Following the new chronostratigraphic division principle mentioned above, according to the division scheme of China Regional Chronostratigraphy (Geological Age) Table (II) (Continental Phase), taking into account the tradition and facilitating the application of achievements, the connotation and division of lithostratigraphy still follow the existing scheme. The boundary of lithostratigraphic group is based on the marker layer with relative isochronous significance, and the relationship between chronostratigraphy and lithostratigraphy is established (Table 1-3-9). According to the above ideas, based on the previous work (Wang Zengyin, 1996), the Carboniferous-Permian stratigraphic system in Bohai Bay Basin is established (Figure 1-3-40).
Late Hercynian tectonic movement after Carboniferous-Permian deposition caused uplift and denudation in some areas, forming parallel unconformity, but most areas were in continuous sedimentary relationship with Triassic, so this project divided the Lower Triassic and Carboniferous-Permian into the same structural layer for research.
(A) the characteristics of rock strata
Lithostratigraphic units pay attention to the consistency of rock characteristics without considering the influence of time, genesis, climatic environment or events (Jiang, 2004). In addition to lithologic combination characteristics, natural gamma logging curve and resistivity curve (Gubei Well 2) are added to the borehole data in the coverage area for division and comparison.
Carboniferous-Permian can be divided into benxi formation, Taiyuan, Shanxi, Xiashihezi, Shangshihe and Shiqianfeng formations (Sunjiagou formation). Triassic is limited in distribution. Only in a few areas such as Linqing Depression, the Lower and Middle Triassic remains, which can be divided into Liujiagou Formation, Heshanggou Formation and Ermaying Formation from bottom to top.
Table 1-3-9 Carboniferous-Permian Division and Comparative Evolution Table of North China Platform
Figure 1-3-40 Carboniferous-Permian stratigraphic system in Bohai Bay Basin
1. Carboniferous-Permian
(1) benxi formation
It is mainly composed of a set of shale and sandstone sandwiched with thin marine limestone, and some areas are sandwiched with coal lines. Its main feature is that there is no coal seam and medium-fine grained timely sandstone. Most of the lower part of benxi formation belongs to limestone platform facies deposits, and the seismic characteristics are strong vibration, high-frequency and low-frequency parallel reflections (Figure 1-34 1, Figure 1-3-42), and most of them show two or three strong reflection events with good continuity. There is purple shale with iron at the bottom, which is a chicken-nest-shaped irregular iron ore layer (Shanxi-style iron ore) and an aluminum shale or bauxite layer (G-layer bauxite layer).
Benxi formation is in integrated contact with the overlying Taiyuan Formation as a whole, with the bottom of stable medium-fine grained timely sandstone as the top boundary of benxi formation. It is in parallel unconformity contact with the underlying Ordovician limestone strata, and the bottom of Shanxi-style iron ore and G-layer bauxite layer or the top of Ordovician limestone denudation surface is taken as the bottom boundary mark of benxi formation.
The spontaneous potential is in the form of medium and low amplitude microwave, with small amplitude change in the lower part and obvious positive anomaly in the upper part. The resistivity increases gradually from bottom to top, and the curve shows the characteristics of funnel+tooth box+finger combination.
Figure 1-3-4 1 Carboniferous-Permian seismic reflection characteristics of line 82-90 in Jiyang Depression
Figure 1-3-42 Characteristics of Carboniferous-Permian seismic reflection in line 568 of Jiyang Depression
(2) Taiyuan Formation
This is a set of coal-bearing sedimentary formations with alternating land and sea. It is mainly composed of sandstone, shale, sandy shale, siltstone, limestone and coal seam. Generally, it contains about 4 layers of limestone, about 3 layers of timely sandstone and multi-layer coal seams with different thicknesses. Integrated contact with the overlying Shanxi Formation.
The main difference between this group and the overlying and underlying strata is that it contains important minable coal seams (usually located in the lower part), multi-layer limestone and medium-fine grained timely sandstone. The top boundary of Taiyuan Formation is 1 medium-fine-grained timely sandstone above marine limestone formed at the turn of Carboniferous-Permian.
The spontaneous potential curve undulates like a microwave, and the lower part is a combination of multiple dentate bells, the upper part is a symmetrical combination of microwave+box+dentate funnel, and the upper part is a combination of box+funnel, showing multiple sedimentary cycles as a whole. The resistivity curve changes widely and slowly, and the lower part is in the shape of a tooth bell with medium and high resistance, and the upper part is a combination of a plurality of boxes and funnels with medium and low resistance.
(3) Shanxi Formation
It is mainly composed of sandstone, shale and clay rock deposited on the mainland, and the coal seam is formed by multi-layer offshore peat swamp. Generally speaking, the strata of Shanxi Formation are dark in color, mostly gray, dark and grayish green, without purple mudstone and limestone, and contain important minable coal seams, which are in integral contact with the overlying Shihezi Formation.
The spontaneous potential curve is microwave-like, with positive anomaly in the middle and low amplitude in the lower part, showing a combination of toothed box+bell+funnel, and microwave-like in the upper part, with little amplitude change. The upper part and the lower part form two cycles. The resistivity curve shows the combined characteristics of medium and high resistance tooth box and funnel.
(4) Lower the stone box group
It is a set of continental clastic rock deposits, which consists of yellow-green and gray-green sandstone, shale, siltstone and sandy mudstone interbedded with unrecoverable coal seams. There is a layer of purple mudstone containing iron, manganese and bauxite at the top, that is, "peach blossom mudstone". It is called "A-layer bauxite" in Northeast China, Liaoji, Hebei Province and the southern foot of Yanshan Mountain, "B-layer bauxite" in Zibo, Shandong Province and "purple porphyry aluminomudstone" in Huaibei. It is in integral contact with the overlying stone boxes.
The vertical variation of spontaneous potential curve is small, showing a symmetrical microwave shape. The lower part of resistivity curve shows a high-resistance dog tooth shape, and the upper part shows a low-resistance tooth+finger+bell combination.
(5) Upper stone box group
It is a set of clastic rock deposits with yellow-green, purple-red sandstone, siltstone, glutenite and mudstone interbedded, and generally does not contain coal seams. However, Huainan, Huaibei, Yuzhou and Pingdingshan in Henan Province have minable coal seams and few purple rock series, which are generally gray, yellow-green siltstone, sandstone and mudstone.
The lower spontaneous potential curve shows a plurality of funnel+bell combinations, and the resistivity curve shows a plurality of medium and high resistance tooth box+bell+finger+tooth funnel combinations. The spontaneous potential curve in the middle is bell-shaped+funnel-shaped+tooth-shaped box-shaped, and the resistance value of the resistivity curve decreases gradually, and the change in the lower part is wider and slower, and the upper part is in a medium-high resistance tooth shape. The upper spontaneous potential curve is symmetrical and low-amplitude microwave-like, with little overall amplitude change, and the resistivity curve is low-resistance tooth-like, reflecting the sedimentary environment with weak hydrodynamic conditions.
The above characteristics reflect the paleogeographic pattern of the North China block during the regression period, from north to south, from west to east, from continental facies to marine facies, and gradually transition to bay and lake facies. This formation is in integral contact with the overlying Sunjiagou Formation. The bottom of sandstone (Pingdingshan sandstone) above chert layer (siliceous layer) is the top boundary of Shangshihe Formation.
(6) Shiqianfeng Formation (Sunjiagou Formation)
It is a river and lake deposit composed of purplish red and grayish purple mudstone, sandstone, fresh water marl (fresh water limestone) and a small amount of gypsum, and fresh water limestone (marl) and gypsum are important marker beds of this group. This group is in integral contact with the overlying Liujiagou Formation. The top of gypsum calcium core above marl is the top boundary of Shiqianfeng Formation.
2. Triassic system
The distribution of strata is limited, and only a few areas in Linqing Depression, Jizhong Depression and Huanghua Depression have Lower and Middle Triassic. Liaohe Depression, Bozhong Depression and Jiyang Depression all lack Triassic.
The Lower and Middle Triassic in Linqing Depression are well preserved and can be divided into Liujiagou Formation, Heshanggou Formation and Ermaying Formation from bottom to top.
(1) Liujiagou Formation
It is characterized by the interbedding of dense red mudstone and sandstone, which is in integral contact with the underlying Carboniferous. In the logging response, the apparent resistivity curve is palm-block, and it is increasing from bottom to top. The spontaneous potential curve is a straight or low-amplitude tooth bell-shaped negative anomaly, and the fluctuation increases from bottom to top.
(2) Heshanggou Formation
The main lithology is purple, brown, light gray fine sandstone, siltstone, siltstone, calcareous sandstone, purple and purplish red mudstone. Lithology gradually thickens from south to north. The apparent resistivity curve is box-shaped, with high resistance, and shows a slow downward trend from bottom to top. The lower part of the spontaneous potential curve is bell-shaped negative anomaly, the negative anomaly at the bottom boundary is obvious, and the upper part is straight.
(3) Ermaying Formation
It is mainly purplish red mudstone interbedded with brown, dark gray fine and siltstone with unequal thickness, and locally mixed with gravel and unequal grained sandstone. The apparent resistivity curve is high or low, which is lower than the underlying stratum as a whole and fluctuates more than the underlying stratum. The negative anomaly of bottom spontaneous potential is obvious.
Jizhong depression is mainly composed of red and brown siltstone and fine sandstone with purple thin mudstone, which can be compared with Liujiagou Formation-Ermaying Formation of Lower and Middle Triassic in North China. The Lower and Middle Triassic in Huanghua Depression are widely distributed, and the lithologic characteristics are purplish red sandstone and siltstone, which are interbedded with mudstone in unequal thickness. The "high resistivity red" stratum in Dongpu sag is a set of continental red clastic rock deposits with high resistivity and dense lithology. Below it is the Shiqianfeng Formation at the top of Permian, which is superimposed by different strata from Paleogene to Neogene. This book temporarily belongs to the Lower and Middle Triassic.
(2) Division of biostratigraphy
1. Carboniferous-Permian biological assemblage characteristics
Through systematic research, conodonts, reptiles, foraminifera, sporopollen, archaea and brachiopods were identified (table 13 10).
Table 1-3- 10 Characteristics of Carboniferous-Permian Bioassemblage in North China
2. Fossil calibration of Carboniferous-Permian short-time strata.
Short-term chronostratigraphy refers to chronostratigraphic units whose time limit is shorter than that of traditional chronostratigraphy such as Benxi, Taiyuan and Shanxi, and it is grade or sub-grade, such as,, etc.
(Equivalent to benxi formation): It is marked by the combination zone of lepidoptera and Dianthus. ]]
(equivalent to the lower part of benxi formation): It is marked by the ridge belt Profusulinella parva, the first appearance of Profusulinella parva is the bottom mark, and the disappearance of Profusulinella is the upper limit. ]]
(corresponding to the upper part of benxi formation): The appearance of (Odonata) and (Odonata) combination zone is the symbol, the first appearance of molecules of (Odonata), (Odonata), Beedenia and other genera is the bottom boundary, and their disappearance is the top boundary. ]]
(Equivalent to the lower part of Taiyuan Formation): The bottom line is the appearance of small broken particles and original small broken particles in the small broken particle quasi-small broken particle-small broken particle combination zone, which marks the development peak of small broken particles and small broken particles. ]]
(corresponding to the lower part of Taiyuan Formation)-(corresponding to the upper part of Taiyuan Formation): It is marked by the appearance of the combination zone of the plant Neurospora ovata-Lepidoptera, or by the combination zone of Brachiopoda Reticulata-Taiyuan Fu-Eomarginefera Pusilla-Choristites Norni. ]]
(equivalent to the upper part of Taiyuan Formation): marked by the combination of psudoschwagerina and sphaeroschwagerina. The pseudofusulinid-pseudooschwegerina subzone in North China is the Permian bottom zone, and the first appearance of pseudofusulinid, pseudooschwegerina or Sphaeroschwagerina molecules is the Permian bottom boundary. ]]
(equivalent to Shanxi Formation): It is marked by the combination zone of the plant Trionyx tricornutum-Trionyx tricornutum-Pteris macrophylla. ]]
(equivalent to the lower part of Shanxi Formation): marked by the Stafela Belt, with the first appearance of Stafelafu population as its lower limit. ]]
(corresponding to the upper part of Shanxi Formation): marked by the Schwagerina Andersensis-s. grande nsis junction zone, bounded by schwager ina peak and a large number of Schwagerina andersensis and S. Grandensis. ]]
(Equivalent to the Lower Shihezi Formation): It is marked by the combination zone of Trionyx tricornutum-Adiantum ferrugineum-Athyrium cathayensis, in which the appearance of Pteris unicornis marks the new development of Cathaysian flora and can be used as a calibration feature. ]]
(equivalent to the lower part of Shangshihe Formation): marked by Lobatanullaria ensifolia-Gigantoclagriei-Fashionitis Hallei, it belongs to the first flourishing development period of Cathaysian flora, with developed types, prosperous genera and species and high degree of differentiation as its main characteristics. ]]
(equivalent to the upper part of Shangshihe Formation): marked by the assemblage zone of lobatanullaria heianensis-gigantoclea hallei-psygmophyllum mltipartitum, it belongs to the second flourishing period of Cathaysian flora, with the obvious decline of Cycadaceae, unclear taxonomic status of Ginkgo biloba and relatively developed plants as an important symbol of this assemblage. ]]
(equivalent to Shiqianfeng Formation): marked by the Ullmania Bronnii-Yuania Magnifolia assemblage zone, the Cathaysian flora declined seriously, gymnosperms dominated, and Mesozoic molecules appeared. ]]
The distribution of Triassic is limited, and only the lower and middle Triassic remains in a few areas such as Linqing Depression. There are few fossils preserved in Liujiagou Formation and Heshanggou Formation, which are consistent with the corresponding stratigraphic characteristics in North China and belong to the biological "dumb layer". Ermaying Formation is rich in charophyte and sporopollen fossils, in which charophyte and charophyte assemblage have typical characteristics of Middle Triassic, among which charophyte includes Stenochara dontzian, S.ovata, S.yuananensis, S.shaikni, etc. Sporopollen includes three-pore spore, striped spore and lateral pore spore.
(3) Strata distribution
1. Carboniferous-Permian
The author surveyed and collected Boshan section in Zibo, Shandong, Liujiang section in Qinhuangdao, Hebei, Niumaoling section in Benxi, Liaoning, Fuzhou Bay section in Jinxian, Liaoning, Junzhuang town in Jingxi, Beijing, Shabagou section in Xingtai, Hebei, Beidayu section in Fengfeng, Hebei and Longgong section in Hebi, Henan, and established eight standard stratigraphic sections. Strata of 28 wells are divided and 28 basic stratigraphic profiles are established. Based on the above lithologic characteristics and stratigraphic paleontology research results, a new stratigraphic system (attached figure 1-3-43, attached figure 1-3-44) is adopted to make stratigraphic correlation between the Carboniferous-Permian standard profile and the main profile in Bohai Bay Basin.
According to the new division scheme, the chronostratigraphy of the study area is divided into (benxi formation), (lower part of Taiyuan Formation), (upper part of Taiyuan Formation), (Shanxi Formation), (lower part of Shangshihe Formation), (upper part of Shangshihe Formation) and (Shiqianfeng Formation) from morning till night. Now its distribution law is described as follows:
1) Early Carboniferous () (benxi formation): The residual thickness changed greatly, from 1 (well Bo 2) to ~ 147m (Niumaoling, Benxi). The thickest areas are Benxi and Fuzhou Bay in the Taizi River Basin of Liaoning Province in the northeast, and the thickness is generally more than 100m ... The thinnest part is located in the edge of the study area, such as Bo-2 well (1m) in the Bohai Bay area in the northeast, Jingu-2 well (3m) in the southeast of Jinzhou City in the west, and Kendong-3 well (3m) east of Kenli in the southeast. Benxi formation is becoming thinner from northeast to southwest.
2) Late Late Carboniferous () (the lower part of Taiyuan Formation): the residual thickness of stratum changes greatly. The thickest part is located near well 1 in Huimin sag of Jiyang depression, with a thickness of 98 meters. The thinnest part is mostly located at the edge of the study area, such as near the well Lete 1 in the south of Lijin, with only 10.0 m left. Near Qinhuangdao10.8m.
3) Early Early Permian () (upper part of Taiyuan Formation): the residual thickness changed greatly. The thickest part is located near Well Zhuanggu 2 in the northeast of Huanghua City, which is 192m, and the thinnest part is located near Well Yi 155 in Gubei buried hill belt of Zhanhua Depression.
4) Late Early Permian () (Shanxi Formation): The residual thickness of strata changes greatly, and the thickest part is located in the area of Zeng 2 well in the south of Tianjin, with a residual thickness of about 300m, which gradually becomes thinner around, but the thickness distribution of each depression is uneven. The thinnest part is located in Guanshen 1 well area in the northwest of Guantao County, with a remaining thickness of 30m. Generally speaking, the remaining thickness of Shanxi Formation in the study area is mostly between 5 1 ~ 1 10m.
5) Early Middle Permian () (Xiashihezi Formation): The residual thickness of strata varies greatly, with the maximum thickness of 5 19.9m, which is located in the area of Well Da 3 in the southeast of Dacheng County on the west side of the study area. Since then, although there is a trend of gradually thinning to the periphery, there is still a phenomenon of uneven residual thickness in each depression. But in general, the residual thickness mostly varies between 10 1 ~ 200m.
6) Late Middle Permian () (lower part of Shangshihe Formation): The maximum residual thickness is located in Guangu 1 well area in the south of Cangzhou City, which is176m. From here, the residual thickness becomes thinner in the surrounding areas of the north, east and south, and the minimum thickness is 16m in the area of Well Chengdong 2 from the east to the west of the Chengdong uplift.
7) Early Late Permian () (upper part of Shangshihe Formation): the maximum residual thickness of Guangu 1 well area in the south of Cangzhou City is 342m. It gradually decreases from the center to the east and surrounding areas, and reaches the well Kengu 12 to the east of Kenli County in the eastern margin of Jiyang Depression, with a remaining thickness of only 20m.
8) Late Late Permian () (Shiqianfeng Formation): Shiqianfeng Formation is absent in most areas and only distributed locally. The largest remaining thickness is located in the area of Well Da 3 in the southeast of Dacheng County, with a thickness of 463m m. From this point of view, it gradually decreases around, for example, the thickness of Qinhuangdao area is 30m, and the thickness of Shengao 1 well in the south margin is 30 m.
From the perspective of the whole region, the distribution of residual thickness of Carboniferous-Permian system has the following laws:
The thickest part of Carboniferous-Permian is located in the area of Well Da 3 in Jizhong Depression (the remaining thickness is1615.5m), and it tends to become thinner around.
Figure 1-3-43 Carboniferous-Permian stratigraphic division and correlation map in Bohai Bay Basin (1)
Figure 1-3-44 Carboniferous-Permian Stratigraphic Division in Bohai Bay Basin (2)
The residual thickness in southeastern Tangshan increased slightly, exceeding 600 m; It gradually descends to the east, down to about 200 meters in Zhuanggu 2 underground, and reaches the northern part of Huanghua-Jiyang Depression. The residual thickness varies greatly, alternating from east to west, and the thickness in some areas can reach 800m m.
It extends to the southeast to the south edge of Nanpi Depression, with a thickness of 9 18m (Donggu 1 well). To Huimin sag, the thickness is alternately distributed from east to west, with a general thickness of 200 ~ 400 m, and the remaining thickness in some areas can exceed 800m (well Qugu 2), and the surrounding thickness of well Kendong 3 is 28m, which is the thinnest place in the whole region.
It extends to the southwest to the southeast of Longyao County, and the remaining thickness is 996.5 meters (well Ju 2), and gradually decreases to about 200 meters to the south. The eastern part of Linqing Depression thickens eastward, with Tanggu 1 well reaching1000 m, and the remaining thickness of Dongpu Depression varies from 200m to 826m m.
The Carboniferous-Permian system in Liaohe depression is distributed in the northeast in a strip shape, and the southern section is located in the west of Liaoyang, with the thickest part reaching about 780m; The northern section is located in Shenyang, and the thickest part can reach about 600m m.
The Carboniferous-Permian in Bozhong Depression is distributed in the northeast, with a large range in the southwest and a large residual thickness, with the thickest reaching about 1400m; The northeast section is thin, small in scope and distributed in a strip shape, with a maximum thickness of about 600 meters.
The above characteristics show that the residual thickness of strata is not only influenced by the original sedimentary background, but also obviously reformed by the later tectonic action. From the bulge to the gentle slope, the stratum becomes new and the range gradually decreases.
2. Triassic system
In terms of stratigraphic contact, the Lower and Middle Triassic in North China (such as Jingcheng, Yanshan, Linqing Depression, Jiyuan Basin, Ordos Basin, etc. ) is in integral contact or parallel unconformity contact with the underlying Shiqianfeng Formation. This shows that the Hercynian movement at the end of Permian did not cause drastic changes in the sedimentary pattern of North China (Chen Hongde et al., 1999). The early and middle Triassic inherited the structural pattern of high northwest and low southeast since the late Hercynian, belonging to a large-scale intracontinental depression basin, and continued the dry-hot climate of the late Permian, developing continental red clastic deposits. Seismic data (Figure 1-3-45) show that the primary sedimentary distribution is similar to that of the Late Paleozoic, and both of them are covered by caprocks (Wang Tonghe et al.,1999; Qi Jiafu et al.,1995; Qi Jiafu et al., 2003). The difference of residual thickness between Lower Triassic and Middle Triassic is the result of later erosion.
Figure 1-3-45 Seismic Profile of JL-78-3 14.5 in Linqing Depression
Within the North China Plate, the residual strata of the Lower Triassic and the Middle Triassic are adjacent to the west and scattered in the east (Figure 1-3-46).
In Ordos basin in the west, the thickness of Lower and Middle Triassic gradually increases from northeast to southwest, and the thickness of Huanxian and Qingyang in the southwest can reach about1100 m. According to the drilling and seismic data in Qinshui basin, the total thickness of Lower and Middle Triassic in the basin can reach 1200 m. The maximum thickness of lower and middle Triassic clastic rocks in Jiyuan-Yichuan-Luoyang area of western Henan Province is over 2500m, and it gradually becomes thinner to around100m. The maximum thickness of Zhoukou sag in eastern Henan is also close to1000 m.
In the eastern part of Bohai Bay Basin, the Lower and Middle Triassic are distributed in depressions and some bulges such as Jizhong, Huanghua, Dongpu and Linqing. The general sedimentary thickness is about 1000m, and Linqing depression can reach1000 m. In addition, the Lower and Middle Triassic is also distributed in Zibo, Zhangqiu, Xiahuayuan in northern Hebei, Chengde in Beijing and Xishan (Qi Jiafu et al., 2003).
Figure 1-3-46 Horizontal comparison of Paleozoic and Triassic in North China.
The Lower and Middle Triassic is a set of purplish red siltstones with fluvial facies. The maximum thickness in Shenxian sag, guanxian sag and Qiuxian sag of Linqing Depression is 2000m, which is characterized by thinning from west to east (Figure 1-3-47), thinning from south to north, and the whole east is missing.
Figure 1-3-47 Isograms of Residual Thickness of Lower and Middle Triassic in Bohai Bay Basin
The upper Triassic is missing in the Bohai Bay basin as a whole, while Ordos, Qinshui basin and Jiyuan basin in western North China are relatively developed.
To sum up, the distribution of residual thickness in the study area is uneven, the residual thickness of the main depressions changes relatively greatly, and the phenomenon of missing is common. The main reason for this phenomenon is that in the original sedimentary period, the basement topography was complex, and it was influenced by many factors such as geological tectonic movement and climate in the later period, which caused obvious regional differences and formed the current distribution pattern of residual thickness.
(4) sedimentary features
After the late Ordovician-early Carboniferous sedimentary discontinuity, the crust of the middle and late Carboniferous declined and the Carboniferous-Permian coal-bearing formations were deposited.
During the sedimentary period in benxi formation, the stratum thickness and carbonate content in the study area gradually increased from west to east. Benxi formation is superimposed on Fengfeng Formation or Majiagou Formation of Middle Ordovician, and is mainly composed of a set of shale and sandstone, with thin layers of marine limestone or coal lines.
During the deposition period of Taiyuan Formation, a set of coal-bearing deposits with alternating land and sea was formed in Bohai Bay Basin, which was mainly composed of sandstone, shale, sandy shale, siltstone, limestone and coal seam.
During the sedimentary period of Shanxi Formation-Xiashihezi Formation, a set of coastal clastic strata was formed. Among them, Shanxi Formation is mainly composed of continental sedimentary sandstone, shale and clay rock, which is an important coal-forming period in North China Basin. The Xiashihezi Formation is a set of interbedded yellow-green and gray-green sandstone, shale, siltstone and sandy mudstone, and the clastic rock deposits are mainly terrestrial and contain non-exploitable coal seams.
During the sedimentary period of Shangshihe Formation-Shiqianfeng Formation, the lithology gradually became thinner from north to south, and changed from gravel sandstone and coarse sandstone to mudstone, siltstone and medium-fine sandstone. Shangshihe Formation is a set of clastic rock deposits with yellow-green and purple sandstone, siltstone, glutenite and mudstone interbedded, and coal seams mainly appear in Huainan area in the south. Shiqianfeng Formation is a set of river and lake sediments, containing purplish red and grayish purple mudstone, sandstone, fresh water marl (fresh water limestone) and a small amount of gypsum, and there is basically no coal seam in the whole area.
During the Early-Middle Triassic, some areas were uplifted and denuded, but the study area basically continued the structural nature and paleogeographic outline of the Late Paleozoic, and the seawater completely withdrew from the southeast. Continental deposits are mainly developed, showing the characteristics of contiguous distribution in the west and sporadic distribution in the east. The Middle and Lower Triassic is mainly distributed in Linqing and Dongpu sags in the southwest and Jizhong and Huanghua sags in the northwest, and it is a set of purple mudstone with fluvial facies.
In the late Triassic, Indosinian movement led to the loss of Upper Triassic in Bohai Bay Basin.
Figure 1-3-48 Carboniferous-Permian evolution diagram (chronostratigraphic framework)
Chronostratigraphic profile (Figure 1-3-48) shows that in the middle and late Carboniferous, after a long period of erosion, North China once again settled and accepted deposition, and transgression first appeared in Xuhuai area and Liaodong-Tangshan area, where the earliest benxi formation was deposited. Later, with the expansion of transgression, it gradually extended to the southern, northern and eastern parts of North China. The general trend of the top and bottom boundaries of Taiyuan Formation, Shanxi Formation and Shihezi Formation is gradually rising from north to south and from east to west, and the times are also changing from old to new. Among them, the marker bed and coal seam also change synchronously with time.
The above diachronic characteristics reflect that during the sedimentary period of benxi formation, transgression first started in Bohai Bay Basin, and then spread to the west and south, reflecting that the basin basement was a dustpan with an opening to the east, and inherited the basin basement morphology in the late Early Paleozoic. From Taiyuan Formation, the southern part of the basin became the origin and destination of transgression, and the basement of the basin was in a state of southward dip.