Stratigraphic analysis of Cambrian stratigraphy in Pigs' Nest, Qinyuan County, East Rim

The Cambrian section of Pigs' Nest in Wanghe Township, Qinyuan County, is one of the main observation points of the group, and the location of the section is shown in Fig. 2-6 (located at the edge of the map).

I. Sedimentary stratigraphy

The strata exposed in this area include the Lower Cambrian Maozhuang Formation, the Middle Cambrian Xuzhuang Formation and Zhangxia Formation, and the Upper Cambrian mesa, Changshan and Fengshan Formations. The Lower Cambrian strata are incomplete and are pseudo-consolidated on the Great Wall System.

1. Maozhuang Formation (O1m,5.5m)

The bottom of the Maozhuang Formation is commonly seen as limonite spots or streaks, which are marked by exposure. The lower part is gray-yellow thinly laminated medium-coarse-grained calcareous (dolomitic) feldspathic quartz sandstone, and the upper part is the same-colored mud-bearing calcareous sandy dolomite with phosphorus-bearing reaction. The fossil Eosoptychoparia kochibei (Walcott) (Giant Wisdom's Oriental Livid Cheekworm) is seen in this group.

2. Xuzhuang Formation (O2x,73.1m)

The lower part of the Formation mainly consists of purplish-red and grayish-green feldspathic quartz sandstone and shale, which are generally phosphorus-responsive and have ripple-scar formations, and the upper part of the Formation contains marine chlorite. Overall, the group is dominated by clastic deposition of terrestrial origin. It contains the fossils Eosoptychoparia sp. (Oriental lifecheek worm undetermined), Kailiella sp. (Little Kaili worm undetermined), Dongbeia sp. (Northeastern worm undetermined), and Kochaspis sp. (Koch's worm undetermined).

The upper part of this group is gray-black and green-gray oolitic dolomitic marl, dolomitic chert, druzy chert, calcareous shale, etc., and is generally dominated by carbonate deposits. The fossils found on the profile are Lonchiougia sp. (Spear-spined Inoue worm undetermined), Lorenzella cf. adaris (Abare Lawrence worm undetermined).

3. Zhangxia Formation (02z,111m)

This Formation is dominated by light gray, gray, and greenish gray oolitic dolomitic cherts with dolomitic cherts, druzy cherts, and marl, and the lower part of it is gray-yellow calcareous shale and thinly laminated clay-bearing striped cherts interbedded with the sedimentation. The fossils are Anomocarella sp. (Small Shoulderless Worm unspecified), Koptura sp. (Cut Tail Worm unspecified), Damesella sp. (Deuterostomus sp. unspecified), and Cyclolorenzella sp. (Round Laurence Worm unspecified).

4. mesa Formation (O3g,36.4m)

The mesa Formation is dominated by greenish-gray, grayish-gray, and grayish-black dolomitic chert and muddy dolomite, with bamboo-leafed dolomitic chert, bamboo-leafed chert, and calcareous shale. With trilobite fossils: Blackwelderia paronai (Para butterfly bug), Blackwelderia sp. (Butterfly bug undetermined species).

5. Changshan Formation (O3C,6.0m)

The bottom part is gray medium-thick layered dolomitic dolomite interbedded with gray-green dolomitic shale, with ripple scar structure. The upper part is gray and gray-green dolomitic shale interbedded with muddy dolomite and dolomitic dolomite, with ripple marks at the dolomite level. The top part is gray and gray-green dolomitic shale. Fossils are seen only in Obolus sp. (garden cargo shell undetermined species).

6. Fengshan Formation (O3f,86.6m)

Gray, grayish-white, and dark-gray dolomite dominates the Formation, with clayey dolomite, and no obvious fossils are seen because of strong dolomitization. This group is in consolidated contact with the overlying Lower Ordovician strata.

Second, stratigraphic analysis

The stratigraphy of this section is relatively complete, and 10 stratigraphic sequences (Fig. 3-10) have been classified.

1. Stratigraphic sequence 1 (7-8 layers)

The bottom boundary of stratigraphic sequence 1 overlaps with the bottom boundary of Cambrian system and the bottom boundary of Maozhuang Formation. Due to the influence of Jinning movement, the Ordos Massif rose to land, so the stratum from Aurignacian to Lower Cambrian Steamed Bun Formation is missing on the section, and at the same time, the markers such as limonite are seen on the boundary part, so the sequence 1 is a type I sequence.

Stratigraphic sequence 1 mainly consists of the stratum of Maozhuang Formation, which is 5.5 m thick, and the calcareous quartz sandstone in the lower part belongs to the deposition of wave bumps during the period of sea advancement, while the muddy dolomite in the upper part belongs to the products of relatively high space available during the period of high water level. Since the section is located at the edge of the uplift, the dense section is not developed.

2. Sequence 2 (9-14 layers)

Sequence 2 is equivalent to a set of stratigraphy of Xuzhuang Formation, with a thickness of 25.3m, and the lower part of this sequence has appeared the seaward stagnant gravel, so this sequence may belong to the type I sequence. The domain of the marine system consists of four quasi-stratigraphic sequences, each quasi-stratigraphic sequence is mud shale → conglomerate-bearing sandstone, sandstone or siltstone depositional sequence, quasi-stratigraphic sequence group is regressive; the more upward, quasi-stratigraphic sequences of the proportion of muddy sedimentation increases, the lower coarse-grained sedimentation upward particles become finer (from the conglomerate to the siltstone), the sand layer becomes thinner. The dense section is relatively typical and consists of gray-green sandy shales that are generally phosphorus-reactive, while the rest of the beds are not as strongly and extensively phosphorus-reactive as this one. The high water table system domain consists of five quasi-sequences, quasi-sequence group is a front accumulation type, characterized by the higher sand layer is thicker, the larger the grain size (from siltstone to sandstone).

3. Sequence 3 (15-17 layers)

Sequence 3 is located in the middle of Xuzhuang Formation, about 26 m thick, the sandstone above the boundary of the sequence contains sea chlorite, which is the product of the sea erosion period, and the horizontal comparison shows that this sequence is a type I sequence. The domain of the marine intrusion system consists of three to four quasi-stratigraphic sequences of mottled shale, sandy shale → calcareous sea chlorite feldspathic sandstone, dominated by land-sourced detrital littoral deposition. The dense section is relatively typical and consists of gray-green shale. The high-water system domain contains four quasi-stratigraphic sequences, each quasi-stratigraphic sequence consists of yellow-green calcareous shale → gray oolitic dolomite-bearing tuff, and the quasi-stratigraphic sequence group has a weak pre-cumulative structure.

4. Sequence 4 (17-19 layers)

Sequence 4 includes the upper part of the Xuzhuang Formation and the lower part of the Zhangxia Formation, which is about 41 m thick, of which the Haijin System consists of grayish-black oolitic dolomitic marl, with the depositional characteristics of the inner terrace-outer terrace, and it contains 5-6 quasi-sequences, but due to the development of thick beds and suture line tectonics, the quasi-sequence characteristics are not very obvious. The dense section is more typical. The dense section is more typical and consists of yellow-gray mud shale. The domain of high water level system contains five quasi-sequences, and the three quasi-sequences near the intensive section are more prominent, showing the sequence of calcareous shale → mud-bearing banded tuff, and the quasi-sequence group is of accretionary type; the upper two quasi-sequences are mainly composed of greyish-black oolitic dolomitic tuff, and the dolomitization indicates that the water body has become shallower and over-exposure has occurred. The sequence is Type I by cross-section comparison.

5. Sequence 5 (20-22 layers)

Sequence 5 consists of the upper part of the Xuzhuang Formation and the lower part of the Zhangxia Formation, and is about 47 m thick, which is marked by the appearance of bamboo-leafed tuffs, and is likely to be a type I sequence. The domain of Haijin Formation is thin and consists of bamboo-leaved chert and dolomitic chert, containing 2 quasi-stratigraphic sequences. The dense section is light gray thinly laminated marl and calcareous shale. The high-water system domain has 5-6 quasi-stratigraphic sequences, each quasi-stratigraphic sequence consists of oolitic dolomitic tuff → dacitic tuff, and the quasi-stratigraphic sequence group is generally in the preaccumulation type.

Figure 3-10 Stratigraphic analysis of Cambrian sequences in the section of Pigs' Nest, Wanghe Township, Qinyuan County, Shanxi

Continued Figure 3-10 Stratigraphic analysis of Cambrian sequences in the section of Pigs' Nest, Wanghe Township, Qinyuan County, Shanxi

1- Dolomite; 2-Mudy dolomite; 3-Mud-bearing dolomite; 4-fine-grained muddy dolomite; 5-mudstone; 6-biotite; 7-olivine-bearing dolomitic tuff; 8-bamboo-leaved tuff; 9-sandy shale; 10 Iron-bearing marine chlorite fine sandstone; 11-calcareous fine sandstone; 12-quartzite sandstone

6. Sequence 6 (22-23 layers)

This sequence is dominated by the middle and upper strata of the Zhangxia Formation, which is about 40 m thick, and is dominated by oolitic dolomitic cherts. The sea-entry system is dominated by oolitic dolomitic cherts, which have the characteristics of a platform margin; the bioclastic content increases in the high-water system, and the degree of dolomitization is a little stronger than that in the lower strata, and the grains are coarser (medium-coarse grains). The quasi-stratigraphic sequence within the stratigraphic sequence is not easy to recognize, and the dense section is not typical. Since there is no obvious sign of clipping or undercutting action, it may belong to type II.

7. Sequence 7 (23-27 layers)

This sequence consists of the top of Zhangxia Formation, the middle and lower part of Mesa Formation, and it is about 30.3 m thick, and the lower part of Sequence 7 has been changed from the light grey dolomitic bioclastic tuff in Sequence 6 to the grey-black dolomitic oolitic bioclastic tuff, and the bioclasts have more bioclasts than those in the lower layers, and the darker color indicates that the space available for them has increased, and the bioclasts indicate that it is a beach-type sediment and is advancing towards the land. It is landward advancing. There are two quasi-stratigraphic sequences in the domain of the marine progression system, and each quasi-stratigraphic sequence presents a calcareous shale → tuff type. The dense section is typical and consists of yellowish green calcareous shale. The high-water system contains six quasi-stratigraphic sequences, and each quasi-stratigraphic sequence exhibits a muddy-striped dolomitic tuff → dolomitic dolomitic tuff type. The bottom boundary of this sequence has no other sign except the cyclonic change, so it may belong to the nature of type II.

8. Sequence 8 (27-32 layers)

Sequence 8 includes the upper part of Mesa Formation, Changshan Formation and the lower part of Fengshan Formation, which is about 29.5 m thick, and there are signs of iron dipping near the bottom boundary of the sequence, so it may belong to the boundary of the sequence of type I. The area of Haijin system contains 5 quasi-circumferential dolomites, which are of the type of the dolomites. The domain of Haijin system contains 5 quasi-stratigraphic sequences, showing mud dolomite → oolitic dolomite or shale → bamboo-leaved tuff type. The dense section is gray and gray-green dolomitic shale, which represents the relatively deeper deposition of the water body. The high-water system domain has four quasi-stratigraphic sequences, characterized by shale → druzy-leaved tuff or muddy dolomite → druzy-leaved tuff, with strong dolomitization in the upper part, which is not easy to subdivide the quasi-stratigraphic sequences.

9. Stratigraphic sequence 9 (33-35 layers)

Stratigraphic sequence 9 consists of the lower part of Fengshan Formation and is about 28.2 m thick, and below the bottom boundary of the stratigraphic sequence, there is a pure and industrially available medium and fine-grained dolomite, indicating that there has been a seaward migration of the exposed and freshwater lenses, and the strata above the boundary of the stratigraphic sequence have become darker in color, grey, grey-black, and the dolomite contains muddy, and the stratigraphic strata contain pyrite nodules, showing the characteristics of deepening of the water body; it belongs to the type I sequence. There are 5 quasi-stratigraphic sequences in the domain of the Haijin system, and each quasi-stratigraphic sequence consists of muddy dolomite and bamboo-leaf dolomite. The dense section is not typical. The high-water system is mainly composed of gray and dark gray medium-coarse-grained dolomite, which is pure and can be used for industrial purposes, and the quasi-stratigraphic sequences are not easy to be identified.

Table 3-9 Boundary properties of Cambrian sequences and main identification marks of the Pigs' Nest Cambrian sequence in Qinyuan County

10. Sequence 10 (36-37 layers)

Sequence 10 is mainly composed of the middle and upper strata of the Fengshan Formation. Below the bottom boundary of sequence 10 is the product of strong dolomitization (Table 3-9), and above it is the flint-bearing dolomite with cave development, which belongs to the boundary of type I sequence. Its top boundary is downward from the bottom boundary of the Ordovician, and there is no reliable evidence as to whether part of the stratigraphy is missing or not, but the giant dolomite is developed below the top boundary of the sequence, so it also belongs to the boundary of the Type I sequence (for a detailed discussion of the top boundary of Sequence 10, see Chapter 4). The sea-entrant domain consists of dark gray flint-bearing dolomite (36 beds) with widespread development of cavities, interbedded with lenticular yellow-green shale, suggesting seawater intrusion but located near the shoreline. Dense sections are not developed. The high-water system domain consists of grayish-white giant thick-bedded megagranular dolomite (available for industrial use) (37 layers), suggesting intense dolomitization after prolonged stratigraphic exposure.

Three, summary

(1) The profile of the Pig's Nest in Qinyuan County is a profile with full exposure of Cambrian strata, in which the Lower Cambrian strata are incomplete, while the Middle and Upper Cambrian strata are complete. Because the section is located in the edge of the terrace during the Cambrian depositional period, the stratigraphic characteristics are not very obvious.

(2) In the Cambrian stratigraphy of this section, 10 complete sequences (and one interstage sequence) were identified, including eight type I sequences and two type II sequences, which are regional and globally comparable; the boundary features of the type I sequences are relatively obvious, while the boundary features of the type II sequences are not obvious.

(3) The two type-II boundaries are located in the middle and upper part of the Zhangxia Formation, and the Zhangxia Period is the regional sea-entry period in the region, so it is inferred that the type-II sequences were developed in the regional sea-entry (water-entry) period, which is reflected in other basins as well (for example, the Songliao Basin).

(4) The low-water system domain is missing in the general sequences of this section.

(5) Dense segments within each stratigraphic sequence are generally atypical or absent in this profile (Table 3-10).

Table 3-10 Characteristics of the dense sections of the Cambrian in the Pigs' Nest, Qinyuan County