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T boundary
Sedimentary Geology 197 (2007) 127 – 140 www.elsevier.com/locate/sedgeo
Geochemistry of the sedimentary rocks from the Nanxiong Basin, South China and implications for provenance, paleoenvironment and paleoclimate at the K/T boundary Yi Yan a,⁎, Bin Xia a , Ge Lin a , Xuejun Cui a , Xiaoqiong Hu a , Pin Yan b , Faqiang Zhang c a
b
Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou, 510640, PR China Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, 510301, PR China c Research Institute of Petroleum Processing, Beijing, 100083, PR China Received 12 March 2006; received in revised form 17 September 2006; accepted 25 September 2006
Abstract Cretaceous and Tertiary clastic sedimentary rocks from the Nanxiong Basin, South China have been analyzed to constrain their provenance, depositional climate and environment. Evidence from discrimination diagrams for sedimentary provenance and tectonic setting show that the Nanxiong Basin sediments were derived from typical continental sources. Geochemical signatures (e.g. Eu/Eu⁎, Th/Ti, La/Ti, Ta/Ti, Yb/Ti and Y/Ti ratios of the claystone) are nearly constant, suggesting the provenance of the Nanxiong Basin remained similar throughout the Late Cretaceous to Early Paleocene (83–56 Ma). In contrast Rb/Ti, Cs/Ti ratios and TOC and CaCO3 concentrations require an obvious change in climate across the Late Cretaceous and Early Paleocene boundary. Singularly higher CaCO3 contents and lower TOC values and Rb/Ti, Cs/Ti ratios in the Late Cretaceous indicate that a long period extreme dry climate occurred at that time in South China. Rb/Ti, Cs/Ti ratios and TOC values escalated and CaCO3 contents decreased in the Early Paleocene suggesting that the climate became relatively wet, which resulted in greater vegetation cover. The lasting extreme dry climate in the Late Cretaceous may provide a clue to the extinction of the dinosaurs in the Nanxiong Basin. © 2006 Elsevier B.V. All rights reserved. Keywords: South China; Nanxiong Basin; Geochemistry; Provenance; Paleoclimate
1. Introduction Geochemical signatures of clastic sedimentary rocks provide important sources of information that record different aspects of provenance, tectonic, environmental ⁎ Corresponding author. Tel.: +86 20 85290293. E-mail addresses: [email protected], [email protected] (Y. Yan). 0037-0738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2006.09.004
and ecological evolution. Although hydraulic sorting, weathering and diagenesis can alter the geochemical composition of basin sediments, there often remains a strong signature to the original source terrain, reflecting the nature of the exposed continental crust (Roser and Korsch, 1988; Rollinson, 1993). Significant improvement has been made in geochemical approaches to monitoring sediment provenance, especially through the
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introduction of discrimination diagrams based on the relationship of major and trace elements (Bhatia, 1983; Taylor and McLennan, 1985; Roser and Korsch, 1986; McLennan and Taylor, 1991; Condie, 1993; Cullers, 1994; Bauluz et al., 2000). Whilst trace element geochemical studies have tended to focus on aspects of sediment provenance, their application to paleoclimate reconstructions has been relatively neglected, with most paleoclimate studies focusing on biogenic components, such as carbonate, opal, organic carbon and authigenic elements (e.g. Elderfield, 1990; Murray and Leinen, 1993). Recent research has suggested that clastic sedimentary trace element records may contain additional new paleoclimate information that provides important new constrains on sediment depositional environments and climate (Wei et al., 2004). Some major and trace elements, such as alkali and alkali earth elements that are water mobile elements and very sensitive to climatic change, which can be used as a valuable proxy of paleoclimate evolution (Wei et al., 2004).
A major target of global paleoclimate studies has been the K/T boundary because this is linked to understanding the phenomena of vertebrate mass extinctions (e.g. Courtillot et al., 1988; Erben et al., 1995; Zhao and Yan, 2000; Zhao et al., 2002). In southern China there are extensive deposits of vertebrate-rich Upper Cretaceous– Cenozoic terrestrial clastic sedimentary rocks that are ideal for such studies. Discovery of large quantities of Cretaceous and Paleocene vertebrate fossils in the Nanxiong Basin in the early 1960s (Young, 1965; Zhao, 1975; Erben et al., 1995), earmarked this basin as a key site. Over the last 30 years, multidisciplinary studies of the K/T boundary in the Nanxiong Basin (Young, 1965; Yang et al., 1993; Erben et al., 1995; Zhao et al., 1998; Zhao and Yan, 2000) have been carried out. Stable carbon and oxygen isotopes were analyzed in dinosaur eggshell samples, collected in two sections of the Nanxiong Basin (Zhao and Yan, 2000). These revealed multiple positive δ18O perturbations during the K/T transition that suggest at least three periods of extreme dry climate with mean
Fig. 1. (A) Regional tectonic map of South China (after Davis et al., 1997); (B) Geological map and locations of profiles in the Nanxiong Basin; (C) Stratigraphic location of the samples from the Nanxiong Basin (Stratigraphy age from Zhang et al., 2000 and Bureau of Geology and Mineral Resources of Guangdong Province, 1988).
Y. Yan et al. / Sedimentary Geology 197 (2007) 127–140
annual air temperatures N 27 °C, similar to the range of 26.6–33.9 °C calculated by Yang et al. (1993) based on δ18O of carbonate rocks (middle and upper Pingling Formation). Although this evidence suggests a drastic environmental change occurred in the Nanxiong Basin during the K/T transition, published stable isotope data have not produced a clear picture because they are fragmentary and speculative and give only limited information (Zhao and Yan, 2000). Whether the drastic environmental change occurred instantaneously or gradually remains unclear (Erben et al., 1995; Zhao and Yan, 2000). In this study, we use major and trace signatures of the bulk sedimentary rocks that span the Cretaceous–Tertiary boundary in the Nanxiong Basin to investigate the behavior of different elements and find more robust constraints on the climatic conditions in South China during the K/T transition. 2. Geological background The Nanxiong Basin belongs to the South China Block, which comprises the Yangtze Craton in the north and Cathysia Block in the south (Fig. 1). The bulk of the Yangtze Craton is Proterozoic–Late Archaean (Ames et al., 1996) formed of mainly low-grade clastic metasedimentary rocks with minor volcanic rocks (Li and McCulloch, 1996) and a non-metamorphic shelf to continental sedimentary sequence that spans the Neoproterozoic to Triassic. From the Late Triassic onwards, sedimentation on the Yangtze Craton was dominated by continental red-beds (Chen and Jahn, 1998). The Cathaysia Block consists of Precambrian basement and a Sinian to Mesozoic sedimentary and volcanic cover. The lower part of the sedimentary cover (Sinian to lower Palaeozoic) is represented by a thick series of folded and slightly metamorphosed marine sedimentary rocks (Ling et al., 1996). From the Devonian onwards, Cathaysia appears to share the same stratigraphy as the Yangtze Craton (Chen and Jahn, 1998). During the Late Jurassic to Cretaceous the region was an active Andeantype magmatic arc subject to block faulting and granite emplacement (Xu, 1990). Since the Late Cretaceous, related to backarc opening (Hawkins et al., 1990), and in its early stages to lithospheric extension of the Cathaysia Block in South China (Chung et al., 1997), a series of NE-trending faults and small continental basins formed. The Nanxiong Basin, developed on pre-Jurassic basement, is a small intermontane basin situated in northern Guangdong Province, South China. The basin is elongated with its axis oriented northeast–southwest (Fig. 1). The topography within the basin is a hilly terrain, with rolling hills less than 50 m high. The Upper
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Cretaceous to Tertiary sedimentary rocks attain a maximum thickness of over 7000 m. Typical basin fill comprises red fluvial-lacustrine clastic sequences that span the Late Cretaceous, represented by the dinosaurbearing Nanxiong Group (Dafeng, Zhutian, Zhenshui and Pingling Formations) and Early Paleocene, represented by the mammal-bearing Luofuzai Group (Shanghu and Nongshan Formations) (Yang, 1998; Zhao and Yan, 2000). These formations are described in more detail below; 2.1. Dafeng Formation Unconformably overlying Paleozoic–Mesozoic granites in the south of the Nanxiong Basin, the Dafeng Formation comprises brown-red coarse-grained clastic sedimentary rocks. Sedimentation is mainly in the form of fluvial–lacustrine red beds and consists of conglomerates, sandstones, with silty mudstone interbeds. Fewer ostracod, such as Cyprois sp. were found in this formation (Ling et al., 2005). 2.2. Zhutian Formatiom The Zhutian Formation is characterized by lacustrine facies fine-grained clastic sedimentary rocks comprise of brown-red, dark-purple muddy siltstone and silty mudstone with yellowish-green calcareous mudstone interbeds. Large quantities of ostracods and charophytes, and minor amount of foraminifers, insects, estheria, bivalves and gastropods were discovered. 2.3. Zhenshui Formation Sedimentation of the Zhenshui Formation is composed of two parts: the lower part of the formation is dominantly in the form of lacustrine facies fine-grained clastic material with restricted fluvial, which mainly comprised of brownred, dark-purple siltstone and silty mudstone with sandstone and conglomerate interbeds; sedimentation of the upper part of the formation is mainly in the form of fluvial–lacustrine red beds and comprises brown-red coarse-grained clastic sedimentary rocks with silty mudstone interbeds. This formation is enriched in vertebrate and dinosaurian eggs and minor amount of ostracods, charophytes, bivalves and gastropods. 2.4. Pingling Formation The sediments of the Pingling Formation are dominantly coarse-grained fluvial with restricted lacustrine facies, comprising purple-red sandstones, conglomerates
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Table 1 XRF data for the major elements (wt.%), ICP-MS data for the trace elements (ppm) and the total organic carbon (TOC) values of the samples from the Nanxiong Basin (PAAS from Taylor and McLennan, 1985) No. wt.% SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI CaO⁎ TC TOC CaCO3 Total ppm Sc Ti V Cr Mn Co Ni Cu Zn Ga Ge Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U La/Yb(N) Eu/Eu⁎ La/Sc Th/Sc Th/Cr Th/Co
NX1 52.48 0.64 12.36 4.17 0.08 2.28 11.26 0.75 2.85 0.17 12.59 0.19 2.42 0.049 19.7 99.63
10.7 4038 121 41.8 622 10.7 23.3 58.3 178 15.3 1.54 135 192 30.8 185 17.4 17.5 258 41.9 77.9 9.71 34.7 6.48 1.24 5.70 0.90 5.74 1.15 3.12 0.47 2.86 0.45 5.07 1.55 26.4 16.6 6.26 9.49 0.62 3.90 1.54 0.39 1.55
NX2 58.58 0.58 8.78 1.25 0.12 1.61 13.07 1.06 1.66 0.12 12.47 0.00 2.86 0.045 23.4 99.30
6.70 3447 171 24.0 863 7.61 15.7 56.0 56.2 9.81 1.35 79.9 198 25.6 264 15.0 9.79 200 32.7 63.2 7.80 28.4 5.43 0.98 4.51 0.75 4.74 0.96 2.68 0.41 2.56 0.39 6.76 1.22 8.86 14.0 13.9 8.28 0.60 4.88 2.08 0.58 1.83
NX3 56.81 0.65 14.16 5.11 0.09 2.67 7.08 1.17 3.17 0.19 9.55 0.44 1.46 0.036 11.8 100.65
12.5 3989 77.7 113 778 14.0 31.3 34.0 107 20.0 1.91 177 157 29.9 172 21.5 20.3 731 48.5 98.3 11.4 39.4 7.63 1.14 6.04 0.96 5.99 1.13 3.12 0.48 3.03 0.46 4.81 2.41 28.6 24.9 7.33 10.4 0.50 3.86 1.98 0.21 1.77
NX4 54.54 0.68 14.98 5.54 0.11 2.59 6.86 0.88 3.67 0.26 9.87 0.49 1.40 0.034 11.3 99.98
14.5 4355 99.0 69.3 867 14.0 32.3 36.6 148 23.0 2.27 216 154 35.5 197 22.5 24.7 359 53.0 107 12.5 44.1 8.57 1.39 6.83 1.11 6.78 1.30 3.68 0.56 3.39 0.52 5.28 2.09 29.4 27.1 7.35 10.1 0.54 3.63 1.86 0.39 1.93
NX5 66.03 0.53 8.00 0.91 0.14 1.27 10.73 1.30 1.35 0.09 10.26 0.00 2.34 0.033 19.2 100.61
– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
NX6 59.65 0.70 14.70 5.08 0.08 2.13 5.19 1.28 3.31 0.21 8.19 0.62 1.01 0.029 8.16 100.52
12.5 4239 114 64.3 606 11.6 26.2 28.9 139 19.7 1.88 184 117 41.8 259 22.8 20.0 372 55.6 102 12.5 43.0 8.24 1.18 6.52 1.03 6.36 1.27 3.52 0.53 3.23 0.50 6.97 2.20 31.2 28.1 7.63 11.1 0.48 4.43 2.24 0.43 2.43
NX7 58.42 0.74 13.35 4.88 0.10 2.40 7.10 1.09 2.97 0.16 9.13 0.30 1.49 0.032 12.1 100.34
11.9 4538 74.1 59.3 739 13.6 26.9 29.9 97.4 18.2 1.85 154 213 30.7 196 20.6 18.3 562 46.7 91.6 11.0 38.4 7.71 1.30 6.26 0.97 5.84 1.15 3.20 0.48 3.04 0.46 5.25 1.71 27.1 21.9 6.28 9.95 0.56 3.92 1.83 0.36 1.61
NX8 55.71 0.73 15.48 5.63 0.08 2.67 6.21 0.62 3.81 0.15 9.57 0.38 1.28 0.030 10.4 100.66
13.9 4436 83.9 152 596 14.1 32.5 25.2 100 20.3 1.89 184 159 26.2 130 18.7 24.1 610 45.3 88.0 10.4 36.5 7.02 1.33 5.75 0.86 5.15 0.99 2.67 0.40 2.62 0.38 2.97 1.46 28.3 19.1 5.98 11.1 0.63 3.26 1.37 0.12 1.35
NX9 62.45 0.65 11.58 3.49 0.10 1.53 7.54 1.11 2.55 0.08 9.02 0.30 1.58 0.027 12.9 100.10
8.75 3948 55.7 52.9 802 9.74 20.1 25.8 64.7 14.9 1.87 130 119 106 300 17.5 13.3 240 59.4 121 13.7 53.5 12.2 2.89 15.9 2.67 17.2 3.48 9.40 1.31 8.47 1.27 8.07 1.46 22.3 17.4 3.13 4.54 0.64 6.79 1.99 0.32 1.79
NX10 59.64 0.59 9.34 2.68 0.11 1.48 11.51 1.19 1.83 0.10 11.35 0.06 2.49 0.037 20.4 99.82
7.06 3522 79.6 11.2 848 7.51 12.8 39.4 45.6 11.0 1.36 88.8 209 23.1 240 14.9 9.02 271 33.4 65.2 7.96 28.6 5.47 0.96 4.34 0.72 4.34 0.83 2.36 0.36 2.25 0.36 6.18 1.19 16.5 14.2 5.83 9.59 0.59 4.72 2.01 1.27 1.89
NX11 70.71 0.59 7.75 1.96 0.14 1.02 7.97 1.16 1.48 0.07 7.85 0.10 1.72 0.033 14.0 100.70
6.13 3769 46.6 45.1 1052 5.94 13.8 41.1 48.6 9.42 1.57 77.9 112 31.2 355 16.0 8.42 183 35.5 64.8 8.42 30.6 6.22 1.12 5.38 0.87 5.54 1.09 3.14 0.49 3.09 0.47 9.22 1.27 15. 9 14.1 4.72 7.45 0.58 5.79 2.29 0.31 2.37
NX24 NX23TOC – Total NX22 NX21CaO* – theNX20 NX19 NX16 of the rock. NX15 TC – Total Carbon, Organic Carbon. amount of CaO incorporated NX17 in the silicate fraction
PAAS – post-Archean Australian shale. wt.%
NX12 58.23 0.72 12.10 4.15 0.07 1.80 9.28 0.75 2.84 0.11 10.59 0.30 1.96 0.034 16.0 100.64
11.6 4959 80.3 51.6 593 10.9 25.8 40.7 82.3 17.2 1.99 157 167 35.8 236 20.0 20.0 349 45.6 90.0 10.9 39.2 7.51 1.38 6.51 1.04 6.42 1.32 3.67 0.57 3.51 0.53 6.18 1.56 29.4 18.8 5.37 8.40 0.59 3.90 1.60 0.36 1.71 PAAS
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Table 1 (continued) NX24 61.77 0.62 10.65 3.16 0.07 1.48 8.91 1.12 2.16 0.10 9.25 0.06 1.93 0.033 15.8 99.29 ppm 8.19 4003 61.4 32.7 594 8.39 16.4 28.9 58.3 13.4 1.66 117 130 32.2 298 17.5 17.2 266 39.1 74.9 9.19 31.9 5.97 1.07 5.03 0.77 4.82 0.93 2.68 0.40 2.47 0.38 7.66 1.40 18.7 16.9 3.65 10.3 0.59 4.78 2.06 0.51 2.01
NX23 58.60 0.74 12.28 4.21 0.07 1.89 8.58 0.95 2.61 0.13 9.29 0.35 1.80 0.036 14.6 99.35
11.1 4820 91.6 92.5 554 11.4 32.2 30.1 124 17.2 2.03 151 131 32.6 260 22.1 19.1 345 46.7 90.1 11.0 38.7 7.32 1.28 6.08 0.94 5.99 1.18 3.36 0.51 3.05 0.47 6.74 1.79 25.5 20.5 4.44 9.88 0.58 4.18 1.84 0.22 1.79
NX22 56.98 0.64 12.64 4.22 0.07 1.97 9.23 0.90 2.84 0.21 9.69 0.38 1.93 0.032 15.8 99.39
11.0 4230 78.1 41.1 581 12.0 22.6 27.2 85.2 17.1 1.97 155 157 28.9 248 18.2 22.5 535 41.5 80.0 9.66 34.5 6.67 1.22 5.75 0.87 5.31 1.04 2.94 0.44 2.67 0.43 6.43 1.42 23.2 17.1 4.11 10.0 0.60 3.75 1.54 0.41 1.42
NX21 53.59 0.61 12.64 4.25 0.07 2.13 10.78 0.84 2.78 0.13 11.83 0.20 2.30 0.033 18.8 99.65
11.6 4011 83.2 60.3 624 12.8 30.9 24.6 104 17.3 1.85 154 167 26.8 174 17.2 20.9 516 38.8 74.2 9.02 31.8 6.05 1.17 5.14 0.82 5.14 0.99 2.78 0.41 2.59 0.41 4.59 1.32 26.1 16.0 3.61 9.69 0.63 3.34 1.38 0.26 1.25
NX20 70.37 0.77 11.32 4.22 0.03 1.57 2.77 0.85 2.96 0.08 5.71 0.61 0.498 0.035 3.85 100.65
8.81 4577 64.8 39.3 192 6.86 16.1 20.4 38.6 15.1 6.86 367 79.1 30.5 344 20.6 102 369 39.5 76.6 9.32 33.1 6.45 1.10 5.29 0.88 5.59 1.11 3.17 0.49 3.14 0.48 9.10 1.63 19.7 17.2 4.77 8.15 0.57 4.49 1.96 0.43 2.51
NX19 66.70 0.70 10.50 3.70 0.03 1.37 6.30 1.02 2.50 0.09 7.72 0.51 1.28 0.040 10.3 100.63
8.25 4413 57.6 36.6 236 6.03 14.3 11.3 38.0 14.6 12.3 229 141 26.9 297 20.2 83.8 309 38.9 72.4 9.05 32.0 6.06 0.99 5.01 0.77 4.90 1.00 2.76 0.44 2.71 0.41 7.75 1.75 15.3 16.5 3.48 9.28 0.54 4.72 2.00 0.45 2.74
NX17 52.88 0.67 15.88 5.61 0.04 2.86 6.77 0.53 4.28 0.14 9.76 0.52 1.44 0.100 11.1 99.42
14.1 4436 109 57.7 402 12.2 31.1 17.3 98.4 24.3 3.45 384 124 31.9 164 21.6 120 421 46.4 90.1 11.0 40.0 7.98 1.42 6.38 0.99 5.89 1.14 3.15 0.46 2.82 0.44 4.27 1.84 21.7 21.9 3.47 10.3 0.59 3.29 1.55 0.38 1.79
NX16 51.88 0.65 12.25 4.32 0.07 3.05 11.14 0.74 2.91 0.10 13.52 0.44 2.55 0.258 19.1 100.63
11.9 4542 89.8 55.5 648 11.6 33.0 40.3 114 18.9 2.90 226 181 29.2 185 21.0 74.9 358 43.4 83.0 10.0 35.8 6.56 1.14 5.20 0.86 5.31 1.02 2.92 0.45 2.71 0.42 4.77 1.73 24.1 19.9 4.03 10.6 0.58 3.63 1.67 0.35 1.71
NX15 56.99 0.64 14.62 4.98 0.05 2.64 6.17 0.91 3.40 0.14 9.26 0.44 1.38 0.152 10.2 99.80
12.6 4226 90.6 98.8 452 13.1 36.2 22.3 130 20.5 2.52 223 116 29.5 178 21.2 56.3 1690 43.2 83.7 10.0 36.0 7.01 1.06 5.74 0.90 5.54 1.07 3.02 0.47 2.80 0.43 4.91 1.86 26.5 20.3 4.16 9.98 0.50 3.41 1.60 0.20 1.54
PAAS 62.80 1.00 18.90 6.50 0.11 2.20 1.30 1.20 3.70 0.16 – – – – – –
16 – 150 110 – 23 55 50 85 20 – 160 200 27 210 19 – 650 38.2 79.6 8.83 33.9 5.55 1.08 4.66 0.77 4.68 0.99 2.85 0.40 2.82 0.44 5 – 20 14.6 3.1 – – 2.38 0.91 0.13 0.63
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and mudstone with siltstone interbeds and enriched in small calcareous concretions, ostracods, charophytes, bivalves, gastropods, vertebrate and dinosaurian eggs. The ostracod fauna are characterized by Porpocypris sphaeroidalis and Parailyocypris taizhouensis. The charophyte fauna are characterized by Latochara curtula–Grovesichara changzhouensis (Zhang et al., 2000). 2.5. Shanghu Formation The Shanghu Formation is distinct from the underlying Pingling Formation in that the sedimentation is dominantly lacustrine facies, fine-grained clastic material with restricted fluvial facies, which dominantly comprise of brown-red muddy siltstone and silty mudstone with sandstone, conglomerate interbeds and common bigger calcareous concretions (Yang, 1998; Zhao and Yan, 2000), ostracods, charophytes, bivalves and gastropods. The ostracod fauna are characterized by Cypris jiangxiensis. Mammal fossils have been discovered, but no dinosaurian eggs have been found in this formation. 2.6. Nongshan Formation The Nongshan Formation is dominantly by lacustrine facies and consists of yellowish-green and dark-gray calcareous mudstone with brown-red muddy siltstone and silty mudstone interbeds, rich in ostracods, charophytes and gastropods. 3. Sample description and analytical method In total 21 samples were collected from the Nanxiong Basin (Fig. 1). Most of the samples are claystone except for sample NX9, which is a siltstone. Major oxide compositions of the samples were determined at the Guangzhou Institute of Geochemistry, Chinese Academy of
Fig. 3. PAAS-normalized REE patterns for the samples from the Nanxiong Basin (PAAS composition from Taylor and McLennan, 1985).
Sciences using X-ray Fluorescence (XRF). Trace element analyses were carried out using a PE Elan 6000 inductively coupled plasma mass spectrometry (ICP-MS). GSD12, GSD9, GSR3, GSR1 and SY-4 were analyzed as standard in the analytical techniques. The analytical precision is better than 5% for the major elements and 5–10% for trace elements. Liu et al. (1996) describe relevant analytical details. In order to calculate the total organic carbon (TOC) and CaO⁎, which is the amount of CaO incorporated in the silicate fraction of the rock, the samples were washed by 1N HCl to remove carbonate, washed three times by de-ionized water and then dried. The C contents of the dry bulk samples (TC) and acid-leached samples (TOC) were measured on an Elementar CHN-O Analyzer in the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (Table 1). The C contents in carbonate can be calculated by subtracting the C contents in acid-leached samples from the C contents in bulk samples, and the CaO contents incorporated in the carbonate can be deduced from the C contents in carbonate. To examine the geochemical characteristics of the claystone from the Nanxiong Basin, enrichment factors
Fig. 2. Average of major and trace element enrichment factor plot (relative to PAAS) for the samples from the Nanxiong Basin (PAAS composition from Taylor and McLennan, 1985).
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for major and trace elements relative to PAAS (postArchean Australian shale; Taylor and McLennan, 1985) were determined. The enrichment factor (Ex⁎) for any element (x) is given by the following: Ex* ¼ ðCx sample=CAl sampleÞ=ðCx standard=CA1 standardÞ
Cx is concentration of the element and CAl is the concentration of Al. The Al normalization is utilized because concentrations of most elements (especially the water-immobile elements) in claystone show correlation with Al2O3, due to hydraulic sorting and absolute concentration can be disturbed by dilution (Bauluz et al., 2000). PAAS is used as the standard. Enrichment factor diagrams, summarized by taking average value are shown in Fig. 2. 4. Results Results of major, trace element analyses for the Nanxiong Basin are summarized in Table 1. 4.1. Major elements The SiO2 contents of most samples from the Nanxiong Basin range from 50 to 70 wt.% and are enriched relative to PAAS. The Al2O3 contents of the samples are all lower than of PAAS (18.9 wt.%), in contrast, the CaO contents are very high and remarkably higher than that of PAAS (1.3 wt.%). Most samples show that the Na2O and K2O contents are slightly enriched relative to PAAS, with average values of 0.96 ± 0.04 wt.% and 2.76 ± 0.54 wt.%.
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4.2. Trace elements 4.2.1. HFS elements and compatible elements Most samples from the Nanxiong Basin show that Th, Y, Nb and Ta contents are enriched relative to PAAS with enrichment factors of 2.04, 1.98, 2.29 and 1.58 respectively. In contrast, Sc, V, Cr, Co and Ni contents show variable degrees of depletion relative to PAAS. 4.2.2. Rare earth elements The enrichment factor relative to PAAS is shown in Fig. 3. The samples show the similar REE patterns to PAAS and enrichment relative to PAAS except for sample NX9, which displays an abnormally high REE content, especially HREE content. Sample NX9 is a siltstone (other samples are clays) and thus it is possible this data is anomalous and related to grain size. It is known that the chemical composition of sedimentary rocks can be influenced by hydraulic concentration of REE-rich weathering-resistant phases such as zircon, Cr-spinel, monazite, and apatite and these may produce irregular chemical variations in some trace elements (Cullers et al., 1979; Mass and McCulloch, 1991). Most of the samples show (La/Yb)N ratios higher than PAAS (8.77) and Eu/Eu⁎ ratios range from 0.5 to 0.6 with little variation, and lower than PAAS (0.66) (Fig. 4). 4.3. Alkalis and alkaline earths The Sr contents of most samples range from 100 to 200 ppm and exhibit random with strata (Fig. 4), probably
Fig. 4. Stratigraphic variation in Sr, Rb, Cs concentrations and (La/Yb)N, Eu/Eu⁎, ratios for the samples from the Nanxiong Basin.
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due to their occurrence in multiple mineral phases and are often affected by the carbonate content (Nesbitt and Young, 1982; Wei et al., 2004). This is consistent with the significant linear correlation between CaCO3 and Sr (r = 0.72). The Rb and Cs contents of most samples range from 100 to 300 ppm and 0 to 150 ppm respectively and show a clear shift in values at the Late Cretaceous– Paleocene boundary. The Rb and Cs contents of Upper Cretaceous samples are lower (b200 ppm and 50 ppm
Fig. 5. Discrimination diagram for sedimentary provenance (A) and sedimentary tectonic setting (B) of the samples from the Nanxiong Basin (after Bhatia, 1983; Roser and Korsch, 1988). F1 ¼ 30:638TiO2 =Al2 O3 –12:541Fe2 O3 ðtotalÞ=Al2 O3 þ 7:329MgO=Al2 O3 þ 12:031Na2 O=Al2 O3 þ 35:402K2 O=Al2 O3 –6:382
respectively) and show significant increases in values (up to ∼350 ppm and 100 ppm respectively) for Paleocene samples. 5. Discussion 5.1. Quartz dilution and provenance It is widely accepted that quartz dilution can signficantly influence the chemical composition of terrigenous sediments (Cullers, 1994). The concentration of most elements (especially the immobile elements) in sediments normally shows correlation with Al2O3 and SiO2, due to a sorting effect and the absolute concentration can be disturbed by dilution (Cullers, 1994). The claystone collected from the Nanxiong Basin for this study show limited grain size variation except for sample NX9, which is a siltstone. Consequently quartz dilution effects should be negligible. This is consistent with the poor correlations between SiO2 and some elements, such as SiO2–REEs (r = − 0.15), SiO2–Nb (r = − 0.20) and SiO2–U (r = − 0.11). In the discrimination diagram for sedimentary provenance (Bhatia and Crook, 1986; Roser and Korsch, 1988), samples plot in the field of quartzose sedimentary provenance (Fig. 5A). This consistent with the large negative Eu anomalies that Eu/Eu⁎ ratios range from 0.5 to 0.6 and higher Th/Sc, La/Sc, Th/Cr and Th/Co ratios relative to PAAS which values range from 1 to 3, 3 to 7, 0 to 0.5 and 0 to 5 respectively (Table 1). In the discrimination diagram of Bhatia (1983), samples from the Nanxiong Basin plot in the passive continental margin field (Fig. 5B). CaO was corrected by 1N HCl leaching and the CaO incorporated in the carbonate has been removed. In the La–Th–Sc discrimination diagram (Condie, 1993) the majority of samples plots in the continental margin field (Fig. 6) and exhibit no major change in source from the Late Cretaceous to the Early Cenozoic. 5.2. Paleoclimate
F2 ¼ 56:5TiO2 =Al2 O3 –10:879Fe2 O3 ðtotalÞ=Al2 O3 þ 30:875MgO=Al2 O3 –5:404Na2 O=Al2 O3 þ 11:112K2 O=Al2 O3 –3:89 F3 ¼ 0:303–0:0447SiO2–0:972TiO2 þ 0:008Al2 O3 –0:267Fe2 O3 þ 0:208FeO–3:082MnO þ 0:14MgO þ 0:195CaO þ 0:719Na2 O–0:032K2 O þ 7:51P2 O5 F4 ¼ 43:57–0:421SiO2 þ 1:988TiO2 –0:526Al2 O3 –0:551Fe2 O3 –1:61FeO þ 2:72MnO þ 0:881MgO–0:907CaO–0:177Na2 O–1:84K2 O þ 7:244P2 O5
Chemical weathering is an important mechanism driving elemental fractionation away from parental bedrock signatures (Nesbitt and Young, 1982). The extent of fractionation depends on bedrock and local weathering conditions tied to climate. Stronger chemical weathering is generally associated with warm and humid climates, whilst more arid climate is generally associated with relatively weak chemical weathering (Nesbitt and Young, 1982).
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geochemistry and mineralogy of siliclastic sediments, these results must be accepted with caution (Cox et al., 1995). In the discrimination diagram for sedimentary provenance (Fig. 5), the samples from the Nanxiong Basin plot in the field of quartzose sedimentary suggesting an obvious sedimentary recycling. Trace elements do not form mineral frameworks, but generally occur in association with clay minerals as sorbed particles on surfaces or included in interlayer cation sites; therefore, their occurrence in mudrocks is not strongly related to mineralogy and bulk composition and can give more reliable information about provenance and weathering (Cox et al., 1995).
Fig. 6. La–Th–Sc discrimination diagram for sedimentary tectonic setting of the samples from the Nanxiong Basin (after Condie, 1993). A = Oceanic island arc; B = continental island arc; C = active continental margin; D = passive continental margin.
5.2.1. CIA values and A–CN–K discrimination Chemical weathering strongly affects the majorelement geochemistry of siliciclastic sediments and labile cations (e.g., Ca2+, Na+, K+) are removed relative stable residual constituents (Al, Ti) during weathering (Fedo et al., 1995). The Chemical Index of Alteration (CIA), which measures the degree of weathering of feldspars relative to unweathered protoliths is a widely used chemical index to ascertain the degree of sourcearea weathering (Nesbitt and Young, 1982). The weathering trend also can be illustrated on an Al2O3, CaO⁎ + Na2O, K2O (A–CN–K) triangular plot, which is useful for evaluating and correcting the effects of Kmetasomatism and giving some information of the composition of the fresh source rock (Fedo et al., 1995) (where CaO⁎ is the amount of CaO incorporated in the silicate fraction of the rock). Samples from the Nanxiong Basin, plotted in A– CN–K space, define a distinct linear array that is quite different from the inferred weathering trend (Fig. 7 solid-line arrow). This suggests that some potassium metasomatism occurred in the claystones from the Nanxiong Basin, slightly lowering the CIA values (Fedo et al., 1995, 1997). Fig. 7 shows CIA reconstruction for the samples from the Nanxiong Basin, which range from 70 to 80. A straight line (Fig. 7 dashed-line) through the data projects onto the feldspar join at a point (Fig. 7 solid circle), which indicates the fresh composition (i.e., plagioclase to K-feldspar ratio) of parental bedrock. However, because geologic systems are not simple and the sedimentary recycling can change the major-element
5.2.2. Elemental ratios During chemical weathering Ti is released from primary minerals but is generally precipitated before it is transported out of the area of weathering (Nesbitt and Markovics, 1997) causing lower Ti concentrations relative to parental bedrock (Peuraniemi and Pulkkinen, 1993). Zr primarily resides in zircon, which is resistant to chemical weathering (Tole, 1985). As a result, Ti and Zr usually behave as conservative elements in a weathering profile, and thus Ti and Zr-normalized ratios help to track the behavior of elements during chemical weathering. For the samples from the Nanxiong Basin increases in Zr with SiO2 contents (r = 0.88) imply that small zircons exist in the sediments. In contrast, the Ti contents show no covariance with SiO2 content (r = − 0.11) and inter-sample variations in Ti are less than Zr; hence elements are normalized to Ti.
Fig. 7. The samples from the Nanxiong Basin plotted in A–CN–K space. Solid-line arrow represents predicted weathering trend; dashedline shows the effect of K addition. Solid circle represents feldspar proportion of fresh parental rocks.
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Nb and Ta are mostly concentrated in some accessory heavy minerals. Both heavy minerals and clays are important hosts for REEs, Y and Th in sedimentary rocks (Condie, 1991). These elements are often believed to be resistant to chemical weathering in sedimentary rocks (Taylor and McLennan, 1985) if chemical weathering is not extreme (Nesbitt, 1979; Nesbitt et al., 1980; Nesbitt and Markovics, 1997). The Th/Ti, La/Ti, Ta/Ti, Yb/Ti and Y/Ti ratios of the samples from the Nanxiong Basin are nearly constant, indicating behavior similar to Ti and are relatively unaffected by weathering (Fig. 8). Altogether the evidence from the discrimination diagrams for sedimentary provenance and tectonic setting show that Nanxiong Basin sedimentary rocks were derived from typical continental sources. Importantly, geochemical signatures (Eu/Eu⁎, Th/Ti, La/Ti, Ta/Ti, Yb/Ti and Y/Ti ratios of the clays) are nearly constant, suggesting the provenance of the Nanxiong Basin remained similar throughout the Late Cretaceous to Early Paleocene. Rb and Cs are very sensitive to climatic influences (Nesbitt et al., 1980) and may become leached during chemical weathering. However most of the dissolved Rb and Cs may be absorbed and fixed in secondary minerals (Weaver, 1967), which explains the fact that weathering products generally have higher Rb and Cs concentrations than parental bedrocks (Peuraniemi and Pulkkinen, 1993). In general, enhanced chemical weathering will result in higher Rb and Cs contents. The Rb/Ti and Cs/Ti ratios of samples from the Nanxiong Basin display an abrupt change at the Upper Cretaceous and Paleocene boundary. The ratios for the Upper Cretaceous sample are lower, but increase abruptly for the Lower Paleocene samples. The following factors may change Ti-normalized ratios in sediments: (1) change of sediment source area, (2) change of chemical weathering in source area, and (3) grain-size sorting during sediment transport. As discussed above, there are no obvious changes in sediment source areas during the Late Cretaceous to Paleocene. There is also no evidence to suggest hydraulic sorting effects significantly influence elemental ratios as most samples have the same clay fraction grain size. This leaves chemical weathering as the most likely explanation for the observed changes in Rb/Ti and Cs/Ti ratios and element records. 5.2.3. Carbonate and TOC contents The composition and source of carbonates and their relationship to the physical and chemical characteristics of lake water have been discussed widely and it has been shown that the CaCO3 contents in lacustrine sediments can be used as a basic proxy for reconstructing paleoclimate (Robbins and Blackwelder, 1982; Millero and
Roy, 1997). Normally, low CaCO3 contents reflect a warm-humid climate, while high CaCO3 contents reflect a cold-dry climate (Muller et al., 1972; Dean, 1999). TOC contents in lacustrine sediments can be used as a proxy for primary biological production in a lake (Sampei et al., 1997) controlled by precipitation and/or temperature (Kumon, 2003). Hence, sequential changes in TOC contents in lacustrine sediments are useful as a proxy for reconstructing changes in paleoclimate (Yamada, 2004). Generally, to reconstruct independent paleoenvironmental changes, consideration must be given to changes in mass accumulation rate. According to Erben et al. (1995), sedimentation rates in the Nanxiong Basin were ∼40 cm/ ka throughout the period spanning the K/T boundary. It can be concluded that both the contents (wt.%) and the mass accumulation rates of TOC have similar tendencies in the Nanxiong Basin. Hence, TOC and CaCO3 contents in lacustrine sediments of the Nanxiong Basin can be a proxy for reconstructing paleoclimatic changes. CaCO3 contents of the samples from the Nanxiong Basin show similar changes to Rb/Ti and Cs/Ti ratios, with higher values during the Late Cretaceous and lower values in the Early Paleocene (Fig. 8). The total organic carbon (TOC) of samples shows very low values during the Late Cretaceous (most are less than 0.05%), but increase (to more than 0.1%) in the upper Lower Early Paleocene (Fig. 8). The change in TOC values is not entirely coincident with the shifts in Rb/Ti and Cs/Ti ratios, which occur at a higher stratigraphic level. TOC value is thought to reflect change in vegetation cover that is generally ahead of weathering responses to climate change (Kumon, 2003). 5.2.4. Paleoclimate As chemical weathering on continents is largely controlled by moisture and temperature, a wet and warm climate may enhance the chemical weathering. The high CaCO3 contents and low TOC values indicate that a predominantly arid climate occurred in South China during the Late Cretaceous to Early Paleocene. Variation of Rb/Ti, Cs/Ti ratios, CaCO3 contents and TOC values imply the climate underwent an obvious change around the time of the K/T boundary. Singularly higher CaCO3 contents and lower TOC values and Rb/Ti, Cs/Ti ratios in Upper Cretaceous sample indicate that an extreme dry climate occurred during the Late Cretaceous in South China, and that the vegetation cover around the Nanxiong Basin was arid. This model is supported by the evidence from stable carbon and oxygen isotopes in dinosaur eggshells collected from the Nanxiong Basin (Zhao and Yan, 2000). The long period extreme dry climate during the Late Cretaceous is consistent with the
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Fig. 8. Variations of Ti-normalized ratios of some major and trace elements and the total organic carbon (TOC), CaCO3 values for the samples from the Nanxiong Basin.
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extinction of the dinosaur population and a significant floral change at the K/T boundary. Many well-preserved nests of dinosaur eggs and eggshell fragments were discovered in the sediments of Upper Cretaceous Pingling Formation, but no evidence has been found in Lower Paleocene Shanghu Formation (Zhao et al., 2002). The ostracod fauna across the K/T boundary is distinct in that the Pingling Formation is dominated by P. sphaeroidalis and P. taizhouensis and the charophyte fauna are characterized by L. curtula–G. changzhouensis. The ostracod fauna in the Shanghu Formation are represented by C. jiangxiensis, while the charophyte fauna are characterized by Nemegtichara prime. However the alternation of different biomes did not occur instantaneously at the K/T transition. Some typical Early Paleocene biomes already appear in the upper section of the Nanxiong Group, such as Hornichara lagenalis and Eucypris (Tong et al., 2002). Furthermore, the extinction of the dinosaurs in the Nanxiong Basin did not occur instantaneously, but was spread out within 250 ka of the boundary with major extinction beginning in the upper section of the Nanxiong Group (Zhao et al., 2002). These evidences together with the variation of Rb/Ti, Cs/Ti ratios, CaCO3 contents and TOC values from the Upper Cretaceous to the Lower Paleocene indicate that the extreme dry climate in the Nanxiong Basin did not occur instantaneously, but started from the upper section of the Nanxiong Group and lasted a long time, synchronous with the gradual extinction of the dinosaurs and the alternation of different biomes at the K/T boundary. Rb/Ti, Cs/Ti ratios and TOC values escalated and CaCO3 contents decreased in the Lower Paleocene suggesting that the climate became relatively wet, which result in greater vegetation cover. 6. Conclusions Geochemical signatures of basin clastic sedimentary rocks provide important sources of information that record different aspects of the basin provenance, tectonic, environmental and ecological evolution. Whilst trace element geochemical studies have tended to focus on aspects of sediment provenance, their application to paleoclimate reconstructions has been relatively neglected. However clastic sedimentary trace element records may contain additional new paleoclimate information that provides important new constrain on sediment depositional environment and climate. Rb/Ti, Cs/Ti ratios and TOC, CaCO3 values of the sediments from the Nanxiong Basin require an obvious change in climate across the Upper Cretaceous and Lower Paleocene boundary. Singularly higher CaCO3 contents
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Geochemistry of the sedimentary rocks from the Nanxiong Basin, South China and implications for provenance, paleoenvironment and paleoclimate at the K/T boundary Yi Yan a,⁎, Bin Xia a , Ge Lin a , Xuejun Cui a , Xiaoqiong Hu a , Pin Yan b , Faqiang Zhang c a
b
Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou, 510640, PR China Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, 510301, PR China c Research Institute of Petroleum Processing, Beijing, 100083, PR China Received 12 March 2006; received in revised form 17 September 2006; accepted 25 September 2006
Abstract Cretaceous and Tertiary clastic sedimentary rocks from the Nanxiong Basin, South China have been analyzed to constrain their provenance, depositional climate and environment. Evidence from discrimination diagrams for sedimentary provenance and tectonic setting show that the Nanxiong Basin sediments were derived from typical continental sources. Geochemical signatures (e.g. Eu/Eu⁎, Th/Ti, La/Ti, Ta/Ti, Yb/Ti and Y/Ti ratios of the claystone) are nearly constant, suggesting the provenance of the Nanxiong Basin remained similar throughout the Late Cretaceous to Early Paleocene (83–56 Ma). In contrast Rb/Ti, Cs/Ti ratios and TOC and CaCO3 concentrations require an obvious change in climate across the Late Cretaceous and Early Paleocene boundary. Singularly higher CaCO3 contents and lower TOC values and Rb/Ti, Cs/Ti ratios in the Late Cretaceous indicate that a long period extreme dry climate occurred at that time in South China. Rb/Ti, Cs/Ti ratios and TOC values escalated and CaCO3 contents decreased in the Early Paleocene suggesting that the climate became relatively wet, which resulted in greater vegetation cover. The lasting extreme dry climate in the Late Cretaceous may provide a clue to the extinction of the dinosaurs in the Nanxiong Basin. © 2006 Elsevier B.V. All rights reserved. Keywords: South China; Nanxiong Basin; Geochemistry; Provenance; Paleoclimate
1. Introduction Geochemical signatures of clastic sedimentary rocks provide important sources of information that record different aspects of provenance, tectonic, environmental ⁎ Corresponding author. Tel.: +86 20 85290293. E-mail addresses: [email protected], [email protected] (Y. Yan). 0037-0738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2006.09.004
and ecological evolution. Although hydraulic sorting, weathering and diagenesis can alter the geochemical composition of basin sediments, there often remains a strong signature to the original source terrain, reflecting the nature of the exposed continental crust (Roser and Korsch, 1988; Rollinson, 1993). Significant improvement has been made in geochemical approaches to monitoring sediment provenance, especially through the
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introduction of discrimination diagrams based on the relationship of major and trace elements (Bhatia, 1983; Taylor and McLennan, 1985; Roser and Korsch, 1986; McLennan and Taylor, 1991; Condie, 1993; Cullers, 1994; Bauluz et al., 2000). Whilst trace element geochemical studies have tended to focus on aspects of sediment provenance, their application to paleoclimate reconstructions has been relatively neglected, with most paleoclimate studies focusing on biogenic components, such as carbonate, opal, organic carbon and authigenic elements (e.g. Elderfield, 1990; Murray and Leinen, 1993). Recent research has suggested that clastic sedimentary trace element records may contain additional new paleoclimate information that provides important new constrains on sediment depositional environments and climate (Wei et al., 2004). Some major and trace elements, such as alkali and alkali earth elements that are water mobile elements and very sensitive to climatic change, which can be used as a valuable proxy of paleoclimate evolution (Wei et al., 2004).
A major target of global paleoclimate studies has been the K/T boundary because this is linked to understanding the phenomena of vertebrate mass extinctions (e.g. Courtillot et al., 1988; Erben et al., 1995; Zhao and Yan, 2000; Zhao et al., 2002). In southern China there are extensive deposits of vertebrate-rich Upper Cretaceous– Cenozoic terrestrial clastic sedimentary rocks that are ideal for such studies. Discovery of large quantities of Cretaceous and Paleocene vertebrate fossils in the Nanxiong Basin in the early 1960s (Young, 1965; Zhao, 1975; Erben et al., 1995), earmarked this basin as a key site. Over the last 30 years, multidisciplinary studies of the K/T boundary in the Nanxiong Basin (Young, 1965; Yang et al., 1993; Erben et al., 1995; Zhao et al., 1998; Zhao and Yan, 2000) have been carried out. Stable carbon and oxygen isotopes were analyzed in dinosaur eggshell samples, collected in two sections of the Nanxiong Basin (Zhao and Yan, 2000). These revealed multiple positive δ18O perturbations during the K/T transition that suggest at least three periods of extreme dry climate with mean
Fig. 1. (A) Regional tectonic map of South China (after Davis et al., 1997); (B) Geological map and locations of profiles in the Nanxiong Basin; (C) Stratigraphic location of the samples from the Nanxiong Basin (Stratigraphy age from Zhang et al., 2000 and Bureau of Geology and Mineral Resources of Guangdong Province, 1988).
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annual air temperatures N 27 °C, similar to the range of 26.6–33.9 °C calculated by Yang et al. (1993) based on δ18O of carbonate rocks (middle and upper Pingling Formation). Although this evidence suggests a drastic environmental change occurred in the Nanxiong Basin during the K/T transition, published stable isotope data have not produced a clear picture because they are fragmentary and speculative and give only limited information (Zhao and Yan, 2000). Whether the drastic environmental change occurred instantaneously or gradually remains unclear (Erben et al., 1995; Zhao and Yan, 2000). In this study, we use major and trace signatures of the bulk sedimentary rocks that span the Cretaceous–Tertiary boundary in the Nanxiong Basin to investigate the behavior of different elements and find more robust constraints on the climatic conditions in South China during the K/T transition. 2. Geological background The Nanxiong Basin belongs to the South China Block, which comprises the Yangtze Craton in the north and Cathysia Block in the south (Fig. 1). The bulk of the Yangtze Craton is Proterozoic–Late Archaean (Ames et al., 1996) formed of mainly low-grade clastic metasedimentary rocks with minor volcanic rocks (Li and McCulloch, 1996) and a non-metamorphic shelf to continental sedimentary sequence that spans the Neoproterozoic to Triassic. From the Late Triassic onwards, sedimentation on the Yangtze Craton was dominated by continental red-beds (Chen and Jahn, 1998). The Cathaysia Block consists of Precambrian basement and a Sinian to Mesozoic sedimentary and volcanic cover. The lower part of the sedimentary cover (Sinian to lower Palaeozoic) is represented by a thick series of folded and slightly metamorphosed marine sedimentary rocks (Ling et al., 1996). From the Devonian onwards, Cathaysia appears to share the same stratigraphy as the Yangtze Craton (Chen and Jahn, 1998). During the Late Jurassic to Cretaceous the region was an active Andeantype magmatic arc subject to block faulting and granite emplacement (Xu, 1990). Since the Late Cretaceous, related to backarc opening (Hawkins et al., 1990), and in its early stages to lithospheric extension of the Cathaysia Block in South China (Chung et al., 1997), a series of NE-trending faults and small continental basins formed. The Nanxiong Basin, developed on pre-Jurassic basement, is a small intermontane basin situated in northern Guangdong Province, South China. The basin is elongated with its axis oriented northeast–southwest (Fig. 1). The topography within the basin is a hilly terrain, with rolling hills less than 50 m high. The Upper
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Cretaceous to Tertiary sedimentary rocks attain a maximum thickness of over 7000 m. Typical basin fill comprises red fluvial-lacustrine clastic sequences that span the Late Cretaceous, represented by the dinosaurbearing Nanxiong Group (Dafeng, Zhutian, Zhenshui and Pingling Formations) and Early Paleocene, represented by the mammal-bearing Luofuzai Group (Shanghu and Nongshan Formations) (Yang, 1998; Zhao and Yan, 2000). These formations are described in more detail below; 2.1. Dafeng Formation Unconformably overlying Paleozoic–Mesozoic granites in the south of the Nanxiong Basin, the Dafeng Formation comprises brown-red coarse-grained clastic sedimentary rocks. Sedimentation is mainly in the form of fluvial–lacustrine red beds and consists of conglomerates, sandstones, with silty mudstone interbeds. Fewer ostracod, such as Cyprois sp. were found in this formation (Ling et al., 2005). 2.2. Zhutian Formatiom The Zhutian Formation is characterized by lacustrine facies fine-grained clastic sedimentary rocks comprise of brown-red, dark-purple muddy siltstone and silty mudstone with yellowish-green calcareous mudstone interbeds. Large quantities of ostracods and charophytes, and minor amount of foraminifers, insects, estheria, bivalves and gastropods were discovered. 2.3. Zhenshui Formation Sedimentation of the Zhenshui Formation is composed of two parts: the lower part of the formation is dominantly in the form of lacustrine facies fine-grained clastic material with restricted fluvial, which mainly comprised of brownred, dark-purple siltstone and silty mudstone with sandstone and conglomerate interbeds; sedimentation of the upper part of the formation is mainly in the form of fluvial–lacustrine red beds and comprises brown-red coarse-grained clastic sedimentary rocks with silty mudstone interbeds. This formation is enriched in vertebrate and dinosaurian eggs and minor amount of ostracods, charophytes, bivalves and gastropods. 2.4. Pingling Formation The sediments of the Pingling Formation are dominantly coarse-grained fluvial with restricted lacustrine facies, comprising purple-red sandstones, conglomerates
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Table 1 XRF data for the major elements (wt.%), ICP-MS data for the trace elements (ppm) and the total organic carbon (TOC) values of the samples from the Nanxiong Basin (PAAS from Taylor and McLennan, 1985) No. wt.% SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI CaO⁎ TC TOC CaCO3 Total ppm Sc Ti V Cr Mn Co Ni Cu Zn Ga Ge Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U La/Yb(N) Eu/Eu⁎ La/Sc Th/Sc Th/Cr Th/Co
NX1 52.48 0.64 12.36 4.17 0.08 2.28 11.26 0.75 2.85 0.17 12.59 0.19 2.42 0.049 19.7 99.63
10.7 4038 121 41.8 622 10.7 23.3 58.3 178 15.3 1.54 135 192 30.8 185 17.4 17.5 258 41.9 77.9 9.71 34.7 6.48 1.24 5.70 0.90 5.74 1.15 3.12 0.47 2.86 0.45 5.07 1.55 26.4 16.6 6.26 9.49 0.62 3.90 1.54 0.39 1.55
NX2 58.58 0.58 8.78 1.25 0.12 1.61 13.07 1.06 1.66 0.12 12.47 0.00 2.86 0.045 23.4 99.30
6.70 3447 171 24.0 863 7.61 15.7 56.0 56.2 9.81 1.35 79.9 198 25.6 264 15.0 9.79 200 32.7 63.2 7.80 28.4 5.43 0.98 4.51 0.75 4.74 0.96 2.68 0.41 2.56 0.39 6.76 1.22 8.86 14.0 13.9 8.28 0.60 4.88 2.08 0.58 1.83
NX3 56.81 0.65 14.16 5.11 0.09 2.67 7.08 1.17 3.17 0.19 9.55 0.44 1.46 0.036 11.8 100.65
12.5 3989 77.7 113 778 14.0 31.3 34.0 107 20.0 1.91 177 157 29.9 172 21.5 20.3 731 48.5 98.3 11.4 39.4 7.63 1.14 6.04 0.96 5.99 1.13 3.12 0.48 3.03 0.46 4.81 2.41 28.6 24.9 7.33 10.4 0.50 3.86 1.98 0.21 1.77
NX4 54.54 0.68 14.98 5.54 0.11 2.59 6.86 0.88 3.67 0.26 9.87 0.49 1.40 0.034 11.3 99.98
14.5 4355 99.0 69.3 867 14.0 32.3 36.6 148 23.0 2.27 216 154 35.5 197 22.5 24.7 359 53.0 107 12.5 44.1 8.57 1.39 6.83 1.11 6.78 1.30 3.68 0.56 3.39 0.52 5.28 2.09 29.4 27.1 7.35 10.1 0.54 3.63 1.86 0.39 1.93
NX5 66.03 0.53 8.00 0.91 0.14 1.27 10.73 1.30 1.35 0.09 10.26 0.00 2.34 0.033 19.2 100.61
– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
NX6 59.65 0.70 14.70 5.08 0.08 2.13 5.19 1.28 3.31 0.21 8.19 0.62 1.01 0.029 8.16 100.52
12.5 4239 114 64.3 606 11.6 26.2 28.9 139 19.7 1.88 184 117 41.8 259 22.8 20.0 372 55.6 102 12.5 43.0 8.24 1.18 6.52 1.03 6.36 1.27 3.52 0.53 3.23 0.50 6.97 2.20 31.2 28.1 7.63 11.1 0.48 4.43 2.24 0.43 2.43
NX7 58.42 0.74 13.35 4.88 0.10 2.40 7.10 1.09 2.97 0.16 9.13 0.30 1.49 0.032 12.1 100.34
11.9 4538 74.1 59.3 739 13.6 26.9 29.9 97.4 18.2 1.85 154 213 30.7 196 20.6 18.3 562 46.7 91.6 11.0 38.4 7.71 1.30 6.26 0.97 5.84 1.15 3.20 0.48 3.04 0.46 5.25 1.71 27.1 21.9 6.28 9.95 0.56 3.92 1.83 0.36 1.61
NX8 55.71 0.73 15.48 5.63 0.08 2.67 6.21 0.62 3.81 0.15 9.57 0.38 1.28 0.030 10.4 100.66
13.9 4436 83.9 152 596 14.1 32.5 25.2 100 20.3 1.89 184 159 26.2 130 18.7 24.1 610 45.3 88.0 10.4 36.5 7.02 1.33 5.75 0.86 5.15 0.99 2.67 0.40 2.62 0.38 2.97 1.46 28.3 19.1 5.98 11.1 0.63 3.26 1.37 0.12 1.35
NX9 62.45 0.65 11.58 3.49 0.10 1.53 7.54 1.11 2.55 0.08 9.02 0.30 1.58 0.027 12.9 100.10
8.75 3948 55.7 52.9 802 9.74 20.1 25.8 64.7 14.9 1.87 130 119 106 300 17.5 13.3 240 59.4 121 13.7 53.5 12.2 2.89 15.9 2.67 17.2 3.48 9.40 1.31 8.47 1.27 8.07 1.46 22.3 17.4 3.13 4.54 0.64 6.79 1.99 0.32 1.79
NX10 59.64 0.59 9.34 2.68 0.11 1.48 11.51 1.19 1.83 0.10 11.35 0.06 2.49 0.037 20.4 99.82
7.06 3522 79.6 11.2 848 7.51 12.8 39.4 45.6 11.0 1.36 88.8 209 23.1 240 14.9 9.02 271 33.4 65.2 7.96 28.6 5.47 0.96 4.34 0.72 4.34 0.83 2.36 0.36 2.25 0.36 6.18 1.19 16.5 14.2 5.83 9.59 0.59 4.72 2.01 1.27 1.89
NX11 70.71 0.59 7.75 1.96 0.14 1.02 7.97 1.16 1.48 0.07 7.85 0.10 1.72 0.033 14.0 100.70
6.13 3769 46.6 45.1 1052 5.94 13.8 41.1 48.6 9.42 1.57 77.9 112 31.2 355 16.0 8.42 183 35.5 64.8 8.42 30.6 6.22 1.12 5.38 0.87 5.54 1.09 3.14 0.49 3.09 0.47 9.22 1.27 15. 9 14.1 4.72 7.45 0.58 5.79 2.29 0.31 2.37
NX24 NX23TOC – Total NX22 NX21CaO* – theNX20 NX19 NX16 of the rock. NX15 TC – Total Carbon, Organic Carbon. amount of CaO incorporated NX17 in the silicate fraction
PAAS – post-Archean Australian shale. wt.%
NX12 58.23 0.72 12.10 4.15 0.07 1.80 9.28 0.75 2.84 0.11 10.59 0.30 1.96 0.034 16.0 100.64
11.6 4959 80.3 51.6 593 10.9 25.8 40.7 82.3 17.2 1.99 157 167 35.8 236 20.0 20.0 349 45.6 90.0 10.9 39.2 7.51 1.38 6.51 1.04 6.42 1.32 3.67 0.57 3.51 0.53 6.18 1.56 29.4 18.8 5.37 8.40 0.59 3.90 1.60 0.36 1.71 PAAS
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Table 1 (continued) NX24 61.77 0.62 10.65 3.16 0.07 1.48 8.91 1.12 2.16 0.10 9.25 0.06 1.93 0.033 15.8 99.29 ppm 8.19 4003 61.4 32.7 594 8.39 16.4 28.9 58.3 13.4 1.66 117 130 32.2 298 17.5 17.2 266 39.1 74.9 9.19 31.9 5.97 1.07 5.03 0.77 4.82 0.93 2.68 0.40 2.47 0.38 7.66 1.40 18.7 16.9 3.65 10.3 0.59 4.78 2.06 0.51 2.01
NX23 58.60 0.74 12.28 4.21 0.07 1.89 8.58 0.95 2.61 0.13 9.29 0.35 1.80 0.036 14.6 99.35
11.1 4820 91.6 92.5 554 11.4 32.2 30.1 124 17.2 2.03 151 131 32.6 260 22.1 19.1 345 46.7 90.1 11.0 38.7 7.32 1.28 6.08 0.94 5.99 1.18 3.36 0.51 3.05 0.47 6.74 1.79 25.5 20.5 4.44 9.88 0.58 4.18 1.84 0.22 1.79
NX22 56.98 0.64 12.64 4.22 0.07 1.97 9.23 0.90 2.84 0.21 9.69 0.38 1.93 0.032 15.8 99.39
11.0 4230 78.1 41.1 581 12.0 22.6 27.2 85.2 17.1 1.97 155 157 28.9 248 18.2 22.5 535 41.5 80.0 9.66 34.5 6.67 1.22 5.75 0.87 5.31 1.04 2.94 0.44 2.67 0.43 6.43 1.42 23.2 17.1 4.11 10.0 0.60 3.75 1.54 0.41 1.42
NX21 53.59 0.61 12.64 4.25 0.07 2.13 10.78 0.84 2.78 0.13 11.83 0.20 2.30 0.033 18.8 99.65
11.6 4011 83.2 60.3 624 12.8 30.9 24.6 104 17.3 1.85 154 167 26.8 174 17.2 20.9 516 38.8 74.2 9.02 31.8 6.05 1.17 5.14 0.82 5.14 0.99 2.78 0.41 2.59 0.41 4.59 1.32 26.1 16.0 3.61 9.69 0.63 3.34 1.38 0.26 1.25
NX20 70.37 0.77 11.32 4.22 0.03 1.57 2.77 0.85 2.96 0.08 5.71 0.61 0.498 0.035 3.85 100.65
8.81 4577 64.8 39.3 192 6.86 16.1 20.4 38.6 15.1 6.86 367 79.1 30.5 344 20.6 102 369 39.5 76.6 9.32 33.1 6.45 1.10 5.29 0.88 5.59 1.11 3.17 0.49 3.14 0.48 9.10 1.63 19.7 17.2 4.77 8.15 0.57 4.49 1.96 0.43 2.51
NX19 66.70 0.70 10.50 3.70 0.03 1.37 6.30 1.02 2.50 0.09 7.72 0.51 1.28 0.040 10.3 100.63
8.25 4413 57.6 36.6 236 6.03 14.3 11.3 38.0 14.6 12.3 229 141 26.9 297 20.2 83.8 309 38.9 72.4 9.05 32.0 6.06 0.99 5.01 0.77 4.90 1.00 2.76 0.44 2.71 0.41 7.75 1.75 15.3 16.5 3.48 9.28 0.54 4.72 2.00 0.45 2.74
NX17 52.88 0.67 15.88 5.61 0.04 2.86 6.77 0.53 4.28 0.14 9.76 0.52 1.44 0.100 11.1 99.42
14.1 4436 109 57.7 402 12.2 31.1 17.3 98.4 24.3 3.45 384 124 31.9 164 21.6 120 421 46.4 90.1 11.0 40.0 7.98 1.42 6.38 0.99 5.89 1.14 3.15 0.46 2.82 0.44 4.27 1.84 21.7 21.9 3.47 10.3 0.59 3.29 1.55 0.38 1.79
NX16 51.88 0.65 12.25 4.32 0.07 3.05 11.14 0.74 2.91 0.10 13.52 0.44 2.55 0.258 19.1 100.63
11.9 4542 89.8 55.5 648 11.6 33.0 40.3 114 18.9 2.90 226 181 29.2 185 21.0 74.9 358 43.4 83.0 10.0 35.8 6.56 1.14 5.20 0.86 5.31 1.02 2.92 0.45 2.71 0.42 4.77 1.73 24.1 19.9 4.03 10.6 0.58 3.63 1.67 0.35 1.71
NX15 56.99 0.64 14.62 4.98 0.05 2.64 6.17 0.91 3.40 0.14 9.26 0.44 1.38 0.152 10.2 99.80
12.6 4226 90.6 98.8 452 13.1 36.2 22.3 130 20.5 2.52 223 116 29.5 178 21.2 56.3 1690 43.2 83.7 10.0 36.0 7.01 1.06 5.74 0.90 5.54 1.07 3.02 0.47 2.80 0.43 4.91 1.86 26.5 20.3 4.16 9.98 0.50 3.41 1.60 0.20 1.54
PAAS 62.80 1.00 18.90 6.50 0.11 2.20 1.30 1.20 3.70 0.16 – – – – – –
16 – 150 110 – 23 55 50 85 20 – 160 200 27 210 19 – 650 38.2 79.6 8.83 33.9 5.55 1.08 4.66 0.77 4.68 0.99 2.85 0.40 2.82 0.44 5 – 20 14.6 3.1 – – 2.38 0.91 0.13 0.63
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and mudstone with siltstone interbeds and enriched in small calcareous concretions, ostracods, charophytes, bivalves, gastropods, vertebrate and dinosaurian eggs. The ostracod fauna are characterized by Porpocypris sphaeroidalis and Parailyocypris taizhouensis. The charophyte fauna are characterized by Latochara curtula–Grovesichara changzhouensis (Zhang et al., 2000). 2.5. Shanghu Formation The Shanghu Formation is distinct from the underlying Pingling Formation in that the sedimentation is dominantly lacustrine facies, fine-grained clastic material with restricted fluvial facies, which dominantly comprise of brown-red muddy siltstone and silty mudstone with sandstone, conglomerate interbeds and common bigger calcareous concretions (Yang, 1998; Zhao and Yan, 2000), ostracods, charophytes, bivalves and gastropods. The ostracod fauna are characterized by Cypris jiangxiensis. Mammal fossils have been discovered, but no dinosaurian eggs have been found in this formation. 2.6. Nongshan Formation The Nongshan Formation is dominantly by lacustrine facies and consists of yellowish-green and dark-gray calcareous mudstone with brown-red muddy siltstone and silty mudstone interbeds, rich in ostracods, charophytes and gastropods. 3. Sample description and analytical method In total 21 samples were collected from the Nanxiong Basin (Fig. 1). Most of the samples are claystone except for sample NX9, which is a siltstone. Major oxide compositions of the samples were determined at the Guangzhou Institute of Geochemistry, Chinese Academy of
Fig. 3. PAAS-normalized REE patterns for the samples from the Nanxiong Basin (PAAS composition from Taylor and McLennan, 1985).
Sciences using X-ray Fluorescence (XRF). Trace element analyses were carried out using a PE Elan 6000 inductively coupled plasma mass spectrometry (ICP-MS). GSD12, GSD9, GSR3, GSR1 and SY-4 were analyzed as standard in the analytical techniques. The analytical precision is better than 5% for the major elements and 5–10% for trace elements. Liu et al. (1996) describe relevant analytical details. In order to calculate the total organic carbon (TOC) and CaO⁎, which is the amount of CaO incorporated in the silicate fraction of the rock, the samples were washed by 1N HCl to remove carbonate, washed three times by de-ionized water and then dried. The C contents of the dry bulk samples (TC) and acid-leached samples (TOC) were measured on an Elementar CHN-O Analyzer in the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (Table 1). The C contents in carbonate can be calculated by subtracting the C contents in acid-leached samples from the C contents in bulk samples, and the CaO contents incorporated in the carbonate can be deduced from the C contents in carbonate. To examine the geochemical characteristics of the claystone from the Nanxiong Basin, enrichment factors
Fig. 2. Average of major and trace element enrichment factor plot (relative to PAAS) for the samples from the Nanxiong Basin (PAAS composition from Taylor and McLennan, 1985).
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for major and trace elements relative to PAAS (postArchean Australian shale; Taylor and McLennan, 1985) were determined. The enrichment factor (Ex⁎) for any element (x) is given by the following: Ex* ¼ ðCx sample=CAl sampleÞ=ðCx standard=CA1 standardÞ
Cx is concentration of the element and CAl is the concentration of Al. The Al normalization is utilized because concentrations of most elements (especially the water-immobile elements) in claystone show correlation with Al2O3, due to hydraulic sorting and absolute concentration can be disturbed by dilution (Bauluz et al., 2000). PAAS is used as the standard. Enrichment factor diagrams, summarized by taking average value are shown in Fig. 2. 4. Results Results of major, trace element analyses for the Nanxiong Basin are summarized in Table 1. 4.1. Major elements The SiO2 contents of most samples from the Nanxiong Basin range from 50 to 70 wt.% and are enriched relative to PAAS. The Al2O3 contents of the samples are all lower than of PAAS (18.9 wt.%), in contrast, the CaO contents are very high and remarkably higher than that of PAAS (1.3 wt.%). Most samples show that the Na2O and K2O contents are slightly enriched relative to PAAS, with average values of 0.96 ± 0.04 wt.% and 2.76 ± 0.54 wt.%.
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4.2. Trace elements 4.2.1. HFS elements and compatible elements Most samples from the Nanxiong Basin show that Th, Y, Nb and Ta contents are enriched relative to PAAS with enrichment factors of 2.04, 1.98, 2.29 and 1.58 respectively. In contrast, Sc, V, Cr, Co and Ni contents show variable degrees of depletion relative to PAAS. 4.2.2. Rare earth elements The enrichment factor relative to PAAS is shown in Fig. 3. The samples show the similar REE patterns to PAAS and enrichment relative to PAAS except for sample NX9, which displays an abnormally high REE content, especially HREE content. Sample NX9 is a siltstone (other samples are clays) and thus it is possible this data is anomalous and related to grain size. It is known that the chemical composition of sedimentary rocks can be influenced by hydraulic concentration of REE-rich weathering-resistant phases such as zircon, Cr-spinel, monazite, and apatite and these may produce irregular chemical variations in some trace elements (Cullers et al., 1979; Mass and McCulloch, 1991). Most of the samples show (La/Yb)N ratios higher than PAAS (8.77) and Eu/Eu⁎ ratios range from 0.5 to 0.6 with little variation, and lower than PAAS (0.66) (Fig. 4). 4.3. Alkalis and alkaline earths The Sr contents of most samples range from 100 to 200 ppm and exhibit random with strata (Fig. 4), probably
Fig. 4. Stratigraphic variation in Sr, Rb, Cs concentrations and (La/Yb)N, Eu/Eu⁎, ratios for the samples from the Nanxiong Basin.
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due to their occurrence in multiple mineral phases and are often affected by the carbonate content (Nesbitt and Young, 1982; Wei et al., 2004). This is consistent with the significant linear correlation between CaCO3 and Sr (r = 0.72). The Rb and Cs contents of most samples range from 100 to 300 ppm and 0 to 150 ppm respectively and show a clear shift in values at the Late Cretaceous– Paleocene boundary. The Rb and Cs contents of Upper Cretaceous samples are lower (b200 ppm and 50 ppm
Fig. 5. Discrimination diagram for sedimentary provenance (A) and sedimentary tectonic setting (B) of the samples from the Nanxiong Basin (after Bhatia, 1983; Roser and Korsch, 1988). F1 ¼ 30:638TiO2 =Al2 O3 –12:541Fe2 O3 ðtotalÞ=Al2 O3 þ 7:329MgO=Al2 O3 þ 12:031Na2 O=Al2 O3 þ 35:402K2 O=Al2 O3 –6:382
respectively) and show significant increases in values (up to ∼350 ppm and 100 ppm respectively) for Paleocene samples. 5. Discussion 5.1. Quartz dilution and provenance It is widely accepted that quartz dilution can signficantly influence the chemical composition of terrigenous sediments (Cullers, 1994). The concentration of most elements (especially the immobile elements) in sediments normally shows correlation with Al2O3 and SiO2, due to a sorting effect and the absolute concentration can be disturbed by dilution (Cullers, 1994). The claystone collected from the Nanxiong Basin for this study show limited grain size variation except for sample NX9, which is a siltstone. Consequently quartz dilution effects should be negligible. This is consistent with the poor correlations between SiO2 and some elements, such as SiO2–REEs (r = − 0.15), SiO2–Nb (r = − 0.20) and SiO2–U (r = − 0.11). In the discrimination diagram for sedimentary provenance (Bhatia and Crook, 1986; Roser and Korsch, 1988), samples plot in the field of quartzose sedimentary provenance (Fig. 5A). This consistent with the large negative Eu anomalies that Eu/Eu⁎ ratios range from 0.5 to 0.6 and higher Th/Sc, La/Sc, Th/Cr and Th/Co ratios relative to PAAS which values range from 1 to 3, 3 to 7, 0 to 0.5 and 0 to 5 respectively (Table 1). In the discrimination diagram of Bhatia (1983), samples from the Nanxiong Basin plot in the passive continental margin field (Fig. 5B). CaO was corrected by 1N HCl leaching and the CaO incorporated in the carbonate has been removed. In the La–Th–Sc discrimination diagram (Condie, 1993) the majority of samples plots in the continental margin field (Fig. 6) and exhibit no major change in source from the Late Cretaceous to the Early Cenozoic. 5.2. Paleoclimate
F2 ¼ 56:5TiO2 =Al2 O3 –10:879Fe2 O3 ðtotalÞ=Al2 O3 þ 30:875MgO=Al2 O3 –5:404Na2 O=Al2 O3 þ 11:112K2 O=Al2 O3 –3:89 F3 ¼ 0:303–0:0447SiO2–0:972TiO2 þ 0:008Al2 O3 –0:267Fe2 O3 þ 0:208FeO–3:082MnO þ 0:14MgO þ 0:195CaO þ 0:719Na2 O–0:032K2 O þ 7:51P2 O5 F4 ¼ 43:57–0:421SiO2 þ 1:988TiO2 –0:526Al2 O3 –0:551Fe2 O3 –1:61FeO þ 2:72MnO þ 0:881MgO–0:907CaO–0:177Na2 O–1:84K2 O þ 7:244P2 O5
Chemical weathering is an important mechanism driving elemental fractionation away from parental bedrock signatures (Nesbitt and Young, 1982). The extent of fractionation depends on bedrock and local weathering conditions tied to climate. Stronger chemical weathering is generally associated with warm and humid climates, whilst more arid climate is generally associated with relatively weak chemical weathering (Nesbitt and Young, 1982).
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geochemistry and mineralogy of siliclastic sediments, these results must be accepted with caution (Cox et al., 1995). In the discrimination diagram for sedimentary provenance (Fig. 5), the samples from the Nanxiong Basin plot in the field of quartzose sedimentary suggesting an obvious sedimentary recycling. Trace elements do not form mineral frameworks, but generally occur in association with clay minerals as sorbed particles on surfaces or included in interlayer cation sites; therefore, their occurrence in mudrocks is not strongly related to mineralogy and bulk composition and can give more reliable information about provenance and weathering (Cox et al., 1995).
Fig. 6. La–Th–Sc discrimination diagram for sedimentary tectonic setting of the samples from the Nanxiong Basin (after Condie, 1993). A = Oceanic island arc; B = continental island arc; C = active continental margin; D = passive continental margin.
5.2.1. CIA values and A–CN–K discrimination Chemical weathering strongly affects the majorelement geochemistry of siliciclastic sediments and labile cations (e.g., Ca2+, Na+, K+) are removed relative stable residual constituents (Al, Ti) during weathering (Fedo et al., 1995). The Chemical Index of Alteration (CIA), which measures the degree of weathering of feldspars relative to unweathered protoliths is a widely used chemical index to ascertain the degree of sourcearea weathering (Nesbitt and Young, 1982). The weathering trend also can be illustrated on an Al2O3, CaO⁎ + Na2O, K2O (A–CN–K) triangular plot, which is useful for evaluating and correcting the effects of Kmetasomatism and giving some information of the composition of the fresh source rock (Fedo et al., 1995) (where CaO⁎ is the amount of CaO incorporated in the silicate fraction of the rock). Samples from the Nanxiong Basin, plotted in A– CN–K space, define a distinct linear array that is quite different from the inferred weathering trend (Fig. 7 solid-line arrow). This suggests that some potassium metasomatism occurred in the claystones from the Nanxiong Basin, slightly lowering the CIA values (Fedo et al., 1995, 1997). Fig. 7 shows CIA reconstruction for the samples from the Nanxiong Basin, which range from 70 to 80. A straight line (Fig. 7 dashed-line) through the data projects onto the feldspar join at a point (Fig. 7 solid circle), which indicates the fresh composition (i.e., plagioclase to K-feldspar ratio) of parental bedrock. However, because geologic systems are not simple and the sedimentary recycling can change the major-element
5.2.2. Elemental ratios During chemical weathering Ti is released from primary minerals but is generally precipitated before it is transported out of the area of weathering (Nesbitt and Markovics, 1997) causing lower Ti concentrations relative to parental bedrock (Peuraniemi and Pulkkinen, 1993). Zr primarily resides in zircon, which is resistant to chemical weathering (Tole, 1985). As a result, Ti and Zr usually behave as conservative elements in a weathering profile, and thus Ti and Zr-normalized ratios help to track the behavior of elements during chemical weathering. For the samples from the Nanxiong Basin increases in Zr with SiO2 contents (r = 0.88) imply that small zircons exist in the sediments. In contrast, the Ti contents show no covariance with SiO2 content (r = − 0.11) and inter-sample variations in Ti are less than Zr; hence elements are normalized to Ti.
Fig. 7. The samples from the Nanxiong Basin plotted in A–CN–K space. Solid-line arrow represents predicted weathering trend; dashedline shows the effect of K addition. Solid circle represents feldspar proportion of fresh parental rocks.
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Nb and Ta are mostly concentrated in some accessory heavy minerals. Both heavy minerals and clays are important hosts for REEs, Y and Th in sedimentary rocks (Condie, 1991). These elements are often believed to be resistant to chemical weathering in sedimentary rocks (Taylor and McLennan, 1985) if chemical weathering is not extreme (Nesbitt, 1979; Nesbitt et al., 1980; Nesbitt and Markovics, 1997). The Th/Ti, La/Ti, Ta/Ti, Yb/Ti and Y/Ti ratios of the samples from the Nanxiong Basin are nearly constant, indicating behavior similar to Ti and are relatively unaffected by weathering (Fig. 8). Altogether the evidence from the discrimination diagrams for sedimentary provenance and tectonic setting show that Nanxiong Basin sedimentary rocks were derived from typical continental sources. Importantly, geochemical signatures (Eu/Eu⁎, Th/Ti, La/Ti, Ta/Ti, Yb/Ti and Y/Ti ratios of the clays) are nearly constant, suggesting the provenance of the Nanxiong Basin remained similar throughout the Late Cretaceous to Early Paleocene. Rb and Cs are very sensitive to climatic influences (Nesbitt et al., 1980) and may become leached during chemical weathering. However most of the dissolved Rb and Cs may be absorbed and fixed in secondary minerals (Weaver, 1967), which explains the fact that weathering products generally have higher Rb and Cs concentrations than parental bedrocks (Peuraniemi and Pulkkinen, 1993). In general, enhanced chemical weathering will result in higher Rb and Cs contents. The Rb/Ti and Cs/Ti ratios of samples from the Nanxiong Basin display an abrupt change at the Upper Cretaceous and Paleocene boundary. The ratios for the Upper Cretaceous sample are lower, but increase abruptly for the Lower Paleocene samples. The following factors may change Ti-normalized ratios in sediments: (1) change of sediment source area, (2) change of chemical weathering in source area, and (3) grain-size sorting during sediment transport. As discussed above, there are no obvious changes in sediment source areas during the Late Cretaceous to Paleocene. There is also no evidence to suggest hydraulic sorting effects significantly influence elemental ratios as most samples have the same clay fraction grain size. This leaves chemical weathering as the most likely explanation for the observed changes in Rb/Ti and Cs/Ti ratios and element records. 5.2.3. Carbonate and TOC contents The composition and source of carbonates and their relationship to the physical and chemical characteristics of lake water have been discussed widely and it has been shown that the CaCO3 contents in lacustrine sediments can be used as a basic proxy for reconstructing paleoclimate (Robbins and Blackwelder, 1982; Millero and
Roy, 1997). Normally, low CaCO3 contents reflect a warm-humid climate, while high CaCO3 contents reflect a cold-dry climate (Muller et al., 1972; Dean, 1999). TOC contents in lacustrine sediments can be used as a proxy for primary biological production in a lake (Sampei et al., 1997) controlled by precipitation and/or temperature (Kumon, 2003). Hence, sequential changes in TOC contents in lacustrine sediments are useful as a proxy for reconstructing changes in paleoclimate (Yamada, 2004). Generally, to reconstruct independent paleoenvironmental changes, consideration must be given to changes in mass accumulation rate. According to Erben et al. (1995), sedimentation rates in the Nanxiong Basin were ∼40 cm/ ka throughout the period spanning the K/T boundary. It can be concluded that both the contents (wt.%) and the mass accumulation rates of TOC have similar tendencies in the Nanxiong Basin. Hence, TOC and CaCO3 contents in lacustrine sediments of the Nanxiong Basin can be a proxy for reconstructing paleoclimatic changes. CaCO3 contents of the samples from the Nanxiong Basin show similar changes to Rb/Ti and Cs/Ti ratios, with higher values during the Late Cretaceous and lower values in the Early Paleocene (Fig. 8). The total organic carbon (TOC) of samples shows very low values during the Late Cretaceous (most are less than 0.05%), but increase (to more than 0.1%) in the upper Lower Early Paleocene (Fig. 8). The change in TOC values is not entirely coincident with the shifts in Rb/Ti and Cs/Ti ratios, which occur at a higher stratigraphic level. TOC value is thought to reflect change in vegetation cover that is generally ahead of weathering responses to climate change (Kumon, 2003). 5.2.4. Paleoclimate As chemical weathering on continents is largely controlled by moisture and temperature, a wet and warm climate may enhance the chemical weathering. The high CaCO3 contents and low TOC values indicate that a predominantly arid climate occurred in South China during the Late Cretaceous to Early Paleocene. Variation of Rb/Ti, Cs/Ti ratios, CaCO3 contents and TOC values imply the climate underwent an obvious change around the time of the K/T boundary. Singularly higher CaCO3 contents and lower TOC values and Rb/Ti, Cs/Ti ratios in Upper Cretaceous sample indicate that an extreme dry climate occurred during the Late Cretaceous in South China, and that the vegetation cover around the Nanxiong Basin was arid. This model is supported by the evidence from stable carbon and oxygen isotopes in dinosaur eggshells collected from the Nanxiong Basin (Zhao and Yan, 2000). The long period extreme dry climate during the Late Cretaceous is consistent with the
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Fig. 8. Variations of Ti-normalized ratios of some major and trace elements and the total organic carbon (TOC), CaCO3 values for the samples from the Nanxiong Basin.
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extinction of the dinosaur population and a significant floral change at the K/T boundary. Many well-preserved nests of dinosaur eggs and eggshell fragments were discovered in the sediments of Upper Cretaceous Pingling Formation, but no evidence has been found in Lower Paleocene Shanghu Formation (Zhao et al., 2002). The ostracod fauna across the K/T boundary is distinct in that the Pingling Formation is dominated by P. sphaeroidalis and P. taizhouensis and the charophyte fauna are characterized by L. curtula–G. changzhouensis. The ostracod fauna in the Shanghu Formation are represented by C. jiangxiensis, while the charophyte fauna are characterized by Nemegtichara prime. However the alternation of different biomes did not occur instantaneously at the K/T transition. Some typical Early Paleocene biomes already appear in the upper section of the Nanxiong Group, such as Hornichara lagenalis and Eucypris (Tong et al., 2002). Furthermore, the extinction of the dinosaurs in the Nanxiong Basin did not occur instantaneously, but was spread out within 250 ka of the boundary with major extinction beginning in the upper section of the Nanxiong Group (Zhao et al., 2002). These evidences together with the variation of Rb/Ti, Cs/Ti ratios, CaCO3 contents and TOC values from the Upper Cretaceous to the Lower Paleocene indicate that the extreme dry climate in the Nanxiong Basin did not occur instantaneously, but started from the upper section of the Nanxiong Group and lasted a long time, synchronous with the gradual extinction of the dinosaurs and the alternation of different biomes at the K/T boundary. Rb/Ti, Cs/Ti ratios and TOC values escalated and CaCO3 contents decreased in the Lower Paleocene suggesting that the climate became relatively wet, which result in greater vegetation cover. 6. Conclusions Geochemical signatures of basin clastic sedimentary rocks provide important sources of information that record different aspects of the basin provenance, tectonic, environmental and ecological evolution. Whilst trace element geochemical studies have tended to focus on aspects of sediment provenance, their application to paleoclimate reconstructions has been relatively neglected. However clastic sedimentary trace element records may contain additional new paleoclimate information that provides important new constrain on sediment depositional environment and climate. Rb/Ti, Cs/Ti ratios and TOC, CaCO3 values of the sediments from the Nanxiong Basin require an obvious change in climate across the Upper Cretaceous and Lower Paleocene boundary. Singularly higher CaCO3 contents
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