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Global and Planetary Change 145 (2016) 78–97

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Global and Planetary Change

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Tectonosedimentary evolution model of an intracontinental flexural (foreland) basin for paleoclimatic research

Xiaomin Fang a,b,c,⁎,JiuyiWanga,b,WeilinZhanga,b, Jinbo Zan a,b, Chunhui Song c, Maodu Yan a,b, Erwin Appel d, Tao Zhang a,b, Fuli Wu a,b, Yibo Yang a,b,YinLua,b a CAS Center for Excellence in Sciences, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, b Key Laboratory of and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China c School of Earth Sciences & Key Laboratory of Western China's Mineral Resources of Gansu Province, Lanzhou University, Lanzhou 730000, China d Department of Geosciences, University of Tübingen, Hölderlinstr. 12, Tübingen D-72074, Germany article info abstract

Article history: Intracontinental flexural (foreland) basin are now frequently used as archives for detailed paleocli- Received 3 December 2015 matic and sedimentary environmental reconstruction, fossil and stratigraphic correlation, and tectonic evolution Received in revised form 14 June 2016 and uplift of basin and orogen. However, sedimentologic characteristics vary considerably in time-space with the Accepted 31 August 2016 evolution of flexural basin, apt to cause misinterpretation of climatic change and stratigraphic correlation. Based Available online 5 September 2016 on high resolution fossil mammal and magnetostratigraphic constraints and sedimentary facies analysis, here we fi Keywords: took the Linxia Basin at the front of the NE Tibetan Plateau as a case to demonstrate and gure out a model how fl Linxia Basin sedimentology and stratigraphy vary temporospatially with the evolution of such exural basin. The results show Intracontinental that the Linxia Basin is a type intracontinental foreland basin subjected to two phases of flexural deformation Magnetostratigraphy exerted by the West Qin Ling (Mts.) and NE Tibetan Plateau to the south. Phase I began latest at the beginning Tectonosedimentary evolution model of the Miocene (23.3 Ma), indicated by a balanced fast flexural subsidence and mostly fine infilling giv- ing rise to the early underlying unconformity. It manifests as an obvious sediment wedge with high filling rate, thickening toward and an occurrence of a mountains-parallel big – shallow lake system along the foredeep, suggesting a less high topography. In the late Phase I, from ~13 Ma to 8 Ma, the subsi- dence and thickening rates began to decrease, accompanied by faults and deformation propagating gradually into the basin, causing gradual basinward migration of the foredeep and its accompanying river-lake system. Since ~8 Ma in Phase II, the West Qin Ling and NE Tibetan began to uplift rapidly and thrust/load onto the Linxia Basin, causing strong mountain , thrust- belt propagation and basin overfilling. This forced the moun- tains-parallel river - lake system to turn to the mountains-perpendicular alluvial - braided river system, and final- ly to an outflow system by the Quaternary onset of the Yellow River in the basin. Concurrent are rapid rotation of the basin, occurrence of growth strata and late unconformities, widespread expansion of boulder conglomerates, great decreasing and increasing sedimentation rates above and before the hanging wall of the -fold system and new supplementary from the thrust-fold system. This demonstrates that in stable climate, same lithologic units such as distinct lacustrine sediments and alluvial conglomerates will migrate basinwards with the foredeep moving into basin, causing a great diachroneity and often misleads to recognize the same lithologic unit in space as one unit in time. A dynamic model is presented that should help to avoid such pitfalls in tectonic basin evolution, especially concerning stratigraphic correlation and paleoclimatic change research. © 2016 Elsevier B.V. All rights reserved.

1. Introduction monsoon and global climatic changes, leading to great progress in our knowledge of the late Miocene – Quaternary Asian monsoon evolution High-resolution loess-paleosol and red-clay sequences on the and mechanism (e.g. Liu et al., 1985; An et al., 1991, 2001; Ding et al., Chinese Loess Plateau and their climatic proxy records, constrained by 1995; Fang et al., 1999; Guo et al., 2000). This success encourages to ex- detailed paleomagnetic, TL/OSL and radiocarbon dating, have widely pand such approaches to Cenozoic basin sediments in order to under- been used for precise stratigraphic constraints for reconstructing Asian stand how the Asian monsoon evolved in pre-late Miocene time and how this evolution is related to global change and uplift of the Tibetan Plateau (e.g. Li et al., 1995; Guo et al., 2002; Fang et al., 2003, 2005, ⁎ Corresponding author at: CAS Center for Excellence in Tibetan Plateau Earth Sciences 2007a; Lu et al., 2004; Dai et al., 2006; Jiang et al., 2007; Sun et al., and Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. 2009a; Qiang et al., 2011; Sun et al., 2011a; Tang et al., 2011). However, E-mail address: [email protected] (X. Fang). such attempts caused large uncertainties and many debates on the

http://dx.doi.org/10.1016/j.gloplacha.2016.08.015 0921-8181/© 2016 Elsevier B.V. All rights reserved. X. Fang et al. / Global and Planetary Change 145 (2016) 78–97 79 stratigraphic ages derived from and fossil mammals al., 2006; Dupont-Nivet et al., 2007; Xiao et al., 2010), the Subei Basin and the sedimentary facies and climatic history in the studied basins, (Gilder et al., 2001; Sun et al., 2005), the Tarim Basin (e.g. Charreau et e.g. the Sikouzi Basin (Young and Chow, 1956; Jiang et al., 2007; Wang al., 2006, 2009a; Huang et al., 2006, 2010; Sun and Zhang, 2009; Sun et al., 2011), the Tianshui Basin (e.g. Guo et al., 2002; Alonso-Zarza et et al., 2009b, 2011b; Zhang et al., 2014, 2015), and the Jungger Basin al., 2009; Peng et al., 2012; Zhang et al., 2013), the Lanzhou Basin (e.g. (e.g. Sun et al., 2004, 2009b; Charreau et al., 2005, 2009b; Ji et al., Opdyke et al., 1999; Qiu et al., 2001; Yue et al., 2001; Xie, 2004; Sun et 2008; Li et al., 2011) (See Fig. 1 for locations). Reasons are not only al., 2011a), the Linxia Basin (Li et al., 1995; Fang et al., 2003; Deng et the difficulties in determining stratigraphic ages due to the lack of dat- al., 2004, 2013; Deng, 2004; Qiu et al., 2004; Garzione et al., 2005), the able volcanic ash layers and fossil mammals, but also because of the per- Xining Basin (Qiu et al., 1981; Lu et al., 2004; Dai et al., 2006; Wu et vasive ignorance of stratigraphic diachroneity, sedimentary facies

Fig. 1. (a) Location of the Linxia Basin in the NE Tibetan Plateau with respect to the in the Tibetan Plateau, modified from Qinghai Geology Bureau (1989) and Fang et al. (2003, 2005). (b) Geologic map of the Linxia Basin showing the distribution of the Cenozoic stratigraphy and its relationship with the tectonic units and major faults which impact the basin; (c) 3- D DEM image of the Linxia Basin in (b) showing the studied sections and their relationships with faults and geomorphology. Note the north direction is just reversed for a striking 3-D view. Cross section a-a’ is presented in Fig. 2a. HLD: Heilinding; GNG: Guonigou; WJS: Wangjiashan; MG: Maogou; HSG: Huaishuguan Fold; YCG: Yinchuangou . Solid (dotted) red lines are surface (subsurface) transpressional faults or thrusts. 80 X. Fang et al. / Global and Planetary Change 145 (2016) 78–97 influence and its temporal-spatial variation with the tectonic evolution Longzhong Basin by the NW-trending Maxian Shan (Mts.) to the of the basins. This is because many of these basins are located in the north and surrounded by the Laji Shan and Leiji Shan (Mts.) to the northern Tibetan Plateau and their rims belong to intracontinental flex- west and the West Qin Ling (Mts.) to the south, with its eastern side ural (foreland) basins. The and sedimentologic characteris- open to the Longzhong Basin. The basin has elevations of about tics (flexural depression center, depocenter, stratigraphic thickness, 1800 m to 2600 m, contrasting with surrounding mountains at eleva- sedimentation rate, and sedimentary facies) vary considerably tions of about 3000–4000 m. Thick Cenozoic sediments up to 1600 m in time and space during basin evolution. This is not familiar by re- filled the basin and are exposed completely by the Yellow River that searchers in paleoclimatology and paleontology and is easily leads to cuts through the sequence (Figs. 1band2). misinterpretation of climatic change and stratigraphic correlation, caus- Former paleomagnetic results determined the sequence comprising ing unnecessary debates when not taken into consideration. Cenozoic fluvio-lacustrine sediments at about 29 to 1.8 Ma, followed by Typical concepts and models of foreland basin evolution in relation eolian loess on river terraces lasting to present (cf. Fig. 3)(Li et al., 1995, to plate (plate collision) have been built up for long time 1997b; Fang et al., 2003). The basin sediments overlie unconformably (e.g., Allen and Homewood, 1986; DeCelles and Giles, 1996). However, on the Caledonian rocks in the basin center and on these are not suitable for direct utilization in intracontinental basins sedimentary rocks in the north. The surrounding mountains since tectonic settings, deformation magnitude and sedimentary facies mostly consist of Cambrian to Permian clastic rocks, carbonates and types, and their associations and evolution are quite different between and Cretaceous intermontane red bed rocks (Gansu Bureau of them. The common dynamics is a tectonic loading in compressional set- Geology, 1989; Fang et al., 2003). ting that will cause subsidence and the development of a thrust-fold The long WE-trending West Qin Ling fault is the most important system in front of the loader. basin-controlling fault that initiated at 50–45 Ma (Clark et al., 2010). In this paper we use the Linxia Basin as a detailed case study to de- To the west, the NS-trending Laji-Leiji Shan fault is another basin-con- velop a dynamic model for a correct reconstruction of stratigraphic trolling fault activated at relatively younger time, possible after 22 Ma and lithofacies records aiming at climatic change studies. The model (Lease et al., 2011). Several small parallel thrust faults root from these demonstrates how stratigraphy and sedimentary facies change in time two controlling faults and reflect basinward propagation of deformation and space during the evolution of an intracontinental flexural basin in from south-to-north and west-to-east forming several rows of thrust- relation to mountain uplift using a conceptual framework and newly folds. Outcrops in the folds were chosen for study, complemented by developed semi-quantitative indicators. drillings in the Heilinding (HLD) anticline at the front of the West Qin The Linxia Basin is located at the northeastern margin of the Tibetan Ling, at Maogou (MG) in the basin center and at Guonigou (GNG) be- Plateau (Fig. 1a), it has the advantage of the most abundant Cenozoic tween them (Figs. 1b and 2). Growth strata were found to occur since mammal fossils in the world (Deng et al., 2004, 2013) and the strati- ~8 Ma and ended at ~1.8 Ma in the Wangjiashan (WJS) section in the graphic sequence has been well dated (Li et al., 1995, 1997a; Fang et eastern limb of the Yinchuangou (YCG) anticline (Fig. 1b) (Li et al., al., 2003). Moreover, the Linxia Basin is the first intracontinental Ceno- 2014). zoic basin in dated by high-resolution magnetostratigraphy and studied by high-resolution paleoclimate records. Recently it was found 2.2. Stratigraphy that in some stratigraphic intervals at some sites the sub-division and correlation of stratigraphy based on fossil mammals and lithology is in The Cenozoic stratigraphy of the Linxia Basin consists mainly of red conflict with results based on magnetostratigraphy (Li et al., 1995; beds of fluvio-lacustrine mudstones and siltstones with some sand- Fang et al., 2003; Qiu et al., 2004; Deng et al., 2004, 2013). Similar stones and conglomerates along basin margins. The stratigraphy is near- cases exist also in the nearby Lanzhou Basin (Opdyke et al., 1999; Qiu ly horizontal in the basin center and folded in basin margins. Previous et al., 2001; Yue et al., 2001; Xie, 2004; Sun et al., 2011a)andtheXining studies have divided the stratigraphy into eight formations based on Basin (Qiu et al., 1981; Lu et al., 2004; Dai et al., 2006; Wu et al., 2006; lithofacies, contacts, paleontology and magnetostratigraphy, which are Xiao et al., 2010). in upward direction the late Oligocene Tala (TL) Fm. (29–22 Ma), the Besides invoking previous magnetostratigraphic sections, we con- early Miocene Zhongzhuang (ZZ) Fm. (21.4–14.68 Ma), the middle Mio- ducted further paleomagnetic work in two new outcrop sections sup- cene Shangzhuang (SZ) Fm. (14.68–13.07 Ma), the late Miocene plemented by two drillings with newly found abundant mammal Dongxiang (DX) Fm. (13.07–7.78 Ma) and Liushu (LS) Fm. (7.78– fossils in the southern Linxia Basin. This helped to establish a well- 6 Ma), the early and late Pliocene Hewangjia (HWJ) Fm. (6–4.34 Ma) time-constrained stratigraphic and geologic transect along south – and Jishi (JS) Fm. or Jishi Conglomerate Bed (3.6–2.6 Ma), and the north direction allowing a detailed stratigraphic correlation and sedi- early Pleistocene Dongshan (DS) Fm. (2.6–1.8 Ma) (Fig. 3)(Li et al., mentary facies analysis. The results demonstrate that the Linxia Basin 1995, 1997a; Fang et al., 2003)(Fig. 3). The sediments were almost de- is a typical intracontinental foreland basin and they show how the stra- posited continuously without a hiatus in most areas except the uncon- tigraphy and sedimentary facies vary during the tectonic evolution of formities between the Paleogene and Neogene stratigraphic sequences the basin. and at the bottom of the JS boulder conglomerate bed between the early and late Pliocene (Fig. 3)(Fang et al., 2003). 2. Geological setting and stratigraphy The stratigraphy can apparently be traced in the basin (Fang et al., 2003). However, due to large lateral lithofacies change different forma- 2.1. Geologic setting tions are characterized by highly variable facies. The typical lithology from the basin margin to the center is represented by the Heilinding The Linxia Basin is a sub-basin of the huge Longzhong Basin forming (HLD) and Maogou (MG) sections below. In a brief view, the stratigra- since the on the base of Caledonian bedrocks of the Qilian phy thickens southward and westward, forming a wedge-shaped Shan orogen and bounded by a series of major strike-slip and basin reservoir close to the Laji-Leiji Shan and West Qin Ling faults transpressional faults, including the Haiyuan-Liupan Shan faults to the (Fang et al., 2003). Moreover, sediments along the basin margin are north and east, the Haiyan fault to the west, and the West Qin Ling generally coarser than in the basin center. Gray to reddish-brown con- fault to the south. Several rows of eastward-extending mountains of glomerates and sandstones with large thickness dominate the basin the NW-trending Qilian Shan (Mts.) stretch into and finally plunge margin whereas in the basin center reddish to brownish fluvio-lacus- below the Longzhong Basin, separating the western part of the trine siltstones and mudstones are dominant. Longzhong Basin into several sub-basins such as the Linxia, Lanzhou Four representative fossil mammal fauna were found in the Linxia and Xining Basins (Fig. 1a). The Linxia Basin is separated from the Basin, the Late Oligocene Dzungariotherium Fauna, the Middle Miocene X. Fang et al. / Global and Planetary Change 145 (2016) 78–97 81

Fig. 2. (a) Simplified geologic cross section AB (see Fig. 1b for location) showing locations of the Heilinding (HLD), Guonigou (GNG), and Maogou (MG) sections and their relationships with basin stratigraphy and tectonics. (b) Detailed stratigraphic succession of the HLD outcrop section (226.5 m) and its relation to the drilling cores HZT (129.2 m) and HZ (477.7 m) at the top and bottom. Inset photographs show field drilling work and lithology of representative cores from cores HZT and HZ. Bt: Luvic horizon; Bw: Cambic horizon; Bk: Calcic horizon. Note that well developed granular ped structure of paleosol Bt horizon is clearly represented, indicating strongly developed luvic cambisol of forest environment (FAO– UNESCO, 1988).

Playbelodon Fauna, the Late Miocene Hipparion Fauna, and the Early nearby Yagou section (Li et al., 1995; Fang et al., 2003; Qiu et al., Pleistocene Equus Fauna, suggesting a deteriorative ecologic environ- 2004)(Figs. 3, 4A, B and C). ment change (Deng et al., 2004, 2013). Each fauna has typical fossil It might be difficult to correlate the stratigraphy by the known for- components that lived in specific time, thus the faunal change gives a mations across the basin as the lithofacies varies largely laterally due general chronologic estimate of the sediment sequence wherever to mountain building and climate changes. Thus, large age ranges of fos- found, and all were correlated to European Land Mammal Zones sil mammals and incomplete finding of them in different sites and sec- (Deng et al., 2004, 2013). The Playbelodon Fauna and Hipparion Fauna tions might lead to misdetermination of stratigraphic ages and thus were also directly found in our HLD and GNG sections, and the miscorrelation of the stratigraphy formations. This might have caused Hipparion Fauna and Equus Fauna were directly found in the WJS sec- some conflicts in age determination of certain stratigraphic intervals tion (Gansu Bureau of Geology, 1989; Li et al., 1995, 1997a; Fang et al., by fossil mammals and magnetic polarity sequences (Li et al., 1995; 2003; Deng et al., 2004). No identifiable fossils were found directly in Fang et al., 2003; Deng et al., 2004, 2013; Qiu et al., 2004). The following the MG section, but the Dzungariotherium Fauna was found in the evidence and arguments will provide more insight on this issue. 82 X. Fang et al. / Global and Planetary Change 145 (2016) 78–97

Fig. 3. Cenozoic stratigraphic sequence of the Linxia Basin, compiled from the Maogou (MG), Wangjiashan (WJS) and Dongshanding (DSD) sections in the Linxia Basin (see Fig. 1bfor locations) from Li et al. (1995) and Fang et al. (2003). X. Fang et al. / Global and Planetary Change 145 (2016) 78–97 83

3. Sections, drillings and new fossil assemblages borehole HZ (477.7 m) in the lower part (Fig. 2b). The boreholes HZT and HZ were drilled in January 2012 and September to December To control how stratigraphy and sedimentary facies change in detail 2011, respectively using an advanced three-in-one tube technology with the tectonic evolution of the basin, we selected and sampled two (Zhang et al., 2012) with average recovery rates of 92% and 96%, respec- fossil-bearing sections supplemented by two drillings at HLD and GNG tively (Fig. 2). The core HZT and outcrop HLD were well connected by in the southern Linxia Basin and combined them with the MG section using a marker layer of ~1 m marl bed at a depth of 120 m in the core in the basin center to obtain a S-N transect for detailed stratigraphy, li- HZT and exposed at the top of the HLD outcrop section (Fig. 2b). This thology, facies and paleomagnetic results (Fig. 2a). The HLD section lithostratigraphic connection was checked by paleomagnetic sampling (35.38° N, 103.32° E) consists of the borehole HZT (129.2 m) in the of overlapping ~10 m interval at the bottom of the core HZT and the upper part, the outcrop HLD (226.5 m) in the middle part, and the top of the HLD outcrop section (see Fig. 7 for details). The core HZ was 84 X. Fang et al. / Global and Planetary Change 145 (2016) 78–97

Fig. 4 (continued).

drilled from the bottom of the HLD outcrop section at a depth of 226.5 m to 134 m from the top is dominated by yellowish brown mudstones in- (Fig. 2b). Eventually, the whole HLD section has an accumulated thick- tercalated with some thin fine conglomerate beds and gray-white ness of 824.7 m along the stratigraphic sequence (Fig. 4A). marlite beds. Centimeter size round-subrounded pebble are scattered Two mammalian fauna were found in the HLD section. The upper in siltstone and mudstone. A weak paleosol was found near the bottom. one is the Hipparion Fauna at the depth of 143 m (23 m in the HLD out- We defined it as the Hewangjia (HWJ) Fm. (Fig. 4A). The Liushu Fm. fol- crop), including Hystrix gansuensis, Promephitis sp., Pleisiogulo sp., lows from 142 m to 263 m characterized by mostly sandstones and silt- Ictitherium sp., Hyaenictitherium wongii, H. hyaenoides, Adcrocuta stones intercalated with thin pebble conglomerate beds and bottomed variabilis, Machairodus sp., Metailurus sp., Felis sp., Hipparion sp., with a thick gray pebble conglomerate bed. Mudstone and moderately Chilotherium wimani, Chleuastochoerus stehlini (Deng et al., 2004). The developed calcic paleosol complexes occur occasionally (Fig. 4A). The lower one is the Platybelodon Fauna in a yellowish-brown sandy con- Dongxiang Fm. makes up the interval from 263 m to 510 m where alter- glomerate bed at the depth of 322 m (202 m in the HLD outcrop), nation of thin grayish fine conglomerate or sandstone and thick reddish which contains a mammal of Platybelodon sp. and fragments of other brown mudstones topped with thick strongly to moderately developed species (Deng et al., 2004)(Figs. 2band4A). luvic paleosol complexes is very distinct. Siltstones and mudstones fre- We chose the GNG section (35.55° N, 103.42° E) as another control- quently contain some fine gravels (Fig. 4A). The Shangzhuang Fm. fol- ling section, which is situated between the HLD and MG sections (see lo- lows from 510 m to 602 m consisting of mostly fine sediments of cations in Fig. 1b). The section consists of a thickness of 211 m Neogene siltstones and strongly to moderately developed luvic paleosol com- sediments with Quaternary eolian loess at the top. Two mammalian plexes. Two thick layers of fine sandstones divide it into two graded cy- fauna, the Hipparion Fauna and Platybelodon Fauna were found at the cles. Conglomerates disappear and no pebbles occur in mudstone and height of 100 m in red-brown silty mudstone and at 88 m in sandy con- siltstone (Fig. 4A). From the base of the SZ Fm. to the bottom of the sec- glomerate, respectively (Fig. 4B). The Hipparion Fauna contains tion at 824.7 m is the Zhongzhuang Fm. which is composed of an up- Dinocrocuta gigantea, Machairodus sp., Tetralophodon sp., Hipparion ward fining cycle of alternative brownish siltstones and reddish- dongxiangense, Shaanxispira sp., Parelasmotherium linxiaense. (Deng et brown mudstones and moderately developed calcic paleosol complexes al., 2004). in the upper and alternating thick reddish-brownish parallel-bedded The 824.7 m thick sequence of the HLD section can be divided into sandstones and massive siltstones in the lower part (Fig. 4A). seven formations by lithology and stratigraphic cycles that were used In the GNG section, four formations were defined. The lowest strata for initial sub-division and nomenclature of the Linxia stratigraphy (Li (0–56 m) is purple red mudstone intercalated by thin greenish-gray to et al., 1995, 1997a; Fang et al., 2003)(Fig. 3). The topmost 20 m of the white marlite which can readily be recognized as the well known “Zebra section is defined as the Pleistocene Dongshan (DS) Fm., composed of Bed”. By paleomagnetic dating it is defined as the SZ Fm. Above it up to yellowish gravel sandstone and mudstone and weakly developed 148 m follows the DX Fm. characterized with several cycles of thin con- paleosol complexes. Underlying the DS Fm. down to 80 m is the Jishi glomerate/sandstone beds and thick yellowish-brown mudstones and Fm. dominated by thick boulder conglomerates intercalated with thin occasionally calcic paleosols. Rounded pebbles and white carbonate gravely sandstone (Fig. 4A). The stratigraphic interval between 80 m nodules scatter within the mudstones/siltstones (Fig. 4B). The LS Fm. X. Fang et al. / Global and Planetary Change 145 (2016) 78–97 85

Fig. 4. A. Stratigraphic division, lithology and sedimentary facies of the synthesized Heilinding (HLD) section. (a) Hand sample of gravely fine sandstone with unequant angular centimeter- scale gravels; gravels are scattered and incorporated within the fine sand matrix (56.7 m, JS Fm., alluvial fan). (b) Core sample of gray-white marlite (121.3 m, HWJ Fm.), overbank in braid- ed river; note the cemented rounded gravels, indicative of subsurface diagenetic in origin. (c–e) Cycled association of sub-facies of basal conglomerates of channel, sandstones of side bar, and siltstone – mudstone topped with paleosols or their complexes of overbank in proximal braided river system (at about 250 m, LS Fm.); enlarged photos showing clear reddish brown luvic Bt and grayish calcic Bk horizons (d), a well-developed paleosol in (c) and ped structure with shining reddish brown clay coatings and black Fe-Mn mottling stains (e) of Bt in (d). (f) Cycled association of channel sandstones and overbank complexes of siltstone – mudstone – paleosol in the middle part of braided river system (315–320 m, DX Fm.). (g) HLD outcrop section showing interbedded conglomerates, sandstones and siltstone-paleosol complexes in braided river system (DX Fm.). (h) Photograph of a core showing the contact between con- glomerate and sandstone in braided river channel (509 m, DX Fm.); note the gravels are relatively well-sorted, equant, rounded to subrounded and slightly imbricated. (i–k) Photographs of cores showing typical Bt and Bk horizons of paleosols (520–690 m, SZ and ZZ Fms.); note granular ped structure in Bt horizon (i), white carbonate nodules and infillings in Bk horizon (i, k) of the paleosols and vertical infilled biologic burrows and small white carbonate nodules and infillings in calcic Ck horizon (calcareous mudstone) (j). Heavy arrow indicates the main flow direction revealed by imbricate structure of gravels.B. Stratigraphic division, lithology and sedimentary facies of the Guonigou (GNG) section. (a, c) Sandy conglomerate beds in braid- ed river channel (LS and DX Fms.); note evident cross-beddings in coarse sandstones overlying conglomerates with imbricated structure. (b, d) Yellowish-gray mudstone with many car- bonate nodules in floodplain (DX Fm.). (e) Well-known “Zebra Bed” in the basal GNG section; note the horizontal greenish-white marlite belts and alternative purple red mudstone, indicative of shallow lake. Heavy arrow indicates main flow direction revealed by imbricate structure of gravels.C. Stratigraphic division, lithology and sedimentary facies of the Maogou (MG) section. (a) Stratigraphy of the HWJ Fm. showing a basal carbonate-cemented sandstone and conglomerate bed of braided river channel and siltstone, mudstone and paleosol as- sociation of the overbank floodplain above (also see b). (c) Representative alternated sub-facies association of basal siltstones of small river channels, overlying overbank mudstone and top paleosol developed during less flooding interval in floodplain (LS Fm.). (d) “Zebra Bed” of the DX Fm. showing alternated horizontal greenish-white weakly laminated marlite belts (f) and purplish massive mudstones of shallow lake. (e) Thick, parallel-bedded sandstones bottomed with sandy conglomerates (230–251 m) underlying the “Zebra Bed”, indicating a big river channel. (g, h) Basal parallel-bedded sandstone of big-river channel (g) and the overlying thin horizontally-bedded or laminated siltstones and mudstones (h) (SZ Fm.). (i, j) Thick loose, yellowish parallel-bedded or cross-bedded sandstones of big river channel (i) and reddish massive or horizontally-bedded siltstones and mudstones of floodplain and shallow lake in the ZZ Fm.; note the undulated erosion surface between the ZZ and TL Fms indicates the occurrence of unconformity. (k–n) Representative photos of the TL Fm. showing thin horizon- tally-bedded siltstone and mudstone of shallow lake (k) and cross-bedded sandstones and massive fine conglomerates (m, n) of braided river channels. A relict paleosol with clear Bt-Bk- Ck profile immediately below the channel sandstone bed is presented (l). Rose diagrams showing statistics of cross-beddings or gravels where available are plotted beside the section for paleoflow indication. 86 X. Fang et al. / Global and Planetary Change 145 (2016) 78–97

(148–198 m) is composed of light brownish carbonate-noduled mud- composition. The statistical directions of the gravel ab faces show two stone with three thin beds of weakly developed paleosols and sandy major northern and northeastern trends (Fig. 4C). siltstone beds. Above 198 m up to 211 m, another fining upward cycle occurs with thin basal grayish sandstone and thick homogenous light 4. Sampling and methods brownish mudstone, characterized as the HWJ Formation. Superimposed on it is the Pleistocene yellowish loose loess of several In the HLD outcrop and GNG sections, oriented block samples were to decadal meters. collected at 25 cm stratigraphic intervals, but where conglomerate ap- The MG section (35°39ʹ2.3″N, 103°16ʹ29″E) is characterized with six peared, this interval was increased to ~1–2 m depending on existing cycles of graded sediments of different features and thus they were ini- lenses of siltstone to sandstone. The block samples were then cut into tially divided into six formations (Figs. 3 and 4C) (Li et al., 1995, 1997a; three cubic sub-samples of 2 × 2 × 2 cm3 in the laboratory. In total, Fang et al., 2003). The first cycle at the bottom was termed as Tala Fm, 769 × 3 and 700 × 3 samples were obtained for paleomagnetic mea- spanning the interval 0–91 m in thickness (29–22 Ma in age) (Figs. 3b surement of the HLD outcrop and GNG sections, respectively. For cores and 4C) (Li et al., 1995; Fang et al., 2003). It consists of two sub-units. HZT and HZ, the plastic tube-cored sediments were first cut into The lower unit (0–25 m) is dominated by coarse sediments of gravely two halves, and then sub-samples of 2 × 2 × 2 cm3 were collected at sandstones and sandstones with clear cross-beddings intercalated ~0.5–1 m stratigraphic intervals, resulting in a total of 815 × 3 samples with purplish-reddish massive siltstones and mudstones in which (169 × 3 for HZT, and 646 × 3 for HZ core). No samples from fine lenses some are subjected to weak pedogenesis but one occurs as a well-devel- in conglomerate were collected due to low recovery rate in conglomer- oped calcic paleosol (Fig. 4C-l–n). The cross-beddings show an average ate. Overall, a total of 2284 stratigraphic levels were obtained for paleo- direction of 51.9° N (Fig. 4C). The upper unit (25–91 m) is predominant- magnetic measurements for all studied outcrops and cores. ly fine sediment of thick bedded massive purplish siltstones in the lower At the paleomagnetic laboratory, remanences and directions were part, horizontal thin bedded or laminated purplish-reddish mudstones measured with a 2G Enterprise Model 760 SQUID magnetometer in and alternated thin mudstone and siltstone in the middle part (Fig. field-free space (b300 nT). Stepwise thermal demagnetization was car- 4C-k), and thick weakly bedded massive purplish mudstones bottomed ried out on one set of samples at 10–50 °C intervals up to 690 °C or until with a thin sandstone bed in the upper part (Figs. 3band4C). the remanence intensity fell beyond the sensitivity limit The second cycle is at 91–188 m (21.4–14.68 Ma) and is named (2 × 10−12 Am2 for 8 cm3 samples) of the cryogenic magnetometer. An- Zhongzhuang Fm. (Figs. 3b and 4C) (Li et al., 1995; Fang et al., 2003). other set of samples (470 for core HZ) was subjected to alternating field Its lower part (91–121 m) has three distinct thick layers of yellowish demagnetization (AfD) in 12 steps from 5 mT to 90 mT (at intervals of gravely sandstones and coarse sandstones developing obvious large 5 mT and 10 mT below and above 30 mT, respectively). All these mea- cross- and parallel-beddings, intercalated with two thin layers of red- surements were performed at the Paleomagnetism Laboratory of the In- dish siltstones and mudstones (Fig. 4C-i). The cross-beddings show an stitute of Geology and Geophysics, Chinese Academy of Sciences. average direction of 59.4° N (Fig. 4C). The sandstones are very loose The intensity of NRM for all samples usually falls within 10−4–10−6 A/m and well sorted. Its middle and upper parts (121–188 m) are alternated (Fig. 5). Previous detailed rock magnetic studies on different sediment reddish horizontally thin-bedded or massive siltstones and mudstones types in the Linxia Basin have demonstrated that the characteristic rem- intercalated with a few calcic paleosols (Fig. 4C-j). anent magnetization (ChRM) was mainly carried by magnetite and he- The third cycle is at 188 –230 m (14.68–13.07 Ma) and was named as matite (Fang et al., 2003, 2007b). For most samples, one low- Shangzhuang Fm. (Figs. 3band4C) (Li et al., 1995; Fang et al., 2003). It temperature component with no consistent directions below 200– has a similar lithology as the ZZ Fm. below and consists of brownish par- 300 °C was removed which we consider as a secondary component allel-bedded fine sandstones intercalated with thin siltstones at 188– (Fig. 5d, g, i, j). A likely secondary component is also removed by AfD 203 m and reddish brown horizontally thin-bedded or laminated or below 30–40 mT (Fig. 5e, f). After removal of the secondary magnetiza- massive siltstones and mudstones at 203–231 m (Fig. 4C-g, h). A weakly tion, many specimens exhibit a linear decay to the origin (Fig. 5i, l). In developed paleosol is found at the top of the massive mudstone (Fig. part of the samples, a stable direction is observed until above 450 °C 4C). (Fig. 5a, d), and fully demagnetization is achieved at 580 °C (Fig. 5b, The fourth cycle is at 231–313 m (13.07–7.78Ma)andwasnamedas d). More common, a large number of samples show a stable component Dongxiang Fm. (Figs. 3band4C) (Li et al., 1995; Fang et al., 2003). It is above 580 °C and decay to origin until 690 °C (Fig. 5g–k), indicating a also composed of lower and upper units with mainly coarse and fine hematite carrier. The hematite component shows no significant direc- sediments. The lower coarse sediments (231–251 m) are horizontally tional difference to the component unblocked between 450 and 580 °C. thick-bedded median sandstones with massive and weak cross-bed- The demagnetization behavior confirmed former rock magnetic re- dings, bottomed with a conglomerate bed (Figs. 3 and 4C-e). The sults, suggesting that the ChRM in the Linxia sediments is carried by upper fine sediments (251–313 m) are a striking unit of the well- magnetite and hematite. We thus calculated the ChRM for each sample known so called “Zebra Bed” characterized with alternated purple mas- by principle component analysis (Kirschvink, 1980), using at least four sive mudstones and massive to weakly horizontal-laminated white- demagnetization steps trending toward the origin above 400 °C or greenish gray marlites, looking like a zebra belt pattern (Fig. 4C-d, f). 40 mT. For the HLD outcrop and GNG sections, virtual geomagnetic The fifth cycle is at 313–376 m (7.78–6 Ma) and was named as poles (VGPs) were calculated from the ChRM directions. Samples with Liushu Fm. (Figs. 3band4C) (Li et al., 1995; Fang et al., 2003). It consists maximum angular deviation larger than 10° and VGP latitudes between mainly of yellowish brown massive or weakly horizontally thin-bedded −30° and 30° were rejected. Because declination data are meaningless siltstones and brownish massive mudstones with some calcic paleosols due to drilling rotation, only inclination data were used for core samples and sandstones (Fig. 4C). to determine polarities. For core samples, directions with maximum an- The sixth cycle is at 376–443 m (6–4.34 Ma) and was named as gular deviation of N15° were rejected. Overall, 1605 ChRM directions Hewangjia Fm. (Figs. 3band4C) (Li et al., 1995; Fang et al., 2003). It con- out of the total 2284 levels (i.e., 70.3%) were accepted from the HLD sists of alternated light brownish yellow massive siltstones and brown- and GNG sections (490 from 769 samples in the HLD outcrop, 573 ish yellow mudstones bearing well developed calcic paleosols and from 700 samples in the GNG section, and 542 from 815 samples in bottomed with a lensed fine carbonate-cemented conglomerate and cores HZT + HZ). coarse sandstone bed (Fig. 4C-a, b). The gravels in the conglomerate The accepted ChRM directions from outcrops were subjected to a pa- bed are mainly 1–2 cm (maximum 10 cm) in size, subrounded, and con- leomagnetic reversal test to evaluate whether the average normal and sist of grayish-greenish sandstones (50%), purple sandstone (20%), reversed poles are statistically antipodal within the 95% confidence quartzite (15%), granites (8%), black silicalites (2%) and schist (1%) in limit. The results indicate a positive test with classification A (Fig. 6a) X. Fang et al. / Global and Planetary Change 145 (2016) 78–97 87

Fig. 5. Representative thermal and alternating field demagnetization diagrams for samples from the HLD and GNG sections. Open (closed) symbols represent vertical (horizontal) projections. NRM—natural remanent magnetization.

(McFadden and McElhinny, 1990). The jackknife technique (Tauxe and variabilis, Machairodus sp., Metailurus minor were found in the Chinese Gallet, 1991) was used to evaluate the robustness of determined polar- standard mammal zone, the Baodean Fauna, which is correlated to the ity intervals in the HLD outcrop and GNG sections. This test yields a J European Land Mammal Zone MN 12 with an age of 7.5–6.8 Ma value of −0.0984 for the HLD outcrop and −0.0700 for the GNG sec- (Agustí et al., 2001; Deng et al., 2004). We therefore correlated N10 to tion, respectively, suggesting that these two records have uncovered C3An.2n. The Hipparion Fauna in the HLD section is therefore estimated N95% of the true number of polarity intervals (Fig. 6b). ChRM directions at 6.3 Ma (Fig. 4). Platybelodon sp. was found at depth of 322 m in the from the cores HZT and HZ were not subject to the reversal and jack- HLD section. This mammal is a typical fossil component of the middle knife tests because only inclination values are available due the uncon- Miocene Platybelodon Fauna, which was widely excavated in the Linxia trolled azimuth during drilling. Basin and was correlated to the European Land Mammal Zone MN 7/8 (Deng et al., 2004) with an age of 13.0–11.1 Ma (Agustí et al., 2001). 5. Results Polarity R19 concurs with this mammal and thus we correlated the polarity intervals R19–R21 and N22–N23 to C5r and C5An, respectively 5.1. Magnetostratigraphy of the Heilinding and Guonigou sections (Fig. 7). Above R18, based on these fossil constraints, two long normal polar- The polarity sequence of the whole HLD section was established ities N19 and N13 can readily be correlated to C5n.2n and C4n.2n, re- based on the inclinations of cores HZT and HZ and VGP latitudes of the spectively. For correlation of the subsequent polarity zones N5–N18 HLD outcrop. Each polarity was defined by at least two subsequent and R5–R18 see Fig. 7. The polarity intervals R3 and R4 suffer from the levels with same polarity. In total, 40 normal and 40 reversal polarities shortage of available samples because of thick conglomerates. However, were identified throughout the entire section, marked as N1–N40 and R4 seems likely correlated to C2Ar, and N3–N4 might record the Gauss R1–R40. The observed polarities are correlated to the GPTS 2012 of chron. Nevertheless, we tentatively correlated N3–N4 to C2An, and Gradstein et al. (2012) (Fig. 7). R1–R2 to C2r (Gilbert chron), respectively (Fig. 7), according to the op- The two fossil mammalian assemblages found in the HLD outcrop timum likelihood. The N1 at the top we correlated to the end of the section provide robust constraints for correlation. As presented above, Olduvai chron, suggesting an age of ~1.8 Ma for upper end of the HLD a latest Miocene Hipparion Fauna was found at the depth of 143 m fall- section. The correlation for the uppermost HLD section is supported by ing into the polarity interval N10. The components of this fauna like lithologic correlation between the HLD section and the WJS and DSD Chilotherium wimani Hyaenictitherium wongii, H. hyaenoides, Adcrocuta sections where a marker bed of thick boulder conglomerates (JS Fm.) 88 X. Fang et al. / Global and Planetary Change 145 (2016) 78–97

Fig. 6. (A) Equal-area projections of the characteristic remanent magnetization (ChRM) directions and mean directions (with oval of 95% confidence) for the HLD outcrop and GNG section. Fisher mean data for normal and reversed directions are presented. (B) Magnetostratigraphic jackknife analysis (Tauxe and Gallet, 1991) for the HLD outcrop and GNG section. The plot indicates the relationship between average percent of polarity zones retained and the percentage of sampling sites deleted, where the slope J is directly related to the robustness of the results. The obtained slopes J have values of −0.0984 and −0.0700 in the HLD outcrop and the GNG section, respectively, suggesting that the sections have recovered N95% of the true number of polarity intervals. occurs in three sections and an early Quaternary Equus Fauna was found consequently represents C6Cn.1n and R40 is correlated with C6Cr, directly above the boulder conglomerates (JS Fm.) in distinct fluvio-la- yielding an age of 23.3 Ma for the bottom of the HLD section, although custrine grayish siltstone unit (DS Fm.) in the WJS section. This places C6Cn.2n was not distinguished in the section. Under this correlation a strong age constraint on the DS Fm. (at the interpreted paleomagnetic framework, other short polarity intervals can be correlated with the ages of 2.5–1.8 Ma) and the underlying marker bed in the JS Fm. (at the GPTS 2012, despite few very short polarity zones are missing due to Gauss Normal Chron at 3.6–2.6 Ma) (Li et al., 1995; Fang et al., 2003) the possible sampling intervals. Overall, the 824.7 m HLD section docu- (Fig. 3). The fine fluvial sediment unit at the top of the HLD section ments Neogene sediments with a range of 23.3–1.8 Ma (Fig. 7). and the below thick boulder conglomerate bed are consequently corre- Eleven pairs of normal and reversal polarities, marked as N1–N11 lated with the DS and JS Fms. in the WJS and DSD sections. This indicates and R1–R11, were identified throughout the GNG section. The lower that our interpretation of the polarities in the uppermost HLD section mammalian fauna, Platybelodon sp., found at thickness of 88 m has (which determines the fine fluvial sediment unit at 2.6–1.8 Ma and been proven to occur in the time equivalent of MN 7/8 (13.0–11.1 Ma) the below boulder conglomerate marker bed at 3.8–2.6 Ma) is plausible (Agustí et al., 2001; Deng et al., 2004). This fossil is just situated at the in a flexural basin where alluvial conglomerates propagate into the bottom of N6, which allows us to correlate N6 to C5An. Consequently, basin, i.e. the earlier conglomerates indicating the approaching the Platybelodon Fauna in the GNG section was determined at mountain. 12.47 Ma. At the height of 100 m, the lowermost level of the Hipparion Below N23, there are no fossil constraints for polarity correlation. Fauna appears within R5, because the Hipparion dongxiangense found in The most noticeable polarities are the long normal intervals N25 and the fauna differs from that found in the HLD section. The Hipparion N26, which are likely representing C5ABn and C5ACn-C5ADn. The nor- dongxiangense was demonstrated as the oldest Hipparion in China and mal polarity zones N32, N34, and N39 are then straightforward correlat- was constrained to be early late Miocene in age, most likely correspond- ed to C5En, C6n. and C6Bn. The last normal polarity interval N40 ing to MN9 (11.1–9.8 Ma) (Deng et al., 2004). Thus we can correlate R5 X. Fang et al. / Global and Planetary Change 145 (2016) 78–97 89

Fig. 7. Magnetostratigraphy of the HLD and GNG sections and their correlations with the GPTS of Gradstein et al. (2012). Gray stripe shows the marlite bed as the maker bed connecting the HZT core and HLD outcrop. VGP: virtual geomagnetic polarity; GPTS: Geomagnetic polarity time scale. Legends for lithology refer to Fig. 4A, B and C.

to C5r.1r-C5r.2r with high certainty, yielding an age of 11.5 Ma for the Alluvial fan sediments are thick and poorly sorted; matrix-support- lowermost level of the Hipparion Fauna in our section, which is the ed pebble to boulder conglomerates are intercalated with thin lenses oldest age for Hipparion passing through the Bering Strait from Northern of red sandstones and siltstones (Fig. 4A-a). The gravels in the conglom- America to the Eurasian land (Sen, 1997). This correlation is supported erate bed are commonly 5–30 cm with largest over 50–100 cm, angular by lithostratigraphic observations which show that the Hipparion Fauna to subrounded, and randomly distributed. In most cases, unequant an- locates just 12 m above the Middle Miocene Platybelodon Fauna, indi- gular gravels are scattered in matrix of mudstones to sandstones. cating its age is not much younger than the Platybelodon Fauna. Accord- These sediments only occur along the margins of mountains and thicken ing to these fossil constraints, the significant polarity intervals N4 and during approaching of mountains. N7–N8 were correlated to C5n.2n and C5ABn-C5ADn. Between them, River sediments manifest as two types, big river channel-bar and the long reverse polarity interval R7 likely represents the long reverse floodplain deposits, and small braided river channel, bar and overbank zones C5r-C5AAr, although short normal zones (e.g. C5Ar.1n, C5Ar.2n, deposits. Big river channel-bar deposits are characterized by very thick and C5AAr.1n) were not identified. Based on this fundamental frame- (often 2–5 m) gravelly coarse sandstones with large cross-beddings work, other polarity intervals were subsequently correlated with the and parallel beddings (Fig. 4C-e, i). Immediately above these sandstones GPTS 2012, determining an age range N 17.3–7.7 Ma for the GNG section are often large river floodplain deposits characterized by thick red or (Fig. 7). brownish red siltstones and mudstones having massive or sometimes weak horizontal or undulated beddings, intercalated with some thin layers of brownish yellow or grayish-greenish fine to median, well 5.2. Sedimentary facies and lithostratigraphy sorted, clast supported sandstones having massive or weak cross-bed- dings or parallel beddings deposited by small tributary sub- on 5.2.1. Identification of sedimentary facies and division of lithostratigraphy the floodplain (Fig. 4C-j). Occasionally, well developed paleosols can According to the diagnostic structure and fabric of sediments be found on the top of mudstones or immediately below a sandstone (Reading, 2009) in the total five studied sections, we identified five sed- bed, or strictly speaking on the top of association of small tributary imentary facies: alluvial fan, river, lake, swamp and eolian loess. sub-river channel sandstones – floodplain siltstones and mudstones, 90 X. Fang et al. / Global and Planetary Change 145 (2016) 78–97 representing floodplain surface that were not subjected to flooding for a and therefore appear mostly in the basin center, such as on the top of relatively long time but to pedogenesis forming paleosols. The types of the Guonigou and Maogou section (Fig. 4B, C). Close to the basin margin, developed paleosols depend on paleoclimate conditions. Big river sedi- in the HLD, no recognizable loess occurs. ments mainly distribute in the basin center. Small braided river channel, bar and overbank deposits consist of char- 5.2.2. Variation of sedimentary facies and lithostratigraphic division in the acteristic small cycles or sub-facies association of channel deposits of thin to studied sections thick grayish fine to coarse conglomerates with massive or weak imbricate Sedimentary facies was recovered for the three studied sections structures and small cross-beddings at the bottom, bar deposits of thin based on lithology and facies characteristics summarized above (Fig. brownish yellow sandstones with massive or weak parallel- and cross-bed- 4A, B, C). In the HLD section, five facies were identified. These are chan- dings in the middle, overbank plain deposits of thin to medium brownish nel and floodplain deposits (related to big rivers), channel and overbank red to brown massive siltstone and mudstone beds in the upper part, and deposits (related to braided rivers), and alluvial fan (facies association one or several weakly- to well-developed brownish red to reddish brown indicative of for approaching proximity of mountains in upward direc- paleosols at the top (e.g. Fig. 4A-c–g, B-a, c, C-m, n). The gravels and tion) (Fig. 4A). The section starts with big river channel deposits of bed- cross-beddings show mostly a northward direction. The channel conglom- ded thick gravely sandstones (~825–740 m in depth). At depths of 740– erate beds mostly have large lateral variation in thickness and occur often 602 m, the cycled siltstones, mudstones and paleosols were interpreted as long large lenses. This facies association occurs in space in front of alluvial as a typical facies association of big river floodplain, representing small fans along the margins of mountains. tributary river channels, overbank flooding sheet flows and paleosol de- All paleosols found in the Linxia Basin are luvic paleosols character- velopment on overbank deposits in watershed area or during intervals ized by a luvic Bt horizon or a cambic Bw horizon and an underlying dis- of less flooding on the floodplain of the big river (Reading, 2009)(Fig. tinct calcic Bk horizon and occasionally additional calcic Ck horizon (e.g. 4A). From 602 m to 510 m, the small cycles of thin fine conglomerate Figs. 2b and 4A-d, i–k, C-l). The Bt horizon is reddish or brownish red, and sandstone beds and reddish siltstones and mudstones with with silty clay or clay texture, shows a ped structure (fine granular paleosols were interpreted as braided river channel and overbank de- soil peds), contains many small root and animal channels, and develops posits (Reading, 2009), which gradually propagated into the basin and many shiny reddish ferric clay coatings and black Fe\\Mn mottling replaced the former floodplain of the big river (Fig. 4A). From 510 m stains on surfaces of peds and walls of channels (Figs. 2band4A-e). to 80 m, similar braided river sediments of cycles of channel conglomer- The Bw horizon is reddish brown or brownish, with silty clay texture, ates, side bar sandstones and overbank siltstones and final calcic shows fine blocky to massive structures, contains some or frequent paleosols dominated the long interval with a long trend of increasing small root channels, and exhibits a few brownish clay coatings and channel deposits and decreasing overbank deposits and paleosol devel- black Fe\\Mn mottling stains. The Bt or Bw horizon is usually 20– opment, suggesting an upward increase of water power. This upward 40 cm thick, depending on preservation, and its upper part is often erod- coarsening trend culminated at intervals 80–20 m where poorly sorted ed away (Fig. 2b). The Bk horizon is immediately below the Bt or Bw ho- boulder conglomerates occurred, which we interpreted it as alluvial fan rizon and is gray or yellowish gray, mostly 10–15 cm thick, and has deposits (Reading, 2009). Above it, the short interval of brownish yel- many white carbonate coatings, small nodules and impregnations low sandstones and siltstones at the top of the section was interpreted which cement the sediments (e.g. Figs. 2band4C-d, i, k). In most as a return to braided river conditions (Fig. 4A). cases, the content of the carbonate pedofeatures below the Bt horizon Since the big lateral facies change and no recognizable stratigraphic is higher, thus much more distinct than that below the Bw horizon. Oc- unit between the MG section in the basin center and the HLD section in casionally, down-leaching carbonate can reach soil parent materials the basin margin, the stratigraphy of the HLD section was divided (Figs. (mudstone or siltstone) below the Bk horizon (calcic Ck horizon with 4Aand7) according to the same criteria of lithologic features and cycles carbonate pedofeatures similar to that in Bk), but its content is less for the MG section (Li et al., 1995, 1997a; Fang et al., 2003): than that in the Bk horizon (Fig. 4A-j, C-l). Such kind of carbonate Zhongzhuang Fm. - bottom cycle of channel to floodplain deposits of pedofeatures is common in this region in Quaternary paleosols, but big river at 824.7–602 m in depth (23.3–18 Ma in age) -; Shangzhuang the Bt horizon is much stronger than that of the Quaternary equivalents Fm. - cycle for a transitional interval from big river floodplain to braided (Fang et al., 1994). In braided river areas, layers of very hard carbonate river deposits at 602–510 m (18–14.8 Ma); Dongxiang Fm. and Liushu concrete or bedded large carbonate nodules called petrocalcic horizon Fm. - cycles for low and high (water power) degrees of braided river de- are observed below paleosols. They are not soil genetic but formed posits at 510–263 m (14.8–9.7 Ma) (more overbank deposits) and 263– from groundwater or river water (Fang et al., 1994). 142 m (9.7–6.5 Ma) (more channel deposits), respectively; Hewangjia Shallow lake sediments are mainly dominated by purple-red or red- Fm. - the cycle representing a return to low degree of braided river de- dish-brown, gray mudstones, siltstones, and thin marlites. Sediments posits at 142–80 m (6.5–4.1 Ma); Jishi Fm. - distinct boulder conglomer- are very fine, well sorted, and composed of clay and silt. They usually ex- ate bed of alluvial fan at 80–20 m (4.1–2.6 Ma); Dongshan Fm. - top hibit thin horizontal beddings or laminated beddings consisting of alter- braided river deposits at 20–0m(2.6–1.8 Ma). ation of reddish-brown/purple-red mudstone and white/green marlite Deng (2004) recognized a thick mainly coarse sediment unit of gray- known as the Zebra Bed or yellowish brown to brown siltstones (Fig. ish-yellowish or rusty loose sandstones (10 m) – mudstones (6 m) – 4B-e, C-d, h, j, k). These beds distribute in the basin center and can lat- conglomerates (15 m) between the DX and LS Fms. at Longdan (see erally be traced over several to decades of kilometers in outcrops. Fig. 1b for location) where a Platybelodon Fauna was found in sand- Lakeside sediments contain massive mudstone, siltstone, and thin stone, and he gave an additional nomenclature of the Hujialiang Fm. sandstone (Fig. 4C). Weak wavy bedding and soft deformation are com- to that unit. Due to large lateral facies variation, such a unit is very diffi- mon in sandstone and mudstone. cult to recognize in our HLD section where the corresponding Swamp deposits contain well preserved dark gray peat and inter- Platybelodon Fauna occurs only in a thin conglomerate bed at 377 m bedded by yellowish/brownish siltstones commonly with thin layers which is only one of many thin conglomerate beds of braided river of horizontal carbonate nodules of 3–10 cm and plant (even tree tim- channel deposits (Fig. 4A). Thus we do not use the nomenclature of ber) relics. It is not a usual type and only found in the topmost sequence the Hujialiang Fm. in our stratigraphic division system. of the WJS and DSD sections, and occurs in association with shallow lake In the GNG section, distinct “Zebra Bed” of stable interbedded purple sediments (Fig. 3a, c). silty mudstones and white marlites occurs at the bottom at thicknesses Eolian loess is yellowish homogenous silt and clay and unconform- 0–56 m (N17.3–14 Ma), which was interpreted as shallow lake deposits ably caps the underlying sediments. These loess deposits were (Reading, 2009). Above it, from 56 m to 148 m (14–8.8 Ma), we interpreted as wind-blown in semi-arid to arid areas (Liu et al., 1985) interpreted the siltstones and mudstones (occasionally with weekly X. Fang et al. / Global and Planetary Change 145 (2016) 78–97 91 developed calcic paleosols) as floodplain, and the interbedded sand- in the basin center to 580 m in the HLD section in a proximal position stones and fine conglomerates as channel deposits of small tributary of the mountains, indicating a wedge body of the sediments in the rivers on the floodplain and intervening braided rivers (Reading, basin with the depocenter near the HLD section (Fig. 8). The corre- 2009)(Fig. 4B). From 148 m to 198 m (b8.8 Ma) we recognized another sponding average deposition rate (roughly equals thickening rate) in cycle of floodplain of siltstones and mudstones bottomed with a thick the HLD section is 41.1 m/Ma, much higher than that in the MG section channel conglomerate bed. The interval 198–211 m consists of final at 15.9 m/Ma. Furthermore, the thickening rate in the HLD section de- cycle of floodplain. Superimposed on it is the eolian loess deposit (Fig. creases upwards from a mean of 47 m/Ma at the interval 22–13 Ma to 4B). The above facies variation in the GNG section constructs a facies as- a mean of 35.0 m/Ma at the interval 13–8 Ma, while in the MG section sociation indicating a distal sedimentary environment. the corresponding time intervals show an increasing trend from a By lithology and facies, the “Zebra Bed” can readily be divided as the mean of 15.4 m/Ma to a mean of 16.4 m/Ma (Fig. 8). The thickening Dongxiang Fm. However, by age it only can be defined as the state of the GNG section which lies between the MG and HLD sections Shangzhuang Fm. (Figs. 4Band7). The above cycle of floodplain, rhyth- is between them (mean of 18.4 m/Ma during 13–8 Ma). We call this in- mically interfered by braided river at 56–148 m, is divided as the terval as Phase I. Dongxiang Fm. The uppermost two cycles of floodplain deposits at Secondly, since 8 Ma, the wedge body of the sediments disappeared 148–198 m and 198–211 m are defined as the Liushu Fm. and but thickened toward the basin (for intervals 8–4.3 Ma, 120 m in HLD Hewangjia (HWJ) Fm., respectively (Fig. 4B). section and 131 m in the MG section). The corresponding thickening The stratigraphic division of the MG section follows previous studies (Li rate of the HLD section decreased dramatically to a mean of 28.4 m/ et al., 1995; Fang et al., 2003)(Fig. 3b). Here we made more detailed facies Ma while the MG section increased nearly two fold to a mean of analysis on the stratigraphic units. The graded sequence at the bottom (0– 28.5 m/Ma, indicating a fast basinward migration of depocenter (Fig. 35 m) consists of poorly sorted massive fine conglomerates to cross-bed- 8). We call this interval as Phase II. ded gravely sandstones and siltstones, and was interpreted as braided Thirdly, in correspondence to the two-phase change of the sediment river channel, side bar to overbank sheet deposits (Reading, 2009)(Fig. body and thickening rate above, the lithology and sedimentary facies 4C). The overlaying purplish-reddish brown mudstones and siltstones and its association show also two phases. In Phase I (ca. 23–8Ma),the with clear horizontal beddings (35–60 m) are interpreted as shallow lake stratigraphy of the HLD section is dominated mostly by fine sediments sediments, representing further reduced relief and sedimentary supply. of siltstones, mudstones and paleosol complexes intercalated with The thick massive weakly bedded purplish silty mudstones (60–91 m) some thin sandstones and fine conglomerates of big river channels were interpreted as floodplain deposit (Reading, 2009). These two units and floodplain in the lower part, and a braided river system in the compose the Tala Fm. (Fig. 4C). For the ZZ Fm., we regarded the lower upper part. The basin center, with the stable “Zebra Bed” of a shallow part (91–121 m) of the loose, well sorted, yellowish gravely sandstones lake occurring in the big river floodplain, was situated in the location and coarse sandstones with obvious large cross- and parallel-beddings as of the GNG section (SZ Fm.) before 14 Ma. It later shifted northwards channel deposits related to a big river, and the upper part (121–188 m) (basinwards) to the MG section (DX Fm.) at 13–8 Ma, indicating an ob- of alternative reddish massive siltstones and mudstones with a few calcic vious diachronous occurrence of the same stratigraphic unit, while the paleosols or with horizontal-beddings as floodplain or short-lived shallow former lake in the GNG section was replaced by a basinward expanding lake deposits on the floodplain (Reading, 2009)(Fig. 4C). The SZ Fm. braided river front (Fig. 8). The flow directions of big rivers in Phase I (188–231 m) has similar lithologic cycle and characteristics to the ZZ Fm. were preferentially in eastward direction (Figs. 1b, 4A, B and C), indicat- below. Thus we interpreted it as another cycle of big river – shallow lake de- ing the big river system paralleling the E-W Qin Ling. In Phase II, the HLD posits (Fig. 4C). The lower part (231–251 m) of the DX Fm. consists of thick section was replaced by coarse sediments of proximal braided river and massive, parallel-bedded or weakly cross-bedded conglomerates to sand- debris flow, and the nearly whole basin was occupied by the expanded stones and was again interpreted as channel deposits of a big river braided river system. The former lake was squeezed to disappear in the (Reading, 2009)(Fig. 4C). Above it, at intervals 251–313 m, we find the MG section (Fig. 8), and might have expanded more northward and striking unit of “Zebra Bed” with stable horizontal-bedded alternated pur- eroded away by the Yellow River there (Fig. 1b). The occurrence of the ple massive mudstones and white-greenish gray marlites (occasionally greatly shrank Paleo-Dongshan Lake around DSD and WJS at 2.6– laminated), and we interpreted it again as rhythmically fluctuated shallow 1.8 Ma (Li et al., 1995)(seeFig. 1b for location and Fig. 3 for evidence) lake deposits (Reading, 2009)(Fig. 4C).TheLSFm.(313–376 m) of pre- might be the preserved relics of the basinward-migrated lake system dominant siltstones and mudstones with some calcic paleosols and sand- since 8 Ma. After 1.8 Ma, the Paleo-Dongshan Lake was terminated by stones represents a transition from lake to floodplain, that can be a pulse tectonic deformation and uplift of the whole Linxia Basin, called interpreted as lakeside and floodplain deposits (Reading, 2009)(Fig. 4C). Qingzang Movement C (Li et al., 1995) and followed by the onset of the The last graded cycle (the HWJ Fm.) at the top (376–443 m) from fine con- Yellow River in the basin and subsequent stepwise incisions until recent glomerate and coarse sandstone beds to very thick brownish yellow mas- forming seven clear terraces (Li et al., 1997b). The termination of the sive mudstones with many well developed calcic paleosols was HLD section at 1.8 Ma (Figs. 7 and 8c) might be a response to the interpreted as floodplain channel and overbank sheet deposits added by Qingzang Movement C. obvious eolian dust input (Fan et al., 2006; Reading, 2009)(Fig. 4C). The north-trend directions of river flows in the studied sections and The facies variation in the MG section reflects two major sedimenta- other sites of the basin in Phase II lend further support to this northward ry facies associations, a braided river - floodplain association at the bot- braided river expansion (Figs. 1 and 4). tom and river-lake-river association in the middle and upper parts, suggesting a general change of deposition from frontal braided river to 6. Discussion distal and basin center. 6.1. Tectonosedimentary evolution of the Linxia Basin and uplift of the NE 5.2.3. Space-time evolution of stratigraphy and sedimentary facies Tibetan Plateau The above detailed paleomagnetic dating, sedimentary facies analyses and stratigraphic division of the three sections (Figs. 4A, B, C and 7)enable The space-time evolution of the stratigraphy and sedimentary facies a detailed correlation of these sections for precise analysis of space-time above suggests that the Linxia Basin began to be flexed as an variations of the stratigraphy and sedimentary facies and the underlying intracontinental foreland basin at latest since the onset of the Miocene tectonic forcing (Fig. 8). Three distinct features can readily be found. from the West Qin Ling to the south. At the early Miocene in Phase I, Firstly, the stratigraphy at intervals 22–8 Ma is obviously thickened the flexural subsidence was very fast and was balanced by the persistent toward the margin of the West Qin Ling from 223 m in the MG section detrital supply from the West Qin Ling and the hinterland of the NE 92 X. Fang et al. / Global and Planetary Change 145 (2016) 78–97

Fig. 8. Correlation of the stratigraphy vs. sedimentary facies of the three studied sections within same time intervals (22, 13, 8 and 4.34 Ma) in the Linxia Basin. Note that the generally decreasing and increasing trends of sedimentation rates in the foredeep and forebulge zones concur with basinward migrations of the foredeep and basin center (lake) and their accompanying expansions of alluvial fan - braided river, together with an upward disappearance of wedge body of sediments in Phases I and II. A 3-D cartoon presentation of processes in Phases I and II is presented in lower left for easy tracing, in which thin and heavy arrows indicate detrital supply direction and uplift, respectively and numbers the replacing positions by moving depozones. Variation of carbonate content of the MG section (Li et al., 1995) is plotted beside the section for lithologic recognition and checking. Legends for lithology refer to Fig. 4A, B and C. D: debris flow of alluvial fan; B: braided river; F: floodplain; L: lake; R: big river. X. Fang et al. / Global and Planetary Change 145 (2016) 78–97 93

Tibetan Plateau, causing very obvious thickening of the sediments as a The uplift of the Maxian Shan, which borders the northern margin of striking wedge body and high and low sedimentation rates in the HLD the Linxia Basin, and the ongoing forebulge propagation (Fig. 1), may and MG sections in the long (foredeep) and short (forebulge) edges of have exerted some influences on the processes above. This happened the wedge body, respectively (Fig. 8). Since the sediments are mostly in many so-called broken foreland basins, where broad subsidence fine detritus and the related facies association was dominated by a and sediment filling were added to the forebulge and back-bulge mountains-parallel big river - shallow lake system along the foredeep zones, did this: e.g., the -related foreland basins in Argentina Si- (Figs. 4A, B, C and 8), we conclude that a low relief floodplain occupied erras Pampeanas (Jordan and Allmendinger, 1986; Ramos et al., most of the basin and the West Qin Ling and NE Tibetan Plateau were 2002); the Iberian -related West Duero Basin (Martín- not so high, but subjected to persistent denudation. The Paleo-Yellow González et al., 2014); and the Italian Apennines-related Po Basin and River through the Linxia Basin along the northern foot of the Qin Ling Ionian foreland basin (DeCelles and Gilest, 1996; Doglioni et al., 1999). in the Eocene - middle Miocene suggested by researchers earlier, The Maxian Shan is the exposed relict of the Qilian Shan extending east- based on stratigraphy of several basins (Lin et al., 2001), corroborates the wards into the western Longzhong Basin (Fig. 1). It is comprised of pre- occurrence of a mountain-parallel river system in the Miocene. At the late Cambrian heavily metamorphosed basement rocks and Early Creta- stage (13–8 Ma) of Phase I, decreased flexural depression, thickening and ceous cover rocks of red beds (Gansu Bureau of Geology, 1989). Tertiary sedimentation rate, probably due to initial fault propagation into the sediments superimpose unconformably upon the pre-Cambrian and basin and fold growth, caused the gradual expansion of a debris flow – Early Cretaceous rocks and are raised to over 2500 m above sea level braided river system along the apron of the West Qin Ling and squeezed by thrust faults (Fig. 1b). During the West Qin Ling and NE Tibet defor- the former big river - lake system such as the lacustrine “Zebra Bed” migrat- mation and uplift, the tectonic stress was transferred to the Maxian ing northwards into the basin (Fig. 8). Only by lithology, it is natural to re- Shan through Linxia Basin basement and the Maxian Shan gard the “Zebra Bed” in the GNG section at N17–14 Ma and in the MG transpressional faults, causing it deformation and uplift. However, the section at 13–8Ma(Fig. 8) as one unit with one stratigraphic name, causing deformation of the northern margin of the Linxia Basin by the south- a great pitfall. This great diachroneity of stratigraphy in the intracontinental ward thrusting of the Maxian Shan thrusts (e.g. F6 and F6′) was much foreland Linxia Basin might explain the contradiction and debate on strati- weaker and later, because 1) the late Miocene-Pliocene red beds are graphic ages and correlations constrained by magnetostratigraphy and fos- found to superimpose on the mountain rocks with no folding but only sil mammals (Li et al., 1995; Fang et al., 2003; Qiu et al., 2004; Deng et al., mild tilting; 2) the total Tertiary stratigraphy thickness near the margin 2004, 2013). Particularly when fossil mammals were not found directly in of the mountain is b400 m and is thinning quickly southwards to the paleomagnetically dated sections, such as the MG section, fitting of b200 m; and 3) the late Miocene-Pliocene red beds near the southern magnetic polarities caused very unreasonable and strange correlation bank of the Tao He (River) still show northward flow direction and a with the GPTS. Main obtained polarities cannot well be correlated with Qin Ling detrital source (Fig. 1b). Thus, the marginal depression caused theGPTSpattern,andtherelationshipsbetweendepthandageandbe- by the Maxian Shan uplift and detrital supply were limited during much tween the lithology and sedimentation rate are out of logic order in regard of the Tertiary. This means that the “counter-basin” thrust-fold belt to relationships between lithology, sedimentation rate and age (within the loading by the Maxian Shan had not acted like the Andes and Alpine same unit and same lithology sedimentation rates show very large fluctua- broken foreland basins (Jordan and Allmendinger, 1986; Ramos et al., tions; high sedimentation rates are not generally correlated with coarse 2002; Martín-González et al., 2014) exerting significant impacts both sediments in strong tectonic settings but with fine sediments of mudstones on subsidence and on sediment filling on the Linxia foreland basin. and siltstones) (Deng et al., 2013). Such biasing and bizarre correlation of However, the Maxian Shan weak depression and its related forebulge obtained polarities with the GPTS to fitfossilmammalagesisalsorepre- rise and the advance of fault F6′ may have contributed to deepening sented in results from the Xining Basin (Wu et al., 2006). of the back-bulge area and slow down of the sedimentation forebulge Since 8 Ma in Phase II, the rapid widespread basinward expansion of the area, which migrated northward from the Linxia foreland basin in debris flow – braided river system (propagation of boulder conglomerates Phase II (Figs. 1band8). Late in the Quaternary, the outflowing Yellow and other coarse sediments) from the rim of the West Qin Ling caused the River will transport all sediments from the basin and Maxian Shan to termination of the former mountains-parallel big river – shallow lake sys- other remote areas (Fig. 1). tem (Fig. 8). This expansion was accompanied by a rapid decrease and in- The sedimentation rate can be also influenced by climatic change. crease of sedimentation rates in the in the forebulge and foredeep zones By comparing sedimentation rate records of the studied sections of the HLD and MG sections, respectively, a rapid clockwise basin rotation with the global climatic record (Zachos et al., 2001), we found that (Fig. 9b, c) and the growth of folds and strata (not clear in the HLD section, the striking global Middle Miocene Cooling (MMC) event at about but obvious in the Huaishuguan (HSG) and Yinchuangou (YCG) ) 14 Ma is associated with a pulse increase of the sedimentation rate (Fang et al., 2003; Li et al., 2014)(seeFig. 1b, c for locations). All these ob- in the Linxia Basin (Fig. 9a, b). The strong erosion caused by the servations collectively suggest that the faults propagated rapidly into the great deterioration of the Linxia climate at the MMC event (Ma et basin forced by the rapid uplift and denudation of the West Qin Ling and al., 1998; Li et al., 2014)waslikelyresponsibleforthepulsedin- NE Tibetan Plateau, leading to folding and uplifting the stratigraphy proxi- crease of the sedimentation rate above. However, the Plio-Quaterna- mal to the mountains. Finally, at about 1.8 Ma, this folding and rising termi- ry long-term global cooling has no clear response in our nated the deposition in the HLD section and let it become a new second sedimentation rate records, which show a coupled persistent de- sedimentarysourceforthebasin(Fig. 8). At the same time, the entire Linxia crease and increase of sedimentation rates of the HLD section in basin was also deformed and uplifted, with an end of the foreland basin de- the foredeep and the MG section in the forebulge at about 8 Ma, re- position, followed by the appearance and subsequent great incision of the spectively (Fig. 9a,b).Thissuggestsafirst order tectonic basinward present Yellow River. All later eroded sediments were brought out of the thrusting-folding forcing rather than climatic forcing. basin (Figs. 1–3)(Fang et al., 2003; Li et al., 1997b), suggesting a re- gional rise of the mountains and the foreland basin. This is supported by fis- 6.2. Model of tectonosedimentary evolution of intracontinental foreland sion track analyses of detrital apatite grains from the Cenozoic sediments of basin the Linxia Basin. Recycled detrital apatite grains occur since about 8 Ma, as indicated by the turnover of a previous decreasing trend of youngest ages of Summarizing the results of the study above enables us to develop a fission tract components to an increasing trend at about 8 Ma, suggesting a model that generalizes the tectonosedimentary evolution of an new recycled sedimentary source (Brandon, 1992; Garver et al., 1999)in intracontinental foreland basin. It could be indicative for future studies the broad basin margin and a rapid uplift of the NE Tibetan Plateau at on climatic change, stratigraphic division and correlation, sedimentary about 8 Ma (Zheng et al., 2003). facies and basin analysis, tectonic evolution, and mountain uplift and 94 X. Fang et al. / Global and Planetary Change 145 (2016) 78–97

subsided accommodation by input sediments (Covey, 1986). This leads to high subsiding and high sedimentation rates in the loading part (i.e., the foredeep) of an intracontinental foreland basin and a bal- anced rise and decreased low sedimentation rate in the opposite part (i.e., the forebulge) of the basin. A name “foreslope zone” is given to the area between them (Fig. 10a, c). The balance between the denuda- tion of the mountains, thus sediment supply and infilling, and flexural depression means the topographic relief between the mountains and basin surface will not be so high. This low relief will suppress the devel- opment of debris flow – braided river system along the mountain rim, but will favor a mountains-parallel big river – shallow lake system along the foredeep. On the forebulge zone small braided rivers will occur. Thus, mostly fine sediments of mudstones to sandstones of big river – shallow lake and distal braided rivers dominate the basin (Fig. 10a). At later stage of Phase I, the balance between mountain uplift, denu- dation, sediment infilling and basin depression began to turn gradually to increasing mountain uplift, probably due to stronger continental push and insistence from the thickened basin crust. The increased uplift will cause faulting and deformation to propagate into the basin, initiat- ing a gradual rise of the basin margin, decreasing subsidence and sedi- mentation rates, basinward expansion of the braided river system, and migration of the mountains-parallel big river - lake system, resulting in diachronous occurrence of a same stratigraphic unit. This process might be accompanied by initial growth of folds and strata, and basin rotation. In Phase II, the balance between mountain uplift, denudation, sedi- ment infilling and basin depression is broken. The mountains began to uplift rapidly and the thrust-fold system propagates fast into the basin, causing rapid rotation of the basin and growth of folds and strata in the basin margin and even near the basin center (Figs. 10b, c). This fast thrusting - folding conducts strong stress and new loading immedi- ately into the basin, giving rise to a new flexural subsiding depression and foredeep in front of the thrust-fold system and further depressing the former foreslope and even the forebulge zones. This process will cause a large decrease of sedimentation rates in the former foredeep zone or even terminate deposition there, which turns into a wedge- top zone (DeCelles and Gilest, 1996) or even an erosional area and act as a second source supplying recycled sediments, increasing the sedi- mentation rate in the foreslope and forebulge zones (Figs. 8 and 10b). This rapid uplift and creation of a new sediment source can be revealed by the turnover of a decreasing trend of youngest ages of fission track Fig. 9. (a) Late Cenozoic global climate change indicated by the oxygen isotope record of components of detrital apatite from persistent denudation (thus synthetic benthic foraminifera of deep sea sediments (Zachos et al., 2001). (b) younging of cooling ages) of the mountains to an increasing trend in Variations of sedimentation rates and (c) mean paleomagnetic declinations of the HLD, the upward sequence of basin sediments due to adding progressively GNG and MG sections. Mean declinations were calculated for every formation of the sections. Note the general rapid decrease (increase) of sedimentation rates concurred in older detrital apatite deposited in and eroded from the new second sed- the HLD section in the foredeep and MG section in the forebulge at ~8 Ma, also iment source (Brandon, 1992; Garver et al., 1999)(Fig. 10c). This com- synchronous with the rapid ~14o clockwise rotation of the Linxia Basin since ~8 Ma, but monly means that the sediment supply is sufficient that the basin is less correlating with climate, suggesting a tectonic rather than climatic forcing at a long- overfilled. On the other hand this process will favor the basinward ex- term time scale. MMC: Middle Miocene cooling event; MMO: Middle Miocene climatic pansion of the proximal debris flow – braided river system and push optimum. and squeeze the former big river – lake system from the former foredeep to the slope and bulge zones and finally terminate it. In conse- geomorphologic configuration based on sediments in such basin types. quence, the mountains-perpendicular debris flow – braided river sys- The tectonosedimentary evolution can be divided in two phases accord- tem predominates the basin, which finally advances and covers the ing to sedimentary and tectonic environments. In Phase I, when an bulge with rivers carrying sediments out of the basin (Fig. 10b, c). The intracontinental mountain began to be activated (rejuvenated) and up- wedge body of sediments in the former foredeep will disappear and a lift due to continental collision, the will thrust onto the new wedge body of sediments will be formed in front of the thrust- basin and push the loaded basin to flexural subsidence. Deposition of fold system in the far basinward or center area (Fig. 10b). sediments from the rising mountain in the subsiding depression will This tectonosedimentary evolution is independent of climatic add new load and cause further subsidence. The subsidence is mostly change which only exerts additional or second order impacts on lithol- fast at the early stage, though it depends principally on the loading de- ogy and sedimentation rate in climatic event. For example, under semi- gree of the old thrust-fold belt (mountains) and rigidity of the loaded arid and arid climates, surface and groundwater contain much ions of basin basement (Beaumont, 1981; Stockmal et al., 1986; Sinclair et al., evaporites and flow from proximal debris flow - braided rivers into 1991). It is probably due to strong block push and rise of the old the distal floodplain of big river and lake, depositing marls (such as thrust-fold belt (mountains) reaching already above sea level, which the “Zebra Bed” in the Linxia Basin; Figs. 4A, B, C and 8) or gypsum will result in corresponding strong erosion that may balance the and even salts in lake, or their nodules in distal braided river and .Fn ta./Goa n lntr hne15(06 78 (2016) 145 Change Planetary and Global / al. et Fang X. – 97

Fig. 10. Tectonosedimentary evolution model of an intracontinental flexural basin. (a) Phase I. Fast flexural subsidence balances with fast infilling sediments denudated from persistent rising but low mountains, resulting in high depression and sedimentation rates and mostly fine sediments (mudstones to sandstones) of mountains-parallel big river – shallow lake system along the foredeep; (b) Phase II. Rapid uplift of mountains is not balanced by further rapid subsidence and infilling of the former foredeep but rising of the former foredeep by rapid propagation of thrusts – folds (also acting as new secondary source for basin sediments) and creation of a new foredeep in the thrust-fold system, causing rapid basinward migrations of the foredeep and basin center (lake) and great expansion of mostly coarse sediments (sandstones to boulder conglomerates) of alluvial fan - braided river system (mountains-perpendicular river system) which finally replaces the former mountains-parallel river – shallow lake system. (c) Schematic diagram illustrating how sedimentation rates in the foredeep and forebulge zones, basin rotation and sedimentary source change with two phases of evolution of flexural basin above. Note a coupled decrease and increase trend of sedimentation rates in the foredeep and forebulge zones occurs from Phase I to Phase II when the foredeep is migrating basinwards into forebulge. This is often accompanied with a rapid basin rotation and infilling of recycled sediments from the new secondary source from the rising thrust-fold system, both caused by rapid propagation of thrusts and folds into the basin. The recycled sediments are indicated by the turnover of a decreasing trend of youngest ages of fission track components of detrital apatite to an increasing trend. This two-phase evolution of basin stratigraphy and sedimentary facies is independent on climatic change, which only exerts superimposed impacts on lithology, summarized in tables plotted in the right. D: debris flow of alluvial fan; B: braided river; F: floodplain; L: lake. White arrows in 3D diagrams show general directions of river flows. 95 96 X. Fang et al. / Global and Planetary Change 145 (2016) 78–97

floodplain sediments (Fig. 10). Under humid-hot climate, surface and GNG sections and helps on field recognition of fossil mammal sites are ground waters are fresh, river and lake will receive more water and or- greatly appreciated. Valuable comments from Prof. Rixiang Zhu and ganic matter, apt to lead swamps occurring on floodplain or lakeside Prof. Erche Wang on initial manuscript and Prof. Gwenn Peron-Pinvidic and forming light coloured, mostly horizontally thin-bedded or laminat- and an anonymous reviewer for revised version are greatly appreciated, ed sediments (Fig. 10). An extreme climatic event will only cause a pulse which improved much the manuscript quality. Especially our cordial impact on lithology and sedimentation rate. 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