Geochronologic Age Constraints on the Middle Devonian Hujiersite Flora

Palaeogeography, Palaeoclimatology, Palaeoecology 463 (2016) 230–237

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Geochronologic age constraints on the Middle Devonian Hujiersite ﬂora of Xinjiang, NW China

Daran Zheng a,b,HongheXub,JunWanga, Chongqing Feng a, Haichun Zhang b, Su-Chin Chang a,⁎ a Department of Earth Sciences, The University of Hong Kong, Hong Kong Special Administrative Region, China b State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, China article info abstract

Article history: The Hujiersite Formation of West Junggar, Xinjiang contain fossil plants that represent significant examples of Received 4 July 2016 Middle Devonian flora in North China. However, previous radio-isotopic age constraint for this fossil-rich forma- Received in revised form 19 September 2016 tion is absent. The best-studied section of the Hujiersite flora occurs at a locality referred to as 251 Hill, which ex- Accepted 11 October 2016 poses numerous examples of fossil plants and information on depositional environment. In this study, we Available online 13 October 2016 investigate the geological background of the 251 Hill section and collect two tuffaceous sandstone beds from a

Keywords: normal fault near the classic fossil outcrop. U-Pb ICP-MS analysis of detrital zircons from these samples gave a Land plants maximum depositional age of 385 Ma (Givetian stage) for the hanging wall and 380 Ma (early Frasnian stage) Zircon for the footwall of the upper Hujiersite Formation. This study thus reports a novel Givetian–early Frasnian age Hujiersite Formation constraint for Hujiersite ﬂora. This geochronologic constraint is consistent with previous age interpretations de- Geochronology rived from biostratigraphic correlations. Devonian © 2016 Elsevier B.V. All rights reserved. Xinjiang

1. Introduction entire Hujiersite Formation. Our geochronologic analysis (ICP-MS U-Pb) of detrital zircons from tuffaceous sandstone samples from the 251 Hill Major advances in land plant evolution and diversification oc- section gave robust and consistent maximum depositional ages for the curred in the Middle to late Devonian. The fossil records contain upper Hujiersite Formation. These ages offer a novel age constraint for tree-sized plants that comprised early forests of the Middle-late De- the Hujiersite flora and may further help constrain the distribution, evo- vonian at different continental localities including North America lution and migration of early land plants, as well as their influence on (Stein et al., 2007, 2012), Spitsbergen (Berry and Marshall, 2015) Devonian palaeoenvironments. and West Junggar, China (Xu et al., 2012b, 2015). The appearance and development of forest ecosystems imparted dramatic changes 2. Geological setting on the Earth system, as evident from the rapid decline of atmospheric carbon dioxide during the Devonian (Berner, 1997; Algeo et al., West Junggar is located in North Xinjiang, northwestern China 2001; Morris et al., 2015). (Fig. 1). The locality is regionally situated in central Asia near the The Hujiersite flora occurs within the Devonian Hujiersite Forma- China-Kazakhstan border, south of the Altai mountain range and tion, West Junggar, Xinjiang, China and contains representative fossil the Yili block (Geng et al., 2009). Palaeogeographically, West Junggar plants including abundant mega-plants that appear to form a small- abutted Siberia, Tarim and Kazakhstan continental fragments (Feng scaled forest inhabiting an alluvial flat or flood plain (Cai and Wang, et al., 1989). Devonian units of West Junggar include the lower 1995; Xu et al., 2015). Based on conchostracans (Liu, 1990) and plant Devonian Hoboksar group, the Middle Devonian Hujiersite Forma- fossils (Cai and Wang, 1995; Xu et al., 2012b, 2014, 2015), most previ- tion and the upper Devonian Hongguleleng Formation (Cai, 2000). ous studies suggested a late Middle Devonian age for the Hujierste For- The Hujiersite Formation was first described and formalized in mation. However, some disagreements exist (Lu, 1997; Cai, 2000; Wang Mangkelu, Hoxtolgay, Hoboksar in 1973 (Lu, 1997) and occurs main- et al., 2004; Xu et al., 2015) and radio-isotopic age determination for this ly in Hoboksar, West Junggar. fossil-bearing formation is absent. The 251 Hill section lies about 20 km north of the town of In order to clarify the age for the Hujiersite flora, we investigate the Hoxtolgay, Hoboksar Mongol Autonomous County, Xinjiang, NW 251 Hill section. This section exposes the most fossil-rich horizon of the China and hosts only the upper member of the Hujiersite Formation (Fig. 1; GPS: 46° 36′ 55″ N, 86° 1′ 6″ E). A small normal fault disrupts ⁎ Corresponding author. the section with the offset of about 0.5 m (Fig. 2). The section consists E-mail address: [email protected] (S.-C. Chang). primarily of tuffaceous conglomerate, sandstone and siltstone,

Fig. 1. Geological map showing the location of 251 Hill in Hoboksar, Xinjiang, NW China.

interbedded with thin coal seams (Fig. 3). Abundant plant taxa 3.2. Zircon separation and cathodoluminescent (CL) imaging (megaplants and spores) were reported from the section (Xu et al., 2015)(Figs. 3 and 4). The samples were crushed and separated to isolate the 80–200 μm The West Junggar area probably represent a land bridge or contigu- grain size-fraction. The detailed preparation procedure is described as ous land area that allowed dispersal and expansion of certain Devonian in Wang et al. (2015a, 2015b). A total of 200 inclusion-free zircon grains lycopsids and aneurophytaleans (Xu et al., 2012a, 2015; Jiang et al., from each sample were then picked under a binocular microscope and 2013). The abundance of miospore Cymbosporites and coal seams in mounted in epoxy resin. Hardened mounts were sectioned and polished the section indicate that lycopsids growing in a tropical and humid en- to expose zircon grain midsections at about 2/3 to 1/2 of their width. vironment represented major constituents of Hujiersite ecosystems (Xu Cathodoluminescent (CL) imaging documented grain morphologies et al., 2014). The degree and quality of plant fossil preservation also in- and internal structure for in situ analysis. dicates burial and fossilization near sites of growth (Feng et al., 2014; Xu et al., 2015). 3.3. U-Pb analysis of zircons

3. Materials and methods U-Pb isotopic data from zircons were obtained through the Depart- ment of Earth Sciences, University of Hong Kong, using a Nu Instru- 3.1. Materials ments Multiple Collector (MC) ICP-MS with a Resonetics Resolution M-50-HR Excimer Laser Ablation System. The analyses used a beam di- Two tuffaceous sandstone samples (H-01, H-02), each weighing ameter of 30 μm and 6 Hz repetition rate for a signal intensity of 4 mV at about 3 kg, were collected between fossil-bearing strata of the 251 Hill mass 238U for the standard zircon 91500. Average ablation time was ca. section (Fig. 2). Sample H-01 was collected from a middle section of 40 s and pit depths reached about 30 μm. The GJ-1 zircon standard the normal fault hanging wall (Fig. 2B, D), while H-02 occurred in the (609 Ma, Jackson et al., 2004) and Harvard reference zircon 91500 lower part of the footwall (Fig. 2C, E). Abundant detrital zircons from (1065.4 ± 0.3 Ma, Wiedenbeck et al., 1995) were used for calibration. the samples are suitable for U/Pb dating. Detailed operational procedures are found in Xia et al. (2011).We 232 D. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 463 (2016) 230–237

Fig. 2. Images of 251 Hill and samples of tuffaceous sandstone. (A) 251 Hill normal fault (arrowed). (B) Footwall location of sample H-02 (white box). (C) Hanging wall location of sample H-01 (white box). (D) Sample H-02 in outcrop. (E) Sample H-01 in outcrop.

used ICPMSDataCal (Liu et al., 2010) to process the off-line signal selec- and oscillatory zoning patterns indicate igneous origin and limited ex- tion, quantitative calibration and time-drift correction. We used a func- posure to sedimentary processes (Fig. 5C). Eighty-six zircon grains tion given in Anderson (2002) to correct for common Pb in Microsoft gave concordant ages ranging from 483 to 380 Ma (Fig. 5D). Six analyses Excel. Isoplot v. 3.0 (Ludwig, 2003) was used to construct concordia di- gave a maximum depositional age of 380 Ma for H-02. The largest sub- agrams and probability density plots. Within the overall detrital age dis- population of ages generally fell between 390 and 380 Ma. The age distribution, we cite 206Pb/238U ages for zircon grains younger than tribution also included several sporadic Ordovician and Silurian ages 1000 Ma and 207Pb/206Pb ages for older grains. Supplementary table (483–403 Ma). lists U-Pb data. 5. Discussion 4. Zircon morphology and age data 5.1. Biostratigraphic age for the Hujiersite flora One hundred zircon grains were separated from the sample H-01. With a few exceptions, zircon grain size ranged from 100 to 200 μm. The Hujiersite flora consists of a number of plant taxa (Table 1)in- Most grains exhibited euhedral morphologies and oscillatory zoning cluding lycopsids Haskinsia hastata (Xu et al., 2008), H. sagittata (Xu et patterns indicating igneous origin (Fig. 5A). Angular grain facets also al., 2008), Leclercqia cf. complexa (Xu and Wang, 2008), L. uninata (Xu suggest minimal abrasion from sedimentary transport and support et al., 2011a), Colpodexylom gracilentum (Xu and Wang, 2011), the interpretation that the tuffaceous sandstone depocenter lay Hoxtolgaya robusta (Xu et al., 2012a)andDrepanophycus minor (Xu et near the magmatic source of zircons. Ninety-four analyses provided al., 2013). Hujiersite flora also include zosterophylls Serrulacaulis concordant ages ranging from 534 to 385 Ma (Fig. 5B). spineus (Xu et al., 2011b)andS.cf.furatus (Xu et al., 2011b), One hundred analyses were performed on sample H-02. Zircon grain iridopteridaleans Compsocradus givetianus (Wang, 2008; Fu et al., sizes were smaller than those from sample H-01. While some grains 2011), progymnosperms Aneurophytom doui (Jiang et al., 2013)and were intact, most exhibited fractured morphologies. Euhedral facets Teraxylopteris sp. (Jiang et al., 2013), uncertain flora Tsaia conica D. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 463 (2016) 230–237 233

Fig. 3. Stratigraphic column showing lithologies, biostratigraphy and sample points in the Hujiersite Formation.

(Wang et al., 2004), Taeniocrada and Barssasia (Dou, 1983; Cai and (Meyer-Berthaud et al., 2003; Xu and Wang, 2008; Xu et al., 2012a). Ini- Wang, 1995), cormose lycopsid rhizomorphs (Xu and Wang, 2016), tially described from the Middle Devonian Panther Mountain Formation megaspore Verrucisporites lui (Xu et al., 2012b) and miospore of eastern New York, USA (Banks et al., 1972), Leclercqia complexa Cymbosporites magnificus (Xu et al., 2014). The Hujiersite flora correlate persisted from early to Middle Devonian (Table 1). Abundant examples with fossils in the Zhifang and Yundukala Formations of East Junggar of early plants and arthropods have led to a Middle Givetian age (Cai and Li, 1982; Dou, 1983; Cai, 2000) and more generally with the co- interpretation for the Panther Mountain Formation (Banks et al., 1972; eval assemblages in North America, South America and western Europe Shear et al., 1987; Shear and Bonamo, 1988). L. complexa also appears (Xu et al., 2015). in the lower Devonian Beartooth Butte Formation of northern Wyoming, AgeconstraintsontheHujiersiteflora can inform paleoenvironmental USA (Tanner, 1983), the Eifelian–upper Givetian Storm Hill Sandstone and taxonomic interpretations. The herbaceous lycopsid Haskinsia Formation of Queensland, Australia (Meyer-Berthaud et al., 2003), the ranges from tropical and temperate paleoclimatic zones in the Eifelian and upper Givetian of Belgium (Fairon-Demaret, 1981), the Devonian suggesting apparent adaption to cooler tropical and Givetian of Germany (Fairon-Demaret, 1980), the lower Campo Chico warmer temperate conditions (Xu et al., 2008). The species Haskinsia Formation of Venezuela (Harvey, 1999) and the Emsian Campbellton hastata and H. sagittata were initially described from the lower Formation of Canada (Gensel and Albright, 2006). Vertebrate fauna indi- Campo Chico Formation of Venezuela (Berry and Edwards, 1996; cate a Pragian age for the Beartooth Butte Formation in Cottonwood Xu et al., 2008) and interpreted as late Givetian to Frasnian in age Canyon (Elliot and Ilyes, 1996; Lamsdell and Legg, 2010) while according to megaplant and spore assemblages (Harvey, 2001; palynological evidence indicates a late Lochkovian to early Pragian age Hammond and Berry, 2005). Appearance of these species within (Tanner, 1983). The type section at Beartooth Butte has meanwhile both the Hujiersite and lower Campo Chico flora indicates temporal been interpreted as Emsian in age (Tanner, 1983; Lamsdell and Legg, overlap of the two floras. Leclercqia exhibits global lower–Middle 2010; Caruso and Tomescu, 2012). The cosmopolitan distribution Devonian distribution, typically appearing in arid paleoenvironments of L. complexa in these areas reflects its relatively persistent Emsian– 234 D. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 463 (2016) 230–237

Fig. 4. Photographs of typical elements of Hujiersite ﬂora, scale bars = 10 mm. (A) Haskinsia sagittata.(B)Haskinsia hastata.(C)Leclercqia uncinata.(D)Colpodexylon doui.(E)Serrulacaulis spineus.(F)Hoxtolgay robusta.

Givetian temporal range. Colpodexylon consists of six species including Banks, 1978; Schweitzer and Matten, 1982). A. doui represents the three (C. cachiriense, C. coloradense and C. camptophyllum) recorded only Aneurophytales found outside the Rheic Ocean area (Jiang et al., from the Campo Chico Formation (Table 1; Edwards and Benedetto, 2013). 1985; Berry and Edwards, 1995), two (C. deatsii and C. trifurcatum) Xu et al. (2014) interpreted a late Emsian to Frasnian age for the from the upper Devonian of New York (Banks, 1944; Grierson and Hujiersite Formation based on systematic analysis of spores from four Banks, 1963) and one from the Hujiersite Formation (Xu and Wang, sections in West Junggar. The palynological assemblage of the 251 Hill 2011). According to Xu and Wang (2011), Colpodexylon gracilentum section consists mostly of Cymbosporites cf. magnificus spores. strongly resembles Colpodexylon sp. from the Campo Chico Formation C. magnificus fossils are abundant in the early Givetian of eastern (Berry and Fairon-Demaret, 2001) and thus affirms a close relationship Melville Island, Canada (Table 1; McGregor, 1994; Richardson and between the two floras. McGregor, 1986) and are locally distributed in the uppermost Eifelian The zosterophyll Serrulacaulis is widely distributed throughout the to lower Givetian strata of Eastern Europe (Arkhangelskaya, 1974; Devonian (Table 1; Xu, 2011; Xu et al., 2011b). Serrulacaulis furcatus oc- Avkhimovitch et al., 1993). Megaspore assemblages from the 251 Hill curs in the lowermost Frasnian of New York (Hueber and Banks, 1979), section only include spores from Verrucisporites, which has been the Givetian of Ronquières, Belgium (Stockmans, 1968)andtheCampo documented in China (Lu and Ouyang, 1978; Xu et al., 2012b), Canada Chico Formation of Venezuela (Berry and Edwards, 1994). Confinement (Chaloner, 1959) and Saudi Arabia (Marshall et al., 2007). of S. furcatus to Givetian or Frasnian strata indicates a similar age for the Verrucisporites lui also appears in the Xichong Formation of Yunnan, Hujiersite Formation. The iridopteridalean Compsocradus (type species: southern China (Xu et al., 2012b). The Xichong Formation contains C. laevigatus) occurs in the Campo Chico Formation (Berry and Stein, later Middle Devonian flora dominated by lycopsids, miospores, fish fos- 2000) and in the Hujiersite further demonstrating correlation of the sils and megaplants indicating a Givetian age (Cai, 2000; Wang et al., two floras. The progymnosperm genus Aneurophyton probably consists 2007). The Xichong flora however appears endemic only to local tropi- of two species: A. doui and the type species A. germanicum,thelatterof cal areas (Wang et al., 2007) and does not easily correlate with coeval which occurred throughout Middle Devonian Euramerica (Serlin and floras from other localities (Xu et al., 2015). Both the Xichong and D. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 463 (2016) 230–237 235

Fig. 5. U-Pb concordia diagrams with select cathodoluminescent (CL) images of zircons and frequency histograms of detrital age populations from sample H-01(A–B) and sample H-02 (C– D). Green bars in histograms represent youngest ages.

Hujiersite floras share examples of V. lui and a Tsaia, taxa of undeter- 6. Conclusion mined position (Wang and Berry, 2001; Wang et al., 2004). Both of these fossils occur in the Hujiersite hanging wall at the 251 Hill suggest- Plant taxa from the Hujiersite Formation of West Junggar, NW China, ing a Givetian age. known as the Hujiersite flora, resemble the Givetian to Frasnian Campo In conclusion, Haskinsia hastata, H. sagittata, Leclercqia complexa, Chico flora of Venezuela and the Givetian Xichong flora of China. The Serrulacaulis furcatus and Colpodexylom gracilentum species and the 251 Hill section hosts well studied examples of the flora along with tuff- genus Colpodexylon all appear in both the Hujiersite and Campo aceous sandstone exposed in both the hanging wall and foot wall of a re- Chico floras. Correlation of the Hujiersite and Campo Chico floras verse fault. The tuffaceous sandstones were collected and analyzed for thus indicates coeval development during late Givetian to Frasnian the detrital zircon age distribution. U-Pb ages obtained by ICP-MS stages. yielded youngest detrital zircon ages 385 Ma for the hanging wall and 380 Ma for the footwall. These age data suggest a Givetian to early Frasnian depositional age for the Hujiersite flora. Moreover, our age 5.2. Depositional age of the Hujiersite flora data will improve our knowledge of the Devonian environment in general, including the link between plants and atmospheric compositions. Geochronologic analysis of two samples of tuffaceous sandstone found in the foot wall and hanging wall of the 251 Hill small normal Acknowledgements fault yielded youngest ages of 380 Ma and 385 Ma, respectively. Youn- gest values in from detrital age populations may not give the most accu- This research was supported by the HKU Seed Funding Program rate depositional age (Anderson, 2005) since the youngest magmatic for Basic Research, Chang's HKU startup funding (201411159057), events must occur prior to deposition (Gehrels, 2014). Youngest ages Open Projects of the Key Laboratory of Economic Stratigraphy and from detrital populations however may offer approximate constraints Palaeogeography, Chinese Academy of Sciences (2016KF05) and the on deposition (e.g. Wang et al., 2014). The concordant ages reported National Natural Science Foundation of China (41272001). This work here are also consistent with general age constraints suggested by pale- is a contribution to UNESCO-IUGS IGCP Project 632. ontological evidence. A Givetian age (385 Ma) for the 251 Hill hanging wall and an early Frasnian age for its foot wall (380 Ma) (after Cohen Appendix A. Supplementary data et al., 2013) represent novel geochronologic constraints for the upper Hujiersite Formation. This duration accords with biostratigraphic inter- Supplementary data to this article can be found online at doi:10. pretations as discussed above. 1016/j.palaeo.2016.10.015. 236 D. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 463 (2016) 230–237

Table 1 Megaplant, miospore and megaspore fossils recorded in Hujiersite Formation and localities hosting similar species used for correlations.

Genus Species Locality Horizon Age References

Haskinsia H. hastata Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Xu et al. (2008) H. hastata Venezuela Campo Chico Fm. Late Givetian or Frasinian Berry and Edwards (1996) H. sagittata Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Xu et al. (2008) H. sagittata Venezuela Campo Chico Fm. Late Givetian or Frasinian Berry and Edwards (1996) Leclercqia L. uninata Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Xu et al. (2011a) L. cf. complexa Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Xu and Wang (2008) L. complexa Venezuela Campo Chico Fm. Mid–late Givetian Harvey (1999) L. complexa Belgium Eifelian and late Givetian Fairon-Demaret (1981) L. complexa New York, USA Panther Mountain Fm. Middle Givetian Banks et al. (1972) L. complexa Wyoming, USA Beartooth Butte Fm. Early Devonian Tanner (1983) L. complexa Queensland, Australia Storm Hill Sandstone Fm. Eifelian–late Givetian Meyer-Berthaud et al. (2003) L. complexa Germany Givetian Fairon-Demaret (1980) L. complexa Canada Campbellton Fm. Emsian Gensel and Albright (2006) Colpodexylon C. gracilentum Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Xu and Wang (2011) C. cachiriense Venezuela Campo Chico Fm. Late Givetian or Frasinian Berry and Edwards (1995) C. coloradense Venezuela Campo Chico Fm. Late Givetian or Frasinian Edwards and Benedetto (1985) C. camptophyllum Venezuela Campo Chico Fm. Late Givetian or Frasinian Berry and Edwards (1995) C. deatsii New York, USA Middle–early late Devonian Grierson and Banks (1963) C. trifurcatum New York, USA Middle–early late Devonian Banks (1944) C. sp. Venezuela Campo Chico Fm. Middle Devonian Berry & Fairon-Demaret (2001) Hoxtolgaya H. robusta Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Xu et al. (2012a) Drepanophycus D. minor Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Xu et al. (2013) Serrulacaulis S. spineus Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Xu et al. (2011b) S. cf. furcatus Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Xu (2011) and Xu et al. (2011b) S. furcatus New York, USA Genesee group Earliest Frasnian Hueber and Banks (1979) S. furcatus Belgium Givetian Stockmans (1968) S. furcatus Venezuela Campo Chico Fm. Late Givetian or Frasinian Berry and Edwards (1994) Compsocradus C. givetianus Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Wang (2008) and Fu et al. (2011) C. laevigatus Venezuela Campo Chico Fm. Late Givetian or Frasinian Berry and Stein (2000) Aneurophyton A. doui Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Jiang et al. (2013) Teraxylopteris T. sp. Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Jiang et al. (2013) Tsaia T. conica Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Wang et al. (2004) T. denticulata Yunnan, China Xichong Fm. Givetian Wang and Berry (2001) Taeniocrada T. cf. decheniana Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Cai and Wang (1995) T. gracilis Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Cai and Wang (1995) Barssasia B. sibirica Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Dou (1983) and Cai and Wang (1995) Cymbosporites C. cf. magnificus Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Xu et al. (2014) C. magnificus Arctic Canada Givetian Richardson and McGregor (1986) C. magnificus Eastern Europe Latest Eifelian to early Givetian Avkhimovitch et al. (1993) Verrucisporites V. lui Xinjiang, China Hujiersite Fm. Givetian–early Frasnian Xu et al. (2012a, 2012b) V. lui Yunnan, China Xichong Fm. Givetian Xu et al. (2012a, 2012b)

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