1 Introduction

Aeolian sandstones are formed by wind, abrasion and diagenesis in arid environments. The two types of aeolian sandstones, buried and residual deposits, are distinguished according to their state of preservation. In buried deposits, sedimentary cover preserves lenticular sand dune structures. In residual deposits, exposure at the surface and diagenesis erase original sedimentary structures. Over the last few years, various Quaternary aeolian deposits have been discovered throughout China including the Ordos Plateau and Qaidam Basin (Hu et al. 2001).

The Qaidam Basin is influenced by westerly winds and both Asian and south-west monsoon systems. As such, it records regional responses to variations in the global climate system. Since the Late Cenozoic, neo-tectonic movements have caused the basin’s depositional centre to migrate east, forming Quaternary deposits in the Sanhu area. Many scholars have investigated recent sedimentary environments of the Sanhu area to conclude that Quaternary deposition consisted primarily of clastic sediments deposited as lakeshore facies (**e and Guan 2000; Guo and Chen 2008; Guo et al. 2011).

This study performed detailed analysis of cores from the Qijia 1 Well. Petrographic analysis using stereo microscopy, scanning electron microscopy (SEM) and chemical analysis methods revealed structures, grain size distributions, textural features and chemical compositions indicative of aeolian sandstones. Discovery of aqueously deposited aeolian sandstones expands the range of known sedimentary environments in the Sanhu area and refines understanding of its paleoclimatic history.

2 Geological setting

The Sanhu area, located in the eastern Qaidam Basin, represents a first-order tectonic unit delineated by three saline lakes: Tai**ar, Suli, and Dabsan Lakes. The Sanhu area covers over an area of 60,000 km2 and lies between the Chuanxingqiu structure to the west and the Nuomuhong area to the east (Fang et al. 2007).

Neogene tectonic episodes shifted the Qaidam depositional centre from west to east. The basin reached its maximum depth of over 3200 m in Suli Lake. The basin consists of three secondary tectonic units differentiated according to thickness and deformation. These include the Salt Lake structural belt, the Suli Lake Depression and the Dabsan Lake Depression (Table 1). The Qijia 1 Well samples a south-westerly limb of the Salt Lake structure belt and extends 3950 m through the Quaternary (Fig. 1). Sedimentary cover consists primarily of Gobi desert deposits including saline and alkali material containing no vegetation. The Qigequan Formation (Q1+2) sampled by the Qijia 1 Well is lithologically dominated by sandstone–mudstone interbeds that reach a total thickness of 1411 m. These include grey, grey black and off-white mudstones, and sandy mudstones, followed by grey, dark grey and off-white argillaceous siltstones, minor pale yellow mudstones, grey carbonaceous mudstones, and off-white siltstones, which are lacustrine sediments. The upper part consists of grey to black carbonaceous mudstones (Fig. 2). Sandstones contain plant debris while mudstones contain debris from ostracod species of lyocypris and Qinghaicypris crassa. These fossils only found in the freshwater-less saline sediment, reflecting low temperature conditions, are the division of the Quaternary and the Neogene in Qaidam Basin (**e and Guan 2000).

Table 1 Stratigraphic system and contact relations for the eastern Qaidam Basin
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Fig. 1
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Geological topographic map of the Sanhu area, Qaidam Basin

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Fig. 2
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Sedimentary characteristics and synthesis column map of the Qigequan Formation from the Qijia 1 Well core

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3 Samples and analysis

Samples analysed included 28 core samples from the Qijia 1 Well of the Sanhu Depression (Qaidam Basin), a sand sample from Mingsha Mountain and an aeolian sand from an area near Qinghai Lake. Each sample was divided into five aliquots for thin section analysis, grain counts, physical property analysis, scanning electron microscopic imaging and compositional analysis.

Thin sections were examined using a Zeiss Axio Scope A1 stereo microscope for the identification of sandstone shapes. Texture and grain morphology were imaged using a TESCAN Vega2 scanning electron microscope (SEM). Samples examined with SEM were washed in 10% hydrochloric to isolate quartz grains. X-ray fluorescence spectrometry (XRF) was used to determine major element compositions.

4 Results

4.1 Sedimentary characteristics

Stereo microscopic imaging showed that sandstone core material from 866.6 m depth (Qijia 1 Well) consisted mostly of sand exhibiting high textural maturity, grain support and pore cementation. Rock types consisted of medium to fine grained lithic quartz sands encapsulated by regular iron clay. Compositions consisted of more than 75% quartz and debris representing between 15% and 20%. Clastic textures categorized as well-sorted and rounded pure quartz sand with single mode grain distribution and few suspension materials (Fig. 3a). A core sandstone sample from 876 m (Qijia 1 Well) exhibited angular to subangular textures and low textural maturity (Fig. 3b). Core sandstones at 866.6 m (Qijia 1 Well) resembled aeolian deposits in the Chinese Loess Plateau (Lu et al. 2001), south-west France (Bertran et al. 2011), and outcrop around Mingsha Mountain (Fig. 3c). In contrast, core material from 876 m differs from the aforementioned sandstone examples. Stratigraphic analysis of Qijia 1 Well cores identified 19 aqueously deposited aeolian horizons within the Qigequan Formation. These layers showed a maximum thickness of 10 m, a minimum thickness of 1.4 m and an average thickness of 4.6 m. Based on the deposition rate (Guo 2008), the depositional age of the aqueously deposited aeolian sandstones (Fig. 3) was determined.

Fig. 3
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Petrographic features of sandstones from core material at 866.6 m (a), 876 m (b) and modern aeolian deposits (c) imaged by stereo microscopy

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4.2 Textural characteristics

4.2.1 Grain size distributions

Core sandstones from 382–386, 866.6, and 1138.8 m gave mean Krumbein phi values of 4–8 and 3–4 Φ, sorting coefficients between 1 and 2, skewness greater than 0.3, and a kurtosis exceeding 1.56. Rock types are categorized as coarse siltstone and siltstone. Cumulative curves (Fig. 4a) show sandstone samples forming two lines indicating the influence of saltation and suspension during transport. The abscissas (Φ) of the truncation points for these two components ranged from 4 to 5 while their ordinates (accumulative content percentage) ranged from 95% to 99.5%. This indicates that core sandstone samples from 382 to 386, 866.6, and 1138.8 m consist primarily of saltation components with a minor suspension component. Transport by a single mode left them well sorted. These sedimentary characteristics overlap with those found in modern aeolian sands from Mingsha Mountain (Fig. 4c) and suffice to discriminate them from lakeshore facies (Goudie 1987; Hu et al. 2001; Mountney and Russell 2004, 2009; Zhang et al. 2011; Bertran et al. 2011, 2016). Aeolian horizons of the Qigequan Formation also contain some clay and mica particles apparently transported by the wind and deposited under lacustrine conditions.

Fig. 4
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Cumulative grain size curves for sandstones from the Qigequan Formation and for sediment from modern aeolian deposits. a Core material from 382 to 386, 866.6, and 1138.8 m. b Core material from 876, 1140–1145, and 1803.08 m. c Sediment from modern aeolian deposits

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Core sandstone from 876, 1140–1145, and 1803.08 m gave Krumbein phi values of 3–4 and 2–3 Φ, sorting coefficient ranging from 3 to 4, a skewness of 0.3, and a kurtosis ranging from 0.9 to 1.11. Grain size distributions categorize these samples as coarse siltstone and siltstone (Table 2). Cumulative curves (Fig. 4b) show sandstone samples forming two lines indicative of saltation and suspension transport modes. The abscissas (Φ) of the truncation points for these two components ranged from 2 to 3 and ordinates (accumulative content percentage, %) ranged from 95% to 99%. This indicates that core sandstones from 876, 1140–1145, and 1803.08 m experienced major suspension transport and only minor saltation. We therefore interpret these core intervals as representing lakeshore facies.

Table 2 Mean graphical textural values for sandstones from the Qigequan Formation and for sediment from modern aeolian deposits
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4.2.2 Morphology and textural characteristics of quartz sands

As a sedimentary process, wind causes rounding of clastic particles. With the exception of subangular particles hosted by linear dunes, sediment rounding, and other signs of abrasion serve to differentiate aeolian from lacustrine deposits. The presence of small, irregular pits in the surface of particles > 100 μm and frosted grain textures indicates wind abrasion. Dish-shaped and disciform pits with both regular and arcuate edges formed in highly rounded particles indicate high-intensity wind. These pits range 10–30 μm in depth in particles 100 μm in diameter (Mongelli et al. 2006; Abdullah and Eswaran 2014; Refaat and Hamdan 2015). Conchoidal fracture of quartz grains is interpreted as evidence of glacial deposition (Mahaney et al. 2001; Cowan et al. 2008; Deane 2010).

Core material from the Qigequan Formation was imaged by scanning electron microscope (SEM). Images showed that samples from 866.6 m (Fig. 5c) and 1138.8 m (Fig. 5d) consisted of moderately well-rounded particles. Grains appeared subrounded to subangular and exhibited surfaces with dish-shaped pits and crescent-shaped pits. Sandstones collected from areas surrounding Qinghai Lake and Mingsha Mountain show similar characteristics (Fig. 5a, b), indicating wind transport over considerable distances (Ou et al. 2008). Core material from 876 m (Fig. 5e) and 1803.08 m (Fig. 5f) does not show these patterns. This material may instead reflect a lakeshore sand dam. Sedimentary features indicate arid environments with significant diurnal temperature variation.

Fig. 5
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Surface features of quartz grains from the Qigequan Formation and modern aeolian deposits. a Modern aeolian sands around Qinghai Lake. b Modern aeolian sands of Mingsha Mountain. c Aeolian core material at 866.6 m. d Aeolian core material at 1138.8 m. e Sand dam material at 876 m. f Sand dam material at 1803.08 m

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4.3 Geochemical characteristics and physical properties

4.3.1 Chemical compositions

Previous studies suggest that wind transport alters clastic grain size distributions, chemical composition, and overall chemical content of aeolian deposits (Winspear and Pye 1996; Wu 2003). Chepil (1956) asserted that wind transport removes Al2O3, CaO, CO2, and organic matter from sediment and causes relative enrichment in SiO2 and Fe2O. Nesbitt and Young (1982) defined chemical index of alteration (CIA) parameters and established an analytical framework for sediments using Al2O3, CaO, K2O, and Na2O values. Honda and Shimizu (1998) found average CIA values of 54.6 in sediments from the Taklimakan desert. Kasper-Zubillaga et al. (2007) found that desert dune sediment composition is primarily influenced by sedimentation and source area rather than by chemical weathering. Ou et al. (2008) compared SiO2, Al2O3, and TOFE (Fe2O3 + FeO) compositions of paleo-dune material, recent dune sands and fluvio-lacustrine sediments. These researchers found that chemical compositions of recent dune sands resembled those of paleo-dune sands. The ancient and recent dune material exhibited relative enrichment in SiO2 and relative depletion in Al2O3 and TOFE (Fe2O3 + FeO) compared to the fluvio-lacustrine sediments. Hamdan et al. (2015) examined correlation between chemical compositions of aeolian sands in Upper Egypt and considered different source areas around Toshka Lake.

Table 3 lists major element compositions for 28 samples of the Qigequan Formation. Aeolian sandstones exhibited low CaO, K2O, Al2O3, Fe2O3, and Mn, and high SiO2, Na2O, and MgO compared to lakeshore sand dam sediment and lakeshore mudflat sediment. This variation probably reflects variation in chemical composition between different sedimentary microfacies associated with the respective sedimentary environments. During periods when humid climatic conditions prevailed, active elements (e.g. potassium and sodium) would dissolve and migrate due to acidic conditions in the aqueous medium. Stable elements (e.g. iron, aluminium, manganese) influenced by chemical weathering in the areas of high elevation would migrate and deposit in the low-lying areas, as opposed to the arid climatic conditions.

Table 3 Chemical composition of sandstones in the Sanhu area, Qaidam Basin
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4.3.2 Paleo-salinity estimate

Estimates of paleo-salinity can be reconstructed from sedimentary chemical parameters to inform paleo-environmental interpretations. Paleo-salinity estimates for closed basinal systems can also capture changes in sedimentary environment. Paleo-salinity indicators tend to give low values for humid sedimentary environments and high values for arid environments (Patrickson et al. 2010). The lacustrine facies developed during deposition associated with the summer monsoon while the aeolian sandstones deposit formed during the winter monsoon (Ou et al. 2008). For this study, we used Cl concentrations to estimate paleo-salinity and discern climatic effects.

The Qigequan Formation core material from Qijia 1 Well gave Cl concentrations ranging from 818 ppm to 25.2%, with mean values of 6234 ppm. Mudstones deposited and preserved within more sandy sediments gave Cl concentrations that could reflect salinity of ground water or surface depositional features (e.g. lakes). Concentrations of Cl from Qijia 1 Well core material indicate relatively high salinity during deposition of the Qigequan Formation. Associated lacustrine environments likely experienced saline to brackish water conditions. These characteristics indicate a relatively arid environment.

4.3.3 Physical properties

Porosity of aqueously deposited aeolian sandstones ranged from 29.9% to 38.7% and permeability ranged from 102 × 10−3 to 581 × 10−3 μm2. Core samples gave a mean porosity value of 33.0% and a mean permeability value of 274 × 10−3 μm2. Lakeshore sand dam material gave porosity values ranging from 21.6% to 30.5% with a mean value of 28.0% and permeability values ranging from 19.7 × 10−3 to 138 × 10−3 μm2 with a mean value of 95.1 × 10−3 μm2. These parameters support the interpretation that the Qijia 1 Well core material represents aeolian sandstones as it exhibits more void space than the lakeshore sand dam material.

5 Discussion

Aqueously deposited aeolian sandstones of the Qigequan Formation are dominated by coarse to fine siltstones. Sandstones exhibit relatively higher SiO2, Na2O, and MgO contents than the sand dam sediments in the lakeshore. Sandstones consist of well-sorted and rounded pure quartz. Microscopic (SEM) imaging of sandstones revealed dish-shaped pits and crescent-shaped pits in the surface of clasts. These characteristics resemble aeolian deposits in the Salawusu River Valley (Ou et al. 2008) and central and south-western Nebraska (Winspear and Pye 1996) in the USA (Table 4).

Table 4 Summary of characteristics of the aeolian sandstones and fluvio-lacustrine sands of Qijia 1 Well in the Qaidam Basin, Salawusu Formation in the Salawusu River Valley and central and south-western Nebraska in USA
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Aeolian sand is mainly deposited on the lakeshore and in deserts. The sand dam on the lakeshore exhibits relatively low maturity, moderately poor sorting and roundness, and strong inheritance of provenance. In contrast, aeolian sand in the desert forms relatively thick layers and exhibits higher quartz content, high maturity and roundness and is well sorted. Based on the evidence listed, this paper infers that aeolian sands of the Qigequan Formation were deposited under aqueous conditions. Regional aeolian sediments are controlled by dynamic interactions between glacial, ice-margin fluvial, aeolian and volcaniclastic processes under the influence of a distinctive climatic regime (Mountney and Russell 2004). Aeolian landforms serve as a long-term integrator of the effects of the strongest wind regimes in a region (Koster 1988). According to the research results, it is speculated that there are mainly two reasons for the formation of aqueously deposited aeolian sandstones in the Quaternary. (1) Pollen data indicates that the Qaidam Basin experienced relatively humid conditions during the Oligocene. The ancient Qaidam Lake gradually formed in the west of basin, which lay at a relatively low elevation at the time. Two episodes of Himalayan tectonic movement uplifted the Qaidam Basin and cut the basin off from ancient sedimentary source areas. Climatic evolution caused the lake to contract and become saline. In the Quaternary, widespread glacial intervals imply low-temperature climate conditions. The nearby Kunlun Mountains and Qilian Mountains host middle to early Pleistocene glacial features and the freshwater-less saline fossils typified by the ostracod Qinghaicypris crassa (2.34–2.52 Ma), indicating that the Qaidam Basin experienced low temperature conditions during the Quaternary. The uplift of the Himalayas and the Qinghai–Tibet Plateau also blocked humid air travelling from the Indian Ocean to the basin (Harris 2006; Boos and Kuang 2010; Molnar et al. 2010). Abundant nutrients contributed to planktonic blooms and associated organic carbon accumulation (Zachos and Kump 2005). Draw down of carbon dioxide contributed to climate change in the Qaidam Basin, which provides the direct conditions for the aeolian sandstones. (2) In the early Oligocene, an ancient lake gradually formed in the western Qaidam Basin due to the warm climate. However, neo-tectonic movements led to the uplift and subsidence of the basin, and the ancient Qaidam Lake was segmented into two parts with one depocentre migrating from west to east. During the Quaternary, lacustrine sediments accumulated mostly in the Sanhu area. An extensive, semi-deep lacustrine depocentre formed in the Tai**ar and Dabsan Lake regions. The present-day lake margin has also inherited layout and other sedimentary features from the early Pleistocene lake, including a northerly lakeshore primarily dominated by mud deposition and marshlands, and a southerly margin dominated by sand deposition. Uplifted sandy lacustrine deposits exposed over a large area provided ample sediment source for the observed aeolian deposits.

Discovery of aqueously deposited aeolian sandstones further strengthens the evidence that Qaidam Basin experienced arid-cold paleoclimatic conditions and strong wind during the Pleistocene.

6 Conclusions

Aqueously deposited aeolian sandstones of the Qigequan Formation are dominated by coarse to fine siltstones. Sandstones exhibit relatively higher SiO2, Na2O, and MgO contents than the sand dam sediments in the lakeshore. Sandstones consist of well-sorted and rounded pure quartz clasts with single mode grain size distribution and few suspension materials. Microscopic (SEM) imaging of sandstones revealed dish-shaped pits and crescent-shaped pits in the surface of grains.

The Salt Lake of Sanhu likely represents the source area for the aeolian sandstones. Glacial deposits and uplift of the Himalayas and Qinghai–Tibet Plateau caused arid-cold paleoclimatic conditions and strong wind in the Qaidam Basin. Sediments partially lithified in the Pleistocene were eroded by stronger winds that developed during this time. Sandstones were transported to the Salt Lake by wind and accumulated in the lake’s depocentre.