Seismic event record in sedimentary strata and its significance

Recording of seismic events in sedimentary strata and its significance

Song Tian Rui

1 the origin of the problem

In the early 1980s, when studying the sedimentary rocks of the Mesoproterozoic Changzhou Gou Formation in the Ming Tombs of Beijing, it was found that there was a "hole-digging" filling structure in the rhythmic layer of tidal flat sandstone and siltstone in the middle of this formation. At that time, it was thought that there were two possible reasons, one was the oldest trace fossil, and the other was the filling pipeline structure caused by the earthquake (Song,, 1985). Later, similar trace fossils were found (Song,,1985; Song He, 1985) and preliminarily identified as: Planolites sp (? ); Because the stratigraphic age of the Changzhougou Formation is more than 65.438+billion years earlier than that of the Ediacaran fauna, it is tentatively named "Suspected Trace Fossils" (Song, 654.38+0.987). As for why the "digging hole" filling structure is particularly developed in late Proterozoic clastic rocks and is also common in Neoproterozoic carbonate rocks, there are different understandings in domestic and foreign literature records. One view is that it is caused by shrinkage crack filling (D.L.Kidder, 1990), and this view is that it is caused by earthquake events (Song, 65438+), Qiao Xiufu and Song, 1994,1996; Song He G. Ainse Le,1996; I.J. fairchild, G. Einsele and Song, 1997). In fact, the influence of earthquake events on sediments has long been noticed, for example, stepped faults (A.Seilacher, 1969), and submarine earthquakes can turn the deposited crust upside down (A.Seilacher,/). M. Green et al., 1994). This paper only discusses the study of earthquake event records.

Quaternary earthquake and modern earthquake records include stratum dislocation, sandblasting, folding and collapse (Ding Guoyu,1982; Feng Xianyue et al., 1982), but there are definitely more Proterozoic earthquakes and tsunamis. Because the records of these large earthquakes are difficult to identify, it is necessary to pay attention to similar sedimentary structures from macro to micro. In addition, because modern seismic records mainly occur on land, it is more difficult to study seismic records in marine sediments, especially micro-sedimentary structures, than stratigraphic methods, such as molar structures, so other explanations have appeared. Fairchild et al. (1992) recognized the correctness of earthquake interpretation after studying the Xingmincun Formation of Dalian Sinian system, and explained it from the mechanism.

2 Records of seismic events in sedimentary strata

2. 1 Earthquake event record belongs to abnormal sedimentary structure.

Normal sedimentary structures occur in the order of denudation-migration-sedimentation-diagenesis, while the sedimentary structures caused by earthquake events are special records that disrupt the normal order. No matter in the ocean or on land, earthquake events caused by faults, volcanic eruptions, large-scale collapses and even celestial impacts will produce macroscopic and microscopic sedimentary structural anomalies, local discontinuities, collapses, brecciation and drainage structures of sedimentary strata when they reach a certain intensity.

2.2 Macro record

198 1 year, the Holocene Sub-committee of the Quaternary Research Committee of China and the Seismological Bureau of Shaanxi Province held the "Symposium on Prehistoric Earthquakes in China" in xi 'an, and discussed the ancient earthquake relics found in Xinjiang, Shaanxi, Beijing, Jiangsu, Yunnan, Sichuan and other provinces (cities, autonomous regions) and their research results, including fault dislocation, sediment deformation, various related geomorphological phenomena and so on. Fault creep, sand liquefaction, sand blasting, liquefied convolute layer and sandy dike in sedimentary strata are the most typical macroscopic signs (Zhu Haizhi,1982; Feng Xianyue et al.,1982; Hong Xiang Fa et al., 1996). The conditions for preserving seismic event records in older sedimentary strata are complicated, and late tectonic activities, magmatic hydrothermal activities and frequent storm activities may affect the accurate judgment of seismic event records (Gong et al., 1988). Abnormal sedimentary structures caused by earthquakes can appear in sandstone, mudstone and carbonate strata, and also in granular layers and mound-depression bedding similar to turbidite and tempestite. Therefore, sand liquefaction, intralayer folds and intralayer micro-faults have become the most characteristic signs to identify earthquake event records. In this paper, the judgment of macroscopic and microscopic signs of earthquake events emphasizes its comprehensiveness. It is necessary to consider not only a single abnormal sedimentary structure, but also its sequence and relationship, as well as the event environment that produces abnormal sedimentary structures, such as the tectonic activity stage of sedimentary strata, the signs of volcanic activity, the sedimentary environment, the accumulation speed and quantity of sediments, and the possibility of celestial matter falling.

(1) fault, syngenetic fault and dislocation displacement

There are many faults in the earth's crust, but not every fault zone can find records of earthquake events. One case is that the active period of the fault has passed and the old remains are no longer preserved, and the other case is that the vibration generated by the fault is small and slow enough to produce abnormal sedimentary structures recorded by earthquake events. But there is a homologous sedimentary basin unit, which is related to faults, syngenetic faults and dislocation displacement. Even the existence of contemporaneous faults in the basin can be inferred from the dislocation displacement at the edge of the basin. A.Seilacher( 1969) first interpreted the stepped fault as seismite; Dislocation displacement can also be called micro-fault. (A. Seilacher， 199 1)。 The interlayer spacing of dislocation displacement is only a few centimeters, which means that the periphery of the basin is far from the earthquake center of the fault, but the center of the basin is accompanied by a series of collapse brecciation phenomena because of the large distance of the fault. For example, the Proterozoic Yanshan trough basin in North China is a sedimentary basin with a history of 654.38+0 billion years. There are dislocations or micro-faults in the Changzhougou Formation of the Ming Tombs in Beijing in the west of the basin (Song et al., 199 1), and Kuancheng in Hebei in the east (Figure 1), and there are also stepped faults at the same level in the center of the basin. The fault distance of the Great Wall syngenetic fault in Huolangyu area of Miyun is hundreds of meters. Wang, sedimentary facies, sedimentary environment, sedimentary model and sedimentary background of clastic rocks in Yanshan Great Wall sedimentary basin, and also on the formation and evolution characteristics of the basin, Ph.D. thesis of Institute of Geology, Chinese Academy of Sciences, 1985.

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(2) Intralayer fold

Intra-layer fold refers to the symmetrical or asymmetrical fold between the upper and lower parallel layers, which can be judged that the earthquake occurred in the middle of the formation of the upper and lower sedimentary layers, with different scales, but they all appeared together with the surrounding earthquake event records. The mudstone in Neogene two-layer sandstone in Wensu County, Xinjiang is a symmetrical intralayer fold, and the wavelength can reach 10m (Song, 1982). The black siliceous rocks in the double-layer dolomite of Mesoproterozoic Wumishan Formation in the Ming Tombs of Beijing are symmetrical intralayer folds with a wavelength of 30cm. The folds in the other dolomite are asymmetric, about1.5m; ; However, in the fold layer of Tuanshanzi Formation, the fold in the layer is only about 40cm. The folded stromatolites of Wumishan Formation, like Chinese cabbage, were also formed by earthquakes. Intra-layer folds can also be superimposed to form dermoid structure (Song, 1988) or slip to form dermoid structure (A. Seilacher, 199 1).

Figure 1 Paleoseismic record of Proterozoic Changzhou Gou Formation in Yanshan.

A. The stratigraphic records of Changzhougou Formation in Busanling, Beijing, in the western section of Yanshan Mountain are mainly characterized by intra-layer faults, sand veins and filled tubular structures. In the flattening and lenticular bedding interbed of tidal flat sedimentary profile, the microstructure includes flame structure, intralayer dislocation, siliceous blind vein and so on. Lf: The middle section of Changzhou Gou Formation is interbedded with tidal flat mudstone and sandstone; Sand liquefaction veins caused by earthquakes were found in the flat lens layer of Changzhougou Formation in the Ming Tombs of Beijing. These sedimentary structures are different from dry fracture structures. Mud cracking is a layered structure, which permeates downward in a V-shape, while sand liquefaction veins permeate downward and upward at the same time. Ls: lenticular sandstone; Fm: Muddy thin bedding, silty thin bedding; Lq( 1): sand liquefaction vein penetrates downward; Lq(u): sand liquefaction vein penetrates upward; Seismic slump breccia (Sb) occurs in Changzhougou Formation, Kuancheng County, eastern Yanshan Mountain. The normal faults in the sandstone layer of Changchenggou Formation in Kuancheng are caused by earthquakes, and these extensional structures conform to the formation mechanism of Yanshanian extensional depression. PL: parallel sandstone bedding; Ift: interlayer normal fault

(3) brecciation in the stratum

Seismogenic breccia is different from normal sedimentary breccia, which can be identified according to the accumulation mode and size of breccia and other seismic records. For example, in western Sichuan and western Yunnan, seismogenic breccia is unconformity with seismogenic breccia (Liang et al., 199 1) and is associated with water system structure (Qiao Xiufu et al., 65438+). In the first two cases, the interbedded breccia is mostly arranged in disorder, while the interbedded breccia produced by the latter is arranged in parallel or radial like spines, which is easily mistaken for the breccia produced by the storm.

(4) Tsunami storm stratification

Typical storm bedding in hilly-depression is generated below the bottom of waves (S. Greewood and D.J. Sherman, 1986), but when an earthquake causes a tsunami, the sea level in the shallow sea-coastal zone may suddenly rise. According to the records, there were two earthquakes and tsunamis along the coast of Honshu, Japan in 1896 and 1933, and the sea level rose by 20 ~ 24m；. 1975, a tsunami hit Caggio Bay in Hawaii, which lasted for more than 4 hours. The peaks of the two tsunamis were the same, which was 15 min. The wavelength caused by tsunami is related to the water depth, which is equal to d, g is the acceleration of gravity, and d is the depth of seawater (B.A.Bolt, 1978). It can be seen that the conditions for the formation of storm bedding-hill-depression bedding under the influence of tsunami exist. This is confirmed by the existence of typical storm bedding such as gully bedding and gradual bedding in the earthquake and tsunami sequence of the Mesoproterozoic Wumishan Formation in the Ming Tombs (Song, 1988).

(5) Drainage structure

There are many records about the phenomenon of sand liquefaction in clastic sediments caused by modern earthquakes, such as "sandstone wall", "sandstone layer" and "sandstone cycle layer" (Zhu Haizhi et al., 1982). Similar sand liquefaction phenomena have been found in the Mesoproterozoic Changzhougou Formation, the Chuanlinggou Formation in the Ming Tombs of Beijing and the Chuanlinggou Formation in Jixian County, and related literatures have also reported (Song et al.,1985; 1986; Song and G. Ainse Le, 1996). Water system structure in carbonate rocks also appeared at the top of the mound structure in the earthquake and tsunami sequence of Wumishan Formation in the Ming Tombs of Beijing (Song and G. Ainse Le, 1996), but on a smaller scale. The drainage structure in dolomitic limestone of Sinian Xingmincun Formation in Jinshitan, Dalian is very common, and it has been proved that it is liquefied mud veins caused by earthquake vibration (Qiao Xiufu et al., 1994, 1996). This kind of sawtooth micrite vein is called "molar tooth structure" abroad (Smith,1968; Connor,1972; Horodyski, 1976), but the explanation of its earthquake origin is the first report in China. I.J. fairchild and others also agree with his conclusion on the cause of the earthquake after his investigation in this area. He believes that the formation process can be divided into six stages: ① the cohesion of the original rock decreases from argillaceous to sandy, but the water saturation increases, and the petrochemical direction is opposite; (2) cracks can be pulled into the basin or groove by gravity; ③ When buried deeply, the degree of cementation increases from top to bottom, and the water content decreases, so the upper part can produce soft sediment deformation, and the lower part can produce cracks under the action of love wave; ④ Xingmincun Formation is composed of alternate layers of tempestite and micrite dolomitic limestone, and the fractures are mostly concentrated in dolomitic limestone; ⑤ Interlayer stratum produces local liquefaction in the groove; ⑥ Under the influence of the earthquake, the cracks produced filling-cementation, bending and fracture, and finally formed a "molar-like structure" (Figure 2).

2.3 Miniature recording

Because earthquake events can produce macroscopic records, they can also have an impact on microscopic scales. However, due to many factors that cause microscopic phenomena, similar phenomena are difficult to identify. Therefore, we should be cautious when judging whether the microstructure is induced by earthquake events. The most important thing is to explore the formation mechanism of microstructure on the premise of determining the macro-structure, and at the same time, to analyze and judge the sedimentary facies and sedimentary environment formed by sediments.

Fig. 2 "molar stripe" structure caused by earthquake vibration.

Different lithology; B. cracking and gravity; C. buried depth; D. different horizons; E. different horizons in the trough period; F. Fault development

(1) Intralamellar dislocation

Micro-layer dislocation caused by earthquake is very common in the lithology of mud-sand interbeds. Due to the acceleration of seismic waves, the bedding is dislocated, but the shear resistance of mud layer is stronger than that of sand layer, so the dislocation structure is generated between mud and sand layers (I.J.Fairchild et al., 1997). Both the Mesoproterozoic Changzhougou Formation and Chuanlinggou Formation in the Ming Tombs of Beijing have microlayer dislocation, accompanied by other macro-seismic event records, and the dislocation distance is 0.25mm, showing a "normal fault" shape. Of course, dislocations in other layers may also appear in seismic rocks in the form of "reverse faults" (A.Seilacher, 199 1).

(2) Microscopic drainage structure

Because the macroscopic records of the earthquake events of the Mesoproterozoic Changzhougou Formation and Chuanlinggou Formation in the Ming Tombs of Beijing are obvious, especially in the Chuanlinggou Formation, it is found that volcanic activity is related to the earthquake records (Song and G. Einsele,1996; Song et al., 2000), it is inferred that a series of filling microstructures are earthquake-induced micro-drainage structures, rather than diagenesis or dry crack filling phenomena. There are three types of micro-drainage structures: ① The coarse silt veins filled downward and upward are arc-shaped or "molar-shaped" (Song,,1985); (2) In the banded rocks of mudstone and siltstone layers, the upper siltstone layer presents a "micro-gyration" structure or a "flame" structure; ③ Microcrystalline siliceous veinlets are inserted into siltstone-sandstone layer, and the formation stages of veinlets are different, and dislocations can be displayed in the layer, which is different from ordinary diagenetic microcrystalline siliceous veinlets. The filler produced by micro-drainage structure or tubular filling structure is relatively coarse particles, because under the same seismic intensity, "the finer the particle composition, the higher its relative density, and the more difficult liquefaction" (Huang Xinggen et al., 1982).

(3) Broken single crystals and lattice dislocations

At present, it is found that there are examples of single-particle lattice fragmentation and lattice dislocation in clastic rocks and carbonate rocks, which are probably caused by violent impact. However, because some particles are terrigenous clastic materials, it is necessary to study the stage of crystal breakage and the background of lattice dislocation. The existence of this microscopic record provides clues for further research in the future.

In a word, there are various records of earthquake events in domestic and foreign literatures (Figure 3). Obviously, the records of seismic events determined in strata are much less than the times of geological events in geological history. "Why is this? (1) Sedimentary scientists may not be able to determine seismites, such as turbidites or tempestites; ② Seismic signs may be destroyed by subsequent storm events or biological interference in most places, especially in fine-grained mudstone deposited in anoxic basins; (3) Because of the unclear mechanism, it is difficult to confirm that the sediments are seismic deformation rather than other reasons "(A.Seilacher, 199 1).

3 Significance of earthquake event record

In the past, there was a lack of definite judgment on seismic event records in sedimentary strata, and it was difficult to understand its significance. In recent ten years, due to the results of various studies, it is reasonable to distinguish the differences between sedimentary strata and seismic records in storm rocks and turbidites, so as to explore stratigraphic correlation and regional seismic correlation. With the increase of accumulated data in the future, the understanding will be deeper.

The significance of earthquake event recording can be summarized in three aspects:

(1) According to historical records (Li Shanbang, 198 1), earthquake occurrence levels can be divided. However, due to the differences in population distribution, cultural development and recording conditions in different provinces, the accuracy of earthquake records is also different (table 1). For example, in Shaanxi province, there are many places where emperors built their capitals, and the recorded data are reliable. However, Xinjiang is vast and sparsely populated, with many ethnic groups. In the early days, nomadic people dominated, and the recording conditions were poor. Only earthquakes have been recorded since 17 16 BC. However, Xinjiang is an earthquake-prone area, and these records are probably incomplete. In addition, Tibet, Qinghai and Heilongjiang were sparsely populated in ancient times, with large areas of mountains and deserts. Although earthquakes occur frequently, there are few historical materials. According to the statistical data compiled by Li Shanbang (198 1), we can still see the difference of earthquake occurrence rate among provinces. This paper is divided into eight grades. Tibet, Taiwan Province Province, Yunnan and Gansu are the earthquake-prone areas in past dynasties. Although the division by provinces and regions is different from that by tectonic units, according to the inheritance relationship between Neogene and Quaternary sediments in geological history, Neogene is thick, and there are many unconformity contact surfaces between Quaternary and Neogene, which is also of guiding significance for earthquake classification (Song, 1986).

(2) The abnormal structures caused by earthquakes in sedimentary strata have certain significance for the prediction of modern seismic zones.

The above-mentioned earthquake events have produced a series of macro and micro abnormal sedimentary structures, and many active seismic faults and sandblasting structures have been reported in Quaternary Pleistocene (Wang,1982; Xiang Hongfa et al., 1996). Therefore, in Quaternary drilling engineering and hydrogeological exploration, it will be beneficial to the prediction of earthquake zones and inherited strong earthquake-prone areas by studying and observing the abnormal structure and frequency caused by earthquakes.

Fig. 3 Seismotectonic records in the literature.

Proterozoic earthquake and tsunami structure in North China (left); Simplified seismic rock (right) (according to A.Seilacher, 199 1)

(3) Large-scale stratigraphic correlation is carried out according to earthquake event records.

During the large-scale stratigraphic correlation, especially for the strata lacking fossils, such as Precambrian, it has been confirmed that there are seismic micrite veins ("molar tooth pattern" structure) in carbonate rocks (Qiao Xiufu et al., 1994,1996,2001). Liquefied filling veins of sand layers are also common in clastic rocks of Mesoproterozoic Changzhougou Formation-Chuanlinggou Formation. Similar phenomena have been found not only in China, but also in the United States, which is not accidental, and may be related to frequent earthquake events caused by global crustal activities in early Proterozoic.

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The fifth chapter is the earthquake sequence of vibration liquefaction of carbonate rocks, originally published in Journal of Geology, Volume 68 1 issue, and published in Journal of Geology, Volume 7, Issue 3 1994. The Sinian system in the south of Liaodong Peninsula has been added to the current issue.

(1) explains the composition difference between argillaceous bright veins and surrounding rocks in carbonate rocks. Song put forward the concept of "blind pulse" in 1996. Combined with the liquefaction simulation experiment, it is pointed out that some liquefied sand veins are connected with the liquefied parent sand layer, and some are separated from the liquefied parent sand layer to form "blind veins", and their composition and particle size are slightly different from the sand layer around their emplacement.

(2) In the research work after 1996, four seismic active periods were identified in the Sinian system in the Xuhuai area of Jiaoliao, and a new Sinian system was compared. The printed text (Figure 20) still retains the original three active periods of earthquakes, without modification, and only sets the age of the lower boundary of Upper Sinian at 550 Ma. For the four active periods of earthquakes, please read the article "Paleoseismic Belt of China-Korea Plate" in Chapter 22 of this book and related articles in this book.

(3) Some maps and photos have been added.

Since the publication of 1994, the original text has attracted the attention of geologists at home and abroad. Plaziat Yate, a French geologist, enthusiastically supported it in his private letter 1995, believing that it was the most profound explanation of geodynamics, and gave a large number of foreign documents and his own earthquake research results in the Red Sea Rift Valley. Fairchild and others also specially investigated the seismic records of Sinian system in Liaodong Peninsula, and published corresponding papers on seismic viewpoints. Many geologists in China have cited this paper many times (one of the papers with high citation rate in geological magazines), which has promoted the study of seismic events in strata. The basic facts and explanations of the paper have been tested by geological practice for more than ten years.

We are glad to see that document 1994 has promoted the depth of stratigraphic research and structural interpretation in China's geological survey. Therefore, in this monograph, the article 1994 is supplemented with information, and some geological records are further explained. The published article is somewhat different from the original text, which is a scientific summary of re-creation.