Introduction to the first author: Chen Hongjun, male, born in 1973, engineer, bachelor, now mainly engaged in marine geological investigation and research, e? Mail: Chen Hongjun, hydz.cn

(Guangzhou Marine Geological Survey Guangzhou 5 10760)

Based on the study of borehole data of ZQ2 and ZQ4 in the northern shelf area of the South China Sea, the seismic sequence division and earthquake prediction of single channel seismic data in this area are carried out. Interpretation and analysis of sedimentary facies. According to the characteristics of borehole strata, seismic reflection and submarine geomorphology, this paper analyzes the ancient coastal positions in the late Middle Pleistocene, the last glacial maximum and the late glacial period, and probes into the ancient coastal changes in this area since the late Middle Pleistocene, which is of great significance for further guiding the quaternary environmental research in this area.

Keywords Pleistocene; Ancient coast of South China Sea; Single-line earthquake

1 preface

The study of global environmental change is one of the important topics in earth science research in this century, and environmental change is closely related to sea level change. The change of ancient coast is a direct reflection of sea level change, and it is very meaningful to understand the change of ancient coast to reveal the environmental change. Feng et al. used borehole data to analyze the sea level change in this area, but the method of identifying coastline by combining single-channel seismic data with borehole data is rare in China. This study attempts to understand the law of sea level change by this means, which is of great significance to the study of environmental changes in the South China Sea since the Late Tertiary.

Analysis of Sedimentary Environment in the Late Middle Pleistocene and Late Late Pleistocene in Holes 2 ZQ2 and ZQ4

2. Analysis of sedimentary environment of1zq2 hole

2. 1. 1 Late Middle Pleistocene

According to the dating, hole 3 1-45.3m of ZQ2 belongs to the upper member of Middle Pleistocene, with coarse lithology and gray extremely fine sand mixed with gravel. Gravel interbeds exist in the range of 35.00 ~ 4 1.3 1m in this section.

Paleontological analysis data show that foraminifera fossils in this section are scarce or missing. There are few nannofossils, belonging to Gephyrocapsaoceanica assemblage, with 5 species, representing the continental sedimentary environment. There are no diatom fossils and ostracods at the top (Feng Zhiqiang et al., 1996). The above data show that this period was a continental sedimentary environment (Figure 1).

Figure 1 Location Map of Boreholes and Seismic Lines in the Study Area

Figure 1 Borehole and Seismic Line Location

2. 1.2 Late Pleistocene

The 7 ~ 16m segment belongs to the upper segment of late Pleistocene. The ESR age of 8m is 165438+30600a. B. P. 65438+ 14C11942 277a.b.p. and 13m when the age is 0.4m. The age difference between 8 ~ 1 1.4m is 542 277a. B.p., the sedimentation rate in this period is calculated to be about 0.013 ~ 0.004m/a.b.p. According to this sedimentation rate, the age difference between 1 1.4 ~ 13m is18655. B.P, the sedimentary thickness in this period should be 242.6~74.6m, but the hole only reveals1.6m. Obviously, there is strong sedimentary erosion in the section of1.4 ~13m. The maximum value of the last ice age was about 18000a. The decrease of sedimentary thickness in this period is the result of low sea level during the last glacial maximum.

2.2 ZQ4 hole sedimentary environment analysis

2.2. 1 Late Middle Pleistocene

32.0 ~ 39.0 m is the upper member of Middle Pleistocene, which is mainly composed of dark gray fine sand and clayey silt. According to the data of sediments and paleontology, Feng Zhiqiang and others think that the ZQ3 hole was a littoral and shallow sea sedimentary environment in the late Middle Pleistocene (Feng Zhiqiang and others, 1996) (Figure 3).

2.2.2 Late Pleistocene

3.5 ~ 1 1m is the upper member of Late Pleistocene, and the lithology is mainly light gray clayey silt. In this stratum, only the freshwater species Cyclotella is regarded as diatom. However, the foraminifera are few in number, mainly Bekazia, and their shells are broken, and many of them are rust-colored. The fossil assemblage of foraminifera is also very strange, including both deep-water foraminifera and shallow-water foraminifera (Feng Zhiqiang et al., 1996), from which it is inferred that foraminifera were transported and deposited in different places. Generally, it reflects the estuarine sedimentary environment.

Fig. 2 ZQ2 borehole histogram, sedimentary facies map and sea level change curve.

(modified according to Feng Zhiqiang et al. 1996)

Fig. ZQ2 borehole comprehensive histogram, sedimentary facies sequence and sea level change (after Feng et al. 1996)

The t.l. age of the hole is 10978 549a at 3m, the ESR age is15500a.b.p. at 3.5m, and the ESR age is 34600a. /kloc-b.p. at 0/0.00m (Figure 3). Calculated by 3 ~ 3.5m, the deposition rate is 8. 3.5 ~ 100 m difference is19100 a. B.P, the corresponding sedimentary thickness should be 14.8m, but in fact the thickness of this section is only 6.5m Therefore, this shows that this area was also affected by the low sea level during the last glacial maximum, when the exposed sea surface in the shelf area was eroded.

To sum up, in the late Middle Pleistocene, the ZQ2 hole was a continental sedimentary environment, while the ZQ4 hole was a coastal shallow sea deposit. In the late Late Pleistocene, both holes ZQ2 and ZQ4 were continental sedimentary environments. Due to the low sea level at the peak of the last glacial period, the thickness of sedimentary layers was reduced due to erosion.

Fig. 3 Borehole histogram, sedimentary facies map and sea level change curve of ZQ4.

(modified according to Feng Zhiqiang et al. 1996)

Fig. ZQ4 borehole comprehensive histogram, sedimentary facies sequence and sea level change (after Feng et al. 1996)

3 seismic reflection characteristics

This area is divided into five reflective interfaces: R0, R 1, R2, R3 and R4 from top to bottom. On this basis, the seismic sequence of single channel seismic profile is divided. Five groups of earthquake sequences a, b, c, d and e are identified. Combined with borehole dating data, the corresponding ages of each sequence are shown in table 1.

Table 1 seismic sequence division in northern South China Sea

3. 1 layer b reflection characteristics

The overall performance is low continuity of amplitude variation? Intermittent, low-frequency reflection and internal chaotic reflection structure (Figure 4) indicate that the lithology of this set of strata has changed greatly. The seismic facies characteristics reflect the relatively high-energy sedimentary environment.

Fig. 4 seismic reflection characteristics of layers b and c

Fig. 4 seismic reflection characteristics of sequences b and c

3.2 layer reflection characteristics

Between ZQ2 and ZQ4 holes, this layer has two seismic reflection characteristics, one is amplitude variation, low continuity and low frequency reflection (Figure 4), and there is random reflection locally in the layer, which is unconformity with the underlying stratum. False integration relationship. Generally, it reflects the high-energy deposition environment. Another feature is medium? Weak amplitude, continuous, intermediate frequency, mat, internal parallelism? A nearly parallel reflective structure (Figure 4), which is in super contact with the upper layer B and the lower layer D ... generally reflects the low-energy deposition environment.

4 identification and characteristics of ancient coast

Through seismic facies? Sedimentary facies analysis, identify the ancient coast on the seismic profile and determine its approximate location. The so-called coastal zone refers to the interaction between land and sea.

4. 1 Late Middle Pleistocene

Seismic facies and sedimentary facies are analyzed in layer C of single channel seismic profile in this area (see attached figure 1 for the profile position). According to different reflection characteristics, it represents different sedimentary environments. On the basis of sedimentary facies analysis, the sedimentary facies map of layer C (Figures 5 and 6) is made, and then the continental facies, coastal facies and shallow sea facies are divided, and the approximate position of the ancient coast in the late Middle Pleistocene is determined (Figure 9 (a)). Combined with the analysis of drilling data, the formation time of the ancient coast is about 280,000 ~ 230,000 A B.P. Generally, it is EW-oriented and located at the modern water depth of-50 ~-120 m.

Fig. 5 reflection of ancient coastline on profile (ZD42 survey line)

Fig. 5 Ancient coastline (ZD42 line) in seismic profile.

4.2 Maximum value of last glacier

The heyday of the last ice age was about180000 ab.p., during which the sea level dropped the most, and the northern continental shelf retreated in a large area, exposing the land (Fan et al., 2004). Sea level drop100 ~ 200m (Wang et al., 1990). The ZD44 section (see Figure 1) located at the slope break of the outer edge of the continental shelf shows that layer B is a set of low-order alluvial fans (Kou et al., 1994) related to river action, indicating that the sea level dropped sharply at the peak of the last ice age, and the ancient coast retreated outside the slope break of the continental shelf. Fig. 9 (b) shows the ancient coast inferred from the position of the low tide fan on the profile.

4.3 Ancient coastline from Late Glaciation to Early Holocene

Fig. 6 Sedimentary facies map of seismic sequence C in the study area

Fig. 6 Sedimentary facies of sequence C in the study area

Fig. 7 Low water level system formed during the last glacial maximum (ZD44 survey line)

Fig. 7 Low tide fan (ZD44 line) formed during the last glacial maximum.

In the late ice age, the climate warmed, the sea level rose, and the ancient coast briefly stopped. Layer A is a set of thin delta deposits formed at that time (Figure 8). Its seismic facies is characterized by parallel upper and lower interfaces, and its interior is gently inclined progradation reflection. This layer and the bottom boundary are in a lower super contact relationship. Topographically, it has a gentle slope. Using the existing seismic data, according to the morphological characteristics and location of the slope, the ancient coast of this period was determined (Figure 9 (c)). The modern water depth of the coastline is about-180m, which is in the northeast direction. The coastline is straight and roughly parallel to the modern coastline.

Fig. 8 Late Glacial Paleocoastline (ZD 12 survey line)

Fig. 8 Ancient coastline at the end of the ice age (ZD 12 line)

5 discussion and conclusion

From the above analysis, it is concluded that in the late Middle Pleistocene, the ancient coast was generally EW-oriented, located at the modern water depth of -50 ~- 120 m, and the sea level dropped sharply at the peak of the last ice age, and the ancient coast retreated outside the shelf slope break zone. From the late glacial period to the early Holocene, the ancient coastline was located at the water depth of about-180m, which was ne-oriented, and the coastline was straight, roughly parallel to the modern coastline.

From the late Middle Pleistocene to the last glacial maximum, the ancient coast retreated from the land to the sea, indicating that the sea level changes in this period generally showed a downward pattern. After the last ice age, the climate became warmer and the sea level began to rise. After the Holocene, in a short period of more than 10,000 years, the ancient coast moved from the shelf fault to the water depth of-180m, and then quickly moved to the water depth of -20m, until the present position (Dr. J.R.Chen et al., 1985), indicating that the sea level has risen rapidly since the Late Glaciation.

The rapid change of coastline is influenced by several aspects: ① the injection speed of terrigenous debris (Wilgus et al.,1991); ② Climate change; (3) Neotectonic movement characterized by land rise and sea fall in this area (Zhang Hunan et al., 1990). Under the comprehensive action of these factors, the ancient coastline gradually retreated to the sea from the late Middle Pleistocene to the Ice Age. In the late ice age, the climate became warmer, the supply of sediments decreased, and the coastline gradually migrated to the land until its present position.

Because the ancient coast is constantly moving back and forth in this area, it forms a regressive coast, so it can be said that the whole sea area has an ancient coast. Transgressive coasts are often easily eroded, leaving mainly regressive coasts. There are not many identifiable ancient coasts, and the resolution of single-channel seismic data is limited, so only relatively late ancient coasts in a certain earthquake sequence can be identified. With the further investigation in the future, it is believed that more ancient coasts will be discovered.

Fig. 9 Distribution map of ancient coastline in different periods.

Fig. 9 distribution of ancient coastline in different periods

References and reference materials

, Feng,. 1985. Study on the -20m ancient coastline in the northern South China Sea. Proceedings of China Quaternary Shoreline Symposium.

Fan, Liao 2005. Changes of Cenozoic paleoenvironment in the northern South China Sea. Journal of Guangxi Academy of Sciences, Volume 2, 2 1,No. 1; 5 1~55

, Feng, Xue, etc. Geological hazards in the northern South China Sea and evaluation of submarine engineering geological conditions. Nanjing: Hohai University Press, 8 1 ~ 152.

Cole, Dudley. 1994. Characteristics of Quaternary paleochannel on the northern shelf of the South China Sea. Journal of Geology, Volume 8, Volume 68, No.3; ; 269~277

Wang. 1990. The Sea of China during the Ice Age? Research status and problems. Quaternary research, No.2.11~124.

Nan, Chen Weiguang, et al. Neotectonic movement and geological environment along the coast of South China. Beijing: seismological press, 234 ~ 238.

C.K. Wilgus et al. 199 1. Principles of sequence stratigraphy, Beijing: Petroleum Industry Press, 62 ~ 63.

Changes of ancient coastline in the northern South China Sea since the late Middle Pleistocene and its geological significance

Chen Wencheng Peng Xuechao

(Guangzhou Marine Geological Survey, Guangzhou, 5 10760)

Abstract: According to borehole analysis and seismic sequence division, three ancient coastlines in different periods are identified. Sedimentary facies analysis of single channel seismic profile in the northern South China Sea, as well as stratigraphic unconformity and seabed morphology. These three ancient coastlines all belong to the middle and late period? Pleistocene, the last glacial maximum and after? The last ice age. According to the research results, we also discussed the changing mechanism of the ancient coastline.

Key words: Middle Pleistocene ancient coastline in single channel seismic profile of South China Sea