IPA15-SG-089 PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-Ninth Annual Convention and Exhibition, May 2015 STRUCTURAL INTERPRETATION OF TECTONICALLY ASSOCIATED NORMAL AND REVERSE FAULTS OF BUKIT BAJANGAN IN KEBUMEN, CENTRAL JAVA Aldo Febriansyah Putra* Salahuddin Husein* ABSTRACT Understanding the tectonic history of Central Java has long been recognized as critical to successful petroleum exploration over the area. Most of the geologic setting and its product in Central Java are intimately linked with subduction tectonics. This paper attempts to analyze the deformation process of Bukit Bajangan, Kebumen, Central Java. The lithology of Halang Formation of Miocene Age, evidenced in outcrop by alternating layers of competent tuffaceous sandstones and siltstones. The association of normal fault and high-angle reverse faulting is of critical interest in this paper. Association of normal and reverse faults can occur when the normal fault has highly-dipping strata that tectonically lock the movement of the other block. Reverse faulting occurs when extension progresses while tectonic locking occurs. Relative sense of movement is determined from slickensides and striation and drag folds. The main fault has an identifiable zone of brecciation. Methods used in this study are structural measurement, paleo stress analysis, and clay modeling. Structural measurement was employed to determine the orientation of structural planes and the dynamics of the structures. The results of study show that paleo stress direction of main fault came from N S direction. This result follows the regional tectonic forces originating from subduction. Clay modeling was used to demonstrate the structures formed based on interpreted basement movement of Bukit Bajangan. Keywords: Normal Fault, Reverse Fault, Structural Diagram, Clay Model, Tectonic Locking, Bukit Bajangan, Kebumen INTRODUCTION The study of structural geology is critical in interpreting deformation processes based on * University of Gadjah Mada geometry. In constructing a structural diagram, understanding the geometry of the structures is the basis. This study begins with determining the geometry of structures and uses this knowledge in constructing the structural diagram. The research emerges in importance since tectonics may generate various structural geometries. Study area is located in Bukit Bajangan, Kebumen, Central Java. The area is located approximately 105 km west of University of Gadjah Mada. The outcrop is a hill with height of 22.5 meters and length of 400 meters. Based on UTM coordinate, the outcrop is located at 0356454; 9150022. The outcrop is susceptible to mass wasting, since mining activities do not pay sufficient effort in implementing safe techniques. Outcrop Observation The outcrop comprises alternating tuffaceous sandstone and siltstone with carbonate content. The outcrop displays a normal fault and reverse fault. Determination of relative movement is based on slickenside and striation, and drag fold. Fault breccia also indicates that movement has occurred. Normal fault displays drag fold, high-angle plane, intensive fault breccia, and conjugation. Meanwhile, the reverse fault has high angle plane, drag folding and fault branching. METHOD This study is separated logically into field observation and laboratory work. Field observation includes structural data collection. Laboratory work includes structural analyses, diagram construction, and clay modeling. Structural data collection was directed towards obtaining data regarding the orientation of beds and structural features. Measurement of bed orientation
was critical since the outcrop displayed opposite dip direction. Structural features measured in the study included fault plane and its properties. Structural analysis in the laboratory is employed to understand the direction of paleo stress of the area. Structural analysis will further our understanding in constructing and understanding the model. Stress analysis will be interpreted to understand the probable geologic phenomenon that created the outcrop. Another method applied in the study was clay modeling. The objective of the modeling was to test the interpretation of strike-slip faults forming Bukit Bajangan. The dimension of the base is 30 cm x 20 cm x 10 cm, while the sedimentary cover is 30 cm x 20 cm x 1 cm. RESULT Structural Observation and Measurement Due to its structural characteristics, the outcrop is divided into three zones: northern zone, fault, and southern zones (Figure 1). Zonation of research area is based on structural features and dip of beds. Northern zone dips to the south with the orientation of N 79 E/ 17; while the Southern zone orientates in N 306 E/ 16. The different dip direction of northern and southern zone is related to extensional stress. In terms of morphology, Bukit Bajangan can be classified as faulted syncline. Fault zone is composed of conjugate normal faults and reverse faults. The main normal fault itself has orientation of N 266 E/ 73. Synthetic fault has orientation of N 270 E/ 76, while the antithetic ones of N 121 E/ 50 and N 112 E/ 45. Otherwise, main reverse fault orientates in N 103 E/ 78. This also branches in N 104 E/ 48 and N 88 E/ 67.Normal fault is regarded as main fault of the outcrop because it displays drag folding, exposed slickenside, fault breccia, and fault gouge (Figure 2). Structural Analysis Structural features of Bukit Bajangan are interesting since normal faults, characterizing extensional stress, exist along with reverse faults. Structural analysis shows that direction of maximum principle stress is N 126 E, intermediate stress N 273 E, and minimum stress N 6 E. Maximum principle stress acts vertically, generating extension in approximately N S direction (Figure 3). Extensional features in Bukit Bajangan also displayed as strata in Northern zone dipping to the south, while Southern zone dips to the north. Consideration of structural analysis of Bukit Bajangan should be brought to regional tectonics view. Based on geologic map published by Geological Research and Development Bandung (Asikin et. al., 1992), Kedungramat Fault of sinistral movement influenced the formation of Bukit Bajangan. Lineament on satellite image provides a picture that Bukit Bajangan lies East of Kedungramat Fault. Extension in N 6 E direction is produced by branching of Kedungramat Fault, generating dextral fault at an angle to the main fault. Due to the trend of Kedungramat Fault is relatively NE SW direction, generated branch fault may trend in E W direction. The association of Kedungramat Fault and the branch fault generated extensional stress in approximately N S direction. Schematic regional tectonics map provides visualization of Kedungramat Fault and its branch (Figure 4). Deformation Mechanism Based on field observations, Bukit Bajangan has drag fold due to normal faulting that plunges beneath the surface. The drag fold is the key to cope with geological structures in Bukit Bajangan. Deformation mechanism begins with extension in approximately N S direction. Initial geological structure generated by this stress was a syncline. As extensional stress progressed, a part of Bukit Bajangan experienced normal faulting. Normal fault in Bukit Bajangan is determined as main fault since drag fold and fault breccia zone can be observed. Normally faulted strata in the drag fold of main fault have orientation of N 301 E/ 71, representing intensive tilting. Drag fold of normal fault plunges into the Earth, locking northern part of the main fault to form conjugate normal faults. As a result, such locking by the drag fold produces compression in N S direction. Tectonic locking causes the northern block to be unstable. To achieve physical equilibrium, it requires faulting with vertical displacement towards lower pressure condition. Another normal fault cannot accommodate stress due to tectonic locking. That is because vertical displacement towards the Earth was disabled. As a result, reverse fault is the only mechanism that can deal with compression due to tectonic locking. Another consideration which supports tectonic locking is projection of fault plane orientations to the subsurface. Structural diagram of
Bukit Bajangan provides visualization of the structural features (Figure 5). Deformed condition of Bukit Bajangan is interpreted to be ductile behavior. The basis of this interpretation is general morphology of the outcrop and obvious drag fold due to normal faulting. The general morphology of Bukit Bajangan shows that mechanism which takes place is buckling. Deformation condition encompasses pressure, temperature, pore pressure, and deformation rate. Pressure and temperature are primary physical factors in deformation, but reconstruction on the position of Bukit Bajangan strata relative to other parts of the area should be considered. Therefore, pore pressure and deformation rate can be considered. Strata of Bukit Bajangan were deposited in submarine environment, enabling high portion of fluid inside the sediment. To generate ductile deformation, pore pressure should be low to enable more intensive contacts between grains. As a consequence, strata of Bukit Bajangan must have undergone considerable compaction to expel the fluid. Hence, the most influential factor in deformation is rate. The slower the rate, the more ductile the deformation would have been. Faults in Bukit Bajangan were generated when the materials could not accommodate the stress by losing cohesion. Clay Model Experiment Clay model experiment aims to observe related geological structures in strike-slip fault system. Davis (2012) states that along with the main strikeslip fault, synthetic and antithetic faults also form as conjugate fault sets. In the experiment, the conjugate fault set involves the antithetic dextral fault (R - shear). Primary concern in this modeling is to prove extensional regime of the area (Figure 6). The clay model consists of two parts: basement and sedimentary cover. Truncated glass represents the basement, while clay represents the sedimentary cover. The dimension of the basement is 30 cm x 20 cm x 10 cm. For the sedimentary cover, the dimension is 30 cm x 20 cm x 1 cm. The basement consists of three zones. In order to achieve extension, Zone A and C are moved, while Zone B remains in place (Figure 4). The result of this deformation is major normal fault system with strike trending in E W direction. Other related structures are uplifted and subsidence areas along the strike of main sinistral fault. Implication to Petroleum Exploration Satyana (2007) mentions Central Java as terra incognita. Petroleum exploration in Central Java doesn t bring exceptional discoveries as other areas of the island. The focus of this study was the application of stress analysis to understand the dynamics of structures based on geometry. The core of the work was to construct deformation processes and their correlation to stress. The model facilitated interpreting the order of fault propagation, although time difference may be narrow. Meanwhile, the order of fault propagation implies different deformation intensity. Highly-intensive deformation is able to grind the rocks to form fault gouge. The fine-grained fault rock, in turn, characterizes the fault to seal or allow hydrocarbon migration. CONCLUSION Field work and laboratory analyses generate several points to conclude the study: Tectonic setting of study area is extension in approximately N S direction. Stress in Bukit Bajangan is related to Sinistral Kedungramat Fault Extension in approximately N S direction is produced by branching of Kedungramat Fault in E W trend with dextral sense. Normal fault becomes the main fault, while the reverse fault is the associated structure. Extension in Bukit Bajangan generates drag fold of normal fault which locks the block north of it. As a result, the northern block experienced compression in N S direction as extension progressed. Reverse fault is a manifestation that the block north of the normal fault cannot subside and spread laterally. Further study on the area should emphasize the stratigraphic correlations. ACKNOWLEDGEMENT This study is a part of Paper Workshop of American Association of Petroleum Geologists University of Gadjah Mada Student Chapter. Thank you to the committee who organized and run the forum. Appreciation to Arba Azzaman, Hercio Camilho Fernandes Xavier, Muhammad Ilyasa Satyadarma, Yan Restu Freski, and Yeftamikha who were great fieldwork and laboratory partners.
REFERENCES Asikin, S., Handoyo, A., Busono, H., and Gafoer, S., 1992, Geologic Map of The Kebumen Quadrangle, Jawa. Davis, G., H., Reynolds, S., J., and Kluth, C., F., 2012, Structural Geology of Rocks and Regions: Third Edition, p. 337. Satyana, A. H., 2007, Central Java Indonesia: Ä Terra Incognita in Petroleum Exploration: New Considerations on the Tectonic Evolution and Petroleum Implication: Proceedings of Indonesian Petroleum Association, 31 st Annual Convention and Exhibition.
Figure 1 - Zonation of Bukit Bajangan based on dip direction and structural geology. This figure shows intensive tilting of strata in the south of normal fault. The plunging drag fold due to normal fault is the key in interpreting tectonic locking beneath the surface.
Figure 2 - Delineation of normal and reverse faults of the outcrop. In clockwise direction, the figure represents delineation of the fault zone, drag folded strata south of normal fault, and delineation of normal fault plane. The figure explains structural features of the outcrop details. Figure3 - This table represents structural data of Bukit Bajangan, Kebumen, Central Java. Bukit Bajangan has trend of normal fault approximately in E W direction. Antithetic fault of Bukit Bajangan also can be observed, represented in number 6 and 7. Structural features of Bukit Bajangan are interesting since normal faults, characterizing extensional stress, exist along with reverse faults. Structural analysis begins with calculating direction of principle stresses. Maximum principle stress direction is N 203 E, intermediate principle stress N 79 E, and minimum principle stress N 344 E. As a result, geological structures in Bukit Bajangan are consequence of extensional stress in N S direction. Counting deviation below 30 indicates that the structure formed in one tectonic phase.
Figure 4 - Stress in Bukit Bajangan is related to Sinistral Kedungramat Fault and its branch. Extension to form Bukit Bajangan is interpreted to originate from branching of the main fault to generate dextral antithetic fault. Schematic regional tectonic map above attempts to visualize the structures (after Asikin et. al., 1992). Figure 5 - Structural diagram of Bukit Bajangan shows plunging drag fold in normal fault zone. Such plunging drag fold resists the movement of the northern block. Therefore, the northern block is tectonically locked. Extension causes the northern block in unstable condition. Reverse fault becomes a manifestation to accommodate stress due to extension.
Figure 6 - (a) This diagram shows fault system related to the formation of Bukit Bajangan. The basement composed of sinistral fault as main fault, and dextral fault as R -shear. Such fault system enables extension in approximately N S direction. (b) The clay model represents sinistral fault as the main fault and dextral fault as the antithetic one. The results of this basement movement are uplifted and extended areas along the main fault. A critical result is the extension in N S direction perpendicular to the antithetic fault. Bukit Bajangan is interpreted to form due to the extension. The reverse fault cannot be modeled with clay due to long history of deformation.