Ijges 1 Vol 6 No 1.Pdf

International Journal of Geology and Earth Science Vol. 6, No. 1, March 2020 Enormous Mass Movements, and Gravitational Tectonics Model of the North Serayu Mountains, Karangkobar Area, Banjarnegara Regency, Central Java, Indonesia Sari Bahagiarti Kusumayudha and Heru Sigit Purwanto Geological Engineering Department, Universitas Pembangunan Nasional Veteran Yogyakarta, Indonesia Email: [email protected], [email protected] Abstract—North Serayu Mountains in the Central Java every place of this region are a group of clay, marl, shale, province, Indonesia, stretches with west - east axis, bordered silt, and sandstones with quite high carbonate content by Slamet volcano to the Bogor Mountains in the west, and included in the Merawu formation, and a group of delimited by Ungaran volcano to the Kendeng Mountains in volcanic rocks. As it is known, the Merawu Formation the east. In the north of these mountains there is coastal alluvial plain of Java, and in the south there is a depression has physical properties of impermeable, plastic, soft, and zone of River Serayu. Overall geomorphostructures of the very sensitive to erosion, while the volcanic rocks North Serayu Mountains form a faulted anticlinorium, behaves elastic, hard, and dense. which one of its flank relatively dipping to the south. In the Due to this geological condition, Karangkobar and bottom part of this ranges are such plastic, clastic, clayey surrounding area is often subjected to relatively sedimentary rocks, Eocene to Miocene aged. While at the catastrophic rock mass movements in the form of upper portion there is a group of elastic, brittle, massive landslides, subsidence, shifting, and creeping. In this volcanic rocks, andesitic to basaltic composition, Pliocene to region there are also several mountains or hills, which Pleistocene aged. A volcano, called Rogo Jembangan stands over the top of the North Serayu anticlinorium, with two of according to the results of previous geological researches, its eccentric cones, namely Mount Telagalele and Mount it is verified that the area really always disturbed by mass Pawinihan, situated in the Karangkobar District. North movements [1]. Serayu anticlinorium with a plastic bedrock, which is There are some mountains and/or hills in the overlain by elastic, hard, and heavy rock has created such a Karangkobar area that very well known by local residents, tectonic model influenced by gravity. Parts of volcano’s namely Mount Telagalele (1291 m), Mount Pawinihan body and volcanic rocks blocks in the Banjarnegara (1232 m), and Mount Lawe (about 700 m above sea Regency area generally move slowly but surely southward level). Those mountains transfer slowly over the base over a giant slip plane in the form of orographic fields. Locally, this gravitational tectonic is manifested as mass rock, relatively to the south, toward the Banjarnegara slides, glides, and creeps, occur any time in the study area. Basin [2]. The travel of these mountains triggers the Orogenetics of the North Serayu Mountains is still on going occurrence of smaller scale movements on their sides in line with the active tectonism of the Java island, caused and/or slopes. by subduction of Indian-Australian plate beneath the In the year 2006, a part of the foot slope of Mount Eurasian plate. As long as that is the case, gravitational Pawinihan collapsed and slided, burying many houses tectonic will continue to run, and mass movements in the and killing 90 people in Sijeruk village. In the year 2012 research area will regenerate to happen. landslide happened again on the southeast slope of Mount Index Terms—anticlinorium, gravitational tectonic, mass Pawinihan, and in 2014 landslide occurred at Jemblung movement, North Serayu Mountains village involving norteastern slope of Mount Telagalele, assassinating 108 people [3]. The occurences of mass movements in Karangkobar I. INTRODUCTION and its surroundings are not likely to cease this time, but will continue to happen. For this reason, it is necessary to Karangkobar and its surrounding area, Banjarnegara study the characteristics of the geological dynamics and Regency, Central Java, are known as an area with local tectonic of the area, so that it can be used as a unstable rocky conditions. It is located from 7 km to 30 reference for landslide disaster mitigation in the km to the north of the Banjarnegara city, the capital city Karangkobar and surrounding areas. of the Banjarnegara regency. Astronomically it is situated o o o o Based on the rationales mentioned above, some, at 109 40’ - 109 45’ East Longitude, and 7 15’- 7 20’ research in order to find out the characteristics, models, South Latitude (Fig. 1). Rocks that exposed in almost and force controlling mass movements in Karangkobar Manuscript received December 13, 2019; revised February 19, 2020. need to be done, with geological point of view. ©2020 Int. J. Geol. Earth Sci. 1 doi: 10.18178/ijges.6.1.1-8 International Journal of Geology and Earth Science Vol. 6, No. 1, March 2020 “slump” [10]. When the involving mass is hard or fresh rock, without any slip plane, then the mass movement is namely rockfall. Whereas if the velocity of mass transfer is slow, high water content, it usually in the form of flow, such as debris flow, soil flow, and mud flow (Fig. 3) [7]. Figure 1. Location map of the study area. II. METHOD OF STUDY This research was conducted based on analytical G = Weight Force methods using secondary as well as primary data from α = Slope inclination Φ = Friction Angle field surveys and mappings. Some aproaches applied in c = Cohesion Force the study was topographic interpretation, remote sensing, surface geologic and geomorphologic mappings, Figure 2. Stress distribution acting on a slope [9]. petrologic analysis, rock/soil properties analysis, and geologic structure assessment. Data used for the analyses were derived from field surveys, mappings, and existing previous studies.The results are further discussed in this paper. Survey and mapping were done to identify the variation of physical characteristics and distribution of the lithology. Geologic structure assessment and measurements were held for recording the strike and dip of bedding, folds, joints or fault planes. Survey and mapping were also used for identifying the morphology Figure 3. Types of mass movements [8], [11]. such as hills, ridges, and valleys. Topographic and image interpretation were utilized for identifying pattern and Classification of rock movement is totally difference orientation of fracture system (joint, fractures, fault) also from that of soil mass. Types of rock mass movement hills and valleys lineaments. include planar slide, wedge, and topple (Fig. 4) [7]. Petrological and petrographic analyzes were carried Whatever the type of movement, stability of such a slope out to determine rock types [4], paleontological analysis is quantitatively represented by its Factor of Safety (FS). was also carried out to decide rock geological age. Determining the factor of safety such a slope is based on Determination of rock age, especially in sedimentary the principle equation: rocks deposited in the marine environment, is based on FS (Factor of Safety) = (1) plantonic foraminifer fossils (Blow, 1969) [5]. On the other hand, geological structures assessment was also ∑ Resisting Force done clopleeting this research by refering to the existence ∑ Driving Force of field indicators such as lithological offset, fault plane, brecciation zone, milonitization, and morphological lineament or scarps [6], [7] Related to rock and soil mass movements, it is ferivied that when there is a disturbance of force balance distribution in such a slope, mass movement will occur Figure 4. Types of rock slope failures: (a) block gliding (planar [8]. Force acting on a certain area is described as stress sliding), (b) wedge, (c) toppling [8]. [8]. The distribution of stresses working on a slope can be illustrated as in Fig. 2. III. RESULTS AND DISCUSSION In general, mass movements are able to be categorized into various types. The classification is based on the A. Geomorphology and Stratigraphy speed of movement, the presence or absence of slip plane, The Karangkobar and surrounding areas are positioned the character of slip plane, and the variety of involving in the physiographic zone of the North Serayu materials. For example, if the speed of movement is rapid, Anticlinorium [2], occupying elevation between 400 m, there is a circular slip plane with rotational sliding, and up to more than 1000 m above sea level. The highest the involving mass is soil, then it can be claasified into point of the area occupies an elevation of 1291 m. The ©2020 Int. J. Geol. Earth Sci. 2 International Journal of Geology and Earth Science Vol. 6, No. 1, March 2020 landscape in Karangkobar and its surroundings can be There is Ligung Formation that unconformably grouped into several geomorphological units, namely the overlaying the Bodas Formation. It is composed of Volcanic Hills morphology unit, the Undulating volcanic deposits from Mount Korakan [2]. The rock unit morphology unit, and the Homoclynal Mountains consists of andesite breccia with lignite insertions, 2 m morphology unit. thick, and there are also plant remains can be found. The Volcanic Hills morphology unit occupies an There is a cross - layering structure, indicating that the altitude of 800 m - 1291 m, slope reaches 10o – 30o or Ligung Formation sedimented in a fluvial environment. about 18% to 58% (steep to very steep) with a height This formation is facies difference from the Jembangan difference of 300 m to 500 m, generally built by volcanic Formation which is a volcanic deposit from Rogo rocks including breccia and lava. The Undulating Jembangan complex. morphology unit is situated at altitude of 405 m to 900 m, The Jembangan Formation consists of laharic breccia, displaying slopes range from 2o – 10o or aproximately 5% pyroclastic breccia, agglomerates, and volcanic ash to 20% with height difference between the lowest and the deposits. Their composition is generally andesite-basaltic, highest elevation of 10 m to 100 m.

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