The evolution of a terrace sequence along the Yellow River (HuangHe) in Hequ, Shanxi, China, as inferred from optical dating

Geomorphology 109 (2009) 54–65

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

The evolution of a terrace sequence along the Yellow River (HuangHe) in Hequ, Shanxi, China, as inferred from optical dating Jia-Fu Zhang a,⁎, Wei-Li Qiu b, Rong-Quan Li b, Li-Ping Zhou a a b

Laboratory for Earth Surface Processes, Department of Geography, Peking University, Beijing 100871, China School of Geography, Beijing Normal University, Beijing 100875, China

a r t i c l e

i n f o

Article history: Received 1 December 2006 Received in revised form 17 January 2008 Accepted 8 August 2008 Available online 12 February 2009 Keywords: Optical dating Fluvial deposits Evolution of terraces Yellow River Hequ area of China

a b s t r a c t Investigations of river terraces are often seriously hindered by difficulties in dating the formation of terraces using conventional and well-established methods such as 14C dating. In this paper, recently developed optical dating techniques were applied to a Yellow River terrace sequence in the Hequ area, Shanxi Province, China. Based on field investigations, four terraces were identified. Systematic sampling for optical dating was carried out on these terrace deposits. The single-aliquot regenerative-dose method was used to determine equivalent dose. By analyzing the degree of bleaching of fluvial samples, maximum and minimum optical ages were obtained for these samples. The minimum optical dates obtained are geomorphologically and stratigraphically consistent with each other, suggesting that the dates are reliable. The formation ages of the four terraces are 3.4 ∼ 10, 20 ∼ 25, ∼ 30 and ∼ 90 ka, respectively. On the basis of these optical dates, the geomorphological evolution of the terrace sequence was established, and the factors affecting the terrace development discussed. © 2009 Elsevier B.V. All rights reserved.

1. Introduction A river terrace represents the past bed level of a river, and terrace formation responds to tectonic movements or climatic changes in an area (e.g. Bridgland, 2000; Pan et al., 2003; Sun, 2005; Hanson et al., 2006; Westaway et al., 2006). Hence, river terraces have been widely investigated by geomorphologists in order to understand the tectonic and climatic history of an area. River terraces have also attracted the considerable attention of archaeologists, because many artifacts are distributed in them (e.g. Bird, 1939; Swisher et al., 1996; Hou et al., 2000; Bridgland, 2000). However, the major obstacle to the investigations is age determination (Stokes and Walling, 2003), because of the lack of terrace deposits suitable for dating using conventional methods such as radiocarbon dating. Even if organic materials found in terrace deposits were dated, it is still very difficult to correlate the radiocarbon dates with the deposition ages of fluvial sediments. This is because it is uncertain whether the organic materials and the formation of sedimentary strata are synchronous. In addition, the radiocarbon dating range also limits its application to sediments younger than ∼ 40 ka. Optically stimulated luminescence (OSL) dating techniques (optical dating) offer a possibility of accurately dating fluvial sediments. The major advantage of optical dating over other radiometric dating

⁎ Corresponding author. Tel./fax: +86 10 62754411. E-mail address: [email protected] (J.-F. Zhang). 0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2008.08.024

methods is that the former dates the last exposure of sediments to light, i.e. it directly dates the deposition age of the main constituents (quartz and feldspar grains) of Quaternary sediments. In particular, the recently developed single-aliquot regenerative-dose (SAR) protocol for quartz (Murray and Roberts, 1998; Murray and Wintle, 2000; Wintle and Murray, 2006) has resulted in an improved precision and accuracy for optical ages of quartz (Murray and Olley, 2002). Although fluvial sediments are not ideal materials for optical dating because of possibly heterogeneous bleaching at the time of deposition, it is still possible to accurately date the sediments by assessing the degree of bleaching of the sediments at deposition (e.g. Wallinga, 2002; Zhang et al., 2003; Jain et al., 2004; Pei et al., 2006). The Yellow River (Huanghe), well known for its tremendous sediment load, originates in the northeast of the Tibetan Plateau, and flows eastwards to the Bohai Sea. It runs across the Loess Plateau of North China and the east of the Ordos Plateau, and results in the formation of large gorges between Shanxi (Jin) and Shaanxi (Shaan) provinces. The history and the river terraces of the Yellow River have been of great interest to a wide community of scholars since 1920s (e.g. Wang, 1925; Barbour, 1933a,b; Pan et al., 1994; Cheng et al. 2002). But further investigations of the evolution of the Yellow River have been hindered by the lack of a reliable chronological framework for the fluvial sequences. In this paper, a terrace sequence of the Yellow River in the Hequ area, Shanxi Province, China, was chosen for dating using luminescence techniques. Here, the terrace age is referred to the formation age of terrace treads. In this area, four river terraces of the Yellow River along the banks of the river were recognized by fluvial

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sediments. By systematic sampling, the underlying fluvial and the overlying wind-blown samples on the terraces were dated using the single-aliquot regenerative-dose protocols (Murray and Wintle, 2000; Wintle and Murray, 2006), and a luminescence chronology for the terrace sequence was established. The geomorphologic evolution of the terrace sequence was then inferred from the optical dates. 2. Geological and geomorphologic setting The Hequ area is located in the northwest of Shanxi Province and borders on Shaanxi Province in the southwest and on Inner Mongolia Autonomous Region in the west and northwest (Fig. 1). Geologically, the Hequ area belongs to the northeast part of the Ordos Plateau which is a huge Mesozoic basin filled with inland clastic sediments of Triassic, Jurassic and Cretaceous ages. From late Early Cretaceous, the basin was uplifted as a whole and became a part of the ancient JinShaan Highlands, and then underwent a planation history until the late Paleogene. A new smaller basin appeared in the same area in the Neogene and began to receive red clay deposits. The Ordos Plateau was formed during the Pliocene and Quaternary owing to both further uplift and the dissection of the Yellow River in the east (Wang et al., 1985). The strata exposed in the river valley around Hequ are almost horizontal except that some local warping causes the strata to be gently tilted with the dip angles of up to 5 ∼ 10° and orientated mainly west or southwest. The strata consist of Triassic shales (mudstones), sandstones and conglomerates. These rocks are weakly cemented and are liable to be eroded. As shown in Fig. 1, the study area is located in the river valley, the northern part of the Yellow River gorge between Shanxi and Shaanxi provinces. The elevation of the river course in this area is about 840 ∼ 850 m a.s.l. (above sea level). The height of the watershed is more than 1000 m a.s.l. The most distinctive landform feature of the Hequ area is that the valley here is quite open and is really an erosional basin with meanders at the bottom. It is one of the places in the Yellow River gorge that has such typical meanders. The name of the town “Hequ” is a Chinese term for meander. In the west and southwest of the surveyed area, there are mountains and hills of rocks eroded heavily by running water. The high peaks have an elevation above 1060 m a.s.l. and lower peaks or hills close to the Yellow River decline to heights of 900 ∼ 950 m a.s.l.. To the east of the river, there is a high platform which has a relatively flat top surface with an even height between 1000 ∼ 1050 m a.s.l. and is covered with thick loess or sandy loess. The edge of the platform is dissected by gullies and the descending slope is smoothed by slope processes. In the central part on both the banks of the river, stream terraces are asymmetrically distributed along the meanders. The meanders at Hequ are not free meanders, but a kind of deformation incised meanders with obvious lateral erosion and downcutting. On the convex banks of the river, bedrock is exposed. Based on field investigations, a sequence of four terraces above the floodplain of the river in this area were identified, and the elevation of the terrace treads is between 850 and 950 m a.s.l. A profile (Z-Z′ line in Fig. 1) along the convex bank of the river was investigated in detail and was systematically sampled for optical dating. The details of the profile are shown in Fig. 2 and described below. 3. River terraces and OSL sampling As shown in Figs. 1 and 2, four terraces along Z-Z′ line are named Terraces T1, T2, T3 and T4 from the lowest level (south) to the highest level (north), respectively. The lowest and the highest terraces (T1 and T4) are generally fill terraces, and the middle terraces (T2 and T3) are strath terraces covered with a thin veneer of alluvium. In order to provide an accurate chronology for the geomorphological evolution of these terraces, systematic sampling was carried out on a total of nine sections on these terraces. These nine sampling

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sections are numbered A to I from south (T1) to north (T4) and shown in Fig. 2. Samples were collected from different terrace deposits including fluvial sediments (channel and floodplain deposits) and overlying sediments such as loess. Fig. 3 shows part of section I in Terrace T4 in which a fluvial sand sample was collected from a sand lens within a gravel layer, and two wind-blown samples from an overlying loess-palaeosol sequence. Samples were taken by pushing or hammering metal cylinders into freshly cleaned sections. It is ensured that the cylinders were completely full in order to avoid mixing during transport. After being removed from the section, the cylinders were immediately sealed tightly at the ends with aluminum foil and plastic tape in order to prevent light exposure and water loss. Loess samples were collected by cutting a block from a section and wrapping it in aluminum foil and plastic tape. 3.1. Terrace T1 The lowest terrace (T1) is a small meander scroll plain (Figs. 1, 2). It is ∼ 1000 m broad and has an elevation of about 6.0 to 10.0 m above the present river channel. The terrace deposits can be represented by the sediments in Sections A, B and C in Fig. 2. The sediments observed in the exposures of the terrace deposits are mainly floodplain sandy silt. Section A is composed of a thin (0.2 ∼ 0.3 m thick) sand layer at the top of the section, which is affected by agricultural/cultural activities, and a pale brown sandy silt layer (N1.5 m thick) with horizontal bedding. OSL samples (HQ-OSL05 and 06) were taken from this layer. Section B is located in a drainage ditch. The exposed part (1.5 m thick) of this section is pale brown sandy silt with discontinuous and horizontal bedding. Samples HQ-OSL07 and 08 were collected. Section C consists of three parts. The lower part is a gravel layer, and 0.8-m-thick deposit is exposed. It consists of channel deposits characterized by cobbles, pebbles and coarse sands. The gravels are rounded, flattened and well-sorted. The middle part of the section is a sand lens of 0.6 m from which a sample (HQ-OSL09) came. The upper part is a disturbed or redeposited sandy silt layer of 0.5 m. 3.2. Terrace T2 Terrace T2 is a relatively extensive upland terrace ∼ 1300 m broad within the convex bank (Figs. 1, 2). Its elevation is about 16 ∼ 23 m above the present river channel. Almost all deposits covered on the strath surface can be observed along the high cliffs on the east bank of the river. The strath is overlain by channel gravels, floodplain silt and sand. From south to north, the change in sediments, the strath surface and terrace tread heights are shown in Sections D, E, F and G in Fig. 2. The exposure of the south section (Section D) indicates that it consists of a N0.9-m-thick gravel layer overlain by a 0.69-m-thick sand layer capped by a 0.26-m-thick layer of silty clay disturbed by agricultural activities. The gravel layer is mainly composed of rounded and flattened cobbles and pebbles. An OSL sample (HQ-OSL10) was taken from the middle of the sand layer. In Section E, the top layer is 0.25-m-thick soil overlying the gravel layer. Within the gravel layer, there is a sand lens of 0.78 m in thickness with a 3 cm reddish brown silty clay bed within it. Sample HQ-OSL11 was collected from the middle of the sand lens. Section F is divided into six sediment layers from top to bottom. (1) a 0.5-m-thick silty soil layer in which cobbles are embedded; (2) a 0.7-m-thick gravel layer containing cobbles, pebbles and sand; (3) a 0.55-m-thick sand lens; (4) a 0.2-m-thick reddish brown laminated silty clay layer; (5) a 0.45-m-thick sand lens; (6) a 1.25-m-thick gravel layer containing cobbles, pebbles and sand. In this section, the bedrock of Triassic sandstones and mudstones is exposed on the river cliff, and its top is mantled by channel deposits (Layer 6). Two OSL samples (HQ-OSL12 and 13) were collected from the two sand lenses, although their thickness is less than 0.6 m.

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Fig. 1. Geomorphological map of the Hequ area showing the Yellow River terraces, the locations of terrace profile (Z-Z′) shown in Fig. 2 and OSL sampling sections.

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Fig. 2. Schematic profile of the Yellow River terraces in the Hequ area (profile Z-Z′ in Fig. 1) showing sampling sections, sample positions and OSL ages.

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Fig. 3. Photograph of a section in Terrace T4 showing a fluvial sand sample which was collected from a sand lens within a gravel layer, and two wind-blown samples from an overlying loess-palaeosol sequence.

The far north section (Section G) is close to Terrace T3. A 6.3-mthick gravel layer containing cobbles, pebbles and sand overlies bedrock. The top layer is 0.7 m of soil disturbed by human activities. A buried soil 0.6 m in thickness lies between the top soil layer and the gravel layer. Sample HQ-OSL12 came from the middle of the buried soil layer in which the parent materials are not fluvial sediment. 3.3. Terrace T3 Terrace T3 is the narrowest one in the surveyed area (Fig. 2). The average height of its tread is ∼ 30 m above the present river channel. The terrace sediments observed in exposures on the bank of the river are shown in Section H. This section consists of three sediment layers. The top layer is a sand dune 3.3 m in thickness. The wind-blown fine sand is characterized by well-sorted, loose sand and lack of bedding. On the top of this layer is a 0.1 ∼ 0.2 m soil. The middle layer is floodplain deposits 2.0 m in thickness. It is composed of interbedded reddish brown clayey silt and yellow brown fine sand with typically horizontal bedding. The lower layer is channel deposits consisting of cobbles, pebbles and sands, and this 3.8 m gravel layer rests on bedrock of Triassic purple shale and grey yellow sandstones. Here, a bedrock outcrop 8.5 m high can be observed. Four OSL samples were taken from this section. Two (HQOSL15 and HQ06-OSL01) of them were from the floodplain sediment layer. The other two samples (HQ-OSL16 and 17) were from the top aeolian layer. The formation age of the terrace tread should be between the burial ages of the sediment from the two units. 3.4. Terrace T4 The highest Yellow River terrace in this area is Terrace T4 (Fig. 2), and the terrace tread has an elevation of ∼86 m above the present river channel. On this terrace 32.3-m-thick fluvial sediments were deposited, and the sediments are overlain by a 4.8-m-thick loess-palaeosol sequence. From Fig. 2, it can be seen that the terrace deposits can be divided into five sediment layers from bottom to top. Layer 1 (32.1 ∼ 37.1 m depth) is a 5.0-m-thick gravel resting on the top of the

Triassic bedrock whose top is ∼52 m above the present river. Layer 1 is covered by Layer 2 (26.1 ∼ 32.1 m depth) which is a floodplain sediment 6.0 m in thickness. Layer 2 is composed of interbedded grayish yellow fine sand, silty sand, and reddish brown clayey silt with perfect horizontal bedding. Layer 3 (17.1 ∼ 26.1 m depth) is a 9.0 m-thick debris flow deposit with red clay pellets. This layer was covered by a 12.3-mthick gravel layer (Layer 4, 4.8∼ 17.1 m depth) with inclined bedding and cross-bedding. Two sand lenses with cross-bedding were found within this gravel layer in this section, and their maximum thickness is 1.0 and 1.5 m, respectively. One of them is shown in Fig. 3. Layer 4 is mantled by a loess-palaeosol sequence (Layer 5, 0 ∼ 4.8 m depth). The upper loess layer is 3.6 m in thickness. The lower palaeosol layer is 1.2 m in thickness, and a carbonate concretion horizon occurs at the bottom of this layer. A total of six OSL samples were taken from this section (Fig. 2). Samples HQ-OSL18 and 19 are floodplain sediments from Layer 2. Samples HQOSL20 and 21 are channel deposits from the sand lenses in Layer 4. Samples HQ06-OSL02 and 03 are palaeosol and loess, respectively. Their positions are shown in Figs. 2 and 3.

4. Optical dating 4.1. Dose rate determination Uranium, thorium and potassium contents of the samples were determined by neutron-activation-analysis (NAA), except that samples HQ06-OSL01, 02 and 03 were analyzed using thick-source alpha counting (a Littlemore Low Level Alpha Counter 7286 with 42-mmdiameter ZnS screens). The potassium content was also measured using flame photometry, and the results are consistent within error limits with the results determined by NAA (Table 1). Different longterm water contents (mass of moisture/dry mass; Aitken, 1985) for the burial period were assumed for these samples, and the relative uncertainties were taken as 20% (see Table 1). An alpha efficiency factor (a-value) of 0.038 ± 0.003 for quartz (Rees-Jones, 1995) was used to calculate the alpha contribution to the total dose rate. Using the revised dose-rate conversion factors of Adamiec and Aitken (1998)

Table 1 Optical dating results. Section

Lab code

Field no.

Depth, m

Grain size, μm

Flame photometry

NAA

Water content, %

K, %

K, %

U, ppm

Th, ppm

Dose rate, Gy/ka

All measured aliquots

Relatively well bleached aliquots

OSL ages, ka

Na

Nb

Max.

De, Gy

De, Gy

Min.

Δ, %

HQ-OSL05 HQ-OSL06 HQ-OSL07 HQ-OSL08 HQ-OSL09

1.50 0.40 1.20 0.30 0.80

90–125 90–125 90–125 90–125 90–125

1.72 1.65 1.80 1.65 1.65

1.72 ± 0.10 1.61 ± 0.09 1.73 ± 0.09 1.77 ± 0.09 1.64 ± 0.09

2.35 ± 0.08 2.01 ± 0.07 2.60 ± 0.09 2.21 ± 0.09 1.73 ± 0.08

9.60 ± 0.21 9.78 ± 0.22 10.40 ± 0.23 9.54 ± 0.21 8.74 ± 0.20

10 10 10 10 10

2.80 ± 0.10 2.67 ± 0.10 3.15 ± 0.11 2.75 ± 0.09 2.57 ± 0.09

23 21 35 22 26

38.13 ± 4.20 18.89 ± 1.46 7.42 ± 1.07 42.07 ± 4.05 30.56 ± 1.02

2 3 4 5 15

23.77 ± 1.63 9.09 ± 0.54 1.58 ± 0.10 19.12 ± 1.21 27.43 ± 0.95

13.6 ± 1.6 7.1 ± 0.6 2.4 ± 0.4 15.3 ± 1.6 11.9 ± 0.6

5.1 ± 0.5 3.4 ± 0.3 0.5 ± 0.04 7.0 ± 0.5 10.1 ± 0.6

62.5 52.1 79.2 54.2 15.1

Terrace T2 D Pku-L585 E Pku-L586 F Pku-L587 Pku-L588 G Pku-L589

HQ-OSL10 HQ-OSL11 HQ-OSL12 HQ-OSL13 HQ-OSL14

0.63 1.02 3.05 1.70 0.90

90–125 90–125 150–250 150–250 90–150

1.65 1.65 1.96 1.96 1.80

1.65 ± 0.09 1.78 ± 0.09 2.10 ± 0.10 2.12 ± 0.10 1.91 ± 0.10

1.75 ± 0.08 1.39 ± 0.09 0.80 ± 0.07 1.35 ± 0.08 1.83 ± 0.08

8.38 ± 0.20 7.36 ± 0.21 4.46 ± 0.16 6.05 ± 0.19 9.97 ± 0.25

10 10 5 5 10

2.55 ± 0.09 2.40 ± 0.09 2.39 ± 0.10 2.64 ± 0.10 2.79 ± 0.10

24 25 24 31 27

64.53 ± 2.68 65.56 ± 3.82 94.83 ± 7.03 75.61 ± 3.98 63.22 ± 1.81

9 8 17 20 18

51.88 ± 4.82 50.56 ± 2.53 81.13 ± 5.73 68.04 ± 3.15 58.01 ± 1.44

25.6 ± 1.4 27.3 ± 1.9 39.8 ± 3.4 28.7 ± 1.9 22.6 ± 1.0

20.3 ± 2.0 21.1 ± 1.3 34.0 ± 2.8 25.8 ± 1.5 20.8 ± 0.9

20.7 22.7 14.6 10.1 8.0

Terrace T3 H Pku-L590 Pku-L779 Pku-L591 Pku-L592

HQ-OSL15 HQ06-OSL01 HQ-OSL16 HQ-OSL17

4.90 4.30 2.55 0.65

90–125 90–125 90–125 90–125

1.65 1.45 1.72 1.72

1.65 ± 0.11

1.98 ± 0.10 2.64 ± 0.36 2.26 ± 0.08 1.23 ± 0.06

7.76 ± 0.20 10.79 ± 1.20 10.10 ± 0.23 7.36 ± 0.20

10 10 10 10

2.48 ± 0.11 2.55 ± 0.23 2.79 ± 0.10 2.44 ± 0.09

22 22 22 22

91.56 ± 3.89 85.95 ± 3.46 74.62 ± 1.53 66.65 ± 2.19

17 15 22 21

85.43 ± 3.55 79.38 ± 2.96 74.62 ± 1.53 65.68 ± 2.06

36.9 ± 2.5 33.7 ± 1.3 26.8 ± 1.1 27.3 ± 1.3

34.5 ± 2.0 31.2 ± 3.0 26.8 ± 1.1 27.0 ± 1.3

6.5 7.4 0.0 1.1

Terrace T4 I Pku-L593 Pku-L594 Pku-L595 Pku-L596 Pku-L780 Pku-L781

HQ-OSL18 HQ-OSL19 HQ-OSL20 HQ-OSL21 HQ06-OSL02 HQ06-OSL03

32.60 27.60 12.30 7.80 4.20 1.80

90–125 90–125 150–250 150–250 4–11 4–11

1.72 1.65 2.12 2.35 1.60 1.55

2.12 ± 0.10 1.50 ± 0.08 0.69 ± 0.06 0.61 ± 0.06 3.35 ± 0.38 3.22 ± 0.33

8.38 ± 0.23 6.75 ± 0.20 2.88 ± 0.12 3.24 ± 0.13 11.59 ± 1.28 8.53 ± 1.10

15 10 5 5 15 10

2.37 ± 0.10 2.20 ± 0.08 2.28 ± 0.10 2.44 ± 0.10 3.50 ± 0.34 3.36 ± 0.33

23 27 23 22 8 8

331.50 ± 13.73 345.52 ± 10.44 261.88 ± 13.62 244.70 ± 10.83 308.46 ± 10.57 268.34 ± 12.85

21 22 18 14

315.44 ± 7.58 317.68 ± 7.89 242.46 ± 9.74 227.47 ± 12.76

139.8 ± 8.1 157.00 ± 7.4 115.0 ± 7.7 97.1 ± 5.7 88.2 ± 9.2 80.0 ± 8.7

133.0 ± 6.3 144.3 ± 6.3 106.4 ± 6.2 90.3 ± 6.1

4.9 8.1 7.5 7.0

1.78 ± 0.10 1.54 ± 0.09

1.75 ± 0.10 1.63 ± 0.08 2.26 ± 0.09 2.43 ± 0.10

J.-F. Zhang et al. / Geomorphology 109 (2009) 54–65

Terrace T1 A Pku-L580 Pku-L581 B Pku-L582 Pku-L583 C Pku-L584

Note that the U and Th contents of samples HQ06-OSL01, 02 and 03 were measured by thick-source alpha counting. The potassium contents determined by flame photometry were used to calculate dose rate. The water contents (percentage of dry sample weight) were assumed, and their relative uncertainties were taken as 20%. For the loess (HQ06-OSL03) and palaeosol (HQ06-OSL02) samples, only one age was given. NAA: neutron activation analysis. △: the relative difference between the maximum and the minimum ages of the samples. a Number of all single-aliquot De obtained. b Number of De of the relatively well-bleached aliquots selected according to Zhang et al. (2003).

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and water-content attenuation factors (Aitken, 1985), the elemental concentrations were converted into effective dose rate. The calculation was performed using the ‘AGE’ program of Grün (2003), in which the calculation of the cosmic ray contribution to the dose rate is included. 4.2. Equivalent dose measurement Quartz grains extracted from the samples were used for luminescence measurement, and they were prepared under subdued red light in the laboratory dark room. The outer layer of a sample block and the two ends of a sample cylinder were first cut away. The remaining sample was treated with hydrogen peroxide to remove organic material, and then dilute hydrochloric acid to dissolve carbonates. For sand-sized samples, the sample was then washed with water to remove clay minerals, followed by drying and sieving to select coarse grains (Table 1) for OSL measurement. Coarse-grained quartz was extracted by immersing the sieved sample in 40% HF for 40 or 80 (determined by IRSL signals from the treated samples after etching in HF) minutes and then 10% HCl. For fine-grained samples such as loess, the sample was then deflocculated using a dilute sodium oxalate solution, and polymineral fine grains (4∼ 11 μm) were isolated by settling the sample in the solution. Finegrained quartz was obtained by treating the polymineral extracts with silica saturated fluorosilicic acid (H2SiF6) at room temperature to dissolve feldspars, followed by a treatment with 10% HCl to remove any fluorides produced. The purity of quartz extracts was checked by IR stimulation (Zhang and Zhou, 2007). The chemically purified quartz was prepared for luminescence measurements by settling the fine grains in acetone onto 0.97 cm diameter aluminum discs, or mounting the coarse grains as a monolayer on discs using silicone oil as an adhesive. For the coarse grains, medium aliquots (5-mm-diameter mask) were created for OSL measurements. All luminescence measurements, beta irradiation and preheat treatments were carried out in an automated Risø TL/OSL reader equipped with a 90Sr/90Y beta source (Bøtter-Jensen et al., 2000). Blue light (470 ± 30 nm) LED stimulation was used for quartz OSL measurements, and IR laser diode (830 ± 10 nm) stimulation for scanning feldspar contamination. Luminescence was detected by an EMI 9235QA photomultiplier tube with three 2.5 mm Hoya U-340 filters (290 ∼ 370 nm) in front of it. The improved single-aliquot regenerative-dose procedure (SAR) (Murray and Wintle, 2000; Wintle and Murray, 2006) was used to measure the single-aliquot equivalent dose (De) of the quartz extracts. The regenerative beta doses include a zero dose used for monitoring recuperation effects and a repeat of the first regeneration dose used for checking the reproducibility of the sensitivity correction (i.e. recycling ratio). A 40 s blue-light stimulation at 280 °C at the end of each cycle was also carried out. Preheats between 160 ∼ 300 °C for 10 s and a cut-heat of 160 °C was applied, OSL signals were measured for 40 s at 125 °C. The value of De was estimated by interpolating sensitivity-corrected natural OSL onto a dose–response curve (Fig. 4) (Duller, 2007). The error on individual De values was calculated using the counting statistics and an instrumental uncertainty of 1.0%. 5. Optical dating results and discussion

Fig. 4. An OSL dose–response curve for a coarse-grained quartz aliquot of fluvial sample HQ-OSL21 obtained using the SAR method (see text). The response is fitted with a combined saturating exponential and linear growth function. In order to observe its dose saturation characteristics, the same procedure with 6 additional doses (702.0, 936.0, 1170, 1560, 0.0, 78.0 Gy, shown as solid squares) was carried out after De measurement. The inset is a dose–reponse curve for a palaeosol sample, HQ06-OSL02. The solid circles are the natural signals.

De values obtained at temperatures above 200 °C are larger than those obtained at low temperatures. This difference in De value between low and high preheat temperatures can also be observed for samples HQOSL19, 20, 21 and 22 (the inset to Fig. 5). In order to confirm the above results, dose recovery tests were performed on the same samples as used for the preheat plateau tests. After they were completely bleached by blue light, beta doses of 58.5 and 273.0 Gy were given to samples HQ-OSL10 and 18, respectively. The same SAR procedure as used for the preheat plateau tests was then carried out to determine the ‘equivalent dose’ (measured dose) of the samples. The ratios of given dose to measured dose as a function of preheat temperature are shown in Fig. 6. For sample HQ-OSL10, the results show that the measured doses are in excellent agreement with the given doses over the whole range of preheat temperatures. For sample HQ-OSL18, the measured doses are in agreement with the given dose only between 220 and 300 °C, and this plateau is similar to the preheat plateau in Fig. 5. Based on these test results and suggestions by Wintle and Murray (2006), preheats of 200 and 260 °C for 10 s were used for the samples from Terraces T1 and T2 and the samples from Terraces T3 and T4, respectively. 5.2. Dose saturation characteristics The upper age limit of luminescence dating depends, to a large extent, on the dose saturation level of dated samples. Fig. 4 shows a dose–response curve for an aliquot of a fluvial sand sample (HQOSL21). The inset to this figure is a curve for a palaeosol sample (HQ06-OSL02) above the fluvial sample. The curves demonstrate that the natural signals from these samples are not saturated. In particular, the palaeosol sample was not saturated at applied regeneration doses of up to 990 Gy. The implication is that all samples from the terraces in this area can be dated using optical dating techniques.

5.1. Preheat plateau and dose recovery tests 5.3. Distribution of De and bleaching analysis In order to examine the effect of preheat temperature on De, preheat plateau tests were carried out on two samples (HQ-OSL10 and 18) between 160 and 300 °C at 20 °C increment. At least 4 aliquots per temperature step were measured by the SAR protocol. The results are shown in Fig. 5. It indicates that there are no effects of preheat temperature on equivalent dose at least between 160 and 260 °C for the relatively young sample (HQ-OSL10), and between 220 and 300 °C for the relatively old sample (HQ-OSL18). For sample HQ-OSL18, the

One of the luminescence properties of fluvial deposits is that they have large De scatter between aliquots (e.g. Olley et al., 1999; Zhang et al., 2003). This large spread in De is believed to be, to a large extent, due to partial or heterogeneous bleaching of the sediments at the time of deposition. On the other hand, the single-aliquot De distribution of a sample may provide information about its bleaching history (e.g. Bailey and Arnold, 2006). Plots of De versus sensitivity-corrected

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selected using the following procedure. The scatter between aliquots in De, natural OSL signals, and the first regenerative-dose-induced OSL signals are represented by relative standard deviations (RSDDe, RSDNOSL and RSDR-OSL), respectively. The measured aliquots are ranked in order of increasing sensitivity-corrected natural OSL intensities. The RSDN-OSL value is calculated with the number of aliquots increasing by one in each step, starting from the two aliquots with the lowest natural OSL. The calculation is repeated until the aliquot is reached, in which the RSDN-OSL value achieves the RSDR-OSL value of all aliquots, and the aliquots before this aliquot are considered to be relatively well bleached. The average Des of the selected relatively well-bleached aliquots of these fluvial samples are listed in Table 1. For the old samples, the average De values of the selected and all the measured aliquots are consistent within error limits, reflecting the small impact of incomplete bleaching of these samples at the time of deposition. In Fig. 7, the RSDDe, RSDN-OSL and RSDR-OSL values for all measured aliquots of the samples are also given. It can be seen that the scatters are reduced with increasing De value. The radial plot (Galbraith, 1990) has also been introduced to show the distributions of single-aliquot or single-grain De estimates (e.g. Olley et al., 1999). The radial plots of Des for the same samples as shown in Fig. 7 are presented in Fig. 8. The two central lines of the shaded regions with 2σ width of the De distribution on the radial plots represent the mean De values of all measured aliquots and the wellbleached aliquots according to Zhang et al. (2003). Compared to the young sample (HQ-OSL07), the majority of the single-aliquot doses for

Fig. 5. Plots of De versus preheat temperature for samples HQ-OSL10 and 18. Each data point represents the mean of at least four aliquots. The inset shows the comparison of De values obtained using preheat temperatures of 260 and 200 °C for samples HQOSL19, 20, 21 and 22.

natural OSL signals have been used for analyzing the bleaching of fluvial sediments by Zhang et al. (2003). Fig. 7 displays examples of the plots for the fluvial samples. The spread in De values for the samples from Terrace T1 is similar to sample HQ-OSL07, these samples were heterogeneously bleached at the time of deposition. The spread in De values for the sand samples from Terraces T2, T3 and T4 is similar to samples HQ-OSL13 and 19, and the impact of heterogeneous bleaching is not important for these older samples. In order to assess the bleaching history of these fluvial samples, a modern sample (HQ-OSL01) from the modern floodplain of the Yellow River in Hequ was tested. The sample was deposited by a flood one week before sampling, and therefore its equivalent dose should be close to zero. A simplified SAR procedure (only two regeneration doses of 0 and 10 Gy) was applied to 75 aliquots, and the data were also plotted in Fig. 7. The average De of 2.4 ± 0.5 Gy and the maximum De of up to 38.2 Gy obtained demonstrate that the sample was poorly bleached. The De distribution is very similar to sample HQ-OSL07. This implies that the samples from Terrace T1 were poorly bleached. However, for the relatively old samples from Terraces T2, T3 and T4, the residual dose of ∼ 2.4 Gy is masked by the scatter of De, suggesting that the residual dose is relatively insignificant for these samples. A similar conclusion was reached by Jain et al. (2004) in their review of optical dating of fluvial sediments. The difference in scatter between the natural and the first regenerated OSL signals obtained by using the SAR protocol is believed to be due to poor bleaching of samples (Zhang et al, 2003). Relatively well-bleached aliquots among all measured aliquots were

Fig. 6. Results of dose recovery tests for samples HQ-OSL10 and 18 (see text). Each data point represents the mean of four aliquots.

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Fig. 7. Plots of equivalent doses versus sensitivity-corrected natural OSL for samples HQ-OSL01, 07, 13 and 19. The scatter in the natural OSL, the first regenerated OSL signals and De (RSDN-OSL, RSDR-OSL and RSDDe,) are given (see text).

the old sample (HQ-OSL19) fall within the shaded regions, and the two regions overlap, indicating that this sample was relatively well bleached at deposition. This is consistent with the simulated results for ‘fluvial’ samples with different ages by Bailey and Arnold (2006).

(0% and 1.1%, respectively) are seen for the two dune sand samples (HQ-OSL16, 17). This excellent consistency between the maximum and minimum ages for the wind-blown sediments also indicates that the bleaching analysis for these samples is successful. The discussion below will be based on the minimum ages.

5.4. OSL ages Analytical data and results of optical dating are listed in Table 1. Two mean De values for all the samples except for the two loess and palaeosol samples (HQ06-OSL02 and 03) are given. The average De value of all aliquots measured for a sample represents the total equivalent dose of the sample, including the dose the sample has received since burial and the residual dose before burial, and its corresponding age can thus be regarded as the maximum age of the sample. The average De value of the well-bleached aliquots selected according to Zhang et al. (2003) represents the true burial dose, and its corresponding age is most likely to be close to the true burial age. However, for a sample (Sample HR5_60) from a relict beach ridge on the Herbert River floodplain in far northern Queensland, Olley et al. (2004) pointed out that the age value obtained using Zhang et al. (2003) was underestimated. Therefore, the age calculated from the selected aliquots should perhaps be considered as the minimum age of the sample. So, the true burial ages should be between the maximum and minimum ages. From Table 1, it can be seen that the relative difference between the minimum and maximum ages is larger than 50% for samples HQOSL05, 06, 07 and 08 from Terrace T1, indicating that the impact of poor bleaching is important. The differences for the samples from Terrace T2 are between 8.0% and 22.7%, suggesting that the impact of incomplete bleaching of these samples is smaller. For the samples from Terraces T3 and T4, the difference of less than 10% implies that for these samples the residual OSL signals for these old samples are relatively insignificant relative to the signals accumulated since the last exposure to light. As would be expected, only small differences

5.4.1. Terrace T1 The five samples from the three sections on Terrace T1 were dated to 0.5 ∼ 10.1 ka. The ages of the two samples in Section A are in stratigraphic order, but the two ages for Section B are not. Section B is located in a ditch, and the sediments might have been reworked by human activities. The minimum age of sample HQ-OSL09 in Section C is older than those in Section A, this is consistent with the geomorphological evolution of this terrace. According to these optical dates, we conclude that Terrace T1 was formed during the Holocene. 5.4.2. Terrace T2 The four fluvial sand samples (HQ-OSL10, 11, 12 and 13) from the sand lens within gravel layers in this terrace were dated to 20.3 ± 2.0, 21.1 ± 1.3, 34.0 ± 2.8 and 25.8 ± 1.5 ka. The ages of samples HQ-OSL12 and 13 in Section F are in stratigraphic order, and are slightly older than samples HQ-OSL10 and 11. This can be explained by their positions in their sections and in the terrace (see Fig. 2). Sample HQOSL14 (20.8 ± 0.9 ka) from Section G is younger than the fluvial sand samples from the terrace. This confirms the field observation that this sample is from a soil layer deposited after the formation of the terrace, and its age is younger than the underlying fluvial sediments. This also implies that all the OSL dates obtained for this terrace are internally and stratigraphically consistent, and are also in good agreement with the chronological evolution of the terrace, i.e. the samples from the rear part of the terrace are older than the samples from the front of the terrace. These OSL ages lead us to conclude that the terrace tread was formed between 20 and 25 ka.

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Fig. 8. Radial plots of equivalent doses for samples HQ-OSL01, 07, 13 and 19. The central lines in the shaded regions (2σ width of the De distribution) on the radial plots represent the mean De of all measured aliquots (upper line and shaded regions) and the mean De of the relatively well-bleached aliquots (lower line and shaded regions) according to Zhang et al. (2003). For sample HQ-OSL01, only one line is given, and represents the mean De of all measured aliquots.

5.4.3. Terrace T3 The two floodplain sandy-silt samples (HQ-OSL15, HQ06-OSL01) from the lower part (laminated silt) of Section H on Terrace T3 were dated to 34.5 ± 2.0 and 31.2 ± 3.0 ka, respectively. The terrace tread is covered by wind-blown fine sands. The two wind-blown samples (HQOSL16 and 17) from the upper part of the section were dated to 26.8 ± 1.1 and 27.0± 1.3 ka, respectively. The terrace tread should be abandoned at between the ages of samples HQ06-OSL01 and HQ-OSL16. Therefore, this terrace is inferred to have formed at approximately 30 ka. On the other hand, the stratigraphic consistency in OSL dates between the fluvial and the wind-blown samples demonstrates the validity of the OSL ages obtained for these samples. 5.4.4. Terrace T4 From the bottom to the top of the section on this terrace, the four fluvial sediment samples (HQ-OSL18,19, 20 and 21) were dated to 133.0± 6.3,144.3±6.3,106.4±6.2 and 90.3±6.1 ka, respectively. The OSL ages of the palaeosol and loess samples (HQ06-OSL02 and 03) overlying the

fluvial sediments are 88.2±9.2 and 80.0±8.7 ka, respectively. They are all in stratigraphic order. The formation of this terrace is between the age of the top fluvial sample (HQ-OSL21) and the overlying palaeosol sample (HQ06-OSL02). Their optical dates indicate that the terrace formed in the Late Pleistocene (about 90 ka). On the other hand, the good stratigraphic consistency and younging upward of the optical ages provides confidence in the reliability of these dates, and also shows that these samples are within the dating range of luminescence, as indicated by the dose– response curves in Fig. 4. 5.5. Evolution of the terraces Based on the geomorphologic investigations and the optical ages obtained for the terrace deposits, the evolution of the terraces and then the Yellow River valley can be inferred. The fluvial history of the Yellow River in the Hequ area can be dated back to at least ∼ 140 ka ago, and be summarized as four incision and accumulation cycles corresponding to the four terraces.

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The OSL ages of the samples from the highest terrace (T4) suggest that the bedrock valley underlying the sediments was formed much earlier than ∼140 ka. After this first incision, the valley was filled with fluvial sediments as thick as 32.3 m. But the accumulation was interrupted sometime between ∼ 140 and ∼110 ka, indicated by a set of debris flow deposits (Layer 3 in Section I) sitting on the floodplain sediments (Layer 2 in Section 1) (Fig. 2). The existence of red clay pellets in the deposits indicates a large palaeoflood followed by a period of very low discharge in the river. From ∼90 to ∼35 ka, the stream not only cut down through ∼ 32 m of its own fill in T4, but also ∼32 m into the underlying soft bedrock, and a new bedrock-valley bottom, the base of the terrace deposits of Terrace T3, was formed. The deposition of the terrace sediments started ∼35 ka ago and ended at ∼ 30 ka, indicated by the ages of the floodplain samples (HQ-OSL15 and HQ06-OSL01) from Terrace T3. Corresponding to Terrace T2, the optical age of the channel deposits (sample HQ-OSL12 from Section F) indicates that the incision started before ∼ 34 ka. The accumulation of channel deposits lasted from ∼ 34 ka ago to ∼ 20 ka. The river cut down ∼ 5 m lower than the top (861 m a.s.l.) of the bedrock of Terrace T3 during this period. It seems that the incision and deposition of channel sediments of Terrace T2 and floodplain sediments of Terrace T3 overlap in time. This can be explained by the idea that channel sediments of Terrace T2 accumulated at the same time as the floodplain sediments on Terrace T3 were deposited when palaeofloods occurred. On the other hand, the overlap in time could also be due to lateral migration and deposition on the meandering river, indicated by a southward younging of optical ages from Section F to D on Terrace T2. The last incision period occurred after ∼ 20 ka with the abandonment of the T2 tread by the river, and the period of the fluvial sediment accumulation in Terrace T1 ended at ∼ 3.4 ka. The optical ages of the samples from Terrace T1 indicate that the terrace tread was abandoned by the river at between 10.1 and 3.4 ka with the lateral shifting of the meandering river.

5.6. Factors affecting terrace evolution The Hequ area within the Ordos Plateau has experienced the tectonic history of the plateau. The formation of the river terraces along the Jin-Shaan gorge are mainly attributed to the regional epeirogenic uplift of the Ordos plateau (Yue et al., 1997; Cheng et al., 2002; Sun, 2005; Zhang et al., 2006), but the effect of climate on the formation of the terraces has been less discussed in the literature. The uplift of the plateau caused the deep incision of the river, and the highest river terrace (T4) in the Hequ area is inferred to be due to tectonic uplift, as evidenced by the high bedrock cliff and the tectoniccontrolled river terraces of similar age in the neighbouring areas such as the Fen-Wei graben (Sun, 2005). For the lower terraces (T3, T2, T1), the optical ages indicate that downcutting and abandonment of terrace treads takes place more rapidly, and the downcutting of the river during these periods is small (Fig. 2). There is no evidence of local neotectonics in this area. Terrace T2 is seldom observed in the other areas of the Jin-Shaan gorge. This suggests that these lower terraces may be produced by both tectonic activity and climate change. The downcutting of the meandering river might be attributed to the uplift of the Ordos plateau. On the other hand, sediments were deposited on the convex bank during the lateral migration of the river. This procedure should be mainly affected by river discharge changes. Especially the formation of Terraces T2 and T1 is controlled by climate change. However, the lack of high-resolution palaeoclimate records in this area means that this assertion is speculative. Clearly, further investigation of the forcing factors for the formation of the lower terraces and lateral migration of the river in this area will be necessary in future research.

6. Conclusions Four Yellow River terraces (T1, T2, T3 and T4) in the Hequ area, Shanxi province, China, were identified according to field investigations. A total of twenty OSL samples were collected from nine sections on these terraces. These samples include the underlying fluvial sediments and the overlying deposits such as loess on the terraces. The degree of bleaching of the samples at the time of deposition except for loess and palaeosol samples were analyzed according to the single-aliquot De distribution, and the maximum and minimum optical ages were then calculated for these samples. The stratigraphical and geomorphological consistency in optical ages for these samples, except for the two samples from a section in Terrace T1, shows that the dating results are reliable. The degree of bleaching of the fluvial samples is an important consideration in dating the relatively young fluvial samples from Terrace T1. However, the residual OSL signals at the time of deposition are relatively insignificant for the old fluvial samples from higher terraces. In addition, systematic sampling is very important to date fluvial sediments in order to obtain reliable ages. This study further demonstrates the value of optical dating techniques in dating fluvial sediments. The formation ages of the terraces, and the abandonment ages of the terrace surfaces by the river, can be summarized as follows. The highest terrace (T4) was formed at ∼ 90 ka, and controlled by tectonic uplift. The three lower terraces (T3, T2 and T1) were formed by climate change and tectonic activity at ∼30 ka, 20 ∼ 25, 3.4 ∼ 10 ka, respectively. More detailed interpretation of the terrace evolution and understanding the response or implication of the terraces to climate changes and tectonic movements are required.

Acknowledgments We thank Y.-Z. Liu and G. Hu for help in sampling. We also thank Prof. B.-Y. Yuan and D.-W. Mo for discussion. We appreciate Geoff Duller, his constructive suggestions significantly improved the quality of the manuscript. We also thank the anonymous reviewer and Jimin Sun for their valuable comments on the manuscript. This work was supported by the National Natural Science Foundation of China (Grant Nos. 40471010 and 49925307).

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