Indonesian Journal on Geoscience

5

Authors: Sri Yudawati Cahyarini

Abstract - Variation of δ13C content in coral skeletons shows the influence of metabolic fractionation in aragonite coral. Understanding coral δ13C variation can thus be useful to more understand e.g. past bleaching event which is further useful for coral health and conservation. In this study, δ13C content in Porites coral from Labuhan Bajau, Simeulue Islands was analyzed. To know the correlation between variation of coral δ13C and light intensity, the monthly varia- tion of coral δ13C is compared to solar radiation and cloud cover. The result shows that for the period of 2003 to 2008, coral δ13C shows it is well correlated (r=0.42 p=0.153) with cloud cover variation in annual mean scale. Meanwhile, in seasonal mean variation, coral δ13C is strongly influenced (r=0.85 p<0.0001) by cloud cover with 1 - 2 month time lag. Comparing to the solar radiation (cloud cover), SST influences dominantly the variation of coral δ13C from southern Simeulue Island waters (LB sample) in an annual mean scale than in a seasonal scale. Keywords: δ13C, coral, Porites, solar radiation, cloud cover, SST

10

Authors: Euis Tintin Yuningsih, Hiroharu Matsueda, Mega Fatimah Rosana

Abstract - The gold-silver ores of western Java reflect a major metallogenic event during the Miocene-Pliocene and Pliocene ages. Mineralogically, the deposits can be divided into two types i.e. Se- and Te-type deposits with some different characteristic features. The objective of the present research is to summarize the mineralogical and geochemical characteristics of Se- and Te-type epithermal mineralization in western Java. Ore and alteration mineral assemblage, fluid inclusions, and radiogenic isotope studies were undertaken in some deposits in western Java combined with literature studies from previous authors. Ore mineralogy of some deposits from western Java such as Pongkor, Cibaliung, Cikidang, Cisungsang, Cirotan, Arinem, and Cineam shows slightly different characteristics as those are divided into Se- and Te-types deposits. The ore mineralogy of the westernmost of west Java region such as Pongkor, Cibaliung, Cikidang, Cisungsang, and Cirotan is characterized by the dominance of silver-arsenic-antimony sulfosalt with silver selenides and rarely tellurides over the argentite, while to the eastern part of West Java such as Arinem and Cineam deposits are dominated by silver-gold tellurides. The average formation temperatures measured from fluid inclusions of quartz associated with ore are in the range of 170 – 220°C with average salinity of less than 1 wt% NaC equiv. for Se-type and 190 – 270°C with average salinity of ~2 wt% NaCl equiv for Te-type. Keywords: epithermal gold-silver deposit, fluid inclusions, selenides, Se-type, tellurides, Te-type, western Java

14

Authors: Erawati Fitriyani Adji, Febrian Asrul, M. Aidil Arham, Bayu Wisnubroto

Abstract - The carbonate on Fika Field has a special character, because it grew above a basement high with the thick- ness and internal character variation. To develop the field, a proper geological model which can be used in reservoir simulation was needed. This model has to represent the complexity of the rock type and the variety of oil types among the clusters. Creating this model was challenging due to the heterogeneity of the Baturaja Formation (BRF): Early Miocene reef, carbonate platform, and breccia conglomerate grew up above the basement with a variety of thickness and quality distributions. The reservoir thickness varies between 23 - 600 ft and 3D seismic frequency ranges from 1 - 80 Hz with 25 Hz dominant frequency. Structurally, the Fika Field has a high basement slope, which has an impact on the flow unit layering slope. Based on production data, each area shows different characteristics and performance: some areas have high water cut and low cumulative production. Oil properties from several clusters also vary in wax content. The wax content can potentially build up a deposit inside tubing and flow-line, resulted in a possible disturbance to the operation. Five well cores were analyzed, including thin section and XRD. Seven check-shot data and 3D seismic Pre-Stack Time Migration (PSTM) were available with limited seismic resolution. A seismic analysis was done after well seismic tie was completed. This analysis included paleogeography, depth structure map, and distribution of reservoir and basement. Core and log data generated facies carbonate distribution and rock typing, defining properties for log analysis and permeability prediction for each zone. An Sw prediction for each well was created by J-function analysis. This elaborates capillary pressure from core data, so it is very similar to the real conditions. Different stages of the initial model were done i.e. scale-up properties, data analysis, variogram modeling, and then the properties were distributed using the geostatistic method. Finally, after G&G collaborated with petrophysicists and reservoir engineers to complete their integrated analysis, a geological model was finally created. After that, material balance was needed to confirm reserve calculations. The result of OOIP (Original Oil in Place) and OGIP (Original Gas in Place) were confirmed, because it was similar to the production data and reservoir pressure. The model was then ready to be used in reservoir simulation. Keywords: reservoir modeling, carbonate, rock and oil types, simulation, Fika Field

6

Authors: Baiq Dewi Krisnayanti and Christopher Anderson

Abstract - New technology is needed to protect the safety and health of communities and the environment at ASGM locations in Indonesia. This technology must be simple, cheap, easy to operate, and financially rewarding. A proven option that should be promoted is phytoextraction, a farming activity that could develop agriculture as an alternative livelihood in ASGM areas. This is a technology where plants are used to extract metals from waste rock, soil, or water. These metals can be recovered from the plant in its pure form, then be sold or recycled. Gold phytoextrac-tion is a commercially available technology, while an international research has shown that phytoextraction will also work for mercury. In the context of this idea, tailings would be contained in ‘farming areas’ and cropped using phytoextraction technology. Gold and mercury would be extracted in the crops, with the remaining mercury burden of the tailings becoming adsorbed to soil constituents. The system would be financially rewarding to ‘gold farmers’. The economic value of this scenario could facilitate the clean-up and management of mercury pollution, reducing the movement of mercury from tailings into soil, water, and plants, thereby mitigating environmental and human risk in the mining areas. The goal of the described research is to promote agriculture as an alterna- tive livelihood in ASGM areas. The gold value of the phytoremediation crop should provide a cash incentive to artisanal farmers who develop this new agricultural enterprise. The benefits will be social, environmental, and economic, as opportunities for education, employment, new business, the containment of toxic mercury, food safety and security, and revenue are all realized. Keywords: gold, phytomining, tailing, new business, phytoremediation, agriculture

11

Authors: Moh. Heri Hermiyanto Zajuli and Hermes Panggabean

The Oligocene Sinamar Formation consists of shale, claystone, mudstone, sandstone, conglomeratic sand- stone, and intercalation of coal seams. The objective of study was to identify the hydrocarbon source rock potential of the Sinamar Formation based on geochemichal characteristics. The analyses were focused on fine sediments of the Sinamar Formation comprising shale, claystone, and mudstone. Primary data collected from the Sinamar Formation well and outcrops were analyzed according to TOC, pyrolisis analysis, and gas chromatography - mass spectometry of normal alkanes that include isoprenoids and sterane. The TOC value indicates a very well category. Based on TOC versus Pyrolysis Yields (PY) diagram, the shales of Sinamar Formation are included into oil prone source rock potential with good to excellent categories. Fine sediments of the Sinamar Formation tend to produce oil and gas originated from kerogen types I and III. The shales tend to generate oil than claystone and mudstone and therefore they are included into a potential source rock. Keywords: Sinamar Formation, Oligocene, organic petrography, biomarker, shale

17

Authors: H.D. Tjia

Most of continental Southeast Asia, that is, Sundaland and Indosinia, achieved a relative tectonic stability by the beginning of the Cenozoic. Since then a strong tectonic activity in Sundaland has been restricted to existing regional fault zones and to regional slow, vertical crustal movements elsewhere that produced small to very large sedimentary basins. On the other hand, regional deformation of Indosinia as a consequence of ductile shearing has continued into the Paleogene. Since the Oligocene, the northern part of Sundaland and Indosinia have been extruded differentially towards southeast along the Red River, Wang Chao (or Mae Ping, or Tonle Sap), and Three Pagodas - Axial Malay fault zones. The initial cause has been attributed to hard collision between Subplate India with Megaplate Eurasia. Plate dynamics in the region have changed substantially since Mid-Miocene as to force wrench-slip reversals along the major fault zones in Sundaland as well as in Indosinia. Concomitant structural inversions are demonstrated on seismic sections. In the core of Sundaland, earlier transtensional wrenching was succeeded by transpressive strike-slip faulting that on major faults of the Malay Basin manifested in reversals of sense. From the Hinge-line fault eastward, the transtensional left wrench slip was succeeded by transpressional dextral slip, while in the region to its west the wrench-slip kinematics was an earlier transtensional right slip followed by transpressional left slip. In the Strait of Malacca and eastern margin of Sumatra, right-lateral wrenching in the Neogene has been common. In certain places it could be established a wrench-slip of transtensional character in Oligocene-Early Miocene, and the transpressional wrench movement occurred mainly during the Middle to Late Miocene. The remarkable coincidence of termination of spreading of the South China Basin in Langhian, and that of the West Philippine and Caroline basins during Mid- Miocene invites further study. Keywords: transtensional vs. tranpressional wrenching, sequence, domains delineation

5

Authors: I. Syafri, E. Budiadi, and A. Sudradjat

Kulon Progo Mountain, located west of Yogyakarta, is known as a unique morphological expression of an elongated dome frequently called “oblong dome”. The structural elements occurring in Kulon Progo Mountain were predominated by a radial pattern. Applying a quantitative method to measure various morphometric elements however, revealed that the regional geotectonic pattern apparently controlled the development of Kulon Progo Mountain. A general picture of the tectonics showed that the mountain building of Kulon Progo was not solely predominated by a vertical undation force; instead it was closely related to the general geotectonics operating in the area. The macro morphological analysis using various types of satellite imageries augmented with field visits unraveled three regional tectonic stages controlled the development of Kulon Progo Mountain. Those are Meratus, Sunda, and Java trends, operating in SW-NE, NNW-SSE, and E-W directions respectively. Keywords: oblong dome, undation theory, regional tectonic pattern, mountain building, Kulon Progo

11

Authors: Suyono

East Ketungau Basin is one of frontier basins in Indonesia. Some of these basins, especially those in eastern Indonesia, have been identified to possess potential of oil and gas. The existing publications of geological fieldworks and extensive exploration in the East Ketungau Basin are limited. The detailed sedimentological and biostratigraphical studies of the sedimentary succession will be used to reconstruct the tectonic and palaeogeographical history of the basin. The sedimentary Mandai Group consists of three facies such as mudstone facies, clean sand facies and alternation between thinly coal seam, coaly shale, and claystone facies. However, each facies characterizes depositional environment of barrier- island and associated strand-plain systems. Keywords: East Ketungau Basin, Palaeogene, Mandai Group

12

Authors: A. Idrus, E. M. Ati, A. Harijoko, and F. M. Meyer

Sedimentary-related manganese layers have been discovered in South Central Timor Regency, Timor Island, Indonesia, which is tectonically active and being uplifted due to north-trending tectonic collision between Timor Island arc and Australian continental crust. The manganese layers of 2 to 10 cm-wide interbed with deep sea sedimentary rocks including reddish - reddish brown claystone, radiolarian chert, slate, marl as well as white and pinkish calcilutite of Nakfunu Formations. Stratigraphically, the rock formations are underlain by Bobonaro Formation. Two types of manganese ores found comprise manganese layers and manganese nodule. The manganese layers strongly deformed, lenticular, and segmented, are composed of manganite [MnO(OH)], groutite [MnO(OH)], pyrolusite (MnO2), lithioporite (Al,Li) MnO2(OH)2, and hollandite [Ba (Mn4+, Mn2+)8O16] associated with gangue minerals including calcite, quartz, limonite [FeO(OH)], hematite (Fe2O3), and barite (BaSO4). Whilst the nodule type is only composed of manganite and less limonite. Geochemically, the manganese layers have grade of 63 - 72 wt.% MnO, whereas the nodule one has grade of 63 - 69 wt.% MnO. Generally, iron in Mn ore is very low ranging from 0.2 to 1.54 wt.% Fe2O3, averaged 0.76 wt.%. Hence, Fe/Mn ratio which is very low (0.003 - 0.069), typically indicates a sedimentary origin, which is also supported by petrologic and petrographic data showing layering structure of manganite and lithioporite crystal/grain. Trace element geochemistry indicates that manganese ore was precipitated in a reduction condition. Rare earth element (REE) analysis of manganese ore shows an enrichment of cerium (Ce) suggesting that the ore is basically originated in a marine environment. The manganese nodule is interpreted to be formed by chemical concretion process of unsoluble metals (i.e. mangan, iron) in seawater (hydrogenous) and precipitated on deep sea bottom. On the other hand, the manganese layer is a detrital diagenetic deposit formed by Mn remobilization in seawater column, precipitated and sedimented on the deep sea bottom. Manganese layers have probably been influenced by ‘hydrothermal process’ of mud-volcano activities, proven by the presence of quartz and barite veinlets cutting the Mn layers, manganite recrystallization to be pyrolusite along veinlets cutting manganite and lithioporite layers, and the presence of pyrite and sulphur associated with Mn layers. Field data also exhibit that the significant manganese layers are mostly found around mud volcanoes. The closely spatial and genetic relationships between manganese layers and mud-volcanoes might also be an important guide for the exploration of Mn deposit in the region. Keywords: manganese nodule, manganese layer, Bobonaro, Timor Island, Indonesia

6

Authors: S. Y. Cahyarini

Sea surface salinity is an important parameter in a climate study. Coral δ18O records δ18O seawater and sea surface temperature (SST). While, coral Sr/Ca records SST only commonly used in a paleoclimate study to reconstruct SST. Thus, paired coral δ18O and Sr/Ca can be used to reconstruct δ18O seawater. δ18O seawater and SSS is linearly correlated, thus reconstructed δ18O seawater further is used to reconstruct sea surface salinity (SSS). Instead of using coral Sr/Ca as SST recorder, paired model (grid) or measured SST data is used to reconstruct SSS. In this study, paired coral δ18O and grid SST data are presented to reconstruct SSS from several different locations across Indonesian sea i.e Simeulue, Mentawai, Bunaken, and Bali. Coral-based SSS reconstructions from those locations are then compared to the grid SSS in the seasonal mean scale. The result shows that annual mean variation of salinity for period of 1958-2008 in Mentawai and Simeulue is 33.25 psu and 33.26 psu respectively, while in Bunaken and Bali is 34.03 psu and 33.47 psu respectively. Correlation coefficient between coral salinity and salinity from model data in the seasonal/monthly mean scale is high i.e R = 0.62 - 0.83. Based on the monthly mean data, corals in the studied area strongly record SSS variation in the monthly or seasonal mean scale. In Mentawai and Simeulue waters, SSS variation is influenced strongly by monsoon. While, in addition to the monsoon, ocean advection also affects seasonal variability of SSS in the Bunaken and Bali waters. Keywords: salinity, coral, δ18O, SST, Simeulue, Mentawai, Bunaken, Bali

6

Authors: I. S. Sutawidjaja

Merapi is an active strato volcano located in Central Java. This volcano is regarded as the most active and most dangerous volcano in Indonesia. Since the twentieth century, the activities have comprised mainly the effusive growth of viscous lava domes and lava tongues, with occasional gravitational collapses of parts of over-steepened domes producing pyroclastic flows, commonly defined as “Merapi-Type”. Since October 2010, however, explosive eruptions of a relatively large size have occurred to VEI 4, and some associated pyroclastic flows were larger and had farther reach than any produced on July 2006. These events may also be regarded as another type of eruptions for Merapi. On October26th, 2010 such event happened, even though it was not caused by pyroclastic flows of the dome collapses, about thirty people were killed including Mbah Marijan, known as the Merapi volcano's spiritual gatekeeper, who was found dead at his home approximately 4 km from the crater. The Yogyakarta Palace subsequently confirmed his death. This time the disaster was caused by a sudden directed blast that took place at 17:02 pm throughout Cangkringan, Kinahrejo Village, at the south flank of Merapi Volcano. The victims were the local people who did not predict the blast threatened their areas, because they believed that the pyroclastic flows from the dome collapses as long as they knew, did not threaten their areas, and pyroclastic flows would flow down following the Boyong River as the closest valley to their village. The blast swept an area about 8 km2, reaching about 5 km in distance, deposited thin ash, and toppled all trees to the south around the Kinahrejo and Pakem areas. The blast that reached Kinahrejo Village seemed to have moderate temperatures, because all trees facing the crater were not burnt. However, the victims were affected by dehydration and blanketed by fine ash. Keywords: Merapi, Merapi-type, pyroclastic flow, directed blast, Kinahrejo Village

10

Authors: D. Sunarjanto, and S. Widjaja

Basement rocks, in particular igneous and metamorphic rocks are known to have porosity and permeability which should not be ignored. Primary porosity of basement rocks occurs as the result of rock formation. The porosity increases by the presence of cracks occurring as the result of tectonic processes (secondary porosity). Various efforts have been carried out to explore hydrocarbon in basement rocks. Some oil and gas fields proved that the basement rocks are as reservoirs which so far have provided oil and gas in significant amount. A review using previous research data, new data, and observation of igneous rocks in some fields has been done to see the development of exploration and basement reservoirs in Indonesia. A review on terminology of basement rock up till the identification of oil and gas exploration in basement rocks need to be based on the latest technology. An environmental approach is suggested to be applied as an alternative in analyzing the policy on oil and gas exploration development, especially in basement reservoirs. Keywords: hydrocarbon, reservoir, basement rock, Indonesia

7

Authors: Y. Ramadhan, K. Channel, and N. R. Herdianita

The researched area, located in the Dieng Plateau, is included into the Holocene Dieng Volcanic Rock Unit. The regional structure in this area is originated from the major caldera with local fault having orientation of SE - NW. Surface manifestations found in the researched area are hot springs located in Bitingan, Sileri, Siglagah, Pulosari, Kaliputih, and Sikidang. Fumaroles occur in Candradimuka and Pagerkandang and mud pools are located in Sileri and Sikidang craters. Temperatures of the hot springs ranges from 43o to 61o C, pH of 6 - 7, and their conductivity are of 38-78 MeV. The type of hotwater is a mixture of bicarbonate, sulfate, and chloride sulfate deriving from condensation of steam. Based on a relative composition of Cl-Li-B, the hot water is originated from four different reservoirs with different rock associations, while their reservoir temperatures vary from 225o to 300o C. Keywords: geochemistry, water type, reservoir, subsurface temperature, Dieng

11

Authors: A.K Permana, C.R Ward, and L.W Gurba

The Permian coals of the South Walker Creek area, with a vitrinite reflectance (Rvmax) of 1.7 to 1.95% (low-volatile bituminous to semi-anthracite), are one of the highest rank coals currently mined in the Bowen Basin for the pulverized coal injection (PCI) market. Studies of petrology of this coal seam have identified that the maceral composition of the coals are dominated by inertinite with lesser vitrinite, and only minor amounts of liptinite. Clay minerals, quartz, and carbonates can be seen under the optical microscope. The mineral matter occurs in association with vitrinite and inertinite macerals as syngenetic and epigenetic mineral phases. The irregular pattern of the vitrinite reflectance profile from the top to the bottom of the seam may represent a response in the organic matter to an uneven heat distribution from such hydrothermal influence. Examination of the maceral and vitrinite reflectance characteristics suggest that the mineralogical variation within the coal seam at South Walker Creek may have been controlled by various geological processes, including sediment input into the peat swamp during deposition, mineralogical changes associated with the rank advance process or metamorphism, and/or hydrothermal effects due to post depositional fluid migration through the coal seam. Keywords: maceral characteristics, vitrinite reflectance, South Walker Creek coals, Bowen Basin

13

Authors: H.D. Tjia

Gunungsewu Karst (also known as Sewu karst in the literature) is synonymous with morphology of a carbonate terrain dominated by hills crowned by accordant-level tops that developed in a humid tropical environment by comparatively more rapid dissolution and denudation. In addition, the hills are sinoid to cone-shaped. Surface drainage is negligible compared to subsurface water flow. Abandoned channel segments and spatial arrangements of karst hills have been found to correspond with fracture patterns that are genetically associated with the regional compression direction of Jawa Island. Images derived from space platforms show many landform patterns that were neither known from ground-based nor from aerial photograph study. Landforms arranged in ring, multi-ring, spiral, polygonal, and long linear to serpentine patterns are common beside the expected depressions of dolines, poljes, and uvalas. The orientations of the long linear ridges appear to change systematically from those near the coast to those located inland. These linear ridges are interpreted as depositional fronts, most likely representing breaker zones. The youngest depositional ridge fronts, located nearest to the present shoreline, are parallel to the geological strike of Jawa Island. Toward the island’s interior, linear depositional fronts deviate in orientation by as much as 40o. This is now interpreted to have resulted from counterclockwise rotation of the Gunungsewu microplate since the late Middle Miocene. Similar CCW rotations are indicated by the paleomagnetic orientations of igneous rocks located farther east in the southern range of the island. Active tectonics is expressed in stage-wise net uplift of Gunungsewu whereas regional tilting appears negligible. Stacked and often paired river terraces (thus suggesting land uplift) have been used to relatively date paleoarcheological finds. Very recent uplift on the coast show up in lazy-V limestone notch profiles, and occasionally by stacked notches. At the Klayar coast, slight northward tilt of a few degrees are expressed as counter-regional inclination of a subrecent abrasion platform. Keywords: Gunungsewu karst, simple to composite landforms, paleo-breaker lines, active tectonics