The Zhujiang Formation in the Baiyun Sag, Pearl River Mouth Basin, China, is formed primarily in a deep-water continental slope environment. Its chronostratigraphic framework is based on biostratigraphy and sequence stratigraphy, and its geological dating is based on micropaleontological data. This makes it difficult to obtain precise absolute ages for various geological events. In this study, gamma ray (GR) well log data from Wells Y1, Y2, and Y3 were used as paleoclimate proxies, and spectral and wavelet analyses were used to conduct cyclostratigraphic research. The results show that the Milankovitch cycles were preserved in the Zhujiang Formation in the Baiyun Sag. Stratigraphic cycles controlled by 405 and 95 ka orbital eccentricity, 40.4 ka orbital obliquity, and 23.5 ka orbital precession cycles can be identified; the signal of stratigraphic cycles controlled by the 405 ka long eccentricity cycle is the strongest. The floating astronomical time scale is constructed based on 405 ka orbital eccentricity cycle tuning of the GR series. The precise durations of the Zhujiang Formation in Wells Y1, Y2, and Y3 are 7.13, 6.93, and 7.18 Ma, and the average deposition rates are 4.68, 5.91, and 5.33 cm/ka, respectively. The Zhujiang Formation was divided into 17 fourth-, 76 fifth-, and 174 sixth-order cycles using the 405, 95, and 40.4 ka orbital periods as the dividing scales, respectively. This study provides a quantitative method for high-precision isochronous stratigraphic division and correlation in deep-water sedimentary systems.
The Milankovitch theory is an astronomical theory involving the relationship between insolation and Earth’s climate. The theory states that periodic changes in Earth’s orbital parameters (eccentricity, obliquity, and precession) can cause periodic changes in insolation, which lead to climate changes and corresponding changes in sedimentary characteristics and the environment . The strata controlled by Earth’s orbital cycles record the Milankovitch cycles. By astronomical tuning of these cycles, a continuous astronomical time scale (ATS) with a resolution of 20–400 ka can be obtained in cyclostratigraphy, which can accurately determine the geological ages and durations of various geological and biological events and realize stratigraphic division and correlation with high precision [2–4]. At present, cyclostratigraphic research results have been obtained from Paleozoic and older strata [5–8] as well as from Mesozoic and Cenozoic strata [9–12]. These strata range from marine to continental [13–16].
The Pearl River Mouth Basin is a Cenozoic petroliferous basin. Few paleomagnetic and isotopic dating data are available for the Neogene. Most researchers have established a chronostratigraphic framework through biostratigraphic and sequence stratigraphic studies [17–22]. The age of the Neogene in the Pearl River Mouth Basin is primarily based on micropaleontological data [19–21]. The age framework was discontinuous and inaccurate because paleontological samples were obtained from drill cuttings. The well log data can be used as a paleoclimate proxy to perform cyclostratigraphy studies, which can be used to obtain high-resolution geological age scales. Liu et al. and Tian et al. conducted related cyclostratigraphy studies using gamma ray (GR) well log data in the Huizhou Sag of the Pearl River Mouth Basin [23–25].
The Baiyun Sag of the Pearl River Mouth Basin has abundant oil and gas resources, and the Zhujiang Formation is an important exploration target layer. The Zhujiang Formation of the Baiyun Sag was primarily formed in a deep-water continental slope environment, and it is difficult to divide and correlate the strata. In this study, we conducted cyclostratigraphy research of the Zhujiang Formation in the Baiyun Sag to establish a high-precision chronostratigraphic framework. Considering that factors such as hiatuses and lateral changes in strata thickness and sedimentary facies led to incomplete records of Milankovitch cycles, we selected three horizontally contrastable drilled wells (Y1, Y2, and Y3) for a comparative study to identify the Milankovitch cycles. We established a high-resolution ATS of the Zhujiang Formation in the Baiyun Sag by astronomical tuning to obtain the sedimentation time and rate. Different Milankovitch cycles were selected as scales to divide high-frequency cycles. This study provides a quantitative method for high-precision isochronous stratigraphic division of deep-water sedimentary systems.
2 Geological background
The Pearl River Mouth Basin is a Cenozoic sedimentary basin located in the northern South China Sea at 18°30′–23°30′N and 113°10′–118°00′E (Figure 1a). It is a typical passive continental margin basin. Its tectonic and sedimentary evolution can be divided into a rift valley stage from the Paleocene to the Eocene, a depression stage from the Oligocene to the Middle Miocene, and a stable subsidence stage from the Late Miocene to the present . The tectonic units are distributed in a southwest to northeast direction, and from north to south, the northern uplift belt, northern depression belt, central uplift belt, central depression belt, southern uplift belt, and southern depression belt occur successively . The Baiyun Sag is located in the eastern part of the central depression belt and covers an area of approximately 25,500 km2 (Figure 1b). It is the largest sedimentary depression in the Pearl River Mouth Basin and contains extremely thick deposits of fluvio-lacustrine, marine-terrestrial transitional, and shallow-deep sea facies.
The Cenozoic strata in the Baiyun Sag, from bottom to top, are the Paleogene Shenhu Formation, Wenchang Formation, Enping Formation, and Zhuhai Formation; the Neogene Zhujiang Formation, Hanjiang Formation, Yuehai Formation, and Wanshan Formation; and the Quaternary strata. The Zhujiang Formation is composed primarily of deep-water continental slope deposits (Figure 2). The lower member is composed of interbeds of light gray sandstone, gray mudstone, and silty mudstone with thin layers of limestone. The upper member is thick gray and silty mudstones interbedded with light gray sandstone and siltstone. In the seismic reflection profile, the Zhujiang Formation is located between T40 and T60. Four calcareous nannofossil zones (NN1–NN4) and five planktonic foraminiferal zones (N4–N8) were found in the Zhujiang Formation [19–21,27]. In the study area, the last downhole occurrence of planktonic foraminifera Globigerina ciperoensis (age 22.9 Ma) was used as the marker of the Paleogene/Neogene boundary , and the last downhole occurrence of the calcareous nannofossil Helicosphaera ampliaperta (age 15.87 Ma) was used as the marker of the Lower and Middle Miocene boundary . Therefore, the Zhujiang Formation was formed in the Early Miocene.
Based on seismic, logging, core, and paleontological data, the Zhujiang Formation in the Baiyun Sag was divided into five third-order sequences (SQ1–SQ5) (Figure 2). During the sequence SQ1, the northern section of the Baiyun Sag accumulated continental shelf marginal delta deposits, and the middle and southern sections accumulated neritic shelf deposits. During the sequence SQ2, the northern section of the Baiyun Sag was composed of delta and neritic shelf deposits, and the middle and southern sections formed in a deep-water continental slope environment, with deep-water fan, gravity flow, and bathyal mudstone deposits. Compared with those in SQ1, the area of delta deposits decreased, and the area of neritic shelf deposits increased during SQ2, reflecting an obvious transgression process. During the SQ3 development period, the rate of relative sea-level fall was small, and the provenance supply was insufficient. The shelf marginal delta deposition did not develop. The northern section of the Baiyun Sag contained neritic shelf deposits, while the middle and southern sections contained bathyal deposits. During the SQ4 development period, the facies belt changed little, consisting of neritic and bathyal mudstone deposits, reflecting a relatively stable or slow fall in sea level. During SQ5, deltas developed on the northern section of the Baiyun Sag, and the middle and southern sections accumulated neritic and bathyal mudstone deposits.
3 Data and methods
3.1 GR well log data
The GR well log data can indicate variation in shale and organic matter content in the strata, which is controlled by the sedimentary environment and sea level change [28,29]. Therefore, GR data can be used as a good paleoclimate proxy . The GR data were obtained from Wells Y1, Y2, and Y3 in the southern Baiyun Sag. The Zhujiang Formation of the three wells is a neritic or bathyal facies with complete sedimentary records and no obvious hiatus. The GR data of the three wells were equally spaced with a spacing of 0.1524 m. Although the GR data can reflect the periodic changes in strata caused by various geological factors, the data also contain signals independent of geological factors. Therefore, the data should be preprocessed to eliminate these signals. We used the following two steps to process the data: first, outliers that were much greater than or less than the mean value were eliminated and replaced with the mean value; then, the data were detrended to eliminate low-frequency background noise.
3.2 Time series analysis
To accurately identify the main stratigraphic cycles, a method combining spectral and wavelet analyses was adopted, and the results of the three wells were compared to determine the stratigraphic cycles. Both spectral and wavelet analyses were performed using the MATLAB software. Multitaper method (MTM) spectral analysis was used to transform the data from the depth domain to the frequency domain. The abscissa in the spectrum diagram represents the frequency, the reciprocal of which is the thickness of the corresponding stratigraphic cycle. The ordinate in the spectrum diagram is the relative power, and a larger value leads to the specific sedimentary cycle appearing more frequently. We chose spectral peaks above 95% confidence and selectively used spectral peaks between 90 and 95% confidence. A continuous wavelet transform was used in the wavelet analysis. Wavelengths with high power and relative continuity in the wavelet spectrum are the thicknesses of dominant stratigraphic cycles. The digital filtering method was used to obtain the stratigraphic cycles of different periods using a Gaussian bandpass filter on the preprocessed GR data. The La2010d orbital solution was established for 15.97–23.03 Ma as an accurate astronomical target .
4.1 Theoretical orbital cycles
Based on the Geologic Time Scale 2012 (GTS2012) , the age of the Lower Miocene is 15.97–23.03 Ma. MTM spectral analysis of the standardized eccentricity, tilt and precession (ETP), which is the sum of the normalized eccentricity, normalized obliquity, and negative normalized precession, was performed to obtain the theoretical orbital cycles for this period (Figure 3a). Spectral analysis shows that the theoretical orbital cycles are the 1,000 ka ultralong, 405 ka long, and 125 and 95 ka short eccentricity cycles; 53.0, 40.4, and 39.4 ka obliquity cycles; and 23.5, 22.2, and 18.9 ka precession cycles (Figure 3b). The main theoretical orbital cycles of the Early Miocene are the 405 and 95 ka eccentricity, 40.4 ka obliquity, and 23.5 ka precession cycles. The proportion of the main theoretical orbital cycles was used as a benchmark for the analysis of the Milankovitch cycles.
4.2 Determination of Milankovitch cycles
Milankovitch cycles were determined by comparing the thicknesses of the dominant stratigraphic cycles with theoretical orbital cycles. If the ratio of the dominant stratigraphic cycle thickness is the same as the ratio of the theoretical orbital cycles, it indicates that the stratigraphic cycles are driven by astronomical orbital cycles, and the Milankovitch cycles are preserved in the strata [33,34]. Based on the strata thickness and paleontological ages, the average sedimentation rates of the Zhujiang Formation in Wells Y1, Y2, and Y3 were estimated to be 4.7, 5.8, and 5.4 cm/ka, respectively, which were used as the constraint conditions for determining the Milankovitch cycles.
The depth of the Zhujiang Formation in Well Y1 is 2869.1–3202.8 m. There are significant spectral peaks at 17.94, 7.94, 4.20, 3.28, 2.11, 1.99, 1.78, 1.70, 1.55, and 1.05 m. The ratios of 17.94, 4.20, 1.78, and 1.05 m are fairly consistent with the ratios of the 405, 95, 40.4, and 23.5 ka cycles (Figure 4a, Table 1). The wavelet spectrum shows that the thicknesses of the dominant stratigraphic cycles are consistent with the results of spectral analysis (Figure 4b), suggesting that the deposition of the Zhujiang Formation is controlled by astronomical orbital cycles. Thus, based on the estimated average sedimentation rate of 4.7 cm/ka from biostratigraphic data, the 17.94 and 4.20 m cycles correspond to the 405 and 95 ka eccentricity cycles, the 1.78 m cycle correspond to the 40.4 ka obliquity cycle, and the 1.05 m cycle correspond to the 23.5 ka precession cycle. From the spectral diagram, the amplitude of the 17.94 m cycle is the largest, indicating that the Zhujiang Formation was primarily controlled by the 405 ka long eccentricity cycle.
|Well||Depth (m)||Frequency||Cycle thickness (m)||Cycle thickness ratio||Orbital cycles (ka)||Orbital cycles ratio|
The results of spectral and wavelet analyses of the GR data from the Zhujiang Formation in Well Y2 show that the ratios of the dominant cycle thicknesses of 25.0, 8.23, 6.25, 2.52, and 1.56 m are consistent with the ratios of the orbital cycles of 405, 125, 95, 40.4, and 23.5 ka (Figure 4c and d, Table 1). Thus, based on the estimated average sedimentation rate of 5.8 cm/ka, the 25.0, 8.23, 6.25, 2.52, and 1.56 m cycles correspond to the 405, 125, 95, 40.4, and 23.5 ka orbital cycles, respectively.
Spectral and wavelet analyses of the GR data from the Zhujiang Formation in Well Y3 show that the dominant stratigraphic cycle thicknesses are 21.98, 5.01, 4.07, 3.09, 2.20, and 1.23 m, and the ratios of 21.98, 5.01, 2.20, and 1.23 m are similar to the ratios of the orbital cycles of 405, 95, 40.4, and 23.5 ka (Figure 4e and f, Table 1). Based on the estimated average sedimentation rate of 5.4 cm/ka from biostratigraphic data, the stratigraphic cycles of 21.98, 5.01, 2.20, and 1.23 m are controlled by the 405 ka long eccentricity, 95 ka short eccentricity, 40.4 ka obliquity, and 23.5 ka precession cycles. The signal of the 21.98 m cycle is the strongest.
A comparison of Wells Y1, Y2, and Y3 shows that the Zhujiang Formation has preserved sedimentary cycles controlled by the 405 ka long eccentricity, 95 ka short eccentricity, 40.4 ka obliquity, and 23.5 ka precession cycles (Table 1), and the signals of the stratigraphic cycles controlled by the 405 ka long eccentricity cycles are the strongest. Therefore, the Zhujiang Formation deposition in the Baiyun Sag was controlled by Earth’s orbital cycles.
4.3 The floating ATS
The 405 ka long eccentricity cycle has been relatively stable in the Cenozoic [35,36], and they are the main cyclic factors controlling the Zhujiang Formation in the Baiyun Sag. Therefore, the 405 ka long eccentricity cycle was selected as the target curve for astronomical tuning to construct the floating ATS. Eccentricity affects the insolation of the entire Earth, and ice ages develop during the orbital eccentricity minima . High GR values correspond to layers with high shale and organic contents, reflecting warm and humid climatic conditions. Thus, high GR values corresponded to high eccentricity values when performing astronomical tuning.
We bandpass filtered the 17.94 m cycles of GR data from the Zhujiang Formation in Well Y1 using a Gaussian bandpass filter (passband: 0.05575 ± 0.0035) and recognized seventeen 17.94 m cycles, expressed as E1–E17 (Figure 5a). Tuning the 17.94 m cycles to the 405 ka orbital cycles, a 7.13 Ma long floating ATS was constructed (Figure 5b). Therefore, the average sedimentation rate of the Zhujiang Formation in Well Y1 was 4.68 cm/ka. The MTM spectral analysis of the 405 ka-tuned GR time series of the Zhujiang Formation shows peaks with confidence greater than 95% at 405, 173, 95, 53, 47, 41, 39.4, 23.5, and 22.2 ka, and the 405 ka cycle peak is the maximum (Figure 6a), which is consistent with the theoretical orbital cycles, indicating that the floating ATS is reliable.
Similarly, a 6.93 Ma long ATS was constructed for the Zhujiang Formation in Well Y2 based on tuning the 25.0 m cycles to the 405 ka orbital cycles (Figure 5c and d), with an average sedimentation rate of 5.91 cm/ka. A 7.18 Ma long ATS was constructed for the Zhujiang Formation in Well Y3 based on tuning the 21.98 m cycles to the 405 ka orbital cycles (Figure 5e and f), with an average sedimentation rate of 5.33 cm/ka. The MTM spectral analysis of the 405 ka-tuned GR time series of the Zhujiang Formation in these two wells shows significant peaks at 405, 125, ∼100, 40.4, 39.4, 23.5, and 22.2 ka, which are consistent with the theoretical orbital cycles (Figure 6b and c).
5.1 The Zhujiang Formation age model
Based on the floating ATS, the sedimentary durations of the Zhujiang Formation in Wells Y1, Y2, and Y3 were 7.13, 6.93, and 7.18 Ma, respectively. These results are consistent with the biostratigraphic data. The maximum difference is 250 ka, which is a reasonable uncertainty and less than the error of one 405 ka orbital cycle for astronomical calibration of the geologic time scale. This shows that the floating ATS of the three wells can be used for high-precision strata division and correlation in the Baiyun Sag.
The 405 ka-tuned GR time series provides a 7.18 Ma long floating ATS for the Zhujiang Formation in Well Y3. Considering the base of the Zhujiang Formation at 3,128 m in Well Y3 as the Oligocene/Miocene boundary and anchored at 23.03 Ma based on the GTS2012 (Figure 7), the ATS age at the top of the Zhujiang Formation is 15.85 Ma, which is similar to the biostratigraphic age of 15.87 and 0.12 Ma later than the GTS2012 assigned age of 15.97 Ma for the boundary of the Early and Middle Miocene . By comparing the seventeen 21.98 m cycles (E1–E17) of the GR series with the 41–57th 405 ka long eccentricity cycles (E40541–E40557) of the La2010d astronomical solution, a corresponding relationship was revealed (Figure 7), indicating that the ATS has high credibility.
5.2 Establishment of chronostratigraphic framework
ATS is continuous and has high precision; thus, it can be used to accurately date geological and biological events. The ATS ages for the tops of third-order sequences SQ1–SQ5 are 22.6, 22.1, 18.56, 16.89, and 15.85 Ma, respectively (Figure 7).
The abundance of planktonic foraminifera and calcareous nannofossils in the Zhujiang Formation of the Baiyun Sag was high. The first occurrence (FO) surface of planktonic foraminifera Globorotalia sicanus and the last occurrence (LO) surface of planktonic foraminifera Catapsydrax dissimilis, Globoquadrina binaiensis, and G. ciperoensis were found in the Zhujiang Formation in Well Y3 at depths of 2,788, 2,838, 2,898, and 3,124 m, respectively. According to the ATS, the four biological events are dated at 16.53, 17.39, 18.56, and 22.97 Ma, respectively (Figure 7). The LO surfaces of calcareous nannofossils Sphenolithus belemnos, Triquetrorhabdulus carinatus, and Disctyococcites bisectus have been found in the Zhujiang Formation in Well Y3 at depths of 2,826, 2,928, and 3,124, respectively. According to the ATS, the three biological events are dated at 17.18, 19.08, and 22.97 Ma, respectively (Figure 7). A comparison of the ATS ages of these seven biological events with those of their predecessors reveals that there is a difference of 0.06–0.72 Ma [18,38–40].
5.3 Division of high-frequency cycles
The high-frequency cycles are controlled by climate and sea-level changes caused by the Milankovitch cycles. The fourth-, fifth-, and sixth-order cycles were controlled by long eccentricity (405 ka), short eccentricity (∼100 ka), and obliquity cycles (∼40 ka), respectively. Selecting different Milankovitch cycles as the standard for the division of high-frequency cycles can realize isochronicity and improve the accuracy of the stratigraphic division and correlation.
Based on the above spectral analysis, the center frequency and bandwidth of the 405 ka long eccentricity cycle in Well Y3 are 0.0455 and 0.01, respectively; those of the 95 ka short eccentricity cycle are 0.1996 and 0.04, respectively; and those of the 40.4 ka obliquity cycles are 0.4549 and 0.05, respectively. Milankovitch curves of different frequency bands were obtained using Gaussian bandpass filtering of the preprocessed GR data. These Milankovitch curves were used as reference curves for the division of the fourth-, fifth-, and sixth-order cycles. Combined with the core and logging data, we divided the Zhujiang Formation of Well Y3 into 17 fourth-, 76 fifth-, and 174 sixth-order cycles (Figure 8).
In this study, by conducting spectral and wavelet analyses to conduct cyclostratigraphic research, a floating ATS was constructed based on 405 ka orbital eccentricity cycle tuning of the GR series in the Zhujiang Formation. Based on the ATS, sequence stratigraphy and biological events were accurately dated, and a high-precision chronostratigraphic framework of the Zhujiang Formation in the Baiyun Sag was established. The following conclusions were drawn:
The Zhujiang Formation in the Baiyun Sag records Milankovitch cycles. The stratigraphic cycles controlled by the 405 and 95 ka eccentricity cycles, 40.4 ka obliquity cycles, and 23.5 ka precession cycles are identified, and the signal of the stratigraphic cycles controlled by the 405 ka long eccentricity cycles is the strongest.
Based on the ATS, the precise durations of the Zhujiang Formation in Wells Y1, Y2, and Y3 are 7.13, 6.93, and 7.18 Ma, respectively. The average sedimentation rates of the Zhujiang Formation in Wells Y1, Y2, and Y3 are 4.68, 5.91, and 5.33 cm/ka, respectively.
The ATS ages of the top of sequences SQ1–SQ5 are 22.6, 22.1, 18.56, 16.89, and 15.85 Ma, respectively. The FO ATS ages of the planktonic foraminifera G. sicanus and those of the LO of the planktonic foraminifera C. dissimilis, G. binaiensis, and G. ciperoensis in Well Y3 are 16.53, 17.39, 18.56, and 22.97 Ma, respectively. The LO ATS ages of calcareous nannofossils S. belemnos, T. carinatus, and D. bisectus in Well Y3 are 17.18, 19.08, and 22.97 Ma, respectively.
Based on the Milankovitch theory, the Zhujiang Formation in the Baiyun Sag can be divided into 17 fourth-, 76 fifth-, and 174 sixth-order cycles.
The authors are grateful to the anonymous reviewers for their valuable comments, and the editors for their careful revisions. All these helped to improve the manuscript significantly.
Funding information: This research was supported by the National Natural Science Foundation of China (Grant No. 41472098) and the National 13th Five-Year Plan’s Major Science and Technology Program of China (Grant No. 2017ZX05032-002-002).
Author contributions: Shangfeng Zhang contributed to the conception of the study and approved the final version of the article for publication, Ping He analyzed the data and wrote the manuscript, Enze Xu helped perform the analysis with constructive discussions, and Chunxia Zhu contributed to the data collection.
Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.
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