Journal of Himalayan Earth Sciences Volume 52, No. 2, 2019 pp. 96-105 Morphotectonic study of a watershed controlled by active fault in Southern Garut, West Java, Indonesia JohanBudi Winarto1,2*, EMI Sukiyah3, Agus Didit Haryanto3 and Iyan Haryanto3 1Geology Agency, Bundung, West java, Indonesia 2Post graduate program of Geology, faculty of Geological Engineering Padajadjaran University, Bandung, West java, Indonesia 3Department of Geoscience, Faculty of Geological Engineering Padajadjaran University, Bandung, West java, Indonesia *Corresponding author's email: [email protected] Submitted date: March 1, 2019 Accepted date: Sep 22, 2019 Published Online: Abstract This research aimed to analyze geomorphological shapes of the Cilaki watershed in Southern West Java in relation to geological structures using a geomorphological approach . The Cilaki watershed is characterized by wide valley shapes in the mid to upstream areas and narrow valley shapes in the downstream area, which shape is like a wine glass. The Cilaki watershed is dominated by Quaternary volcanic deposits, while in the downstream area Tertiary sedimentary rocks are exposed. The Cilaki watershed appears to be controlled by active fault, but it isn't known how its stage of activities. The morphotectonic analysis focuses on the influences of geological structures on the shape of the watershed using remote sensing method. The tectonic frame is determined by tectonical analysis base on Southern West Java tectonic setting. We divide the morphotectonic study of the Cilaki watershed into three parts: 1) the quantitative characteristics of the geomorphology; 2) morphometrical analysis; and 3) characteristics of the geological structures. The shape and boundaries of the Cilaki watershed are determined by their structural influences. The lithology of the Cilaki watershed is controlled by resent tectonic activities with class 1, because both the Quaternary volcanics and the Tertiary sedimentary rocks are systematically cut off by faults. The morphotectonic analysis offers a method that may support other studies concerned with neotectonics. Keywords: Active fault, Cilaki watershed, Morphotectonic, Remote sensing. 1. Introduction years. A potentially active fault is a fault that had moved in the past 2 million years. And the This study aims to define the influences of inactive fault is a fault that has not been moved active fault structures on the shape and in the past 2 million years. Whereas the active boundaries of the Cilaki watershed in the fault is define a fault that moves in the Cisewu areas, southern West Java. The Quaternary period and has the potential to move subduction of the Indian-Australian tectonic back in the future (Sukiyah et al., 2016). plate underneath the Eurasia plate (Fig. 1a) has shaped the geological setting of Java (Darman The present study used data from field a n d S i d i , 2 0 0 0 ; Ve r s t a p p e n , 2 0 1 0 ; observations and remote sensing method. Simandjuntak, 2015). The development of Morphotectonic approaches and various regional north-south oriented stress fields has applications are mentioned in older text books resulted in compressional faulting in West Java and publications on geomorphology and during the Plio-Pleistocene, with dominating geology (e.g., Doornkamp, 1986; Keller et al., fault orientations directed North-South, West- 1996; Burbank and Anderson, 2009; Sukiyah et East, Northwest-Southeast, and Northeast- al., 2016) concerned tectonic study for many Southwest (Hilmi and Haryanto, 2008). The applications. The scope of our research is to reasearch area is affected by strongly regional study the role that remote sensing can play in tectonic activity with evidence for deformation morphotectonic analysis and their relation to processes and associated with active faults. As the shape of watershed. Several researchers an active fault is define refer to Keller et al., have used the morphotectonic analysis (1996) that faults have moved in the past 10,000 (Doornkamp, 1986; Dehbozorgi et al., 2010; 96 Permana et al., 2015; Sukiyah et al., 2016) to West-Java is part of an island arc i d e n t i f y g e o l o g i c a l s t r u c t u r e s a n d characterized by tectonics and volcanism morphotectonic characteristic with varying (Simandjuntak, 2015). The research area is part implementation in West Java areas. Here we of the Southern Mountain Zone of West-Java report a new perspective of the genetic of the (Van Bemmelen, 1949). It is a mountainous watershed base on remote sensing in area with moderate to high slopes and plains morphotectonic study. occupying the coastal area. Stratigraphically, the research area is composed of the Cretaceous 2. Geological setting basement rock, Tertiary sedimentary rocks, Quaternary volcanic deposits and Holocene The research area is a part of the Garut and Alluvium, (Alzwar et al., 1992). Several active Pameungpeuk geological map regional sheet faults are found in West Java (Fig. 1c), such as (Alzwar et al., 1992) and Sindang Barang and the Baribis fault, the Cimandiri fault and the Bandar Waru geological map regional sheet Lembang fault (Van Bemmelen, 1949; Hilmi (Koesmono et al., 1996). The Cilaki watershed and Haryanto, 2008). The study of the area is in Cisewu regency, Southern Garut, correlation of a shape of watershed and West Java, Indonesia (Fig. 1b). The Cilaki geological structures in the research area has watershed and surrounding area is an area that thus far never been carried out in detail. is densely populated and is a popular tourist Previous studies concerning landscape, destination. However, the area suffers from lithology and geological structure have shown frequent landslides and earthquakes. that the research area has been affected by tectonic activity from the Cretaceous up to recent (Hilmi and Haryanto, 2008). Fig. 1. (a) The regional geological setting of Indonesia; (b) the location of the research area in Cisewu regency, Southern Garut West Java Province; and (c) the major faults of West Java are the Baribis Fault, the Cimandiri Fault and Lembang Fault. 97 3. Materials and method lineaments analysis is used to determine geological structure characteristic. There is a T h e m e a s u r e m e n t s o f s e v e r a l different method to interpret and identify the morphometry parameters were performed result analysis of the geological structures and using a Geographic Information System (GIS); the morphometric measurements, but most can topographic maps (scale 1:25.000), geological be eliminated by basic geomorphological maps scale 1:50.000 (Remote sensing method), a n a l y s i s ( D o o r n k a m p , 1 9 8 6 ) . T h e regional geology map scale 1:100.000, digital morphotectonic approach was used to imagery data from 2016 Aster DEM imagery determine tectonic activities of the Cilaki and 2017 Google Earth imagery. We were used watershed. remote sensing method and combined with field observation, which is processed at 4. Result and discussion Geological Agency laboratory Indonesia. The tectonic geomorphology has been 3.1. Morphometry method developed as basic reconnaissance tool to identify a geological structure area effectively The main aspects of geomorphological (Bull, 2007). The geology map of the research analysis are morphography, morphogenetic and area was reproduced by investigation from m o r p h o m e t r y ( Va n Z u i d a m , 1 9 8 5 ) . remote sensing analysis and field observation, Morphotectonic is defined as a description of as shown in Figure 2. The investigation of the the relationship between the tectonic processes geological structure is used to reveal the active and the surficial processes resulting in the fault influences on the Cilaki watershed f o r m a t i o n o f g e o m o r p h i c f e a t u r e s conditions in the research area. (Doornkamp, 1986; Keller and Pinter, 1996; Bull, 2007; Burbank and Anderson, 2009). It's 4.1. Morphotectonic characteristic usually related to Quaternary period which recent tectonic activities and active fault. In recent study, other application of Morphometric measurements are used as a part morphotectonic used to understanding of the of a geomorphological analysis that uses genetic of the watershed. The Cilaki watershed quantitative measurements to determine of covers an area of 385.1 km². The morphotec- shape and dimension of landforms, tonic units of the Cilaki watershed can be sub- environments, living organisms or other objects divided into two periods of tectonic activity. (Van Zuidam, 1985). The geomorphic index is a One part is on structural activity resulting from criteria to evaluate the occurrence of recent movement tectonic in the Tertiary period, that is tectonic activity also known as neotectonic represented by faults structure cut off the activity (Doornkamp, 1986). Tertiary sedimentary rock unit. The other represents a response to post Quaternary period Author has used 6 geomorphic indices to isostatic affected and represented by faults determine the tectonic activity , such as structure cut off the Quaternary volcanic rock bifurcation ratio (Rb), Basin shape index (Bs), unit. Mountain-front sinuosity (Smf), Valley height- width ratio (Vf ratio), Drainage density (Dd) 4.1.1. Quantitative geomorphology and Circularity ratio (Rc). Table 3 is showing the results of Cilaki watershed morphometry The Cilaki watershed was characterized calculation. Using this approach, the spatial by quantitative geomorphology analysis. The variation in tectonic activity period can be morphography unit of Cilaki watershed were identified. classified into 6 units based on valley shape, elevation and surface areas in km2. The results 3.2. Geological structure analysis of the morphometry measurement are shown in Table 1. The various of landscape is a response A path of isostatic frame and geological of the uplift occurrence and deformation structures adjustment analysis in research area processes.