Coloration Mechanism of the Early Cretaceous Hongshanwan Landform in the Lanzhou Basin, China: Constraints from Geochemistry and Detrital Zircon U-Pb Geochronology

Abstract

The Early Cretaceous Hongshanwan landform in the Lanzhou Basin hosts distinctive multicolored rhythmic sedimentary layers, yet the factors controlling their coloration remain debated. This study integrates mineralogical observations, whole-rock geochemistry, and detrital zircon U-Pb geochronology to investigate the controls on sediment coloration and basin evolution. Sharp and stratigraphically consistent color boundaries indicate that coloration was largely established during sedimentation and early diagenesis, with limited influence from late-stage weathering. Geochemical data suggest that the sediments were predominantly derived from intermediate-to-mafic igneous rocks under low-to-moderate chemical weathering and dominantly oxidizing conditions. Reddish-brown strata are mainly colored by fine-grained authigenic hematite formed during early diagenesis, whereas bluish-gray and pale-yellow layers inherit their colors from calcareous and mafic components with limited post-depositional alteration. Detrital zircon age distributions reveal three principal age populations (1322–1994 Ma, 331–376 Ma and 217–286 Ma), providing first-order constraints on provenance evolution and episodic sediment supply linked to multiple orogenic cycles in a back-arc foreland basin setting. Overall, the multicolored stratigraphy reflects a coupled influence of provenance composition, depositional redox state, diagenetic processes, and tectonic forcing, offering new insights into the origin and evolution of continental red-bed systems in inland basins of northern China.

1. Introduction

Sedimentary rocks, as the final products of the Earth’s surface material cycle, record complex geochemical information from source weathering to transport, deposition, and lithification [1,2,3,4,5,6,7]. The spatial and temporal distribution and evolution of their compositional characteristics profoundly influence the shaping of landforms and provide crucial insights into the properties of provenance rocks, climate, topography, weathering processes, transport mechanisms, and diagenetic effects [5,8,9,10,11,12,13].

In continental sedimentary basins, red-bed deposits are widely developed, and their color variations are primarily associated with the distribution of iron oxides within the sediments and their subsequent diagenetic evolution. Previous studies have shown that iron oxides such as hematite and goethite commonly form and precipitate on the surfaces of detrital grains during sedimentation and early diagenesis, constituting an important mechanism responsible for the coloration of red beds and multicolored sedimentary rocks [14,15]. The formation and transformation of iron-bearing minerals are typically controlled by multiple factors, including the redox conditions of the depositional environment, weathering processes in the source area, and climatic variations. Consequently, multicolored stratified sedimentary sequences often record complex couplings among sediment provenance, paleoclimate, and basin evolution.

Similar multicolored stratified sedimentary systems occur in many regions worldwide. For example, typical red-bed outcrops are well developed along the northern piedmont of the Qilian Mountains in northwestern China, including the “Ink Danxia” landscape in the Lanzhou area (Figure 1a) and the “Rainbow Danxia” in the Zhangye UNESCO Global Geopark [16] (Figure 1b). At the margin of the Junggar Basin in Xinjiang, the Wucaitan (“Five-Colored Beach”) in Burqin also exhibits distinct multicolored sedimentary bedding structures [17] (Figure 1c). In other parts of the world, Vinicunca (Rainbow Mountain) in the Peruvian Andes (Figure 1d) displays prominent color-banded structures formed by sedimentary rock layers composed of different minerals, whose development is closely related to sedimentary processes, diagenesis, and subsequent tectonic uplift [18,19]. Across the Eurasian continent, the complex sedimentary systems and tectonic units in the Altai region of Russia reflect a close relationship between sedimentation and tectonic evolution [20]. In the Caucasus region, iron-rich layered color bands occur within the Khizi–Siyazan shale deposits of Azerbaijan, and these structures are closely associated with redox conditions of the depositional environment and the distribution of iron minerals [21] (Figure 1e). In addition, the multicolored sedimentary strata developed in the Serranía de Hornocal range in northwestern Argentina—often referred to as the “Mountain of Fourteen Colors”—also exhibit distinct stratified color bands produced by variations in lithology and mineral composition [22] (Figure 1f).

The Lanzhou Basin is located in the eastern segment of the Qilian Mountains, northwestern China (Figure 2a,b), and represents one of the important continental sedimentary basins along the northeastern margin of the Qinghai–Tibet Plateau. Early Cretaceous red-bed deposits are widely developed within the basin. Among them, the Hongshanwan landform represents a distinctive sedimentary geomorphological type characterized by typical multicolored rhythmic stratification. Previous studies suggest that the Hongshanwan landform is mainly composed of sandstone, siltstone, and mudstone, and its formation is closely related to the tectonic tilting of the sedimentary strata. Under prolonged slope wash erosion and weathering processes, the strata have evolved into gently inclined slope landforms with clearly developed rhythmic bedding structures [23].

Existing studies on the Hongshanwan landform have primarily focused on aspects such as sedimentary facies, sequence stratigraphy, sedimentary–tectonic chronology, paleopedology, and paleontological fossils [24,25,26,27,28,29]. However, the coloration mechanism of the multicolored sedimentary rocks is still largely attributed to variations in redox conditions driven by paleoclimatic changes [30]. Systematic analyses of other potentially important factors—such as the diversity of sediment provenance, the intensity of weathering in the source region, and the coupled influence of tectonics and climate—remain relatively limited.

In recent years, rock geochemical methods have been successfully applied to the study of fine-grained clastic rocks [31,32,33]. Geochemical signatures are widely employed as essential indicators for distinguishing provenance areas [1,34,35,36,37,38,39]. Key elements such as Co, Cr, Ni, Sc, Th, V, Y, and rare earth elements (REEs) have low mobility during transport and thus serve as critical markers for determining the provenance of clastic sediments [40,41]. These elements are also applied to reconstruct paleoclimatic conditions [12,42] and to assess ancient weathering processes [3,8,31]. Weathering indices commonly employed in these reconstructions include the Chemical Index of Alteration (CIA), Al₂O₃ content, K₂O/Na₂O ratio, Index of Compositional Variability (ICV), Chemical Weathering Alternative Index (CPA), and Plagioclase Index of Alteration (PIA). Additionally, the major elements of the rock can reveal the paleo-humidity and the correlation between mineral composition and weathering trends [5,43,44]. Recent studies on the detrital sediments have also been extensively applied to examine their relationship with landform features [9,45,46,47,48,49], providing new perspectives for quantifying the formation mechanisms of the Hongshanwan landform.

Figure 2. Geological setting and local maps of the study area. (a) Location map of major peripheral blocks and study area around the Tibetan Plateau. (b) Structural and geological sketch map of Lanzhou basin, revised from [50,51].

Figure 2. Geological setting and local maps of the study area. (a) Location map of major peripheral blocks and study area around the Tibetan Plateau. (b) Structural and geological sketch map of Lanzhou basin, revised from [50,51].

Accordingly, this study takes the Hongshanwan landform strata in the Lanzhou Basin as the research object. By integrating mineralogical analysis, whole-rock geochemistry, and detrital zircon U-Pb geochronology, together with geochemical indices such as the Chemical Index of Alteration (CIA), the Index of Compositional Variability (ICV), and ratios of redox-sensitive elements, this study systematically investigates sediment provenance, weathering intensity, and depositional environmental characteristics. The objectives of this study are to: (1) clarify the provenance characteristics of the sediments and their material basis for the formation of the multicolored rhythmic stratification in the Hongshanwan deposits; (2) reveal the controlling mechanisms of paleoclimatic conditions and redox environments on the development of multicolored rhythmic sedimentation; and (3) explore the influence of tectonic evolution in the eastern Qilian Mountains on the sedimentary system and the formation of the Hongshanwan landform, thereby providing new geochemical constraints for understanding the evolution of red-bed sedimentary systems and reconstructing the Early Cretaceous depositional environment.

2. Geological Background

The Lanzhou Basin is a Late Mesozoic intracontinental basin located on the northeastern margin of the Tibetan Plateau, formed as an Early Cretaceous rift basin on a Jurassic sedimentary foundation. The basin is part of the eastern section of the Central Qilian Uplift in the Qilian Orogenic Belt, bordered to the north by the North Qilian “multi-phase fault activity zone” and to the south by the South Qilian Uplift unit (Figure 2a,b) [27,52]. After the early Paleozoic orogeny, the region experienced regional extension during the late Paleozoic, with marine strata deposited on the basement rocks [53]. By the Permian, the ancient Qilian and Longxi blocks had formed a continuous east–west structure, which underwent surface erosion. During the Triassic, the region was influenced by the Indonesian orogeny, and early Jurassic sediments were deposited in local intermontane basins [50,51]. In the Early Cretaceous, the East Asian region experienced a northwest–southeast extension, forming a series of northeast–southwest trending extensional basins, characterized by thick continental sediments and metamorphic core complexes, reflecting the rifting of the North China Craton [27,54]. The basin underwent a faulted expansion phase during the early Yanshan orogeny, gradually transitioning from a rift to a depression. By the late Early Cretaceous, the basin began to uplift due to the Yanshan orogeny, marking the end of its evolutionary history [51]. During the Late Cretaceous, the collision between the Lhasa and Qiangtang blocks, oblique convergence along the southeastern margin of the Eurasian Plate, and the compaction of fine-grained sediments together drove the development of a new fold system within the Hekou Formation [27]. Since the Cenozoic, the Lanzhou Basin has continued to be influenced by the remote effects of compressional stress from the Indian Plate towards the Eurasian Plate, contributing to ongoing deformation associated with the northeastern growth of the Tibetan Plateau [55].

The basement of the Lanzhou Basin is composed of Pre-Paleozoic metamorphic and igneous rocks. The overlying strata, from bottom to top, consist of the Jurassic Xiangtang Formation, the Cretaceous Hekou Group, and early Cenozoic terrigenous sediments [50,51,56,57,58,59,60] (Figure 2b, Figure 3a). Among them, the Early Cretaceous Hekou Group is dominated by sedimentary facies of alluvial fan, delta, and littoral–shallow lake environments, which mainly consist of red clastic deposits interbedded with thin layers of mudstone and siltstone, formed under arid climatic conditions between approximately 143 to 109 Ma [27,60].

3. Materials and Methods

3.1. Sample Collection and Preparation

Field sampling was conducted stratigraphically at the outcrop of the Upper Member of the Second Formation of the Early Cretaceous Hekou Group (K₂H^2b) in Shuiju Danxia, Lanzhou Basin (Figure 2a). A total of 66 samples were collected, including siltstone and dolomitic mudstone. Among them, 3 samples were used for thin section analysis, 12 samples for whole-rock geochemical analysis, 1 sandstone sample for detrital zircon U–Pb dating, and 50 samples for mineralogical composition analysis.

3.2. Analytical Methods

3.2.1. XRD and SEM Analysis

Semi-quantitative X-ray diffraction (XRD) analysis was performed on 50 selected samples, including 17 red-brown samples (H1–H17), 16 blue-gray samples (L1–L16), and 17 pale-yellow samples (Y1–Y17). Analyses were carried out using a Rigaku SmartLab9 rotating anode XRD with operating conditions of 40 kV and 100 mA, using Cu Kα radiation. Stepwise scanning was conducted at a speed of 4°/min over a 2θ range of 3°–85°. The analytical uncertainty of XRD mineral analysis is estimated to be 2%. The relative content of each mineral was estimated using the K-value method, which represents the positive correlation between the content of a mineral and the intensity of its characteristic diffraction peak. Subsequently, selected section samples were analyzed using field emission scanning electron microscopy (FESEM) with a FEI Quanta 450 FEG Scanning Electron Microscope. The operating conditions included an accelerating voltage of 20 kV, an emission current of 10 µA, and a working distance of 8.5 mm. A secondary electron imaging detector (SE2) was used to observe inelastically scattered electrons generated near the sample surface, while an AsB detector was employed to reveal chemical compositional differences among minerals. To improve electrical conductivity, the samples were coated with a ~10 nm thick platinum layer prior to analysis. Preparation of the XRD and SEM samples and the microscopic observations were carried out at the Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences.

3.2.2. Major Element Analysis

Twelve fresh, unweathered samples (YQh01–04 for red-brown siltstone, YQl01–04 for blue-gray dolomitic mudstone, and YQy01–04 for pale-yellow dolomitic mudstone) were crushed to a powder finer than 200 mesh using a sample crusher and analyzed using an X-ray fluorescence spectrometer (XRF) at the Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences. The procedure included drying the powdered samples at 105 °C, accurately weighing them, mixing with lithium metaborate flux, and fusing at 1050 °C to produce glass beads for XRF analysis. Loss on ignition (LOI) was determined by measuring the weight loss after ignition at 1000 °C. The analytical precision for major elements was within ±5%.

3.2.3. Trace Element and REE Analysis

Trace and rare earth element concentrations were measured using a Nu AttoM high-resolution inductively coupled plasma mass spectrometer (HR-ICP-MS) at the Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences. Precisely weighed sample powders were digested using a HNO₃-HF acid mixture in high-pressure Teflon vessels. Reference materials BHVO-2, AGV-2, and W-2 were used for quality control, with relative errors for all elements below 5%.

3.2.4. Detrital Zircon U–Pb Dating

Samples were crushed to about 80 mesh, followed by magnetic and heavy liquid separation. Zircons were hand-picked under a binocular microscope, mounted in epoxy resin, and polished to expose internal structures. Cathodoluminescence (CL) imaging, along with transmitted and reflected light microscopy, was used to observe internal features and select suitable areas for U-Pb dating. Analyses were performed at the Laboratory of the Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, using LA-ICP-MS with a laser spot diameter of 30 μm, energy density of 5–7 J/cm², and frequency of 6–7 Hz. Standard zircon 91500 (²⁰⁶Pb/²³⁸U = 1065.4 ± 0.6 Ma; [61]) was used as an external standard, and zircon GJ-1 (²⁰⁶Pb/²³⁸U = 599.8 ± 1.7 Ma; [62]) was used to monitor data quality. Trace element calibration was performed using NIST SRM 610. Isotopic ratios and element concentrations were processed using the GLITTER 4.0 software, and common lead correction followed the method of Andersen [34]. Zircon U-Pb ages and probability density plots were generated using IsoplotR [63].

3.3. Chemical Weathering Indices

In this study, various weathering indices were used to evaluate the paleoenvironmental conditions of the Hongshanwan landform sediments.

The Chemical Index of Alteration (CIA) was used to assess the paleoclimatic conditions, calculated as a mole fraction according to Formula (1) [12]:

CIA = [Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O)] × 100

(1)

In this equation, CaO* refers to the CaO content in silicate minerals only (excluding carbonates and phosphates). Since CO₂ data were not obtained, the proportion of CaO in carbonate components could not be determined. To calculate CaO*, the molar proportions of CaO and Na₂O were compared. If CaO ≤ Na₂O, the molar proportion of CaO was accepted; if CaO > Na₂O, the molar proportion of Na₂O was assumed to be equal to that of CaO [64].

The Index of Compositional Variation (ICV) was used to evaluate the maturity of the sediment, calculated as a mole fraction according to Formula (2) [3]:

ICV = [Fe₂O₃ + K₂O + Na₂O + CaO + MgO + TiO₂]/Al₂O₃

(2)

The Plagioclase Index of Alteration (PIA) was also used to trace the weathering of plagioclase in the sediments, calculated as a mole fraction according to Formula (3) derived from CIA [8]:

PIA = 100 × [(Al₂O₃ − K₂O)/(Al₂O₃ + CaO* + Na₂O − K₂O)]

(3)

4. Results

4.1. Geology of the Hongshanwan Landform

The Hongshanwan landform is primarily located in the central part of the Zhuanglang River and Xianshui River basins (Figure 3a). Based on the color rhythms, the landscape can be divided into two types. One type consists of a red-brown and blue-gray stratigraphic sequence from the lower part of the second member of the Hekou Group (H₁H^2a), with a nearly 1:1 thickness ratio of interbedded layers forming the Hongshanwan landform landscape (Figure 3b). The other type is characterized by a three-colored sequence of red-brown, blue-gray, and pale yellow from the upper part of the second member of the Hekou Group (H₁H^2b), with an approximate 2:1:1 thickness ratio, forming the Hongshanwan landform landscape (Figure 3c).

The second member of the Hekou Group is characterized by transgressive–regressive lacustrine cycles, marked by interbedded fine sandstones with wave-generated cross-bedding, blue-gray thin-bedded limestones and mudstones, and medium- to thick-bedded fine sandstones with trace fossils and mud cracks indicative of littoral to shallow lake–semi-deep lake depositional systems [51,58,65,66]. These deposits formed primarily under arid and hot climatic conditions during 127.41–124.19 Ma [67,68].

The lower part of the second member of the Early Cretaceous Hekou Group (K₁H^2a) is distributed along the northern and southern margins of the basin within the study area (Figure 3a). In the north, it is unconformably overlain by the Paleogene Xiliugou Formation, and in the south (south of the Yellow River), it unconformably contacts the Ordovician Wusushan Group. Lithologically, it consists of red-brown to purplish-red medium–thick-bedded siltstones and silty mudstones interbedded with gray to blue-gray medium–thin-bedded silty mudstones and calcareous mudstones, often displaying ripple marks and burrows [51].

The upper part of the second member (K₁H^2b) conformably overlies the lower strata (Figure 3a) and together forms a medium-scale open anticline–syncline fold structure trending NW-SE (Figure 3a,d). Lithologies include red-brown medium–thick-bedded silty mudstones interbedded with pale yellow and blue-gray thin-bedded dolomitic mudstones (Figure 3b).

The red-brown silty mudstones (Figure 4a,d–e) exhibit angular to sub-angular textures and are composed of mineral clasts such as feldspar, quartz, mica, and calcite, with grain sizes mainly between 0.005 and 0.05 mm and some up to 0.05 to 0.10 mm, accounting for about 35% of the rock volume. The iron-bearing matrix (Cly+Lim), primarily clay minerals (Cly), dispersed ferruginous material (Lim), and clasts smaller than 0.005 mm make up about 65%. Iron oxides appear as secondary goethite staining in clay minerals, imparting a reddish-brown hue.

The blue-gray dolomitic mudstones (Figure 4b, f–g) locally display lenticular structures and are composed of angular to sub-angular grains primarily between 0.005 and 0.05 mm, and secondarily between 0.05 and 0.10 mm, accounting for ~10% of the rock volume. Micritic dolomite (Dol), with micritic texture and high-order interference colors, occurs in vein-like forms or as rhythmically interlayered with clay-rich layers, with layer thicknesses of 0.5–2 mm, and comprises ~20% of the rock. The clay matrix (Cly), composed of clay minerals and fine felsic, calcareous, and ferruginous clasts (<0.005 mm), alternates rhythmically with dolomitic layers, constituting ~70% of the rock. Bioturbation structures (Dom) are also observed.

The pale-yellow dolomitic mudstones (Figure 4c, h–i) are composed of grains mainly in the 0.005–0.05 mm range, and secondarily in the 0.05–0.10 mm range, accounting for ~10% of the rock. Micritic dolomite occurs similarly as in the blue-gray type, mainly as veins or rhythmically layered with clay layers (0.5–2 mm thick), comprising ~20% of the rock. The clay matrix consists of clay minerals and <0.005 mm felsic, calcareous, and ferruginous debris, rhythmically interbedded with dolomite-rich layers, and makes up ~70% of the rock.

4.2. Mineral Composition

The sediments of the Hongshanwan landform are predominantly composed of muscovite, albite, and clay minerals, with minor iron-bearing detrital components such as hematite (Figure 5). Quartz is present in the mineralogical assembly (Figure 6), but is not illustrated in the SEM image (Figure 5) due to its relatively low proportion. In the red-brown samples (H1-H17), muscovite (average 37.68%), albite (average content 16.60%), chamosite content (average 12.12%) and analcime (average 8.12%) are the dominant minerals, while calcite (average 4.31%) is present in lower amounts. In the blue-gray samples (L1-L16), muscovite (average 35.45%), chamosite (average 15.93%) albite (average content 14.87%) and dolomite (average 7.40%) are significantly enriched, whereas analcime (average 6.00%) and calcite (average 3.37%) are relatively low compared to the red-brown samples. In the pale-yellow samples (Y1-Y17), muscovite (average 30.42%), chamosite (average 14.68%), albite (average content 14.62%) and dolomite (average 7.00%) also occur as dominant minerals, and analcime (average 4.18%) and calcite (average 4.56%) are the minor minerals. On the other hand, the scanning electron microscope observation is inconsistent with the RXD data. The minerals in the red-brown rocks mainly include hematite, sericite, illite, calcite and barite. The blue-gray samples consist of illite, sericite, calcite, barite and minor magnetite and zircon. The minerals in the pale-yellow samples mainly include plagioclase, quartz, calcite and minor hematite, barite and rutile (Figure 6).

4.3. Whole-Rock Geochemistry

The data presented in Table S1 in the Supplementary Materials provide a comprehensive summary of XRF and ICP-MS analytical results, including major element contents (wt.%) and trace and rare earth element values (ppm). In addition, paleoenvironmental indices for the Hongshanwan landform sediments of the Lanzhou Basin are listed, along with reference values for the upper continental crust (UCC) [69] and post-Archean Australian shale (PAAS) [13,70].

Taken as a whole, major element data indicate that the sediments are characterized by low SiO₂ content (average ~43.2 wt.%), moderate Al₂O₃ levels (15–17.5 wt.%), and notably enriched Fe₂O₃ (7.95–11.4 wt.%), which are significantly higher than the UCC and PAAS standards [13,70]. However, the three sediment types of the Hongshanwan landform show distinct differences in major element composition (Figure 7). Red-brown samples (H01–H04) are relatively enriched in Al₂O₃ (average 17 wt.%), Fe₂O₃ (average 10.8 wt.%), FeO (average 2.4 wt.%), and K₂O (average 4.02 wt.%) compared to blue-gray and pale-yellow samples. Blue-gray samples (L01–L04) have the lowest contents of Fe₂O₃ (average 8.71 wt.%), FeO (average 1.95 wt.%) and Na₂O (average 2.35 wt.%), but the highest contents of CaO (average 6.01 wt.%) and MgO (average 4.04 wt.%). Pale-yellow samples (Y01–Y04) show the highest SiO₂ content (43.9 wt.%) and intermediate contents of Fe₂O₃ (average 9.61 wt.%), FeO (average 2.14 wt.%), CaO (average 5.59 wt.%) and MgO (average 3.6 wt.%).

The red-brown samples are enriched in large-ion lithophile elements (LILEs) such as Rb (average 135 ppm) and Cs (average 1.6 ppm), as well as transition trace elements (TTEs) including Cr (average 119 ppm), Zn (average 101 ppm), and Co (average 35 ppm). The blue-gray samples show relative enrichment in U (average 6.9 ppm), Hf (average 5.7 ppm), and Ni (average 43 ppm). The pale-yellow samples exhibit co-enrichment of high-field-strength elements (HFSEs) and LILEs, including Nb (average 13 ppm), Ta (average 1.5 ppm), Th (average 21 ppm), Zr (average 156 ppm), Sr (average 505 ppm), and Ba (average 474 ppm).

All samples from the Hongshanwan landform display right-inclined chondrite-normalized REE patterns in the spider diagram (Figure 8), characterized by enrichment in light rare earth elements (LREEs) and relative depletion in heavy rare earth elements (HREEs), with an average (LREE/HREE) ratio of 7.59. The samples show negligible Ce anomalies (δCe ≈ 0.99) and weak Eu anomalies (δEu ≈ 1.07), indicating limited fractionation during deposition and diagenesis. Overall, the REE patterns exhibit pronounced HREE depletion with slight enrichment toward Yb, suggesting a dominant control of source composition rather than post-depositional processes.

Significant variations in REE compositions are observed among different lithological units. The red-brown samples are characterized by relatively high total REE contents (ΣREE = 172.48 ppm on average), strong LREE enrichment (LREE = 150.19 ppm; LREE/HREE = 6.77), and HREE depletion (HREE = 22.28 ppm). They show moderate (La/Yb) _N values (average 2.99), low (Gd/Yb) _N values (0.05), weak negative Eu anomalies (δEu ≈ 0.89), and no significant Ce anomaly (δCe ≈ 0.91). The blue-gray samples exhibit relatively lower ΣREE (144.04 ppm), but the highest LREE/HREE ratios (average 8.47), indicating stronger fractionation. They display slight positive Eu anomalies (δEu ≈ 1.31), no Ce anomaly (δCe ≈ 1.01), and higher (La/Yb) _N values (average 5.03), suggesting a mixed provenance with enhanced contribution from mafic components. The pale-yellow samples show REE characteristics similar to those of the red-brown samples, with comparable ΣREE (172.47 ppm), LREE (152.25 ppm), and HREE (20.22 ppm), and an average LREE/HREE ratio of 7.54. Their REE patterns display weak positive Eu and Ce anomalies (δEu ≈ 1.00; δCe ≈ 1.05), indicating a similar provenance and geochemical behavior.

Overall, the REE spider diagram highlights a consistent LREE-enriched pattern with minor interlayer variations, reflecting a predominantly intermediate–mafic provenance with limited influence from post-depositional alteration.

4.4. Detrital Zircon U–Pb Geochronology

The detrital zircon U–Pb age results of the upper member of the Lower Cretaceous Hekou Group are shown in Table S2 in the Supplementary Materials. Ages range from 217.3 ± 2.45 Ma to 2449.3 ± 20.6 Ma. Both the probability density plots (PDPs) and kernel density estimates (KDEs) exhibit three major age peaks at 242 Ma, 349 Ma, and 1837 Ma (Figure 9a–c). The zircons can be grouped into: 217.3–286.1 Ma (Permian–Triassic, 29 grains, 60.4%), 331.5–376.4 Ma (Devonian–Carboniferous, 6 grains, 12.5%) and 1322.3–1994 Ma (Mesoproterozoic, 10 grains, 20.8%). Cathodoluminescence (CL) images of the zircons reveal variable grain sizes and sub-angular to sub-rounded shapes, with distinct oscillatory zoning indicating magmatic origin [71,72,73]. Th/U ratios > 0.1 further support a magmatic origin [74].

5. Discussion

5.1. Elemental Abundance and Correlation

5.1.1. Elemental Abundance

Geochemical analyses indicate that the sediments of the Hongshanwan landform in the Lanzhou Basin are characterized by low SiO₂, moderate Al₂O₃, and significantly enriched Fe₂O₃ contents, accompanied by high concentrations of iron-bearing detrital minerals and clay minerals. This enrichment in iron components likely reflects not only the influence of strongly oxidizing depositional environments but also the precipitation of iron-bearing minerals such as hematite and goethite under arid–oxidizing conditions [75]. The red-brown samples exhibit notably high concentrations of large-ion lithophile elements (LILEs) Rb and Cs, as well as transition trace elements (TTEs) Cr, Zn, and Co, and are rich in muscovite and quartz, suggesting a felsic granitic provenance that underwent a certain degree of weathering and sorting [76]. The simultaneous increase in K₂O, CaO, and MgO may correspond to rapid erosion in orogenic belts [77,78]. The relative enrichment of CaO and MgO, especially in the blue-gray samples, is associated with elevated contents of chlorite and dolomite, indicating enhanced dolomitic carbonate components characteristic of evaporitic lacustrine deposits or basaltic provenance materials (e.g., residuals from basalt weathering) [9,13]. The co-enrichment of high-field-strength elements (HFSEs) and LILEs in pale-yellow samples suggests input from felsic volcanic provenance of crustal origin [31,79]. Moreover, all samples show significant enrichment of Cr and Co compared to the upper continental crust (UCC), indicating the presence of mafic-ultramafic components in the provenance area, likely related to gabbroic or ultramafic rocks [80,81]. The rare earth element (REE) distribution pattern shows enrichment of light REEs (LREEs) and depletion of heavy REEs (HREEs), forming a right-sloping pattern, which also supports a dominantly felsic provenance with only weak sorting or sedimentary recycling [13,76].

5.1.2. Elemental Correlations

The correlation heatmap of major, trace, and rare earth elements (REEs) in the sediments of the Hongshanwan landform (Figure 10) reveals several meaningful geochemical relationships. Al₂O₃ is significantly negatively correlated with MnO, MgO, CaO, and Sr, but shows generally positive correlations with transition trace elements (e.g., Cr, Co, Zn) and both light and heavy REEs. This indicates that clay minerals and aluminosilicates are the principal hosts for REEs and TTEs and that they are primarily derived from the weathering residues of felsic igneous rocks [13,76]. In contrast, carbonate-related elements such as MnO, MgO, and CaO show strong positive correlations with high-field-strength elements (HFSEs) including Th, Zr, Hf, and U, as well as with Eu. This pattern suggests that portions of the REEs and HFSEs may co-precipitate with carbonate minerals or derive from heavy minerals (e.g., zircon, monazite) with a felsic crustal provenance [8,31]. Additionally, the positive correlations between Na₂O, K₂O, P₂O_5, and REEs, as well as the co-variation of large-ion lithophile elements (LILEs) such as Ba, Rb, and Cs with REEs, further support the interpretation that these elements are mostly hosted in clay minerals and are well preserved during the weathering process [80]. The distribution of Eu shows strong positive correlations with Sr, U, and carbonate-related components (MnO, MgO, and CaO), but negative correlations with clay-associated elements such as Al₂O₃ and Fe₂O₃. This suggests differential partitioning of Eu among mineral phases, with its geochemical behavior controlled by redox conditions and the degree of plagioclase fractionation [12,75].

5.2. Paleoweathering and Paleoclimatic

5.2.1. Weathering Indices

The Chemical Index of Alteration (CIA) is a widely recognized proxy for paleoclimatic conditions, as it quantifies the ratio of alumina to unstable oxides and directly reflects the degree of chemical weathering [82]. The CIA values of sedimentary rocks in the Hongshanwan landform average 56.27, with comparable values across the three color types: red-brown (55.37–57.71, average 56.41), blue-gray (55.07–58.96, average 56.61), and pale yellow (54.74–57.52, average 55.77). These values suggest that the provenance area underwent weak chemical weathering under hot and arid climatic conditions.

The Index of Compositional Variability (ICV) reflects sediment maturity and serves as an important parameter for evaluating sediment recycling and prolonged weathering in the provenance region [3,83]. The ICV values for the three sample groups in the Hongshanwan landform are also similar, with ranges of 2.01–2.04 (average 2.03), 2.18–2.29 (average 2.21), and 1.79–2.43 (average 2.17), respectively. The overall average ICV value of 2.14 indicates that the sediments are compositionally immature. When plotted on the ICV-CIA diagram, these values reflect the combined influence of climatic conditions on the weathering intensity and geochemical maturity of the clastic sediments. The data indicate that the sediments formed under a hot and arid climate are characterized by weak chemical weathering and low geochemical maturity (Figure 11a).

Fedo et al. [8] proposed the Plagioclase Index of Alteration (PIA) by modifying the CIA formula to specifically assess the degree of plagioclase weathering in sedimentary environments. In the study area, PIA values for the three sample types are as follows: 57.50–61.00 (average 59.04) for red-brown, 56.85–62.48 (average 59.02) for blue-gray, and 56.39–60.22 (average 57.80) for pale-yellow samples. The overall average PIA value for the Hongshanwan landform is 58.62, which similarly indicates that the provenance rocks underwent weak chemical weathering [84,85].

On the A–CN–K ternary diagram (Figure 11b), the data points from the study area’s sandstones plot nearly parallel to the A–CN line and slightly above the average upper continental crust (UCC) values, gradually deviating from the plagioclase–K-feldspar join. This pattern suggests that the parent rocks experienced incipient-to-moderate weathering, accompanied by enrichment of Al₂O₃ during sediment transport.

5.2.2. Element Ratios

Studies have shown that the Th/U ratio in most upper continental crust (UCC) rocks generally ranges between 3.5 and 4.0 [5]. In this study, the Th/U ratios of the samples are relatively low, with an average value of 2.77, which may indicate that the provenance area experienced weak chemical weathering or limited sedimentary recycling. On the Th/U vs. U plot (Figure 12a), the data correspond to initial-to-moderate weathering conditions in the provenance region. Furthermore, the cross-plot of CIA and Th/U ratios provides additional support that the provenance rocks were formed under arid climatic conditions (Figure 12b).

The mean Sr/Cu ratio of 19.21 in the study area suggests an arid climate setting [42]. Additionally, the Sr/Cu vs. Th/U cross-plot confirms that most samples were deposited under hot and dry conditions (Figure 12c). The SiO₂/ (Al₂O₃ + K₂O + Na₂O) ratio is also considered a valuable paleoclimatic proxy [86,87]; however, due to the relatively low SiO₂ contents of the samples, its applicability is limited in this study and is therefore not further discussed. Moreover, the relatively high Rb/Sr ratio (average 0.35) further supports arid climatic conditions in the study area [88].

In summary, based on a combined system of weathering indices and geochemical characteristics, it is determined that the bedrock of the Hongshanwan landform sedimentary sequence in the Lanzhou Basin experienced initial-to-moderate chemical weathering under arid and hot paleoclimatic conditions. This conclusion is consistent with previous studies on the Hekou Formation, which suggested a predominantly arid depositional environment based on analyses of rock color indices [68] and paleontological assemblages [51,52,89,90].

5.3. Paleoredox Conditions

The Cu/Zn ratios of the red-brown, blue-gray, and pale-yellow samples from the study area are 0.92, 0.16, and 0.26, respectively, all indicating relatively low values (average of 0.45), which suggests oxidizing depositional conditions (Figure 13). Based on the U/Th ratio, redox conditions of the Hongshanwan geomorphic sediments can be inferred: U/Th > 1.25 indicates a reducing environment, 0.75–1.25 suggests suboxic-to-dysoxic conditions, and U/Th < 0.75 implies an oxidizing environment [88,91,92]. In this study, the U/Th values of the three sample types are 0.46, 0.44, and 0.31 (average 0.40), all below 0.75, further supporting deposition under oxic conditions (Figure 13). The Ni/Co ratios of the Hongshanwan siltstones and mudstones range from 1.03 to 1.68 (average 1.30), also indicative of oxidizing conditions (Figure 13).

Additionally, the Ce/Ce* ratios in the three sample types range from 0.73 to 1.00 (average 0.89), showing a weak positive Ce anomaly that further supports oxic depositional environments [3] (Figure 13). As shown in the stratigraphic profile, all three color types exhibit geochemical signatures consistent with oxidizing conditions. This supports the interpretation that the Hongshanwan sediments were deposited predominantly under oxic settings and that the high iron content under such conditions likely contributed to the development of reddish fine-grained clastic rocks. Similar conclusions have been drawn by Cai et al. [89] through studies of paleontology and lithofacies paleogeography.

Contrary to traditional views that associate blue-gray and pale-yellow colors with reducing conditions [93,94], the present study finds no evidence of reduction in these layers. Thin-section petrography reveals high dolomite content in these samples, which may reflect carbonate precipitation due to declining lake levels, affecting color development. This aligns with previous colorimetric studies that linked chromatic variation to lake-level changes [68]. The observed rhythmic alternation of sediment colors may also reflect episodic tectonic pulses during sustained uplift of the Qilian Mountains [95].

5.4. Paleotectonic Setting and Provenance

5.4.1. Paleotectonic Setting

The ancient tectonic environment was analyzed using whole-rock major and trace element geochemical compositions [96,97]. As shown in Figure 14, which illustrates tectonic environment discrimination [96], the Hongshanwan landform sediments from the Lanzhou Basin exhibit characteristics indicative of an island arc tectonic setting. The study samples display a high Al₂O₃/SiO₂ ratio (average 0.38) and low TiO₂ content (average 0.62), consistent with typical continental island arc (CIA)/active continental margin (ACM) tectonic environments [96].

The detrital zircon crystallization-to-deposition age cumulative distribution diagram (Figure 15) shows that all samples plot within the convergent/collisional tectonic setting, indicating that the basin was likely situated in a forearc, trench, back-arc, or foreland basin setting [98]. The presence of a secondary age peak around 2000 Ma suggests the recycling of ancient sediments, thereby ruling out a forearc basin origin. The Hekou Group is rich in fossils, especially ostracods dominated by Cypridea, forming the Cypridea–Lycopterocypris–Djungarica assemblage. This fossil association is consistent with Early Cretaceous lacustrine ostracod assemblages in China and abroad [52], lacking deep-marine deposits and containing shallow-marine fossils, thus excluding a trench basin interpretation.

Additionally, the increasing textural and compositional maturity of the clastic rocks and the transition from unstable to stable depositional environments are consistent with typical foreland basin evolution [99]. The provenance characteristics inferred from detrital zircon age groups and their associated tectonic backgrounds indicate a combination of collisional orogeny and extensional intracontinental orogeny. Geochemical data also reveal a mixed provenance signature dominated by felsic with minor mafic contributions, influenced only weakly by recycled sediments.

In summary, the upper section of the second member of the Hekou Group in the Early Cretaceous primarily received sediment from orogenic belts with minimal sedimentary recycling, along with inputs from arc-related terranes and stable continental blocks. The basin is best interpreted as a back-arc foreland basin.

5.4.2. Provenance

Provenance discriminant analysis (Figure 16, after [97]) shows that sediments from the Hongshanwan landform in the Lanzhou Basin are mainly derived from mafic to intermediate igneous sources, with red-brown samples plotting in the mafic volcanic field and blue-gray samples in the intermediate volcanic field. Elevated K₂O and Rb likely reflect enrichment of clay minerals (e.g., muscovite, Figure 6), and low SiO₂ (<45%) along with positive Eu anomalies preclude a purely felsic provenance (Figure 8).

Trace element ratios and rare earth element (REE) compositions further substantiate a mafic-to-intermediate provenance. In the Th/Co-La/Co diagram (Figure 17a), the samples plot near the upper continental crust (UCC) and Post-Archean Australian Shale (PAAS) values, reflecting a generally mixed source. Importantly, however, specific elemental ratios strongly indicate a significant mafic contribution: the Cr/Th and Th/Co ratios are 8.60 and 0.50, respectively, both higher than the UCC values, and the relatively low Th/U ratio (average 2.77) further implies the involvement of mafic rocks [1,31,80].

The average LREE and HREE contents are 143.74 ppm and 19.27 ppm, respectively. While such enrichments and relatively high (La/Yb) _N values (average 3.65) might traditionally be interpreted as a felsic signature, the significantly low SiO₂ content (<45 wt.%) suggests that this REE enrichment is primarily driven by the adsorption of clay minerals (e.g., illite) during weathering, rather than a granitic provenance. This is corroborated by the weak-to-positive Eu anomalies (Eu/Eu*≥ 0.85 to >1 in most samples), which definitively point toward a mafic-to-intermediate source [69]. In the ΣREE vs. (La/Yb) _N discrimination diagram (Figure 17b), the red-brown and pale-yellow samples plot prominently near the tholeiitic basalt field, confirming a mafic volcanic origin. Meanwhile, the blue-gray samples plot between the alkaline basalt and calcareous mudstone fields, reflecting a mixed intermediate-to-mafic provenance. Additionally, the Ni vs. Cr diagram (Figure 17c) and the Eu/Eu* vs. (Gd/Yb) _N diagram (Figure 17d) consistently indicate that these intermediate–mafic source rocks were primarily formed in the late Archean to post-Archean. Notably, these mafic source rocks provided abundant iron-bearing minerals, which served as the material basis for the subsequent oxidation and characteristic red coloration of the Hongshanwan landform.

Detrital zircon U–Pb ages indicate that zircon populations in the Hongshanwan landform sediments of the Lanzhou Basin can be grouped into three main stages: Mesoproterozoic–Paleoproterozoic (1322–1994 Ma), Devonian–Carboniferous (331–376 Ma), and Permian–Triassic (217–286 Ma). These age populations, together with geochemical signatures, suggest that the sediments were predominantly derived from intermediate-to-mafic igneous rocks, with minor contributions from felsic rocks and ancient crystalline basement.

During the Mesoproterozoic–Paleoproterozoic, Proterozoic intrusions in the eastern Qilian Mountains and the Maxianshan area along the southern margin of the basin likely record the Columbia supercontinent rifting event [100]. Paleocurrent data indicate a general south-to-north transport direction [90], suggesting that intermediate–mafic igneous bodies in the southern basin (e.g., Maxianshan) served as major sediment sources, mixed with minor input from older basement materials.

From the Middle Cambrian to Silurian, the Qilian region experienced complex subduction-related magmatism. Southward subduction of the North Qilian oceanic crust along the Taolaishan–Upper Heihe line occurred from the Late Early Cambrian to Early Ordovician, followed by the development of Ordovician–Silurian back-arc basins [56,101,102,103,104]. Intermediate-to-mafic magmatic rocks formed during this period likely provided an important sediment source, with limited felsic input from contemporaneous granitic intrusions.

During the Devonian–Carboniferous, magmatic activity weakened and was characterized by localized emplacement of S-type granites in the Maxianshan area [100,103,105]. However, the regional tectonic regime transitioned to intracontinental orogeny after the Middle–Late Devonian [104], and the Qilian Mountains remained a key provenance dominated by intermediate-to-mafic lithologies, consistent with paleocurrent reconstructions [90].

In the Permian–Triassic, following the Caledonian orogeny, the Qilian region entered an intraplate tectonic setting. Mafic–ultramafic intrusions in the Zongwulong area of southern Qilian during the Late Paleozoic and Mesozoic [103,106] likely supplied abundant detritus to the basin. Trace element and REE characteristics of the Hongshanwan sediments further indicate significant input from mafic sources during this stage, reinforcing a provenance dominated by intermediate–mafic igneous rocks.

5.5. Coloration Mechanism of the Hongshanwan Landform

The most distinctive characteristic of the Hongshanwan landform is the development of well-exposed multicolored rhythmic sedimentary sequences. Variations in the color of clastic rocks are widely recognized to be closely linked to mineral composition, provenance characteristics, depositional environment, and diagenetic processes [107,108,109]. Integrated mineralogical, geochemical, and detrital zircon U-Pb geochronological results from this study provide constraints on the mechanisms controlling the formation of these multicolored strata.

The reddish-brown strata are characterized by a provenance dominated by intermediate-to-mafic igneous rocks, accompanied by the enrichment of hematite, dark clay minerals, and iron-rich detrital components (Figure 5 and Figure 6). Iron-bearing phases in these strata likely originate from two principal sources: (1) primary iron-bearing minerals derived from the weathering of mafic source rocks, which impart an inherited coloration; and (2) authigenic iron oxides, particularly hematite, formed during early diagenesis. During sediment burial, Fe²⁺ released from clay minerals into pore waters was oxidized at redox interfaces and subsequently precipitated as fine-grained hematite with strong pigmenting capacity [110,111].

The dominant reddish coloration of the red-brown units can therefore be interpreted as the combined result of inherited color from iron-rich detrital minerals, authigenic pigmentation during sediment accumulation and early diagenesis, and minor secondary modification during post-depositional alteration or surface weathering. Previous studies indicate that coarse-grained, detrital hematite particles are generally less effective pigments than finely disseminated authigenic hematite. Moreover, the development of laterally extensive and uniformly colored red beds over prolonged geological timescales requires a sustained supply of iron and stable oxidizing conditions [110,111]. The relatively high Fe₂O₃ (average 10.74 wt.%) and FeO (average 2.39 wt.%) contents in the red-brown strata support this interpretation. In addition, the red-brown layers exhibit well-developed, laterally continuous bedding that coincides with color boundaries (Figure 3b–d), which contrasts with the irregular patchy or vein-like features typically associated with secondary oxidation [108,109]. These observations suggest that authigenic pigmentation during early diagenesis played a dominant role in the development of the red-brown coloration, whereas inherited and secondary colors were of subordinate importance.

Weathering indices and redox-sensitive geochemical proxies indicate that the source rocks experienced low-to-moderate chemical weathering under arid to semi-arid climatic conditions, and that sediment deposition occurred predominantly under oxidizing conditions. Such an environment is favorable for the oxidation of Fe²⁺ to Fe³⁺ in pore waters and the precipitation of iron oxides during early diagenesis. Consequently, the red-brown coloration of the Hongshanwan strata is interpreted to be primarily controlled by the formation of authigenic hematite, with only limited influence from inherited detrital iron minerals and late-stage secondary processes.

In contrast, the blue-gray strata are enriched in CaO and MgO and contain abundant clinochlore and dolomite, suggesting derivation from a mixed provenance involving calcareous mudstone and mafic volcanic rocks. These strata display stable color boundaries that are parallel to bedding and laterally persistent (Figure 3b–d), indicating limited post-depositional color modification. Their bluish-gray coloration is therefore interpreted to be mainly inherited from carbonate minerals and dark mafic components rather than produced by diagenetic iron oxidation.

Detrital zircon U-Pb age distributions indicate that the multicolored rhythmic sedimentary sequences of the Hongshanwan landform reflect episodic variations in sediment supply linked to tectonic activity in the surrounding orogenic belts, particularly the Qilian Mountains. Following basin formation, the Lanzhou Basin experienced multiple phases of tectonic deformation [29,55], resulting in tilting and structural modification of the strata. Together with dominant surface processes such as slope-controlled sheet flow, these tectonic and exogenic factors contributed to the modification of the original sedimentary architecture and ultimately shaped the present-day geomorphological expression of the Hongshanwan landform.

6. Conclusions

(1) Geochemical characteristics indicate that the sediments were predominantly derived from intermediate-to-mafic igneous rocks, with minor contributions from felsic rocks and recycled Precambrian basement materials. Trace element ratios and REE patterns (LREE/HREE = 7.59) reflect a mixed but mafic-influenced provenance. Detrital zircon U–Pb ages define three major age populations (1322–1994 Ma, 331–376 Ma, and 217–286 Ma), indicating a composite provenance linked to multiple tectono-magmatic events in the Qilian orogenic system.

(2) Weathering indices (CIA = 56.27, ICV = 2.14) and A–CN–K ternary plots suggest a low degree of weathering in the provenance area, dominated by unstable minerals. Ratios such as U/Th, Ce/Ce*, and Ni/Co indicate an overall oxidizing depositional environment.

(3) The red-brown coloration of the strata is primarily controlled by the presence of authigenic hematite formed during early diagenesis. Iron was likely mobilized in pore fluids under reducing conditions as soluble Fe²⁺ released from iron-bearing minerals during early burial. Subsequent exposure to oxidizing conditions promoted the oxidation of Fe²⁺ to Fe³⁺ and the precipitation of hematite coatings on detrital grains and within the clay matrix. This process ultimately produced the characteristic red coloration of the strata. Similar mechanisms of iron mobilization and hematite precipitation have been widely reported in continental red-bed successions and are considered a fundamental process responsible for red-bed pigmentation [14,15]. In contrast, the blue-gray and pale-yellow strata are dominated by carbonate minerals, with colors inherited from provenance rocks, showing stable hues and minimal redox variation.

(4) Tectonic discrimination diagrams and detrital zircon age spectra indicate that the sedimentary system developed in a back-arc foreland basin within a continental island arc setting. Episodic tectonic activity drove pulses in sediment supply, producing multicolored rhythmic cycles. Subsequent tectonic uplift and deformation, combined with slope-dominated surface processes, shaped the present geomorphic features of the Hongshanwan landform.

(5) The Hongshanwan landform provides a quantitative model for interpreting colorful red-bed mountain landscapes elsewhere, such as Vinicunca (Peru), the Altai Mountains (Russia), and the Quebrada de Humahuaca-Hornocal range (Argentina), demonstrating the universal interplay of provenance, weathering, diagenesis, and tectonics in controlling sedimentary color patterns.