Ages of Jurassic Volcano-Sedimentary Strata in the Yanshan Fold-And-Thrust Belt and Their Implications for the Coal-Bearing Strata of Northern China

International Geology Review

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Ages of Jurassic volcano-sedimentary strata in the Yanshan Fold-and-Thrust Belt and their implications for the coal-bearing strata of northern China

Hai-Long Gao, Yue Zhao, Shuan-Hong Zhang, Jian Liu, Hao Ye, Guo-Can Wang & Jian-Min Liu

To cite this article: Hai-Long Gao, Yue Zhao, Shuan-Hong Zhang, Jian Liu, Hao Ye, Guo-Can Wang & Jian-Min Liu (2019) Ages of Jurassic volcano-sedimentary strata in the Yanshan Fold- and-Thrust Belt and their implications for the coal-bearing strata of northern China, International Geology Review, 61:8, 956-971, DOI: 10.1080/00206814.2018.1481459 To link to this article: https://doi.org/10.1080/00206814.2018.1481459

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ARTICLE Ages of Jurassic volcano-sedimentary strata in the Yanshan Fold-and-Thrust Belt and their implications for the coal-bearing strata of northern China Hai-Long Gaoa,b, Yue Zhaob,c, Shuan-Hong Zhangb,c, Jian Liub, Hao Yed, Guo-Can Wanga and Jian-Min Liub aSchool of Earth Sciences, China University of Geosciences, Wuhan, China; bInstitute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, China; cKey Laboratory of Paleomagnetism and Tectonic Reconstruction of Ministry of Land and Resources, Beijing, China; dThree Gorges Geotechnical Consultants Co., LTD, Wuhan, China

ABSTRACT ARTICLE HISTORY Jurassic coal-bearing strata are widely distributed in the North China Craton (NCC) and other areas Received 9 May 2018 of northern China. These coal-bearing strata were previously considered to be Early–Middle Accepted 23 May 2018 – Jurassic in age based on plant fossils, particularly the fossil assemblage of Coniopteris KEYWORDS Phoenicopsis. Since coal-bearing strata are interbedded with volcanic units in the basins of the Coal-bearing strata; Yanshan Fold-and-Thrust Belt (YFTB), northern NCC, isotopic dating of the volcanic units can stratigraphic correlation; therefore provide age constraints on the coal-bearing strata and the Coniopteris‒Phoenicopsis northern China; Yanshan assemblage. In this paper, we performed a systematic geological survey and present the results of Fold-and-Thrust Belt zircon U–Pb dating of the volcanic units and a pluton in typical basins of the YFTB. These data, combined with the results of previous studies, indicate that the ages of the Nandaling/ Xinglonggou, Haifanggou, Jiulongshan, and Tiaojishan/Lanqi formations are 180–168, 169–161, 161–157, and 161–153 Ma, respectively. The ages of the interbedded coal-bearing Yaopo and Beipiao formations are constrained to be 169–161 and 177–169 Ma, respectively. Our results demonstrate that both the coal-bearing strata and the Coniopteris‒Phoenicopsis assemblage are Middle Jurassic in age, which is younger than that previously considered. This fossil assemblage plays a critical role in age constraints on the Jurassic coal-bearing strata. The refinement of its age permits a more precise dating of the coal-bearing strata, especially in northwestern China, where datable interbedded volcanic units are lacking.

● Stratigraphical framework for the Jurassic strata of Yanshan region is established. ● The fossil assemblage of Coniopteris‒Phoenicopsis occurred in Middle Jurassic. ● The Jurassic coal-bearing strata in northern China are mainly Middle Jurassic.

Introduction measures in Europe are usually interlayered with marine strata (Cox and Sumbler 2002; Husinnec and Read The global average temperature throughout the 2007), and their ages have been well constrained to Jurassic was significantly warmer than at present (Ree be Middle Jurassic. However, the coal-bearing strata in et al. 2004; Sellwood and Valdes 2006, 2008; Deng et al. northern China are non-marine, making it difficult to 2017), which provided favourable conditions for the determine their ages due to the highly endemic nature, formation of coal measures. Jurassic coal-bearing strata moderate to poor preservation of Jurassic fossils are widespread throughout Europe (e.g., Fisher and (although the strata are highly fossiliferous), and lack Hancock 1985; Petersen 1993; Petersen and Andsbjerg of volcanic units (Hendrix et al. 1995). Although many 1996; Petersen and Brekke 2001; Barron et al. 2012), studies have been conducted, as a result, there is gen- northern Africa (e.g., Baioumy 2009; Edress and Abdel- eral agreement that the strata are as old as Triassic and Fatah in press), central Asia (e.g. Moore and Esmaeili as young as Middle Jurassic (e.g., Mi et al. 1980; Wu and 2012; Rajabzadeh et al. 2016), western and southern Zhang 1983; Zhang 1983; Chen et al. 1984; He and Wu Mongolia (Sun et al. 2009), and northern China (e.g., 1986; Wu and Zhou 1986;Liet al. 1988; HBGMR 1989; Vakhrameyev and Doludenko 1977; Hendrix et al. 1995; Zhang and Li 1990; BBGMR 1991; QBGMR 1991; Hendrix Wang et al. 2005, 2012; Dai et al. 2009;Aoet al. 2012;Li et al. 1995; Deng et al. 2003, 2017; Liu and Li 2006; et al. 2014b, 2018; Guo et al. in press). The coal-bearing Zhang and Jiang 2010). Fortunately, the contemporary

CONTACT Yue Zhao [email protected]; [email protected] No. 11 South Minzudaxue Road, Haidian District, Beijing 100081, China Supplementary data can be accessed here. © 2018 Informa UK Limited, trading as Taylor & Francis Group INTERNATIONAL GEOLOGY REVIEW 957

Figure 1. Distribution of Jurassic strata in northern China (modified after Deng et al. 2003). coal-bearing strata in the YFTB (Figures 1 and 2) are (Figure 3). Zircon U–Pb dating of volcanic units by laser interlayered with volcanic units, providing age con- ablation–inductively coupled plasma–mass spectrome- straints on the basis of isotopic ages of volcanic units try (LA–ICP‒MS) and sensitive high mass-resolution ion

Figure 2. Simplified tectonic map of the Yanshan Fold-and-Thrust Belt, showing major structures and plutons (modified from Darby et al. 2004; Liu et al. 2013). The study areas are labelled and include (from west to east) the Western Hills of Beijing, Xiabancheng and Jin–Yang basins. Abbreviations are as follows: NCB = North China Block; SCB = South China Block; WHB = Western Hills of Beijing; XBCB = Xiabancheng Basin; BPB = Beipiao Basin; JYB = Jin–Yang Basin; YP = Yamenzi Pluton. 958 H.-L. GAO ET AL.

Figure 3. Previous stratigraphic correlation of Jurassic lithostratigraphic units based on fossils in northern China (complied mainly from Mi et al. 1980; Wu and Zhang 1983; Chen et al. 1984;Liet al. 1988, 2014c; HBGMR 1989; LBGMR 1989; Zheng and Zhang 1990; BBGMR 1991; QBGMR 1991; Hendrix et al. 1995; Deng et al. 2003, 2017;Xuet al. 2005, 2013; Zhang et al. 2016).

microprobe (SHRIMP) were performed with an aim to China. In the volcano-sedimentary basins, the volcanic establish the Jurassic stratigraphic framework of the units provide age constraints on the coal-bearing strata YFTB, and provide age constraints on both the coal- and fossil assemblages therein; while the ages of the bearing strata and their fossil assemblage zones. The coal-bearing strata in northwestern China were mainly Coniopteris–Phoenicopsis plant fossil assemblage is determined based on fossils, without isotopic age con- widely distributed in Eurasia and plays a crucial role in straints. Therefore, refinement of the fossil assemblage constraining the ages of Jurassic coal-bearing strata is helpful in constraining the age of the coal-bearing (Sze 1955, 1959; Harris 1964;Miet al. 1980; Chen et al. strata in basins that lacking datable volcanic units. 1984). Refinement of plant fossil ages enables more Three typical basins (from west to east, the Western precise dating of terrestrial coal-bearing strata, espe- Hills of Beijing, Xiabancheng, and Jin-Yang basins) in cially in northwestern China where the interbedded the YFTB were selected for study (Figure 2(b)), with the volcanic units are lacking. aim of establishing the stratigraphic framework and providing age constraints on both the coal-bearing strata and their fossil assemblages. Geological background The YFTB is located in the northern NCC and is considered to be an intraplate deformation belt that Regional geology formed during the late Permian to Early Triassic after Jurassic coal-bearing strata are widely distributed in the collision of the Mongolian arc terranes and the NCC NCC and other areas of northern China. Divided by the along the Solonker Suture Zone (Figure 2(a); e.g., Taihangshan, the Jurassic basins are classified into two Chen 1998; Davis et al. 2001; Xiao et al. 2003; Wang types: huge, stable basins in the western part of north- et al. 2007; Liu et al. 2013). The belt experienced a key – ern China; and small, intermontane basins in the east- transformation from the E W-trending Paleo-Asian tec- ern part (Figure 1; Deng et al. 2003; Liu and Li 2006;Li tonic domain to the NNE-trending Paleo-Pacific tectonic et al. 2018). The Jurassic stratigraphic successions in domain (e.g., Zhao 1990; Zhao et al. 1994, 2004a, 2004b; these basins are characterized by coal measures in the Zhai et al. 2004; Dong et al. 2008), and a series of lower part and red clastic beds in the upper part volcano-sedimentary basins formed during the (Figure 3). Volcanic units are present in the basins of Mesozoic (Liu et al. 2004; Liu 2007). The Jurassic strati- eastern North China, but not in those of northwestern graphic successions in the YFTB are characterized by INTERNATIONAL GEOLOGY REVIEW 959 two volcanic units interlayered with coal-bearing strata. the Lanqi Formation in the Beipiao Basin (Chang et al. The Middle Triassic–Jurassic stratigraphy in previous 2009). researches is shown in Figure 3.

Samples and methods Chronostratigraphic control Sampling and description of cross-sections The ages of the early Mesozoic strata have pre- Volcanic and clastic deposits occur alternately in the viously been constrained by fossils. The Xingshikou volcano-sedimentary basins of the present study, mean- Formation (consisting of fluvial gravels) was ing that the volcanic units can provide valuable abso- assigned to the Late Triassic (Mi et al. 1984)or lute age constraints on the stratigraphic framework. Early Jurassic (Liu 1988) based on plant and fish Jurassic volcanic rocks and plutons in typical basins of fossils, respectively. The coal-bearing Yaopo and the YFTB have been dated to establish the stratigraphic Beipiao formations were considered to be Early– framework in the YFTB. Middle Jurassic (Chen et al. 1984;HBGMR1989; In the Western Hills of Beijing, geochronological stu- BBGMR 1991)andEarlyJurassic(Miet al. 1980;Wu dies have been performed on the Nandaling and and Zhang 1983;LBGMR1989)inagebasedonflora Tiaojishan formations, while data on the latter are and sporopollen. The underlying Nandaling/ more robust than those on the former (e.g., Davis Xinglonggou Formation was therefore constrained et al. 2001; Liu et al. 2006; references therein). The to the Early Jurassic based on the ages of the over- Nandaling volcanic rocks disconformably overlie the lying units. Xingshikou Formation (BBGMR 1991) and unconform- Isotopic age dating of volcanic or pyroclastic beds ably overlie the Paleozoic strata (Bao et al. 1983; Yang can be used to constrain the ages of interbedded strata et al. 2006). The Tiaojishan Formation overlies the (Meng et al. 2014). The detrital zircon grains in the Jiulongshan Formation (Figures 3, 4(a) and 5; Wong Xingshikou Formation of the Xiabancheng Basin are as 1927, 1929) across an angular unconformity. Andesite young as 197 ± 6 Ma (Liu et al. 2012), implying an Early samples were collected from the lower part of the Jurassic age. Inherited zircon grains from the Nandaling Nandaling Formation in the Fachengkou Section (sam- flood basalt in the Western Hills of Beijing yielded a ple 150904–1) and from the base of the Tiaojishan peak age of 174 ± 8 Ma, which was inferred to repre- Formation (sample 010907–4). The sampling locations sent the eruption age of the Nandaling Formation are marked in Figure 5. (Zhao et al. 2006). A biotite 40Ar–39Ar age of In the Xiabancheng Basin, the Yamenzi syenite pluton is 180 ± 2 Ma was reported from the base of the overlain by the Jiulongshan Formation across a nonconfor- Nandaling Formation in northern Hebei (Davis et al. mity (Figure 6).Theageofthepluton provides a reliable 2001), and a similar SHRIMP zircon U–Pb age of constraint on the age of the Jiulongshan Formation. 177 ± 4 Ma was obtained from the equivalent Sample 1021 was collected from the pluton (Figure 6). Xinglonggou Formation in the Beipiao Basin in western Figure 7 provides cross-sections of the Haifanggou Liaoning (Yang and Li 2008). An LA–ICP–MS zircon U– Formation in Nanpiao County at the southeastern mar- Pb age of 163 ± 1 was reported from a tuff layer in the gin of the Jin–Yang Basin. The Haifanggou Formation in lower part of the Jiulongshan Formation (Chen et al. Nanpiao was previously considered to be the Early 2014; Zhang et al. 2014). Chang et al.(2014) reported a Jurassic Xinglonggou Formation. A typical Middle plagioclase 40Ar–39Ar age of 167 ± 1 Ma for interbedded Jurassic fossil assemblage (Coniopteris–Phoenicopsis) tuff in the middle part of the Haifanggou Formation in was identified in the sedimentary layers, and this for- the Beipiao Basin. Recently, Huang (2017) also reported mation was reassigned to the Middle Jurassic a similar zircon U–Pb age of 167 ± 1 Ma from the Haifanggou Formation (LBGMR 1989). The Haifanggou tuffaceous breccia at the base of the Haifanggou Formation in Nanpiao County overlies the Triassic Formation. The lower age limits of volcanic rocks from Hongla or Houfulongshan formations, and underlies the Tiaojishan Formation (in western Beijing and north- the Late Jurassic Lanqi Formation across angular uncon- ern Hebei) are 161–157 Ma (Davis et al. 2001; Cope formities (Figure 4(b); LBGMR 1989). The Haifanggou 2003; Zhao et al. 2004b; Liu et al. 2006; Cope et al. Formation is divided into three units: conglomerates 2007). Two plagioclase 40Ar–39Ar ages of 161 ± 1 Ma in the lower part (Figure 4(c)); volcanic and pyroclastic and 159 ± 1 Ma were obtained from tuff at the base of rocks in the middle part (Figure 4(d‒g)); and coal- 960 H.-L. GAO ET AL.

Figure 4. Field photographs showing contact relations and samples from the Yanshan Fold-and-Thrust Belt: (a) angular unconformity beneath the Tiaojishan Formation in the Western Hills of Beijing; (b) angular unconformity beneath the Haifanggou Formation in western Liaoning; (c) conglomerate at the base of the Haifanggou Formation; (d) sample 15132; (e) sample 15127; (f) sample 08077; (g) sample 08079–2; and (h) coal measures in the upper part of the Haifanggou Formation. Abbreviations are as follows: J2y = Middle Jurassic Yaopo Formation; J3t = Upper Jurassic Tiaojishan Formation; J2–3h = Middle–Upper Jurassic Haifanggou Formation; and T2h = Middle Jurassic Houfulongshan Formation. bearing sandstones and mudstones in the upper part collected from the Dawopu–Aijiagou and (Figure 4(h)). Cross-sections and sample locations are Houfulongshan–Pandaogou sections. Field photo- marked in Figure 7. Four andesitic samples were graphs of the samples are shown in Figure 4. INTERNATIONAL GEOLOGY REVIEW 961

Figure 5. Geological sketch map of the Western Hills of Beijing and cross-section (modified from BBGMR 1991). A–A’: Fachengkou section.

Figure 6. Geological sketch map, photograph and cross-section of the Yamenzi Pluton in the Pingquan region of northern Hebei (modified from Liu et al. 2006, 2012). 962 H.-L. GAO ET AL.

Figure 7. Geological sketch map and cross-sections of the western Liaoning area. (a) The Jin–Yang Basin; and (b) the Nanpiao County, western Liaoning (modified from Liaoning Bureau of Geology and Mineral Resources (LBGMR) 1989;Heet al. 2007).

Methods et al. 2010) and GLITTER 4.0 (Macquarie University) soft- ware. Zircon U–Pb concordia and weighted mean age The samples were cleaned prior to zircon separation, plots were produced using Isoplot/Ex_ver3 (Ludwig and the separated zircon grains were mounted in epoxy 2003). The analytical results are listed in resin, polished, and gold-coated. The grains were Supplementary Table 1. photographed in transmitted and reflected light, and Zircon SHRIMP U–Pb analyses were conducted using imaged by cathodoluminescence (CL) (Figure 8). The a SHRIMP II at the Beijing SHRIMP Centre, Beijing, China, zircon U–Pb isotopic analyses were conducted using following the procedures and methods of Williams et al. an Agilent 7700 LA–ICP–MS instrument at Sample (1998) and Jian et al.(2003). Isoplot version 3 (Ludwig Solution Analytical Technology, Wuhan, China, and 2003) was used for off-line data processing. The analy- using an Agilent 7500 LA–ICP–MS instrument at the tical results are listed in Supplementary Table 2. State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. The analytical proce- dure is described in detail by Liu et al.(2008, 2010). The Results off-line data selection and integration of background and signals, time-drift correction, and quantitative cali- The zircon grains from the volcanic rocks and pluton bration were conducted using ICPMSDataCal 8.3 (Liu are euhedral with sizes of 20–200 μm and length:width INTERNATIONAL GEOLOGY REVIEW 963

Figure 8. Cathodoluminescence images of representative zircons from six samples of Jurassic volcanic rocks from typical basins of the Yanshan Fold-and-Thrust Belt. Circles show the location of analyses, and the resulting age is noted below each zircon.

ratios of 4:1. The analyzed grains exhibit a wide range the other 18 grains yield a concordia age of 157 ± 2 Ma of Th and U contents, with Th/U ratios of 0.14–4.52. and a weighted mean age of 157 ± 3 Ma (MSWD = 0.26; Most of the zircon grains are elongate columnar in Figure 9(b)). The analyzed ages of the Yamenzi syenite shape, and others are short columnar in CL (Figure 8). pluton (Figure 6; sample 1021) are 168–153 Ma, and 12 The majority of the grains are igneous zircons, and of 15 analyzed grains yield a concordia age of those with obvious oscillatory zoning in CL and smooth 161 ± 2 Ma (MSWD = 2.2) and a weighted mean age signals in ICPMSDataCal 8.3 were selected for isotopic of 161 ± 3 Ma (MSWD = 0.58; Figure 9(c)). Sample 15132 age calculation. The age data are listed in is an autochthonous andesite collected from the lowest Supplementary Tables 1 and 2, and the concordant volcanic unit overlying the conglomerates at the base analyses are shown in concordant and weighted mean of the Haifanggou Formation (Figures 4(d), 7 and 10). plots in Figure 9. Seven concordant analyses of 16 grains from this sam- Andesite sample 150904–1 was collected from the ple yield a concordia age of 169 ± 1 Ma and a weighted base of the Nandaling Formation (Figure 5). In total 19 mean age of 169 ± 2 Ma (MSWD = 1.6; Figure 9(d)). of the 25 analyses yield a weighted mean age of Sample 15127 is an andesite collected from the base of 172 ± 2 Ma (MSWD = 1.7; Figures 9(a) and 10). the Haifanggou Formation (Figures 4(e), 7 and 10), Andesite sample 010907–4 was collected from the which yields ages of 160–170 Ma. Nine concordant Tiaojishan Formation near Malan (Figure 5). Six zircon analyses yield a concordia age of 164 ± 1 Ma and a grains are inherited, and 14 concordant analyses from weighted mean age of 164 ± 2 Ma (MSWD = 1.2; 964 H.-L. GAO ET AL.

Figure 9. Zircon U–Pb concordia and weighted mean age plots for samples from Jurassic volcanic rocks and the pluton in the Yanshan Fold-and-Thrust Belt, North China. (a) Sample 150904–1, from the lower part of the Nandaling Formation; (b) sample 010907–4, from the base of the Tiaojishan Formation; (c) sample 1021 from the Yamenzi Pluton; (d) sample 15132, from the base of the Haifanggou Formation; (e) sample 15127, from the lower part of the Haifanggou Formation; (f) sample 08077–1, from the middle part of the Haifanggou Formation; and (g) sample 08079–2, from the middle part of the Haifanggou Formation.

Figure 9(e)). Sample 08077–1 is an andesite collected the other 13 concordia ages are 170–156 Ma with a from the lower part of the Haifanggou Formation concordia age of 166 ± 3 Ma and a weighted mean age (Figures 4(f), 7 and 10). Six inherited zircon grains of 164 ± 2 Ma (MSWD = 0.78; Figure 9(f)). The andesitic from the 22 analyses yield ages of 2500–2210 Ma, and sample 08079–2 was collected from the middle part of INTERNATIONAL GEOLOGY REVIEW 965 the Haifanggou Formation (Figures 4(g), 7 and 10). Jurassic magmatic activity in the YFTB is therefore con- Eight concordant analyses of the 16 analyzed grains strained to 180–168 Ma (Figure 11). yield a concordia age of 165 ± 2 Ma and a weighted mean age of 165 ± 3 Ma (MSWD = 2; Figure 9(g)). Jiulongshan Formation The andesite sample 010907–4 was collected from the base of the Tiaojishan Formation in the Western Hills of Discussion Beijing and yields a zircon SHRIMP U–Pb age of Ages of volcanic strata 157 ± 3 Ma (Figures 5 and 11), providing an upper limit age for the Jiulongshan Formation. In northern Nandaling/Xinglonggou formation Hebei, the reported lower age limit of the Jiulongshan The Nandaling and Xinglonggou formations were pre- Formation is 163 ± 2 Ma (Chen et al. 2014; Zhang et al. viously considered to be equivalent strata (Figure 2; 2014). The emplacement age of the Yamenzi Pluton LBGMR 1989;Xuet al. 2005). Yang and Li (2008) (sample 1021) is 161 ± 3 Ma. The Jiulongshan reported a SHRIMP zircon U–Pb age of 177 ± 4 Ma for Formation nonconformably overlies the pluton, thus a tuff sample collected from the Xinglonggou suggesting that the Jiulongshan Formation is younger Formation (Figure 11). The andesite sample 150904–1 than 161 ± 3 Ma (within error of the 163 ± 2 Ma age). collected from the base of the Nandaling Formation in For convenience, we provisionally refer to the age of the Fachengkou Section yields a LA–ICP–MS zircon age 161 ± 3 Ma for the lower limit age of the Jiulongshan of 172 ± 2 Ma (Figures 10 and 11). We reported zircon Formation in this paper. The lower age limits of the U–Pb ages of 175–168 Ma for the Nandaling Formation Tiaojishan/Lanqi Formation are 161–157 Ma (Zhao from the Caihongqiao Section in the Western Hills of et al. 2004a; Liu et al. 2006; references therein; Ma and Beijing (Figures 10 and 11; Gao et al. 2018). Davis et al. Zheng 2009; Chang et al. 2014;Yuet al. 2016). (2001) reported a biotite 40Ar–39Ar age of 180 ± 2 Ma Therefore, the age of the Jiulongshan Formation is for silicic tuff in northern Hebei (Figure 11). The earliest constrained to 161–157 Ma.

Figure 10. Correlation of stratigraphic columns from the western Beijing and western Liaoning areas. Abbreviations are as follows: J3l–j = Longmen + Jiulongshan formations; J3l = Lanqi Formation; J3t = Tiaojishan Formation; J2–3y = Yaopo Formation; J2–3h = Haifanggou Formation; J2n = Nandaling Formation; J1 x = Xingshikou Formation; T1h = Hongla Formation; T3h = Houfulongshan Formation. Data are sourced from: (I) Gao et al.(2018); (II) this paper; (III) Zhao et al.(2006); (IV) Chang et al.(2009); (V) Chang et al.(2014). 966 H.-L. GAO ET AL.

Figure 11. Revised stratigraphic correlation of Jurassic strata in northern China (descriptions and data are from Li et al. 1988; Davis et al. 2001; Deng et al. 2003; Peng et al. 2003;Ge2004; Zhao et al. 2004a; Liu et al. 2006; Yang et al. 2006; Zhao 2007; Yang and Li 2008; Chang et al. 2009;Geet al. 2010;Liet al. 2013, 2014c;Hu2014; Meng et al. 2014; Huang 2017). ‘?’ represents the debated stratigraphic contact relationship.

Haifanggou Formation Hills of Beijing are the same as, or similar to, the In the western Liaoning area, samples from the lower to Yorkshire Flora (Harris 1964; Chen et al. 1984). Some middle parts of the Haifanggou Formation yielded ages typical Early–Middle Jurassic species occur in both of 169–164 Ma (Figure 9). The lower age limit of the tuff Yorkshire and western Beijing, such as Coniopteris at the base of the Lanqi Formation is 161 ± 1 Ma hymenophylloides, C. tatungensis, and Ehoracia lobifolia (Chang et al. 2009). Therefore, the age of the (Chen et al. 1984). Some species that are commonly Haifanggou Formation is constrained to 169–161 Ma. preserved in the Triassic–Early Jurassic strata are also found in the lower part of the Yaopo Formation (Chen Tiaojishan/Lanqi Formation et al. 1984). The Yaopo Formation was therefore con- Previous studies have constrained the eruption ages of strained to the Early–Middle Jurassic on the basis of the the volcanic units of the Tiaojishan/Lanqi Formation to above observations (Chen et al. 1984; BBGMR 1991). Mi ~161–153 Ma (Liu et al. 2006; references therein; Ma et al.(1980) constrained the Beipiao Formation to the and Zheng 2009; Chang et al. 2014;Liet al. 2014a, 2016; Early Jurassic based on large numbers of Neocalamites, Yu et al. 2016). Dictyophyllum, and Clathropteris, few Conopteris, and no typical Triassic species. Abundant floral fossils (includ- ing Coniopteris) were identified in the Haifanggou Ages of coal-bearing strata Formation and assigned as Middle Jurassic flora (Mi The coal-bearing strata in the YFTB were previously et al. 1980; Pan 1983). The dating of strata on the considered to be Early–Middle Jurassic in age due to basis of slowly evolving flora is not sufficiently accurate; the abundant plant fossils correlating with the Jurassic instead, isotopic ages of volcanic units can provide Yorkshire Flora in the UK (Harris 1964; Chen et al. 1984; more precise age estimates. Zhao et al. 2002). The Jurassic strata in the UK are The ages of the coal-bearing strata can be con- subdivided into stages based on the widely distributed strained by dating the volcanic units. As shown in and fast-evolving ammonites (Cox and Sumbler 2002). Figure 3, the overlying strata are the Longmen, The Yorkshire Flora occurs mainly in the Middle Jurassic Jiulongshan, and Haifanggou formations, and the Ravenscar Group, whose age is authentic owing to the underlying strata are the Nandaling and constraints of ammonites in marine strata. More than 20 Xinglonggou formations. Their ages are listed in species (~20% of the identified fossils) in the Western Supplementary Table 3. The age of the Jiulongshan INTERNATIONAL GEOLOGY REVIEW 967

Formation is 161–157 Ma (Zhao et al. 2004a;Liet al. (2) The Jurassic coal-bearing strata in the YFTB are 2014a; this study) and the upper age limit of the mainly Middle Jurassic in age, not Early Jurassic Nandaling Formation is 168 ± 2 Ma (Gao et al. as was previously suggested. The Beipiao 2018). Therefore, the age of the coal-bearing strata Formation in western Liaoning (177–169 Ma) is in the Western Hills of Beijing is constrained to older than the Yaopo Formation in the Western 168–161Ma.TheagesobtainedfortheHaifanggou Hills of Beijing (168–161 Ma). Formation (169–164 Ma) and the reported age of the (3) The Coniopteris–Phoenicopsis fossil assemblage, Xinglonggou Formation (177 ± 4 Ma; sample collected previously considered to be Early–Middle Jurassic from the middle part of the formation; Yang and Li in age, is revised to the Middle Jurassic based on 2008) suggest that the age of the Beipiao Formation our dating of volcanic units in the YFTB. The is 177–169 Ma (Figure 9). The Beipiao Formation is revised age of this fossil assemblage suggests therefore older than the Yaopo Formation. The dis- that the coal-bearing strata in northern China are parity in ages of the two formations explains the mainly Middle Jurassic in age. abundant Coniopteris in the Yaopo Formation, and the scarcity of the fossil in the Beipiao Formation. The coal-bearing strata in the YFTB were formed mainly during the Middle Jurassic, not the Early Geolocation information Jurassic as previously suggested (Chen et al. 1984; The study area is located at the northern margin of the NCC Mi et al. 1984;LBGMR1989; BBGMR 1991). (39°30ʹ–42°30ʹN, 114°–123°E). As one of the typical Middle Jurassic fossil assemblages, Coniopteris–Phoenicopsis, occurs in the upper Acknowledgments part of the Beipiao Formation in the YFTB (177– 169 Ma; Yang and Li 2008; this study). This fossil assem- We gratefully acknowledge the anonymous reviewers and Dr blage has been considered to be Early–Middle Jurassic Robert J. Stern (editor-in-chief) for their valuable and construc- in age in China (Sze 1955, 1959; Chen et al. 1984). Some tive reviews and comments which greatly improve the manu- script. We thank Zi-Bo Fu, Qi-Qi Zhang for their help in the field strata containing this fossil assemblage, such as the investigation. Xiao-Ming Liu and Xiao-Li Shi are acknowledged Yongdinghe and Fuxian formations (Figure 3), are still for technical support during the zircon U–Pb analyses. considered to be Early Jurassic in age. Many Jurassic units in northern China were assigned older ages by previous studies, which has been proved by recent Disclosure statement studies (Ge 2004; Yang et al. 2006; Zhao 2007;Ge No potential conflict of interest was reported by the authors. et al. 2010;Liet al. 2013, 2014c). Based on the plant fossil, the stratigraphic correlations are revised and shown in Figure 11. Funding A global arid climate was indicated by the appearance This study was financially supported by the fund from the of Late Jurassic evaporates in Europe and northern Africa, Ministry of Science and Technology of People’s Republic of and the mottled and red Upper Jurassic sediments in China, under Grant [Number, 2016YFC0600108-4] and [the central Asia and northern China (Vakhrameyev and National Natural Science Foundation of China], under Grant Doludenko 1977; Zheng and Zhang 1990). The basins in [number 41725011 and 41572204]. northern China experienced compressional deformation and were completely or partially uplifted during the References Middle–Late Jurassic (Hendrix et al. 1992;Jolivet2015; Van Der Voo et al. 2015;Fanget al. 2016; Yang et al. Ao, W.H., Huang, W.H., Weng, C.M., Xiao, X.L., Liu, D.M., Tang, 2017). The disappearance of coal-bearing strata was exa- X.Y., Chen, P., Zhao, Z.G., Wan, H., and Finkelman, R.B., 2012, cerbated by both the arid global climate and the uplift of Coal petrology and genesis of Jurassic coal in the Ordos Basin, China: Geoscience Frontiers, v. 3, no. 1, p. 85–95. northern China during the Late Jurassic. Baioumy, H.M., 2009, Mineralogical and geochemical charac- terization of the Jurassic coal from Egypt: Journal of African Earth Sciences, v. 54, p. 75–84. doi:10.1016/j. Conclusions jafrearsci.2009.03.003 Bao, Y.G., Xie, D.Y., Chen, Z.B., and Mo, B.T., 1983, On the Yanshan movement in Beijing area: Acta Geologica Sinica, (1) The ages of the Nandaling and Haifanggou for- v. 2, p. 195–204. In Chinese with English abstract. mations in the YFTB are constrained to Barron, A.J.M., Lott, G.K., and Riding, J.B., 2012, Stratigraphical 180–169 Ma and 169–161 Ma, respectively. framework for the Middle Jurassic strata of Great Britain 968 H.-L. GAO ET AL.

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