Identification of Milankovitch Cycles and Their Sedimentary Responses in Fine-Grained Depositional Strata on the Southwestern Margin of the Songliao Basin

Abstract

A series of fault depressions developed in the Kailu area on the southwestern margin of the Songliao Basin, where thick lacustrine fine-grained sedimentary rocks were widely deposited during the initial faulting stage in the Early Cretaceous. These formations serve as the primary source rocks within the depressions. To investigate the depositional cyclicity framework, paleoenvironmental conditions, and source rock development patterns of fine-grained sedimentary strata, this study focuses on the Naiman Sag, selecting Well Nai-10 for wavelet transform and spectral analysis based on natural gamma ray logs. Combining core, well logging, and geochemical element analyses, Milankovitch cycles within the Yixian Formation were identified. The relationship between theoretical orbital periods and sedimentary cycles in a single well was established, enabling the high-precision identification and classification of fine-grained sedimentary cycles. Furthermore, the study explores the sedimentary response to orbital forcing and the development patterns of source rocks. The results indicate that fine-grained sedimentary strata exhibit distinct Milankovitch cyclicity, with a strong correlation between astronomical periods and sedimentary cycles. Using the 100 kyr short eccentricity cycle as the tuning curve, an astronomical timescale and high-frequency cyclic division for the target interval were established. Under the control of long eccentricity cycles, sedimentation exhibits strong response characteristics: near the peak of short eccentricity cycles, the climate was warm and humid, redox conditions were strong, and precipitation was high, facilitating organic matter accumulation. Based on this response relationship, two ideal enrichment models of mudstone and shale under different paleoclimatic conditions are proposed, providing valuable insights for identifying high-quality source rocks and unconventional hydrocarbons in hydrocarbon exploration.

1. Introduction

The Milankovitch cycles (Milankovitch oscillations) were proposed by Milankovitch during his study of Quaternary glaciations [1]. This theory suggests that Earth’s climatic periodicity is controlled by orbital parameters such as eccentricity, obliquity, and precession. Variations in orbital parameters cause differences in the solar radiation received by Earth, leading to distinctions between warm and cold or humid and arid climates. These climate changes further influence water stratification, sediment infill, and the composition and population density of organisms within lakes. Generally, fine-grained sedimentary strata have limited stratigraphic resolution due to their homogeneous lithology and lack of markers. However, when constrained within a chronostratigraphic framework, they exhibit good continuity and high resolution, allowing for the recording of sedimentary cyclic variations driven by astronomical forcing and other factors. Meanwhile, the formation and preservation of organic matter in fine-grained sediments are influenced by paleoclimatic conditions. Consequently, cyclic climate variations partially control the differences in organic matter abundance within strata. Therefore, studying Milankovitch cycles in fine-grained sedimentary strata and establishing a high-resolution sequence stratigraphic framework and cyclic model can help investigate the sedimentary responses of lacustrine fine-grained deposits to orbital forcing and the organic matter enrichment patterns, which hold significant scientific value in advancing the understanding of lacustrine fine-grained sedimentation.

Domestic and foreign scholars have made significant progress in applying Milankovitch cycle theory to the division and correlation of high-frequency sequence stratigraphy, as well as in exploring the relationships between orbital forcing, paleoclimate changes, and organic matter enrichment [2,3,4,5,6,7]. For instance, Dong Xinjun et al. [2] investigated deep lacustrine fine-grained sediments in the fourth member of the Shahejie Formation in the Dongying Depression. By employing wavelet and spectral analysis of natural gamma logging data, they identified Milankovitch cycles and confirmed that long eccentricity cycles played a dominant role in controlling the evolution of these sediments. Similarly, Peng Jun [3] and Lin Shanshan [4] recognized Milankovitch cycles in the lacustrine fine-grained strata of the Jiyang and Huanghua Depressions in the Bohai Bay Basin, using proxies for organic matter abundance and paleo-redox conditions to analyze orbital responses. In the South Turgay Basin of Kazakhstan, Shi Juye et al. [5] identified well-preserved Milankovitch cycles in sedimentary sequences, with astronomical forcing exerting a marked influence on depositional processes. Yang Xue et al. [6] further contributed by identifying Milankovitch cycles in the deep-water fine-grained sediments of the first member of the Qingshankou Formation in the Songliao Basin, establishing stratigraphic features shaped by astronomical control. Park Jeffrey et al. [7] used the Global Atmospheric Circulation Model (GCM) to simulate the responses of the Mesozoic Cretaceous climate and the Milankovitch cycle, revealing how the Milankovitch cycle affects the hydrological cycle and then triggers regional-scale climate change and sedimentary evolution. During the early rift stage of the Naiman Depression, the Yixian Formation developed extensive lacustrine organic-rich fine-grained sediments. These thick mudstone intervals reached thermal maturity, forming a key hydrocarbon-generating system in the region. Although many scholars have extensively studied the lithological and geochemical characteristics of the Yixian Formation [8,9,10,11], systematic investigations into its deep-water fine-grained sedimentation remain scarce. Specifically, there is a lack of a stratigraphic cyclicity framework, limited understanding of the depositional paleoenvironment and paleoclimate, and an unclear model for source rock development. To address these gaps, this study applies Milankovitch cycle theory to construct a high-resolution sequence stratigraphic framework for the target strata, clarify the sedimentary responses to astronomical forcing, and establish a source rock development model. These efforts aim to advance the understanding of fine-grained sedimentation on the southwestern margin of the Songliao Basin and support both conventional and unconventional hydrocarbon exploration.

2. Regional Geological Setting

A series of fault depressions has developed on the southwestern margin of the Songliao Basin, bordering Jilin Province to the east and Liaoning Province to the south. The region has a triangular shape, extending approximately 300 km from north to south and about 370 km from east to west, with a total area of 57,469.76 km². Its geographical coordinates range from 119°04′ E to 123°43′ E and from 42°29′ N to 45°00′ N. The Naiman Sag is a secondary negative tectonic unit on the southwestern side within the faulted depression group. It is mainly located on the southeastern boundary of Inner Mongolia Province (Figure 1a). Its formation is primarily controlled by the Hongshan–Balihan and Xilamulun River faults. It is adjacent to the Zhangsanyuanzhi–Xinmiao and Lujiapu depressions to the northwest and the Baxiantong depression to the east, forming a narrow NNE-trending belt [8,9,12,13]. In terms of structural and sedimentary evolution, it shares similarities with small Meso-Cenozoic sedimentary basins in eastern China, characterized by three main features: small area, strong segmentation, and significant fault-controlled subsidence. Controlled by boundary faults and two major internal faults, the Naiman Sag is divided from west to east into three secondary structural units: the western steep slope belt, the central depression belt, and the eastern gentle slope belt [12,13,14] (Figure 1b). The Lower Cretaceous in the sag hosts the main hydrocarbon-bearing strata, and the following formations developed in ascending order: the Yixian Formation (K1y), Jiufotang Formation (K1jf), Shahai Formation (K1sh), and Fuxin Formation (K1f) (Figure 1c). In the subsidence center in the northern part of the sag, the Yixian Formation developed extensive lacustrine fine-grained sedimentary strata with favorable hydrocarbon generation and expulsion conditions, serving as the primary hydrocarbon source system within the sag.

3. Theory and Methods

3.1. Data and Sample Selection

During a given geological time period, variations in Earth’s orbital parameters occur gradually and remain relatively stable. The proportional relationships among these parameters—eccentricity, obliquity, and precession—are critical indicators for identifying Milankovitch cycles in sedimentary records. It is widely accepted that, when stratigraphic periodicities exhibit ratios corresponding to these three orbital components, the strata are considered to have been influenced by Milankovitch forcing. However, three essential conditions must be met for a reliable astronomical interpretation: (1) Sedimentary continuity: the sequence must be continuous and free from significant hiatuses or unconformities to ensure the integrity of the cycle analysis. (2) Relatively stable sedimentation rate: only when the rate is stable can thickness ratios in the depth domain accurately reflect time ratios. (3) Climatically sensitive proxies: the selected indicators must reflect variations in climate or lake level. The thick mudstone layers of the Yixian Formation in the Naiman Depression represent stable, organic-rich lacustrine fine-grained deposits that are particularly sensitive to climatic fluctuations, making them well suited for studying Milankovitch cycles and their sedimentary expressions. In such studies, data selection is critical: proxies that effectively capture climate variability are prioritized. Among these, the natural gamma ray (GR) logging curve stands out due to its high resolution and ability to preserve climate-related signals, making it a preferred proxy for astronomical cycle identification [15,16]. In selecting a representative well, preference is given to those located in structurally stable settings with minimal tectonic disturbance and complete stratigraphic records [17,18]. Well Nai-10, the focus of this study, is situated in such a stable area, characterized by continuous well logging data and abundant core samples. This research utilizes GR log data from the thick mudstone interval of the Yixian Formation in Well Nai-10 as a proxy for analyzing orbital-scale sedimentary cycles. By integrating logging data, major and trace element geochemical analyses, and petrographic observations, the study investigates stratigraphic cyclicity and shale depositional responses governed by astronomical forcing. The GR data were acquired using the CPLog logging system and processed using CIFLog 2.0 software. The samples collected this time were mainly from the depth of 2045–2080 m of Well Nai-10, with a sampling interval of one sample every 0.5 m. The major and trace elements and TOC experimental analyses were carried out on the collected samples. For mineralogical study, thin sections were prepared by cutting samples perpendicular to bedding, mounting them in resin on glass slides, drying them in an oven, and examining them under an OLYMPUS microscope (Olympus Corporation, Tokyo, Japan)at magnifications of 10×, 20×, and 50×.

3.2. Signal Analysis

Before signal analysis, data preprocessing is typically required to prepare the dataset for further analysis. The preprocessing steps include interpolation (using Lagrange polynomials to ensure equidistant data points for time series analysis), detrending (subtracting the mean of the dataset), noise removal (applying third-order and higher-order cumulants to suppress low-frequency components and enhance high-frequency signals), and additional detrending (using least squares fitting to remove upward low-frequency trends). These preprocessing steps can be performed using tools such as Matlab 2024 or Acycle 2.7. After preprocessing, the data are subjected to wavelet transform and spectral analysis in Matlab to extract Milankovitch cycle information from the lacustrine fine-grained sediments of the Yixian Formation, enabling a detailed analysis of sedimentary cycles.

Spectral analysis decomposes complex multi-frequency signals in the time domain into sine wave components of different frequencies. Essentially, it transforms spatial domain signals into the frequency domain [19]. Common spectral analysis methods include Fast Fourier Transform (FFT), the Maximum Entropy Method (MEM), and the Multi-Taper Method (MTM). In spectral analysis, the relative amplitude indicates the strength of the GR signal. A higher relative amplitude corresponds to stronger cyclicity, meaning that a particular stratigraphic thickness appears more frequently in the region. The purpose is to assess whether high-amplitude, non-random frequency bands align with Milankovitch cycle periods. By analyzing the amplitude values and corresponding frequencies in the spectrum, the cycle thickness for a given frequency in the strata can be calculated as 1/frequency. Since Earth’s orbital signals enter the sedimentary system in a non-linear manner and may be coupled with other signals, spectral analysis alone cannot fully determine whether a distinct Milankovitch cycle exists in the studied interval, necessitating further wavelet analysis. Wavelet transform extracts multi-scale information embedded in well log signals, enabling cyclic information segmentation and extraction [20]. Ultimately, by analyzing the relationship between the cyclic thickness corresponding to orbital periods and energy clusters in the wavelet spectrum, it is possible to determine whether the studied strata exhibit orbitally controlled sedimentary cycles.

3.3. Calculation of Theoretical Orbital Period

Previous studies have demonstrated that astronomical cycles controlled the deposition of fine-grained sedimentary strata within multiple continental lacustrine basins across China [21,22,23,24,25]. The Milankovitch cycle theory suggests that periodic variations in Earth’s orbital parameters (eccentricity, obliquity, and precession) drive cyclic climate changes. These variations are preserved in climate-sensitive strata, allowing them to be identified in well log curves [26,27,28]. Over the past 250 million years of geological history, the most stable cycle has been the 405 ka long eccentricity cycle. Short eccentricity, obliquity, and precession cycles are influenced by Earth–Moon gravitational interactions and tidal forces, with their periods gradually increasing slightly over geological time. However, the ratio between these cycles remains relatively stable within specific geological periods and is reflected in variations in sedimentary structures, sedimentary features, and depositional thickness. Overall, the ratio of long eccentricity to short eccentricity, obliquity, and precession is approximately 20:5:2:1 [29,30]. This study adopts the Cretaceous astronomical cycle model proposed by Laskar [31], integrating previous studies’ [32] quantitative divisions of long eccentricity, short eccentricity, obliquity, and precession cycles to calculate the theoretical orbital parameters of the Cretaceous (

Table 1. The theoretical parameters of the Milankovitch cycle in the Cretaceous and their ratios (following reference [18,19]).

Name	Theoretical Cycle/kyr				Cycle	Ratio
Eccentricity	405	1.00	—	—	—	—	—	—	—
	125	3.24	1.00	—	—	—	—	—	—
	100	4.05	1.25	1.00	—	—	—	—	—
Obliquity	48.0	8.44	2.60	2.08	1.00	—	—	—	—
	40.0	10.13	3.13	2.5	1.20	1.00	—	—	—
	37.5	10.8	3.33	2.67	1.28	1.07	1.00	—	—
Precession	22.5	18.00	5.56	4.44	2.13	1.78	1.67	1.00	—
	18.4	20.01	18.00	5.43	2.61	2.17	2.04	1.22	1.00

). These parameters are used to identify Milankovitch cycle characteristics in the stratigraphic record and serve as the basis for astronomical cycle calculations.

3.4. TOC and Element Geochemical Analysis

To determine the TOC content, the sample must first be ground to 200 mesh, then loaded into a ceramic crucible. The sample is rinsed with 5% dilute hydrochloric acid to remove inorganic carbon. After drying, a combustion aid and iron filings are added to the crucible, which is then burned in an instrument at 1100 °C to obtain the TOC value. The instrument used for TOC measurement in this study was the Leco-CS744 Carbon Sulfur Analyzer (LECO, Laboratory Equipment Corporation, San Jose, CA, USA).

For major element determination, the selected samples were first ground to 200 mesh and then dried at 105 °C for 3 h. After drying, the samples were cooled to room temperature in a desiccator. Subsequently, 0.70 g of each sample was mixed with 7.00 g of lithium borate (Li₂B₂O₇) flux and fused in a furnace to form glass discs. Major element concentrations were measured using a PANalytical PW2424 X-ray fluorescence (XRF) spectrometer (PANalytical B.V., Almelo, The Netherlands) with an analytical error of less than 5%. For trace element analysis, 0.1 g of the dried sample was first subjected to a desalting treatment. Then, 40 mg of the treated sample was digested in a sealed vessel using a 1:10 mixture of HF and HNO₃ at 180 °C for 24 h. After digestion, the solution was evaporated to near dryness, and the residue was redissolved and diluted with 5% HCl. Trace element concentrations were analyzed using an iCAP Q inductively coupled plasma mass spectrometer (ICP-MS) (Thermo Fisher Scientific, Waltham, MA, USA) with an analytical error also below 5%.

4. Results

4.1. Cyclic Response Characteristics of Paleoclimatic Environments

By analyzing the geochemical characteristics and vertical ratios of elemental concentrations in sediments, the paleodepositional climatic environments experienced by the sediments can be characterized [33,34,35,36,37]. For example, Na and Sr are mobile elements, while Al and Rb are relatively immobile. Under warm and humid conditions, increased surface precipitation leads to the leaching of Na and Sr, resulting in a decrease in W(Na)/W(Al) and an increase in W(Rb)/W(Sr). Conversely, in a cold and arid environment, W(Na)/W(Al) increases, while W(Rb)/W(Sr) decreases. Therefore, the relative values of W(Na)/W(Al) and W(Rb)/W(Sr) can be used to indicate relative variations in paleoclimate conditions. Similarly, V and Ni are redox-sensitive elements, and the ratio W(V)/W(V + Ni) is commonly used to indicate the redox conditions of the environment. Under anoxic conditions, V is more readily enriched in the environment, resulting in a higher W(V)/W(V + Ni) ratio. Conversely, under oxic conditions, Ni tends to accumulate before V, leading to a lower W(V)/W(V + Ni) ratio. Additionally, the Fe/Mn ratio is often used as a proxy for paleo-water depth. Under warm and humid conditions, Fe exists mainly in colloidal form in lake basins, whereas, under cold and arid conditions, Mn is more easily enriched. Therefore, a higher Fe/Mn ratio indicates a humid climate and corresponds to deeper paleo-water conditions [38,39,40,41]. The data indicate that the paleoclimatic environment during the deposition of the Yixian Formation in the study area underwent periodic fluctuations. Within the 2045–2075 m coring interval, four distinct peaks in paleoclimatic proxy indicators were identified (Figure 2), suggesting four episodes of relatively warm, humid, and reducing lacustrine conditions. Correspondingly, organic matter also showed four phases of notable enrichment, aligning with these paleoenvironmental shifts.

4.2. Milankovitch Cycle Analysis

In the spectral analysis plot (Figure 3), four primary frequency points (A, B, C, and D) were identified with confidence levels exceeding 90%. Their relative amplitudes decrease in sequence, with frequencies of 0.028, 0.102, 0.272, and 0.559 m⁻¹, corresponding to cycle thicknesses (1/frequency) of 35.71, 9.98, 3.68, and 1.79 m, respectively. The cycle thickness ratio is 1:2.05:5.48:19.96, which closely approximates 1:2:5:20, indicating a strong correlation among the long eccentricity, short eccentricity, obliquity, and precession cycles. Previous studies [29,30,31,32,42,43,44] have shown that, if the precession/obliquity/short eccentricity/long eccentricity cycle thickness ratio approaches 1:2:5:20, the presence of Milankovitch cycles can be preliminarily inferred. Additionally, wavelet transform analysis was performed on this interval to verify the findings. The wavelet transform results (Figure 3b) indicate that the spectral energy at the 35.7 m cycle thickness, corresponding to long eccentricity, is the strongest and exhibits good continuity, followed by short eccentricity. This indirectly confirms that high-amplitude, non-random frequencies align with Earth’s orbital cycles. Furthermore, as shown in Table 1, the theoretical periods of 18.4 ka, 37.5 ka, 100 ka, and 405 ka were identified, with a ratio of 1:2.04:5.43:20.01, which closely matches the observed cycle thickness ratio of 1:2.05:5.48:19.96, with an error of less than 1%. This further confirms that the stratigraphic deposition was controlled by astronomical cycles, with the corresponding orbital periods of long eccentricity, short eccentricity, obliquity, and precession being 405 ka, 100 ka, 37.5 ka, and 18.4 ka, respectively.

4.3. Identification and Division of High-Frequency Sedimentary Cycles

Based on the integrated data analysis and previous research experience in high-frequency sequence stratigraphy using Milankovitch orbital cycle theory [26,32,45,46,47], the long eccentricity cycle curve corresponds to the medium-term base-level cycle, classified as a fourth-order cycle with a duration of 405 kyr. The short eccentricity cycle curve corresponds to the short-term base-level cycle, classified as a fifth-order cycle with a duration of 100 kyr. The obliquity cycle curve corresponds to the ultra-short-term base-level cycle, classified as a sixth-order cycle with a duration of 18.4 kyr. The maxima of the filtered curve were used as the basis for base-level cycle division. In combination with the Jurassic–Cretaceous boundary (ca. 145 Ma) at the base of the Yixian Formation, as constrained by U–Pb dating in comparable basins [48,49], this boundary was used as an anchor point. Incorporating relevant sequence stratigraphic theories, high-frequency cycle division and correlation were achieved in the study area.

As shown in Figure 4, a total of six 405 kyr long eccentricity cycles were identified from the long-eccentricity-filtered curve of Well Nai-10. These correspond to six fourth-order sequences. These cycles align with six high-energy clusters in the wavelet time–frequency spectrum, with each cycle having an approximate thickness of 35 m, representing six medium-term base-level cycles. Similarly, the short eccentricity filtering curve identified 24 short eccentricity cycles of 100 kyr, each with a cycle thickness of approximately 9 m, corresponding to 24 short-term cycles. The sedimentation duration is calculated by multiplying the number of identified cycles by the duration represented by each cycle. Using the bottom age of the Yixian Formation as an anchor point and referencing the short eccentricity cycle filtering curve, the deposition duration of the studied interval was estimated to be approximately 2.4 Ma from bottom to top. Based on the sedimentation rate of each short eccentricity cycle, the average sedimentation rate of the lower Yixian Formation was calculated to be 8.36 cm/kyr. Additionally, the filtering curve indicates that, in terms of orbital parameter modulation, when the peak amplitude of the long eccentricity (405 kyr) filtering curve is high, the short eccentricity (100 kyr) amplitude also increases, as observed near the first, second, fifth, and sixth long eccentricity cycles. Moreover, obliquity and precession exhibit the same pattern, demonstrating the modulation effect of long-period cycles on short-period cycles.

4.4. The Response of Paleoclimatic Environments to Astronomical Cycles

Comparative data analysis reveals that eccentricity cycles at both the 405 kyr (long) and 100 kyr (short) scales exert significant control over variations in the paleo-lake environment, mineral composition, and organic matter accumulation. The long and short eccentricity curves show strong correlations with the ratios of paleoclimatic proxy indicators and total organic carbon content.

Long-term eccentricity primarily governs climatic variability. During the half-cycles corresponding to its maxima, the climate tends to be relatively warm and humid, whereas, during minima, it becomes colder and drier. As a result, lake basins experience deeper water levels and more reducing conditions during eccentricity maxima, which, in turn, leads to elevated geochemical ratios such as Rb/Sr, V/(V + Ni), and Fe/Mn. These conditions favor the formation of minerals under reducing environments and promote the effective accumulation of organic matter. In contrast, variations in short-term eccentricity are associated with enhanced seasonal dynamics, such as intensified monsoonal precipitation. This process increases terrestrial input via rivers, which transports abundant nutrients and enhances lake productivity. Under reducing conditions in deeper parts of the water column, organic matter preservation and pyrite formation are also favored. Consequently, higher concentrations of total organic carbon (TOC), clay minerals, and pyrite are commonly observed. Comparative analysis (Figure 5 and Figure 6) shows that intervals near the maxima of short-term eccentricity cycles correspond to increased proxy values and greater organic matter abundance. During the warm humid half-cycle, layers associated with short eccentricity maxima exhibit average values of 0.65 for Rb/Sr, 0.55 for V/(V + Ni), and 53 for Fe/Mn, a clay mineral content exceeding 70%, an average pyrite content of 5.3%, and an average TOC of 4.2%. In contrast, during short eccentricity minima, the average Rb/Sr declines to 0.38, V/(V + Ni) to 0.31, and Fe/Mn to 38, the clay mineral content remains above 40%, the pyrite content averages 4.1%, and the TOC falls to 3.3%—significantly lower than during the maxima (Figure 6). A similar pattern is also observed during the cold dry half-cycle (Figure 6).

5. Discussion

Organic Matter Preservation Patterns Controlled by Astronomical Cycles

The trace elements, organic matter abundance, and mineral composition of lacustrine sediments undergo periodic changes in response to variations in paleoclimate, paleo-water depth, and sediment supply [50,51], leading to differences in organic matter accumulation patterns at different stages. Earth’s orbital parameters to some extent regulate the patterns of Earth’s climate changes. When eccentricity approaches its maximum, Earth’s orbital trajectory becomes more elliptical, increasing the difference between perihelion and aphelion. This enhances the precession amplitude regulated by eccentricity, leading to greater seasonal variations, with shorter winters and prolonged summers. As a result, Earth’s surface receives more solar radiation, leading to a warmer climate. At the same time, surface evaporation increases, precipitation becomes more abundant, and surface runoff intensifies, transporting a large number of nutrients into the lake basin, which benefits algae and plankton growth, significantly enhancing lake productivity. Laminated and bedded structures are more likely to develop under strong seasonal variations (Figure 7). During this period, some Fe(II) remains in deep water, undergoing rapid deposition without immediate oxidation, leading to its rapid accumulation in the deeper parts of the lake basin. When encountering hydrogen sulfide retained at the lake bottom, Fe(II) reacts to form pyrite precipitates, resulting in higher pyrite concentrations. Additionally, an abundant sediment supply provides a solid material foundation for organic matter accumulation. A higher lake level provides ample space for organic matter accumulation. The weakly reducing conditions of the bottom water in the lake basin create favorable conditions for the burial and preservation of organic matter. The thin sections of mudstone samples near the eccentricity maximum primarily show dark mudstone with well-developed organic-rich laminae (Figure 8a–d). Therefore, fine-grained strata deposited under warm humid climatic conditions exhibit high organic matter abundance and significant hydrocarbon generation potential.

When eccentricity approaches its minimum, Earth’s orbital trajectory becomes nearly circular, reducing the difference between perihelion and aphelion. The precession amplitude, regulated by eccentricity, also decreases, resulting in weaker seasonal contrasts characterized by prolonged and colder winters and shorter summers. As a result, Earth’s surface receives relatively less solar radiation, leading to a drier and colder climate. Laminated or platy structures tend to develop under weak seasonal variations (Figure 9). At this stage, surface evaporation decreases, precipitation diminishes, surface runoff declines, and river transport capacity weakens, leading to reduced sediment supply. The paleo-water body becomes shallower, algae and plankton populations decline, and lake productivity decreases. Meanwhile, in relatively shallow water zones, Fe(II) is rapidly oxidized to Fe(III) due to the oxidizing environment, reducing the reaction between Fe(II) and sulfur ions and thereby decreasing pyrite precipitation. Additionally, the reduced sediment supply limits organic carbon input, and the arid climate inhibits algae and plankton growth, resulting in relatively low TOC content in the corresponding sedimentary strata, which are characterized by laminated silty mudstone and massive calcareous mudstone. The thin sections of mudstone samples near the eccentricity minimum primarily show sandy mudstone with poorly developed organic-rich laminae (Figure 8e–h).

6. Conclusions

(1) During the sedimentation of the Yixian Formation in the Naiman Sag, the paleoclimatic environment exhibited periodic changes. In the core section between 2045 m and 2075 m, four periods of relatively warm and humid reducing environments were observed in the lake basin, accompanied by four corresponding phases of organic matter enrichment.

(2) The eccentricity cycle is in good agreement with the climate fluctuation cycle, indicating a significant influence of astronomical orbital cycles on the fine-grained sedimentary strata of the Yixian Formation. The strata preserve well-defined Milankovitch cycles, with a total of 6 long eccentricity cycles, 24 short eccentricity cycles, 60 obliquity cycles, and 121 precession cycles identified.

(3) The eccentricity cycle exerts a significant regulatory effect on the precession cycle, influencing the distribution patterns of lacustrine mudstones and shales. A long eccentricity cycle can be divided into a warm humid half-cycle and a cold dry half-cycle. During the warm humid half-cycle, the climate is relatively mild with distinct seasons, increased precipitation, enhanced surface runoff, and strong terrestrial input, making it an ideal period for organic matter enrichment. In contrast, the cold dry half-cycle features a colder and drier climate, reduced precipitation, limited surface runoff, and weaker terrestrial input, which is unfavorable for organic matter accumulation. Based on the relationship between astronomical orbital cycles and lacustrine fine-grained sedimentation, two ideal depositional models for lacustrine mudstones and shales were developed, providing insights into the deep lacustrine fine-grained sedimentation process.