The Cretaceous-Paleogene calcareous nannofossils in southern Tibet were mainly carried out by Xu Yulin et al. (Xu Yulin et al.,1992; Xu Yulin, 2000), the corresponding calcareous nannofossil belt was established and compared with Sissingh( 1977) fossil belt (CC belt). In addition, Lan et al. (2000) also studied the calcareous nannofossils of the Middle Cretaceous calcareous nannofossils belt and the boundary between Sainorman and Toulon periods in Gamba area, southern Tibet, and studied the distribution of calcareous nannofossils of the Albian and Santo periods. According to the existence of marker species, five first-time events were identified, and six calcareous nannofossil belts were established, namely, from the bottom to the top, Prediscosphaera cretacea belt, Eiffel Thursturriseifeili belt, Lithraphidites acutum belt, Gartnerago obliquum belt, Quadrum gartneri belt and Lucianorhabdus cayeuxii belt. At the same time, through the intercontinental comparison, it is suggested that the initial appearance of Inonotus obliquus should be used as a sign to divide the boundary between the Selenormand period and the Tulong period in this area.
Foreign countries have made good achievements in the study of calcareous supermicroorganisms near the boundary between Jurassic and Cretaceous. The main work and achievements are closely related to the progress of DSDP and ODP. Drilling at DSDP and ODP stations reveals that calcareous nannofossils in boundary strata are well preserved. Relatively speaking, the research in this field in China is still blank, and the work of calcareous supermicrobial strata has not been carried out during this period, mainly because the distribution of marine strata in China is very limited during this period; Secondly, compared with the stratigraphic samples of DSDP and ODP, the only marine strata in China in this period often suffered from strong structural uplift and weathering erosion, and individual tiny nannofossils were easily destroyed, thus affecting identification and classification. Based on this premise, it is urgent for geologists in China to conduct more in-depth and detailed research.
In this work, shale and silty shale samples collected from the J-K boundary stratum in Gyangze area and the south bank of Yangzhuoyongcuo in Langkazi County were deeply studied in the laboratory, and various methods were adopted for two years. After many failures, more than 500 slides were made only by microscope observation, and abundant calcareous nannofossils were finally found, which made up the blank of calcareous nannofossils near the J-K boundary in China.
4. 1.4. 1 analysis method
Calcareous nannofossils are different from ordinary microfossils in sampling, processing and observation because of their tiny volume and fine structure. Therefore, the methods of treatment and observation and research are introduced in detail below.
(1) Analysis method of observing samples with optical microscope
The processing method of calcareous nannofossil samples is very simple and special. Because of its delicate texture and small size, violent chemicals cannot be used, and only gravity differentiation can be used. The process is mainly divided into two steps: batch sampling and enrichment.
1) Disperse the sample: fully disperse the sample, thus precipitating particles with the size of ultrafossils. The method is:
(1) Take 3 ~ 4 fresh samples that have been broken into rice grains and put them into water for soaking and diffusion, or put them into water after soaking in xylene. The most ideal sample is a soft rock sample with low hardness, which can even be crushed with nails. If it is consolidated hard rock, it needs to be broken into two pieces in advance. Cut 3 ~ 4 samples with the size of rice grains on its cross section with a screwdriver, break them into powder in a mortar, and then soak them in a beaker with 20mL of water.
(2) If the immersion is difficult to diffuse, the sample can be boiled in water, or the small beaker soaked in the sample can be placed on an ultrasonic vibrator for several minutes to twenty or thirty minutes, and facilitated diffusion can be used. In order to avoid the damage of ultrasonic vibration to fossils, it is more appropriate to use the frequency of 28kHz and the power of 5 W. If the sample is difficult to disperse because of the high clay content, a small amount of sodium carbonate can be added to boil it.
During the whole treatment process, special attention should be paid to the pH value of the treatment solution. On the one hand, it can avoid the dissolution and destruction of ultra-micro fossils with fine calcareous skeleton in low pH liquid, and at the same time, because alkaline medium can keep clay dispersed, it is also convenient to handle. The most favorable solution is pH =9.4. Therefore, it is necessary to add baking soda (4g per 20L water) and sodium carbonate (3g per 20L water) to the distilled water for treatment, so as to make the pH value reach 9.4. Tap water or distilled water should not be used directly.
2) Enrichment: It is an important step in the sample treatment process to remove particles and organic matter that are too coarse and too fine, and to enrich ultramicro fossils.
Add 30% hydrogen peroxide to the sample (while adding baking soda to keep the pH value of the medium at about 9.4), and after heating for 65438±0h hours, if the dark sample turns light gray, it indicates that the organic matter has been oxidized. Centrifuge, pour out the upper liquid, then add Na2CO3 for cleaning, and centrifuge again, and so on for many times. If the organic content is not high, this step can be omitted. Coarse particles can be removed by screening or precipitation. In the screening method, the dispersed samples are washed on a fine sieve with a pore size of 0.035mm or 0.04mm (i.e. 300 mesh), the coarse particles remaining on the sieve are discarded, and the washed liquid under the sieve is taken for further analysis. The precipitation method is to precipitate the ground sample in baking soda water solution for 1 ~ 2 min, discard the precipitated coarse particles, and take the above liquid for further analysis. There are different methods for further enrichment, such as beaker method, dropper method and filter paper method (see Stradner et al.,1961; Hay,1977; Haq， 1978； Ji,1981; Tongji University Marine Microbiology Laboratory,1982; Hao Yichun et al.,1993; Bown et al.,1998; Hardenbor et al.,1998; Bornemann et al., 2003).
This experiment was carried out in the laboratory of China Geo University (Beijing) College of Oceanography, and many latest and most commonly used calcareous nannofossils were treated, made, observed and analyzed.
First, the usual smear method is adopted. First, put a small amount of samples (the size of rice grains) on the glass slide, drop 1 ~ 2 drops of distilled water, spread them evenly with disposable toothpicks or small plastic sticks, dry them on an electric heating plate, and then seal them with neutral resin glue to make a long-lasting glass slide. Sealants are Canadian gum (refractive index 1.52), and then they are magnified under a polarizing microscope. This method is simple and fast, and only needs a small amount of sediment (generally about 1g). This is a very quick and effective method to determine whether there are fossils and observe the composition of fossil communities.
Because the abundance, differentiation and preservation of calcareous nannofossils in J-K boundary strata are not as good as those in Cenozoic and modern marine sediments, calcareous nannofossils can hardly be found in the slices made by the above general treatment methods. After that, various enrichment methods such as concentration and precipitation were adopted. Now, choose one of the methods, the specific steps are as follows:
A. Sample treatment and preparation of thin slices
(1) Take a rock sample, cut off the polluted part of the surface and use its fresh surface.
(2) For soft samples, clean rock samples are cut into many small particles. Or scrape off about 20mL of rock powder with a screwdriver or knife and put it into a 50mL beaker.
(3) Crush the consolidated hard rock into two pieces in advance, scrape some rice grain-sized samples on its cross section with a screwdriver, grind them into powder in a mortar and put them into a 50 ml beaker.
(4) Add about 20mL of buffered distilled water (pH =9.4) into a beaker filled with rock powder, and fully stir with a glass rod to make a suspension.
(5) For samples that are difficult to diffuse after soaking, it is advisable to put the small beaker soaked in the sample on a small ultrasonic vibrator (frequency 28kHz, power 5W) to vibrate for 5s, and if necessary, it can vibrate for several minutes or even 20 ~ 30 min to facilitated diffusion.
(6) After the stirred suspension is allowed to stand for 30 seconds, the supernatant is poured into a second beaker; Stir the remaining suspension evenly, let it stand for 1 ~ 2 min, and then pour the supernatant into the third beaker to make the intermediate supernatant; The sediment left at the bottom is a low turbid liquid.
(7) Absorb the supernatant, middle supernatant and lower turbid liquid with a dropper and drop them on the prepared glass slide. Each liquid is sampled from different layers from top to bottom, and gently dripped on five slides, so that the suspension is evenly distributed on the whole cover glass. And put this slide on a normal temperature electric heating plate.
(8) heating the electric heating plate to dry the suspension. Pay attention to the low temperature (40 ~ 50℃) as much as possible, and after a certain heating and drying time, so as to avoid strong convection of moving particles in suspension.
(9) Drop a drop of sealant (refractive index 1.52) in the center of the slide.
(10) Stick the cover glass on the glass slide. When sticking the cover glass, put the cover glass with sealant face down, gently put it on the sample loaded with the glass slide, and gently press the cover glass with tweezers or glass rod to make the sealant spread over the whole surface of the cover glass. At this time, be careful not to leave bubbles between the cover glass and the glass slide.
(1 1) Let it stand at room temperature for a period of time to solidify the sealant. Make a glass slide for identification under the microscope, and then stick a label on the glass slide with the sample number, origin and other contents, that is, make a glass slide that can be preserved for a long time.
B. Microscopic observation, identification and photographing
Because calcareous nannofossils will show special extinction phenomenon under orthogonal polarizing microscope, they will be observed, identified and photographed under orthogonal polarizing microscope/oil immersion lens with magnification of 0/000 times. More than 600 views were randomly selected to observe and identify calcareous nannofossils. In order to ensure the uniformity and accuracy of fossil classification and identification, some samples were selected for scanning electron microscope observation.
(2) Analysis method of observing samples with scanning electron microscope (SEM)
Scanning electron microscope can directly observe the structural details of calcareous nannofossils, so it is also a common analytical method.
Firstly, the samples were treated by concentrated precipitation method to enrich calcareous nannofossils. The method steps are the same as the sample treatment methods (1) ~ (6) observed by optical microscope. After that, the difference is that the enriched supernatant, middle supernatant and lower suspension are dripped on the special sample table for scanning electron microscope for full drying. Then, after spraying gold in vacuum, the dried samples on the stage can be observed and photographed. For specific methods, please refer to the "Technology" section in the book "Biostratigraphy of Calcareous Nanofossils" (Bown et al., 1998). The gold spraying, observation and photographing of this scan were carried out in the experimental center of China Petroleum Exploration and Development Research Institute and the scanning electron microscope room of China Geo University.
4. 1.4.2 calcareous microorganisms in the study area
In this study, 55 samples from 5 profiles in Gyangze-Langkazi area were analyzed, and 550 calcareous microfossils were processed and sliced. Some samples were observed by scanning electron microscope (SEM) and 50 scanning photos were taken. Some species that are difficult to identify were observed by optical microscope and scanning electron microscope. Each slice has more than 600 observation horizons, and the abundance of calcareous nannofossils is estimated according to the standards defined by Hay( 1977) and Miriam Cobianchi et al. (1997):
A = richness: 6 ~ 10 species per view; C = common: 1 ~ 5 species/view;
F = few: 1 species/1 ~ 10 views; R = Rare: 1 species/1 1 ~ 300 View.
In this study, calcareous nannofossils were found for the first time in Jiabulakou and Jiabula sections in Gyangze, and Sangxiu Formation in Linxi section in Langkazi County (Plate I), especially in Jiabulagou section (Table 4.3). Many types belong to global molecules and intercontinental molecules, which provide a basis for the division and correlation of this set of strata. Compared with calcareous supermicrobes in other parts of the world at the same time, the biological abundance and diversity in the study area are relatively low, and the Ellipsagelosphaeraceae biota is the main one.
Table 4.3 Distribution of calcareous nannofossils in Jiabulagou Mouth and Jiabula Section of Gyangze
Note: J is the mouth of Jiabulagou; JF is part A-B; A means that fossils are rich in content; C means that the fossil content is medium; F means low fossil content; R indicates that the fossil content is scarce (A: 6- 10 specification per view; C:65438+ 0-5 specimens per view; F: 1-1specimen in the field of view; R: 1 The sample is within 1 1-300 field of view).
Characteristics of (1) elliptic algae-eating biota
The characteristics of Ellipsagelosphaeraceae biota are that the stones are round, oval and double-shield, and the lines on the shield plates overlap each other. Under the orthogonal polarizing microscope, both baffles have interference images. It can also be divided into Watznaueria, Cyclagelosphaera, Manivitella, Ellipsagelosphaera and other genera. In this study, it was found that Watznaueria was dominant, followed by Cyclagelosphaera and Manivitella.
According to the identification, the genus Watznaueria includes six species, namely, Watznaueria barnesae, Watznaueria fossacincta, Watznaueria ovata, Watznaueria manivitae, Watznaueria cf. manivitae and Watznaueria biporta. There are two species of Cycladophaera, namely Cyclagelosphaera margerelii and Cyclagelosphaera deflandrei. There are 1 species of Monopteridae, namely Monopteroidea.
The main difference between Watznaueria, Manivitella and Cyclagelosphaera is that the former two are oval, while the latter is round and sub-round. The main difference between Watznaueria and Manivitella is that the latter has a large and empty central area. Watznaueria barnesae is the dominant species in the genus Watznaueria, and the abundance of a single species in each section is more than 40%, followed by Watznaueria fossacincta, Watznaueria ovata, Watznaueria manivitae, Watznaueria cf. manivitae and Watznaueria biporta. This is consistent with the statement that Watznaueria barnesae is the most common Cretaceous pebble in poorly preserved combinations (Perch-Nielsen, 1985).
Taxonomically speaking, the six species of Watznaueria are distinguished according to their individual size, and Watznaueria barnesae, Watznaueria fossacincta and Watznaueria ovata are distinguished according to whether there is a central hole or not, and the size of the central hole increases in turn. Raspberry is very big and can be easily separated from raspberry and raspberry. Watznaueria cf. manivitae is also very large, generally above 8μm, with a small or closed central hole, which is different from Watznaueria manivitae. Watznaueria biporta has two large holes in the central area. The central area of Watznaueria britannica in Britain has a crossbar, which can be distinguished from the above six kinds.
Oedogonium is a genus with birefringent distal shield in Oedogonium ellipticum. Under the polarizing microscope, the distal shield of Cyclogelopaphaera is bright, which is different from that of Markalius. The two species found in the study area, Cyclagelosphaera margerelii and Cyclagelosphaera deflandrei, are easy to distinguish. The former is smaller, and the lower distal shield of polarization microscope is brighter, while the latter is larger, and the color of polarization microscope is yellow.
Mani Vitera is oval, and the edge area of the stone consists of two layers of rings, which is characterized by a large and hollow opening in the central area.
The biodiversity in the study area is relatively low, which is often considered as a typical unstable condition and nutrient-rich cold surface water (Okada et al.,1973; Brand,1994; Melinte et al., 2001). Watznaueria barnesae is a dominant species, which is common and abundant in most environments throughout Cretaceous. It has been proved to be a very soluble and adaptable world species. It is an energetic ecological species and can adapt to new habitats as soon as possible (Mutterlose,1991; Melinte et al., 200 1). In addition, Watznaueria barnesae is dominant and is usually regarded as a sign of superimposed diagenesis (Roth,1986; Ross et al., 1986).
(2) The horizon distribution and age of calcareous supermicrobial assemblage in Early Cretaceous.
A. Jabra Group
The gray-dark gray shale and silty shale at the bottom of Jiabulakou Formation in Gyangze area are rich in calcareous nannofossils Speetonia colligata, Calcicalathina oblongata, Watznaueria barnesae and Watznaueria fossacincta. Tree shrew, tree shrew, tree shrew, tree shrew, tree shrew, tree shrew, tree shrew, tree shrew, tree shrew, tree shrew, tree shrew, tree shrew Among them, Watznaueria barnesae、Watznaueria fossacincta、Watznaueria ovata、Watznaueria manivitae、Watznaueria cf. manivitae、Cyclagelosphaera margerelii、 Biscutum constans, Manivitella pemmatoidea and Diazomatolithus lehmanii are world species. Ladybug annulata, ladybug with spotted wings, ladybug with long beak, ladybug with no beak, ladybug with six teeth in Darkmouth, puffin with multiple teeth, puffin with Neostis skrjabini, puffin with Neostis skrjabini, etc. It's a Tethys species.
There are abundant species in the world, among which the species of Watznaueria is dominant, and the abundance of Watznaueria in each slice is as high as 60% ~ 90%, followed by other genera and species, followed by Cyclagelosphaera margerelii, Biscutum constans and Manivitella pemmatoidea. Manivitella pemmatoidea appeared in Berriasian—Cenomanian period, Biscutum constans appeared in Cretaceous period, and Watznaueria and Cyclagelosphaera have a long time span, but they are usually considered as typical species in the low latitude combination of late Jurassic and early Cretaceous. Bown et al. (1998) considered that Watznaueria britannica was the dominant species in the late Jurassic Tithonian, and the genus Watznaueria was still dominant in the early Cretaceous, but Watznaueria britannica was often replaced by Watznaueria barnesae and Watznaueria fossacincta. After careful identification, Watznaueria britannica was not found in this study area, but it was rich in Watznaueria barnesae and Watznaueria fossacincta, indicating that the age of this study area was early Cretaceous.
The number of Tethys species is relatively small, but most of them are of stratigraphic significance. Australopithecus minor and Australopithecus minor are standard zone fossils of the early Cretaceous Berriasian period, but they are few in number and extremely low in abundance in this study area. Cyclagelosphaera deflandrei is an endemic species in Tethys sea area, mainly distributed in the sediments of early Early Cretaceous (Perch-Nielsen, 1985). Polyccostella senaria is a bainite fossil in the early Cretaceous, and Gartner( 1977) thinks that Polyccostella senaria is an excellent indicator fossil for identifying bainite in offshore sediments. Speetonia colligata is a fossil from the late Berriasian-Trif stage, and Calcicalathina oblongata is a fossil from the early Varangian stage to the early Trif stage. Six-toothed dragon appeared in Cretaceous.
Abundance and differentiation of calcareous nannofossils in the lower part of Jiabula Formation (2 ~ 4 layers) in Jiabula section are much lower than those in Jiabula Goukou section. It produced Vaznauba Barnabas, Vaznauba fossacincta, Vaznauba cf. manivitae, Vaznauba biporta, Cyclagelosphaera margerelii, Cyclagelosphaera deflandrei, Biscutum constans, No calcareous nannofossils were found in Polycostella senaria, Manivitella pemmatoidea, Diazo Rubble, lehmanii and Calcium, mainly because black shale was exposed in the lower part of Jiabula Formation. From the point of view of paleontology, most black shales lack such ultraconodonts and calcareous ultrafossils, but they are dominant in pelagic carbonates and have been considered by many scholars as an ecotype of malnutrition (Coccioni et al.,1992; Barr, 1994).
Shale and siltstone in the lower part of Jiabula Formation in Linxi section of Langkazi County contain a small amount of calcareous nannofossils Watznaueria barnesae, Tubodiscus verenae and Manivitella pemmatoidea. Manivitella pemmatoidea is a fossil from Berriasian to Cenomanian in the early Cretaceous, and Tubodiscus verenae is a fossil from Valanginian in the early Cretaceous. Therefore, the lower age of Jiabula Formation in this area is Early Cretaceous.
The comprehensive analysis of the fossils in the lower part of Jiabula Formation in Gyangze and Langkazi areas shows that the age of this fossil is transitional, with both Jurassic molecules and Cretaceous members, but it still mainly reflects the face of the early Cretaceous fossil assemblage, that is, the early Cretaceous period from Berrias to Walangeni. This fossil assemblage corresponds to Sissingh (1977) fossil zonation CC1~ the lower part of CC3, and Hardenbor et al. (1998) fossil zonation NJK-D to NK-3 (Figure 4.3; Table 4.4).
Table 4.4 Comparison of calcareous nannofossil assemblages (belts) between southern Tibet and other regions
B. Sangxiu Group
The shale in the lower part of Sangxiu Formation in Linxi section of Langkazi County contains a small number of calcareous nannofossils, such as Calcicalathina oblongata, Speetonia colligata, Diazomatolithus lehmanii, Polycostella senaria and Watznaueria barnesae. The abundance and diversity of fossils are much lower than that of Jiabula Formation in Gyangze area, and the genus and species are the same as some fossils of Jiabula Formation. According to the above analysis, the bottom of the Sangxiu Formation is the same as that of the Jiabula Formation, belonging to the early Cretaceous Berias-Walangjina period, which is equivalent to the sissing( 1977) fossil zonation of the lower part of the CC 1 ~ CC3 belt and the Hardenbol. Table 4.4).
This time, samples 13 were collected in Kadong section of Langkazi county, and * * slices 130 were made. After careful identification, calcareous nannofossils were not found in the lower parts of Sangxiu Formation and Jiabula Formation, probably because most of the lower parts of Sangxiu Formation and Jiabula Formation in Kadong section were exposed to black shale, and the paleo-marine environment was not conducive to the survival of calcareous nannomicrobes.
To sum up, after careful analysis and comparison with calcareous nannofossil assemblages (belts) in other parts of the world in the same period, the calcareous nannofossil assemblages in the lower part of Jiabula Formation and the lower part of Sangxiu Formation in the study area belong to the early Cretaceous Berias-Varangian period, which is equivalent to the fossil zonation of Sissingh( 1977), the lower part of CC 1 ~ CC3 belt and Hardy in Tethys Sea area.