LẠNG SƠN CITY: A LATENT SITE IN NATURAL HAZARDS

LÊ CẢNH TUÂN1, NGUYỄN LINH NGỌC1, ĐẶNG VĂN BÀO2

1Scientific Institute of Geology and Mineral Resources
2Faculty of Geography, University of Natural Sciences, Hà Nội National University

 

Abstract: The Lạng Sơn City is situated in a syncline, which is formed by limestone of the two Bắc Sơn (C-P2 bs) and Đồng Đăng (P3 đđ) Formations. These limestones, of rather high purity, suffered strong impact of fault activity, especially of active faults. The strong activities of the Cao Bằng - Tiên Yên Fault during neotectonic stage, with crushed zones of 700-5000 m in width, created a favourable environment for moving the groundwater from the Lạng Sơn basin to areas of lower base level of erosion. In the city area, there is a thin Quaternary bed, covering the uneven and not homogeneous surface of a karstic topography. The covered karstic surface is, at present, higher than the base level of erosion at Lạng Sơn and much higher than the base level of erosion of the Kỳ Cùng River lower course, where the escape of groundwater is able to occur. The favourable premises of karstification in the Lạng Sơn City area are latent causes of natural hazards.

I. INTRODUCTION

Lying in the East Bắc Bộ, the Lạng Sơn City is famous not only as a centre having commercial exchange with China, but also a city having great touristic potentiality: in this city, there are many classified vestiges, such as Mai Pha Culture, Phai Vệ Cave, Tam Thanh Cave, etc.. On the tectono-structural side, the Lạng Sơn City lies on a syncline of Paleozoic sediments, formed by limestone of the Bắc Sơn Formation (C-P2 bs) [2]. In the 70' of the past century, basing on results of his geomorphologic studies, Prof. Đào Đ́nh Bắc has been considering that the Lạng Sơn City area is a karstic plain of typical tectonic origin [1].

On the topographic side, the Lạng Sơn City area has the form of a rather flat depression, bearing the feature of a karstic plain. The topographic surface of this depression is eroded, and its margin is formed by Mesozoic sediments. The centre of the city is formed by pure limestone (easy to be dissolved and corroded). In this area, the water environment is rich in CO2, faults have high destructive force, the climatic regime is humid-tropical, the tectono-structural characteristics cause high level of crushing to basement rocks, the activities of biological world, especially, with human impact, are high; these are favourable premises for the karstification process. The strong development of karstification has been leading to series of characteristic topographic forms, such as funnels, pits, subterranean cavities, etc... that are objects related to latent natural hazards presented for discussion in following parts by authors of this article.

II. SKETCH OF POSITION OF THE LẠNG SƠN DEPRESSION ON THE REGIONAL STRUCTURE PLAN

Some authors have been considering that, the Lạng Sơn Depression was developed during Paleozoic. In fact, it is not completely like that; the anticline directly related to the formation of the Lạng Sơn Depression belongs to Mesozoic. At present, it is destructed by Mesozoic tectonic movements, its relics consist of sediments of its two limbs with clear bed attitude presented on the structural plan (Fig. 1). The results of topographic analysis show that the Lạng Sơn Depression has a rhombic form, extending in north-south direction. From Nà Tâm in the north to Nà Tu in the south, its length is about 8 km. From Nà Chang in the west to P̣ Luông in the east, its width reaches 4 km [3]. The whole rhombic depression is the dome of the Mesozoic anticline, the basement of which is a syncline formed by Paleozoic karstic rocks. The rocks forming the syncline consist of limestone of the Bắc Sơn and Đồng Đăng Formations, that is strongly crushed. This structure completely conforms to the study of Vũ Văn Chinh [13], that is related to the time of restriction of sedimentary basins situated along the Cao Bằng - Tiên Yên Fault.

Some geological literatures consider that the Lạng Sơn spreading depression is a Quaternary depression occurring in the western corner, formed by two nearly perpendicular with one another: the Cao Bằng - Tiên Yên and Thương River ones with their centre as Lạng Sơn [11]. In general, the Lạng Sơn basin is an anticline controlled by faults. The greatest fault is the Cao Bằng - Tiên Yên one, which is an object interested by many geological works, among them there are the works of [2, 7, 9-12]. When studying the mechanism of the Cao Bằng - Tiên Yên Fault during the neotectonic stage, Vũ Văn Chinh considered that the neotectonic stage began since 32 Ma, during this the Cao Bằng - Tiên Yên Fault was developed in two phases [12]. In the first phase, the impact of faults moved the blocks according to the pull-apart mechanism for enlarging the basin and settling sediments. In the second phase, the faults moved, that restricted the basin, caused the dislocation and folding of sediments in the basin. The studies of [10] have been giving similar remarks, in addition, considering that the beginning time of formation of the Cao Bằng - Tiên Yên Fault was at the end of Permian. Discounting the lineaments on satellite photographs, we receive assemblage of lineaments developed with high density in northwest-southeast direction (310-130°). Based on geophysical materials, the Cao Bằng - Tiên Yên Fault having manifestations of present activity lies in coincidence with a seismic zone of medium rank with Mmax = 5.6 - 6.0 Richter degree, with the focal depth of 25-30 km (Nguyễn Đ́nh Xuyên, 1994).

Along the destructive zone, occur Neogene and Quaternary basins containing loose continental sediments. Destructive zones of faults are composed of fractured and cataclastic zones that are developed in northwest-southeast direction and rather clearly expressed oneself on Landsat and Spot satellite images with the width of, sometimes, over 5 km. The left-lateral movement in the Oligocene-Miocene tectonic phase has been supposed as about from 1000 to 5000 m; the right-lateral movement in the Pliocene-Present tectonic phase - about 500-700 m (based on geomorphologic materials). The Lạng Sơn area was divided into two geological blocks by the Cao Bằng - Tiên Yên Fault, that have different deformation characteristics. The northeast limb of this fault lies in the compression stress field, where are developed closed fissures which have little capacity of containing and transporting groundwater; while the southwest limb, especially the Lạng Sơn City, lies in the spreading stress field, where are developed splitted fissures which are favourable for containing and transporting groundwater.

III. STRUCTURAL CHARACTERISTICS OF THE LẠNG SƠN DEPRESSION

Since the 90' of the past century, many geological research works have been clarifying the structural nature of the Lạng Sơn Depression, especially its basement structure, and unanimously considering that limestone of the Đồng Đăng Formation (P3 đđ) unconformably covers that of the Bắc Sơn Formation (C-P2 bs), both of them are covered by Quaternary sediments. The works of geological mapping, among them the typical is the project of Survey on urban geology of Lạng Sơn City and our research works carried out since 1993 year, have been allowing the authors of this paper to establish a transversal section of the Lạng Sơn basin for understanding the deep structure of this area, at the same time linking parameters, interpreting the correlation of the basement structure with latent natural hazards.

The rocks participating in the structure of the Lạng Sơn Depression consist mainly of Paleozoic karstic formations, distributed in the centre, with Mesozoic rocks in the two limbs. The Bắc Sơn Formation is composed of light-grey, thick-bedded to massive limestone, penetrated by calcite veins, with interbeds of variegated calcareous breccias, 600-700 m thick. The Đồng Đăng Formation is characterized by black-grey limestone, brecciated limestone, 150-200 m thick [2]. At present, the structure was broken, faults are strongly developed, cutting carbonate formations and together with tectonic movements making the basin subsided. With the above characteristics of the Cao Bằng - Tiên Yên Fault and its present activities, the basement of the depression has been divided into many small blocks. In the regional scale, this area is the contact area between two structuro-facial zones - the Sông Hiến and An Châu Depressions [9]. The Lạng Sơn Depression was formed by the pull-apart mechanism, related to the right-lateral slip of the Cao Bằng - Tiên Yên Fault, with Quaternary sediments deposited in it. The surface of the depression is rather even, therefore it is still named as Lạng Sơn basin [3], or Lạng Sơn karstic plain [1]. Carbonate sediments in the Lạng Sơn Depression were strongly folded and dislocated, forming series of fold in the influence confines of the melange zone situated along the fault; just that time was the time point of forming the syncline at the centre of the Lạng Sơn basin.

The previous study results have been confirming that carbonate sediments of the Bắc Sơn Formation are rather diversified, from limestone, dolomitic limestone, organic limestone, usually grey to light-grey coloured, massive to thick-bedded [6]. The results of drilling for bauxite prospection of the Geological Party No.3 in the Tam Thanh area have been showing that the Quaternary cover is very thin and does not surpass 10 m, and the depth of boreholes does not control the thickness of limestone. The analytic results on limestone quality have been giving that its CaO content is over 50%, showing the purity of limestone, that is favourable for the karstification process.

During the Quaternary, the Lạng Sơn Depression passed through at least 3 times of uplift for forming terraces and alluvial banks along the Kỳ Cùng River [4], that is proved by 3 water traces carved on the limestone cliff situated near the Khánh Khê Bridge and existing river terraces; surely, there exist at least 3 stages of karstic cave related to 3 times of topographic raising in the Lạng Sơn area. These karstic cave stages have the relation between one another both on the surface and in the depth. In general, the Lạng Sơn Depression has a very thin Quaternary cover, with the greatest thickness not surpassing 20 m (according to drilling material of the Project of survey on urban geology of Lạng Sơn City the greatest thickness of Quaternary sediments covering Paleozoic limestone is 17.1 m (KT.15) [6].

In the depression, there exist series of subsided pits, that are remains of the lacustrine regime controlling this area in the past. Because of this lacustrine regime, the uppermost horizon of the depression consists of weak soil layer, that is unfavourable condition for construction work. The tectonic forces control also the activity of stream in the Kỳ Cùng River and cause processes of washout and deposition. Our results of survey during the years from 1993 to recent time have been showing that limestone in the city centre exists in two levels: the upper overlies the local base level; the lower is equal or underlies it. Especially, the Kỳ Cùng River flows to the north, crosscutting the Cao Bằng - Tiên Yên Fault at Na Sầm, where the altitude is about 175 m, while in Lạng Sơn City it is of 250 m, it means the difference in altitude between these two areas is 75 m, and the groundwater is able to move from Lạng Sơn to Na Sầm following systems of fissures, among them the most important is the system of opened fissures of the Cao Bằng - Tiên Yên Fault. This is just the latent character in natural hazards of subterranean karstic horizon in Lạng Sơn. At present, in karstic caves of the Tam Thanh and Nhị Thanh touristic districts water flows from the caves to the Kỳ Cùng River in the place of the Lạng Sơn Polyclinic. The Kỳ Cùng River at the city centre corresponds only to its middle course. So, the difference in base level between the city centre and the area, where the river crosscuts the Cao Bằng - Tiên Yên Fault, causes the movement of groundwater along the crushed and fissure zones from the south northwards. This is the cause speeding up the activity of subterranean karst in the Lạng Sơn Depression.

Through many material sources, it is able to affirm that the Lạng Sơn City is situated on a karstic horizon. The problem on the distribution of subterranean karst in the Lạng Sơn area still exists and has not answer yet. The subterranean karst contains abundant water resources, but is latent in natural hazards.

III. LATENT HAZARDS IN THE LẠNG SƠN CITY AND ADVISES SERVING ITS SUSTAINABLE DEVELOPMENT

In karstic areas, some natural hazards usually occur, such as: flash flood, mud and rock flow, earth collapse, missing or superfluity of micro-elements (arsenic, iodine, etc.), pollution of water sources, leakage and loss of water in reservoir, subterranean collapse, desertification. As regards the Lạng Sơn Depression, the most worthy hazards are earth collapse, pollution of water sources, subterranean collapse and desertification. In the framework of a paper, we touch upon only the structure of basement rocks of the Lạng Sơn Depression, that consist completely of karstic rocks which are related to the collapse of karstic caves; the oscillation in season of the water level, making the surficial horizon, formed by Quaternary sediments, easy to happen the shrinkage of volume; the Cao Bằng - Tiên Yên Fault with series of fissures and crushed zones of 700-5000 m in width playing the role of drainage channels which speed up the karstification in the depth. A complete karstic system includes two main zones, namely the zone of erosion and corrosion, related to the input factor and the zone of deposition, related to the output factor. Based on this viewpoint, the Lạng Sơn Depression belongs to the erosional zone and is related to the input factor. It is the reason to affirm that the Lạng Sơn Depression is latent in natural hazards.

On the surface of the Lạng Sơn Depression, there are Quaternary sediments, consisting mainly of clay, silty clay, silt mixed with sand and grit-gravel with a thickness not surpassing 20 m, having the sinking coefficient varying from 1.58 to 3.13 kg/cm², among them the largest distributive area belongs to Lower-Middle Holocene clay and loam having the sinking coefficient varying from 1.98 to 2.53 kg/cm². Construction works having the loading capacity surpassing this value make easy to occur the collapse of subterranean karstic caves, especially when the extraction of groundwater reaches the large scale. This is also the area where limestone is exposed directly on the surface of relief in the form of relic peaks or forms steep cliffs in the two banks of the Kỳ Cùng River (in the section where there is army quarter, and the section extending from Tiên Pagoda to Mai Pha Bridge). Karstic caves surely have streams which converge in the Kỳ Cùng River that is observed in area of the Tam Thanh and Nhị Thanh caves, where emitted water flows into the stream running near the Lạng Sơn Polyclinic, then discharges into the Kỳ Cùng River. The areas, where the cave system is strongly developed, are much vulnerable, and latent in such phenomena as collapse of subterranean karst (as in Hà Tây, Phú Thọ, Quảng Trị, etc...). Through our investigation we think that, it is necessary to study in details the rule of karstic distribution during the planning process of construction, because it is a factor causing the loss of stability of the geological environment. Apart from this, the enlargement of groundwater extraction in this area needs a detailed study on the appearance of the above said collapse and a calculation of most rational extracted water quantity, otherwise, there will be great consequences for construction works.

Some previous geological literatures have been touching upon such topographic forms as funnels, pits [3], that are peculiar forms of karst. At present, some of them were filled up for the construction. In the years before 1990, in rice fields there still were remaining limestone mounds rising upon the surface, but at present the urbanization process has been effacing all and residential districts appear with many high buildings, that are developed even upon limestone massifs, as in the Tam Thanh and Nhị Thanh touristic districts. The basement structure of the Lạng Sơn Depression consists of carbonate sediments of great thickness (700-1000 m [12]). The results of analysis of limestone in Lạng Sơn area has been giving: intensity of saturate antipressure is 34-75 kg/cm² [6]. In this area, limestone suffers strong karstification, forming a developed cave system, with crushed and fractured zones, that, combined with the monsoon-tropical climate, are favourable premises for speeding up the development of karstification. During Holocene, karstic pits were filled up with Quaternary deposits, which are loose sediments, easy to be collapsed when the calcareous basement is dissolved and washed out.

In the northeast of the Lạng Sơn Depression, there is the Cao Bằng - Tiên Yên Fault, which still is active in present time [8, 11, 12]. Data of the Institute of Geophysics have been showing that along this fault earthquakes of 5.5 Richter degree are able to occur. If this phenomenon becomes real, the consequences will be immeasurable.

According to our materials of analyzing maps of topography of different generations, combined with the interpretation of airborne photographs and field survey materials, almost all wells of pressure water is now filled up. The Tiên Well (Angel Well), a touristic site of the Lạng Sơn City, previously is an artesian well, but at present, its water level is lowered about 2-3 m, sometimes deeper, and the water rises only in rainy season.

Nowadays, the Lạng Sơn City extracts water mainly from the carbonate formation. The phenomenon of land cracking happened in the Vĩ Thượng Village, Hoàng Đồng Commune in 1998, that has been interpreted by some geologists as related to the extraction of groundwater from the limestone aquifer. Similar event took place in the Hamlet III, Đồng Xuân Commune, Thanh Ba District, Phú Thọ Province in 29/4/2004 [5]. We should think that, it is necessary to early plan for preventing the hazard of earth subsidence, especially in residential districts.

The karstic environment is characterized by a double spatial system with shortage of surficial water, but abundance of groundwater, that is vulnerable to all impacts and not protected by water-sealing beds, therefore the water source is easy to be polluted by different factors penetrating through caves, crevices, fissures. And it is very difficult to overcome the consequences caused by pollution. It is necessary also to rise the consciousness of the community in the protection of environment. Related to this direction, there is the Vietnamo-Belgian Project: “Developping the karstic mountainous rural area in Northwest Việt Nam by long lasting management of the country and community education: Conditions and advantages” (VIBEKAP). It was successfully realized in the Scientific Institute of Geology and Mineral Resources.

This paper is completed with the financial support by the Project of Basic Research bearing the code 7.170.06.

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