Sequence Stratigraphy Analysis of Cambrian in Zhuwopeng, Qinyuan County, Eastern Margin

Cambrian section of pig shack in Wanghe Township, Qinyuan County is one of the main observation points of our research group. The location of this section is shown in Figure 2-6 (at the edge of the map).

I. Sedimentary strata

The strata exposed in this area are Maozhuang Formation of Lower Cambrian, Xuzhuang Formation and Zhangxia Formation of Middle Cambrian, Gushan Formation, Changshan Formation and Fengshan Formation of Upper Cambrian. Among them, the lower CAMBRIAN strata are incomplete and pseudo-integrated on the Great Wall System.

1 .maozhuang formation (O 1m, 5.5m)

Brown iron ore spots or stripes can be seen at the bottom of Maozhuang Formation, with exposure traces. The lower part is grayish yellow thin-layer medium-coarse grained calcareous (dolomite) feldspar timely sandstone, and the upper part is the same color argillaceous calcareous sandy dolomite, which has phosphorus-containing reaction. Fossil Eosooptychoparia Kochibei (Walcott) was found in this group.

2. Xuzhuang Formation (O2x, 73. 1m)

The lower part of this formation is mainly purplish red and grayish green feldspar, timely sandstone and shale. Sandstone generally has phosphorus reaction and corrugated structure, and the upper part contains glauconite. Generally speaking, terrigenous clastic deposits are dominant. Fossil-bearing Eosoptychoparia sp. (unknown species of oriental Bucca orientalis), Kailiella sp. (unknown species of Carex), northeast grass. (unknown species in Northeast China) and Kochaspis sp. (unknown species of worms in robert koch).

The upper part of this formation is gray-black, blue-gray oolitic dolomitic marl, dolomitic limestone, bamboo-leaf limestone, calcareous shale, etc. Generally, carbonate deposits are dominant. Fossils found on the profile include: Lonchiougia sp. (unknown species of Oxytropis Oxytropis inoculation bacteria) and Lorenzella cf. Adaris (unknown species of Laurenti Abare).

3. Zhangxia Formation (02z,11m)

This formation is dominated by light gray, gray and blue gray oolitic dolomite limestone, with dolomite limestone, bamboo-leaf limestone and marl, and grayish yellow calcareous shale and thin argillaceous banded limestone in the lower part. Fossils include Anomocarella sp .(Ankleptora sp.) and Koptura sp. (Ceratosaurus species. ), Delta Serra species. (Dermatidae. ) and Cyclolorenzella sp.

4. Gushan Formation (O3g, 36.4m)

Mainly blue-gray, gray, gray-black dolomitic limestone and argillaceous dolomite, with bamboo-leaf dolomitic limestone, bamboo-leaf limestone and calcareous shale. Trilobite fossils: Blackwelderia paronai (Papilionidae), Blackwelderia sp.

5. Changshan Formation (O3C, 6.0m)

The bottom is gray medium-thick layered bamboo dolomite mixed with gray-green dolomite shale with corrugated structure. The middle and upper parts are mixed with gray, gray-green dolomite shale, argillaceous dolomite and bamboo-leaf dolomite, and the dolomite layer is wavy. The top is gray and gray-green dolomitic shale. Only Obolus sp. It was found in fossils.

6. Fengshan Formation (O3f, 86.6m)

It is mainly gray, grayish white and dark gray dolomite mixed with argillaceous dolomite. Due to strong dolomitization, no obvious fossils are found. This formation is in integral contact with the overlying Ordovician strata.

Second, sequence stratigraphic analysis

The strata in this profile are relatively complete, and the 10 sequence is divided by * * (Figure 3- 10).

1. sequence 1 (7 ~ 8 layers)

The bottom boundary of sequence 1 coincides with the bottom boundary of Cambrian and Maozhuang Formation. Due to the influence of Jinning movement, the Ordos block rose to the land, so the strata from Sinian to Lower Cambrian Shantou Formation were missing in the profile, and limonite was seen at the boundary, so the 1 sequence was type I sequence.

Sequence 1 is mainly composed of Maozhuang Formation strata, with a thickness of 5.5 meters, and the calcareous chronotropic sandstone in the lower part belongs to wave weathering deposition during transgression; The argillaceous dolomite in the upper part belongs to the product with relatively large accommodation space during the high water level period. Because the profile is located at the edge of the uplift, the dense section is not developed.

2. Sequence 2 (layer 9 ~14)

Sequence 2 is equivalent to a set of strata of Xuzhuang Formation, with a thickness of 25.3 meters. There are transgressive residual gravels in the lower part of this sequence, so this sequence may belong to type I sequence. The transgressive system tract consists of four parasequences, each of which is a sedimentary sequence of shale → gravelly sandstone, sandstone or siltstone, and the parasequence group is regressive. Upward, the proportion of argillaceous sediments in parasequence increases, the coarse-grained sediments in the lower part become thinner (from gravel to siltstone) and the sand layer becomes thinner. The tight section is typical and consists of gray-green sandy shale, which generally reacts with phosphorus. The reaction between other horizons and phosphorus is not as strong and extensive as here. The high water level system tract consists of five parasequences, and the parasequence group is progradational, with thick sand layer and large grain size (from siltstone to sandstone).

3. Sequence 3( 15 ~ 17 layer)

Sequence 3 is located in the middle of Xuzhuang Formation and is about 26m thick. The sandstone above the sequence boundary contains glauconite, which is the product of transgression. Through lateral correlation, the sequence is type I sequence. The transgressive system tract consists of 3 ~ 4 variegated shales, sandy shales → calcareous glauconite feldspathic sandstone subsequences, which are mainly terrigenous clastic coastal sediments. The tight section is relatively typical and consists of gray-green shale. The high water system tract contains four parasequences, each of which is composed of yellow-green calcareous shale → gray oolitic dolomite limestone, and the parasequence group is weakly progradational.

4. Sequence 4( 17 ~ 19 layer)

Sequence 4 includes the upper strata of Xuzhuang Formation and the lower strata of Zhangxia Formation, with a thickness of about 41m.. Among them, the transgressive system is composed of gray-black oolitic dolomitic marl, which has the sedimentary characteristics of inner platform-outer platform and contains 5 ~ 6 parasequences, but the parasequence characteristics are not obvious because of the thickness of the layer and the development of suture structure. The tight section is typical and consists of yellow-gray shale. The high water level system tract contains five parasequences, and the three parasequences near the tight section are characterized by calcareous shale → argillaceous banded limestone sequence, and the parasequence groups are superimposed. The above two parasequences are mainly composed of gray-black oolitic dolomitic limestone, and dolomitization means that the water body is shallow and overexposed. After transverse comparison, the sequence is type I.

5. Sequence 5 (20 ~ 22 layers)

Sequence 5 includes the upper strata of Xuzhuang Formation and the lower strata of Zhangxia Formation, with a thickness of about 47 meters. This sequence is marked by the appearance of bamboo-leaf limestone, which may belong to type I sequence. The transgressive system tract is thin, composed of bamboo limestone and dolomite limestone, and contains two quasi-sequences. The dense section is light gray thin marl and calcareous shale. There are 5 ~ 6 parasequences in the high water system tract, each parasequence is generally composed of oolitic dolomitic limestone → bamboo-leaf limestone, and the parasequence group is generally progradation type.

Figure 3- 10 Sequence Stratigraphic Analysis of Cambrian in Zhuwopeng Section of Wanghe Township, Qinyuan County, Shanxi Province

Figure 3- 10 Sequence Stratigraphic Analysis of Cambrian in Zhuwopeng Section of Wanghe Township, Qinyuan County, Shanxi Province

1-dolomite; 2- argillaceous dolomite; 3- argillaceous dolomite; 4- Fine-grained argillaceous dolomite; 5— Mudstone; 6- Biological debris; 7— Oolitic dolomitic limestone; 8- Bamboo-leaf limestone; 9— Sandy shale; 10-iron-bearing glauconite fine sandstone; 11-calcareous fine sandstone; 12- quartzite sandstone

6. Sequence 6 (Layer 22-23)

The sequence is dominated by the middle and upper strata of Zhangxia Formation, with a thickness of about 40m. The transgressive system tract is dominated by oolitic dolomite limestone, which has the characteristics of platform margin shoal; The content of bioclastic in the high water level system tract increases, and the degree of dolomitization is slightly stronger than that in the lower strata, and the particles are coarse (medium coarse). Quasi-sequences in sequences are not easy to identify, and dense segments are not typical. Because there is no obvious sign of cutting or undercutting, it may belong to type ⅱ.

7. Sequence 7 (Layer 23-27)

This sequence consists of the top of Zhangxia Formation and the middle and lower strata of Gushan Formation, with a thickness of about 30.3 meters. The lower part of Sequence 7 changed from light gray dolomitic bioclastic limestone in Sequence 6 to gray black dolomitic oolitic bioclastic limestone. Bioclastic limestone is more than the underlying stratum, and the dark color indicates that the accommodation space is increased, and most of the bioclastic limestone indicates that it is beach-like deposit, advancing towards the land. There are two parasequences in transgressive system tract, and each parasequence is calcareous shale → limestone. The tight section is typical and consists of yellow-green calcareous shale. The high water system tract contains six parasequences, and each parasequence is characterized by argillaceous banded dolomitic limestone → dolomitic bamboo-leaf limestone. The bottom boundary of this sequence has no other signs except cyclic changes, and may belong to type II nature.

8. Sequence 8 (27 ~ 32 layers)

Sequence 8 includes the upper Gushan Formation, the lower Changshan Formation and Fengshan Formation, with a thickness of about 29.5 meters ... There are exposed signs such as iron impregnation near the bottom boundary of the sequence, which may belong to type I sequence interface. The transgressive system tract contains five parasequences, namely argillaceous dolomite → oolitic dolomite or shale → bamboo-leaf limestone. The dense section is gray and gray-green dolomitic shale, which means that the water body is deeply deposited. There are four parasequences in the high-water system tract, which are characterized by shale → bamboo-leaf limestone or argillaceous dolomite → bamboo-leaf limestone, and the upper part is strongly dolomitized, so it is difficult to subdivide the parasequences.

9. Sequence 9 (33 ~ 35 layers)

Sequence 9 consists of the lower strata of Fengshan Formation, with a thickness of about 28.2 meters. There are pure medium-fine dolomite under the bottom boundary of the sequence, which can be used in industry, indicating that fresh water lenses have been exposed and migrated into the sea. Above the sequence boundary, the strata become dark, gray and gray-black. Dolomite contains mud and strata contain pyrite nodules, showing the characteristics of deepening water body. It belongs to type I sequence. There are five parasequences in the transgressive system tract, and each parasequence consists of argillaceous dolomite and bamboo-leaf dolomite. Dense segments are not typical. The high water level system tract is mainly composed of gray and dark gray medium-coarse dolomite, which is purely industrial and difficult to identify quasi-sequence.

Table 3-9 Characteristics and main identification marks of Cambrian sequence interface in Zhuwopeng, Qinyuan County

10. Sequence 10 (36 ~ 37 layers)

Sequence 10 is mainly composed of the upper and middle strata of Fengshan Formation. Below the bottom boundary of sequence 10 is the product of intense dolomitization (Table 3-9), and above it is dolomite with flint developed in karst caves, which belongs to type I sequence interface. Its top boundary is lower than the bottom boundary of Ordovician, and there is no reliable evidence whether some strata are missing. However, there is thick dolomite under the top boundary of sequence, so it also belongs to type I sequence boundary (see Chapter 4 for a detailed discussion of the top boundary of sequence 10). The transgressive system tract is composed of dark gray dolomite containing flint (36 layers), in which karst caves are widely developed with lenticular yellow-green shale, indicating that seawater intrudes but is located near the coastline. Dense segments are not developed. The high water level system tract is composed of gray-white giant dolomite (available for industrial use) (37 layers), which indicates that strong dolomitization occurred after long-term exposure of the stratum.

Three. abstract

(1) The Zhuwopeng section in Qinyuan County is a relatively complete section of Cambrian strata, in which the Lower Cambrian strata are incomplete and the Middle-Upper Cambrian strata are complete. Because the profile is located at the inner edge of the platform during the CAMBRIAN sedimentary period, the sequence characteristics are not obvious.

(2) A total of 10 complete sequences (and 1 intertemporal sequences) have been identified in the Cambrian strata in this section, including 8 type I sequences and 2 type II sequences, and the sequence properties are comparable regionally and globally. The characteristics of type I sequence interface are obvious, while the characteristics of type II sequence interface are not obvious.

(3) The two type II boundaries are located in the middle and upper part of Zhangxia Formation, which is a regional transgression period in this area, so it is inferred that the type II sequence developed in a regional transgression (transgression) period, which is also reflected in other basins (such as Songliao Basin).

(4) Generally speaking, every sequence in this section lacks low water level system tract.

(5) The dense part in each sequence of the profile is usually atypical or non-existent (Table 3- 10).

Table 3- 10 Characteristics of CAMBRIAN Compact Profile of Pig Wopeng in Qinyuan County