Journal of Asian Earth Sciences 159 (2018) 24–33

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Journal of Asian Earth Sciences

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Magnetochronology of the mammalian faunas in the Lanzhou T Basin, Northwest ⁎ Peng Zhanga,b, Hong Aoa,c, , Mark J. Dekkersd, Zhisheng Ana,b, Lijuan Wanga, Yongxiang Lie, Shihang Liua,f, Xiaoke Qianga, Hong Changa, Hui Zhaoa,g a State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China b Beijing Normal University, Beijing 100875, China c Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA d Paleomagnetic Laboratory ‘Fort Hoofddijk’, Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The Netherlands e State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China f University of Chinese Academy of Sciences, Beijing 100049, China g Department of Geography, Shaanxi Normal University, Xi'an 710062, China

ARTICLE INFO ABSTRACT

Keywords: The fluvio-lacustrine sequence in the Lanzhou Basin, located at the northeastern margin of the Tibetan Plateau, Magnetostratigraphy is a rich source of Oligocene– mammalian fossils, critical to our understanding of the terrestrial Asian Lanzhou Basin and environmental evolution. While the Miocene mammalian faunas have been dated with magne- Land mammal fauna tostratigraphy, the numerical age of the Oligocene faunas remains controversial. Here, we present new high- Paleoenvironment resolution magnetostratigraphic records from two fluvio-lacustrine sections in the central Lanzhou Basin, which provide age constraints for two Oligocene mammal assemblages in the Lower Xianshuihe Formation, the Nanpoping and Xiagou Faunas. The Lower Xianshuihe Formation is suggested to span from polarity chrons C12r to C7n, ranging from ca 31 Ma to 24 Ma. The Early Oligocene Nanpoping Fauna correlates to the C12r–C11n.2n polarity chron intervals, yielding an age of ∼31–30 Ma, while the Late Oligocene Xiagou Fauna correlates to chrons C7An–C8n.2n, with an age of ∼26–25 Ma. This magnetostratigraphy provides a more accurate age than biochronology for the Oligocene giant mammal contained in the Nanpoping Fauna and sheds new light on the chronology and distribution of Indricotheriinae in Eurasia. The faunas, in particular the Nanpoping Fauna, suggest a mixed setting of woodlands and grasslands associated with a humid environment in the Lanzhou Basin during Early Oligocene, in contrast to its present arid environment with few woodlands.

1. Introduction covers an area of about 300 km2 (Fig. 1b). Three formations are re- cognized: from old to young respectively, the Xiliugou, Yehucheng, and Knowing the first occurrences of terrestrial in regions Xianshuihe Formation (Zhai and Cai, 1984; Yue et al., 2001). The Xi- across the globe with different paleoclimatic conditions is an important liugou Formation consists of red massive sandstone and gravelly component in our understanding of their evolution and expansion. sandstone layers. The Yehucheng Formation is distinctive in the dark Because of the step-wise uplift of the Tibetan Plateau associated with red monotony of its gypsiferous mudstone, siltstone and sandstone the Indo-Asia collision during the Paleocene (Yin and Harrison, 2000), units. The Xianshuihe Formation consists mainly of red mudstone and is well-developed long continuous Cenozoic fluvio-lacustrine strata are associated with several sandstone packages (Fig. 2). It can be further exposed in northwestern China (Zhai and Cai, 1984; Fang et al., 2003). subdivided into the Lower, Middle and Upper Xianshuihe Formations The Lanzhou Basin at the northeastern margin of the Tibetan Plateau (Qiu et al., 1997). (Fig. 1a) is a rich source of Oligocene–Miocene mammalian fossils and Recent magnetostratigraphic records have provided age constraints has attracted great attention from geologists and paleontologists since for the Xianshuihe Formation, Yehucheng Formation, and Upper the 1920s (Yang and Bian, 1937; Qiu and Gu, 1988; Qiu, 1989; Flynn Xiliugou Formation (Yue et al., 2001; Zhang et al., 2014; Ao et al., et al., 1999). The basin is a north-northwest trending syncline and 2016; Wang et al., 2016; Zhang et al., 2016). The Miocene faunas, such

⁎ Corresponding author at: State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China. E-mail address: [email protected] (H. Ao). https://doi.org/10.1016/j.jseaes.2018.03.021 Received 29 August 2017; Received in revised form 9 March 2018; Accepted 20 March 2018 Available online 21 March 2018 1367-9120/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/). P. Zhang et al. Journal of Asian Earth Sciences 159 (2018) 24–33

Fig. 1. Schematic location of Central and Eastern Asia (a), and a geologic map of the Lanzhou Basin (b). Red and purple five-pointed stars indicate fossil sites of ; white dashed line indicates the inferred northern margin of the arid belt in the Oligocene; XG, Xiagou fossil site, HYT, Huangyangtou fossil site, NPP, Nanpoping fossil site in (b); YD, Yongdeng section, XSH, Xianshuihe section, DTG, Duitinggou section; JMZ, Jimazhuang section; FHS, Fenghuangshan section in (b). The DTG and JMZ sections are investigated in this study. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) as the Xingjiawan (XJW) Fauna, Duitinggou (DTG) Fauna, Zhangjiaping Xianshuihe Formation has a thickness of ∼233 m and records polarity (ZJP) Fauna, and Miaozuizi (MZZ) Fauna in the Upper and Middle subchrons C6Cn.2r–C5En in the DTG section, ranging in age from 23 to Xianshuihe Formations have also been dated recently (Ao et al., 2016; 18 Ma (Zhang et al., 2016). Here we focus on the Lower Xianshuihe Zhang et al., 2016). However, the ages of the older Oligocene Xiagou Formation, which has a thickness of ∼150 m in the DTG section and (XG) Fauna and Nanpoping (NPP) Fauna remain controversial, because ∼97 m in the JMZ section. The Lower Xianshuihe Formation seems the currently available magnetostratigraphic records (Opdyke et al., essentially continuous in the DTG section; only a few erosional levels 1998; Flynn et al., 1999; Qiu et al., 2013; Zhang et al., 2014) have are observable that may be related to (short duration) hiatuses. Detailed ambiguous correlations to the geological polarity time scale (GPTS) lithological comparison with the DTG section and field investigations (Hilgen et al., 2012; Vandenberghe et al., 2012). Here, we establish new indicate that the top and bottom of the JMZ section feature potential high-resolution magnetostratigraphic records from two fluvio-lacus- sedimentary hiatuses. trine sections in the central Lanzhou Basin, aiming to provide updated The NPP and XG Faunas were initially excavated in the villages of precise ages for the XG and NPP Faunas. Nanpoping (36°08′32.8′′ N, 103°36′6.0′′ E, ∼8 km southwest of the DTG section) and Ganjiatan (36°14′36.7′′ N, 103°35′3.8′′ E, ∼m3k 2. Geological setting northwest of the DTG section) (Qiu et al., 1998; Wang and Qiu, 2000a, b; Qiu et al., 2001), respectively (cf. Fig. 1b for their locations). The In this study, the previously investigated stratigraphic section of the NPP Fauna was unearthed from a thick yellow sandstone package at the Xianshuihe Formation at DTG (Zhang et al., 2016) is extended down- bottom of the Xianshuihe Formation. This package/layer is an im- ward and complemented by a new parallel section at Jimazhuang portant guide bed for within-basin correlation and can be traced across (JMZ). The JMZ section (36°12′29.8–30.5″ N, 103°36′56.5–57.9″ E) lies the entire Lanzhou Basin (Zhai and Cai, 1984; Qiu et al., 2001; Yue 1.5 km south of the DTG section (36°13′10.6″–12′55.1″ N, et al., 2001). Also, the thick red mudstone sequence yielding the XG 103°36′39.6–48.2″ E). Both sections lie on the eastern side of the Fauna between that yellow sandstone package and packages of white Lanzhou Basin syncline; the bedding attitudes of the Xianshuihe For- gravelly sandstone can be readily recognized in the field (Zhai and Cai, mation are west-dipping with fairly low tilts of ∼30°. The Middle 1984; Yue et al., 2001; Xie, 2004). As suggested by previous studies and

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Fig. 2. Photographs of the DTG (a and b) and JMZ (c and d) sections.

Table 1 List of Xiagou (XG) Fauna and Nanpoping (NPP) Fauna in the Lanzhou Basin (Qiu et al., 1997, 2013; Xie, 2004; Li et al., 2017).

Fauna Species

XG Amphechinus sp., Amphechinus cf. A. rectus, Amphechinus cf. A. minimus, Soricidae indet., Desmatolagus cf. D. gobiensis, Ordolagus sp., Sinolagomys kansuensis, Sinolagomys cf. S. major, Tsaganomys altaicus, Tataromyindae indet., Tataromys plicidens, Tachyoryctoides sp., Eucricetodon sp., Heterosminthus lanzhouensis, Parasminthus spp., Parasminthus asiae-centralis, Parasminthus tangingoli, Parasminthus parvulus, Sinosminthus sp., Yindirtemys xiningensis, Yindirtemys ambiguous, Yindirtemys granger, Litodonomys huangheensis, Didymoconus berkeyi, Aprotodon sp. NPP Desmatolagus pusillus, Ochotonidae gen. et sp. indet., Ordolagus teilhardi, Aplodontidae, Tsaganomys altaicus, Tataromys plicidens, Tataromys sigmodon, Tataromys minor, Eucricetodon asiaticus, Parasminthus tangingoli, Parasminthus parvulus, Yindirtemys granger, Bounomys bohlini, Bounomys ulantatalensis, Monosaulax sp., Anomoemys lohiculus, Karakoromys sp., Steneofiber sp., Allacerops cf. A. turgaica, Paraceratherium huangheense sp., Schizotherium ordosium, Aprotodon lanzhouensis, Paraentelodon sp.

Notes: The fossils of Paraceratherium huangheense sp. were excavated in the yellow sandstone package in the bottom part of the Lower Xianshuihe Formation at Huangyangtou (cf. Fig. 1b for the location) during our field investigation in 2013, and reported by Li et al. (2017). This is the first time that the Paraceratherium fossils were found in the Lower Xianshuihe Formation. confirmed by our new field investigation, the fossiliferous layers of the Thermal demagnetization up to 680 °C included a maximum of 18 steps NPP Fauna can be stratigraphically correlated to the yellow sandstone with intervals of 50 °C below 580 °C and 20–30 °C above. After each layer at the bottom of the DTG (36°12′55.1″ N, 103°36′48.2″ E) section demagnetization step, the remaining NRM was measured with a hor- and JMZ section (36°12′30.5″ N, 103°36′57.9″ E), while the XG Fauna is izontal pass-through 3-axis 2-G cryogenic superconducting magnet- linked to the upper part of the red mudstone around 250–270 m stra- ometer (model 755-R) housed in a magnetically shielded space (< 300 tigraphic level (0 m is the top of the section; 36°13′11.4″ N, nT). The paleomagnetic measurements were carried out at the Institute 103°36′44.9″ E) in the DTG section, and at 20–30 m (0 m is the top of of Earth Environment, Chinese Academy of Sciences, Xi’an. the section; 36°12′30.2″ N, 103°36′57.9″ E) in the JMZ section. The Demagnetization results were evaluated by orthogonal diagrams mammal species of the NPP and XG faunal assemblages are listed in (Zijderveld, 1967); the principal component direction for each sample Table 1 (Qiu et al., 1997; Xie, 2004; Qiu et al., 2013; Li et al., 2017). was computed using a least-squares linear fitting technique (Kirschvink, 1980). The principal component analysis (PCA) was done with the 3. Sampling and methods PaleoMag software developed by Jones (Jones, 2002).

In order to obtain samples that were as fresh as possible, the upper 4. Results 30–50 cm of the outcrop was removed (usually) to eliminate potential weathering effects and other vegetation disturbance. 546 and 195 block The NRM intensity of the samples is usually of the order of – – – samples were taken from the DTG (with intervals of 20–30 cm) and JMZ 10 4–10 3 A/m, while the background value is less than 10 6 A/m. The sections (with an interval of 50 cm), respectively; they were oriented characteristic remanent magnetization (ChRM) directions were selected using a magnetic compass in the field. From each oriented block, two from at least four (but typically 6–15) consecutive demagnetization cubic specimens of 2 cm × 2 cm × 2 cm were prepared in the labora- steps starting from at least 250 °C (with upper temperatures of tory for stepwise thermal demagnetization. ≥450 °C) with a maximum angular deviation (MAD) less than 15° for a Stepwise thermal demagnetization of the natural remanent mag- line fit. For most samples in this study, after progressive removal of a netization (NRM) was performed with a TD-48 thermal demagnetizer. low-temperature component (usually a viscous magnetization) between

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Fig. 3. The direction and intensity evolution of the NRM during stepwise thermal demagnetization for selected samples from the DTG and JMZ sections. Solid (open) circles represent horizontal (vertical) planes. The numbers refer to the demagnetization temperatures in °C and NRM is the starting natural remanent magnetization.

100 and 250 °C, a high-temperature ChRM is isolated during the steps (Hoffman, 1992). up to 680 °C (Fig. 3). The ChRM decays uni-vectorially toward the The polarity sequence of the DTG section is based on the VGP la- origin of orthogonal plots. Consistent with previous results (Zhang titudes of the remaining 362 ChRM directions. Polarity zones are de- et al., 2015, 2016; Ao et al, 2016) of the Oligocene–Miocene red beds in fined by at least two successive levels of the same polarity. Together the Lanzhou Basin, the demagnetization results indicate that hematite is with the previous 269 ChRM directions from the upper part of the DTG a dominant ChRM carrier, while magnetite is a minor ChRM carrier as section (Zhang et al., 2016), a total of 18 pairs of normal (N1–N18, 378 indicated by no significant inflection of remanent magnetization at the samples) and reversed (R1–R18, 253 samples) polarity zones are Curie temperature of magnetite (∼580 °C) in the intensity evolution identified in the DTG section (Figs. 4 and 5). The 378 normal ChRM diagrams (Fig. 3). Among the 546 and 195 demagnetized samples from directions yield an overall mean of declination D = 15.8° and inclina-

DTG and JMZ sections, 370 and 123 samples yielded reliable ChRM tion I = 24.3° (k = 13.5, α95 = 2.0°; k is the precision parameter and directions, respectively. Virtual geomagnetic pole (VGP) latitudes were a95 is the radius of 95% confidence cone around the mean direction) calculated from these ChRM directions for both sections. Eight samples before tilt adjustment and D = 3.3° and I = 34.7° (k = 13.7, α95 = from the DTG section and 7 from the JMZ section that have VGP lati- 2.0°) after tilt adjustment. The 253 reversed ChRM directions yield an tudes divergent more than 30° from the means of normal and reversed overall mean of D = 204.4° and I = –18.2° (k = 11.3, α95 = 2.8°) be- VGP latitudes (red triangles in Fig. 4) were not used to determine the fore tilt adjustment and D = 194.6° and I = –35.7° (k = 12.5, α95 = polarity, as they may have recorded a transitional geomagnetic field 2.6°) after tilt adjustment. After tilt correction, the 631 ChRM directions

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Fig. 4. Lithostratigraphy and magnetostratigraphy of the DTG (this study and Zhang et al. (2016)) and JMZ sections in the Lanzhou Basin and their correlations to the geomagnetic polarity timescale (GPTS). VGP, Virtual geomagnetic pole latitudes; gray dots, lines and polarity zones, data from Zhang et al. (2016); black dots, lines and polarity zones, data from this study; red triangles, VGP latitudes that more than 30° away from the means of normal and reversed VGP latitudes.

cluster in approximately antipodal normal and reversed polarity or- (k = 12.6, α95 = 5.0°) before tilt adjustment and D = 353.2° and ientations (Fig. 5a), passing the reversals test (McFadden and I = 40.3° (k = 12.1, α95 = 5.0°) after tilt adjustment. The 43 reversed McElhinny, 1990). The jackknife technique (Tauxe and Gallet, 1991)is ChRM directions yield an overall mean of D = 209.5° and I = –13.1° a useful tool to test the robustness of a magnetic polarity zonation. The (k = 14.8, α95 = 5.9°) before tilt adjustment and D = 199.4° and obtained jackknife parameter (J) is −0.2945 (Fig. 6a), which falls I=–30.4° (k = 14.8, α 95 = 5.9°) after tilt adjustment (Fig. 5b). The within the range of 0 to −0.5 that was recommended for a robust ChRM directions of the JMZ section fail the reversals test, but the low magnetostratigraphic data set by Tauxe and Gallet (1991). This sup- value of J (−0.2189, Fig. 6b) and the same polarity pattern as that of ports a primary origin of the ChRM and the reliability of our magne- the DTG section still testifies to the reliability of both magnetostrati- tostratigraphy. graphic records, minor differences are inferred to be due to differences VGP latitudes calculated from 116 ChRM directions are used to in continuity and the presence of hiatuses, primarily in the JMZ section. establish the polarity sequence of the JMZ section (Fig. 4). Five normal and four reversed polarity zones are identified in the JMZ section 5. Discussion (Fig. 4). The normal polarity zones N1 and N2–N4 in the JMZ section correlate to polarity zones N12 and N13–N15 in the DTG section, re- 5.1. Correlation to the GPTS spectively. The lowermost long normal polarity zone N5 in the JMZ section correlates to two short normal polarity zones N16 and N17 in Although a precise age cannot be estimated from the mammalian the DTG section (Fig. 4). The 73 normal ChRM directions yield an faunas excavated in the Lanzhou Basin, the faunas themselves provide overall mean of declination D = 14.5° and inclination I = 32.7° an approximate chronology as a starting point for our

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Fig. 5. Equal-area projections of the ChRM directions of the DTG (a) and JMZ (b) sections before (in-situ) and after (tilt-corrected) tilt correction. Solid/open symbols represent downward/upward inclinations; N, number of samples; D, declination; I, inclination; α95, radius of 95% confidence cone around the mean direction; k, precision parameter. magnetostratigraphy. The NPP Fauna is thought to be an Early DTG section is near-continuous from the uppermost part of C12r to Oligocene fauna based on the occurrence of the large mammal C7n.2n polarity chrons, ranging from ca 31 to 24 Ma. Likewise, the JMZ Schizotherium ordosium (an extinct relative of the horse) and small section spans from polarity chrons C11n.2n to C7An, ranging from mouse-like mammals Tataromys and Bounomys, which are usually found ∼30 Ma to 25 Ma (Fig. 4). Possibly due to a sedimentary hiatus, po- in Oligocene strata, and the absence of advanced Tachyoryctoides (a larity chrons C11r and C12n are observed at the bottom of the DTG mouse-like mammal) and Sinolagomys (a rabbit-like mammal) (Qiu section but are missing at the bottom of the JMZ section. Likewise, et al., 1997; Xie, 2004). The presence of the rodents (e.g., Tataromys sp. polarity subchrons C7n.2n and C7r are recorded in the uppermost part and Yindirtemys grangeri) and carnivores (e.g., Didymoconus berkeyi) of the Lower Xianshuihe Formation in the DTG section, but are absent indicates a Late Oligocene age for the XG Fauna (Qiu et al., 1997; Wang in the uppermost JMZ section. and Qiu, 2000). Thus, the NPP and XG Faunas suggest that the Lower Our new high-resolution magnetostratigraphic records (Fig. 4) are Xianshuihe Formation ranges from Early Oligocene to Late Oligocene. based on robust correlations to the GPTS, in contrast to a previous DTG Moreover, our recently established magnetostratigraphy suggests that magnetostratigraphic record (Fig. 7 a) that is typified by more ambig- the overlying Middle Xianshuihe Formation in the DTG section spans uous correlations to the GPTS (Opdyke et al., 1998; Flynn et al., 1999; from 23 to 18 Ma (Fig. 4)(Zhang et al., 2016). Based on the established Qiu et al., 2013). Our magnetostratigraphy considerably improves the biochronology and overlying magnetostratigraphy, we can readily polarity chron structure of the DTG section and allows identification of correlate the magneto-sequence of the Lower Xianshuihe Formation in some short-duration polarity chrons that were overlooked in the pre- the DTG section to the GPTS. A distinctive pattern of two long normal vious magnetostratigraphic record. The normal polarity chron N4 of the intervals (N11 and N13) separated by two reversed intervals and a previous magnetostratigraphy is possibly comparable to polarity chrons shorter normal interval (N12) enables unequivocal correlation to po- N5 –N8 of the present magnetostratigraphy, but the two intervening larity chrons C7n.2n–C8n.2n (Fig. 4). The underlying two normal in- reversed polarity chrons are missing in the previous magnetostrati- tervals (N14 and N15) separated by a long reversed interval (R14) graphy. The long normal polarity chron N5 of the previous DTG mag- correlate to polarity chrons C9n–C10n.2n. The lowermost N16–N18 netostratigraphy probably relates to polarity chrons N9–N10 of the portion is correlated to C11n.1n–C12n (Fig. 4). In this correlation, we present magnetostratigraphy, but again, the intervening reversed po- note that two expected short reversed polarity subchrons (C8n.1r and larity chron is missing; that interval would correspond to C6cn.1r in the C10n.1r) in the GPTS are missing in the DTG section, which might be GPTS. The long normal polarity chron N6 identified between 150 and due to a minor hiatus in the sedimentary record (Charreau et al., 2005). 210 m by Opdyke et al. (1998) possibly corresponds to the present This absence of short polarity chrons is rather common in the Cenozoic polarity chrons N11–N14 and three intercalated reversed polarity sedimentary records in western China (Charreau et al., 2005; Dai et al., chrons are missing. Below, two normal polarity zones (N16 and N17) 2006; Xiao et al., 2012). Thus, the Lower Xianshuihe Formation in the identified in this study that correspond to polarity chrons C11n.1n and

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Formation provides robust age constraints for the NPP and XG Faunas. The XG Fauna is stratigraphically correlated to the middle part of the red mudstone at around 250–270 m and thus to chrons C7An–C8n.2n, yielding an estimated age of ∼26–25 Ma. It is correlated to the Tabenbulakian stage of Land mammalian ages of Asia (ALMA), the middle Arvernian stage of Land mammalian ages of Europe (ELMA), and the middle Ar2 stage of Land mammalian ages of North America (NALMA). The XG Fauna is essentially synchronous with the Xiejia Fauna in the Xining Basin (Dai et al., 2006; Xiao et al., 2012). Both contain predominantly small mammals, such as Sinolagomys, Para- sminthus and Yindirtemys (Li et al., 1981; Qiu et al., 1997, 2013). The NPP Fauna is stratigraphically correlated with the thick yellow sand- stone package (357.9–383 m) at the bottom of the Xianshuihe Forma- tion (Fig. 4), which spans from the C12r to C11n.2n polarity chrons. Thus, the age of the NPP Fauna is estimated to be around 31–30 Ma. It is contemporaneous with the latest Hsandagolian–earliest Tabenbula- kian stage of ALMA, the Suevian stage of ELMA, and the late Whit- neyan–earliest Arikareean 1 (Ar1) stage of NALMA. Most of the Oligocene-Miocene mammalian faunas unearthed in the Lanzhou Basin have now been well dated (Flynn et al., 1999; Yue et al., 2001; Qiu et al., 2013; Zhang et al., 2014; Ao et al., 2016; Zhang et al., 2016), which allows to build a magnetochronological sequence for these faunas (Fig. 8). The XJW Fauna is correlated with the boundary between C4n.2n and C4r.1r, or between C5n.2n and C5r.1r, with an age of8Maor11Ma(Fig. 8)(Ao et al., 2016). The DTG Fauna excavated from the sandstone package in the DTG section is located in the up- permost part of the polarity chron C6n, with an age of 18.9 Ma (Zhang et al., 2016). The ZJP Fauna is located between polarity chron C6An.2n and C6AAr.1n (∼21.3–20.6 Ma), and the MZZ Fauna occurs between polarity chron C6Cn.2n and C7n (∼24–23.2 Ma) (Zhang et al., 2016). Combining an age of ∼26–25 Ma for the XG Fauna and ∼31– 30 Ma for the NPP Fauna, there seems to be significant mammal flourishing from Early Oligocene to Late Miocene in the Lanzhou Basin, with suitable environmental conditions (Fig. 8). The taxonomic composition and faunal properties of mammals are sensitive to environmental conditions (Matthews et al., 2011; Zhang et al., 2016). Both typical grassland (e.g., Tataromys and Bounomys) and woodland species (e.g., Paraceratherium) are identified in the mamma- lian faunas (Table 1, Fig. 8); the grassland species account for more than half in most Faunas except for the XJW and MZZ Faunas (Fig. 8). A mixed setting of woodlands and grasslands in the Oligocene can thus be inferred from the composition of the mammals in the Lanzhou Basin, in contrast to the present cold and dry conditions. This paleoclimatic in- ference is supported by more than 20 species of large plant fossils and semi-humid indicative features of the cuticle of Populus davidiana Dode in the yellow sandstone package at the bottom of the DTG section (Sun et al., 2004; Yao et al., 2010). Palynological results show dominant arboreal plants and rare xerophytes in the lower part of the red mud- stone in the DTG section, which indicate rather warm and humid Oli- gocene climate in the Lanzhou Basin that is similar to the present cli- mate in Southeast China (Miao et al., 2013). Likewise, the neighboring Fig. 6. Magnetostratigraphic Jackknife (J) analysis (Tauxe and Gallet, 1991) for the DTG Linxia Basin (south of the Lanzhou Basin) had dominant woodland and JMZ sections. species in the Oligocene (Deng, 2004, 2011). At present, however, both the Lanzhou and Linxia Basins have become much less suitable for C11n.2n, are missing in the previous low-resolution magnetostrati- mammals and vegetation because of the enhanced aridity and cooling graphy. Our established entire DTG magnetochronology is comparable during Late Cenozoic times. to that of the Xianshuihe section (Wang et al., 2016) in the western The precise magnetostratigraphic dating of these mammalian faunas Lanzhou Basin and to that of the Xiejia and Tashan sections (Dai et al., also provides a chronological base for further study of the mammalian 2006; Xiao et al., 2012) in the neighboring Xining Basin (Fig. 7). The migration in East Asia during the Oligocene. Indricotheriinae is a sub- regional comparability of these magnetostratigraphic records provides family of , one of the largest mammals that ever lived, additional evidence for their accurate and reliable correlations to the equaling the medium-sized sauropod dinosaurs in size (Fortelius and GPTS. Kappelman, 1993). The first Indricotheriinae described was Para- ceratherium bugtiense (Pilgrim, 1908) from ‘Chur Lando’ in the Bugti 5.2. Implications for mammal migration and paleoenvironment Hills of central Pakistan, south of the Tibetan Plateau. The fossil bearing strata were suggested to span from the early Early Oligocene to the late Our high-resolution magnetochronology for the Lower Xianshuihe Late Oligocene (Welcomme et al., 2001; Antoine et al., 2003, 2004). In

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Fig. 7. Lithostratigraphy and magnetostratigraphy of the (a) DTG section (Opdyke et al., 1998), (b) DTG section (this study and Zhang et al. (2016)), (c) JMZ section (this study), and (d) Xianshuihe section (Wang et al., 2016) in the Lanzhou Basin and of the (f) Xiejia (Dai et al., 2006) and (g) Tashan (Xiao et al., 2012) sections in the adjacent Xining Basin, and their correlations to the (e) GPTS (Hilgen et al., 2012; Vandenberghe et al., 2012). The correlations of Xianshuihe, Xiejia and Tashan magnetostratigraphic records to the GPTS are the same as in their original, published studies (Dai et al., 2006; Xiao et al., 2012; Wang et al., 2016), but the previous DTG magnetostratigraphy (Opdyke et al., 1998) is re-interpreted here based on our improved polarity chron structure and regional stratigraphic comparisons.

Fig. 8. A composite diagram with all mammalian faunas in the Lanzhou Basin within the framework of the GPTS and the variability of the total number of different mammalian species and percentages of grassland and woodland species for these mammalian faunas (Zhang 1993; Xie, 2004; Ao et al., 2016; Zhang et al., 2016). QTG, Quantougou Fauna.

31 P. Zhang et al. Journal of Asian Earth Sciences 159 (2018) 24–33 the north of the Tibetan Plateau, other genera of Indricotheriinae were Perissodactyla) from the Late Oligocene of north-central Anatolia (Turkey). Zool. J. also recorded in the –Oligocene of Kazakhstan (Gromova, 1959; Linn. Soc. 152, 581–592. Ao, H., Zhang, P., Dekkers, M.J., Roberts, A.P., An, Z.S., Li, Y.X., Lu, F.Y., Lin, S., Li, X.W., Kordikova, 1994), Mongolia (Osborn and Berkey, 1923), and China, the 2016. New magnetochronology of Late Miocene mammal fauna, NE Tibetan Plateau, latter in for example, Yunnan (Chow, 1958; Chiu, 1962), Inner Mon- China: mammal migration and paleoenvironments. Earth Planet. Sci. Lett. 434, golia (Chow and Chiu, 1963, 1964), Xinjiang (Chow and Xu, 1959; Ye 220–230. Charreau, J., Chen, Y., Gilder, S., Dominguez, S., Avouac, J.P., Sen, S., Sun, D.J., Li, Y.A., et al., 2002, 2003) and Gansu (Chiu, 1973; Qiu et al., 1997, 2004). So Wang, W.M., 2005. Magnetostratigraphy and rock magnetism of the Neogene Kuitun far, however, numerical ages for those mammalian faunas including He section (northwest China): implications for Late Cenozoic uplift of the Tianshan Indricotheriinae have remained unavailable, with just a biochronology mountains. Earth Planet. Sci. Lett. 230, 177–192. order (Antoine et al., 2004; Lindsay et al., 2005). In the Nanpoping Chiu, C.S., 1962. Giant from Loping, Yunnan, and discussion on the taxonomic characters of Indricotherium grangeri. Vertebr. Pal. Asiat. 6, 57–71. Fauna of the Lanzhou Basin, the Paraceratherium is dated at Chiu, C.S., 1973. A new genus of giant rhinoceros from Oligocene of Dzungaria. Sinkiang. ∼31–30 Ma, consistent with previous biochronology. After the first Vertebr. PalAsiat. 11, 182–191. Chow, M.C., 1958. Some Oligocene mammals from Lunan. Yunnan. Vertebr. PalAsiat. 2, appearance in the Lanzhou Basin, the Indricotheriinae was also found in – – 263 267. the MZZ Fauna, which has been dated to 24 23.2 Ma (Zhang et al., Chow, M.C., Chiu, C.S., 1963. A new genus of giant rhinoceros from Oligocene of Inner 2016). The precise dating of the Indricotheriinae in the Lanzhou Basin Mongolia. Vertebr. PalAsiat. 7, 230–239. supports the idea that Indricotheriinae lived in Asia throughout the Chow, M.C., Xu, Y.X., 1959. Indricotherium from Hami basin. Sinkiang. Vertebr. PalAsiat. 3, 93–96. Middle Eocene to Late Oligocene (Fig. 1)(Osborn and Berkey, 1923; Chow, M.C., Chiu, C.S., 1964. An Eocene giant rhinoceros. Vertebr. PalAsiat. 8, 264–267. Qiu et al., 2004; Antoine et al., 2008). Dai, S., Fang, X.M., Dupont-Nivet, G., Song, C.H., Gao, J.P., Krijgsman, W., Langereis, C., Under the influence of strong Westerlies, winter monsoon, and Zhang, W.L., 2006. Magnetostratigraphy of Cenozoic sediments from the Xining Basin: tectonic implications for the northeastern Tibetan Plateau. J. Geophys. Res. northern hemisphere glaciation, the present Asian arid belt has a large 111. expansion, extending to > 50°N latitude (Fig. 1a). However, there are Deng, T., 2004. Evolution of the Late Cenozoic mammalian faunas in the Linxia Basin and abundant Eocene–Oligocene Indricotheriinae fossil sites in the northern its background relevant to the uplift of the Qinghai-Xizang Plateau. Quat. Sci. 24, 413–420. part of the present arid belt (Fig. 1a). According to the taxonomic Deng, T., 2011. Diversity variations of the Late Cenozoic mammals in the Linxia Basin and composition and faunal properties of the mammals, the In- their response to the climatic and environmental backgrounds. Quat. Sci. 31, dricotheriinae-bearing faunas indicate humid and warm environment at 577–588 (in Chinese with English abstract). those sites, in contrast to the present arid environment. The northern Fang, X.M., Garzione, C., Van der Voo, R., Li, J.J., Fan, M.J., 2003. Flexural subsidence by 29 Ma on the NE edge of Tibet from the magnetostratigraphy of Linxia Basin, China. margin of the arid belt was possibly displaced to ∼40°N latitude (da- Earth Planet. Sci. Lett. 210, 545–560. shed line in Fig. 1a) during the Oligocene when northern hemisphere Flynn, L.J., Downs, W., Opdyke, N., Huang, K.N., Lindsay, E., Ye, J., Xie, G.P., Wang, glaciation topography did not exist (Zachos et al., 2001) and Westerlies X.M., 1999. Recent advances in the small mammal biostratigraphy and magnetos- tratigraphy of Lanzhou Basin. Chin. Sci. Bull. 44, 105–118. and winter monsoon were possibly not so strong as at present. Fortelius, M., Kappelman, J., 1993. The largest land mammal ever imagined. Zool. J. Linn. Soc. 108, 85–101. 6. Conclusions Gromova, V.I., 1959. Gigantskie nosorogi. Izd. Akad, Nauk SSSR. Hilgen, F.J., Lourens, L.J., Van Dam, J.A., 2012. The Neogene Period. In: Gradstein, F.M., Ogg, J.G., Schmitz, M.D., Ogg, G.M. (Eds.), The geologic time scale. Elsevier, Detailed paleomagnetic investigations on the fossiliferous Lower Amsterdam, pp. 923–978. Xianshuihe Formation were carried out in the Lanzhou Basin. The Ho ffman, K.A., 1992. Dipolar reversal states of the geomagnetic field and core-mantle dynamics. Nature 359, 789–794. correlation of the magnetostratigraphy to the GPTS provides a uniquely Jones, C.H., 2002. User-driven integrated software lives: “Paleomag” paleomagnetics constrained Oligocene chronology for the DTG section and the asso- analysis on the Macintosh. Comput. Geosci. 28, 1145–1151. ciated mammalian faunas. The Lower Xianshuihe Formation in the DTG Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of palaeomag- netic data. Geophys. J. R. Astron. Soc. 62, 699–718. section spans from the uppermost part of C12r to the C7n.2n polarity Kordikova, E., 1994. Oligocene vertebrate assemblages in central Kazakhstan: faunas, chrons, ranging from ca 31 to 24 Ma. The magnetostratigraphic age biogeography and paleoecology. Hist. Biol. 8, 191–208. model provides more precise age constraints for the associated NPP and Li, C.K., Qiu, Z.D., Wang, S.J., 1981. Discussion on Miocene stratigraphy and mammals – XG faunal assemblages than previous biochronology: ∼31–30 Ma for from Xining Basin. Qinghai. Vertebr. PalAsiat. 19, 313 320 (in Chinese with English abstract). the NPP Fauna and ∼26–25 Ma for the XG Fauna. This makes it pos- Li, Y.X., Zhang, Y.X., Li, J., Li, Z.C., Xie, K., 2017. New fossils of paraceratheres sible to precisely place these terrestrial faunas in the international (Perissodactyla, Mammalia) from the Early Oligocene of the Lanzhou Basin, Gansu – mammal zones. In addition, the faunal compositions suggest that the Province, China. Vertebr. PalAsiat. 55, 367 381. Lindsay, E.H., Flynn, L.J., Cheema, I.U., Barry, J.C., Downing, K., Rajpar, A.R., Raza, S.M., Oligocene paleoenvironment in the Lanzhou Basin was characterized by 2005. Will Downs and the Zinda Pir Dome. Palaeontol. Electronica 8, 1–19. a mixed setting of woodlands and grasslands, in contrast to the current Matthews, T., Rector, A., Jacobs, Z., Herries, A.I., Marean, C.W., 2011. Environmental semiarid-arid environment with few forests and sparse vegetation. implications of micromammals accumulated close to the MIS 6 to MIS 5 transition at pinnacle Point cave 9 (Mossel Bay, Western Cape Province, South Africa). Palaeogeogr. Palaeoclimatol. Palaeoecol. 302, 213–229. Acknowledgements McFadden, P.L., McElhinny, M.W., 1990. Classification of the reversal test in palaeo- magnetism. Geophys. J. Int. 103, 725–729. fi Miao, Y.F., Wu, F.L., Herrmann, M., Yan, X.L., Meng, Q.Q., 2013. Late early Oligocene This study was nancially supported by the National Basic Research East Asian summer monsoon in the NE Tibetan Plateau: evidence from a palynolo- Program of China (2013CB956402), National Natural Science gical record from the Lanzhou Basin, China. J. Asian Earth Sci. 75, 46–57. Foundation of China (41174057, 41290253 and 41704071) and the Opdyke, N.D., Flynn, L.J., Lindsay, E.H., Qiu, Z.X., 1998. The magnetic stratigraphy of the Yehucheng and Xianshuihe Formations of Oligocene- Miocene age near Lanzhou city. Open Grant of the State Key Laboratory of Loess and Quaternary Lanzhou, China, Paleontology Working Meeting Lanzhou, pp. 51. Geology (SKLLQG1535). We are grateful for the reviews and the edi- Osborn, H.F., Berkey, C.P., 1923. Baluchitherium grangeri, a giant hornless rhinoceros tor’s comments, which significantly improved this paper. from Mongolia. Am. Museum Nat. History. Pilgrim, G.E., 1908. The Tertiary and Post-tertiary Freshwater Deposits of Baluchistan and Sind: With Notices of New Vertebrate. Government Printing. References Qiu, Z.X., Gu, Z.G., 1988. A new locality yielding mid-Tertiary mammals near Lanzhou. Gansu. Vertebr. PalAsiat. 26, 198–213 (in Chinese with English abstract). Qiu, Z.X., Wang, B.Y., Deng, T., 2004. Indricotheres (Perissodactyla, mammalia) from Antoine, P.O., Shah, S.I., Cheema, I.U., Crochet, J.Y., de Franceschi, D., Marivaux, L., Oligocenein Linxia Basin, Gansu. China. Vertebr. PalAsiat. 42, 177–192. Métais, G., Welcomme, J.L., 2004. New remains of the baluchithere Paraceratherium Qiu, Z.X., 1989. The Chinese Neogene mammalian biochronology—its correlation with bugtiense from the Late/latest Oligocene of the Bugti hills, Balochistan, Pakistan. J. the European Neogene mammalian zonation. In: Lindsay, E.H., Fahlbusch, V., Mein, Asian Earth Sci. 24, 71–77. P. (Eds.), European Neogene mammal chronology. Springer, New York, pp. 527–556. Antoine, P.O., Welcomme, J.L., Marivaux, L., Baloch, I., Benammi, M., Tassy, P., 2003. Qiu, Z.X., Qiu, Z.D., Deng, T., Li, C.K., Zhang, Z.Q., Wang, B.Y., Wang, X.M., 2013. First record of Paleogene Elephantoidea (Mammalia, Proboscidea) from the Bugti Neogene Land Mammal Stages/Ages of China. In: Wang, X.M., Flynn, L.J., Fortelius, Hills of Pakistan. J. Vertebr. Paleontol. 23, 977–980. M. (Eds.), Fossil Mammals of Asia: Neogene Biostratigraphy and Chronology. Antoine, P.O., Karadenizli, L., Saraç, G., Sen, S., 2008. A giant rhinocerotoid (Mammalia, Columbia University Press, New York, pp. 29–90.

32 P. Zhang et al. Journal of Asian Earth Sciences 159 (2018) 24–33

Qiu, Z.X., Wang, B.Y., Qiu, Z.D., Heller, F., Yue, L.P., Xie, G.P., Wang, X.M., Engesser, B., Xie, G.P., 2004. The tertiary and local mammalian faunas in Lanzhou Basin Gansu. J. 2001. Land mammal geochronology and magnetostratigraphy of mid-Tertiary de- Stratigr. 28, 67–80 (in Chinese with English abstract). posits in the Lanzhou Basin, Gansu Province, China. Eclogae Geol. Helv. 94, 373–385. Yang, Z.J., Bian, M.N., 1937. Cenozoic geology of Yongdeng, Gaolan Gansu. Bull. Geol. Qiu, Z.X., Wang, B.Y., Qiu, Z.D., Xie, G.P., Xie, J.Y., Wang, X.M., 1997. Recent advances Soc. China 13, 199–201 (in Chinese). in study of the Xianshuihe Formation in Lanzhou Basin. In: Yongsheng, T. (Ed.), Yao, N.N., Ma, F.J., Yan, D.F., 2010. Reconstruction of palaeoclimatic parameters from Evidence for Evolution-Essays in Honor of Prof. China Ocean Press, Beijing, the Oligocene in Lanzhou Basin, Gansu Province. Gansu Geol. 19, 20–25 (in Chinese Chungchien Young on the Hundredth Anniversary of His Birth, pp. 177–192 (in with English abstract). Chinese). Ye, J., Meng, J., Wu, W.Y., 2003. Discovery of Paraceratherium in the northern Junggar Qiu, Z.X., Wang, B.Y., Xie, J.Y., 1998. Mid-Tertiary chalicothere (Perissodactyla) fossils Basin of Xinjiang. Vertebr. PalAsiat. 41, 220–229. from Lanzhou, Gansu, China. Vertebr. PalAsiat. 36, 297–318. Ye, J., Meng, J., Wu, W.Y., Wu, S.Y., 2002. The discovery of Late Eocene mammal fossils Sun, B.N., Yan, D.F., Xie, S.P., Cong, P.Y., Xin, C.L., Yun, F., 2004. Palaeogene fossil from Burqin of Xinjiang. Vertebr. PalAsiat. 40, 203–210. Populus leaves from Lanzhou Basin and their palaeoclimatic significance. Chin. Sci. Yin, A., Harrison, T.M., 2000. Geologic evolution of the Himalayan-Tibetan orogen. Annu. Bull. 49, 1494–1501 (in Chinese with English abstract). Rev. Earth Planet. Sci. 28, 211–280. Tauxe, L., Gallet, Y., 1991. A jackknife for magnetostratigraphy. Geophys. Res. Lett. 18, Yue, L., Heller, F., Qiu, Z., Zhang, L., Xie, G., Qiu, Z., Zhang, Y., 2001. 1783–1786. Magnetostratigraphy and pavleo-environmental record of Tertiary deposits of Vandenberghe, N., Hilgen, F., Speijer, R., 2012. The Paleogene Period. In: Gradstein, Lanzhou Basin. Chin. Sci. Bull. 46, 770–773. F.M., Ogg, J.G., Schmitz, M.D., Ogg, G.M. (Eds.), The geologic time scale. Elsevier, Zachos, J.C., Pagani, M., Sloan, L., Thomas, E., Billups, K., 2001. Trends, rhythms, and Amsterdam, pp. 855–921. aberrations in global climate 65 Ma to present. Science 292, 686–693. Wang, B.Y., Qiu, Z.X., 2000a. Dipodidae (Rodentia, Mammalia) from the lower member Zhai, Y.P., Cai, T.L., 1984. The tertiary system of Gansu Province. Gansu Geol. 00, 1–40. of Xianshuihe Formation in Lanzhou Basin, Gansu, China. Vertebr. PalAsiat. 38, Zhang, P., Ao, H., An, Z.S., Wang, Q.S., 2015. Rock magnetism properties of Oligocene 10–35. sediments in the Lanzhou Baisn. Chin. J. Geophys. -Chinese edition 58, 2445–2459. Wang, B.Y., Qiu, Z.X., 2000b. Micromammal fossils from red mudstone of lower member Zhang, P., Ao, H., Dekkers, M.J., Li, Y.X., An, Z.S., 2016. Late Oligocene-Early Miocene of Xianshuihe Formation in Lanzhou Basin, China. Vertebr. PalAsiat. 38, 266–281. magnetochronology of the mammalian faunas in the Lanzhou Basin—environmental Wang, W.T., Zhang, P.Z., Liu, C.C., Zheng, D.W., Yu, J.X., Zheng, W.J., Wang, Y.Z., Zhang, changes in the NE margin of the Tibetan Plateau. Sci. Rep. 6. H.P., Chen, X.Y., 2016. Pulsed growth of the West Qinling at ∼30 Ma in northeastern Zhang, X., 1993. New discovery of the mammalian fossils in Baode period of later Tibet: evidence from Lanzhou Basin magnetostratigraphy and provenance. J. Miocene epoch in Lanzhou Basin. Acta Geol. Gansu 2, 1–5 (in Chinese with English Geophys. Res. Solid Earth 121, 2235–2257. abstract). Welcomme, J.L., Benammi, M., Crochet, J.Y., Marivaux, L., Métais, G., Antoine, P.O., Zhang, Y.B., Sun, D.H., Li, Z.J., Wang, F., Wang, X., Li, B.F., Guo, F., Wu, S., 2014. Baloch, I., 2001. Himalayan Forelands: palaeontological evidence for Oligocene Cenozoic record of aeolian sediment accumulation and aridification from Lanzhou, detrital deposits in the Bugti Hills (Balochistan, Pakistan). Geol. Mag. 138, 397–405. China, driven by Tibetan Plateau uplift and global climate. Global Planet. Change Xiao, G.Q., Guo, Z.T., Dupont-Nivet, G., Lu, H.Y., Wu, N.Q., Ge, J.Y., Hao, Q.Z., Peng, S.Z., 120, 1–15. Li, F.J., Abels, H.A., Zhang, K.X., 2012. Evidence for northeastern Tibetan Plateau Zijderveld, J.D.A., 1967. AC demagnetization of rocks: analysis of results. In: Collinson, uplift between 25 and 20 Ma in the sedimentary archive of the Xining Basin D.W., Creer, K.M., Runcorn, S.K. (Eds.), Methods in paleomagnetism. Elsevier, New Northwestern China. Earth Planet. Sci. Lett. 317–318, 185–195. York, pp. 254–286.

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