Genesis of the Upper Jurassic Continental Red Sandstones in the Yongjin Area of the Central Junggar Basin: Evidence from Petrology and Geochemistry

Abstract

The sandstone sections in the Upper Jurassic red beds of the Yongjin area in the central Junggar Basin are important oil and gas reservoirs. The debate over whether red beds are of primary depositional or secondary diagenetic origin persists, leading to uncertainties in the interpretation of reservoir sedimentary facies. This study uses core samples and employs thin section microscope observations, scanning electron microscopy, X-ray diffraction, and major and trace element analyses to investigate the formation period and paleoclimate conditions of red beds and explore the origin of red sandstone. The Upper Jurassic red beds are mainly deposited in arid delta plain environments. The framework grains of the red sandstone are composed of quartz (averaging 22.6%), feldspar (averaging 16.3%), and rock fragments (averaging 36.7%). The rock fragments in the sandstone are mainly composed of intermediate basic volcanic rocks and cryptocrystalline acid volcanic rocks, which are rich in mafic silicate minerals such as olivine, pyroxene, ilmenite, and magnetite. In situ hematitization of ilmenite is observed in the rock fragments, suggesting that the in situ alteration of mafic silicate minerals in the parent rock is the main source of iron ions for hematite. Tiny hematite crystals (2.1 μm) are observed in clay mineral micropores via SEM. Abundant mixed-layer illite/smectite clay indicates early smectite transformation, providing a minor source of iron ions for hematite. Hematite in the red sandstone occurs as a grain-coating type, predating quartz overgrowth, feldspar overgrowth, and (ferroan) calcite and (ferroan) dolomite precipitation. Residual hematite coatings between detrital grain point contacts indicate that hematite is a product of syn-sedimentary or very early diagenetic precipitation, ruling out the possibility that red sandstone formation was caused by later atmospheric water leaching during the fold and thrust belt stage. The average chemical index of alteration (CIA) for the red sandstone is 52.2, whereas the CIA for the red mudstone averages 59.5, and the chemical index of weathering (CIW) reached a maximum of 69. These values indicate that the rocks have undergone mild chemical weathering in arid climates. Additionally, the ratios of trace elements indicate that the water bodies were in an oxidizing state during the sedimentary period. The arid climate and oxidative water conditions were ideal for hematite preservation, thus facilitating red bed formation. The red bed sediments in the study area represent a direct response to the Late Jurassic aridification event and can be compared to global climate change. The results have important implications for stratigraphic correlation and interpretation of reservoir sedimentary facies in the study area while also providing a valuable case study for global research on red beds.

1. Introduction

Red beds have been reported in numerous regions worldwide. Notable examples include the Jurassic Navajo red sandstone of the Colorado Plateau in the United States [1], the Cretaceous red sandstones of the Neuquén Basin in Argentina [2], the Triassic Buntsandstein red sandstones of Central Europe and Germany [3], the early Cambrian red sandstones in the Aydıncık region of southern Turkey [4], and the Permian red sandstones in the Ordos Basin of China [5].

The lithology of the red beds varies, potentially comprising conglomerates, sandstones, siltstones, and mudstones. Essentially, the red beds exhibit distinctive coloration due to the presence of iron oxides—primarily hematite crystals—coating the clastic particles [4,6,7,8]. Consequently, red beds are commonly considered indicators of a strongly oxidizing environment [7]. They primarily develop within fluvial, lacustrine, and aeolian deposits in continental basins [9,10,11,12,13,14].

In addition to serving as indicators of oxidizing environments, red sandstones are important hosts for oil, gas, and metal resources globally [11]. They play a crucial role in the formation of metallic and hydrocarbon deposits within sedimentary basins [11,15,16,17,18]. Consequently, extensive research has been conducted on the genesis of red beds [9,19,20,21,22,23]. As early as the early twentieth century, Tomlinson [19] investigated the genesis of red beds and discovered that the coloring agents within these deposits had already formed during sedimentation and were transported and deposited as mechanical sediments. Krynine [24], Folk [25], and Besly [26] further examined the conditions for the formation of red beds, suggesting that these deposits developed in hot/arid or hot/humid sedimentary environments, closely related to paleoclimate conditions. However, Walker [21] and Eren [8] reported that the infiltration of early oxidizing fluids post-sedimentation can lead to the alteration of iron-rich silicate minerals, resulting in the formation of hematite, which can also give rise to red beds. They argued that the presence of red beds more accurately reflects the environmental conditions after deposition rather than a specific climatic type, as these formations can develop under varying climatic conditions. Miot et al. [27] and Eren et al. [4] found that microorganisms can induce hematite precipitation during early diagenesis. Jiang et al. [28] discovered that during the burial of diagenesis, goethite can be dehydrated to form hematite as the temperature rises. Zhang et al. [29] discovered that hematite can be formed during tectonic uplift. These studies indicate that hematite can form at different stages of burial diagenesis. Recently, Shahrokhi et al. [30] and Liu et al. [31] used advanced inductively coupled plasma mass spectrometry to perform in situ elemental content analysis and quantitative diffuse reflectance spectroscopy to distinguish different types of ferrooxides and demonstrated that hematite is jointly controlled by terrigenous input, early diagenetic oxidation, and clay mineral modification. They proposed that hematite has multiple sources and refuted previous views about sediments turning red during late diagenesis, suggesting diagenetic hematite forms only in the early diagenetic stage. Overall, researchers attribute the formation of continental red beds mainly to hot/arid or hot/humid environments, inherited colors from source rock weathering products, and burial diagenesis [32]. Notably, most research has focused on outcrops at the surface, with few investigations into the genesis of red beds in deeper subsurface environments.

Globally, oil and gas exploration is developing to deep and ultra-deep depths [33,34,35]. Deep and ultra-deep oil and gas reservoirs are diverse and include carbonate, clastic, and volcanic reservoirs. Deep marine carbonates and continental deltaic clastic reservoirs are key exploration objects [33,35]. In deep oil and gas reservoir research, the development mechanisms of deep carbonate reservoirs, such as deep burial leaching and early diagenetic permeability reflux, have been extensively studied [34,36,37]. In contrast, the development mechanisms of deep clastic reservoirs have received less attention. Red sandstone, as a key clastic reservoir, is widely developed in the deep zones of oil-bearing basins, for instance, the deep red sandstone reservoirs in the Permian Rotligende formation in the North German Basin of Germany and the Shahejie formation in the Bohai Bay Basin of China, both exceeding 4000 m in burial depth. Previous studies have mostly focused on outcropping red sandstone, with less attention on the deep red sandstone. Therefore, studying the genesis of deep red sandstone reservoirs can help us understand the development mechanisms of deep clastic reservoirs.

The Yongjin area in the hinterland of the Junggar Basin is rich in deep and ultra-deep Jurassic oil and gas resources and has great exploration and development potential. The Upper Jurassic strata are characterized by a sequence of red beds, with red sandstone serving as the primary reservoir for oil and gas. The origin of these red beds has long been a subject of debate. One perspective posits that red beds are formed through oxidation during the simultaneous transport and deposition of sediments under arid climatic conditions [38,39]. Another viewpoint suggests that red beds developed post-deposition, resulting from the uplift of the basin during the Late Jurassic, which brought the strata close to the surface, where they were subjected to weathering and leaching by atmospheric water [40]. Recent research [41] posits that the lower portion of the red beds at the top of the Jurassic is formed by oxidation during deposition, whereas the upper section resulted from superimposed groundwater oxidation on the preexisting localized red beds developed during sedimentation. The controversy surrounding the genesis of Upper Jurassic red beds has resulted in uncertainties regarding the depositional environment of the reservoir in the study area, thereby limiting the characterization and prediction of hydrocarbon reservoirs.

On the basis of detailed petrological and geochemical studies of Upper Jurassic red sandstone, this paper discusses the formation period and coloring mechanism of the red sandstone, investigates the geochemical characteristics of the red beds to infer the paleoclimate and paleoenvironmental conditions during their formation, and further discusses the genesis of the red beds at the top of the Jurassic in the Yongjin area. This study is not only important for deep reservoir prediction and exploration in the Yongjin area but also serves as a valuable case study for global research on red beds.

2. Geological Setting

2.1. Tectonic Background

The Junggar Basin is the second largest inland basin in China; it is located in the northern part of the Xinjiang Uygur Autonomous Region and is a regional negative tectonic unit surrounded by folded mountain ranges (Figure 1a). The basin is bordered on the north by the Hala Alate Mountains and the Qinggelidi Mountains, west by the Zhayier Mountains and the Yilinheibiergen Mountains, and east by the Kelameili Mountains and the Bogeda Mountains. It is characterized by a marginal basement that is higher than the interior, with the northern margin elevated relative to the southern margin. The northern section experiences uplift, whereas the southern section subsides, constrained by concealed faults, resulting in a configuration that resembles an isosceles triangle [42,43,44,45,46]. The basin primarily comprises six first-order structural belts: three uplift belts, namely, the Western Uplift Belt, the Luliang Uplift Belt, and the Eastern Uplift Belt; two depression belts, the Central Depression and the Wulungu Depression; and the Southern Foreland Thrust Belt of the Northern Tianshan Mountains [47]. The Central Depression can be further subdivided into four depressions and six uplifts, as illustrated in Figure 1a.

The Yongjin area is located in the southeastern part of the Central Depression of the Shawan Sag, within the basin’s interior, at the southern flank of the Chemo Uplift and the western segment of the Fukang Sag (Figure 1a) [48]. The Chemo Uplift is an important structure that formed during the Yanshanian period and spans the Dzungarian Basin. Its evolutionary process can be divided into four main stages: the initial formation stage from the late early Permian to the Early Jurassic; the intense uplift and erosion stage during the Middle to Late Jurassic; the concealed burial stage in the Early Cretaceous; and the folding and decline stage from the Late Cretaceous to the Neogene [49,50,51].

2.2. Jurassic Stratigraphy

Drilling data indicate that the Jurassic thickness in the Yongjin area ranges from approximately 430 to 630 m. From top to bottom, it includes the Kalaza Formation (J₃k), the Qigu Formation (J₃q), the Toutunhe Formation (J₂t), the Xishanyao Formation (J₂x), the Sangonghe Formation (J₁s), and the Badawan Formation (J₁b) (Figure 1c). The Lower Jurassic comprises the Badawan Formation and the Sangonghe Formation. The Badawan Formation is primarily composed of gray-black mudstone, sandstone, and siltstone, with thicknesses ranging from approximately 150 to 200 m. The Sangonghe Formation is primarily composed of black coal seams, mudstone, and gray conglomerate, with thicknesses ranging from approximately 100 to 220 m (Figure 1c). During the Middle to Late Jurassic, the formation of the Chemo Uplift resulted in considerable uplift and erosion in the Yongjin area of the basin. This process led to the absence of the Upper Jurassic Kalaza Formation and the Qigu Formation, whereas the Middle Jurassic Xishanyao Formation is preserved only in the central-eastern part of the study area. This formation is primarily composed of gray sandstone and mudstone interbedded with black coal seams and has a thickness of approximately 100–110 m.

The red beds overlie the Xishanyao Formation. No evidence of an obvious unconformable contact between the two units was found in the drill core. The red beds are mainly composed of brownish-red sandstone and mudstone, with intercalations of oil-bearing gray sandstone. The thickness of the residual red beds decreases from south to north, gradually pinching out toward the north, with a range of 62–102 m (Figure 2) [48,52,53,54]. The red beds are in unconformable contact with the overlying gray formations of the chalk group (Figure 1c). Currently, the chronostratigraphic affiliation of this set of red beds remains a matter of debate [38]. Xie et al. [55] conducted a comprehensive analysis using characteristics such as paleobiological fossil assemblages, clay minerals, and logging curves. They concluded that the red beds belong to the Xishanyao Formation and formed as a weathering crust through weathering and leaching processes after sedimentation. Yang Zhi et al. [56] argued that the sedimentation period of the Toutunhe Formation was characterized by arid and hot conditions, which could have led to the formation of such red beds, whereas the sedimentation period of the Xishanyao Formation was characterized by a warm and humid climate, making the deposition of red beds unlikely. Therefore, this paper provisionally refers to this set of strata as the “Upper Jurassic”. This naming aims to reflect the uncertainty in the current academic discussion and provide a foundation for further research.

2.3. Sedimentology

Currently, the upper red beds of the Jurassic system represent the primary stratigraphic target for oil and gas exploration and development in the Yongjin region. Previous studies have extensively examined the sedimentology of red bed sections via drill cores, logging, and seismic data [41,48,53,57,58].

Observations of drill cores indicate that the lithofacies types and vertical distributions of red bed sections are generally consistent. The upper part is characterized by thick layers of brownish-red mudstone, primarily exhibiting massive bedding, with local occurrences of horizontal bedding. Bioturbation structures are also observed within the mudstone, primarily comprising a fossil assemblage of Palaeophycus, Planolites, and Sellaulichnus. These bioturbation structures are formed by worms during habitation, foraging, and crawling on sedimentary substrates [59]. The central region is predominantly composed of brownish-red mudstone interbedded with reddish-brown and light reddish-brown fine sandstone, very fine sandstone, and siltstone. The primary sedimentary structures observed include parallel bedding and ripple bedding. In the lower section, dark gray and gray medium sandstone, along with very fine sandstone, are interbedded with brownish-red mudstone. The gray sandstone features parallel bedding and cross-bedding. Notably, the gray sandstone is invariably saturated with oil. Furthermore, no plant debris has been observed in red mudstone, red sandstone, or oil-bearing gray sandstone (Figure 3).

The interpretation of these sedimentary environments remains highly controversial. Some scholars suggest that red beds developed primarily in littoral and shallow lacustrine sedimentary environments under humid to semi-arid conditions [60,61]. The alternating appearance of reddish-brown and gray colors may reflect fluctuations in the paleoclimate between humid and arid conditions. However, paleontological evidence from the upper red bed section suggests that, in contrast to the Xishanyao Formation, this section is nearly devoid of spore-pollen fossils and exhibits scarce paleontological traces. This idea indicates that the paleoclimate during the deposition of the red beds was likely characterized by persistent aridity [58]. The widespread development of reddish-brown mudstone further indicates the sustained development of arid environments. Therefore, other scholars believe that the upper red bed section primarily formed under arid conditions as shallow braided river delta deposits, gradually transitioning upward into shallow meandering river delta deposits [52,53,58].

3. Materials and Methods

To analyze the reasons for the formation of the upper red beds in the Deep Jurassic, we made a detailed observation of the cores from nine drilled wells in the study area. It was found that the cores from these nine coring wells showed consistent developmental characteristics from top to bottom. The upper section of the core consists predominantly of thick-bedded red mudstone with evident blocky bedding. The central region is predominantly composed of grey medium sandstone, fine sandstone interbedded with reddish-brown fine sandstone, very fine sandstone, and siltstone. Among these, the gray sandstones primarily exhibit parallel and cross-bedding, while the red sandstones mainly display parallel and ripple bedding. The lower section is characterized by red mudstone interbedded with red muddy siltstone. Overall, the cores exhibit multiple fining-upward sequences. Based on the above observations, we collected samples from nine wells (Y2, Y6, YJ1-1, YJ1-3, YJ12, YJ14, YJ3-X17, YJ15, and YJ303). This collection included 7 samples of red mudstone and 23 samples of red sandstone. The sandstone samples cover fine sand, very fine sand, and siltstone with calcareous and muddy cementation. These samples were used for thin section petrology, SEM, XRD, and major–trace element characterization analyses.

3.1. Petrography

To obtain the petrographic characteristics of the red beds in the study area, standard thin sections were prepared from 22 core plugs and rock samples collected from eight wells, including Y2, YJ1-1, and YJ12, in the Yongjin area. The thin sections were stained with a mixture of alizarin red and potassium ferrocyanide to differentiate between different types of authigenic carbonate minerals. Under an Axio Scope A1 ZEISS petrographic microscope (Carl Zeiss Microscopy Deutschland GmbH, Oberkochen, Germany), the content of clastic particle components was quantified via the point counting method. At the same time, the petrographic structural characteristics of the red sandstone were observed, including particle sorting, rounding, contact relationships, types of cement, and their relative spatial coexistence relationships.

3.2. SEM

Eight representative sandstone samples were selected. Portions of these samples were prepared as probe slices, while the remainder were cut into units approximately 1 cm in diameter and polished. After the samples were covered with a conductive coating (carbon coating), they were adhered to the specimen stage of the electron microscope via a conductive adhesive. Under an acceleration voltage of 15 kV and a spot size of 5.5, the working distance was adjusted to approximately 7 mm. The surface mineral morphology of the particles was observed via a Nova NanoSEM 450 scanning electron microscope (Thermo Fisher Scientific, Hillsborough, NJ, USA), resulting in backscattered and morphological images of the samples. The energy spectrum elemental maps of the sample minerals were obtained using an X-ray energy spectrometer equipped with an X-MAXN80 instrument (Oxford Instruments, Oxford, UK). Additionally, two unpolished fragments of red sandstone were selected. After the surfaces were coated with a conductive film (approximately 17 nm thick), they were adhered to the sample stage of a scanning electron microscope using a conductive adhesive. The mineral morphological characteristics of the particle surfaces were observed via a ZEISS scanning electron microscope (Carl Zeiss Microscopy Deutschland GmbH, Oberkochen, Germany) at an accelerating voltage of 15 kV.

3.3. XRD Diffraction

To identify the types of clay minerals in the samples, ten representative samples of red sandstone and mudstone were selected for XRD diffraction analysis of the clay minerals. The selected samples were ground to 200 mesh via a disk vibration mill. For each sample, 10 g were weighed and mixed with 200 mL of deionized water. After thorough mixing, 10% H₂O₂ was added to remove organic matter, followed by the addition of 2% hydrochloric acid (HCl) to remove carbonates. Once the impurities were removed, deionized water was added for three rounds of centrifugation, ensuring that the pH of the centrifuged samples was between 5 and 7. The samples were transferred into a beaker, and deionized water was added for stirring. After standing, the middle portion of the clay particles (particle size < 2 μm) was extracted. The extracted clay underwent titration and was dried to prepare standard, ethanol, and high-temperature slides for XRD analysis, which revealed the clay mineral composition of the red bed mudstone and sandstone.

3.4. XRF Spectrum

To obtain the geochemical characteristics of the red beds, we selected 12 representative red sandstone samples and 2 red mudstone samples. The concentrations of major and trace elements in these samples were measured via wavelength-dispersive X-ray fluorescence spectroscopy (WD-XRF). Additionally, 4 red mudstone samples were selected for separate trace element analysis. The selected samples were ground to a particle size of 200 mesh and then dried in an oven at 105 °C for 12 h. After drying, the samples were cooled in a desiccator for 2 h. Subsequently, 0.6 g of the standard sample was mixed uniformly with 6.0 g of lithium tetraborate/borate (2:1) in a Pt–Au crucible and mold. Four drops (0.18 mL) of a 0.12 g/mL ammonium bromide solution were added, and the mixture was melted via an electric fusion device at 1050 °C for 19 min, followed by cooling for 285 s to form glass discs. The loss on ignition (LOI) value was measured by igniting 0.6 g of the dried sample in a muffle furnace at 1000 °C for 1.5 h. The prepared glass discs were then analyzed via a PANalytical AXIOS Minerals (Malvern Panalytical, Almelo, The Netherlands) instrument equipped with a rhodium anode X-ray tube and an excitation power of 4 kW in an XRF spectrometer. The detection limits (LODs) and quantification limits (LOQs) for each element were automatically determined via SuperQ 4.0 software. On the basis of the measurement results and the LOD/LOQ data, a quantitative analysis of the elemental content in the samples was conducted, yielding geochemical data for the red layers [62].

Observations of unpolished rock fragments and X-ray diffraction analysis of clay minerals were performed at the Chinese Academy of Geological Sciences, whereas the remaining experiments were conducted at the Institute of Geology and Geophysics, Chinese Academy of Sciences.

4. Results

4.1. Petrographic Characteristics of Red Sandstone

Thin section statistical data indicate that the Upper Jurassic red sandstone in the study area predominantly comprises very fine-grained sandstone, followed by medium-grained fine sandstone, very fine sandstone, and fine-grained very fine sandstone. The sandstones exhibit poor to moderate sorting, with rounding primarily characterized as subangular to subrounded, reflecting a low degree of textural maturity. The particle contacts are predominantly point lines, with a minor proportion exhibiting line contacts.

According to the sandstone classification scheme proposed by Folk [63], the red sandstone in the study area is classified primarily as feldspathic litharenite, with a minor presence of litharenite (Figure 4). The content of detrital quartz is relatively stable and consists primarily of monocrystalline quartz, ranging from 12.9% to 23.3%, with an average of 17.9%. Polycrystalline quartz and chert content range from 1.8% to 9.3%, averaging 4.6%. The feldspar is mainly potassium feldspar, with a relatively low content ranging from 6.2% to 12.3% and an average of 9.2%. Plagioclase content ranges from 2.8% to 12.9%, with an average of 7.0%. The rock fragment content is the highest, ranging from 27.5% to 48.3%, with an average of 36.9% (

Table 1. Composition of rock components in the Upper Jurassic red sandstone of the Yongjin area in the Junggar Basin.

Well	Depth	Qtz		Fsp		Rf
	/m	SCQ/%	PCQ/%	Kfs/%	Plg/%	Vrf/%	Srf/%	Mrf/%
Y2	5961.1	21.0	4.4	10.6	11.8	18.5	0.0	13.3
Y2	5963.9	21.3	6.1	7.0	7.0	26.9	0.4	16.5
Y2	5966.8	17.9	6.8	9.5	6.3	14.4	0.0	25.1
Y2	5967.5	16.8	4.4	9.8	7.1	28.0	0.0	12.4
Y2	5968	19.2	4.5	11.3	6.3	25.3	0.0	8.9
Y2	5969.4	16.8	2.5	9.1	7.7	22.4	0.0	14.4
Y2	5974.2	17.9	1.8	11.9	4.3	17.6	0.5	16.6
Y6	6026.1	14.2	4.3	10.0	10.8	21.0	0.0	17.9
Y6	6026.8	14.8	5.5	6.5	8.3	11.5	0.0	16.0
YJ1-1	5832.9	17.4	3.0	8.2	12.9	12.6	0.2	19.9
YJ1-3	5978.0	16.7	4.1	8.1	5.6	23.8	0.0	18.8
YJ12	5774.5	20.9	4.5	11.1	7.7	14.5	1.9	15.1
YJ12	5779.4	17.2	2.4	8.8	7.3	24.7	0.8	12.8
YJ12	5781.5	23.3	2.9	11.8	7.9	14.9	0.5	12.5
YJ12	5783.1	16.1	5.5	7.2	5.0	15.1	0.0	17.6
YJ12	5787	15.5	5.7	6.2	7.4	29.3	0.7	14.8
YJ14	5504.8	12.9	7.2	9.7	3.8	23.5	0.7	17.2
YJ3-X17	5733.9	21.5	3.8	10.6	6.9	23.7	0.9	11.8
YJ3-X17	5735.5	13.9	4.8	8.8	6.6	26.5	0.5	13.8
YJ3-X17	5741.2	22.3	2.9	12.3	7.1	19.8	0.0	10.2
YJ303	5734.4	16.3	5.0	6.2	2.8	31.6	2.0	14.7
YJ303	5752.4	20.0	9.3	6.9	3.7	24.4	0.2	13.0

Note: SCQ—Monocrystalline quartz; PCQ—Polycrystalline quartz; Kfs—Potassium Feldspar; Plg—Plagioclase; Srf—Sedimentary Rock Fragments; Vrf—Volcanic Rock Fragments; Mrf—Metamorphic Rock Fragments.

). The volcanic rock fragments are the most common, ranging from 11.5% to 31.6% and averaging 21.3%; metamorphic rock fragments average 15.2%, and sedimentary rock fragments are the least abundant, averaging 0.4% (Table 1). According to the continental sandstone framework mode triangular diagram of Dickinson [64] (Figure 5), the sandstone data in the study area are mainly plotted in the transitional arc area, with a few data plotted in the dissected arc area. Sandstone framework grain development indicates the source is from a magmatic arc with moderate denudation. The Qm-F-Lt diagram shows that monocrystalline quartz (Qm) content is low and polycrystalline rock fragments (Lt) content is high, indicating plutonic rocks are less eroded, with volcanic rock fragments being the main component.

Thin section study shows that volcanic rock fragments are predominantly extrusive rock fragments, with a minor amount of intrusive rock fragments. Extrusive rock fragments are mainly medium andesite fragments (Figure 6a,c) and acid cryptocrystalline extrusive rock fragments (Figure 6h). Andesite fragments show long tabular plagioclase phenocrysts and their matrix appears black under orthogonal polarization. Acidic extrusive rock fragments mainly show porphyritic texture, with a cryptocrystalline felsic matrix and black spots on the surface. Mafic basalt fragments are common, displaying intergranular textures with triangle interstices filled with dark minerals (Figure 6d,g). Intrusive rock fragments are mainly altered basic diabase fragments (Figure 6f,g) and are less common. Metamorphic rock fragments are mainly metamorphic sandstone, schist, phyllite, and mud slate fragments with medium and low metamorphic degrees. Sedimentary rock fragments are mainly argillaceous rock fragments. The sandstone components have low compositional maturity, indicating a proximal, rapid-accumulation setting.

4.2. Diagenetic Authigenic Mineral Characteristics of Red Sandstone

The authigenic minerals in the Upper Jurassic red sandstone include fine particles of hematite, quartz overgrowth, feldspar overgrowth, authigenic carbonate minerals, analcime, and various authigenic clay minerals.

4.2.1. Hematite

Under microscopic observation of red sandstone thin sections, the surfaces of the detrital grains are generally covered by opaque reddish-brown material, whereas the interstitial spaces are filled with a substantial amount of opaque reddish-brown clay matrix (Figure 7a). Under reflected light, these opaque reddish-brown coatings and the red clay matrix filling the pores exhibit vibrant red and orange-red luster (Figure 7b), which is attributed to the infiltration of hematite into the clay matrix. Additionally, some grain surfaces develop black opaque spots (Figure 7a), which also display a red luster under reflected light (Figure 7b), indicating that these grains have undergone a process of alteration associated with hematitization.

4.2.2. Authigenic Carbonate

Microscopic observations reveal that three main types of carbonate cements have developed in the red sandstone of the study area. The first type is a substrate-type cement of sparry calcite, which appears pink after being stained with a mixture of alizarin red S and potassium ferricyanide. This sparry calcite occupies nearly all the intergranular pore spaces, with only a reddish-brown hematite film developed between the sparry calcite cement and the grains (Figure 7d). This type of calcite is usually considered to have formed during early diagenetic processes that occurred simultaneously with sedimentation. The second category comprises iron calcite, which is embedded in the pores in a granular mosaic texture and appears purple under plane-polarized light. This ferroan calcite fills the residual intergranular pores formed during the growth of authigenic feldspar and quartz overgrowth (Figure 7e,g), often engaging in replacement interactions with the surrounding grains (Figure 7g). The third category comprises ferroan dolomite cements that exhibit rhombohedral grain shapes, typically filling larger intergranular voids, and a reddish-brown hematite film grows at the contact points with the grains (Figure 7h).

4.2.3. Secondary Quartz Overgrowth

The phenomenon of quartz overgrowth is commonly observed in the red sandstone of the study area, primarily exhibiting a rimmed growth pattern, which is predominantly produced in a single generation.

Microscopic observations revealed that the edges of the quartz overgrowth particles were uniformly surrounded or partially encircled by growth. Additionally, a thin film of hematite is observed to grow between the quartz overgrowth and the grains, followed by the external growth of the calcite cement (Figure 7e). This petrological relationship indicates that quartz overgrowth formed after the deposition of the hematite film and prior to calcite cementation, suggesting an earlier formation time.

4.2.4. Authigenic Feldspar

In the red sandstone of the study area, feldspathic cement is also widely developed, primarily in two forms: relatively overgrowth (Figure 7e) and authigenic feldspar microcrystals (Figure 7f,g). The authigenic feldspar microcrystals are more prevalent than the feldspar overgrowth, predominantly exhibiting a plate-columnar morphology on the grain surfaces. Both the feldspar overgrowth and the authigenic feldspar microcrystals evidently grow outside the hematite film.

4.2.5. Analcime

Under microscopic observation of red sandstone, analcime cement is occasionally observed, typically coexisting with iron calcite in larger intergranular pores. Some analcime surfaces exhibit the development of dissolution pores (see Figure 7i), which formed later than the hematite.

4.2.6. Clay Minerals

On the basis of detailed observation of polished thin sections of red sandstone via scanning electron microscopy, we found that the pores and particle edges predominantly developed amorphous mixed-layer illite/smectite clays and fibrous illite clays. The interstitial spaces between the particles were filled with a considerable amount of clay matrix rich in quartz and feldspar fragments (see Figure 8a,b). Some bright, fine crystal fragments are dispersed within the clay matrix (see Figure 8a,c), and energy dispersive spectroscopy confirms that these fragments are primarily composed of hematite crystals (see Figure 8f). Within the intergranular pores, considerable amounts of fibrous illite, flaky chlorite cement, and amorphous mixed-layer illite/smectite clay minerals are present, interspersed with fragments of quartz and feldspar (see Figure 8c). Among the micropores of the clay matrix, substantial growth of isolated microcrystalline hematite particles in a flaky, clastic form can be observed (see Figure 8d). This distribution imparts characteristics to the clay matrix within the pores that are analogous to those of the clay matrix on the surface of the particles. X-ray energy dispersive spectroscopy revealed the partial substitution of iron (Fe) by titanium (Ti) in hematite (see Figure 8f). Additionally, the development of fragmented titanium oxide particles was observed within the clay matrix (see Figure 8c).

Observations of unpolished red sandstone samples via scanning electron microscopy yielded results consistent with those obtained from polished thin sections. On the surface of the detrital grains, we identified flaky chlorite clay and abundant fibrous illite clay (see Figure 9a). The particle surfaces exhibited extensive development of a clay matrix, which contained numerous fine granular quartz grains, feldspar fragments, and amorphous mixed-layer illite/smectite clays. Within these clay matrices, small hematite crystal fragments were also widely distributed (see Figure 9b).

4.3. X-Ray Diffraction Patterns of Clay Minerals in Red Sandstone and Mudstone

X-ray diffraction analysis was conducted on red bed samples from the study area, and the results are presented in

Table 2. Relative abundance of clay minerals in the Upper Jurassic red beds of the Yongjin area, Junggar Basin.

No.	Well	Lithology	Depth (m)	S (%)	I/S (%)	Ill (%)	K (%)	Chl (%)
1	Y2	sandstone	5963.9	0	43	28	6	23
2	Y2	sandstone	5967.5	0	43	28	6	23
3	Y2	sandstone	5968.0	0	36	33	6	25
4	Y6	sandstone	6026.8	0	31	38	7	24
5	Y6	sandstone	6027.4	0	33	30	5	32
6	Y6	mudstone	6013.4	0	44	35	5	16
7	Y6	mudstone	6014.7	0	45	39	5	11
8	Y6	mudstone	6015.7	0	39	42	7	12
9	Y6	mudstone	6017.5	0	40	43	6	11
10	Y6	mudstone	6020.7	0	42	42	5	11

Note: Ill: illite; Chl: chlorite; I/S: mixed-layer illite/smectite; S: montmorillonite; K: kaolinite.

. The findings indicate that the Upper Jurassic red bed mudstone in the Yongjin area predominantly contains four types of clay minerals: mixed-layer illite/smectite, illite, chlorite, and kaolinite. Among the clay minerals, the illite/smectite mixed layer has the highest content, with an average of 42.0%. This component is followed by illite, which has an average relative content of 40.2%. The chlorite content ranges from 11.0% to 16.0%, with an average content of 12.2%, indicating a relatively low abundance. Kaolinite is not well developed, with a distribution range of 5.0% to 7.0%. The clay mineral development in the red sandstone is analogous to that in the mudstone, which is characterized predominantly by the presence of mixed-layer illite/smectite clay minerals, with an average relative content of 37.2%. The contents of illite and chlorite are lower than those of illite/smectite, averaging 31.4% and 25.4%, respectively. Kaolinite is relatively rare, with an average content of 6.0%. The distribution of the clay mineral content obtained via X-ray diffraction (XRD) analysis is consistent with that observed via scanning electron microscopy (SEM).

4.4. Geochemical Characteristics

The major and trace element characteristics of red mudstone and sandstone are presented in

Table 3. Relative abundance of major elements in Upper Jurassic red beds in the Yongjin area, Junggar Basin.

No	Well	Depth (m)	Lithology	SiO2	TiO2	Al2O3	TFe2O3	MnO	MgO	CaO	Na2O	K2O	P2O5	LOI	Total	CIA	CIW
				wt.%	wt.%	wt.%	wt.%	wt.%	wt.%	wt.%	wt.%	wt.%	wt.%	wt.%	wt.%
1	Y2	5961.1	sandstone	76.54	0.29	11.3	2.04	0.03	0.8	0.7	3.72	2.27	0.09	1.55	99.34	52.8	59.7
2	Y2	5963.9	sandstone	78	0.31	10.25	2.05	0.06	0.83	1.12	3.52	1.97	0.09	1.96	100.15	49.4	55.1
3	Y2	5966.8	sandstone	66.47	0.47	10.76	3.43	0.28	2.51	4.21	3.19	2.17	0.1	6.59	100.18	50.2	56.4
4	Y2	5967.5	sandstone	78.14	0.31	10.18	2.22	0.04	0.7	0.96	3.39	2.1	0.1	1.6	99.73	50.5	56.9
5	Y2	5969.4	sandstone	75.86	0.44	11.16	3.1	0.04	0.87	0.75	3.52	2.23	0.12	1.6	99.68	53.4	60.3
6	Y2	5974.2	sandstone	72.11	0.31	10.67	2.25	0.07	0.7	3.61	3.58	1.99	0.1	4.03	99.42	55.1	62.0
7	YJ1-1	5832.9	sandstone	73.08	0.29	10.15	2.72	0.13	0.8	3.83	2.93	2.34	0.09	4.14	100.5	50.2	57.3
8	YJ12	5779.4	sandstone	68.5	0.39	10.19	2.82	0.29	2.31	4.07	3.14	2.17	0.1	6.37	100.34	49.6	56.0
9	YJ14	5504.8	sandstone	76.91	0.36	10.65	2.32	0.04	0.86	1.25	3.29	2.3	0.1	2.1	100.18	49.7	56.2
10	YJ3-X17	5735.5	sandstone	73.37	0.35	10.27	2.51	0.08	0.65	3.28	3.36	2.01	0.09	3.68	99.65	56.1	63.6
11	YJ303	5734.4	sandstone	70.98	0.42	11.05	3.13	0.12	0.82	3.88	3.26	2.28	0.11	4.31	100.36	52.8	59.8
12	YJ303	5752.4	sandstone	66.45	0.71	15.5	4.82	0.03	1.28	0.51	3.59	3.37	0.13	3.37	99.77	59.7	69.5
13	YJ303	5732	mudstone	56.52	0.73	17.72	7.4	0.09	2.62	1.62	3.59	3.59	0.17	6.29	100.33	56.9	65.0
14	YJ1-3	5966	mudstone	58.23	0.7	17.25	7.26	0.07	2.46	1.21	3.25	3.67	0.17	5.23	99.5	59.2	68.6

Note: CIA and CIW represent the chemical index of alteration and the chemical weathering index, respectively. The specific calculation formulas are provided below.

and

Table 4. Relative abundance of trace elements in Upper Jurassic red beds in the Yongjin area, Junggar Basin.

No	Well	Depth (m)	Lithology	Ba	Co	Cr	Ni	Sr	V	Zn
				(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)
1	Y2	5961.1	sandstone	565.20	6.40	26.60	9.40	151.00	34.90	26.20
2	Y2	5963.9	sandstone	860.10	8.00	157.90	119.50	153.60	32.60	24.50
3	Y2	5966.8	sandstone	372.20	13.60	63.80	12.90	189.30	49.50	38.60
4	Y2	5967.5	sandstone	433.80	8.10	37.00	11.10	146.30	34.50	27.20
5	Y2	5969.4	sandstone	628.90	11.00	51.70	15.90	156.60	56.50	35.70
6	Y2	5974.2	sandstone	1263.00	14.00	58.70	14.20	167.10	41.00	34.90
7	YJ1-1	5832.9	sandstone	384.30	13.40	130.60	46.10	191.80	41.60	33.40
8	YJ12	5779.4	sandstone	761.60	8.70	114.90	52.80	180.00	53.40	37.30
9	YJ14	5504.8	sandstone	506.40	6.80	75.00	92.40	141.30	44.10	29.40
10	YJ3-X17	5735.5	sandstone	1017.10	7.50	46.00	7.40	183.30	47.80	30.30
11	YJ303	5734.4	sandstone	396.70	17.40	297.00	229.90	210.90	65.70	39.80
12	YJ303	5752.4	sandstone	414.00	12.70	55.10	26.60	164.20	103.30	66.00
13	YJ303	5732.0	mudstone	274.40	20.70	64.00	44.60	190.80	117.00	107.00
14	YJ1-3	5966.0	mudstone	502.30	19.40	89.00	57.30	170.70	110.50	98.10
*15	YJ301	5514.0	mudstone	1567.00	12.62	113.30	37.62	345.10	69.38	50.41
*16	YJ301	5582.0	mudstone	1975.00	12.16	75.74	37.55	383.80	59.45	69.57
*17	YJ1-1	5821.7	mudstone	279.60	17.68	100.60	49.37	142.60	95.36	102.50
*18	Y6	6024.6	mudstone	530.00	17.66	142.90	43.90	128.40	97.89	88.68

Note: *15 to *18 are for the collected data.

. The major element analysis indicates that red sandstone has a high SiO₂ content, ranging from 66.45 wt.% to 78.14 wt.%, with an average of 73.0 wt.%. The next most abundant element after SiO₂ is Al₂O₃, which ranges from 10.15 wt.% to 15.5 wt.%, with an average concentration of 11.0 wt.%. The contents of Na₂O (2.93 wt.% to 3.72 wt.%), K₂O (1.97 wt.% to 3.37 wt.%), and MgO (0.65 wt.% to 2.51 wt.%) decrease in decreasing order. The variations in CaO and loss on ignition (LOI) are considerable, ranging from 0.51 to 4.21 wt.% and from 1.55 to 6.37 wt.%, respectively. These variations are attributed primarily to the differing carbonate contents within the red sandstone samples. The loss of carbonates during calcination contributes to the broad range of LOI values. Additionally, the variability in the Fe₂O₃ content (2.04 to 4.82 wt.%) is influenced by the heterogeneity of the red sandstone samples, with some samples exhibiting higher iron-rich clay contents in their pores, whereas others only display a thin coating of hematite on the particle surfaces, resulting in substantial intergranular porosity. The average Fe₂O₃ content is 2.80 wt.%. Furthermore, the samples contain trace amounts of TiO₂ (0.29 to 0.71 wt.%), MnO (0.03 to 0.29 wt.%), and P₂O₅ (0.09 to 0.13 wt.%).

Compared with the red sandstone samples, the red mudstone samples present lower SiO₂ (average content of 57.4 wt.%) and CaO (average content of 1.4 wt.%) levels, indicating a lower carbonate content in the red mudstone. Conversely, the Fe₂O₃ and TiO₂ contents are considerably higher than those in the sandstone samples, with average values of 7.3 wt.% and 0.7 wt.%, respectively. This comparison suggests that the clay contains a greater proportion of hematite fragments. The relatively high contents of Al₂O₃ (average 17.5 wt.%), K₂O (average 3.6 wt.%), and MgO (average 2.5 wt.%) can be attributed to the greater development of clay minerals within the mudstone. In contrast, the levels of Na₂O (average 3.4 wt.%), MnO (average 0.1 wt.%), and P₂O₅ (average 0.2 wt.%) do not substantially vary.

The trace element contents of the tested samples exhibit notable correlations and regularity. The red sandstone is characterized by a high concentration of Ba (average of 481.3 ppm), moderate to high levels of Sr (123.1 ppm), Cr (59.7 ppm), and V (32.3 ppm), and low concentrations of Co, Ni, and Zn. In contrast, the mudstone samples display higher concentrations of Ba (average of 695.3 ppm), V (average of 85.6 ppm), and Zn (average of 65.3 ppm) than the sandstone samples do. This phenomenon is attributed primarily to the effects of diagenesis, with insignificant differences observed in the concentrations of other elements.

To better understand the concentrations of major and trace elements in the red bed samples, the average elemental data were compared with the upper continental crust (UCC) [65,66] data. The results (Figure 10a,b) indicate that, with a few exceptions, the samples exhibit a similar pattern to UCC normalization.

For the sandstone samples, the concentrations of major elements such as Ti, Al, Fe, Na, K, and P tend to be depleted compared with those in the UCC, while Mn and Ca exhibit significant variability, likely due to carbonate cement in some red sandstone samples. Compared to UCC, trace elements like Co, Sr, V, and Zn show depletion, while Cr and Ni exhibit significant variability. This may be due to the contact with gray sandstone in some samples. For the mudstone samples, Fe, Al, and Mg are relatively enriched, and Ca and P are depleted. Ba, Cr, Ni, and Zn show enrichment trends compared to the UCC, while Sr and V are depleted.

5. Discussion

5.1. Formation Time of Red Beds

The timing of the formation of red beds is crucial for understanding the genesis of these deposits. Various explanations have been proposed for when sedimentary rocks and soils become red. Krynine [24] and Folk [25], among others, suggested that the coloration of the rocks occurred before deposition, indicating that the sediments were already red at the time of deposition. In 1976, Walker [21] proposed a new perspective, suggesting that typical red beds are products of diagenesis. According to his view, the sediments were not red at the time of deposition. Over time, the iron-bearing particles within the sediments came into contact with the groundwater in an unbalanced state. This interaction led to the reddening of the rocks. This process occurs during the very early stages of diagenesis. Researchers in the academic community have subsequently proposed that red beds form during various stages of diagenesis. For example, Eren et al. [4] were the first to reveal the phenomenon of microbial-induced precipitation of hematite during the early diagenetic stage. Scholars such as Zhang et al. [29] proposed that red beds undergo reddening during the weathering process in the epigenetic diagenetic stage. These authors suggested that the uplift of the basin caused the strata to enter the phreatic oxidation zone, thereby triggering the weathering process of hematitization. In addition, Lian [28] conducted laboratory heating simulation experiments and proposed that red beds may experience thermal metamorphism and reddening due to high-temperature dehydration during the middle to late diagenetic stages. However, the recent study by Shahrokhi et al. [30] challenges the idea of red bed formation during the late diagenesis stage. They argue that post-sedimentary rock reddening mainly occurs during the early diagenesis stage. These studies enhance our understanding of red bed formation timing and provide a solid theoretical basis for further research in related fields.

The formation time of Upper Jurassic red beds in the study area has been heavily researched. It remains unclear whether the formation of red beds was primarily due to oxidation during the sedimentary process, the result of atmospheric water leaching and erosion following the uplift of the strata at the end of the Jurassic, or a combination of both factors. Currently, no consensus exists in the academic community. Jiao et al. [40] observed under a microscope that no hematite was found at the contact points between particles in the red beds; instead, hematite alteration was more prevalent in the intergranular pores. These authors suggested that hematitization occurred later than the initial compaction and that the red beds were oxidation products from a noncontemporaneous sedimentary period. However, our study revealed hematite films at clastic particle points and point-line contacts in red beds (Figure 7c). This petrological relationship clearly indicates that the hematite films formed prior to considerable compaction [20,67]. Microscopic observations of diagenetic authigenic minerals indicate that some rock fragments undergo hematitization alteration on their surfaces. Furthermore, before the extensive development of early quartz overgrowth, the reddish-brown hematite uniformly coated the surfaces of the clastic particles. This phenomenon is evidenced by the occurrence of quartz overgrowths encapsulating hematite films. (Figure 7e). Moreover, within the interparticle pores, authigenic feldspar microcrystalline, analcime, ferroan–calcite, and ferroan dolomite are observed to grow extensively on top of the hematite films (Figure 7f–i). Even in dense calcite-cemented sandstones, hematite films are visible between particles and pore calcite (Figure 7d). This calcite is typically considered to have formed during the early stages of diagenesis, concurrently with the deposition of clastic particles.

These observations indicate that hematite precipitated very early, with growth on the surfaces of clastic particles preceding cement formation. The subsequent diagenetic alteration does not cause the red coloration of the rock. Therefore, we posit that the sediments acquired their red characteristics during deposition or shortly after, in the early diagenetic stage. Research by Jiang [32] on the Middle Jurassic continental red beds in the Sichuan Basin supports this hypothesis, showing that an oxidizing environment during or soon after deposition can cause hematite precipitation. Hu’s work [68] on Cretaceous red beds also confirms that hematite can form during syn-sedimentary to early diagenetic stages.

Iron is commonly present in the layered silicate structure as divalent or trivalent forms and may also occur as exchangeable cations in the interlayers of layered clay minerals [31,32]. Sediment provenance analysis shows that the red sandstones in the study area contain abundant volcanic rock fragments, mainly andesite, basalt, and acidic cryptocrystalline extrusive rock fragments, which underwent extensive hematitization (Figure 6b,e,i). These volcanic rock fragments, especially andesite and basalt, are difficult to identify as to their original minerals due to extensive hematitization. It is inferred that they were altered from olivine, pyroxene, ilmenite, and magnetite. The hematitization of ilmenite (Figure 6i) further confirms that in situ alteration of iron-rich minerals is the main source of iron ions for hematite, a conclusion supported by many studies [6,20,21]. Scanning electron microscopy observations revealed that the surfaces and interstitial spaces of the particles in the red sandstone were primarily occupied by amorphous clay minerals. These clay minerals infiltrate particle surfaces and interparticle pores during sedimentation, and hematite crystals form as fine dispersions within clay micropores (Figure 8). As hematite has a fine grain size, we cannot observe its surface morphology, which hinders the analysis of its formation and relationship with clay minerals. Liu et al. [69] used high-resolution transmission electron microscopy (HRTEM) to observe the relationship between hematite and clay minerals, a method worth our reference. Their research reveals that the interplanar spacing of hematite matches the lattice stripe spacing of kaolinite. This is mainly due to the release of iron ions during the conversion of montmorillonite to kaolinite, which are adsorbed on the kaolinite surface and precipitate to form hematite. In this regard, we infer that during the sedimentation or early diagenesis of the red beds in the study area, the transformation of smectite to illite would release Fe and Mg, thus providing an iron source for hematite in the clay matrix. The widespread occurrence of mixed-layer illite/smectite clay minerals provides strong evidence for this inference, indicating that smectite was once extensively converted to illite. The iron in clay mineral lattices is limited, making them a minor iron source in red beds [6,20,21,25]. Overall, the iron in hematite mainly comes from the transformation of volcanic rock fragments during the syn-sedimentary and early diagenetic stages, especially those unstable iron-bearing silicates (ilmenite, magnetite, olivine, pyroxene). The conversion of montmorillonite to illite is a secondary source of iron. These minerals release iron ions in situ under suitable geological conditions, leading to hematite formation.

5.2. Paleoenvironment and Paleoclimatology of Red Beds

The traditional view holds that sedimentary rock layers with a distinctive red color are formed under arid to semi-arid climatic conditions, with dry and oxidizing conditions that help preserve hematite. Early studies on modern red deserts also support the view that red beds form under hot and arid climatic conditions [20,67]. However, with more research and the discovery of red beds in different regions, this traditional view has been challenged. Some studies suggest that red beds do not necessarily indicate specific paleoclimate conditions [70] and that red beds can also form in nonarid environments. For example, under warm and humid climatic conditions, red sediments can form through good drainage conditions and specific soil chemical processes [21]. As shown in Table 5, the formation of red beds involves various complex environmental factors and can occur in different settings. It should not be reduced to one model but should be interpreted in regional studies [70].

The content and composition of clay minerals are often used to reconstruct paleoenvironments [71,72,73,74,75]. Li et al. [41] and He [38] observed the clay mineral assemblages and their content variations in the red bed section of the study area. They reported that the clay minerals exhibited a distribution characterized by high illite and relatively high mixed-layer illite/smectite contents, low amounts of chlorite and montmorillonite, and a lack of kaolinite. They concluded that this assemblage indicates that the sedimentation period of the red beds in the Yongjin area was a hot and arid oxidizing environment, which experienced weak alkaline and weak chemical weathering processes. The X-ray diffraction and scanning electron microscopy results of the clay minerals from the red bed samples indicate that the primary clay minerals are amorphous mixed-layer illite/smectite and fibrous illite, with average contents of 42% and 40%, respectively. Additionally, a small amount of flaky chlorite clay has developed, while montmorillonite and kaolinite are absent. These findings are consistent with previous research results. However, beyond 3000 m depth and 100 °C, considerable diagenetic changes in clay minerals occur [73,75]. In the Yongjin area, the deep burial of red beds leads to considerable diagenetic effects on clay mineral contents and types, which are often ignored when interpreting the paleoenvironments of clay minerals. Although the findings of our study regarding clay minerals are generally consistent with those of previous research, we contend that clay minerals alone cannot be used to definitively reconstruct paleoenvironments and that their environmental indicators may be inaccurate.

Table 5. Geochemical indicators for red bed formation in different regions globally.

Region	System	Geochemical Index	Index Range Characteristics	Red Bed Genesis	References
Songliao Basin, China	Cretaceous	CIA	56.2~67.8	The early weathering is weak, and it is formed in an oxygen-rich water environment in an arid climate	Song et al., 2015 [76]
		V/cr	1.0~1.7
		Ni/Co	1.9~2.5
		U/Th	0.1~0.2
		Sr/Cu	1.0~10.0
Sichuan Basin, China	Jurassic	CIA	66.2–75.3	Weathering of source rocks	Jiang et al., 2023 [32]
		CIW	77.5–86.6
Lufengchuan Street Basin, China	Jurassic	CIA	62.5–78.0	The weathering conditions are moderate, and the red beds are mainly formed in the post-sedimentary oxygen-rich sedimentary water	Wang et al., 2024 [77]
		U/Th	0.16–1.03
		V/Cr	0.50–1.85
Cala Viola, Italy	Permian	CIA	86.8–98.5	Non-weathering formation, good drainage, and oxidative sedimentary water conditions promote the formation of red beds	Sheldon et al., 2005 [70]
		Al2O3	4.99%–20.65%
		CaO	0.03%–0.18%
		Sr	36–272 ppm
Mersin area, Turkey	Miocene	Mn/Fe	0.003–0.023	It is formed in an alternating wet and dry climate environment, in an oxidizing water sedimentary environment	Eren et al., 2015 [67]
		Ni/Co	2.3–9.2
		Sr/Ba	0.3–2.2
Transvaal and Olifantshoek Basin, South Africa	Paleoproterozoic	CIA	72–78	Formed by early weathering and post-deposition oxidation	Land et al., 2017 [78]
		Al2O3	12.9–20.3
		CaO	0.01–1.5
		MgO	0.2–2.3

Table 5. Geochemical indicators for red bed formation in different regions globally.

Region	System	Geochemical Index	Index Range Characteristics	Red Bed Genesis	References
Songliao Basin, China	Cretaceous	CIA	56.2~67.8	The early weathering is weak, and it is formed in an oxygen-rich water environment in an arid climate	Song et al., 2015 [76]
		V/cr	1.0~1.7
		Ni/Co	1.9~2.5
		U/Th	0.1~0.2
		Sr/Cu	1.0~10.0
Sichuan Basin, China	Jurassic	CIA	66.2–75.3	Weathering of source rocks	Jiang et al., 2023 [32]
		CIW	77.5–86.6
Lufengchuan Street Basin, China	Jurassic	CIA	62.5–78.0	The weathering conditions are moderate, and the red beds are mainly formed in the post-sedimentary oxygen-rich sedimentary water	Wang et al., 2024 [77]
		U/Th	0.16–1.03
		V/Cr	0.50–1.85
Cala Viola, Italy	Permian	CIA	86.8–98.5	Non-weathering formation, good drainage, and oxidative sedimentary water conditions promote the formation of red beds	Sheldon et al., 2005 [70]
		Al2O3	4.99%–20.65%
		CaO	0.03%–0.18%
		Sr	36–272 ppm
Mersin area, Turkey	Miocene	Mn/Fe	0.003–0.023	It is formed in an alternating wet and dry climate environment, in an oxidizing water sedimentary environment	Eren et al., 2015 [67]
		Ni/Co	2.3–9.2
		Sr/Ba	0.3–2.2
Transvaal and Olifantshoek Basin, South Africa	Paleoproterozoic	CIA	72–78	Formed by early weathering and post-deposition oxidation	Land et al., 2017 [78]
		Al2O3	12.9–20.3
		CaO	0.01–1.5
		MgO	0.2–2.3

The elemental composition of clastic sedimentary rocks, particularly mudstones and siltstone–mudstone, is often used to reflect paleoclimatic conditions during deposition [64,79]. The use of sandstone or its combination with mudstone to determine the paleoclimatic environment during the sedimentary period has been successfully applied in many studies [80,81,82]. Commonly used elemental ratio indicators like Fe/Mn, CIA, and CIW are of great significance for analyzing sedimentary paleoclimate environments. Thus, the precision of elemental measurements is crucial for the accuracy of inference results. However, conventional X-ray fluorescence spectrometry cannot distinguish iron oxides of different valence states and is prone to errors during measurement. In contrast, diffuse reflectance spectrometry can identify various iron oxide minerals and their contents, and micro area in situ geochemical testing offers more precise elemental content data [30,69]. Notably, the elemental compositions of red bed sandstones and mudstones are highly susceptible to diagenetic alteration during burial, resulting in significant discrepancies between their current elemental compositions and those during deposition. Consequently, indices calculated based on major and trace element measurements may not accurately reflect the paleoclimate during deposition. In contrast, isotope analysis techniques, such as δ¹³C, δ¹⁸O, δ⁷Li, and δ^{⁹⁸/⁹⁵}Mo isotope tracking, are less affected by post-depositional diagenetic evolution. They can more precisely indicate the depositional conditions and better identify the paleoclimate during red bed sedimentation [77,83]. Moreover, utilizing spore–pollen assemblages can help reduce the uncertainty of elemental ratios when reconstructing paleoclimate.

Owing to experimental limitations, this study primarily employs X-ray fluorescence spectrometry to measure major and trace elements in red bed sandstones and mudstones. The paleoclimate of the Jurassic top red bed sediment in the study area is reconstructed based on elemental distribution patterns and characteristics of related elemental ratios. Advanced methods will be incorporated in future research for validation.

During the sedimentary process, with increasing weathering intensity, major elements such as Al, Fe, and Mn typically increase in concentration as oxides, whereas the concentrations of elements such as Ca, Mg, Na, and K tend to decrease gradually [66]. Therefore, previous studies often used major element oxide contents to infer the climate during sedimentary processes. The UCC normalization test results of major element oxides in the mudstone samples indicate that the average contents of Al₂O₃, Fe₂O₃, and MnO have increased compared with the standard values of the upper crust, although the changes are insignificant. Conversely, the average contents of CaO and Na₂O decreased, whereas the K₂O content remained relatively stable. The UCC normalization results for the sandstone samples show that the contents of CaO, Na₂O, MgO, and K₂O have decreased, but again, the reductions are unsubstantial. This phenomenon suggests that the red beds in the study area have undergone slight weathering. In the study area, the red bed samples exhibited relative enrichment of Fe and stable levels of Mn. Fe is prone to oxidation and subsequent precipitation under arid conditions, leading to its accumulation. The chemically stable Mn element is present at relatively constant concentrations and precipitates in large amounts only under conditions of intense evaporation. Therefore, the Fe/Mn ratio can be used as an indicator of paleoclimatic conditions. In the study area, the Fe/Mn ratio is relatively high, with average values of 47.4 for sandstone and 94.9 for mudstone, indicating an arid sedimentary environment.

The chemical index of alteration (CIA) and the chemical weathering index (CIW) are commonly used indicators for assessing the intensity of weathering source rocks [84]. The CIA and CIW values are derived from the relative ratios of oxides, with their calculation formulas provided in (1) and (2). The combined use of these indices allows for a more accurate assessment of the extent of chemical weathering experienced by sandstone and mudstone. CaO* refers to the calcium oxide content in silicates, and the method proposed by McLennan [66] in 1993 has been applied for its correction. The CIW index is calculated via the method introduced by Harnois et al. in 1988 [85], which aims to eliminate the influence of potassium exchange during diagenesis on the results:

$$CIA={\displaystyle \frac{molar{(Al}_2{O}_3)}{molar\left({Al}_2{O}_3+Ca{O}^{*}+{Na}_{2}O+K_{2}O\right)}}\times 100$$

(1)

$$CIW={\displaystyle \frac{molar\left({Al}_2{O}_3\right)}{molar\left({Al}_2{O}_3+Ca{O}^{*}+{Na}_{2}O\right)}}\times 100$$

(2)

The CIA values for the red sandstone samples range from 49.4 to 56.9, whereas the mudstone values range from 59.2 to 59.7, with average values of 52.2 and 59.5, respectively. These results are consistent with those of previous studies [38] and fall within the range of weak chemical weathering under arid climatic conditions. In addition, this study utilizes the A-CN-K ternary diagram (where A, CN, and K represent the molar contents of Al₂O₃, CaO* + Na₂O, and K₂O, respectively) to analyze the effects of potassium substitution during the diagenesis process [66,86,87]. As shown in Figure 11, the CIA values of the sample points range between 50 and 60, indicating that they have not experienced substantial weathering compared with the upper crust CIA standard value of 47. However, the sample points deviate from the weathering trend line parallel to A–CN, indicating that the red beds in the study area have undergone slight potassium alteration. To address this finding, the study employs a combined approach using the CIA and CIW indices. The CIW index, which is insensitive to potassium alteration (Table 3), is used to mitigate the impact of potassium alteration on the calculations during diagenesis. The average CIW value is 59 for the red sandstone samples but 69 for the mudstone samples, indicating a transition from weak chemical weathering under arid conditions to moderate chemical weathering under warm, humid conditions. Therefore, the comprehensive analysis suggests that the red beds experienced weak chemical weathering during the depositional period of the study area under arid climate conditions.

The distribution patterns of trace elements in rocks are important tools for inferring paleoclimate and paleoenvironmental conditions during sedimentation. The trace element analysis reveals that the average concentrations of Ba in the red sandstone and mudstone are 481.3 ppm and 695.3 ppm, respectively, whereas the average concentrations of Sr are 123.1 ppm and 210.8 ppm, respectively. This comparison indicates that Ba is more enriched than Sr is, with a ratio of approximately 1:4. Owing to the greater mobility of Sr than Ba, Ba is more likely to precipitate in high-salinity waters. This phenomenon suggests that the sedimentary water body had elevated salinity, indicating a relatively arid climate at that time.

Additionally, the contents of trace elements such as V, Cr, Co, Ni, and Zn in mudstone are sensitive indicators of the redox conditions of the sedimentary water body. Notably, the ratios of V/Cr, Ni/Co, and V/(V+Ni) have been widely used to determine the paleo-redox conditions of sedimentary environments [88,89]. The test data of the red beds indicate that V and Cr are more enriched than Co, Ni, and Zn. The ratio characteristics (Figure 12) reveal that in the Upper Jurassic formations, the distribution of the trace element ratio V/(V+Ni) ranges from 0.61 to 0.81, with an average value of 0.69. This value is close to the boundary between weakly oxidizing and oxidizing environments. The V/Cr and Ni/Co ratios range from 0.61 to 2.98 and 2.15 to 3.09, with average values of 1.32 and 2.74, respectively (

Table 6. Trace element results for the Upper Jurassic red bed mudstone in the Yongjin area.

Trace Element	RE	WOE–WRE	OE	Min–Max	Average	Sources
V/(V+Ni)	>0.84	0.6~0.84	<0.6	0.61–0.81	0.69	Hatch J R et al., 1992 [88]
V/Cr	>4.25	2.0~4.25	<2.0	0.61–2.98	1.32	Jones B et al., 1994 [89]
Ni/Co	>7.0	5.0~7.0	<5.0	2.15–3.09	2.74	Jones B et al., 1994 [89]

). These values indicate an oxidizing environment. This finding suggests that during the deposition of the Upper Jurassic, the basin waters were predominantly oxidizing, leading to the oxidation of magnesium–iron silicate minerals and iron-bearing clay minerals, resulting in the release of Fe³⁺ and reddening of the sediments.

In paleoclimate interpretation, this study uses the chemical indices (CIA and CIW) to evaluate chemical weathering intensity during deposition and combines the Fe/Mn ratio for climate inference. The V/Cr, Ni/Co, and V/(V+Ni) ratios help deduce paleo-redox conditions, while the Sr/Ba ratio is used for paleosalinity assessment. Integrated analyses reveal an arid to semi-arid depositional environment with oxidizing water conditions in the study area. However, the study has limitations, such as a lack of analysis of paleobathymetry and paleotemperature indicators. In order to compensate for these shortcomings, the depth and temperature of ancient water bodies can be reconstructed using the carbon isotope (δ¹³C) and oxygen isotope (δ¹⁸O) values of the sedimentary water, as well as the mass fraction of the trace element strontium (Sr). In addition, the U/Th ratio and isotopes like δ⁷Li and δ⁹⁸/⁹⁵Mo are used to reconstruct redox conditions. The Sr/Cu ratio, a common indicator for climate humidity, can cross-validate weathering intensity indicators. Due to experimental limitations, this study lacks advanced geochemical indicators for verification, potentially causing some result deviations. In recent years, Tian [90] used spore–pollen assemblages to reconstruct the Jurassic paleoclimate of the Junggar Basin. Results show the climate shifted from warm/wet in the early Jurassic to hot/arid in the Late Jurassic. During Late Jurassic sedimentation, the climate was already relatively arid. Liu [91] used geochemical proxies like Rb/Sr, Sr/Cu, and U/Th to reconstruct the paleoclimate of the central area in the Junggar Basin during the Late Jurassic. Results show that the climate gradually turned from semi-arid to arid. In addition, Luo et al. [92] integrated geochemical proxies (e.g., Sr/Cu, U/Th), rare earth elements, and heavy mineral assemblages to reconstruct the Middle–Late Jurassic climate sequence in the Junggar Basin. Their reconstruction confirmed the shift from a warm/wet to an arid climate, and indicated that the Late Jurassic climate became dry. The paleoclimate reconstruction results in this study align with recent findings by other scholars on the Middle–Late Jurassic paleoclimate in the Junggar Basin. This consistency demonstrates the reliability of using geochemical indicators like CIA, CIW, V/Cr, and Ni/Co for paleoclimate reconstruction in this study.

Additionally, Liu et al. [93] found red sediments in their study of the modern Loess Plateau and suggested that this red sediment mainly deposits in arid to semi-arid regions. As rainfall increases, loess layers appear redder due to more hematite formation, creating red soils. The classic Luochuan section shows this. We compared geochemical indicators of modern Luochuan loess deposits and red beds in our study area, finding similarities [94,95]. This suggests that red bed sediment in the study area is consistent with modern red soil sediment in northern areas, both forming in semi-arid to arid oxidative aquatic environments.

The arid paleoclimate indicated by the Upper Jurassic red beds in the study area aligns with regional paleoclimate conditions. Global studies have shown that the Jurassic climate experienced significant fluctuations, with a sudden temperature drop in the early Middle Jurassic that persisted until the Late–Middle Jurassic, followed by a gradual rise, leading to the formation of a high-temperature platform in the Late Jurassic [96]. During the Early Jurassic, northern China experienced a lush, humid climate, which transitioned to a tropical semi-arid to arid climate by the Late Jurassic [97]. Therefore, the terrestrial red beds at the top of the Jurassic in the Yongjin area of the Junggar Basin directly reflect the Late Jurassic aridification event, allowing comparison with global climate changes.

5.3. Insights from Red Bed Sedimentary Facies

Previous studies on the Jurassic red bed sedimentary facies in the Yongjin area of the Junggar Basin have been relatively systematic. Fang et al. [60] and Fan [61] analyzed sediment core colors, sedimentary structures, and sequences and concluded that red beds primarily comprise lacustrine facies characterized by red siltstone and mudstone, as well as shallow lake facies dominated by red mudstone and gray-green sandstone. The main sedimentary environment is semi-arid to semi-humid lacustrine. The alternation of reddish-brown and gray sandstone reflects changes in paleoclimate between humid and arid conditions. Huang [53] and Song et al. [58] analyzed lithology characteristics, sedimentary structures, logging curves, and sedimentary numerical simulations. They concluded that the lower part of the red beds is characterized by a shallow-water braided river delta face, whereas the upper part, due to a reduced sediment supply, features a shallow-water meandering river delta face. These authors suggested that the sedimentation of red beds occurred during a period of persistent aridity, with water turbulence being the reason for the alternation of red and gray sandstones. When the water level was low, the sediments were exposed to the surface and underwent oxidation, resulting in the formation of red sand bodies. The findings of this study on element distribution patterns suggest that the sedimentation environment of the red layer in the study area was oxidizing and arid, which was conducive to the formation of hematite but not gray sandstone. The CIA and CIW calculations revealed that the sedimentary rocks experienced mild chemical weathering due to arid conditions, indicating a persistently dry climate during sedimentation, aligning with regional climatic changes. Additionally, macrocore observations reveal that the red beds are primarily composed of red sandstone and mudstone, with no other colored mudstones. The interbedded gray sandstone contains no plant fossils. The sedimentary structures of gray and red sandstones are similar and are characterized primarily by parallel lamination, planar cross-lamination, and ripple lamination. These features indicate shallow water conditions and high flow velocities during deposition, suggesting a fluvial depositional environment. These characteristics suggest an arid climate and a predominantly terrestrial environment with slight water depth variation during red bed sedimentation. Moreover, we observed widespread indications of oil in the gray sandstone, which we speculate is a result of petroleum intrusion and bleaching processes [3,98], rather than variations in sedimentary water depth. In summary, our analysis inferred that the top of the Jurassic red beds in the study area was characterized by gentle paleotopography, a persistently arid paleoclimate, shallow sedimentary water primarily located on land, and a proximal source, indicative of an arid deltaic plain sedimentary environment.

6. Conclusions

This study utilizes core samples collected from boreholes and employs thin section analysis, scanning electron microscopy, clay mineral X-ray diffraction, and major–trace element analysis to investigate the formation period, paleoclimate, and sedimentary facies of the Upper Jurassic red beds in the Yongjin area, as well as the genesis of the red beds. The results indicate the following.

(1)

Red beds formed during the earliest diagenetic stage, either during sedimentation or shortly after, with the growth of authigenic cement occurring later than the development of hematite.

(2)

The distribution patterns of major and trace elements, along with core observations, indicate that the paleostructure was gentle during the deposition of the red beds. The paleoclimate remained persistently arid, with shallow, oxygen-rich depositional waters, characterizing the overall sedimentation as belonging to an arid deltaic plain.

(3)

Petrographic analysis revealed that the iron in hematite mainly comes from the transformation of volcanic rock fragments during the syn-sedimentary and early diagenetic stages, especially those unstable iron-bearing silicates (ilmenite, magnetite, olivine, pyroxene). The conversion of montmorillonite to illite is a secondary source of iron. This study can provide new insights for understanding global red bed sedimentary mechanisms and late Jurassic paleoclimate reconstruction. Notably, the grey organic-rich sandstone interspersed in the red bed sequence may pose a potential interfering factor. Lithofacies and geochemical evidence show that these grey sandstones and adjacent red beds are not significantly different in initial sedimentary environment conditions. We propose a new genetic model: the anomalous coloration of gray sandstone may be due to secondary bleaching caused by hydrocarbon fluid migration in the later diagenetic stage. In order to test this hypothesis, follow-up research plans focus on the geochemical mechanism of the bleaching process and its spatiotemporal evolution.