Exploration of geological events in the middle Proterozoic Chuanlinggou period of the Ming Tombs in Beijing

Study on Geological Event Records of Proterozoic Chuanlinggou Formation in the Ming Tombs of Beijing

Song and Zheng and Zhang

It was first published in Geological Review, Volume 46, No.4, 2000; The following modifications and supplements have been made in this book: ① After simplifying the graph 1, the relationship between volcano-earthquake events and Re-Os isotope analysis values is highlighted; ② Two micrographs were added to illustrate the evidence of volcanic activity in the dolomite stratum of Chuanlinggou Formation; (3) Added text description and explanation; ④ Supplemented Luck's related literature on Os isotope research. This article is very different from the original, and it is a re-creation on the basis of the original.

1 stratum overview

1. 1 Rock Strata of Chuanlinggou Formation

Chuanlinggou Formation of Mesoproterozoic Great Wall System in the Ming Tombs of Beijing consists of black shale, silty mudstone, sandstone, sand shale rhythmic layer containing dolomite lens and dolomite, with a thickness of about 50m (along the strike). There is an obvious discontinuous interface between the bottom and Changzhou Formation, and there may be a discontinuous interface in the lower part, and the top is in a continuous sedimentary relationship with Tuanshanzi Formation.

1.2 Geological Event Interval of Chuanlinggou Formation

The field profile of Chuanlinggou Formation in the Ming Tombs of Beijing is divided into two black intervals and a yellow interval in the middle after many observations and sampling analysis. The black interval is dominated by shale, silty mudstone and sandstone, and the yellow interval is medium-thick layered breccia dolomite. From the upper part of the lower black interval to the yellow interval is the geological event interval (Figure 1).

The lithologic association of Chuanlinggou Formation in Ming Tombs in Beijing is obviously divided into three parts, which can be distinguished by profile observation and confirmed by the material composition of rocks. The rhythmic layer structure of the upper and lower black intervals dominated by clastic rocks is developed, which may be the reflection of the earliest seasonal sedimentary cycle change. There is an 80cm thick igneous rock in the middle and upper part of dolomite in the yellow interval, and its lithology is coarse rock. Due to the late intrusion of alkali intrusive rock-syenite and the distribution of Proterozoic alkaline volcanic rocks (Yu Jianhua et al., 199 1), the age of volcanic rocks or subvolcanic rocks in this layer deserves attention. Because the isotopic age of volcanic rocks or subvolcanic rocks seen in the Mesoproterozoic section of Jixian County has been measured to be about 65438±0.8 Ga, there are different understandings of their occurrence. One view holds that they belong to volcanic rocks of Chuanlinggou Formation, and the other view holds that they are bedrock or dyke invaded in the later period (Chen et al., 65,438+0980).

Figure 1 Paleovolcano-earthquake event interval of Mesoproterozoic Chuanlinggou Formation in the Ming Tombs of Beijing.

1-sandstone; 2— Siltstone; 3- shale; 4- dolomite; 5- brecciation; 6- volcanic rocks; 7— Changzhougou Formation; 8— Chuanlinggou Formation; 9— Tuanshanzi Formation; 10-rhenium-osmium isotope analysis sampling point

The Chuanlinggou Formation of the Ming Tombs in Beijing has only 0.5 m coarse-grained porphyry. Porphyry is composed of 0.2 ~ 0.4 mm potassium feldspar and some larger biotite crystals, and the parallel bedding distribution of 0. 1 ~ 0.3 mm strip apatite authigenic crystals is common. The short columnar feldspar matrix less than 0.03mm is also oriented, showing the flow direction when the stratum is formed. The chemical composition of trachyte is: silica 256.3%, alumina 14.2%, titanium oxide 20.7%, iron oxide10.8%, iron oxide10.8%, manganese oxide 0.07%, calcium oxide 5.5%, magnesium oxide 2.2% and K2O 9.6% The analysis of rare earth elements is (unit: 10-6): la 46.45, ce 82.75, pr 9.08, nd 38.92, sm 6.35, eu 1.77, GD 4.9 1, TB 0.63, dy 2.92. Yb 0.98, Lu 0. 18, Y 1 1.9, SC 7.25, showing the characteristics of rich light rare earth; The analysis of trace elements is (unit: 10-6): Ti4 196, Mn507, P25 15, Ba2 128, Co 18, Cr158. More than 30 rock samples collected from Chuanlinggou Formation were analyzed by thin section analysis, lead isotope analysis and rhenium-osmium isotope analysis. It is considered that there is a lot of evidence that there is a correlation between volcanoes and earthquake events, which can be reflected in a series of macro, micro and ultra-micro phenomena, especially in dolomite rocks above and below coarse rocks. Typical volcanic microcrystals-potash feldspar and glass feldspar were found. Volcanic crystal chips and volcanic glass chips parallel to the rock layer are also found in micrite dolomite, such as breccia dolomite (99620-1 7c) above the coarse rock layer (not marked in figure1,and its position can be inferred by referring to the marked figures), and micrite dolomite (19620-65433) is above it. Whether there is volcanic eruption or Yanshanian diabase intrusion in Chuanlinggou Formation of Great Wall System in North China has always been controversial. However, the identification of rock slices in the Ming Tombs is not the composition and structure of diabase, but the characteristics of trachyte. In particular, the composition of crystal debris and glass debris erupted by volcano is consistent with that of trachyte, which is more confirmed. It can be inferred that during the formation of Yanshan aulacogen, although volcanic activity reached its climax in Dahongyu period, a series of earthquakes and volcanic activities began in Changzhou period and Chuanlinggou period.

2 signs of geological events

Song et al. (1987, 199 1) reported that there were landslides and sand liquefaction caused by earthquakes in Chuanlinggou Formation. Through systematic observation and sampling analysis, it is considered that the records of geological events in Chuanlinggou Formation are a combination.

2. 1 macro tag

Near the lower part of the black interval and the middle yellow interval, there are very typical sand liquefaction structures (Figure 2a) and intra-layer slump structures (Figure 2b). These two sedimentary structures are very similar to earthquake-induced anomalies and artificial simulated earthquake structures, reflecting that the rock strata were impacted by seismic waves in the quasi-syngenetic or early diagenetic stage (Song, 1988). At the same time as the above geological events are recorded, there are intra-layer fault structures. Seilacher( 1969) first determined that the small step-like faults in the stratum were caused by earthquake events, and similar intra-layer dislocation structures and step-like faults were also found in Changzhougou Formation in the Ming Tombs and Changzhougou Formation in Yamenzi, Kuancheng. When sandstone and silty mudstone appear alternately with tubular filling structure, it is also a manifestation of sand liquefaction (Song et al., 1985, Song et al., 1985). The yellow interval in the middle of Chuanlinggou Formation in the Ming Tombs is thick layered dolomite, but it can be found that there are hidden breccia structures in it. Because of a series of abnormal phenomena related to the whole event interval, this paper thinks that these concealed breccias were formed by earthquakes in the quasi-contemporaneous or early diagenetic period of dolomite, and the microscopic phenomena seen in dolomite slices also confirm their special occurrence.

2.2 Micro-mark

According to the microscopic observation and identification of dozens of large glossy surface and rock slices, it is found that a series of microscopic marks accompanying the macro earthquake events, including small drainage structures and vein filling, generally appear between silty mudstone and coarse siltstone interlayer. Generally speaking, coarse silt is prone to sand liquefaction and drainage veins, because the sandy component contains more water than the muddy component (Figure 2a). Another kind of dislocation in microlayer may also be caused by earthquake. These two microscopic signs can be seen not only in the Chuanlinggou Formation of the Ming Tombs, but also in the upper part of the Chuanlinggou Formation in Jixian County (Figure 2d). It is worth pointing out that most of the dislocations in the microlayer are normal faults (Figure 2c), which is consistent with the macroscopic intrastratal faults or stepped faults seen in the Changzhougou Formation, and also reflects the total tensile stress during the formation of Yanshan Depression. In the dolomite of the yellow interval of Chuanlinggou Formation in the Ming Tombs, there is also a very common and obvious self-crushing breccia structure. These self-fragmenting breccias are not ordinary intrasedimentary clastic breccias, but are triggered by earthquakes. The illusion of volcanic crystal debris and glass debris is also seen in dolomite, suggesting that the earthquake may be caused by volcanic eruption.

Fig. 2 Records of paleoearthquakes and volcanic activities in Chuanlinggou Formation.

A. Sand body flow (Wt) and liquefaction vein (Lq) caused by earthquake, and the photos clearly show that the liquefaction vein originated from the horizontal liquefiable layer (they are connected); B. Soft sedimentary sliding caused by earthquake; C. Intra-layer micro-dislocation (Mft) caused by earthquake, and its upper and lower layers are parallel bedding; D. Earthquake-induced upward extending liquefaction veins (Lq(u) and downward extending liquefaction veins (Lq( 1)). These micro-liquefaction veins, like macro-liquefaction veins, have upper and lower branches, not dry cracks; E. the oldest fossil (Planolites sp. ) is found in Changzhou Formation under Chuanlinggou Formation. According to Professor Du Rulin, a famous fossil expert A.Seilacher confirmed that the discovery of the event layer of Chuanlinggou Formation may be the reason to prevent the upward expansion of trace fossils; K-feldspar-glass feldspar volcanic crystallite (Vc), breccia block (Br) and intralamellar fault (FT) were found in dolomite breccia under trachyte. (sample number: 99620-17c); G. Volcanic crystal detritus (Vc) and volcanic glass detritus (Vg) exist sporadically in micrite dolomite, and their arrangement is consistent with dolomite bedding, which obviously falls from volcanic ash in the air and is not carried by detritus (specimen number: 99620- 19b).

These volcanic glass fragments and volcanic crystal fragments are not only found in interlayer detritus-bearing dolomite (Figure 2f, sample number: 99620- 17c), but also in micrite dolomite (Figure 2g, sample number: 99620- 19b). Volcanic crystals are potassium feldspar, and rhombic cryolite authigenic crystals can also be seen. It should be pointed out that the arrangement of volcanic crystal chips and glass chips is consistent with rock bedding, but the water body is stable when micrite dolomite bedding is formed, so the volcanic crystal chips and glass chips come from volcanic ash falling from the sky, not from land-based scouring sources, indicating that there is indeed volcanic activity in Chuanlinggou Formation. Volcanic activity will also cause earthquakes, and at the same time, there will be sand liquefaction flow and liquefaction veins (Figure 2a), and there will also be soft sedimentary slip (Figure 2b), which is not only manifested in the macro structure, but also in the micro structure, intra-layer faults (Figure 2c), intra-layer liquefaction veins (Figure 2d) and brecciation (Figure 2f).

2.3 Geochemical signs

A few samples from Chuanlinggou Formation were analyzed for Re, Os, 187Re/ 186Os and 187Os/ 186Os, and the Ir content and trace elements of Chuanlinggou Formation were compared with mudstone of Changzhou Formation and Hongshuizhuang Formation to explain the nature of geological events in Chuanlinggou Formation.

From the table 1, it can be seen that the isotope increase of Re and Os is abnormal in the geological event interval.

Table 1 Re and Os analysis of Chuanlinggou Formation in Ming Tombs, Beijing

At present, a large number of documents have discussed the relationship between Ir isotope anomalies and geological events. Chemical concentration method and neutron activation analysis are used to compare the mudstones of Changzhou Formation (Chz B-5), Chuanlinggou Formation (Chu B- 1) and Hongshuizhuang Formation (Ho B- 10a), and it is proved that the mudstone of Chuanlinggou Formation has the highest Ir content (Song et al., 19 1). In addition, the mudstone of Chuanlinggou Formation contains Au11.3×10-9, which is obviously higher than the average value of mudstone. The Ir contents of Duandao Formation, Changzhougou Formation and Hongshuizhuang Formation are 0.5× 10-9, 0.36× 10-9 and 0.1×/kloc-0-9 respectively. The author thinks that the anomalies of Re, Os and isotope ratio, Ir and Au content are related to volcanic activity, not caused by extraterrestrial events.

Trace elements contained in mudstones of Chuanlinggou Formation, Changzhou Formation and Hongshuizhuang Formation are analyzed and compared by plasma spectrum, which has certain enlightenment to the environment of geological events of Chuanlinggou Formation (Table 2).

Trace element plasma spectral analysis data of silty mudstone of Chuanlinggou Formation, Changzhou Formation and Hongshuizhuang Formation.

It is worth pointing out that the content of rare earth elements in the strata developed by earthquake slump structure and sand liquefaction structure is very high. For example, the black argillaceous siltstone with the sample number of 99620- 12 is 33.2× 10-9, which is more than 30 times higher than the Clark value of re 1× 10-9. It is also 4 ~ 5 times higher than the upper and lower floors of the event interval; In particular, the isotope analysis value of 187Re/ 186Os reaches 548, which is about 10 times higher than that of the upper rock, and the value of 187Os/ 186Os reaches 7.5, which is 3-4 times higher than that of the upper rock (see figure/kloc) According to J.M.Luck et al. (1983):187 OS/186 OS ~1as the mantle source, ~ 65438.

Chuanlinggou Formation was formed in a lagoon that transited from clastic coastal zone to carbonate coastal zone (Song et al., 1996), so its sediments have a certain inheritance relationship with Xiachangzhougou Formation, as can be seen from the comparison of chemical elements listed in Table 2. The mudstone of Hongshuizhuang Formation is mainly fine clastic, sandwiched between two sets of carbonate rocks of Wumishan Formation and Tieling Formation, which is quite different from the elements contained in Chuanlinggou Formation. Relatively speaking, Changzhougou Formation and Chuanlinggou Formation are terrigenous, and Hongshuizhuang Formation is terrigenous. Generally speaking, Al, K, Na, Ti, Ba, Ga, V, Th, Ce, La and nd are elements from near land, while Ca, Mg and Mn are elements from far land. According to the contents of elements listed in Table 2, the ratios of Changzhou Gou Formation mudstone (Chz B-5) to Chuanlinggou Formation mudstone (Chu B- 1) and Hongshuizhuang mudstone (Ho B- 10a) are obtained respectively, and CHZB-5/CHUB- 1 approaches 1. It is worth pointing out that the three peak areas are: K, Ti, th, Ce and La. The existing data show that authigenic potash feldspar, authigenic sphene and pyrite often appear in the near-terrestrial source area and are minerals containing K and Ti. Recently, authigenic monazite (Song, 1999) was discovered in the Sinian mudstone in Dalian. Therefore, it is speculated that authigenic monazite may also exist in the mudstone of Changzhou Formation and Chuanlinggou Formation in the Ming Tombs of Beijing, and its chemical elements are mainly Ce, La and nd, with a small amount of Th and U, which also provides a possibility for U and Th-Pb isotopic dating.

Table 3 Content ratio of some fingering elements in mudstone of Chuanlinggou Formation, Changzhou Formation and Hongshuizhuang Formation

Fig. 3 Content ratio of several fingering elements in mudstone of Chuanlinggou Formation, Changzhou Formation and Hongshuizhuang Formation.

Chub-1-mudstone of Chuanlinggou Formation; CHZB-5 mudstone of Changzhou Gou Formation; Hob- 10a- mudstone of Hongshuizhuang Formation

Fig. 4 Seismic event records and stratigraphic correlation of Chuanlinggou Formation.

B- Beijing; J- Jixian county; K- kuancheng; Q- quyang; 1-silty mudstone or shale; 2- sandstone shale interlayer; 3- sandstone; 4- Dolomite lens or rock layer; 5- argillaceous dolomite; 6- iron ore deposit; 7- Volcanic or subvolcanic rocks; 8— Records of earthquake events; 9— Sedimentary discontinuity

From Figure 3, we can also learn that the geological events of Chuanlinggou Formation were caused by seismic activities near the land source, and their characteristics are different from those of Wumishan Formation (Song, 1988) and Dalian Sinian Xingmin Village (Fair-child, et al., 1997), which are considered to be relatively far from the land.

3 Comparison of earthquake events

Intra-layer slump, sand liquefaction filling veins and corresponding microstructures of Chuanlinggou Formation in the Ming Tombs of Beijing are also common in the upper part of Chuanlinggou Formation in Jixian County, Tianjin. Qiao Xiufu et al. (1994, 1996) made a large-scale comparison of liquefaction veins caused by the Sinian earthquake in Dalian. Zhao et al. (1997) trace and compare the Sinian system in Yanshan area, and think that the east-west sedimentary changes of Chuanlinggou Formation are great and its extension is unstable. If the Jixian section in Tianjin is the most complete in Yanshan Depression, then the Chuanlinggou Formation in the Ming Tombs in Beijing is only equivalent to the upper part of the Chuanlinggou Formation in Jixian section (Figure 4).

4 Discussion and conclusion

(1) Before the large-scale volcanic eruption in the heavy rain period in North China, there were volcanic activities accompanied by earthquake events in Chuanlinggou period, and macro and micro earthquake events were recorded in the corresponding horizons of Chuanlinggou Formation in the Ming Tombs, Jixian and Kuancheng areas of Beijing.

(2) Possible metazoan remains have been found in the upper part of the Changzhougou Formation (Song et al.,1985; Song et al., 1986) (Figure 2-e) is incredible, because its geological age is about 1.0 Ga earlier than the emergence of Ediacaran fauna. However, recent research shows that trace fossils before 1.0 Ga have been found in India (Seilacher et al., 1998). Therefore, it is speculated that the geological events of Chuanlinggou period in Yanshan area may be the reason why epigenetic trace fossils of Changzhou Gou Formation are generally difficult to continue. It is reported that trace fossils have also been found in the Dahongyu Formation, so a large number of volcanic eruptions may also make it difficult to continue.

(3) Authigenic monazite is found in the Precambrian mudstone in Dalian, and it is possible to find authigenic monazite because of its high cerium content, while the Chuanlinggou Formation and Changzhou Formation of the Ming Tombs in Beijing have high cerium content. Authigenic monazite contains elements such as U and Th-Pb, which provides the possibility for U and Th-Pb isotopic dating.

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