REGIONAL GEOLOGY AND STUDY OF LANDSLIDE IN SIRNAJAYA AND SURROUNDING AREA, GUNUNGHALU DISTRICT, WEST BANDUNG REGENCY, WEST JAVA

I. E. Putra, B. Brahmantyo, Misbahudin

Geological Engineering Study Program

Faculty of Earth Sciences and Technology

Institut Teknologi Bandung

 

Abstract – Research area is located in Sirnajaya, Gununghalu District, West Bandung Regency, West Java. It is part of Rongga Plateau, a transition among Southern Mountain Zone, Bandung Depression Zone, and Rajamandala Ridge. There are three formations which developed in the research area, they are Cimandiri Formation, Sindangkerta Part of Cimandiri Formation, and Beser Formation. But, according to Tabri (1989), the formation that exist in the area is only Saguling Formation. Geomorphological unit of research area is divided into Sirnajaya Homoclinal Ridge, Bunijaya Homoclinal Ridge, and Cidadap River Plain. Research area is also grouped into  four lithological units such as Sandstone-claystone Unit, Sandstone Unit, Tuffaceous-sandstone Unit, and Breccia Unit. There is Cihurang thrust fault in this area which has direction of SW-NE. Based on Map of Mass Movement Susceptibility Zone, Western Java, it is marked as yellow zone or has potential of medium to high mass movement. The landslide in this area had ever covered the route of Bandung-Gununghalu. The most significant landslide is in Cihurang. Wetness and slope gradient are dominant factor of landslide.

Keywords : Rongga Plateau, Landslide.

Sari – Lokasi penelitian berada di Desa Sirnajaya, Kecamatan Gununghalu, Kabupaten Bandung Barat, Jawa Barat. Daerah ini disebut juga sebagai Plateau Rongga yang merupakan zona transisi antara Zona Pegunungan Selatan, Zona Depresi Bandung, dan Punggungan Rajamandala. Formasi yang terdapat di daerah ini yaitu Formasi Cimandiri, Anggota Sindangkerta Formasi Cimandiri, dan Formasi Beser. Tetapi, menurut Tabri (1989), formasi di daerah ini hanya Formasi Saguling. Satuan geomorfologi di daerah penelitian terdiri dari Satuan Punggungan Homoklin Sirnajaya, Satuan Punggungan Homoklin Bunijaya, dan Satuan Dataran Teras Sungai Cidadap. Sedangkan satuan litologinya yaitu Satuan Batupasir-batulempung, Satuan Batupasir, Satuan Batupasir-tufan, dan Satuan Breksi. Struktur yang berkembang di daerah penelitian adalah sesar naik Cihurang berarah baratdaya-timurlaut. Berdasarkan Peta Zona Kerentanan Gerakan Tanah Jawa Bagian Barat, daerah penelitian termasuk ke dalam zona kuning atau potensi gerakan tanah menengah-tinggi. Seringnya tanah longsor di daerah ini pernah menyebabkan terputusnya jalan utama Bandung-Gununghalu. Lokasi longsor yang signifikan karena berpengaruh terhadap penduduk berada di daerah Cihurang. Kondisi keairan dan kemiringan lereng menjadi faktor dominan yang menyebabkan longsor.

Kata Kunci: Plateau Rongga, Longsoran.

 


 

 

INTRODUCTION

The research area has beautiful view such as waterfall, farmland, and plantation. But, it also has potential of landslide. Based on the Map of Mass Movement Susceptibility Zone, Western Java, generally this location is marked as yellow zone or middle-high mass movement potential as shown in Figure 1. At any time, landslide can occur, particularly it is caused by high wetness and erosion (Djadja et al., 2003 in Putra, 2009). Last landslide occured on Februari 1st, 2009 which covered the route of  Bandung-Gununghalu.

Field investigation is conducted to get geological condition in this area. Data from this investigation are used to make a geological map so it can predict geological history. Then, geological condition in this area such as type, position, geological structure, weathering degree, and slope angle are used to analyze the landslide.

Fell et al. (2008) defined that landslide is rock, debris, or soil mass movement to the bottom of the slope. This movement is influenced by some factors such as slope angle, lithology, wetness, land use, or caused by earthquake. Difference of landslide movement depends on type of material. Landslide can be grouped into five types they are falls, topples, slides, spreads, flows, and composites (Cruden and Varnes, 1996).

Early evidence of landslide is marked by fracture on the top of slope that relatively perpendicular with motion direction. Once water fills the fracture, it makes horizontal force increases which causes landslide. While  on bottom slope, ground accumulation occured.

LOCATION AND METHODOLOGY

Research area is located at 7000’30”-7002’30” LS and 107016’30”-107018’30” BT which is about 14.6 km2. Administratively, it covers two districts and three villages. Northern of this area is Cibedug area, Rongga District and southern of the area is Cidadap River as separating zone, it is part of Sirnajaya and Bunijaya area, Gununghalu District, West Bandung Regency, West Java. This area is included in the Geological Map of The Sindangbarang and Bandarwaru Quadrangles 1:100.000 scaled (Koesmono et al., 1996) and topography map 1208-543. Almost all of research area is covered by PT Montaya’s tea plantation and other part as civilians farm land.

Three main methods are used in this research, namely preparation, field investigation, and data processing. Preparation includes study of literature, morphological analysis using topography map, Google Earth Image, DEM ESRTM, and width determination of research area. Field investigation such as data collection for types, unit, and distribution of lithology, layer orientation, geological structure, weathered soil condition, and landslide zones of study area. While, for landslide observation includes making of cross section, measurement, geometry sketch, and profile. In addition, information from civillians about landslide history is correlated with geological data to assume the factor of landslide. Furthermore, data processing in laboratorium and studio includes analysis of rock, mineral composition, and making of geological map, cross section and stratigraphy profile.

REGIONAL GEOLOGY

Physiography

Research area is transition zone between Bandung Depression Zone and Southern Mountain Zone (van Bemmelen, 1949). The lithology in this area is composed old volcanic deposit at Tertiary age and Quartery volcanic product.

Pannekoek (1946, op cit van Bemmelen, 1949) considered this area as Rongga Plateau which is transition zone between Gununghalu Volcanic Area and Batujajar Plain. The plateau is characterized by mature hilly surface. In the north-eastern side, this plateau descends until Batujajar Plain which is part of Bandung Zone (van Bemmelen, 1949) as shown in Figure 2. Whereas, Southern Mountain Zone is characterized by coarse texture, remains of old volcanic at Late Miocene-Pliocene age (Dam, 1994), and also eroded dome like Gununghalu hills that extends to south-east.

Regional Stratigraphy

This area is included in Sindangbarang and Bandarwaru Geological Map (Koesmono et al, 1996) (Figure 3). When it is correlated with ideal stratigraphic section of West Java (Martodjojo, 1984), there are nine formations from old to young:

  1. Jampang Formation
  2. Rajamandala Formation
  3. Cimandiri Formation
  4. Sindangkerta Part of Cimandiri Formation.
  5. Bojonglopang Formation
  6. Bentang Formation
  7. Kolaberes Formation
  8. Beser Formation
  9. Quartenary Volcanic Deposit.

The formations which exist in the research area are Cimandiri Formation, Sindangkerta Part of Cimandiri Formation, and Beser Formation.

Cimandiri formation is characterized by interbedded claystone, grey siltstone and yellowish brown sandstone. It also characterized locally by volcanic mudflow which consists of tuff, andesite-breccia, and tuffaceous-breccia. The layers are in Cilanang Valley that showed Middle-Miocene age on fluvial to transition zone. The thickness of the formation is 400 m. This formation name is taken from Cimandiri River (Sukamto, 1975). While Sindangkerta Part of Cimandiri Formation is characterized by yellowish-grey pumiceous-tuff, tuffaceous-sandstone, and tuffaceous-breccia. Pumice fragment of 2.5 cm is usually is exist. An outcrop is well exposed in Sindangkerta. The thickness is about 100-500 m. But, according to Tabri (1989), it is an upper part of Saguling Formation with contact of younger Cimandiri Formation in Parakanwayang.

Saguling Formation itself is characterized by breccia and sandstone which age is Early to Middle Miocene and parallel with Cimandiri Formation on its upper part. In Parakanwayang, this formation is characterized by interbedded tuffaceous-sandstone and claystone with intercalation tuf and siltstone that deposited by gravitational mechanism and turbidity current on an instable basin in Middle to Outer Neritic Zone (Tabri, 1989).

While, Beser Formation is charcterized by tuffaceous-breccia and lava which is composed by andesite up to basalt (Alzwar et al., 1992 in Putra, 2009). Lava of Kendeng Mountain interbeds with lahar deposits which are andesite-breccia and tuffaceous-breccia (Koesmono et al., 1996). The maximum size of breccia component is more than 1 m. Matrix consists of grey crystal tuff or tuffaceous-sandstone. There is badly laminated claystone, dark grey, which is shaped like a lense. The depositional environment is terrestial to shallow marine. Lower part of this detritus interfingers with Kolaberes Formation and upper part of Bentang Formation. The thickness of this formation is about 750 m.

Regional Structure

Regional structure in research area based on Sindangbarang and Bandarwaru Geological Map is fault, fold, and joint which are found on lithology with the age of Oligo-Miocene to Quartenary (Koesmono et al., 1996). Fault consists of shear fault which has NW-SE direction or Sumatera pattern and normal fault which has the direction of NE-SW or Meratus pattern. Shear fault cut across normal fault so it can be interpreted that it is younger than normal fault (Ratman & Gafoer, 1998 in Putra, 2009).

Fold pattern which is found is anticline that has direction of SW-NE and W-E, syncline with SW-NE direction, and flexure with W-E direction. Joint is well developed on andesite of Oligo Miocene to Quartenary age.

The structure of Late Miocene lithology produced uplift and then andesite intruded it at Pliocene age on Bentang Formation. Those tectonics formed two different patterns. The fold that occured on Cimandiri Formation formed an anticline and syncline. While, the fault occured on Beser Formation, Bentang Formation, Kolaberes Formation as normal and shear fault (Koesmono et al., 1996).

Based on regional geology, the study area is characterized by young to mature hills with coarse texture because it is closer with South Ridge Zone. The lithology characterized as a compound of epiclastic from Cimandiri Formation and volcanic material from Beser Formation. Structure in this area is probably influenced by Sumatera pattern.

GEOLOGY OF RESEARCH AREA

On detailed scale, landscape in study area generally has young up to mature hills with characteristic such as wave reliefs and slope that is scraped by water. Gentle morphology is located around Cidadap River as river plain. The elevation of research area is about 975-1200 m above sea level.

Slope Angle

Generally, slope angle of study area is about 150-450, with an assumption, the steep slope is dip slope, while gentle slope is back slope. Steep slope itself is showed by tight contour while wide-apart contour shows gentle slope. So, it can be concluded that the rock layers have strike direction of SW-NE and dip direction of SE.

Geomorphological Unit of Research Area

Based on classification of Bentuk Muka Bumi (BMB) from Brahmantyo and Bandono (2006), the research area has three geomorphic units, they are (Figure 4):

  1. Sirnajaya Homoclinal Ridge

This unit covers more than 50% of research area. It is ranging southern to eastern of  research area. It is characterized by uniform back slope pattern which showed similar dip direction  with bit steep to steep slope angle (8-55%). The mature hills can be seen clearly with intensive erotion. Young river until transition from young to mature in this unit is generally downstream.

  1. Bunijaya Homoclinal Ridge

This unit covers north-western of research area, and is cut by Cidadap River Plain. It is characterized by bit steep to very steep topography (15-100%) and formed longitudinal ridge which has direction of SW-NE with uniform backslope pattern. Very steep topography is located in river valley and dip slope edge. While in back slope has usually bit steep ones.

  1. Cidadap River Plain

This unit is characterized by near flat up to gentle topography. It covers throughout Cidadap River which flows from eastern to western of the area, similar with initial dip of this area.

Stratigraphy of Research Area

Based on field observation data and laboratory analysis, rock unit in study area can be divided into four units such as Sandstone-claystone Unit, Sandstone Unit, Tuffaceous-sandstone Unit, and Breccia Unit. Generally, weathering grade of all of them is fresh to moderately weathered (Moye, 1955 in Cornforth, 2005) as shown in Table 1 and Figure 5.

  1. Sandstone-claystone Unit

This unit covers southwestern of map, and is cut by Cidadap River which flows to southwestern of area. Its lithology consists of interbedded tuffaceous-sandstone and claystone which is a product of turbidity current (Tabri, 1989). There are lamination and gradded bedding structure which is turbidity sequence.

  1. Sandstone Unit

This unit is distributed on southeast of previous unit. Stratigraphycally, it is older unit. It is ranging till northern of the area. Cidadap river cut the middle part of this unit, whereas its watercourse flows parallel with the dip of rock unit or subsequent (Lobeck, 1939 in Putra, 2009). The unit that formed Wavy Ridge Unit consists of interbedded tuffaceous-sandstone and breccia with intercalation of glauconite sandstone, siltstone, and tuf. The thickness of this unit is 400 m and it has N720E/230.

  1. Tuffaceous-sandstone Unit

This unit covers northeastern part of the area and east of sandstone unit. Stratigraphycally, this unit is younger than previous layers. The maximum thickness of this unit is 250 m.

  1. Breccia Unit

This unit covers southern part of research area untill the middle part. Stratigraphycally, this unit is the  youngest unit. It formed subparalel pattern with flow pattern parallel to bed plane. Reconsruction of section showed that the thickness of this unit is more than 300 m and has generally N400E/300.

Structure of Research Area

Regionally, the geological structure has direction of NE-SW as normal fault. This fault cut Beser and Cimandiri Formation. There is a fault structure found in Cidadap River which named as Cihurang Fault. Its movement is left handed thrust. If this fault is continued based on its layer plane and morphology, it will cut Sandstone and Tufaceous-sandstone Unit. But, it did not cut Breccia Unit so it is difficult to determine whether the fault is still active or not.

This fault and deformation of all unit of the research area were caused by compression occurred at the end of Middle-Miocene when the Southern Mountain Zone uplifted (Dam, 1994 and van Bemmelen, 1949).        

LANDSLIDE STUDY

Location         

Based on sketch of landslide (Cruden and Varnes, 1996), at least there are five landslides of research area which can be observed in the field and has potential to slide again. Two of them is located in Cihurang and the others occur in Lengkong, Cicurug Satu, and Cidadap (Figure 6).

Based on observation table of landslide locations as shown in Table 2, there is a tendencies that landslide only occured on weathered ground or sandstone. Landslide location on Breccia Unit also occured on its sandstone layer. Based on observation in field and information from civilians, it can be concluded that landslide is more often occured at the rains.

Main factor of landslide in the research area is the increasing of wetness and its slope material. Slope angle does not become a significant factor because on Breccia Unit with angle more than 450, landslide does not happen although its condition is partly weathered.

Analysis 

The most significant location of landslide is located in Cihurang, Cibedug Village, Rongga District, West Bandung Regency. In this location, landslide often occurred e.g. on March 5th 2005 which drag an Elf to fall to Cidadap River on its bottom and killed two victims. The last incident was occurred on February 1st 2009 which its detritus covered Gununghalu-Bandung route (Figure 7).

Rock mounds around landslide in Cihurang were used by civilians to stiffen the slope under the road. Because there were many landslide trace and present of the main way on the slope, so this location is taken to identify the components and the factor of landslide. The sketch of the landslide become case design to know the slope condition.

Based on geological map, the location occured on sandstone unit with slope length of 70 m and height of 31 m from slope base (Figure 8). Its slope material consisted of tuffaceous-sandstone with intercalation weathered siltstone on bottom and weathered soil on top. This slope is passed by main way that connected this area with others, including Bandung. It is also used as a route for Bunijaya-Ciroyom Elf. At slope base, there is a piece of  farm land. The river’s influence for slope stability does not effect significantly.

Sketch of the figure shows cross section of landslide which can be observed and interpreted in the field. There is some parts of landslide  which can not be observed because of the change of land use of the slope.

The landslide sections of this location which can be identified as shown in Figure 9, they are:

  1. 1.      Crown: The practically undisplaced material adjacent to the highest parts of the main scarp.
  2. 2.      Main Scarp: A steep surface on the undisturbed ground at the upper edge of the landslide caused by movement of the displaced material away from the undisturbed ground.
  3. 3.      Top: The highest point of contact between the displaced material and the main scarp.
  4. 4.      Minor scarp: A steep surface on the displaced material of the landslide produced by difference of movements within the displaced material.
  5. 5.      Main body: The part of the displaced material of the landslide that overlies the surface of rupture between the main scarp and the toe of the surface rupture.
  6. 6.      Surface of rupture: The surface forms the lower boundary of the displaced material below the original ground surface.
  7. 7.      Toe of surface of rupture: The intersection (usually buried) between the lower part of the surface of rupture of a landslide and the original ground surface.
  8. 8.      Displaced material: Material displaced from its original position on the slope by movement of the landslide. It forms both the depleted mass and the accumulation. It is shown as a dotted line in the figure.
  9. 9.      Flank: The undisplaced material adjacent to the sides of the rupture surface. Compass directions are preferable in describing the flanks but if left and right are used, they refer to the flanks as viewed from the crown.
  10. 10.  Original ground surface: The surface of the slope.

Based on section sketch and its parts, it can be predicted that the type of main landslide is a slow motion creep, because minor landslides are occured more often in this area, although its spacing is short. It is also proven by some big trees that lean over slope base. But, the type of minor landslide is a slides which occured suddenly with high velocity. That is can be proven from history of landslide in this area.

So that landslide in Cihurang need to be attended and other location which near civillians. Strengthening and stabilization of slope have to be conducted to prevent main way, as civillians economic track, covered with slope material.

CONCLUSION

 

Research area has formations which developed are Cimandiri Formation, Part of Sindangkerta CImandiri Formation, and Beser Formation. But, according to Tabri (1989) this area is included in Saguling Formation. Geomorphology in this area is divided into Sirnajaya Homoclinal Ridges, Bunijaya Homoclinal Ridges, and Cidadap River Plain. Lithologal unit is grouped into four units such as Sandstone-claystone Unit, Sandstone Unit, Tuffaceous- sandstone Unit, and Breccia Unit. Geological structures in this area are thrust fault and tilting layers. The fault cut all of rock unit. Landslide occured in five locations, two of them is located in Cihurang, the remaining is occured in Lengkong, Cicurug Satu, and Cidadap. Landslide occured more often on wet slope gradient more than 20% and on weathered medium to fine clastic sedimentary rock or on its weathered soil. Wetness, slope gradient, and material become dominant causes of landslides. Not all parts of landslide can be observed because it is covered by vegetation and the cange of land use, especially old landslide. Modern strengthening is needed to prevent or reduce landside effect including old landslide, particularly on location that passed by transportation way or near residence.

REFERENCES

Brahmantyo, B., dan Bandono, 2006, Klasifikasi Bentuk Muka Bumi (landform) untuk Pemetaan Geomorfologi pada Skala 1:25.000 dan Aplikasinya untuk Penataan Ruang, Jurnal Geoaplika vol. 1 no. 2 hal. 71-78, Bandung.

Cruden, D.M., & Varnes, D.J., 1996, Landslide Types and Processes, Landslides: Investigation and Mitigation. Published as article.

Fell, R., Corominas, J., Bonnard, C., Cascini, L., Leroi, E., dan Savage, W.Z., 2008. Guidelines for Landslide Susceptibility, Hazard and Risk Zoning for Land Use Planning, Journal of Engineering Geology 102 (2008) hal. 85-98, Elsevier B.V.

Koesmono, M., Kusnama, & Suwarna, N., 1996, Peta Geologi Lembar Sindangbarang dan Bandarwaru, Jawa skala 1:100.000 edisi ke-2, Puslitbang Geologi, Bandung

Putra, I.E., 2009, Tugas Akhir: Geologi dan Studi Longsoran Desa Sirnajaya, Kecamatan Gununghalu, Kabupaten Bandung Barat, Jawa Barat, ITB, Bandung.

Van Bemmelen, R.W., 1949, The Geology of Indonesia: Martinus Nijhof, 1A: 645-654.

 

 Keterangan dan Gambar lebih jelasnya dapat diakses di http://eng-geology.pusku.com/

LAMPIRAN

PETA LOKASI

 Figure 1 Map of Mass Movement Susceptibility Zone, Western Java (Badan Geologi, 2004). Research area is marked by yellow zone or potensial of medium-high mass movement.

 Figure 2 Physiography of West Java (van Bemmelen, 1949). Blue side shows research area

Figure 3 Geological Map of Sindangbarang and Bandarwaru Quadrangles (Koesmono et al., 1996).

Figure 4 Geomorphological Map of Research Area (Putra, 2009).

Figure 5 Geological Map of Research Area (Putra, 2009).

Table 1 Stratigraphic Column of Research Area (Putra, 2009).

Figure 6 Landslide location in Cicurug Satu (a) and Lengkong (b). Both occured on sandstone layer of Breccia Unit which slope material is composed by weathered ground.

gd

bg

                (a)                                                                          (b)

 bgsa

nmh

Figure 7 Trace of Lanslide in Cihurang occurred on February 1st, 2009. (a) Way body which covered by landslide material. (b) Landslide slope. (Putra, 2009).

                Figure 8 (a) Landside which occurred in Cohering and (b) its sketch. (Putra, 2009). The blue side is the newest landslide trace.

 Figure 9 Sketch of landslide in Cihurang. (a) Cross section N-S. (b) Its above appear. (Putra, 2009). Annotation for each of number is written in text.

 Table 2 Observation result of all location of landslide (Putra, 2009).

No.

Location

Coordinate

Unit

Slope Gradient

Wetness

Use Land

Landslide Type

Slope Height

Slope Material

Strengthening

S

E

1.

Cihurang

7001’01.1”

107017’18.1”

Sandstone

45%

Medium

Rice field and way

Slides

25 m

Weathered soil and rock

Bamboo trees

2.

Lengkong

7002’15.2”

107018’19.6”

Breccia

26%

Dry

Rice field

Slides

13 m

Soil

3.

Cicurug Satu

7002’20.1”

107017’21.3”

Breccia

30%

Wet

Rice field

Slides

20 m

Soil

4.

Cidadap

7001’27.7”

107017’33.2”

Tuffaceous-sandstone

40%

Dry

Tuber plantation

Slides

20 m

Soil

Bamboo laths

5.

Cihurang

7000’56.9”

107017’13.9”

Sandstone

38%

Dry

Brush wood

Slides

14 m

Weathered and soil rock

 

 

 

2 pemikiran pada “REGIONAL GEOLOGY AND STUDY OF LANDSLIDE IN SIRNAJAYA AND SURROUNDING AREA, GUNUNGHALU DISTRICT, WEST BANDUNG REGENCY, WEST JAVA

  1. Interesting. Do you have any information about the same topic on Dusun Tangsi Jaya, desa Gunung Halu, kec. Gunung Halu? thanks. ropi

    Suka

Komentar ditutup.