A set of possible earthquake-tsunami sequences in the Precambrian carbonate strata of the Ming Tombs in Beijing

A Probable Earthquake Tsunami Sequence in Precambrian Carbonate Strata of the Ming Tombs District,Beijing

Song Tianrui

The original article was published in the Chinese version of "Science Bulletin" 1988 8 issues; English version Kexue Tongbao, Vo L 33, No.13. The following modifications and additions have been made in this book: ① Changed the original black-and-white photos into color photos and descriptions; ② Added the hilly-sag layers in the storm sedimentary structures of the Baiyun Obo Group in Inner Mongolia and compared them with the series of earthquake-tsunami structures. , indicating that general storm sedimentary structures are small in scale and do not have the sequence of earthquake-tsunami structures; ③ further explains the earthquake origin mechanism of molar-shaped drainage veins; ④ Based on the research results of the past ten years, some supplements have been made to the text. This article is the first domestic report on seismic events in the Mesoproterozoic strata, and provides a systematic description and explanation of the seismic recording phenomena. In 1986, John Rodgers, an academician of the National Academy of Sciences, was invited to inspect the study area of ​​this article and agreed and supported the view of earthquake-tsunami sequences and the catastrophic interpretation of geological records.

There are many abnormal sedimentary structures in the Precambrian carbonate formation of the Ming Tombs in Beijing (Song Tianrui et al. 1985). The author believes that there is a set of possible earthquake-tsunami sequences among them. .

Abnormal sedimentary structures include a series of soft sedimentary deformations, which not only have the significance of geological mutation events, but also have the significance of predicting modern earthquake disasters to a certain extent in some areas (Song Tianrui, 1986) .

1 Geological location

There are twelve groups in the unmetamorphosed Meso-Neoproterozoic strata of the Ming Tombs in Beijing (Song Tianrui et al. 1985), from bottom to top: Changzhougou Formation (Chc), Chuanlinggou Formation (Chch), Tuanshanzi Formation (Cht), Dahongyu Formation (Chd), Gaozhuang Formation (Chg), Yangzhuang Formation (Jxy), Wumishan Formation (Jxw ), Shuishizhuang Formation (Jxh), Tieling Formation (Jxt), Xiamaling Formation (Qbx), Changlongshan Formation (Qbc) and Jingeryu Formation (Qbj) (Fig. 1, left); among them, the Wumishan Formation is the most A thick formation group composed mainly of stromatolite dolomitic rocks. The earthquake-tsunami sequence is located in the lower part of the Wumishan Formation and represents a catastrophic event in the Precambrian period (Figure 1, right).

2 Types of earthquake-tsunami structures

(1) Zigzag folds

This structure appears in the parallel stromatolite-like dolomitic rock basement. Above, when viewed from the side, the fold waves look like zigzags, and contain seismically collapsed breccias in the strata.

(2) Spiny breccia

Laminated algal mats (stromatolites) that were once disturbed became many fragments, appearing as radial breccias. From the side Look, each breccia has a spiny shape.

Figure 1 Precambrian sedimentary strata of the Ming Tombs in Beijing

Left: lithological column; right: earthquake-tsunami sequence of the Wumishan Formation

1—gneiss; 2—sandstone; 3—shale; 4—argillaceous dolomite; 5—silicified dolomite; 6—sandy dolomite; 7—dolomite; 8—marl; 9 — Limestone; 10 — Conformity; 11 — False conformity; 12 — Unconformity

(3) Inner folds

Between the parallel carbonate rock layers above and below, there are Symmetrical or asymmetrical folded rock formations composed of chert or dolomite.

(4) Cross-overlapping folds

Two fold systems are crossed-overlaid and pressed together, which shows that the directions of the two sets of fold axes are perpendicular to each other. On the upper level of the large folds, a group of small folds are superimposed, forming a "dragon skin-like" protrusion.

(5) Mound-building structures with graded bedding and mound-like cross-bedding

Mound-building structures with graded bedding are caused by debris flows and turbidity currents ;Typical moundlike bedding is found in the vicinity of mounds. The mound-building structure is what is called the Jinlang mound-like layer in Chapter 5 of this book.

Figure 2 "Slab Breccia" formed by earthquake-tsunami

a. "Slab Breccia" in dolomite is generated between the upper and lower parallel layers, "Slab Breccia" The intersection angle between "spiny breccia" and bedding can reach 40°~60°; PL: parallel bedding, PL (Dol): dolomite layer; SB: "spiny breccia"; b. "spiny breccia" contained in silicified rocks "Gravel", it is possible that the silicification phenomenon occurred after the formation of the "spiny breccia" of dolomite, because the upper and lower parallel rock layers have been silicified, and uneven silicification color bands appear at the edge of the "spiny breccia", PL(Si): Parallel silicified bedding; SB: There are gradient dark shadows around the silicified "spiny breccia"; c. The partially silicified dolomite contains cone-shaped steeply upright "spiny breccia" that is narrow at the top and wide at the bottom, PL (Dol): Parallel bedding of dolomite, SB: "Lateral breccia"; d. Linear "Lateral breccia" contained in dolomite, PL(Dol): Parallel bedding of dolomite, SB: " "Slaty breccia"

Figure 3 Intra-layer folds and mounding structures formed by earthquakes and tsunamis

a. Sliding due to earthquakes occurs between two parallel dolomite layers Intra-layer folds maintain the sliding trend of the original soft sedimentary deformation, PL (Dol): parallel dolomite layers; Flt: folds formed by intra-layer sliding; b. Mound-building structures formed by earthquakes and tsunamis, accompanied by involutions at the bottom The gradient layer composed of clastic particles has an upward finer grain sequence. Typical storm bedding appears on both sides: hummocky cross bedding and swell cross bedding, forming mounds. Later, liquefied drainage veins (called molar tooth abroad) appeared on the top of the mound due to water drainage. This was formed because the intraclasts contained more water than the dolomite lamina and were affected by earthquakes; MD: Mound-building structure, Gdb : Gradient grain sequence layer; HK: Storm hill bedding; MT: Liquefied micro-sparkling vein structure; c. The "dragon skin structure" caused by the internal bending of the bedding caused by the earthquake and the cross-folding on the surface of the rock layer, DL (Dol ): Parallel dolomite layer, Flt: intralayer folds; d. The hilly-sag bedding produced by storms in the dolomite layer of the Bayan Obo Group in Inner Mongolia is smaller in scale than the earthquake-tsunami ones, and there are no storm hilly layers at the top and inside Liquefaction drainage vein; PL (Dol): parallel bedding of dolomite, GW: good weather layer; BW: bad weather layer

3 Earthquake-tsunami sequence

Earthquake-tsunami sequence It is composed of five units or five types of earthquake-tsunami structures that occur in the Wumishan Formation (see Figure 1, right). The first unit "a" (serrated folds) and the second unit "b" (spinous breccia) usually appear in the lower part of the sequence; the third unit "c" (inner folds) often exists in the middle of the sequence ; Unit 4 "d" (cross-overlapping folds) and unit 5 "e" (mound-building structures with mound-like cross-bedding) are found in the upper part of the sequence.

What this article describes here is a complete earthquake-tsunami sequence. Therefore, these five units cannot appear in a section everywhere; but one or several units can appear along the stratigraphic trend. Contrasting (Song et al., 1985).

4. Explanation of earthquake-tsunami sequences

Some soft-sediment deformation sedimentary structures and submarine shell inversions can be temporarily attributed to seismic events (Seilacher, 1984). However, earthquake- Each unit in the tsunami sequence may be generated by a combination of basement shaking and seawater turbulence in a relatively short period of time. This article assumes that the longitudinal waves (P waves) of earthquakes can produce rock breccias (such as in units "a" and "b") on diagenetic sediments; but the inner folds are mainly affected by shear waves (S waves). , produced in semi-lithic plastic sediments (such as "c" unit); as for the "d" unit (cross-overlapping folds), its origin may be more complex. In addition to P waves and S waves, there may be Rav The influence of Love waves and Rayleigh waves. It is worth pointing out that debris flows, turbidity currents, and hummocky cross-bedding are reported to exist in deep-sea sedimentary environments (Walker, 1984; Kasters, 1984). According to the characteristics of the "e" unit, there are graded bedding in the mounds together with mound-like cross-bedding, accompanied by algal debris and stromatolites, which can be considered to be a shallow marine environment. Therefore, in the fog During the Mishan Formation stage, affected by earthquake-tsunami storms, turbidity currents may have appeared in the shallow sea environment; and shell overturning may occur in time when the ocean bottom is affected by seismic waves (Seilacher, 1984).

The earthquake and tsunami sequence in the middle and lower sections of the Mesoproterozoic Wumishan Formation in the Ming Tombs of Beijing extends far away, from Jixian in the east to Xishan in the west. In particular, the "sandy breccia" is not a general storm effect. It can be explained because storm breccias also appear in the Gaozhuang Formation and Wumishan Formation, which are characterized by a small flattened inclination angle of the breccia, with an angle of only 10° to 25° and an angle of 3 to 5cm from the normal layer. The gravel flakes are distributed in an imbricated shape, but the angle between the "slab breccia" formed by earthquakes and tsunamis and the parallel bedding is mostly between 40° and 90° (Fig. 2a), and may also appear in silicified rocks (Fig. 2b ), can also appear locally in the shape of cone piles that are narrow at the top and wide at the bottom (Figure 2c), or even appear as thin linear "spiny breccias" distributed between parallel dolomite layers (Figure 2d). These strong dynamic effects are not ordinary storms. It can be formed by scouring force; "spiny breccia" also appears along with intra-layer folds (Figure 3 a), sometimes on a huge scale; there are also bidirectional fold axes crossing to form a "dragon skin structure", which appears as a small intra-layer bend in longitudinal section (Figure 3c); Earthquakes and tsunamis can trigger waves of more than 10m, mound-building structures (Figure 3b), and associated intraclastic grain sequence graded layers and hummock-sag bedding; which are particularly worth pointing out. A typical molar tooth appears on the top of the mound structure. This is because the intraclastic particles in the lower part contain a lot of water. Affected by the liquefaction of the earthquake sand bodies, the released water migrates from bottom to top. The shape is exactly the same as that of the molar discharge veins in the seismic rocks of the Xingmincun Formation of the Sinian System in Dalian. As for the hilly-sag-shaped bedding, it is also much larger than that formed in normal storms. For this reason, we specially used Comparing the typical hilly-sag bedding formed by general storms found in the dolomites of the Bayan Obo Group in Inner Mongolia (Fig. 3d), it can be seen that good weather forms normal parallel bedding (GW), and bad weather forms large-scale bedding. Smaller hilly-sag bedding (BW), and is not accompanied by a series of severe earthquakes such as "spiny breccia", "intra-layer folds", "mound-building structures" and "grain-sequence graded layers" structure. It is not accompanied by liquefaction leakage veins, etc.

It must be pointed out that although the "mound-building structure" caused by the earthquake-tsunami and the "mound-like bedding" formed by the storm are similar in appearance, they are actually different. First, the "mound" body of the "mound-building" The material composition inside and outside the mound is different. The inside of the mound is composed of graded layered coarse intraclastic sand, and the outside of the mound is a fine-grained mud layer; while the inside and outside of the mound-like bedding "mound" are fine-silver intraclastic sand with the same basic grain size. ; Secondly, the "molar tooth" formed by liquefaction and water leakage appears on the top of the "mound" body of "Building Mounds", but there is no "molar tooth" in Storm "Mound"; Thirdly, two vertical teeth can appear on the top of the "mound" body of "Building Mounds" Overlapping, while the "mounds" of the storm only appear to be connected laterally; these all reflect that the hydrodynamic force of the "mound-building" is extremely strong, while the storm is relatively weak (see Figure 3b, d).

References

Song Tianrui. About paleoseismic information in sedimentary rock formations. See: Progress in Astronomical Geology (eds. Zhang Qinwen, Xu Daoyi). 1986, 95～104

Song Tianrui and Gao Jian.1985.Tidal sedimentary Structures from Upper Precambrian rocks of the Ming Tombs District,Beijing(Peking),China,Precambrian Research, 29(1985):93～107