According to the traditional coal-forming theory or the viewpoint of most coalfield geologists in the past, coal-forming occurred in the retrogradation period of the retrogradation cycle of water inflow. The core idea of this coal-forming model is that the evolution of coal-accumulating basins has stages. In the later stage of this stage, the active clastic system in the sedimentary system is abandoned, which makes most or all of the basins fall into swamps and then swamp peat. Coal formation occurs in areas suitable for peat accumulation, and coal is formed when the crustal subsidence area is preserved. It can be said that many coal seams in the world were formed in the process of regression or offshore coal-forming environment, and the coal measures formed under the condition of regression require that the basin subsidence cannot stop (Diessel, 1992), and the subsidence will occur in the whole peat generation range, even in the direction of the basin. Therefore, this will lead to the formation of coastal plain depressions, and the accumulation rate of peat is in balance with the settlement accommodation rate. However, the detrital material bypasses the peat swamp or reaches the coastal edge by the way of river crossing the swamp, so that the peat in the front of the progradation delta or the back of the barrier system migrates to the basin and provides a new peat accumulation area. Unless a sudden flood event causes the development of peat to stop or the ash content to increase, in general, peat accumulation will last for the whole regression period until the active clastic system (such as alluvial system development) in the next stage of basin evolution is revived, which stops the development of peat swamp.

The "continental coal model" can better explain that the coal-forming environment is that peat bogs develop into coal under continental conditions or when the basin retreats. In the theoretical system of coalfield geology, coal-forming theory is the most important part. The position of peat swamp is neither water nor land, and swamp is the transitional environment between water and land. In the process of coal formation, such a transitional environment is very critical. But the question is how long this transitional environment can last in the process of coal formation and basin evolution, which is related to the final result of coal formation, so the water system will be the most critical factor restricting the development of peat swamp and the final coal formation. Therefore, water system is the most important factor in coal-forming theory. For basin water system, the previous coalfield geology theory rarely involved, which limited the further development of coal-forming theory. Therefore, it is far from enough to explain the coal-forming process only by the retreat of water area and the expansion of peat swamp to the center of the basin.

2. Coal formation during transgression and coal formation in transgressive system tracts.

Coal-forming mechanism of (1) transgression period

Transgression and regression are two control mechanisms for the formation of alternating land and sea coal seams, so it can be inferred that the composition of coal seams has changed many times. When discussing the filling function of a large sedimentary basin, it can be assumed that the sedimentary datum (under which sediments can be preserved) is close to the sea surface, or more accurately, it is the wave datum in normal weather. In the special coal-forming environment, the sedimentary datum level can be regarded as consistent with the groundwater level. Because the alternate environment between land and sea is hydrologically related to sea level, the position of groundwater surface in most coastal plains is not much different from sea level. Further inland, the groundwater level rises with the average ground inclination angle. In many peat-forming environments, the surface fluctuation determines the best profile of adult rivers, which is formed when erosion and accretion reach a balance. As shown in figure 15- 1, the optimal river profile is tangent to the sea level and rises to the source. Sea level rise, such as from T0 to T 1A, not only creates more sediment accumulation space on the original submerged coastal plain, but also reduces the flat part of the river and shortens the river profile due to the rise of the sedimentary datum. Due to erosion, rivers develop to land to achieve a new balance. On the other hand, if the sea level drops from T0 to T 1B, the lower coastal plain will be eroded, and river erosion will lead to upstream alluvial deposition.

Fig. 15- 1 lateral transfer schematic diagram of optimal river profile under sea level fluctuation (vertically enlarged) (Diessel, 199 1).

Because it plays an important role in controlling groundwater, and the change of relative sea level affects the slope of the river. Therefore, the relationship between seawater dynamics and coal measures is far more than the actual contact between peat and seawater. In the coal seam formed by low peat bog, the original datum plane position is lithologic interface, and many strata are consistent with the original datum plane position in coal-bearing seam deposition.

Sloss (1962) defined the shape of clastic rock lithologic body as a function of the following parameters by using the concept of datum plane: q-the number of detritus entering the sedimentary site per unit time; R refers to the acceptance value, which is expressed by the settlement rate or the space increased below the settlement datum level per unit time (equivalent to the "accommodation space" referred to in Vail( 1987)); M—— the sediment provided to the deposition site, reflecting its structure and composition; D- dissipation coefficient, which is used to indicate the rate at which the sediment (the unacceptable part below the sedimentary base level) is transported away from the sedimentary site.

When the above theory was applied to various transgression-regression models in the early stage, sloss thought that m was a constant, because statistical data showed that the structure and composition of sediments supplied to great basin did not change much for a long time. However, when studying the transformation of coal-bearing strata, including inorganic to organic or organic to inorganic, coal seams and their interlayer sedimentary rocks should be considered respectively. In view of the characteristics of sedimentary rocks between coal seams, the material coefficient can appear as the subscript of Q, which leads to the difference between Q clastic rocks and Q peat. Assuming that the climate and other factors affecting the plant source are very favorable for plant growth, the beginning and end of peat accumulation depend largely on the acceptance value (R) and the quantity (D) of detrital materials (Q detrital rocks) supplied or removed from the sedimentary basin. In the process of peat accumulation, it is assumed that the rising speed of groundwater level is close to the accumulation speed of plant residues, so that peat forms a necessary space (R) without being oxidized (also a form of D) or submerged. As shown in figure 15-2. Under the condition of transgression-regression, the rise, fall or stillness of the sea surface will lead to peat accumulation. Its mechanism is shown in figure 15-3. It is assumed that when the sea level rises, the lagoons and peatlands behind the island will develop to the coastal plain. The coastal erosion caused by the storm and the formation of overflow fan in the lagoon make it migrate to the land direction (T 1), and the barrier sand beach that is constantly redeposited on thin offshore deposits also migrates. Even in the case of moderate load, the compression of peat is very large, which makes the sea level rise even stronger. Above the erosion surface is the residue of sedimentary gravel. The product of this process is the formation of thin but widely distributed coal seams with obvious marine characteristics.

Before peat accumulation (figure 15-2A), terrigenous materials are deposited first, usually in the form of alluvial fan or overflow deposition, and generally located on the wave base, so it is easy to be further dissipated. Although some trees or other plants can develop at this time, root soil rocks can also be formed. However, due to the low groundwater level, the fragments of plant debris cannot be preserved due to oxidation or erosion. Therefore, peat cannot be formed (q peat -d = 0). The rising sea level makes coastal wetlands develop to land, and the conditions for peat formation are shown in figure 152B. As long as the growth rate of plants is consistent with the rising speed of sea level, peat can be formed continuously. However, with the further transgression, the increase of sedimentary space caused by the difference between the increased acceptance value (R) and the material supply (Q peat or debris) in the sedimentary basin cannot be filled by the accumulation of plants. As shown in figure 15-2C, in the central area of the figure, the development of peat stopped due to the over-deep water coverage. The actual width of the central area of Figure 15-2 depends on the inclination angle of this area. Coastal plain and piedmont environment with relatively steep slope angle can form a narrow and diachronic coal seam gathering zone, while in a vast and flat zone (such as delta plain, according to jankowski 199 1), diachronic phenomenon is so weak that the coal seams formed in a large area seem to be simultaneous. The transgressive coal measures are characterized by terrigenous deposits, with root rocks on the floor of coal seams and lakes or lagoons on the roof, which can be covered by marine deposits or replaced by erosion.

Fig. 15-2 schematic diagram of different stages of coal seam formation under the condition of transgression and regression (according to Diessel, 199 1).

The coal measures formed under the condition of regression require that the basin deposition can not stop, and continue to sink in the whole range of peat formation, even more towards the basin. This situation will lead to the formation of coastal depressions. In the coastal low zone, the acceptance value (R) is balanced with the accumulation rate of plants (Q peat), and the debris from the land bypasses the higher peat or passes through the peat in the form of rivers and is deposited in the coastal zone, preparing a new peat accumulation place for the peat to migrate to the basin at the front of the progradation delta or at the back of the barrier system (Figure 15-3T2). Peat accumulation will continue throughout the regression period, except that the ash content in coal is increased due to occasional floods or shale and other terrigenous debris interlayers are formed in coal, until the alluvial facies begins to develop and the energy level of active debris system increases and stops.

Figures 15-2C, d and e summarize the formation conditions of coal measures before regression, during regression and after regression, which is contrary to the previous regression. As a result, the coal seam is bifurcated. Before the accumulation of regressive peat began (figure 15-2C), the central parameter area in the figure was covered with water. At this stage, the organic matter deposition can be ignored, while the clastic deposition is formed in the estuary, delta front and coastal environment behind the island. With the advance of the coastline, peat began to accumulate in the swamp of the inlet (figure 15-2D). When the basin continues to slowly subside and the subsidence speed is accelerated due to compaction, the supply speed of plant residues and their weathering reduction reach a balance until peat is buried by the front of continental sediments, as shown in figure 15-2E. At this point, it returns to the state at the beginning of the loop, as shown in figure 15-2A (figure 15-3T2).

Figure 15-3 Ideal model for formation of Yongli coal seam and related strata in Sydney Basin (Diessel, 199 1)

(2) The nature of transgressive coal seam with marine roof.

The model shown in figure 15-2C shows the important differences between two kinds of transgressive coal seam environments. In the central reference area of the figure, transgression appears as flooding the top of the coal seam, but the seawater does not reach the peatland far to the left of the figure. Although these peats were also formed during transgression, they also experienced the rise of groundwater level caused by sea level rise, which led to the gradual change from dry to wet state. This shows that some coal seams or their land-extending parts were formed in the process of transgression, but they did not actually touch the seawater. Therefore, in theory, transgressive coal seams with marine roof and without marine roof can be distinguished.

Here, the transgressive coal seam with marine roof and the transgressive coal seam without marine roof should be discussed separately. However, it should be clear that the swamp environment is often connected with the ocean through transitional lagoons, bays and estuaries.

Seawater intrusion into coastal peatlands is not only reflected in the sediments above the coal seam, but also in the coal seam itself. The common situation is that the sulfur content of pyrite in the upper part of coal seam profile increases, so it can be distinguished whether the coal seam is affected by seawater. The number of coal seams affected by seawater in coal-bearing sequence depends on the frequency and duration of transgression recorded in stratigraphic column.

Next, we will discuss the coal seams that are considered to be formed during transgression and eventually covered by marine sediments. It is speculated that the accumulation of peat is equivalent to the migration of sediments to land during transgression. For example, peat is a part of transgressive system tract (TST), so the genetic relationship between coal seam formation and transgression should be reflected in the nature of coal seam.

1. Chemical characteristics

The chemical method of coal is to draw the percentage of carbon and hydrogen on the Seiler diagram. Seller diagram is a binary X-Y diagram combining chemical elements (such as C, O, H) and energy (such as volatiles, unit energy and crucible expansion index). As shown in figure 15-4, a curve belt extends horizontally from the right side of the chart to the center, and then rapidly extends downward to 0% H and100% C. Because a large number of analyses show that most coal parameters can be drawn within this limit, this belt is called normal belt or bright coal belt. Because the hydrogen content in some coals is higher than the normal content of the coal rank (represented by C), the coal mined in this belt belongs to the high hydrogen area. Coal with lower than normal hydrogen content belongs to low hydrogen area, and coal is discharged at a lower position.

Figure 15-4 Cele diagram of two kinds of coal seams affected by seawater (Diessel, 199 1)

Another chemical feature influenced by marine strata is high sulfur content and low sulfur isotope ratio. Coal and its nearby sediments show the relationship between sulfur isotope ratio and sedimentary environment, which is very close to modern peat.

Most researchers believe that the distribution of sulfur in marine roof coal is influenced by seawater, and the high sulfur in coal seam began in peat period. The sulfur content in coal is closely related to the distance between the top of coal seam and the bottom of overlying marine layer closest to coal.

2. Mineralogical characteristics

Sulfur contained in seawater is weakened by bacteria to generate H2S, or reacts with organic matter to generate organic sulfur, or reacts with ferrous to generate contemporaneous iron sulfide precipitate to directly generate pyrite, or like some FeS2 variants, this substance is unstable and converted into pyrite. Many researchers have realized that a remarkable feature of coal seams affected by seawater is high pyrite, which is also observed in modern peat sediments.

According to Cohen's research, the classification from fresh water to marine phase shows that pyrite content is the highest in brackish water peat, the middle in marine layer and the lowest in fresh water peat. This also proves that the content of pyrite is high when fresh water peat is covered with high sulfur peat, but it has no effect at the bottom of fresh water peat. In the process of transgression, peat is soaked by seawater due to accretion or flooding, and the unique or enriched minerals in coal are dolomite, calcite and apatite.

As we all know, clay mica has experienced marine environment. Clay mica constitutes the matrix of fine particles in shale, while kaolinite with relatively low content is a large crystal aggregate or a conversion product of lithic silicate that is partially or completely kaolinized before deposition.

3. Coal and rock characteristics

There is no difference between the coal and rock entities of marine roof coal seam and other coal seams, but the enrichment degree of coal and rock components is different, especially in the upper part of coal seam profile, and its coal and rock structure is rich in dark coal and rock types. The rise of groundwater level can be manifested as the increase of detrital inert substances in sapropelic coal in sub-position, off-site and occasionally. The degree of plant tissue damage is usually related to the increase of pH value of peat affected by seawater, because this kind of peat is more suitable for bacterial activities than fresh water peat. The evidence is that the preservation degree of coal seam tissue affected by seawater is very low. As a result, the vitrinite content of debris increases, while the vitrinite content decreases.

Under the condition of near neutrality, the biological density decreases due to the destruction of bacteria. As a result, the microscopic components of chitin group based on clastic vitrinite are relatively rich. In the delta plains and coal seams affected by seawater, the contents of sporophyte and cutinite are generally high.

In the transgressive coal seam with marine roof, the distribution of high fluorescence intensity and low reflection intensity in the coal seam profile is not uniform, but concentrated in the place most affected by seawater, such as the upper part of the coal seam profile. As shown in figure 155, all the properties in the two coal seams show obvious transgression characteristics. In Pelton coal seam directly covered by marine sediments, vitrinite reflectance decreases (a) and vitrinite fluorescence increases (b) and (c). These changes indicate that the activity ability of anaerobic bacteria is enhanced upward. Other characteristics include usually higher pyrite (G) and higher vitrinite content (E), which are also the characteristics of the formation of transgressive coal. Pyrite mainly exists in the form of syngenetic small stones, most of which are preserved in the matrix of clastic vitrinite, but it shows high fluorescence when it approaches pyrite.

Although many coal seams affected by seawater show an increase in crustacean content, some of them show a decrease in crustacean content due to chemical erosion. This is because the pH value of peat water rises above the median value, such as long-term contact with alkaline seawater, but its preservation situation deteriorates rapidly with the increase of alkalinity.

Three. Episodic coal-forming process

Episodic coal accumulation was put forward by Mr. Zhang Pengfei and Professor Shao Longyi of China University of Mining and Technology (1992) according to the theory of coal formation by transgression when studying Carboniferous-Permian in southern China. They noticed that some thick coal seams in the land-sea interaction environment were distributed in different facies areas in a large area (hundreds to thousands of square kilometers). At the same time, it is also noted that the formation environment of some large-scale continuous coal seams is not necessarily related to the sedimentary environment of underlying sediments in coal seams, and this large-scale coal accumulation across different facies areas is expressed by episodic coal-forming theory. Because coal formation and coal accumulation in the process of transgression mainly occurs in the rising stage of sea level, at this time, the regional datum level rises with the rising of sea level, which provides accommodation space conducive to coal formation and enables thick coal seams to gather. Therefore, it can be proved that the most widely distributed coal seam in sedimentary cycle may be formed in flood period, and the thickest coal seam or limestone stratum in sedimentary cycle may be formed in maximum flood period. This large-scale coal accumulation is caused by regional or even global sea level (datum) changes, which can span different sub-environments, different sedimentary facies belts and even different basins. This theory emphasizes the synchronization of environmental coal accumulation in coastal plain and episodic coal accumulation during episodic sea level rise.

Episodic coal formation is closely related to sea level change. In the episodic coal-forming process, a sedimentary event and several sedimentary events it contains can form coal seams with a certain distribution scale. The coal seams formed by large-scale transgression events (such as the third or second transgression events) often have a large regional or basin-wide distribution scale, while the coal seams formed by the second transgression process (such as the transgression events of level 4 or below) have a small regional distribution scale. The former is equivalent to the largest flood period deposit in sequence stratigraphy and genetic stratigraphy, while the latter is equivalent to normal flood surface deposit. Therefore, the thick coal seams distributed widely are mostly the products of the main episodic coal-forming period, representing the maximum flood surface deposition, and the coal seams distributed in a small range are the products of the secondary episodic coal-forming period, representing the normal flood surface deposition. Between two large-scale transgressions, there may be multiple secondary transgressions, forming multiple secondary coal accumulation curtains, and the superposition of multiple secondary coal accumulation curtains forms a higher-level coal accumulation curtain. Combined with the principle of sequence stratigraphy, episodic coal-forming processes can be divided into coal-forming processes corresponding to different levels of sea level change, and the migration law of cohesive coal center and the distribution scale of coal seams in a sea level change cycle can be predicted within the sequence stratigraphic framework (attached figure 15-6).

Figure 15-5 Vertical profile distribution of several coal and rock components in Greta and Pelton coal seams of Greta coal measures in New South Wales affected by seawater (Diessel, 199 1).

Figure 15-6 Formation Process of Coal-bearing Cycle (Quasi-sequence) and Sea Level Change in Land Surface Sea Environment (according to Shao Longyi 1997)

Hao Liming et al. (2000) put forward the superposition method of cyclic frequency curve according to episodic coal-forming process, which states that in the coastal areas of craton basin, the change of sedimentary environment caused by sea level change has an important influence on coal accumulation, and coal accumulation in coastal areas is extremely important during sea level rise. Because in this area, the rise and fall of sea level will lead to corresponding changes in the sedimentary environment, and it is this change that makes the area without coal-forming conditions likely to become a favorable coal-accumulating area, and it can also make the originally favorable coal-forming area become unfavorable for coal-forming. In the land direction of the coastline, the rise of sea level provides suitable accommodation space for peat accumulation. If the rising speed of sea level (base level) is suitable for the accumulation speed of peat, peat can accumulate continuously and form thick coal seam. This usually happens when the sea level rises rapidly. During this period, a favorable coal accumulation area can be formed until the sea level rise rate greatly exceeds the peat accumulation rate and the peat is submerged. In the seaward direction of the coastline, the decline of sea level will make the submerged area unsuitable for peat accumulation shallow, even expose the surface, forming an exposed surface. When the sea level rises again, if the sea level rise rate and peat accumulation rate are properly configured, peat can continue to accumulate, and such areas can also become favorable coal-accumulating areas, usually when the sea level rise rate is low. Generally speaking, during the sea level rise, coastal areas, whether on the land side or on the ocean side, may show periodic sedimentary characteristics. On the contrary, in the land and deep sea areas far away from the coastline, it is difficult for the general change of sea level to change the sedimentary environment accordingly, and it is also difficult for a favorable coal-accumulating environment to appear. In this case, the number of coal-bearing sedimentary cycles can be less, thus forming a relatively stable and single sedimentary sequence. Considering the above factors, we can learn the information of sea level change by studying the cyclicity of drilling holes (or field measured profiles) in different areas. By comparing the information of several such points, we can intuitively get the change of sea level in an area, and then understand the migration law of coal accumulation centers and the distribution range of coal accumulation curtains in this area.

Fourth, the event coal-forming process.

The general law of stratigraphic overlap has long been accepted by people and has become an important basis for restoring the characteristics of ancient sea level change. The theory of average variation has always been one of the pillars of geological theory. For example, people generally accept the definition of transgression: transgression is a geological process in which seawater gradually invades the land and the coastline gradually retreats. Reflected in sedimentation, we can see that marine sediments overlap from marine facies belts to continental facies in turn, and in vertical sequence, it is a relatively complete continental → marine sequence; On the other hand, regression sequence is a relatively complete phase sequence. However, in the process of studying sea level fluctuation, transgression and regression, it is found that mutation or catastrophe is also an important phenomenon in the sedimentary dynamic mechanism of the basin. The formation and development of event stratigraphy has brought new ideas for basin filling and sedimentary analysis. In other words, some transgressions are of the nature of events, and the changes of water system are also caused by many factors. The water characteristics of continental basins are very different from those of offshore and sea basins, and the so-called water inflow and outflow laws are also very different. When considering transgression and regression, it is often a one-way advance and retreat, while when considering continental basins, it is often to look at the changes of the overall basin waters.

The new theory of geological science brings new ideas to coal geology, such as sequence stratigraphy, and also to the field of coal geology. Although sequence stratigraphy has brought revolutionary influence to earth science, especially sedimentology and stratigraphy, the idea of uniform change still dominates when discussing sequence formation. The main idea of classical sequence stratigraphy is still the regular change of sea level rise and fall. This sequence model with continental margin basin as a typical basin must be demonstrated according to the clear mechanism of sea level rise and fall. Therefore, it will inevitably lead to the leading role of variational theory in sequence interpretation. This can be seen from a large number of research results of sequence stratigraphy put forward in recent years. Some researchers compare the sequence formation mechanism of continental lake basin and marginal sea basin, trying to find the same explanation from them, that is, the continental sequence is also explained by sea level change, which seems a bit reluctant.

The rise of event sedimentology and event stratigraphy provides a new idea for explaining the overlapping phenomenon of discontinuous or completely different sediments caused by unexpected events in geological history. The research idea of event deposition, or mutation, is very important for explaining some key interfaces in sedimentary sequence, explaining the asymmetry of sequence structure and explaining some key horizons in sequence. Therefore, for the coal-bearing sequence, analyzing the coal-bearing sedimentary cycle and coal-forming process from the point of view of events or disasters can explain the reasons for the termination of peat accumulation due to sudden or unbalanced changes in water system. Because the genetic relationship between coal seams and deep-water deposits has not been satisfactorily explained, the explanation of the direct contact between deep-water deposits or marine deposits and coal seams in geological history is still controversial, and there is no scientific explanation from the coal-forming mechanism.

According to the Carboniferous-Permian continental marine filling deposits in northern China, shallow marine facies suddenly covered the continental sediments in a large area, and there was an obvious lack of phase sequence between them. Some scholars put forward the event transgression or exosome transgression (He Qixiang et al., 199 1), which is a rapid and sudden seawater intrusion event. The basin where transgression occurred needs specific background conditions, including paleoclimatic conditions at that time.

Firstly, due to the special paleogeographic background of the late Paleozoic epicontinental sea basin in North China (such as extremely flat basin basement, etc.). ), the process of transgression is often characterized by rapid invasion, which is "event" compared with the ordinary process of transgression. In the sedimentary records, sedimentary assemblages with completely different water depths are in direct contact (figure 15-7), during which there is obvious phase sequence loss (non-erosive discontinuous loss), and the transgressive layer is in. Marine limestone directly covering shallow water or exposed sediments is common in the coal-bearing strata of late Paleozoic in North China, and it has multicycle, which is the result of typical sudden transgression. This sudden transgression played an important role in the development and termination of peat bogs and the accumulation and preservation of peat in continental basins.

Figure 15-7 Contact relationship between transgressive coal seam and transgressive deposit (according to Chen Zhonghui, 1993)

Secondly, the late Paleozoic sea level change in the epicontinental basin of North China is characterized by high frequency and high complexity, that is, the late Paleozoic sea level fluctuation cycles are frequent, the sequence is complex and superimposed with each other, and under its control, a set of so-called alternating deposits between land and sea are formed, which generally conforms to the characteristics of "cycles and cycles" and is unique, that is, the rapid transgression process and oscillation frequency constitute the composite sea level change: in the long-term sea level fluctuation cycle, there are many short-term regressions in the long-term regression process. Similarly, there may be short-term regression changes in the process of transgression. The change of sea level is essentially different from the process of transgression and regression, which is the expression of the relative change of coastline after the comprehensive action of sea level rise and fall, basin tectonic subsidence and climate.

In addition, transgression deposits cover the whole basin, and large-scale transgression events have a profound impact on the whole basin system, including the transformation of sedimentary system tracts and the destruction and adjustment of ecosystems.

Carboniferous and Permian are important coal-forming periods in the world, and paleoclimate provides background conditions for the prosperity of plants, but glacier activity is also an important influencing factor, which directly affects the fluctuation of sea level.

Generally speaking, the late Paleozoic sea level change in the epicontinental sea basin of North China (i.e. sudden transgression and high-frequency composite sea level change cycle) controlled the filling sedimentation and coal accumulation in the epicontinental sea basin, and the coal accumulation characteristics were obviously different from other types of basins.

In view of the close relationship between transgressive assemblage and coal seams found in many areas, many transgressive deposits are considered as event transgressive deposits, and Li Zengxue and others (1995) put forward the viewpoint of coal formation by transgressive events. The basic contents of this view are as follows: the combination of marine sediments and coal seams is controlled by the cycle of sea level change, and peat bogs may develop on the basis of the original exposed soil in the early stage of transgression; This kind of peat swamp is a shallow water environment but not a typical water area after the seawater in the epicontinental sea basin has withdrawn for a period of time. In fact, it is a special swamp environment because it is exposed to soil or the seawater has withdrawn incompletely. Because this environment lasts for a long time, plants grow and spread, and peat bogs develop further; This kind of peat swamp is different from the peat swamp on the mainland and is often attacked by seawater. Peat was preserved after large-scale transgression. According to the research of Li Zengxue and others (200 1, 2002, 2003 and 2004), there is the following combination relationship between coal seam and transgressive layer: marine thin limestone/thick coal seam combination is formed in low sea level change cycle, and marine thick sediment/thin coal seam combination is formed in high sea level change cycle. In the sequence stratigraphic framework, the coal seam of transgressive system tract is located at the bottom of system tract, and the coal seam formed by regression is located at the top of high system tract. It can be said that the development of coal seam is related to the turning period in sea level fluctuation, and transgression coal formation has become an important feature of coal formation in continental offshore basins. In the period of low sea level change, the time suitable for peat swamp development lasts for a relatively long time. Although sea level fluctuation has an important influence on peat accumulation, peat accumulation can be carried out stably and eventually become coal. The isochronism of coal-forming in transgression events has also been confirmed by the isochronism comparison of biological assemblage, geochemical characteristics and geophysical data between marine sediments and coal seams in a large epicontinental sea basin in North China.