(2) Sedimentary environment and sedimentary facies
1. Paleocontinental margin sedimentary area
The sedimentary areas of ancient continental margin and carbonate platform are the basic characteristics of sedimentary types in this area. The former can be divided into tidal flat facies and nearshore shoal facies according to the rock type, bedding, structure, structural characteristics and biological association of sedimentary rocks. The latter can be divided into microfacies such as confined sea, platform shoal and open sea.
The sedimentary area on the edge of ancient land mainly refers to the tidal action zone distributed along the edge of ancient land and ancient island. This zone has the characteristics of extremely shallow seawater, sometimes bare, strong evaporation, high salinity, unsuitable for biological reproduction, and weak energy of intermittent water flow, and often accepts terrigenous sand, mud and phyllodes from ancient land denudation areas. Muddy, silty dolomite, algal dolomite, gypsum dolomite, muddy or banded micrite limestone and bamboo-leaf limestone are often formed, and siltstone and shale are often mixed. There are many thin, flaky or fine-grained structures, and there are many insoluble components in limestone. Rock color generally has strong oxidation or oxidation color, in which particles (such as gravel chips) often have oxidation edges. Sometimes accompanied by foreign biological debris and sand debris, particles and so on. Few fossils, simple combination. Under the action of tidal current, flat gravel and biological debris are mostly arranged in directional or imbricate, fan-shaped, chrysanthemum-shaped or vortex shape. Generally, dry cracks, gypsum-salt pseudocrystals, raindrop marks, karst breccia, bird's eye view, tents and other shallow water signs, as well as climbing marks, vertical hidden holes and other structures are developed. Diagenetic marks such as sand seepage, gravity cementation and dissolution formed by fresh water leakage are common. Microfacies can be further divided according to sedimentary position, seawater salinity, water energy and sediment properties.
(1) tidal flat facies
Including the supratidal zone, intertidal zone, lagoon environment in the supratidal zone and intertidal zone, and sometimes some shallow subtidal zones. The supratidal environment is closest to the edge of the ancient land, and only the storm climax can drown it. In arid and hot climate, the "capillary concentration" caused by strong evaporation on flat tidal flat often forms silt, silty dolomite, micrite argillaceous dolomite or algal mat dolomite, forming yunping subfacies. When a large number of terrigenous materials are mixed, mud cloud flat, sand cloud flat and even mud flat subfacies can be formed. In addition to the common sedimentary characteristics of tidal flat facies, argillaceous or cloud-like banded micrite limestone is a typical microfacies in intertidal zone and some shallow tidal environments, and sometimes there are tidal channels or tidal pools, and its microfacies are mainly bamboo-leaf limestone.
In a word, the beach environment is close to ancient land and islands, sometimes bare, sometimes submerged by extremely shallow water. The middle energy mainly comes from tidal action and is intermittent, lacking favorable conditions for biological reproduction, and the microfacies combination is simple, which varies with the geomorphological conditions of the ancient continental margin. The identification and delineation of tidal flat facies is the main basis for judging the position and erosion degree of ancient land, so its research significance is very important in the vast area where tidal flat facies is extremely developed in the eastern part of North China platform.
In the early stage of transgression in the early Cambrian, the sedimentary areas of the ancient continental margin were relatively developed. The southern, northern and northeastern parts of eastern North China are mainly composed of clastic rocks and clay rocks, while the land islands or underwater uplift areas near the eastern and western parts are mainly composed of supratidal clouds and huiyunping. Houjiashan Formation in Huaibei and Xinji Formation in Zhengzhou are the earliest tidal flat deposits. Because the transgression has just started, these facies belts are located in the north of Huaiyang ancient land, so they are mainly phosphorus-containing timely sandstone, conglomerate, siltstone and mudstone, while carbonate tidal flat deposits are mainly argillaceous limestone, quasi-syngenetic dolomite, breccia dolomite and purple shale in southern Shandong and southwestern Shandong. Changzhi, Linxian, Fanxian and Zaozhuang areas belong to the mud cloud flat subfacies in the supratidal and intertidal zones, with a total thickness of more than 33 ~ 70m. There are few biological debris in the rock, and there are finger structures such as stripes, cracks, bird's eyes and deformation bedding. Gypsum cloud has been developed in Pei county, and gypsum layer accounts for more than 10%. During the period of Shantou and Maozhuang, the scope of transgression increased and spread to the whole region, and then tidal flat subfacies were widely developed. The eastern region is mainly purplish red and purple silty shale and calcareous shale, mixed with thin micrite limestone. Tidal flat deposits still exist in the vast area east of Ordos ancient uplift from Xuzhuang period to Fengshan period. For example, in the early and middle period of Xuzhuang in the east, the plain areas exposed by drilling holes such as Tangshan, Jingxi, Quyang to Fengfeng, Liaogu 1, Gu2, Dong 1 and Da 1 inherited the tidal flat environment from Shanhaiguan to the west. The area to the west of Wenshui and Qinyuan in the eastern margin of Ordos ancient uplift, controlled by Liulin and Lishi remnant island, has received a large number of terrigenous coarse-grained debris from the steamed bread period to the Upper Cambrian, and developed sand-bearing clouds, of which Niyunping mud dolomite in Linxian County accounts for more than 60%. In Linxian, Lishi and the west of Zhongyang, more than 60% of sandy argillaceous micrite dolomite on the tide is also deposited. The rhythmic deposition of Fengshan Formation in the east of the platform is composed of argillaceous micrite limestone and calcareous shale, and it is also a typical intertidal sedimentary product.
Figure 1-3-2 Sedimentary facies division and main characteristics of Lower Paleozoic in North China
In tidal flat environment, sediments are mostly composed of regressive sedimentary sequences. For example, the thickness of Lower Ordovician Yeli Formation-Liangjiashan Formation in Pingshun and Fengfeng areas is 178 ~ 4 1 1m, which is mainly grayish yellow fine silty dolomite, stromatolite dolomite and breccia dolomite, with a small amount of bamboo-leaf dolomite. There are gypsum interlayers in Tangyi area, polygonal fractures in rock layers, interbedded breccia structure and bird-eye structure.
The Yeli Formation in Quyang, Dengfeng and Luxi areas is composed of medium-thin bedded micrite, spherulite micrite, bamboo-leaf micrite, argillaceous banded micrite and shale interlayer. Some structures, such as scouring, cracking, bird's eye, wormhole and gypsum pseudocrystal, are found in the rock, while biological debris is common in trilobites, sea lilies, ostracods, gastropods, spicules and so on, but the content is low. This set of microfacies constitutes a tidal flat sedimentary combination from tide to intertidal zone. Liangjiashan Formation evolved from biotite micrite limestone and argillaceous banded micrite limestone with medium thickness and flint to cloud banded limestone and micrite dolomite or breccia micrite dolomite. The microfacies assemblage shows the regressive sedimentary rhythm from the subtidal zone to the supratidal zone. Dairy Pass-Qishan Niyunping consists of fine-grained and silty dolomite containing flint, which is an inherited cloud level formed by the sustainable development of tidal flat environment of Yeli Formation. The bottom of Xiamajiagou Formation (formerly known as Jiawang Formation) is composed of brownish yellow micrite dolomite and light gray micrite limestone, with gravel at the bottom, which is distributed in many areas in the east of North China Platform. Shallow water and exposed traces, such as fine layers, dry cracks, bird's eyes and gypsum pseudocrystals, are common in rocks, but there are few biological debris, which should belong to supratidal mud and yunping subfacies. In Lishi, Hejin, Huixian, Xiyang-Wutai, Hancheng-Mengyin, Puyang, Handan, Yangquan and Xuhuai, there are Yunping or Ni Yunping. The content of terrigenous materials in quasi-syngenetic dolomite varies from place to place, and can reach as high as 30% in Xuzhou and Dengfeng. The Middle Shangmajiagou Formation is located in the north of Huaiyang ancient land, around Wutai and Luxi ancient uplift, with tidal flat deposits composed of thin micrite limestone, Bao Yun porphyritic micrite limestone and micrite dolomite.
Tidal flat and some depressions in intertidal zone are easy to accumulate water after low tide and are replenished when high tide, thus forming intertidal zone and intertidal lagoon environment. If there are shoals and dams in the sedimentary area on the edge of the ancient continent, lagoons often appear behind the beach dams. Tidal flat depression is small in scale and low in medium energy, and most of them are typical still water deposits. Dolomite and argillaceous micrite limestone in early Cambrian sandstone and mudstone, and silty shale and siltstone in Yunping and Huiyunping may be the products of lagoon salinization or desalination. Due to the barrier of the beach dam, the large lagoon behind the beach has poor seawater circulation, weak water energy and scarce organisms. The depth of water body is related to topographic relief, and there may be subtidal deposition when the water body is deep. Due to the backflow and infiltration of salinized seawater, micrite dolomite, spherulite dolomite, micrite dolomite and gypsum rock can be formed in lagoon environment.
Due to the development of tidal flat environment and platform shoal, and the lagoon environment is spatially attached to the above two environments, such lagoon environment sedimentary products can appear in the areas where tidal flat is distributed above the platform, behind the beach bar and between the platform shoals. Purple silty shale and micrite dolomite of Shantou Formation in the east of the platform contain halite pseudocrystals in a large area and gypsum salts in some areas. In some areas where the stratum thickness and lithofacies change greatly in a short distance, such as Jinan, Shandong and Shimenzhai, Hebei, it should belong to lagoon deposits. There are gypsum layers in the Ordovician tidal flat sedimentary facies of Dong 1 well, Liaogu 1 well, Tanggu 1 well and Fengfeng area in Linqing Depression, and there are three gypsum layers in Liangjiashan Formation of Liaogu 1 well. Therefore, this area should be dominated by alternating deposition of intertidal lagoons and confined seas, with occasional intertidal environment. The appearance of mud in Changhan Formation, Linfen high in the west of Yunping and Liaocheng high in the east shows that lagoons exist in the low-lying intertidal zone or supratidal zone on both sides of Niyunping highland. These two lagoon tidal flat sedimentary areas continued until Shangmajiagou period, and the gypsum layer content in their sedimentary rock series reached more than 50%.
In a word, the environmental products of intertidal zone and supratidal lagoon are mostly distributed in tidal flat sedimentary facies, and their distribution range and thickness are generally smaller than those of tidal flat sedimentary products, but their microfacies combination and sedimentary facies marks are far clearer than those of tidal flat facies.
(2) Shoal facies
This facies is located outside the sedimentary area of the ancient continental margin, mostly in the intertidal zone, and can partially extend to the subtidal zone. The coastal shoal environment is characterized by stable tidal current, but intermittent water energy is weak, shallow water, normal salinity, proper amount of foreign biological debris deposition, incomplete sorting and frequent exposure signs. The microfacies assemblage is dominated by bamboo-leaf limestone, banded micrite limestone or spherulite micrite limestone, and sometimes there is a small amount of siltstone or shale. Bamboo leaf limestone is a kind of granular limestone mainly composed of flat gravel, accompanied by biological debris such as trilobites and sea lilies, and a small amount of sand and particles. It is full of mud crystals, and sometimes there are a few bright crystals. Flat gravel mainly comes from micrite limestone, which is pre-fractured after short-distance transportation and redeposition in subtidal zone and intertidal zone. When there is a storm, the flat gravel often stands upright, or forms a rhythmic deposit of interbedded micrite limestone and micrite limestone arranged radially or chrysanthemum, which represents clear weather. This thin gravel limestone interlayer or lens is actually the product of storm flow. Symmetrical wave marks, horizontal bedding, mud cracks, wormholes and other structures can be seen in banded limestone and micrite limestone.
As early as the Neogene, the eastern part of the platform had the nature of low-energy beach in intertidal zone. For example, the granular micrite dolomite limestone with a thickness of 40 ~ 90m in Fujunshan Formation in Jingxi and Tangshan areas, and the granular micrite dolomite limestone with a thickness of 10m or more in Queen Jiashan Formation in Dabei, Xuzhou, are mainly composed of gravel, sand, trilobites, echinoderms and gastropods. In the middle and late period of Xuzhuang, Taiyuan, Mingshui, Dongming, Peixian, Pingdingshan and other places contain radioactive oolites, cloud oolites and granular oolites. Trilobite debris and terrigenous sand are common among grains, and the grain content is generally 10% ~ 16%, and the local oolitic interlayer can reach about 40%. The obvious and universal exposure signs, the performance of atmospheric fresh water infiltration and diagenesis, the occasional occurrence of Chinese cabbage stromatolites and the widespread existence of terrigenous mud and sand indicate the intertidal environment of coastal shallow water. The lithology of Xuzhuang Formation in Jingxi, Suxian and other places is similar, but the gravel content has increased, and the total particle size is between 7% and 20%. Staggered beds, dry cracks and insect tracks are also common.
This kind of deposit is very developed in Gushan Formation and Changshan Formation. Tangshan Formation, Jingxi Formation, Wutai Formation and Changshan Formation in Shandong Province are mainly composed of argillaceous limestone, argillaceous banded micrite limestone and a large number of bamboo-like limestone. Bamboo-shaped gravel chips are mostly oxidized, with high content and directional or random arrangement. Gravel chips contain biological chips such as trilobites and a small amount of oolitic grains, quartz sand, bright crystals or mud crystals to fill the gaps. There are dry cracks and vertical wormholes in micrite limestone, with occasional symmetrical wave marks. The banded limestone often transits to intermittent banded and chain limestone, and sometimes it can form "bamboo-like" micrite limestone. Although this structure has been interpreted as diagenetic compaction and crushing, it is not difficult to find that the cuttings are still dislocated and displaced at close range. They are formed in intermittent weak turbulent water with insufficient hydrodynamic conditions or short action duration. Therefore, the widespread existence of "bamboo-like" structure reflects the basic characteristics of the sedimentary environment of coastal tidal low-energy shoals. Columnar and hemispherical stromatolites usually develop in this facies belt. The height of stacked columns is generally less than 20cm, and the width of columns is only a few centimeters. The columns are concentric or granular oolites, sand chips and trilobite chips (Figure 1-3-3).
Figure 1-3-3 Columnar stromatolites of Gushan Formation in Xiaweidian, Xishan, Beijing
Large-scale transgression in early Ordovician led to the transformation from coastal shoal to platform shoal. After Huaiyuan Movement, quasi-syngenetic dolomite was widely developed in Majiagou in the early stage of transgression. According to the quasi-beach with particle content of 10% delineated by North China Petroleum Bureau and other units in 1987, the particles are mainly algae chips, sand, gravel chips and nucleated stones. As far as their relationship with tidal cloud flat is concerned, quasi-beaches should be low-energy shallow sediments in intertidal zone. Whether this kind of low-particle carbonate rocks can be used as microfacies of the sedimentary environment of underwater shoals or coastal intertidal shoals is still worth discussing. Generally speaking, the Ordovician was the biggest transgression period of the North China platform since Cambrian, and the platform itself was denuded and leveled for a long time, so it is understandable that the environmental differentiation of coastal shoals in the whole region is not obvious.
2. Carbonate platform sedimentary area
This sedimentary area is the inner side of the vast continental shelf closely adjacent to the sedimentary area of the ancient continental margin, and is separated from the shallow sea basin with many platform margin facies areas. Most of its sedimentary interfaces are between the low tide surface and the wave bottom, and some of them directly reach the redox interface. Sedimentation mainly occurs in the subtidal zone, and the seabed topography and water depth change greatly. The water depth can range from several meters to tens of meters, but generally it will not exceed 100 meters. The salinity of seawater is normal, slightly increased in low-lying areas, and organisms are common, while the energy of water body is between weak and medium, and low energy is dominant in tidal waters. This sedimentary area is very developed in North China Platform. According to its underwater landform-seawater unobstructed state, water depth and energy, it can be subdivided into limited intertidal sea, open intertidal sea and platform shoal.
(1) Limited Inter-beach Sea (Limited Sea)
With the change of seabed topography, due to the shelter and relatively low-lying shoals, a limited inter-beach marine sedimentary environment is often formed. Although seawater is connected with Guang Hai, its circulation is blocked, and its salinity is slightly higher, which is not conducive to the biological development of Guang Hai, and there are few fossil species. The main rocks are micrite limestone, spherulite micrite limestone, argillaceous banded micrite limestone, spotted micrite limestone and quasi-contemporaneous dolomite. It is characterized by the lack of high-energy particles. The common biological detritus in rocks are mostly halophytes. When this sedimentary area is closed by coastal beach bar or underwater uplift, lagoon deposits dominated by dolomite can be formed due to the surge of salinity.
In the late Fujun Mountain in the east and north of the platform, the confined sea developed relatively. From Shanhaiguan, Tangshan and Jingxi to Yixian, the thickness of Fujunshan Formation is reduced from 150m to 15m, and the upper strata are mainly gray, dark gray medium-thick bedded micrite limestone and micrite-micrite limestone. A small amount of spherulite micrite limestone is common in Beijing area. The rocks are horizontally layered and there are few organisms. Juxian area is mainly composed of cloud-like micrite limestone, silty shale and dolomite. Well Renqiu Gu 2, Well Liaogu 1 and Fengfeng area are interbedded with thin micrite limestone and shale. The sedimentary environment of Zhangxia Formation is mainly platform shoal and intertidal sea, which is limited if it is surrounded by many beaches. For example, the lower part of Zhangxia Formation in Jingxi and Datong, and the upper part of Tangshan and Yixian all belong to Zhangxia Formation, but their rocks and rocks generally have the characteristics of low content of high-energy particles, single organism, horizontal bedding, few scouring traces on the surface, occasional particles and undeveloped insect tracks. All of them reflect the limited marine environment covered by ancient land, underwater uplift or platform shoal.
Although the Lower Ordovician and North China Platform can be divided into areas dominated by quasi-contemporaneous post-dolomite in the south, areas dominated by micrite limestone, biotite micrite limestone, speckled limestone, argillaceous banded limestone and limestone with flint bands or nodules in the north, and western areas outside this work area, dolomite is mostly quasi-contemporaneous post-metasomatic dolomite, and its original rock lithology is mostly biotite micrite limestone and spherulite micrite limestone, so Tangshan.
The extensive transgression of the Middle Ordovician connected the limited ocean with Guang Hai, and the lithologic differentiation between the north and south of the eastern platform was broken into dolomite and limestone, forming a shallow-water sedimentary environment with more complicated lithologic combination. For example, micrite limestone deposited in Well Ren-2, Well Gu-2 and Xiajiagou of Quyang Formation in western Shandong Province is mixed with marl and dolomite, with horizontal bedding, containing trilobites, sea lilies, cephalopods and other fossil debris, and the bottom is generally scoured, so it should be deposited alternately in confined sea and shelf sea. In the west outside the work area, Majiagou was blocked by the ancient lands of Wulange, Alashan, Qingyang and Yan 'an in the early stage, which belongs to a typical limited marine deposit.
(2) Open sea area between beaches (high seas)
Refers to the open area in the middle or outside of the platform and the vast shallow water area where the platform and the open sea are smooth. Because the North China platform is characterized by the development of platform shoals, the open shallow platform between shoals can be called open inter-beach sea. Its sedimentary interface is mostly located between low tide surface and wave base surface, with normal salinity, water depth of several meters to tens of meters and medium energy. It is mainly composed of granular limestone, granular limestone and micrite argillaceous limestone, and sometimes contains shale and siltstone interlayer. The type of particles in rocks is single, and occasionally high-energy particles such as internal debris and oolitic particles are seen. Rich in fossils, including ostracods, mollusks, echinoderms, trilobites and brachiopods. Among them, the relative content of narrow-salt organisms is small. Common horizontal bedding, occasionally oblique bedding. Occasionally, small single-column and hemispherical layered structures, common horizontal wormholes and biological stirring structures appear.
Mudstones in some thick intervals of Maozhuang and Xuzhuang periods in the east of the platform are mixed with siltstone, glauconite fine sandstone and sandy limestone, with weak reduction color and more marine fossils, which may belong to offshore non-clear water deposits. In Liao Yan and Xuhuai areas, micrite limestone with algae chips, micrite limestone with biological chips, chlorite micrite sandy limestone and nodular micrite limestone in this formation should also be offshore products. The transgression reached its climax in the Zhangxia period of the Middle Cambrian, and the underwater uplift was mostly shoal, and the sea environment between beaches was relatively developed. It is characterized by that the content of oolite is generally less than 30%, the rocks are mainly dark gray micrite limestone, the shallow sea is rich in biological content, and the clay rock content changes greatly. For example, in some rock sections of Zhangxia Formation in Quyang, Renqiu and parts of Shaanxi, bioclastic micrite limestone or algal micrite limestone is dominant, with clay interlayer 10% or more, oolite less than 20%, and high organic matter content in rocks.
In the early Ordovician, Tangshan and Jingxi areas in the northern part of the platform were mainly composed of gray thick biological micrite limestone, banded micrite limestone mixed with bamboo-like gravel chips or sandy micrite limestone, and there was no oxidation edge. Trilobites, cephalopods, gastropods, echinoderms, brachiopods and other fossils are common in rocks, and sometimes horizontal wormholes can be seen, which is a typical offshore deposit. The offshore sediments in Xuzhou-Zhengzhou area are characterized by yellow-green or gray-green shale; In the plain areas such as Renqiu, Gang59 and Gu2 Well, there are banded micrite limestone, dolomite containing flint nodules and its interlayer, and granular limestone is not well developed. There are brachiopods, echinoderms, bryophytes and other Guang Hai biological detritus in the dolomite of western Shandong and Laiwu, which are deposited in the open sea and underwater bamboo beach with horizontal bedding and wormholes. The Xiamajiagou Formation in Tangshan, Jingxi, Quyang, Renqiu and other places is dominated by micrite limestone, with occasional dolomite limestone and marl, among which common fossils are brachiopods, echinoderms and cephalopods, which are horizontally bedded and alternately deposited in open sea and confined sea. The offshore of the upper part of Shangmajiagou Formation consists of biotic micrite limestone, flint micrite limestone and mica micrite limestone, with cephalopods as the main organisms.
The micrite limestone in the upper part of Fengfeng Formation contains many normal marine fossils, and only remains in Fengfeng and Luxi.
In a word, the offshore of eastern North China is mostly controlled by underwater shoals, which are distributed among beaches. Most of the western part of the platform outside the work area is blocked by the ancient land, ancient uplift or platform front facies belt, and distributed behind it. Vertically, it appears alternately with shoals and confined seas. In space, it is mostly distributed in a semicircle around the ancient uplift belt, while in the eastern work area, it is generally distributed in a strip or irregular equiaxed shape.
(3) Platform shoal
There are many underwater uplifts of different sizes and shapes on the North China platform. Most of these areas are above the wave base, with shallow water bodies and long-term exposure to medium-energy seawater, which makes them become beach sedimentary environments and form shoals isolated from the platform. Platform shoals are mainly subtidal deposits with high energy band, with common flash oolitic limestone, flash sandy limestone and flash bioclastic limestone or flash amphibole and algal debris limestone. Sometimes granular bedding, cross bedding, columnar and palmately stromatolites are found. Diagenetic phenomena, such as flowing water wave marks, hail marks, oolitic molds and gravity cementation, are well developed.
Figure 1-3-4 Cambrian sedimentary model of eastern platform
During the Xuzhuang period in eastern North China, thick oolitic limestone was distributed in Beijing, Quyang, Renqiu and Juxian, indicating that the underwater shoal environment appeared intermittently. By the Zhangxia period, oolitic beaches were widely distributed, from Tangshan, Jingxi and Quyang to Fengfeng, and oolitic limestone appeared in different parts, reflecting that shoals developed intermittently in the vertical direction with crustal movement and distributed regularly in the horizontal direction (Figure 1-3-4).
In addition, there are some shoals in Hejin, Hongdong, Xiyang and Pinglu around Lvliang Land Island, and Xuhuai, Songshan and Luoyang in the south of the platform. In the hidden areas such as Huanghua, Jiyang and Renqiu sag, some beaches are also developed by drilling. Well Rugang 59 and Well Tang2 are thick oolitic limestone with stable lithology.
During the Zhang Xia period, shallow water and high energy beach environment was widely developed, and most beaches were confined to ocean or open sea environment. The shoal environment is stable, with thick oolitic limestone and high water energy. Most of them are bright oolitic or pea-shaped limestone, and bamboo gravel beach or sand beach are rare. Shallow oolitic beach is easy to emerge from the water surface, and there are traces of outcropping in the thick massive oolitic limestone of Zhangxia Formation in Zhang Xia, Xuhuai and Fengfeng. Diagenetic marks such as atmospheric fresh water leaching and fresh water cementation are also common.
The Yeli shoal is mainly bamboo-like gravelly limestone. According to the single-factor quantitative statistical comprehensive analysis advocated by Feng Zengzhao, Quyang, Jintang, Luzhong and Juxian are all shallow based on the grain content of 15%. In bamboo limestone or dolomite, the gravel content is above 50%, and gypsum is the main filler, and bright crystals appear from time to time. Gravel chips are mostly flat, round and moderately sorted without oxidation edge, and sand chips and debris are common in gravel. The water body is generally shallow and the salinity is normal. The second large-scale transgression in the Middle Ordovician formed a sedimentary environment dominated by Yunping restricted platform and open platform, with occasional bamboo limestone, but with small thickness and lateral instability, and basically no shoal sedimentary characteristics. Outside the work area, in the west of the platform, controlled by Ordos paleouplift, Qinling Qilian trough and Huaiyang ancient land, the sedimentary area is narrow, the platform shoal is atypical, and the platform edge slope is more developed than the basin. In the east of North China platform east of Ordos paleouplift, the platform margin facies and basin facies are not typical.