Sedimentary system and its sedimentary facies characteristics

Many previous studies have shown that (Li Zengxue et al., 1995, 1997, 2000, 2001; He Zhiping, 2005; Song Lijun et al., 2004), the Carboniferous-Permian system in North China Platform developed a set of marine-continental coal-bearing rock series that transited from sea to land. In the late Carboniferous, it was a mixed sedimentary system of offshore carbonate shelf and barrier dam, and it developed into a delta sedimentary system in the early Early Permian. In the late Early Permian and early Late Permian, it was basically a continental alluvial sedimentary system, and in the late Late Permian, it developed a continental river and lake deposit.

On the basis of field profile, macro-sedimentary facies analysis of borehole cores and summary of lithofacies types, five sedimentary systems (1 1 sedimentary facies types) are identified according to the vertical combination relationship and plane distribution of various lithofacies (Table 2.2).

Table 2.2 Carboniferous-Permian sedimentary system and sedimentary facies units in the study area and its periphery

2. 1.2. 1 fluvial sedimentary system and sedimentary facies

Water system plays an important role in coal-bearing rock series and is developed in various types of coal-bearing rock series. In the long-term settlement of wet areas, a large amount of sediment carried by rivers accumulates, forming alluvial plains in the middle and lower reaches of rivers, especially in offshore areas. River sedimentary system can be subdivided into secondary sedimentary facies types such as river bed, shoal, natural dike, flood plain and crevasse fan, and their sediments have their own characteristics. River deposits are widely developed in the upper Permian Shihezi Formation and Shiqianfeng Formation at the eastern foot of Taihang Mountain and the southern foot of Yanshan Mountain, as well as in the strata corresponding to the lower Shihezi Formation, the upper Shihezi Formation and Shiqianfeng Formation in the late early Permian and late late late Permian in Beijing and Xinglong area. According to the development degree of flood basin, the river sedimentary system can be further divided into meandering river and braided river, in which braided river channel sandstone is developed, flood basin is not developed, sand and mud content is relatively high, and meandering river is the opposite. Sedimentary facies units include river bed stagnant sedimentation, edge beach (or heart beach), natural dikes, crevasse fans, flood basins and other sedimentary units, which are in a positive grain order structure with upward tapering in the vertical direction (Figure 2.7).

Fig. 2.7 Vertical sedimentary model of meandering river in the study area

(1) river bed retention deposit

The particle size of sediments carried by rivers varies greatly. In addition to suspended sediment, mud and other substances that make up the upper sedimentary cycle, sand is the most important sedimentary object. The transportation of coarse gravel debris is very slow, and only short-distance transportation is carried out during the peak period of river flow. Therefore, under normal circumstances, due to the scouring and sorting action of running water, fine-grained materials are constantly taken away, while coarse-grained gravel is left at the bottom of the river bed and produced as a lens, which is called river bed retained conglomerate deposition. It is usually located at the bottom of the river sedimentary profile and gradually transits upward to the beach deposit.

In the river bed retained conglomerate deposition, there are channel staggered fine conglomerate facies and large plate staggered gravelly coarse sandstone facies. Rock types are mainly fine conglomerate and gravelly coarse sandstone, with poor sorting, poor grindability, low composition maturity, massive bedding and scouring contact with underlying rocks. The sand bodies are mostly small lenses, the gravel is obviously oriented, and the maximum plane is inclined upstream of the river.

(2) Beach deposition

The beach is the most important flowing landform of meandering river, and it is the inevitable product of lateral transfer. The river spirals forward in the river course, constantly scouring and eroding the outer dike, and because of the lateral circulation, the objects carried in the river are brought to the inner dike for deposition. At first, only shallows formed. With the continuous development of the river in lateral transfer, the shoal also grows, forming a wide beach in the inner bay of the river. The intensity of underflow in lateral movement gradually weakens, which causes the mechanical differentiation of the transported object. In the lower part of the beach, which is also close to the center of the river, coarse-grained debris is deposited, while in the upper part of the beach far from the center of the river, fine-grained substances are deposited. The thickness of beach sediments is similar to the depth of river bed, and its width depends on the size of river, so beaches with large rivers are widely developed.

The beach deposits in the study area and its periphery are characterized by the development of coarse sandstone facies in plate and trough cross bedding and sandstone facies in trough cross bedding. Lithology is mainly medium-coarse grained feldspar timely sandstone with low maturity and high matrix content. Large-scale plate cross bedding and lateral migration cross bedding are developed in sedimentary structures, and trough cross bedding can also be seen, reflecting the unidirectional flow of water (Figure 2.8).

Figure 2.8 Sedimentary characteristics of meandering river channel in the study area

(3) Natural dike deposition

The natural dike was formed because the river overflowed the bank during the flood. When the river becomes shallow and the flow rate decreases, a large amount of transported materials carried by the river quickly deposit on the shore, forming a natural dike. The grains of rocks deposited on natural dikes are finer than those deposited on marginal beaches and coarser than those deposited in flood basins far away from rivers. They are mainly composed of siltstone and argillaceous rocks. The most prominent feature of lithologic combination is that silty sand and argillaceous layer form thin interbeds. Sandstone layer is generally tens of centimeters thick, and argillaceous layer is generally several centimeters to tens of centimeters thick.

Horizontal bedding fine sandstone facies and sandy bedding fine sandstone facies are developed in the study area and its periphery, which are composed of fine feldspar timely sandstone, silty mudstone and mudstone. See parallel bedding and sand bedding, containing a small amount of plant fragments and root fossils, and occasionally see biological caves.

(4) crevasse fan deposition

When the water level is high, excessive flood will break the natural embankment and form a tongue-shaped deposit of dendritic water system on the slope near the plain, which is called crevasse fan. Crevasse fans sometimes extend to neighboring flood basins. Crevasse fan deposit is a kind of tongue-shaped sand body with lens cross section. The thickness is generally not large, from a dozen centimeters to several meters. The grain size is usually coarser than that of the adjacent riparian sediments, and it is mainly composed of fine sand and some silt-sized substances. The bedding is mainly various small cross bedding, and it may also be medium cross bedding. Other common structures are scouring and filling structures, and plants or other biological debris brought by river water. The sedimentary rocks of crevasse fan in the study area are mainly composed of coarse sandstone and medium-fine sandstone. The sand body is a lenticular body that does not extend too far laterally, and has typical small sand bedding, which is often washed with the underlying stratum.

(5) Flood basin deposition

Flood basin is the lowest part of alluvial plain, including flood lake and flood swamp. Like other stagnant basins, Honghu Lake is mainly suspended sediment brought by flowing water during flood period. The size, shape and location of the river-flooded lake depend on the development history of the alluvial plain, and the length is generally much larger than the width.

In the study area, the sediments in the flood basin are characterized by the development of siltstone facies containing plant debris and purplish red and purplish gray massive mudstone facies. The broad and flat alluvial sedimentary area formed by micro-floods in the flood basin is mainly composed of fine-grained sediments accumulated vertically, generally composed of mudstone, silty mudstone and siltstone interbedded, with horizontal or horizontal wave bedding in some areas, and rich in plant fossil fragments and root fossils. According to the characteristics of sediments, it can be subdivided into floodplain, floodplain lake and riparian swamp (riparian swamp), in which the floodplain is dominated by sandy sediments, riparian lake is dominated by muddy sediments, and riparian swamp is also dominated by muddy sediments, but generally there are peat deposits, forming coal lines or coal seams.

2. 1.2.2 Delta depositional system and sedimentary facies

Delta is one of the oldest concepts in geology. As early as 400 BC, Herodotus and others used the term "delta". He found that the alluvial plain at the mouth of the Nile River was very similar to the Greek letter "delta" in shape, and the word delta came into being at that time. The original definition of delta was put forward by Barrel( 19 12), who believed that "the delta part injected by rivers or built only by permanent water bodies belongs to the sediments on the land surface". Xu Jinghua (1979) defined the delta as: at the mouth of the river that enters the sea and the lake, the slope gradually becomes gentle, and a large amount of sediment is accumulated, and the river forms many branches. Usually, the sedimentary area that is often influenced by the river below the first tributary is called delta. Generally speaking, at the intersection of rivers and oceans or lakes, it is deposited into a cone-shaped large sedimentary body, that is, a delta. Delta is a region dominated by terrigenous clastic deposits, which belongs to the transitional environment between land and sea. Delta deposition is the result of the interaction between river action and ocean action in the estuary area, and a large number of sediments provided by rivers and the settlement of sedimentary areas are the most important factors for its development. The ideal δ shape is conical, but it is difficult to achieve the ideal shape in practice. Influenced by climate, current, sediment, estuary process, waves, tides, current, wind, shelf slope and the structure and geometry of sedimentary water basin, the environment of delta sedimentary area is extremely complicated. After the river bifurcates, it enters the sedimentary range of this area from the first bifurcation. There are lake and swamp sediments between bifurcated rivers, and coastal sediments are mixed near the coast. Therefore, the sediments in the delta environment are mixed-called delta system.

Delta deposits are developed in the study area from Shanxi Formation to Shangshihe Formation, and the delta cycle is clear vertically, indicating that it is obviously affected by sea level change. The coal seam is well developed and its lateral distribution is continuous. Delta system includes sedimentary facies such as upper delta plain, lower delta plain, delta front, pre-delta and inter-delta bay (Figure 2.9).

Figure 2.9 Sedimentary characteristics of meandering river delta in the study area

(1) Upper Delta Plain Facies

Theoretically, the delta plain is located between the first branch of the river and the highest lake (sea) surface, which is the onshore part of delta deposition and the extension of the river-related sedimentary system in coastal areas. Its sedimentary environment and characteristics have many similarities with river facies, which is a microcosm of river facies to some extent. The lithology is mainly sandstone, siltstone and mudstone (including peat and coal). Sandy deposition and the formation of peat and coal are important characteristics of this subfacies. Sandy debris has poor sorting and complex bedding structure, which varies with different environments. See raindrop prints, cracks, footprints and other bedding structures. There are few biological fossils, most of which are freshwater animal fossils and plant residues. The rock mass is lenticular with great lateral variation. The distributary channel and swamp deposits constitute the main body of this subfacies, which is an important difference from ordinary rivers. The upper delta plain subfacies can be further divided into distributary channel, natural dike, crevasse fan, flood basin, bay lake and other sedimentary types.

1) distributary channel deposit: this facies constitutes the main body of the upper delta plain, and is relatively developed in Shanxi Formation and Shihezi Formation in this area, mainly developing low-angle cross-bedding medium sandstone facies and trough cross-bedding medium sandstone facies. The rock type is mainly medium-grained sandstone, and the positive grain sequence becomes finer upward. The bottom is composed of stagnant layer containing coarse sandstone and obvious scouring surface, and large-scale plate-like and trough-like cross bedding is developed. Vertically, it is usually related to natural dikes and flood basins (Figure 2. 10).

Fig. 2. Sedimentary characteristics of distributary channel in10 delta plain

2) Natural riverbank deposit: it develops on both sides of the riverbank and vertically above the distributary channel, and is formed by the accumulation of coarse-grained materials suspended in flood water on both sides of the river during flood period. Horizontal bedding fine sandstone facies and sand bedding fine sandstone facies are developed, and the rock types are mainly siltstone and fine sandstone. There are small cross bedding and horizontal bedding, and biological agitation structure can be seen locally.

3) Crevasse fan deposition: the lithology is mainly fine sandstone and argillaceous siltstone, which is formed by cutting natural dikes along the river bank by dendritic water system. Due to rapid accumulation, the detritus in rocks has poor sorting, low roundness and low maturity, and often contains more miscellaneous bases. There are small groove cross bedding and sand cross bedding, and sometimes there is horizontal bedding, which is fan-shaped in plane and small lenticular in section.

4) Flooding basin sediments: siltstone facies, gray and dark gray mudstone facies and carbonaceous mudstone facies containing plant debris are developed, mainly lake and swamp sediments. Rock types are composed of mudstone and silty mudstone, with a small amount of plant fossils and debris, and siderite nodules are common. The sedimentary structures are mostly horizontal bedding, horizontal wave bedding or massive bedding.

5) Bay Lake: When the basin continues to decline and the distributary channel of the upper delta plain is submerged for a long time, the distributary bay will be transformed into a local bay lake. Its lithofacies types are mainly gray-gray black siltstone, argillaceous siltstone, mudstone and carbonaceous mudstone containing plant debris. The sediments are mainly lakes and swamps, siderite nodules are common, and coal seams are developed. The sedimentary structures are mainly horizontal bedding, low-angle wavy bedding and massive bedding.

(2) Lower Delta Plain Facies

The lower delta plain is located between the highest lake (sea) surface and the lowest lake (sea) surface, which is equivalent to the land-water transition zone of the delta plain. Compared with the upper delta plain, the scale of distributary channel sand body in the lower delta plain continues to decrease. In the process of distributary channel extending to the sea (lake), the channel widens, the depth decreases, the bifurcation increases, the velocity slows down and the accumulation speed accelerates. The sediments are mainly sand and silt, with a small amount of silt. Staggered bedding, wavy bedding and scouring and filling structures are often developed, and intra-layer deformation structures can be seen. The cross section of sand body is lenticular, and it becomes fine-grained deposition in the transverse direction.

The lower delta plain is formed by crevasse fan deposits, distributary bay filling deposits and natural dikes formed by underwater and surface materials, which has obvious characteristics of distributary channel and distributary bay development. The distributary channel deposits are closely related to the distributary bay deposits vertically and horizontally. The distributary bay deposits are mainly dark gray to black mudstone, but also irregularly distributed limestone and siderite.

The lower delta plain facies in the study area mainly includes distributary channel, distributary bay and other sedimentary units (Figure 2. 1 1).

Fig. 2. 1 1 Sedimentary characteristics of lake delta in the study area

1) distributary channel deposit: represented by medium-coarse sandstone, fine sandstone, siltstone and mudstone, it has scouring surface, large-scale plate cross bedding and large-scale trough cross bedding, mainly developing low-angle cross bedding medium sandstone facies and trough cross bedding medium sandstone facies, and the scale of bottom scouring surface becomes smaller. Trunk fossils and mud gravel are common on the scouring surface, and the direction of ancient water flow is mainly one-way.

2) distributary bay deposit: this is a brackish water body separated by natural dikes or swamps, which is connected with open sea water to a certain extent, and develops sand layer fine sandstone facies, gray and dark gray mudstone facies and carbonaceous mudstone facies. Its sediments are mainly dark mudstone, carbonaceous mudstone and silty mudstone, often with sand lens formed by cracks, horizontal bedding, horizontal wavy bedding and small sand bedding, a small number of plant roots and leaves fossils, and more siderite nodules. In brackish water-brackish water environment, it is often vertically associated with distributary channels and distributary estuaries.

(3) Delta front facies

The delta front is the underwater part of the delta, which is distributed in a strip shape in the delta plain to the sea edge. In the process of marine delta formation, because the density of river water is less than that of water storage body, after leaving the delta plain, the low-density river water will be suspended on the high-density sea water in the form of plane jet, and the muddy and sandy sediments carried by it will gradually settle and accumulate near the estuary, forming estuary dams with sandy sediments and far-away sand dams with silty sediments.

1) Underwater distributary channel: it is a natural extension of distributary channel in underwater delta plain. The underwater distributary channel is often massive with unclear grain sequence, and the electric logging curves are mostly box-shaped and some are bell-shaped.

2) Estuary bar deposition: Estuary bar, also known as distributary estuary bar, is located at the mouth of distributary channel, with the highest deposition rate. Muddy sediments were washed away by seawater and winnowing, and sandy sediments were preserved. Therefore, the sediments of the estuary dam are mainly composed of fine sand and silty sand, which are well sorted, pure in quality, with well-developed trough cross bedding and medium-thick layers. It can be seen that there are water wave marks and wave-making marks. As the delta advances to the sea, the estuary dam covers the silt deposits in the pre-delta, and there are few biological fossils.

The sandbar deposits in the estuary of the study area are mainly composed of relatively clean fine sandstone and medium sandstone, with medium-good sorting and low argillaceous matrix content, which may be the result of repeated screening of estuary waves and tides. Mudconglomerate facies, trough-staggered fine sandstone facies, trough-staggered medium sandstone facies and plate-staggered medium sandstone facies are developed. The bedding types include wedge-shaped cross bedding, low-angle cross bedding and large trough-shaped cross bedding, and sometimes swirling bedding and * * * mud gravel can be seen. The direction of paleocurrent is greatly influenced by rivers and tidal currents, and it often has landward or multidirectional components. The profile of sand body is lenticular, the grain size becomes coarse from bottom to top, and the logging curve is often inverted bell-shaped, with abrupt change or gradual change at the top and gradual change at the bottom. The top of the delta plain is often cut by distributary channels, and flows vertically to Chang Shengcheng distributary bay (Figure 2. 12).

Fig. 2. Sedimentary characteristics of delta front and pre-delta in12 study area

3) Distant bar deposition: Distant bar is the marginal part of delta front environment inclined to the ocean. It consists of striped silt, clay and fine sand; Sedimentary structure is characterized by horizontal bedding composed of silt and clay, but cross bedding, filling structure and erosion surface are also common. There are common hidden caves, biological disturbance layers and crusts of different sizes. In the sedimentary sequence, it is located below the mouth bar and above the pre-delta mudstone, forming a sequence that gradually thickens from bottom to top.

The deposits of the far sand bar in this area are characterized by the development of sand-grained bedding fine sandstone facies, parallel bedding fine sandstone facies and biological disturbance siltstone facies. Small sand bedding is characterized by interbedding of medium-fine sandstone with siltstone, silty mudstone and mudstone, thin interbedding of sand and mud and some biological disturbance structures. Together with estuarine bar deposits, it constitutes a sequence of upward coarsening of grain size.

(4) prodelta facies

Prodelta facies is located in front of the delta front, which is a continuous extension of the delta tongue to the sea. Especially in the process of mixing low-density river water with high-density seawater in marine delta, coarse sandy and silty sediments are deposited near the estuary, and muddy sediments can be further transported by suspension, thus forming pre-delta muddy sediments on the seaward side of the delta front.

Prodelta facies in the study area is represented by gray and dark gray mudstone facies, which is mainly composed of fine-grained sediments such as silty mudstone, mudstone and siderite thin layer, with horizontal bedding and occasional animal fossils. Pre-delta and delta front deposits are generally characterized by upward thickening, which are typical inverted cones on logging curves and are easy to identify.

(5) Interdelta bay facies

Interdelta bay refers to the bay between large delta flowers, which has a sedimentary environment from brackish water to normal seawater. The water body here is limited by the delta flowers on both banks, with low water energy and shallow water, and the sediments are often fine, mainly mudstone, carbonaceous mudstone and siderite mudstone. When the water body becomes shallow, the groundwater level is higher than the sea level, and peat bogs can develop and form thick coal seams.

2. 1.2.3 Lake sedimentary system and sedimentary facies

The lake system in this area is mainly developed in the upper and lower Shihezi Formation, characterized by a set of extremely thick layered gray silty mudstone and siltstone deposits, with large formation thickness and low sand content. Horizontal bedding and sand bedding are developed, mainly deposited in some coastal and shallow lakes (Figure 2. 13).

2. 1.2.4 barrier-lagoon sedimentary system and sedimentary facies

Barrier island, lagoons and beaches are located in the transitional zone between land and sea. Like deltas, they belong to the transitional facies group between land and sea. They are a comprehensive sedimentary system. The sedimentary environment of barrier-lagoon system mainly includes barrier dam (or island), offshore shelf in front of barrier island, tidal flat behind barrier island, lagoon, tidal delta and scouring fan (Figure 2.66

(1) barrier dam facies

The barrier dam deposits in the study area are mainly coastal deposits, which are developed in the strata of benxi formation and Taiyuan Formation in the Late Carboniferous. The lithology is mainly fine, medium and coarse sandstone, with medium-good sorting and high composition maturity, which tends to be in the reverse grain order of upward thickening in the vertical direction. Low-angle cross-bedding sandstone facies is developed, and large-scale cross-bedding generally exists in sedimentary structures, especially low-angle cross-bedding (Figure 2. 15).

(2) Overthrow the fan phase

It grows in the lagoon behind the bar, and the coarse sediment on the bar is carried to the lagoon with low energy by storm waves. Lithology is generally fine sandstone, which is well rounded but poorly sorted. The sand body is medium-thick to thick layered, the top-bottom interface changes suddenly, and the logging curve becomes thicker upward. Biological disturbance structures are developed in sandstone, and the bedding structure becomes unclear due to biological disturbance. Even the rocks themselves are mixed in composition due to the development of biological disturbance structures, which are often vertically associated with lagoon deposits.

Figure 2. 13 Sedimentary characteristics of dark argillaceous rocks in lakes in the study area

Fig. 2. 14 Sedimentary sequence of offshore carbonate shelf-barrier bar-tidal flat of Taiyuan Formation

Fig. 2. 15 sedimentary characteristics of bar sand in the study area

Figure 2. 16 Sedimentary characteristics of tidal flat-lagoon in the study area

(3) lagoon facies

Mainly developed in the Upper Carboniferous benxi formation and Taiyuan Formations in the study area, the lithology is mainly black-gray black mudstone and siltstone, containing more siderite oolites and nodules, indicating that it is a reducing environment, often containing biological fossils, but relatively monotonous, and brachiopods and molluscs are more common. In addition, there are biological caves, and the bedding type is mainly horizontal bedding representing calm, with occasional lenticular bedding development. The lagoon is a closed or semi-closed shallow water basin, which is separated from Guang Hai by sand bars and connected with Guang Hai by waterways. The water medium conditions are changeable, mainly brackish water-brackish water (Figure 2. 16).

(4) Tidal flat facies

Tidal flat facies is one of the important sedimentary facies in this area, especially the upper Carboniferous Taiyuan Formation. Tidal flat deposits usually have a vertical sequence of upward tapering, with sand flat at the bottom and mixed flat and mud flat at the top. Tidal flat deposits equivalent to modern coastal deposits (Wang Ying et al., 2003).

The beach is located near the low tide line between the average high tide line and the average low tide line. Most of the sediments are sandy materials, where tidal action is strong and belongs to high-energy environment. Sediments are generally composed of pure sand, and the rock types are mainly fine and medium grained sandstone, mixed with thin siltstone. Sandstone sorting is medium-good, featuring bedding structure, low-angle cross bedding, vein bedding and sand bedding. A small number of siderite nodules and plant fossils were found locally, and biological disturbance structures and hidden caves can be seen.

The mixed flat is a transitional deposit between the mudflat and the sand flat, where the siltstone facies with biological disturbance is developed, and the lithology is mainly siltstone, mostly with thin interbeds of light-colored fine sandstone and dark mudstone or sandy mudstone, and tidal bedding such as waves, veins and lenses is developed, especially with thin interbeds of sand and mud, which is typical of common biological disturbance structures.

Ni Ping is located near the high tide line and above the tidal flat, belonging to a low-energy environment. It is characterized by the development of gray, dark gray mudstone facies and carbonaceous mudstone facies. It is mainly composed of mudstone, carbonaceous mudstone and sandy mudstone, with occasional silty sediments brought by spring tide, including plant fossils, and well-developed biological disturbance structures. The bedding types are mostly horizontal bedding and gentle slope bedding.

2. 1.2.5 Sedimentary system and facies of offshore shelf

(1) offshore muddy continental shelf

The offshore area is located outside the barrier bar and below the normal wave bottom, with low water energy but normal salinity. Biological disturbance siltstone facies and gray and dark gray mudstone facies are developed. The sediments are mainly mudstone and siltstone, with horizontal bedding and sand bedding, containing narrow salt animal fossils and biological disturbance structures, which are transformed into offshore carbonate shelf and barrier bar sediments downward and upward respectively (Figure 2. 17 ~ Figure 2. 19).

Fig. 2. Sedimentary characteristics of offshore shelf in17 study area

Fig. 2. Sedimentary characteristics of offshore muddy shelf in18 study area

Fig. 2. Sedimentary characteristics of offshore carbonate shelf in19 study area

(2) Offshore carbonate shelf

There are 12 ~ 14 layers of limestone and marl in benxi formation and Taiyuan formations in southwest Shandong Province, mainly composed of some bioclastic granular mudstone and argillaceous granular rock, with relatively developed bioclastic. Biofossils usually include in-situ assemblages, such as foraminifera, ostracods and brachiopods, as well as some biological debris from different places. Bioclastic types are also very rich, with different sizes, mainly brachiopod, Ostracoda, algae debris and mollusk debris. In addition, the development reflects the limited calm-relatively open and turbulent trace fossils, such as Zoophycos and Rhizocorallium, which are often * * *, and common horizontal, inclined and vertical caves. Limestone bedding is generally undeveloped, but sometimes hill-shaped cross bedding and grain-order bedding can be seen, reflecting that the sedimentary environment is often affected by storm waves. When there is a storm, the hydrodynamic conditions change greatly, forming storm sediment-storm rock. After storm deposition, it turned into fine weather deposition. The former forms granular rock and argillaceous granular rock with scouring surface, granular bedding and hilly cross bedding, while the latter forms stucco rock with biological disturbance structure.

The above characteristics reflect that the facies is mainly formed in the sedimentary environment of offshore carbonate shelf below the low tide surface, and its hydrodynamic conditions are generally weak, but it is often affected by storm surge. The salinity of seawater is basically normal, with good biological development and rich organic matter. The limestone of benxi formation-Taiyuan Formation in the study area was basically formed in this environment.