found this transition from palynological evidence across the bentonite layer dated around 23.5 Ma (He et al., 2012). Thus, it is reasonable to argue that this transition from relatively warm or humid to cold or dry at about 100 m (Fig. 3) is representing the O/M boundary. The relatively low CIA values of marl samples are consistent with the drier episodes during relatively warm/humid stage in which decreased precipitation caused relatively low rates of chemical weathering, low yield of clastic sediment, and salinization of the ancient lake. All these would promote the deposition of marl (Gierlowski-Kordesch et al., 2013; McLennan, 1993; Murphy and Wilkinson, 1980; Talbot, 1990). 5.2. Provenance of the oil shale 5.2.1. Sedimentary sorting and recycling Sedimentary sorting and recycling can cause enrichment of heavy minerals, thus the evaluation of trace elements mainly hosted by heavy minerals (e.g., Zr in zircon) is a useful way to analyze sedimentary processes (Ghosh and Sarkar, 2010; McLennan et al., 1993). Ratios of Zr/Sc and Th/Sc and their cross plot in the present study are used to
evaluate the sorting and recycling of the provenance materials (McLennan et al., 1993). The Zr/Sc values of the Lunpori samples show variation among lithologies (Fig. 6; Table 3): ratios of marl samples (4.63–8.17) are lower than those of oil shale and mudstone samples (8.2–13, except samples D-25 and D-32), probably because of reduced clastic input during marl deposition. Th/Sc of most Lunpori samples ranges from 0.7 to 1.6, except D-25, which has the highest ratio of 2.86. The cross plot of Zr/Sc and Th/Sc (Fig. 6) shows that sorting and recycling were minor during deposition of the Dingqinghu Formation revealing very little of the clastic fraction in the formation that came from older sedimentary units or experienced long-distance transportation.
5.2.2. Source lithotypes In Fig. 6, all Lunpori samples show linear relationship along the compositional variation line, suggesting felsic but mixed source rocks (McLennan et al., 1993). Samples are also plotted in Al2O3–(CaO⁎ + Na2O)–K2O (A–CN–K) diagrams (Fig. 7) by lithologies to differentiate the source lithotypes (Ghosh and Sarkar, 2010; McLennan et al., 1993; Wang et al., 2006). In Fig. 7, samples of different lithologies show the same linear trend suggesting that the source area was quite stable (Bock et al., 1998; Lee, 2009); calculated weathering trends of several different sediment-source rocks are also illustrated in the diagrams. Intersection of the Lunpori sample trend line with feldspar indicates the feldspar proportion of the unweathered source rock. The near granodiorite intersection indicates that the granodiorite source dominated the provenance. However, the Lunpori trend lines are not parallel to the A–CN side or other calculated weathering trends, probably because of K metasomatism during diagenesis (Fedo et al., 1995; Michalopoulos and Aller, 1995). Nevertheless, the deviation of
P. Ma et al. / International Journal of Coal Geology 147–148 (2015) 58–70
65
Table 4 Rare earth elements (REE) concentrations (in ppm) and some parameters of samples. Sample La no.
Ce
D-1 D-2 D-3 D-4 D-5 D-6 D-7 D-8 D-9 D-10 D-11 D-12 D-13 D-14 D-15 D-16 D-17 D-18 D-19 D-20 D-21 D-22 D-23 D-24 D-25 D-26 D-27 D-28 D-29 D-30 D-31 D-32 D-33 D-34
65.1 8.42 73.2 10.2 63.7 8 72.6 9.45 15.8 1.82 63.6 7.81 62.3 7.88 16 2.06 59.5 7.43 54.9 6.8 14.4 1.85 57.3 7.61 21.1 2.62 15.9 2.08 56.8 7.23 71.7 8.7 51.6 6.4 55.2 7.01 55.9 7.08 52.8 6.78 21 2.61 63.7 7.84 51.1 6.3 48.2 5.72 78 10.4 51.6 6.46 69.6 8.46 66.4 8.31 58.8 7.39 49.6 6.16 65.7 8.46 62.2 7.94 68.1 8.23 64.2 7.97
36.7 42.8 33.6 39.7 8.17 34.5 35.1 8.75 34.7 29.9 6.87 32.4 11.3 8.22 32.3 40.8 28 31.1 30.3 29.5 10.4 34 27.3 25.4 41.7 27.6 37 36.6 32.1 27 36.6 35.5 37.3 33.7
Pr
Nd
Sm
Eu
Gd
Tb
Dy
32 38 29.8 34.7 7.18 28.6 29.3 8.16 27.8 25.3 8.01 28.1 10.2 8.57 26.6 33.3 23.5 25.9 27.8 25.1 10.9 30 23.6 21.8 42.6 24.6 31.2 31.4 28.4 23.4 31.3 29.8 31.5 29.8
5.84 7.16 5.67 6.51 1.39 5.2 5.5 1.65 5.2 4.65 1.94 5.35 1.98 1.85 4.76 6.22 4.4 4.66 5.24 4.74 2.48 5.47 4.37 4.04 9.5 4.71 5.7 5.63 5.07 4.53 5.79 5.26 5.68 5.55
1.08 1.25 0.99 1.14 0.28 0.90 0.98 0.36 0.85 0.80 0.42 0.95 0.37 0.38 0.82 1.12 0.78 0.81 1.00 0.86 0.56 0.97 0.78 0.75 1.89 0.85 1.03 1.00 0.92 0.83 0.98 0.93 1.03 1.03
5.12 5.92 5.08 5.43 1.32 4.32 4.83 1.62 4.28 3.88 1.9 4.58 1.85 1.76 4.11 5.63 3.71 4.04 4.73 4.09 2.48 4.7 3.85 3.7 9.38 4.22 4.89 4.76 4.41 3.86 4.86 4.33 4.84 4.95
0.87 4.58 1.01 5.14 0.83 4.57 0.94 4.89 0.22 1.21 0.74 3.85 0.78 3.98 0.28 1.6 0.65 3.22 0.65 3.25 0.36 2.12 0.76 3.92 0.32 1.72 0.31 1.73 0.66 3.34 0.92 4.97 0.63 3.3 0.66 3.39 0.78 3.96 0.69 3.53 0.44 2.63 0.80 4.11 0.64 3.4 0.62 3.17 1.81 11.6 0.71 3.58 0.80 4.15 0.78 3.98 0.73 3.86 0.69 3.62 0.83 4.37 0.73 3.77 0.79 4.07 0.86 4.45
Y
Ho
Er
Tm
Yb
Lu
ΣREE
LREE
24 25.2 23.6 25.5 7.68 20.2 20.3 9.32 15.7 17.1 11.8 20.3 9.14 9.13 18 27.2 17.2 18.2 20.9 18.7 15 22 18.6 16.8 68.5 18.6 22.2 22.2 21.7 20.4 23.2 21 24.5 24.1
0.89 0.99 0.92 1.00 0.25 0.79 0.79 0.35 0.64 0.67 0.43 0.79 0.36 0.35 0.68 0.99 0.65 0.69 0.79 0.70 0.55 0.83 0.68 0.65 2.43 0.72 0.84 0.81 0.79 0.74 0.87 0.77 0.81 0.89
2.33 2.46 2.32 2.46 0.658 2.07 2.06 0.887 1.69 1.74 1.06 2.04 0.907 0.878 1.76 2.48 1.71 1.82 1.96 1.78 1.37 2.06 1.79 1.67 5.99 1.81 2.09 2.02 1.92 1.91 2.21 1.98 2.01 2.21
0.48 0.48 0.46 0.50 0.14 0.41 0.42 0.19 0.35 0.36 0.22 0.41 0.19 0.18 0.37 0.52 0.35 0.38 0.40 0.36 0.29 0.43 0.36 0.34 1.18 0.37 0.43 0.43 0.41 0.39 0.46 0.41 0.41 0.45
2.73 2.77 2.79 2.98 0.87 2.50 2.46 1.17 2.04 2.11 1.31 2.54 1.18 1.10 2.18 3.12 2.09 2.26 2.39 2.26 1.80 2.55 2.16 2.10 6.60 2.22 2.65 2.51 2.46 2.34 2.76 2.46 2.48 2.72
0.40 0.39 0.38 0.41 0.12 0.35 0.34 0.18 0.29 0.30 0.18 0.36 0.16 0.15 0.31 0.44 0.28 0.32 0.33 0.32 0.26 0.35 0.29 0.29 0.83 0.29 0.35 0.35 0.33 0.32 0.37 0.34 0.35 0.38
190.54 216.97 182.70 208.21 47.11 175.84 177.01 52.57 164.33 152.41 52.88 167.40 63.40 52.58 159.92 208.10 144.60 156.44 163.55 152.21 72.76 179.80 145.23 135.25 292.41 148.34 191.39 187.17 169.29 145.79 188.76 177.42 192.10 183.25
149.14 41.40 3.60 0.99 172.61 44.36 3.89 1.14 141.76 40.95 3.46 0.89 164.10 44.11 3.72 0.98 34.64 12.47 2.78 0.69 140.61 35.23 3.99 1.02 141.06 35.96 3.92 1.05 36.98 15.59 2.37 0.55 135.48 28.85 4.70 1.26 122.35 30.06 4.07 1.05 33.49 19.39 1.73 0.39 131.71 35.69 3.69 0.94 47.57 15.83 3.01 0.71 37.00 15.58 2.38 0.55 128.51 31.41 4.09 1.09 161.84 46.26 3.50 0.97 114.68 29.92 3.83 0.99 124.68 31.75 3.93 1.02 127.32 36.23 3.51 0.94 119.78 32.43 3.69 0.96 47.95 24.81 1.93 0.43 141.98 37.82 3.75 0.98 113.45 31.78 3.57 0.93 105.91 29.33 3.61 0.89 184.09 108.32 1.70 0.47 115.82 32.52 3.56 0.92 152.99 38.40 3.98 1.03 149.34 37.84 3.95 1.08 132.68 36.61 3.62 0.96 111.52 34.27 3.25 0.85 148.83 39.94 3.73 0.98 141.63 35.79 3.96 1.07 151.84 40.26 3.77 1.11 142.25 41.00 3.47 0.91
HREE
L/H
Ce LaN/YbN Eu anomaly anomaly −0.032 −0.044 −0.063 −0.044 −0.011 −0.051 −0.050 0.010 −0.071 −0.055 0.014 −0.044 −0.038 −0.003 −0.060 −0.050 −0.042 −0.054 −0.025 −0.035 0.023 −0.046 −0.046 −0.038 −0.027 −0.047 −0.037 −0.042 −0.037 −0.029 −0.061 −0.038 −0.034 −0.034
−0.068 −0.092 −0.048 −0.063 −0.024 −0.049 −0.063 −0.061 −0.068 −0.051 −0.032 −0.075 −0.048 −0.052 −0.067 −0.057 −0.051 −0.064 −0.055 −0.065 −0.032 −0.046 −0.046 −0.035 −0.064 −0.050 −0.042 −0.056 −0.055 −0.052 −0.065 −0.068 −0.047 −0.044
LREE: La, Ce, Pr, Nd, Sm, Eu; HREE: Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu; L/H = LREE/HREE; Eu anomaly = log[2EuNE / (SmN + GdN)]; Ce anomaly = log[2CeN / (LaN + PrN)]; Elements with subscript N represent the concentration normalized to the PAAS (Taylor and McLennan, 1985).
the Lunpori trend line is not very large, suggesting weak diagenetic influence. Thus, it is not quantitatively analyzed here. Apart from the provenance clues from Figs. 6 and 7, some other major, trace elements, and REE also can be used to determine provenance because of their stable behavior during source rock weathering and sediment transport, diagenesis, and metamorphism (McLennan et al., 1993). The Al2O3/TiO2 ratio is used widely to identify the source lithotypes (Dai et al., 2011, 2015b; Hayashi et al., 1997). Mafic, intermediate, and felsic igneous rocks are characterized by respective Al2O3/TiO2 ratios of 3–8, 8–21, and 21–70 (Hayashi et al., 1997). For the Lunpori samples, Al2O3/TiO2 ratios range from 21.64 to 29.78, indicating a uniform source area that mainly consists of felsic igneous rocks (Dai et al., 2013, 2015b; Hayashi et al., 1997). Co/Th ratios can be used to distinguish felsic and mafic sources, as felsic sources tend to have low Co/Th values (Amorosi et al., 2002;
Cullers, 2002). The Co/Th ratios of Lunpori samples range from 0.2 to 1.39 (Table 3), lower than that of UC (1.65) and PAAS (1.37) (Fig. 8), suggesting felsic-prone source area. La/Sc is also sensitive to source composition (Gu et al., 2002). All of the samples are plotted on a Co/Th versus La/Sc diagram modified after Condie (1993) and Gu et al. (2002) (Fig. 8A). Samples are concentrated in the felsic source field near granodiorite and adamellite, except D-21 and D-25 which both have Co/Th ratios close to average granites. Similarly, Cr/Th ratios are plotted against Sc/Th ratios in Fig. 8B (after Condie and Wronkiewicz, 1990). Low Cr/Th and Sc/Th ratios indicate felsic provenance (Table 3). The samples fall into the felsic source field, between granodiorite and adamellite, and close to the PAAS. Generally, Cr/Th ratios of all samples correlate well with Sc/Th ratios and show a linear relationship, suggesting that (1) the Cr/Th ratio is primarily controlled by provenance, and (2) there were probably mixed felsic source rocks that dominated the clastic input (Condie and
Fig. 5. Distribution patterns of REE normalized to PAAS (Taylor and McLennan, 1985).
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relative contributions of different source lithologies to the sediments were unknown, REE patterns of sediments were clearly inherited from the source area. Most of the source samples are characterized by balance between HREE and LREE, negative Eu and Ce anomalies, which are closely similar to the REE patterns of the Lunpola samples (Fig. 5).
5.3. Drainage system reconstruction
Fig. 6. Diagram of Th/Sc versus Zr/Sc for the samples. Note that most of the samples except D-25 and D-21 distribute near UC and PAAS, along the compositional variations line, and far from sediment recycling area (McLennan et al., 1993). This indicates that samples tend to have a proximal mixed felsic provenance.
Wronkiewicz, 1990). Regional geologic surveys (Pan et al., 2004) indicate that proximal felsic source areas are mainly distributed in the Baingoin area at the south of the basin (Fig. 1). The Baingoin batholiths are mainly composed of Cretaceous granodiorite and adamellite (Gao et al., 2011a,b). 5.2.3. Tectonic setting To further refine the source area, La–Th–Sc, Th–Sc–Zr/10, Th–Co–Zr/ 10, and Ti/Zr versus La/Sc discriminatory plots were also used to determine the tectonic setting of the source area (Ghosh and Sarkar, 2010; Wang et al., 2014b; Fig. 9). In these diagrams, most of the Lunpori samples are plotted into the continental arc and active continental margin fields, except samples D-25, D-21, and D-11, which do not fall into any distinguished fields. The D-21 and D-11 samples are marl with little terrestrial clastics, thus they are away from the Zr/10 apex in Th–Sc–Zr/10 and Th–Co–Zr/10 diagrams. The abnormally high Th content of sample D-25 could cause deviation approaching the Th apex. The continental arc and active continental margin tectonic setting is also in accordance with results from geochronology and geochemistry of the Baingoin batholiths. The batholiths were intruded during several episodes caused by the southern subduction of the Bangong–Nujiang Tethyan Ocean seafloor and collision between the Lhasa and Qiangtang terranes (Gao et al., 2011a; Zhu et al., 2009). The REE distribution of samples from the Baingoin batholiths (Gao et al., 2011a,b) is also illustrated in Fig. 10. In order to make a more direct comparison, they are also normalized to the PAAS. Although
The main river system originated from the Baingoin batholiths and carried clastics northward to the basin. However, it did not directly drain into the ancient Lunpola Lake. In this period, to the south of Lunpola Lake, the ancient Baingoin Lake also received late Oligocene to early Miocene sediments from the south and the depocenter was at the western part of the basin around the area of the present Baingoin Co, as constrained by Ban-1 and Ban-2 boreholes (Luo et al., 1996; Mu, 1992; Xia, 1979; Fig. 1). Xia (1979) reported that drilled core in the Ban-1 borehole on the west lakeshore of Baingoin Co (Fig. 1B) recovered 405 m of sedimentary sequence consisting of brown to gray fluvial–lacustrine shale, mudstone, and sandstone. Similar palynological assemblages for age control were found in Ban-1 core as the Dingqinghu Formation of the Lunpola Basin (Sun et al., 2014a; Wang et al., 1975; Xia, 1979). Wang et al. (2011b) and Sun et al. (2014b) investigated the organic geochemical features of oil shale collected from the Lunpori section and argued that the Lunpola Lake at this stage was a reducing, stratified, and hypersaline paleolake. However, evidence from coeval sediments of the ancient Baingoin Lake suggests dominant fresh to brackish water, because large amounts of freshwater prone ostracodes (Xia, 1979) and gastropod fossils (Zeng and Hu, 1985) were found in the Ban-1 core. Therefore, we summarize that the ancient drainage system of the Lunpola area contained a main river originating from the Baingoin batholiths to the south, the freshwater to brackish ancient Baingoin Lake, and the hypersaline ancient Lunpola Lake (Fig. 11); it is reasonable that the two ancient lakes were connected by the river that allowed water and clastics to drain into the Lunpola Lake across the Baingoin Lake. This is similar to the modern south part of Silin Co lake system, in which freshwater and saline lakes are hydrologically connected (Guan, 1984; Meng et al., 2012). The connection area of the two ancient lakes was most likely at the western margin (Fig. 11), because it was the only area in the Lunpola Basin that developed river delta deposits during this period (Du et al., 2004). The intervening ridge of the exposed Mesozoic strata (Figs. 1, 11) probably has already existed as a low positive landform at late Oligocene to early Miocene, because several small fan-delta front sand bodies implying intermittent stream are also identified along it (Ma et al., 1996; Fig. 11). However, this ridge must be away from the main river system, given its minor to miniscule contribution to clastics input (Fig. 11).
Fig. 7. Al2O3–(CaO⁎ + Na2O)–K2O plot of different lithologic samples (after McLennan et al., 1993), compared with PAAS and UC. The dashed line with arrow represents the calculated weathering trend of different source rock (Fedo et al., 1995; McLennan et al., 1993; Nesbitt and Young, 1989).
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Fig. 8. Diagrams of (A) Co/Th versus La/Sc (Condie, 1993; Gu et al., 2002) and (B) Cr/Th versus Sc/Th (Condie and Wronkiewicz, 1990) for the samples from the Lunpola Basin. Data of different kinds of igneous rocks are from Condie (1993) and Shi et al. (2005). Note that all most all of our samples as well as UC and PAAS fall into the felsic source area.
5.4. Organic matter accumulation The occurrence and characteristics of ancient lakes are the combined result of potential accommodation created by tectonics and sediment plus water input determined by paleoclimate (Carroll and Bohacs, 1999; Carroll et al., 2006; Smith et al., 2008; Fig. 11). Carroll and Bohacs (1999) proposed that three types of lake-basin result based on empirical observations are: overfilled, balanced-fill, and underfilled,
each having a principal lacustrine facies association including fluvial lacustrine (overfilled), fluctuating profundal (balanced-fill), and evaporative (underfilled) (Fig. 11). In the Lunpola Basin, gray–brown finely laminated oil shale layers alternate with mudstone and marl without too much sandy contribution in the entire sequence. Sedimentary structures indicating paleocurrent are rare in the Dingqinghu Formation of Lunpori area (Wang et al., 2011b). All these suggest a profundal and stable paleolake condition
Fig. 9. La–Th–Sc, Th–Sc–Zr/10, Th–Co–Zr/10, and Ti/Zr versus La/Sc discriminatory plots (Ghosh and Sarkar, 2010). Almost all of our samples fall into the area of continental arc and active continental margin.
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Fig. 10. REE distribution patterns of granodiorite (A) and adamellite (B) samples collected from the Baingoin batholiths normalized to PAAS. Black and gray lines represent samples of different intrusion periods. Data from Gao et al. (2011a,b).
away from an estuary. For the Baingoin Basin, two upward coarsening subsequences could be seen at the equivalent strata (Xia, 1979), indicating the progradation of the marginal facies into the lake (Carroll and Bohacs, 1999). Compared to the poor source rocks of the Baingoin Basin, whose total organic matter (TOC) is all less than 1% (Luo et al., 1996), the Dingqinghu Formation in the Lunpola Basin has much better source rocks with type I kerogen mainly from lacustrine algae, high TOC up to 14.49%, and N 500 hydrogen indexes (HI) (Sun et al., 2014b; Wang et al., 2011b). Considering that all samples collected from the Lunpola Basin have gammacerane indexes ranging from 1.36 to 25.26 and b 0.5 pristine/phytane ratios (Wang et al., 2011b), there is no doubt that the Dingqinghu Formation was deposited in hypersaline balanced-fill Lunpola Lake as fluctuating profundal facies association (Carroll and Bohacs, 1999, 2001). To sum up, during late Oligocene–early Miocene, the paleoclimate shifted from a relatively warm or humid condition to a cold or dry condition, and the source area only experienced moderate to weak chemical weathering. In this kind of paleoclimate, the river systems were restrictedly developed and sediments only originating from the Baingoin batholiths to the south flowed into the ancient Baingoin Lake first to form an overfilled lake basin, and then spilled northward to the Lunpola Lake to accumulate fluctuating profundal facies (Fig. 11). Therefore, even though mixed coniferous–broadleaved forest surrounded the drainage area (Sun et al., 2014a; Wang et al., 1975), terrestrial organic matter transported to the Lunpola Lake was minor in volume, and lacustrine algae was able to proliferate as the main organic source and preserve in the stratified hypersaline lake. Intense photosynthesis consumed large amounts of CO2, promoting the paleoproductivity and the deposition of carbonates (Cohen, 2003; Fig. 4). The cooling event constrained by CIA is also in agreement with the reduction of TOC and HI for samples from the upper Dingqinghu Formation (Sun et al., 2014b; Wang et al., 2011b), which probably resulted from the decline of the ancient lake productivity in the cooler climate. 6. Conclusions
Fig. 11. (A) Illustrative lake-type model. In overfilled basins, water and sediment exceed potential accommodation; fluvial–lacustrine facies develop in the freshwater lake basins. In balanced-fill basins, water and sediment are equal to potential accommodation; fluctuating profundal facies occupy in saline water lakes. In underfilled basins, water and sediment could not fill the potential accommodation, evaporative facies are dominant here. P/E = precipitation/evaporation (modified from Carroll and Bohacs, 1999). (B) Illustrative paleogeographical map of the ancient Lunpola and Baingoin Lakes. The location of delta in ancient Baingoin Lake is speculated, the location of sand bodies in the ancient Lunpola Lake are mapped out according to the study of Du et al. (2004), Lin (2012), and Ma et al. (1996).
1) Most of the marl, oil shale, and mudstone samples in the Lunpori section have a weak-moderate weathered source with minor sorting and recycling. 2) The geochemical results indicate that the sedimentary sources were derived from the Cretaceous Baingoin batholiths, a former active continental magmatic arc. 3) The ancient freshwater Baingoin Lake coexisted with the hypersaline Lunpola Lake in a connected drainage system within the same climatic zone shifting from a relatively wet or humid condition to a cold or dry condition across the Oligocene/Miocene boundary. 4) Water and sediments with minor terrestrial organic matter from the overfilled Baingoin Basin flowed to the Lunpola Basin. Blossom of lacustrine algal and reducing, stratified, and hypersaline lake conditions promoted the occurrence and accumulation of organic matter.
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Acknowledgments This research was financially supported by the National Key Project for Basic Research of China (No. 2011CB403007) and the National Natural Science Foundation of China (No. 41202059). Pengfei Ma is supported by the scholarship from China Scholarship Council (No. 201406400014). This work benefited from discussion with Prof. Alan R. Carroll of University of Wisconsin-Madison. We are grateful to Lisa Lesar from University of Wisconsin—Geology Museum and Edanz Group for language editing. Constructive comments on the manuscript by Editor-in-Chief Prof. Shifeng Dai and two anonymous reviewers are gratefully acknowledged. References Amorosi, A., Centineo, M.C., Dinelli, E., Lucchini, F., Tateo, F., 2002. Geochemical and mineralogical variations as indicators of provenance changes in Late Quaternary deposits of SE Po Plain. Sediment. Geol. 151, 273–292. Bauluz, B., Mayayo, M.J., Fernandez-Nieto, C., Gonzalez Lopez, J.M., 2000. 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