Mesoproterozoic and Neoproterozoic sequence stratigraphy

Mesoproterozoic and Neoproterozoic in this area are the first set of caprock deposits above metamorphic rock series. It is widely exposed in Yanshan area, represented by Jixian section of national nature reserve. Jixian section is famous for its large thickness, continuous strata, clear contact between top and bottom, complete development of various formations, simple structure and no metamorphism, and has always been the standard for stratigraphic division and correlation between China and Neoproterozoic. Jixian section is divided into Mesoproterozoic Great Wall System, Jixian System and Neoproterozoic Qingbaikou System from bottom to top, * * * including 12 group, and its characteristics are shown in Figure 1-3.

Figure 1-3 Histogram of Mesoproterozoic and Neoproterozoic in Jixian Section

The research on sequence stratigraphy of Jixian section began in the late 1990s. Huang Xueguang and others [2] re-measured the Jixian section with the emphasis on the systematic sequence stratigraphy of Jixian section, and divided the sequence in detail. The data in this section are all taken from the research results. The research results show that Jixian section is divided into five first-order sequences, 12 second-order sequences and several third, fourth and fifth-order sequences. Because the lithostratigraphic units of Jixian profile series and group level are mainly divided into obvious sedimentary discontinuity (parallel unconformity) and sharp change of lithofacies, the boundaries of lithostratigraphic units are often basically consistent with the sequence boundaries of different levels, and only some transitional boundaries are slightly different from the sequence boundaries. Therefore, this section still describes their sequence stratigraphic characteristics by series and groups.

First, the Great Wall system.

The Great Wall System consists of Changzhougou Formation, Chuanlinggou Formation, Tuanshanzi Formation, Dahongyu Formation and Gaoyuzhuang Formation from bottom to top, with a total thickness of 4,283 meters. ..

1. Changzhougou Formation

Changzhou Gou Formation is a set of coarse clastic rock deposits, with a thickness of 1 109 m ... It is covered by angular unconformity on garnet amphibolite gneiss of Zunhua Group in Archean. The bottom of Changzhougou Formation is composed of fluvial dark purplish red conglomerate, composite glutenite and composite sandstone. The maturity of rock composition and structure is very low, and staggered layers converge in the same direction, indicating that terrigenous debris comes from Shanhaiguan ancient land in the southeast of this area. The rock maturity and structural maturity of the timely sandstone above the initial flood surface are obviously higher than that of the underlying fluvial composite sandstone, and the beach staggered layers such as fishbone and fishbone are developed, which are beach facies deposits and constitute a transgressive system tract. The maturity of upward sandstone continues to increase, and the quartzite section in the middle of Changzhougou Formation is the largest transgressive deposit. The upper part of Changzhougou Formation is composed of a series of rhythms composed of seasonal sandstone and thin silty shale. The upward sandstone changed from medium-thick layer to thin layer, which became thinner and thinner, and the silty shale gradually became thicker, forming a high water level system tract. When the ratio of sand to mud is 1: 1, it transits to Chuanlinggou Formation. Therefore, the Changzhougou Formation constitutes a secondary sequence.

The bottom of Changzhougou Formation is regional unconformity, which is tectonic strengthening unconformity. A set of fluvial facies deposits above the unconformity surface belong to low-tide system tract, so the sequence bottom boundary of Changzhou Gou Formation belongs to SB 1 sequence interface.

2. Chuanlinggou Formation

On the lithostratigraphic unit, the bottom of Chuanlinggou Formation is in a transitional relationship with Changzhou Formation. In Jixian section and its adjacent areas, there are signs of exposure caused by sea level drop, such as dry crack structure, near the above interface, but there is no obvious phenomenon of river rejuvenation, so the interface should be SB2 type.

Chuanlinggou Formation is a set of fine clastic rocks and mudstone deposits. According to the rock characteristics, it can be divided into three lithologic sections with a total thickness of 647 meters. The first section is rhythmic deposition, which consists of fine sandstone, siltstone and shale. From bottom to top, fine sandstone gradually decreases and shale increases, forming an upward thinning sedimentary sequence. Its sedimentary characteristics show that it was formed in intertidal zone and belongs to transgressive system tract. The second member is shale with flat bedding, which is deposited in low-energy still water environment in lagoon basin, and the second member is the most transgressive deposit with a thickness of hundreds of meters. The third section is mainly black flaky silty shale with sandstone lens at the top. The third section inherited the sedimentary environment of the lagoon in the second section, but the seawater became shallow and turned to the open environment, which constituted the high water level system tract deposition.

3. Tuanshanzi Formation

The Tuanshanzi Formation and the underlying Chuanlinggou Formation in Jixian section are in an overall transitional relationship, with shallow seawater and no signs of exposure. Tuanshanzi Formation in Jixian section is mainly composed of iron-bearing dolomite, which is divided into two sections with a total thickness of 4 15m. The lower rhythm is composed of silty shale and siliceous argillaceous iron-bearing dolomite, and its changing trend is that the upward argillaceous decreases and becomes micrite dolomite. The upper member is mainly rhythmic deposition of interbedded silty dolomite and tabular dolomite sandstone, and the iron content of dolomite is obviously lower than that of the lower member. The upper part of the second member is intercalated with sandstone, tuff and stromatolite dolomite, and sedimentary structures such as wavelet marks and salt pseudocrystals can be seen on the horizon. The top fracture structure is extremely developed, and multiple layers of fracture layers form exposed zones.

From the perspective of sequence stratigraphy, although Tuanshanzi Formation is also divided into three fourth-order sequences and two third-order sequences, from the perspective of higher-order sequences, Tuanshanzi Formation is only a part of the high water level system starting from the third member of Chuanlinggou Formation. Therefore, Chuanlinggou Formation and Tuanshanzi Formation together constitute a secondary sequence.

4. Dahongyu Formation

In lithostratigraphy, the Dahongyu Formation has a transitional relationship with the underlying Tuanshanzi Formation. However, the exposed zone composed of multi-layer dry-cracked structures in the transitional zone is obviously characterized by large-scale sea level decline and sequence unconformity. The sequence boundary type should be SB2 type.

Dahongyu Formation generally constitutes a sedimentary cycle from sandstone to potassium-rich shale to siliceous banded dolomite, which is divided into three lithologic sections with a total thickness of 5 16m.

The first and second members of Dahongyu Formation are composed of three secondary cycles: sandstone → potassium-rich tuffaceous fine sandstone, siltstone → dolomite-bearing sandstone, with different layers of potassium-rich basalt and volcanic breccia. Take the bottom secondary loop as an example (Figure 1-4). The lower part is light purple, light yellow-brown flaky thin layer, containing tuffaceous sandstone, with pinnate, wedge and fishbone cross bedding, and the bottom has fracture structure. The middle part quickly transits to gray-white, light yellow-brown thick feldspar-bearing timely sandstone, with parallel bedding development, and each single layer constitutes a sequence and an accretion sequence group. The sandstone monolayer in the upper part of it becomes thinner rapidly, and there are plate-shaped, wedge-shaped and fishbone-shaped staggered layers in the layer. At the top, it is rhythmic deposit composed of light yellow tuffaceous sandstone (bottom) and purplish red silty tuffaceous shale, which is rich in potassium and is called "potassium-rich shale". The crack structure at the top is developed. It can be seen that the above-mentioned second-order cycle is equivalent to a second-order sequence. The lower part of the cycle is a transgressive system tract, the massive layered sandstone in the middle is the largest transgressive deposit, and the upper part is a high-water system tract. The top fracture structure is developed without obvious erosion discontinuity, which is SB2-type sequence boundary. Therefore, the first and second members * * * include three fourth-order sequences.

Figure 1-4 Histogram of the First and Fourth Sequences of Dahongyu Formation in Jixian County

TST-transgressive system tract; MFS-maximum submerged surface; High water level system tract; SB2—- sequence boundary

The third member of Dahongyu Formation consists of gray-white siliceous banded dolomite, stromatolite dolomite, sandy dolomite and siliceous rocks. The truncated-cone stromatolite dolomite at the top of Dahongyu Formation is the largest transgressive deposit, indicating that the high water level system tract on it has been eroded and no longer exists. Therefore, the third member of Dahongyu Formation is an incomplete sequence with only transgressive system tract.

5. Gaoyuzhuang Formation

As mentioned above, there is obvious sedimentary discontinuity between the Gaoyuzhuang Formation and the underlying Dahongyu Formation. In Jixian section, there is a layer of sandstone with a thickness of about 3m at the bottom of Gaoyuzhuang Formation directly covering the truncated stromatolite dolomite at the top of Dahongyu Formation, and the high water level system tract of a second-order sequence at the top of Dahongyu Formation has been eroded. From Jixian to the east, in the areas of Qinglong County and Qianxi County of Hebei Province, the timely sandstone above the bottom of Zhuang Formation is transformed into conglomerate several meters thick. This parallel unconformity is called Qinglong uplift. Therefore, the bottom boundary of Gaoyuzhuang Formation should be SB 1 sequence boundary. Gaoyuzhuang Formation is a set of carbonate sediments, with a thickness of1546m. According to the rock characteristics, it can be divided into four subgroups, namely, Guandi Subgroup, Sangshuanya Subgroup, Zhang Jiayu Subgroup and Huanxiusi Subgroup from bottom to top.

1) Guandi subgroup: includes two secondary cycles. The next secondary cycle starts from the timely sandstone at the bottom and goes up to argillaceous dolomite, dolomite with flint zone and nodules and stromatolite dolomite. The last secondary cycle starts from dark gray manganese-bearing dolomite siltstone and goes up to manganese-bearing sandy dolomite, manganese-bearing dolomite, manganese-bearing stromatolite dolomite and siliceous rocks. These two secondary cycles constitute two tertiary sequences respectively.

2) Sangshuanya Formation: It is also composed of two secondary cycles from argillaceous dolomite to dolomite. The next secondary cycle consists of manganese-bearing silty dolomite (so-called "manganese-bearing shale") and massive dolomite, and the last secondary cycle consists of gray thin-thin argillaceous dolomite and thick micrite dolomite. Dry fractures are found in manganese-bearing shale at the bottom of this subgroup, and seawater is rapidly shallowing in dolomite at the top, so it also constitutes a third-order sequence.

3) Zhang Jiayu Subgroup: It is the thickest of the four subgroups, with a thickness of 778m. The rhythm of this subgroup is well developed, mostly some small rhythms with a thickness of tens of centimeters to 1 ~ 2m, which are composed of flaky argillaceous dolomite and medium-thick carbonate rocks (dolomite and limestone), and the bedding is horizontal and straight. Stromatolites and siliceous bands are not found in this subgroup in Jixian section. In the deep part of the original basin in Jixian, Xinglong, Kuancheng and other places, nodular limestone with different sizes is widely distributed in the flaky argillaceous dolomite in the lower part of this sub-group, which is the symbol of platform basin facies and the sediment in the maximum flooding period. The top is deposited by lower slope facies, middle slope facies and upper slope facies in turn.

4) Huanxiusi sub-group: The lower part of the sub-group is rhythmic deposit, which is composed of black globose algae, asphaltene coarse-grained dolomite limestone (bottom) and dark gray wavy algae mat medium-grained dolomite limestone (top), and formed in semi-closed subtidal zone to intertidal zone. The upper part of the sub-group gradually transitions to the rhythm composed of gravel dolomite containing siliceous lumps (bottom), dolomite containing cat's eye associated nodules (middle) and siliceous breccia (top). The top layer of rhythm is a siliceous crust, and karst breccia is also found in many places. It can be seen that Huanxiusi sub-group itself does not constitute a single sequence, but only a part of the high water level system tract from the upper part of Zhang Jiayu sub-group.

Above the end of Zhuang Formation, the whole region rose, especially Luanxian County, which is called Luanxian Rising (see later).

6. Summary

The whole Great Wall System is divided into two sections by three structural strengthening unconformities (angle unconformity at the bottom of the Great Wall System, parallel unconformity higher than Qinglong at the bottom of Zhuang Formation, and Luanxian at the top of the Great Wall System), which constitute two first-order sequences respectively. The lower Changzhougou Formation, Chuanlinggou Formation, Tuanshanzi Formation and Dahongyu Formation form a first-order sequence, in which the shale in the second member of Chuanlinggou Formation is the largest flood-prone deposit, with transgression system tract below it and high water level system tract above it. According to the available isotopic age data, the time limit is 1800 ~ 1600 Ma. The upper member of Gaoyuzhuang Formation itself constitutes a first-order sequence, in which the tumor-bearing limestone member of Zhang Jiayu Subgroup is the largest transgressive deposit, with transgressive system tract below it and high water level system tract above it. According to the available isotopic age data, its time limit is 1600 ~ 1400 Ma.

Second, the system of gathering talents.

Jixian Formation includes Yangzhuang Formation, Wumishan Formation, Hongshuizhuang Formation and Tieling Formation from bottom to top, with a total thickness of 4300m m..

1. Yangzhuang Formation

Above the end of Zhuang Dynasty, the area rose to land. There are two layers of conglomerate at the bottom of Yangzhuang Formation in Luanxian County on the margin of Shanhaiguan ancient land. Gravel has good roundness and complex composition, and is in parallel unconformity contact with the lower Gaoyuzhuang Formation. This interruption is called "Luanxian Rising". So its bottom boundary is SB 1

There was another transgression in Yangzhuang period. The sea area in this area was blocked by Changping-Huairou underwater uplift, and a large lagoon basin was formed near Shanhaiguan ancient land, and red rock series represented by purple silty argillaceous dolomite was deposited. No red rock series deposits were found outside the lagoon to the west and north of Changping-Huairou underwater uplift. The red rock series of the lagoon is the most developed in Jixian section, with a thickness of 1048m, which is divided into three sections.

A paragraph consists of a series of rhythms, each of which is from top to bottom:

3) Siliceous hard shell layer;

2) light gray medium-thick siliceous banded fine-grained dolomite;

1) grayish white, purplish red sheet-thin silty argillaceous dolomite.

The above rhythm transits from intertidal zone-supratidal zone to subtidal zone-intertidal zone-supratidal zone from top to bottom, indicating that the sea level is gradually rising.

The second member is mainly purplish red silty argillaceous dolomite with a small amount of gray and white silty argillaceous dolomite, which is a typical member of Yangzhuang Formation. The purplish red thick massive silty argillaceous dolomite in the middle of the second member belongs to lagoon basin facies.

The third paragraph is also composed of a series of rhythms, but the rhythm pattern is different from the first paragraph. The basic prosodic layer is from top to bottom:

4) siliceous rocks;

3) Gray siliceous banded algal mat dolomite, sometimes containing small stromatolites such as goblet stromatolites;

2) Gray massive crystalline microbial dolomite with typical tuff structure. The bottom of this layer is not thick microcrystalline dolomite;

1) grayish white, purplish red flaky silty argillaceous dolomite.

The change trend of the above rhythm from the lower part of the third paragraph is that the second layer gradually becomes thinner and even disappears at the top of the third paragraph. Rhythm is mainly in 1) and 4) layers, and only the third) layer is not thick, showing the evolution process of sea level gradually decreasing until it is briefly exposed.

To sum up, the sea level of the first member of Yangzhuang Formation gradually rose during the sedimentary period, forming a transgressive system tract, and the purple layered silty argillaceous dolomite in the lagoon basin in the middle of the second member is the largest transgressive deposit. Above it is a high water level system tract, and the characteristics of sediments show that the sea level is gradually decreasing. Therefore, the Yangzhuang Formation itself constitutes a second-order sequence, which also includes three third-order sequences and six fourth-order sequences.

2. Wumishan Formation

After the short-term exposure of Yangzhuang at the end of the period, when the Wumishan Formation was deposited, the climate changed to humid, which was a lagoon tidal flat environment. The remarkable characteristics of Wumishan Formation different from other formations are as follows. The first feature is the huge thickness of the stratum. Taking Jixian section as an example, the thickness of Wumishan Formation is 2848.45m, accounting for 1/3 of the total thickness of Mesoproterozoic and Neoproterozoic in Jixian section, which is the thickest set of strata in Jixian section. The second feature is that almost all of them are composed of dolomite, and more than half of them are dolomite of microbial origin. The third feature is that the sedimentary rhythm is particularly developed. The thickness of small rhythm is only 1 ~ 2m, and the thickness of large rhythm is tens of meters. The Wumishan Formation, which is thousands of meters thick, is almost composed of rhythms of different sizes. According to preliminary statistics, Wumishan Formation in Jixian section has 4 13 rhythms.

In Jixian section, the rhythm of Wumishan Formation consists of the following five basic layers from top to bottom:

5) Top layer: siliceous rocks, which are exposed siliceous shells on the tide;

4) Upper layer: microbial dolomite, ordinary pseudobare stromatolite, goblet stromatolite and other small stromatolites deposited in intertidal zone;

3) Intermediate layer: layered crystalline dolomite, common microbial rock structures such as chondrites and clots, and some large-giant cone stromatolites, which are deposited in the subtidal zone;

2) Lower layer: gray-white fine-grained dolomite and dark gray striped microbial dolomite, sometimes cup-shaped stromatolite can be seen, deposited in intertidal zone;

1) Bottom layer: grayish white argillaceous dolomite, containing terrigenous sand and gravel chips, deposited in the tidal zone.

As can be seen, the above rhythm shows the reciprocating change of intertidal zone → intertidal zone → subtidal zone → intertidal zone → supratidal zone, which just shows a basic sea level change cycle. Therefore, each rhythm constitutes a most basic sequence.

The higher order sequence is determined by comparing the differences between a series of basic sequences. This difference is mainly reflected in the change of stromatolites, combined with other sedimentary characteristics. Generally speaking, the stromatolites in the middle layer are huge in volume, mainly distributed in the upper, middle and lower layers, and the overall rhythmic thickness is also large, showing the relatively highest sea level period, which determines the maximum flooding period. On the contrary, there is no conical stromatolite in the middle layer, even the middle layer is very thin, and only small stromatolites such as small cup stromatolites are seen in the upper layer (lower layer). The rhythmic top and bottom layers are developed, and the whole rhythmic thickness is also small, which is a period of relative sea level decline. By comparing 4 13 basic sequences in Jixian section, Wumishan Formation can be divided into 17 fourth-order sequences and 4 third-order sequences, Wumishan Formation itself constitutes a second-order sequence, and the bottom sequence interface is SB2 type.

3. Hongshuizhuang Formation

In Jixian section, Hongshuizhuang Formation has a transitional relationship with the underlying Wumishan Formation. The argillaceous dolomite at the top of Wumishan Formation gradually increased, and the single layer became thinner, and then transited to the thin-layer argillaceous dolomite at the bottom of Hongshuizhuang Formation. However, from the point of view of sequence stratigraphy, there are some sedimentary structures such as hill ripple and lenticular bedding near the boundary between Wumishan Formation and Hongshuizhuang Formation, which indicates that the sea level drops and the sea water becomes shallow. More importantly, from the perspective of regional contact, there is obvious sedimentary discontinuity between Wumishan Formation and Hongshuizhuang Formation. In western Liaoning, some people call it Lingyuan Movement. There are flint conglomerate and breccia coarse sandstone at the bottom of Hongshuizhuang Formation in Yixian area, north of Taihang Mountain, with a thickness of only 6.2 1m, which are also in parallel unconformity contact. In addition, in terms of regional distribution, the stratigraphic distribution range of Hongshuizhuang Formation is obviously smaller than that of Wumishan Formation. It can be seen that from a regional perspective, the period from Wumishan to Hongshuizhuang is also a major turning point of sea level fluctuation. Therefore, SB2-type sequence boundary is formed.

Hongshuizhuang Formation is a set of shale deposits, gray, dark gray, yellow-green, etc., and its thickness is very small, only 1 14m. The regional distribution is stable, the thickness changes little, and the lithofacies is single. The transgressive system tract sequence in the lower part of Hongshuizhuang Formation consists of argillaceous shale → dolomitic shale, and the thickness of each sequence is only 10 cm to several cm. The basin facies yellow-green shale with a thickness of more than 70 meters in the middle is the largest flood deposit. The shale on it is deposited in the high water system tract, and fine sandstone lenses begin to appear on its upper part, showing the trend that the sea level gradually decreases and the seawater becomes shallow.

4. Tieling Formation

Tieling Formation is divided into Daizhuangzi Subgroup and Laohuding Subgroup, and there is obvious regional parallel unconformity sedimentary discontinuity between the two subgroups, which is called Tieling Uplift. Tieling Formation is 290 meters thick.

The bottom of Daizhuangzi Sub-formation is thin sandstone, which has a rapid transition relationship with the underlying Hongshuizhuang Formation, that is, the sandstone lens sandwiched in shale at the upper part of Hongshuizhuang Formation increases rapidly and transits to the timely sandstone at the bottom of Daizhuangzi Sub-formation, that is, it changes from basin facies shale to subtidal high-energy sand dam. Therefore, from the point of view of sequence stratigraphy, the deposition of Daizhuangzi Formation is only the continuation of the upper high water level system tract of Hongshuizhuang Formation.

Except for the timely sandstone with a bottom thickness of 10m, the Dai Zhuangzi Ya Formation is mainly composed of (micro) manganese-bearing dolomite mixed with thin argillaceous sandstone. The general trend is that dolomite single layer becomes thinner and manganese content decreases, from stromatolite to gravel. On the contrary, argillaceous sandstone increases and tidal bedding such as lenticular bedding develops. Emerald glauconite shale appears at the top, with occasional cracks. The evolution of sediments in Daizhuangzi sub-formation from subtidal high energy band to intertidal zone-supratidal zone shows the characteristics of high water system tract with continuous sea level decline, which constitutes a part of high water system tract from the upper part of Hongshuizhuang formation.

The parallel unconformity between the upper and lower sub-groups of Tieling Formation shows that the shale at the top of Daizhuangzi Sub-group is covered by the purplish red iron sand conglomerate at the bottom of Laohuding Sub-group, which turns into oolitic hematite in Yanqing County, Beijing, and is called "Sihai" iron mine. Although the bottom boundary is obvious and discontinuous, the cutting caused by river rejuvenation is not obvious, so the bottom boundary should be SB2 type.

The lower part of Laohuding sub-group is a subsequence composed of gravel dolomite limestone and flaky argillaceous limestone, while the upper part of limestone increases and the argillaceous composition decreases, and then transits to limestone, containing irregular wall stromatolites, etc. , thus forming a transgressive system tract. The largest flood deposit is layered array layered limestone. The high water level system tract consists of Tieling laminaria limestone → palmately Jixian laminaria limestone → columnar Jixian laminaria limestone → humpling pyramidal laminaria limestone, and stromatolites decrease from bottom to top until they disappear. Therefore, Laohuding subgroup is another typical example of stromatolite change reflecting sea level change, which is superior to other sedimentary indicators and has become the main line of sequence stratigraphy research.

5. Summary

Throughout the Jixian system, it can be clearly divided into two parts. Xiayangzhuang Formation and Wumishan Formation have similar rhythmic deposits, similar microbial rock deposits and similar basin tectonic background. The sedimentary basin continued to sink, forming extremely thick sediments. The thickness of Yangzhuang Formation and Wumishan Formation is 3896m, which is 9: 1 different from that of Hongshuizhuang Formation and Tieling Formation (404m). The sedimentation is continuous, but influenced by the unbalanced depression in the basin, the formation thickness and lithofacies change greatly. In these two groups, the layered purplish red silty argillaceous dolomite in the lagoon basin in the middle of Yangzhuang Formation is the largest flood deposit, forming a first-order sequence. According to the available isotopic age data, its time limit is 1400 ~ 1200ma. The upper Hongshuizhuang Formation and Tieling Formation were deposited in a relatively stable basin tectonic background. Affected by the small subsidence of the basin, the sedimentary facies is stable, the stratum thickness is small and has little change, and there is regional parallel unconformity between the two subgroups of Tieling Formation. The overall characteristics of strata have belonged to the typical craton basin type. These two groups constitute another first-order sequence, and the basin facies shale of Hongshuizhuang Formation is the largest flood-prone deposit. According to the available isotopic age data, its time limit is 1200 ~ 100 oma.

Third, the blue-white oral system

Qingbaikou Formation includes Xiamaling Formation, Changlongshan Formation and Jingeryu Formation from bottom to top, with a total thickness of 430m.

1. Xiamaling Formation

In Jixian section, the top of Tieling Formation is purplish red limestone, and siliceous crust and karst breccia appear locally, all of which are the results of long-term weathering and erosion. The bottom of Xiamaling Formation is iron-bearing glutenite with the property of bottom conglomerate. Therefore, there is obvious sedimentary discontinuity between the Xiamaling Formation and the underlying Tieling Formation, which is a parallel unconformity contact, called the Qinyu Movement, and is a SB 1 type sequence interface.

The Xiamaling Formation in Jixian section is an upward tapering sedimentary sequence composed of a series of rhythms of sandstone → silty shale. The upper sandstone decreases, the silty shale increases, and the gray shale appears at the top, with the thickness of 198m. Compared with the contemporary strata of Xishan in Beijing, the Xiamaling Formation in Jixian section is only equivalent to the first section of Xishan section, and the upper two to four sections have been eroded. From the point of view of sequence stratigraphy, the Xiamaling Formation in Jixian section is only a part of the lower transgressive system tract of the Xiamaling Formation in Beijing Xishan section, and the upper part of the transgressive system tract (including the maximum flood deposition) and the high water level system tract have been eroded.

2. Changlongshan Formation

As mentioned above, there is parallel unconformity contact between Changlongshan Formation and the underlying Xiamaling Formation, which is called Yuxian Uplift. Due to the obvious rejuvenation of the river, the underlying Xiamaling Formation has been cut to varying degrees, so it is a SB 1 type sequence interface.

Changlongshan Formation is a set of coarse clastic sediments, which consists of lower fluvial composite glutenite, middle marine glauconite sandstone, glauconite siltstone and upper variegated shale, forming an upward tapering sedimentary sequence, which can be divided into three fourth-order sequences and 1 third-order sequences.

3. Jingeryu Formation

The Jingeryu Formation in Jixian section is represented by duck egg blue thin-medium marl. The lower sequence consists of purple argillaceous shale and marl, the middle sequence consists of duck egg cyan calcareous shale and marl, and the upper sequence consists of marl and limestone. The Jingeryu Formation is in parallel unconformity contact with the overlying Cambrian Changping limestone, which belongs to Jixian movement.

The Jingeryu Formation itself constitutes a third-order sequence, and the Jingeryu Formation and the underlying Changlongshan Formation together constitute a second-order sequence. Qingbaikou system constitutes a first-order sequence. According to the available isotopic age data, its time limit is 1000 ~ 800 Ma.