In recent years, there have been reports of life remains in Precambrian layered flint rocks in Asia, Africa, Australia and the United States, such as spheres, filaments and cups similar to fungi and algae. At the same time, there are various organic substances, amino acids and oil and gas displays. Therefore, there has been a lot of evidence about the genesis of layered flint rocks, which has been debated for a long time, mainly focusing on the conclusion of organic genesis. This understanding also provides a new basis for finding energy in Mesoproterozoic and Neoproterozoic sedimentary rock series.
1. Mesoproterozoic and Neoproterozoic layered flint rocks in North China
Compared with other parts of the world, Mesoproterozoic and Neoproterozoic in North China (about 810.80 billion years ago) have thick deposits, complete development and basically no metamorphism, including various forms of flint rocks (Figure 4-3). In order to find the primary oil and gas reservoirs in North China, the authors have successively surveyed the Mesoproterozoic and Neoproterozoic strata in Beixiao Tomb in Quyang, Ming Tombs in Changping, Heishan Temple in Zhuolu, Ancient Ovary in Chicheng and Xiageyu in Xuanhua. The morphology, occurrence, distribution, composition and structure of flint were observed in the field. It is recognized that flint rocks widely distributed in Mesoproterozoic and Neoproterozoic are formed by primary deposition and post-deposition metasomatism, and cannot be generalized. Layered and layered flints with stable lower position and no metasomatic texture under the microscope are mainly primary deposits, which may be mainly organic due to the existence of life remains, residual organic matter and asphaltene impregnation. The following is a description from the perspective of petrology.
(1) algal cell flint: The mineral composition is cryptocryst, microcrystal chalcedony. It shows the biological structure. The cell size of globose algae is 1-20μ m, and they are distributed singly or in clusters (Figure 5-2). The center of the spherical section is a dark cell nucleus rich in organic matter, and the outside is wrapped by a light cell wall. Usually stained with asphaltene, it is brown. There are sometimes rich dark spots in or around cells (Figure 5-5), and well-defined structures can be seen in many cells (Figure 4-4, Figure 6-6). In addition, in different lithology with similar profiles, the shape, size and distribution of cells are very stable, so it is impossible to explain the reason or result of material redistribution during silicification. Cell proliferation and division can often be seen (Figure 4-4), which further shows that it is caused by protozoa.
The algae cell flint of Wumishan Formation in Beixiao Tomb of Quyang belongs to eukaryotic cells by biological identification, and some species and genera are divided (Zhang Yun, 1978).
Figure 4-3 Main morphological characteristics of Mesoproterozoic and Neoproterozoic flint in North China
1-tuberculous; 2- striated layer; 3- small cup; Quadrupyramid shape
The ultrastructure of algae cells was observed under scanning electron microscope. The natural section is slightly irregular spherical (plates 7-2 and 7-3), and the structure of siliceous minerals constituting cells is uneven. The core is a nearly round block-shaped time, the wall is an irregular granular time, and it is surrounded by a block-shaped time or a homogeneous time (R.L.Folk, 1952), which seems to be composed of three different sizes of time. The unbroken spherical objects are mainly composed of small spherical time aggregates (plate 8- 1, 2,3), and the time of small spherical objects is about 1nm.
The above observation results show that the crystallinity of siliceous minerals in and around the algae core is different, which may be related to the nature and quantity of original organic matter.
In the pure dark siliceous rocks at the bottom of Wumishan Formation in Quyang, Hebei Province, there are also higher-level algae trace microstructures, similar to dendritic siliceous algae fossils, rich in organic matter and dark brown (plate 5-3) (Liu, 1976).
The sequence of algal cell flint rocks is stable, mainly occurring in the middle and lower part or bottom of Wumishan Formation, among which the results obtained by the author in the sections of Beixiao Tomb in Quyang, Hebei Province, Heishan Temple in Zhuolu and Ming Tombs in Changping are the most clear. Tianjin Institute of Geology and Mineral Resources (Zhang Pengyuan, 1979) reported that a polynuclear green algae fossil was found in the black flint at the bottom of Wumishan Formation in Jixian County, which is also equivalent to this horizon.
(2) Algal stromatolite flint rock: Siliceous algal stromatolite, like carbonate rock and stromatolite, is flaky, columnar or conical. Silicification was often used to explain its causes, but now it seems that it is not entirely true, depending on the specific situation. For example, in Yangzhuang Formation-Wumishan Formation, there is a columnar or cup-shaped siliceous stromatolite (Figure 4-3), which is mainly distributed in layered and layered flint rocks. There is no obvious metasomatism in macroscopic observation of outcrop, and its basic layer is composed of dark and light siliceous striations. Microscopically, the dark basal layer is rich in organic matter, often stained yellow by asphaltene, with characteristic algal fiber structure and fibrous chalcedony radial, which passes through the basal layer (plate 5- 1) and does not show metasomatism. Sometimes, in the dark stripes, there are microscopic structures with speckled algae traces (Plate 5-4).
Figure 4-4 Schematic diagram of microscopic characteristics of algae cells
(3) Algae-like flint rocks: Algae mainly refer to oolitic or nuclear rocks, which are found in carbonate rocks and flint rocks in Mesoproterozoic and Neoproterozoic, and some original structures in carbonate rocks are replaced by siliceous substances. However, some of them are more similar to the original siliceous biogenesis. The diatom oolitic grains are slightly irregular concentric rings, and are composed of dense dark spotted algae, with the size of1-2 μ m. The oolitic grains are wrapped by comb-like timing or dolomite, forming later than the oolitic grains (Figure 5-6).
Therefore, these primary flint rocks are not completely formed in still water or deep water environment (including organic or inorganic origin), but may also be formed in more turbulent shallow water environment.
All the above phenomena are confirmed by thin sections and electron microscope photos (plate 7- 1, 4; This provides some evidence for the organic origin of Mesoproterozoic layered flint rocks.
2. Mesoproterozoic and Neoproterozoic flint rocks in other areas at home and abroad
According to Min Yushun of Guiyang Institute of Geochemistry (1978), Min Yushun et al., 1978, Ultrastructure and Evolution Model of Some Siliceous Rocks in China, Guiyang Institute of Geochemistry (published internally).
According to a comprehensive report, although the siliceous rocks of Precambrian Anshan Group, Liaohe Group, Wutai Group and Hutuo Group in Northeast China and North China have undergone different degrees of metamorphism, many life remains and organic matter similar to bacteria and algae have appeared, which proves that life has existed in Archean to Proterozoic basins in China. Moreover, spherical nannofossils (3-26 microns in diameter) similar to cells evolved from small to large in structure and size, from simple to complex, and in Mesoproterozoic and Neoproterozoic, the symmetry was obviously enhanced, the structure was clearer, and eukaryotes appeared.
In Precambrian flint rocks in North America, South Africa and Australia, fossil remains such as filamentous bodies, goblets and spheroids of bacteria and algae, as well as organic substances such as carbon, amino acids and hydrocarbons (Engel, 1968, Lebege, 1967, 1973, Knoll and Bargh) have also been found. K lankarma (1970) lists the exact evidence of Precambrian life remains in America, Africa and Australia. , most of which are related to flint (Table 4-4).
Table 4-4 Ultrafossils from Precambrian Flint Abroad
(According to Langkawi, 1970)
In space, the life remains in Precambrian flint rocks in Asia, Africa, Australia and the United States are mainly spherical, and have certain similarities in size and shape, indicating that these primitive microorganisms were all over the world at that time.
3. Organic genetic model
For a long time, people have tried to explain the origin of layered flint rocks distributed in a large area of Precambrian by pure chemical methods such as marine chemical precipitation, lake drought evaporation and volcanic eruption deposition. However, there are also many unexplained problems, such as the simple chemical composition of flint, the continuous supply of silicon dioxide, the rich life and organic remains, and the physical and chemical conditions of water medium.
A large number of life remains were found in Precambrian flint rocks in Asia, Africa, Australia and the United States, which provided strong evidence for their organic origin. How do these lower microorganisms deposit silicon dioxide?
One is a biological accumulation model similar to diatom, and there are modern deposition models to follow. The experimental results show that the saturated solubility of amorphous silica is about 65438 0.20 mg/L at room temperature, and only supersaturated silicic acid sol can coagulate to form silicic acid gel precipitation. The content of silicic acid in natural surface water (rainwater, rivers, lakes and seawater) varies greatly, and it is usually difficult to saturate. Most silicic acid is absorbed by siliceous organisms, such as diatoms, sponges and radiolarians. It is most suitable for deposition, burial and preservation in the form of bioclastic, and it is a main generation mode of siliceous rocks since Mesozoic. The reproduction of this living body in Precambrian may be very amazing, and it has strong diagenesis. It is only because these primitive, very low-level delicate creatures are often difficult to preserve traces in the long geological process. Xu Jinghua (1979) pointed out that a large number of siliceous rocks in the Precambrian system were mainly formed by diatoms, and listed the modern sedimentary model of the Black Sea. The samples taken from the Black Sea show that the iron ore layer and the siliceous layer form a fine layered structure, and both siliceous and organic matter are composed of diatoms. Iron ore deposits used to be siderite, and some became hematite. It is considered that the seawater of the Quaternary Black Sea may be the same as that of the Precambrian.
The second is the biological and biochemical alternate deposition model of siliceous stromatolites. That is to say, lower bacteria and algae secrete mucus sheath substances through photosynthesis, and capture and bind silica colloidal particles in water to precipitate them. Or change the physical and chemical conditions (pH value) of the water medium through photosynthesis, so as to precipitate silicon dioxide. This biological and biochemical deposition mechanism has been confirmed by the light and dark basic layers of Precambrian siliceous stromatolites, the microstructure of algae traces and rich organic matter.
In addition, according to the research results of Precambrian ferrosilicon formation in North America and Neoproterozoic flint-carbonate formation in China, some layered flint rocks have obvious signs of shallow water turbulent deposition, such as oolite structure, internal clastic structure, cross bedding and wavy lines. In the past, these marks were mostly emphasized as metasomatic residual structures, which did not represent the formation environment of siliceous rocks. In fact, some signs are distributed in layered flint rocks with stable horizon and unclear explanation, which belong to primary sedimentary signs, so Precambrian flint rocks are not only deposited in deeper water, but also some clastic flint rocks, which may be formed in shallow water environment or even the product of tidal action zone.
4. Conclusion
(1) The existence of life remains in Precambrian flint rocks is exact, ranging from 630 million years to about 3.5 billion years. It is likely that life appeared on the earth shortly after the formation of the stable crust, that is, biological and biochemical effects existed from the formation of the oldest sedimentary rock series, and flint rock is the oldest and most widely distributed sedimentary rock series.
(2) Since all the Precambrian organisms belong to single-celled microalgae without bones and shells, they are not exactly the same as the siliceous organisms after CAMBRIAN, so it is more appropriate to call them cryptobiotic rocks (or chert rocks) (Min Yushun et al., 1978). Its evolution can be compared with siliceous rocks of chemical origin. The widely circulated strakhov diagram (H.M.Ctpaxob, 1949) should be revised in the textbook of sedimentary petrology.
(3) Diatoms can be used as source rocks, and there are many examples in Cenozoic, especially in the western United States. The Montreux Formation is an important source rock with a large oil production. The sedimentary environment of Montreux Formation is similar to that of modern California Bay. A large number of planktonic diatoms propagate to anoxic environment, and organic matter is preserved and converted into oil and gas. These examples provide evidence for the genesis, sedimentation and oil and gas generation of Precambrian biogenic siliceous layers.
Judging from the development of Precambrian, about 2 billion years ago, which was roughly equivalent to the beginning of Mesoproterozoic in China, it entered the transformation period from prokaryotes (without nuclei) to eukaryotes (with nuclei). This sedimentation reached a material climax, especially in Wumishan Formation (about12-1400 million years ago), where a large number of siliceous organisms were deposited, as follows. Therefore, in combination with the reality of China, we should not only pay attention to the study of oil generation and storage conditions of carbonate rocks, but also pay attention to the study of dark flint rocks widely distributed in them, which is also of great theoretical significance to the study of the origin of life and the evolution of organic matter.