A Chronological Study of the Miocene Shanwang Diatomaceous Shale in Shandong Province, Eastern China

Abstract

The varve chronological approach has been applied to older ages (pre-Quaternary) in the Shanwang Basin for the first time. This study focuses on the analysis of diatom shale from the Shanwang Basin, which was formed in Maar Lake (a volcanic crater basin, often filled by a lake). The lacustrine sediments of the basin encapsulate comprehensive geological information. By identifying species and providing systematic paleontological descriptions of diatoms in the profile, two genera and seven species were recognized. A microscopic examination of the thin sections revealed five types of laminae and couplets. On this basis, the study adopted the artificial semi-automatic counting method of the laminae. The calculation results show that the age of the diatom shale section in the Shanwang Basin is 18.524–17.985 Myr B.P, the deposition time is 0.54 Myr, and the deposition rate is 4.06 cm/Kyr. Finally, through the comparative discussion of various dating methods, it can be concluded that the varve chronology is a more accurate and reliable dating method than other dating methods. The research findings contribute to our understanding of the geological history of the region.

1. Introduction

The existence of chronological history reflecting human social development and geological age tables concerning Earth’s evolution underlines the significant role of the time scale. The initial definition of a varve was proposed by Swedish geologist De Geer in 1910 during his study of glacial lakes. It refers to a sedimentary combination composed of two distinct layers [1]. With the deepening of the study, the laminae extended to all sedimentary environments with “year” cycles [2,3,4,5,6,7,8]. Since the structure and composition of varves are influenced by lake hydrology, climate, biology, etc., different lakes have different varves [9]. Therefore, the thickness, composition, and structural changes of the varves provide rich information on paleoenvironmental changes. Continuous laminae sequences can establish an accurate varve chronology, provide reliable time scales for the accurate reconstruction of climate events, and also provide an important basis for high-resolution paleoenvironmental research [1,2,3,4,10]. In addition, the sun is the main external driver of the Earth’s climate and environmental change, and the formation of the varve is controlled by the Earth’s seasonality as well as climate change [11]. In turn, the varve can serve as a good geological vector for restoring climate change in sedimentary basins.

There have been relatively few studies of older (i.e., pre-Quaternary) chronostratigraphic layers [12]. The Shanwang Basin is a lacustrine sedimentary basin with the relatively complete preservation of pre-Quaternary grain layers, making it ideal for chronostratigraphic studies. The horizontal laminae involving a continuous laminae dominating appears in the diatom shale portion of the Shanwang Basin. The lacustrine laminae are mainly formed by periodic changes in sediment components caused by terrestrial inputs, chemical conditions, and spatiotemporal variations in biological activities [13]. The diatom laminae in the Shanwang Formation are mainly composed of diatoms, which are the result of the seasonal prosperity of diatoms. Owing to the nature of Maar Lake in the Shanwang Basin, there is less contact with the environment outside the lake, which leads to a general tendency for the water environment of the entire lake to be calmer. This tremendously benefits the production and preservation of the varve.

Based on the results of previous studies on the chronology of the Shanwang Basin, this article uses the method of varve chronology to calculate time as well as identify and classify the annual lamination in the Shanwang Basin for the first time. It also briefly analyzes the current state of the study and talks about the traits and preservation factors of lacustrine varve. On this basis, the research value of lacustrine varve is thoroughly explored, and the deposition time of the diatom shale section in the Shanwang Basin is precisely calculated depending on the type of varve, which further clarifies the age of the diatom shale section in the Shanwang Basin. This article systematically studies the dating methods of lacustrine varve before the Quaternary period, compares the results with those of other dating methods, and discusses the reasons for the different dating results. We hope to use this to promote the development of lacustrine varve research in domestic studies.

2. Geological Background

The Shanwang Basin is a nearly circular basin developed in the Miocene with an area of less than 1 km², located in the eastern part of the North China Craton and the middle part of the Tan-Lu fault zone (Figure 1) [14]. The Shanwang Basin is a national key paleontological protection area. A large number of animal and plant fossils [15,16] are in the diatomite layers of the Shanwang Basin, so it is known as the “Fossil Treasure House”.

According to actual observations, the old and new strata in the Shanwang Basin are, respectively, the Wanjiazhuang Formation of the Archean Taishan Group, the Neogene basic volcanic rock, and its sedimentary interlayer. The characteristics of these strata are very clear. Researchers have mostly applied the trifurcation method to these exposed layers. In this method, the surface layer of the Shandong Region is taken as a typical example. The rock layers are divided from bottom to top into the Niushan Formation, Shanwang Formation, and Yaoshan Formation [17] (Figure 2). The Shanwang Formation is not only the most important fossil production horizon but also gradually formed based on the basalt of the Niushan Formation. From the lower part of the formation, the corresponding components are mainly yellow sandstone and tuffaceous breccia, and these sandstones only appear in the marginal area of the basin without sufficient contact with the Niushan Formation, where Miocene vertebrate fossils have their lowest occurrence in this succession. In the middle of the formation, the main components are gray-white diatom shale, algal tuff, and marl layer, and there are many fossil types in the diatom shale. In the upper part of the formation, its main components are green mudstone, brown carbonaceous mudstone, and top sandy gravel. In this study, the diatomaceous shale in the lower to middle part of the Shanwang Formation is mainly selected for research (Figure 2).

This article fully combines other scholars’ research on the stratigraphy of the Shanwang Formation and completes the detailed division of the exposed diatomite layer (21.85 m) in the Shanwang Basin with the help of field measurements. The corresponding number of layers is 32 (Figure 3a,b), and the sequence number of each layer is set from bottom to top. Since the first layer at the bottom is not exposed, the field measurement work cannot complete the data measurement. After observing with professional tools, the unexposed portion is grey-black mud shale with the underlying basalt of the Niushan Formation.

3. Testing Methods

3.1. Method of Making Thin Sections

To better observe the stratification and count, suitable rock samples are selected and sent to the laboratory for thin-section production. Choosing rock samples from the adjacent layers above and below allow for convenient comparative research. Over 100 slices are produced for this study. The key steps are as follows:

Step 1: Select the rock samples with superior laminae and assign numerical identification according to the horizon and segment into 10 × 20 × 30 mm square blocks. The samples are relatively loose and fragile in the study area. To ensure smooth cutting and sectioning, the samples are pre-treated using methyl methacrylate and initiator benzoyl peroxide as binders.

Step 2: Clean the samples, mainly to wash off the dust and other impurities that may have contaminated the sample during storage.

Step 3: Place the samples in a drying oven and slowly heat them to 70–110 °C to dry out the moisture from both the surface and the interior of the sample.

Step 4: After removal, observe the binding condition of the sample and then roughly grind the sample to a thickness of 1–2 mm.

Step 5: Attach the sample to the glass slide using Canadian adhesive or resin adhesive (if the sample bonding is not good, boil it with Canadian adhesive for better results). Place the sample on a grinding plate and finely grind it to a thickness of 0.03 mm.

Step 6: Clean the ground sample thoroughly, cover it with a cover glass, stick and fix it with adhesive, and then place it in a drying oven for drying. The production of thin slices is successful.

3.2. Purification Method of Diatoms

The diatoms are separated into genera and species so that the morphology of the ones could be more clearly observed in the study region. This will enable us to comprehend the biological behaviors of the diatoms that are present in the study area and give us a solid foundation for investigating the Shanwang Basin’s Early Miocene palaeoclimate. Diatom samples from sediments are processed in this study using Håkansson’s (1984) approach. The key steps are as follows:

Step 1: Place the sample in an oven at 50~65 °C for drying. After completion, weigh 12 g of the sample and place it in a beaker. Add 10~15% HCl to the beaker containing the diatom sample and stir evenly to remove calcium from the sediment. After the sample fully reacts with hydrochloric acid, stir evenly and add distilled water, and let it stand for 12~24 h. Repeat adding distilled water three times to wash off any excess hydrochloric acid.

Step 2: Add a concentration of 30% H₂O₂ to the sample to remove organic matter. If the reaction is severe, add a small amount of distilled water or anhydrous ethanol. After the initial reaction between the sample and H₂O₂, heat it in a 60 °C constant temperature water bath for 1~2 h. Afterward, take the sample out of the water bath and observe it under a microscope. After the microscopic examination reaches the standard, wash it with distilled water three times (the washing step is the same as step 1).

Step 3: Add about three times the sample volume of anhydrous ethanol to the sample, stir evenly with a glass rod, apply evenly to the glass slide, drop 1~2 drops of Naphrax glue (refractive index 1.73), and cover the slide. Heat the prepared sheet on an electric heating plate (150~200 °C) until the bubbles inside the slice disappear. After the sheet cools down, store it in a sample box for identification. In addition, the samples obtained after experimental treatment can be made into scanning electron microscope-thin slices for observation.

4. Systematic Paleontology and Laminae of Diatom Shale Section in Shanwang Basin

4.1. Systematic Paleontology of Diatom Shale Section in Shanwang Basin

In the Miocene, the lower average annual temperature (about 9.5~11.3 °C) [18] in the Shanwang area of Shandong Province [19] was suitable for a shorter period of diatom growth and prosperity. Therefore, the prosperity of diatoms was significantly controlled by seasons and thus had seasonal significance. This research identified and classified diatom genera and species in the profile to better understand the genesis and types of diatom stratification in the diatomaceous shale segment of the Shanwang Basin. Numerous species and variants of the Centricae and Pennatae classes were recognized, and some diatoms are shown in Figure 4.

    Bacillariophyta

      Centricae

        Coscinodiscales

          Coscinodiscaceae

            Melosira Agardh

              Melosira islandica f. curvata vel spiralis (Her.) Müller

This kind of algae shell is cylindrical and is connected to a tight chain group. The chain body is curved, with a diameter of about 10.5 μm and a height of about 12 μm. Its morphological characteristics include a developed shell cover, thick wall, small false ring groove, slightly flat ring groove, deep ring body, shorter neck, and arranged dots in parallel in the longitudinal direction. The dots have two types of thicknesses. There are about eight dots in 10 μm, and each dot is about 8–10. The surface of the shell is flat, and the dots are thicker at the edge of the shell. The living environment is freshwater lakes, alkaline water bodies, and it relies on passive floating for survival.

              Melosira granulata var. curvata Grun

This kind of algae shell is cylindrical and is connected to a tight chain group with the spines at the edge of the shell disc. The chain body is curved, with a diameter of about 7.35 μm and a height of about 12.5 μm. Its morphological characteristics include a developed shell surface, thick wall, V-shaped ring groove, and obvious neck. Irregular dots are often square or round, with about 10–12 dots within 10 μm, and the surface of the shell is flat. The living environment is eutrophic freshwater lakes, alkaline water bodies, and it relies on passive floating for survival.

              Melosira islandica Müller var. islandica

This kind of algae shell is cylindrical and is connected to a tight chain group, with a diameter of about 10.5 μm and a height of about 12 μm. Its morphological characteristics include a developed shell cover, thick wall, small false ring groove, slightly flat ring groove, deep ring body, shorter neck, and the dots are arranged in parallel in the longitudinal direction. The dots have two types of thicknesses. The shape of the dots is generally round. There are about eight dots in 10 μm, and each dot is about 8–10. The surface of the shell is flat, and the dots are thicker at the edge of the shell. The living environment is freshwater lakes, alkaline water bodies, and it relies on passive floating for survival.

              Melosira distans (Her.) Ktz. var. distans

The algae shell is short and cylindrical and is connected to a tight chain group by the edge of the shell disc. The diameter is about 14 μm and the height is about 5.5–6 μm. Its morphological characteristics include a developed shell cover, thick wall, V-shaped ring groove, and obvious neck, and its irregular dots are often square or circular. There are about 12 dots within 10 μm. Each dot has about five dots, and the surface of the shell is flat. The living environment is freshwater lakes, alkaline water bodies, and it relies on passive floating for survival.

              Melosira granulata (Her.) Ralfs var. granulata

This kind of algae shell is cylindrical and is connected to a tight chain group with the spines on the edge of the shell disc. The diameter is about 7.35 μm and the height is about 12.5 μm. Its morphological characteristics include a developed shell cover, thick wall, V-shaped ring groove, and obvious neck, and the irregular dots are often square or circular. There are about 10–12 dots within 10 μm. Each dot has about 10 dots, and the shell surface is flat. The living environment is freshwater lakes, alkaline water bodies, and it relies on passive floating for survival.

    Pennatae

      Araphidiales

        Fragilariaceae

          Fragilaria Lyngby

                 Fragilaria brevistriata Grun.

The shell surface is lanceolate, with an elongated beak-like shape at the end. It is 21–29 μm long and 3–5 μm wide. The horizontal lines are very short, with 12–14 within 10 μm, and the pseudo shell seam is wide and lanceolate. The living environment is mainly benthic, littoral (lakeshore), and epiphytic.

                 Fragilaria virescens Ralf. var. virescens

The shell surface is linear, with an elongated end resembling a beak, measuring 21–38 μm in length and 5–6 μm in width. The shell surface lines are arranged in parallel, with 14–17 lines within 10 μm, and a narrow pseudo ridge seam. The living environment is mainly benthic, littoral (lakeshore), and epiphytic.

The diatom shale section of the Shanwang Basin is mainly composed of Melosira, the main species of which are Melosira islandica, Melosira granulata, and their curved varieties, with fewer Melosira distans. They are freshwater planktonic types reflecting deeper water bodies. Melosira islandica and Melosira granulata require water bodies to be freshwater to weakly alkaline. Melosira islandica reflects a stable and cooler lake water period, with little or no external water flow. Fragilaria is a freshwater nearshore eutrophic species, preferring low temperature and slightly cold water bodies, indicating that the lake basin is not large in scope. Overall, diatoms prefer a low temperature and colder water bodies in the study area, belonging to the autumn thriving type.

4.2. The Laminae of the Diatom Shale Section in Shanwang Basin

4.2.1. Characteristics of Laminae

The section of diatomaceous shale presents a well-developed series of diatom laminae across the entire profile [20]. A macroscopic analysis of the second section of the Shanwang Formation demonstrated predominantly horizontal stratification (Figure 5). This observation indicates that the lake’s internal water environment remained placid, allowing for steady sedimentation.

In order to observe the laminae more effectively, a macro-to-micro approach was used. Initially, the laminae were collected in the field and then close observation and measurements were employed to identify centimeter-level laminae. Subsequently, the microscope observation of laminae in rock slices revealed millimeter-level or even micron-level laminae. The following is a description of some layers of the profile.

Layer 7 is a carbonaceous shale with a thickness of about 10 cm, which is located at 17.3 m in the profile. The profile appears gray-black with horizontal bedding. Close observation reveals alternating black and white laminations with thicknesses between 2 and 5 cm. Upon magnification, finer laminations are seen, but they are not prominent under the microscope (Figure 6). Quartz particles are scattered inside (Figure 7), suggesting aeolian deposition (deposited by wind).

Layer 11 is a black diatomaceous shale at a depth of 16 m. As shown in Figure 8, the portion is depicted as dark gray. After taking samples, distinct laminations with a thickness of about 5 cm were discovered. These were identified as diatom-organic matter laminations through a microscopic analysis.

At 14 m in the profile, the 13th layer is a gray diatom shale with a thickness of almost 1.5 m. The color appears gray-white on the profile. When the rock sample was examined, it was discovered to have 2–4 mm thick layers of black and white. Using a microscope, it was discovered that the laminae are visible and that micrometer scale laminae with a thickness of 100–300 μm are present (Figure 9).

The 20th layer is formed of gray-white diatomaceous shale with a thickness of approximately 1.2 m. It is situated at a depth of 8 m in the profile. The laminae have two nodules growing inside of them. The outcrop on the profile is off-white and grey. Black and white laminae are seen in the rock samples. Microscopical examination revealed the laminae to be distinct. The laminae of the diatom debris are recognized, and the debris is primarily composed of clay minerals (Figure 10).

Layer 22, at 6.3 m, is a gray-white diatomaceous shale. The diatom laminations in this layer are well-preserved. As shown in Figure 11, after field observation, the section appeared gray-white. An examination of rock samples revealed alternating dark and light laminations, with a thin and distinct lamination. Under the microscope, most of these laminations were annual, making them characteristic laminations of this profile.

4.2.2. Types and Characteristics of Couplet

The formations of different types of lamination couplets are mainly due to variations in diatom, clay, and organic matter content ratios, as well as the thickness of deposited single-type laminae.

An organic matter-detrital couplet contains virtually no diatoms or other biota. Their formation indicates that the environment and climate of this period were not suitable for the survival of diatoms and other organisms, or that diatoms were in dormancy. This led to the cessation of white diatom layer deposition, replaced by the regular input of terrigenous clay and brief thriving of other oligotrophic diatoms. The thickness of the organic matter layer is greater than that of the clastic layer, and the thickness of the whole layer is larger. This indicates that the number of organisms in the lake is less, and the deposition rate of lake sediments is larger under this condition. Under unipolar light, the black part of the thin section is composed of organic matter and the other part is debris. Upon magnification, no diatoms were observed in the thin section (Figure 12a).

The thick diatom-thin organic matter couplet is dominated by diatoms and organic matter. The formation of such laminae indicates that diatoms were in a prosperous period, and the supply of organic matter and nutrients were very sufficient. Adequate nutrients make the diatom layer develop thicker. The massive growth of diatoms consumes organic matter and nutrients inside the lake, resulting in more diatom laminae than organic laminae. It is also a manifestation of diatom prosperity. Through the observation of the thin section, it was found that the number of diatoms began to increase, while the content of organic matter decreased, and the number of diatom layers was more than the number of organic matter layers (Figure 12b).

The thick diatom-thin debris couplet is mostly composed of diatoms and other debris. Compared with the first two types of laminae, organic matter and nutrients decreased during this period. Observing the interior of the laminae revealed the diatom laminae are generally relatively thin. Compared to the diatom organic matter laminae, this implies that either the development of diatoms is gradually impeded, or the sedimentation rate of terrestrial fine-grained clay rises. Through thin section observation, it was found that the number of diatoms exceeds that of debris, and the organic matter content is already very low or almost non-existent (Figure 12c).

The thin diatom-thick organic matter couplet is dominated by diatoms and debris, and the thickness of the organic matter layer inside the couplet is greater than that of the diatom layer (Figure 12d). This couplet is located in the middle and lower part of the profile, and its thickness is very thin. Its appearance shows that diatoms begin to develop and reproduce due to the change in the climate and environment. Diatoms belong to the early stage of development and reproduction, and the number is not large. The thickness of diatom laminae is smaller than that of organic laminae.

Thin diatom-thick debris is mainly composed of diatoms and detritus. Inside the laminae, the thickness of the diatom laminae is less than that of the detrital laminae (Figure 12e). Its formation shows that the abundance of diatoms in the stratum continues to decrease compared with the thick diatom-thin clastic laminae, and the clastic material is deposited in large quantities. The lake environment is not suitable for the growth of diatoms, which slows down the growth rate of diatoms.

The organic layer is rich in organic matter, but no laminae. The thin sections observed under high magnification showed that the rock composition is relatively simple and uniformly distributed, and there is no laminae (Figure 12f).

5. Varve Chronology Dating Method and Calculation of Age

5.1. Varve Chronology Dating Method

The varve count is the key to the establishment of a varve chronology [2,21]. The accuracy of varve counting depends on the correct distinction of the type, composition, and structure of the varve [21]. Choosing a suitable counting method is the most important aspect of establishing a varve chronology. At present, two methods are mainly used for varve counting in the laminae age series recorded by the Varve Database (VDB) [22]: the first method is to use a digital camera to take high-quality images of fresh sediment, and then valve Photoshop to manually count the varve; the second method is to use the freeze-drying method-resin fixation to make large thin sections of sediment petrography, analyze the characteristics of the varve under the microscope, and carry out the manual counting of the varve. The photo counting method is a simple and feasible method for the layers with obvious marker layers or varve with high deposition rates. Zhou et al. (2007) used this method to count the number of layers of Sugan Lake sediments and measured the thickness of the layers [23].

The method of manual counting is often used in the varve count, which is also a commonly used method in the study of the varve chronology in the domestic arena [24,25]. At present, there are also foreign scholars who use automated [26] and semi-automated [27] software programs to count the laminae of fresh sediments or core thin sections’ scanned images, and obtain more reliable varve chronology data [22,28]. Compared to manual counting, automated counting methods provide a fast and objective counting method which can greatly reduce the time required for counting varves [21]. From a chronological perspective, the limitation of this technique is that it is only applicable to the counting of varve with clear “year” boundaries [29]. Lake sediments are affected by many factors, such as geology, hydrology, climate, and biology, and their developed laminae are sometimes irregular. Even in the same borehole, the characteristics of sedimentary laminae at different depths are not the same [23,24]. Therefore, semi-automatic counting can be used for varve sediments with obvious structural characteristics, regular development, and good preservation, but manual counting verification is still an indispensable link.

This study uses thin-slice lamination counting combined with the lamination quality of the response layer segments to calculate the lamination age and segment division of the profiles within this range. The counting work adopts the method of manual repeated counting of laminae. In order to make the laminae dating work more accurate and reliable, this study adopts the following methods:

(1)

According to the stratification of the section, the rock slices are observed under the microscope, the types of laminae displayed in each thin section are recorded, and the annual laminae are determined. By classifying the diatom species in the study area and combining the biological habits of diatoms, it is possible to identify these thin black and white laminae as seasonal striated mud deposits, with one black and one white layer forming a complete annual depositional rhythm [28]. These diatoms grow fastest in the autumn of a year (cool climate), and only once a year [30]. Through this characteristic of diatoms, it can be determined that a pair of layers in the diatom shale section of the Shanwang Basin is a varve. For example, the sedimentary combination formed by the combination of the diatom layer and organic matter layer in the diatom-organic matter layer is a varve.

(2)

Photos are taken of rock-thin sections through a microscope, the photos with obvious features (such as distinct layer types and easy counting, etc.) are selected, and then the selected photos are imported into CorelDRAW X6 software for laminae counting. The counting method is in millimeters, that is, the number of laminae are counted in the range of 1 mm (in actual operation, some photos failed to include all laminae within 1 mm, but the range of 500 μm could also be used for laminae counting) (Figure 13). When counting, the method of manually counting multiple times is used, and the final value that tends to be stable and the last two consistent counting results are taken. The average thickness of a single annual lamination is taken. Thicker layers can be measured by the function of marking the length of the line in CorelDRAW and combined with the standard scale in the photo.

(3)

The diatomaceous shale section of the Shanwang Basin is divided by observing the type of each layer. According to the type of laminae, lithology, and other factors, the section is divided into five parts. Then, the age is calculated based on the layer couple type and thickness of each part. The principle of division is: (1) The diatomaceous shale section is divided according to the type and thickness of the layer couple, and those of the same type and couplet thickness are divided into one section; different types of couplets but the same thickness of couplets are divided into one part. In this study, the two types of thick diatom-thin organic matter and thin diatom-thick organic matter in the profile are different, but the thickness of the couplets is very similar. The two types of couplets are divided into one part for age calculation. (2) Through microscopic observation, the mudstone at the bottom of the profile has a couplet structure, but the mudstone in the middle and upper part has no couplet or is not obvious. The deposition rate of the same lithology in the deposition is basically the same. In this study, the mudstone is divided into a part for age calculation. (3) There are also two layers of sandy mudstone in the profile.

(4)

According to the thickness of the varve in each layer, the deposition time of each layer is calculated, and the error correction of the dating results is carried out in combination with the laminae quality. Finally, according to the deposition time of each layer, the deposition time of the whole diatom shale section can be obtained.

5.2. Calculation of Age of Diatom Shale Section in Shanwang Basin

The thin diatom-thick debris couplet is composed of diatom and debris and distributed at the top and middle upper part of the profile. The lithology is mainly a grayish-white diatom shale and grayish-black diatom shale. The layers divided into this part are 30, 29, 27, 25, 24, 22, and 20, with a total thickness of 6 m. Viewed through a microscope, the layers that are obvious and easy to distinguish are selected as the marker layer (Figure 14), and the corresponding varves are found. After 10 times of manual counting within 200 mm scale calibration, the counting results tend to be stable and the last two results are consistent. The final number of laminae within 200 μm is 10. After calculating the average, we know that the thickness of the varve is about 20 μm, of which the diatom layer is about 12 μm thick and the detritus layer is about 8 μm thick. It can be calculated that the deposition rate in this layer is 1000/20 = 50 a/mm, that is, 50 Kyr/m, 2 cm/Kyr. The deposition time of this part is 50 Kyr/m × 6 m = 300 Kyr.

The main components of the thick diatom-thin debris couplet are diatoms and detrital clay minerals. According to the laminae, it can be shown that the development of diatoms is more prosperous at this stage (Figure 13). The lithology is gray-white diatom shale, gray-black diatom shale, and dark diatom shale. The layers divided into this part are 19, 18, 17, 16, and 14, with a total thickness of 3.82 m. Photographed by a microscope, it is found that the layer formed by the thick diatom laminae and the thin debris laminae is the varve. In the range of 500 mm, the number of laminae is 15 after 10 times of repeated counting. After calculating the average, the thickness of the varve can be obtained as about 33 μm. The diatom layer is about 20 μm, and the detritus layer is about 13 μm. Therefore, it can be calculated that the deposition rate in this layer is 1000/33 = 30 a/mm, that is, 30 Kyr/m, 3.3 cm/Kyr. The deposition time of this interval is 30 Kyr/m × 3.82 m = 114.6 Kyr.

The main components of the diatom-organic matter couplet are diatom and organic matter. The lithology is gray diatom shale, dark diatom shale, gray-black diatom shale, and black shale. The layers divided into this part are 13, 12, 11, 7, 5, and 4, and the total thickness is 4.83 m. The corresponding varve is identified under a microscope (Figure 15). The laminae are counted in the range of 200 μm. After repeated manual counting, the number of laminae within 200 μm is 4, and then the average number is taken to obtain the varve thickness of about 50 μm. The deposition rate in this layer is 1000/50 = 20 a/mm, that is, 20 Kyr/m, 50 cm/Kyr. Therefore, the deposition time of this part is 20 Kyr/m × 4.83 m = 96.6 Kyr.

The composition of the organic matter-debris annual couplet is mainly organic matter, and a small amount of debris material (Figure 16). The lithology is black mudstone, green mudstone, dark mudstone, light mudstone, gray mudstone, and brown-gray mudstone. The layers divided into this part are 31, 28, 26, 23, 21, 15, 9, 6, 3, 2, and 1, with a total thickness of 6.59 m. Through the observation of thin sections under the microscope, the thickness of a single lamina is larger, and the transition between laminae and laminae is better. This indicates that the sediment is deposited rapidly and stably during this period, and there is no obvious sedimentary discontinuity. To sum up, this layer of lamina is well preserved and has a single lithology, belonging to the best-quality laminae. The varve in this layer is thicker and can be measured directly. By measuring the five selected thin slices with obvious laminae characteristics and then taking the average, the annual lamination thickness can be obtained to about 500 μm, and the deposition rate can be calculated to be 1000/500 = 2 a/mm, i.e., 2 Kyr/m, 500 cm/Kyr. So, the deposition time of this part is 2 Kyr/m × 6.59 m = 13.18 Kyr.

In addition to the abovementioned varve couplet, the section of diatom shale in the Shanwang Basin contains sandy mudstone with a total thickness of 0.61 m. This study draws on the scholars‘ summary of the deposition rates of different lithologies (

Table 1. Sedimentation rate data of different lithologies (m/Myr).

Bubnoff						Prokoph		Li Qingmou		Xu Daoyi
Geologic era	Duration (Myr)	US maximum	European maximum	Valid maximum	Shelf	Mudstone	50~70	Mudstone	7.31	Xishan section	Taiyuan shanxi formation	4.9
erm.	45	62	224	100	13
Lower carb.	40	188	210	150	13	Sandstone	290	Fine sandstone	48.7		Benxi jinci formation	5.6
Upper carb.	40	51	88	50	13	Egret member Formation	60~110

) combined with the astronomical chronology method to obtain the deposition rate of 4.98 cm/Kyr, and selects 48.7 m/Myr, that is, 4.87 cm/Kyr, as the deposition rate of this part. Therefore, the deposition time of this part can be calculated to be 12.5 Kyr.

The deposition rate of each part of the profile in the research region has been determined, as shown in Figure 17 (sandy mudstone is not shown in the figure due to its thin thickness). In this study, the diatom shale section of the Shanwang Basin is divided into five parts, and the deposition time is calculated separately (

Table 2. Deposition time data of the profile.

Layer Number	Thickness (cm)	Deposition Rate (kyr/m)	Deposition Time (kyr)	Layer Number	Thickness (cm)	Deposition Rate (kyr/cm)	Deposition Time (kyr)
31	40	2	0.8	16	28	30	8.4
30	1	50	0.5	15	42	2	0.84
29	80	50	40	14	158	30	47.4
28	165	2	3.3	13	219	20	43.8
27	110	50	55	12	31	20	6.2
26	64	2	1.28	11	26	20	5.2
25	78	50	39	9	50	2	1
24	43	50	21.5	7	12	20	2.4
23	30	2	0.6	6	30	2	6
22	162	50	81	5	145	20	29
21	12	2	0.24	4	50	20	10
20	126	50	63	3	101	2	2.02
19	26	30	7.8	2	65	2	1.3
18	40	30	12	1	60	2	1.2
17	129	30	38.7	8, 10	61	1/4.98	12.5

). The total deposition time of the diatom shale section in the Shanwang Basin is 538.68 Kyr, or around 0.539 Myr, when the deposition times of each part are added together.

6. Discussion

In order to verify the accuracy of the results of the chronology of the laminae, the comparison of different dating methods is adopted for verification. Yu et al. (2017) used a magnetic chronology to calculate the sedimentation time and sedimentation rate of the diatomaceous shale section in the Shanwang Basin, which were 0.468 Myr and 4.67 cm/Kyr, with ages in the range of 18.524 Myr B.P.~18.056 Myr B.P [31]. Yu et al. (2021) calculated the deposition time and deposition rate of the diatom shale section in the Shanwang Basin as 0.44 Myr and 4.98 cm/Kyr [32]. Yu et al. (2019) used the method of solar radiation and magnetic susceptibility tuning to obtain 21 corresponding relationships between depth and age in this section [33]. On the basis of 21 depth points, this study recalculates the age of these depths by using the method of varve chronology (

Table 3. Correlation between depth of diatom shale profile and laminar age in Shanwang.

Depth (m)	Age (Myr B.P.)	Depth (m)	Age (Myr B.P.)	Depth (m)	Age (Myr B.P.)
1.5	18.056	8.4	18.267	15.3	18.459
2.0	18.057	9.2	18.306	16.2	18.476
3.2	18.074	10.4	18.342	17.6	18.480
4.3	18.115	11.4	18.372	18.3	18.493
5.2	18.143	12.5	18.394	19.1	18.509
6.6	18.199	13	18.409	20.2	18.521
7.2	18.229	14.6	18.445	21.6	18.524

). Then, it compares the age results calculated by different dating methods at the same depth (Figure 18), where the slopes of the scatters obtained by different dating methods represent their respective deposition rates. In general, the deposition rate of the diatom shale section of the entire Shanwang Basin obtained by the two dating methods is basically consistent. In the early to middle stage (profile 21.85–8 m) of the diatom shale section in the Shanwang Basin, the laminae age is slightly larger than the tuning age; in the late stage (beginning from 8 m of the profile), the age of the laminae dating method is slightly smaller than that of the tuning dating method. During this period, the laminae type is the diatom-detritus type, and the diatom gradually changes from the prosperous period to the extinction period, and the deposition rate also decreases sharply. This indicates that the deposition rate of the profile is affected by the prosperity degree of diatoms, while the abundance of diatoms is influenced by the climate and environment. Therefore, the sedimentation rate calculated by varve chronology can indirectly reflect the climate, environment, and other conditions during the corresponding period.

There are slight differences in the results between varve chronology dating and the tuned age. The first reason is that diatoms, as aquatic plants, are highly sensitive to changes in the climate and environment, and their magnetic susceptibility cannot be compared with them. The second reason is that most of the sediments in the diatom shale section of the Shanwang Basin are diatom-containing sediments. The biological activity of diatoms can directly affect the deposition rate of sediments in the study area. The thickness of the varve is controlled by the biological activity of diatoms.

In terms of accuracy, using the solar radiation and magnetic susceptibility tuning method to obtain the corresponding relationship between depth and age in the profile, it is necessary to make the maximum value of magnetic susceptibility correspond to the minimum value of solar radiation. However, the change of solar radiation is regular. In order to complete it corresponding to the maximum value of magnetic susceptibility, this method is very limited in the depth corresponding to the profile, and can not calculate the age of continuous depth, which has great limitations. The method of the varve chronology is different, and it has the advantage of independent dating. In a section, as long as the corresponding varve is found, combined with the thickness of the annual lamination, the continuous deposition time and deposition rate of the whole section can be calculated. Therefore, a one-to-one correspondence between any depth point on the profile and age can be established.

The sedimentary time of the diatom shale section in the Shanwang Basin obtained by the study of the varve chronology is 0.54 Myr, which is different from the previous two dating results. Because the previous two dating methods calculate the average deposition rate of the profile, the deposition time of the profile is calculated. However, the method of the varve chronology is to calculate different deposition rates according to the laminae types of different layers in the section, obtain the deposition time of each layer, and finally calculate the deposition time of the whole section. In summary, the author believes that the varve chronology dating method is a highly reliable dating method in addition to the cumbersome statistics.

7. Conclusions

• Using the varve chronology, it is determined that the diatomaceous shale section in the Shanwang Basin has a sedimentary period of around 0.54 Myr and a sedimentation rate of 4.06 cm/Kyr. Further research reveals that the Shanwang Basin’s diatomaceous shale portion is 18.524–17.985 Myr B.P. It gives a precise time period for the investigation of the diatomaceous shale section’s paleoenvironment and paleoclimate in the Shanwang Basin.
• The ages of 21 points are recalculated by using the varve chronology, and the results agree with those obtained from solar radiation and magnetic susceptibility tuning. In addition, according to the biological habits of diatoms, the deposition rate obtained by the method of varve chronology can well reflect the climatic and environmental conditions in the corresponding period, which can show that the varve chronology is accurate and reliable. Although the age calculated by the varve chronology is different from other dating methods, through comparative verification, it can be concluded that the varve chronology dating method is relatively more accurate among these methods, and the age calculated by this method is more in line with the actual age of the diatomaceous shale section in the Shanwang Basin.
• It can be deduced that the climate of the diatom shale section of the entire Shanwang Basin shifted from cold to warm by examining the laminae types in the research area’s profile and combining them with diatoms’ biological behaviors. This finding offers paleontological support for future research on the paleoenvironment and paleoclimate of diatom shale in the Shanwang Basin.