Indonesian Journal on Geoscience

6

Authors: I. S. Sutawidjaja, O. Prambada and D. A. Siregar

Mount Sinabung, located in Karo Regency, North Sumatra Province, is a strato volcano having four active craters. Since its latest eruption about 1,200 year ago, a phreatic eruption occurred on August 27th, 2010. The eruption took place in Crater-I, which was initiated by a greyish white plume and then followed by black plumes as high as 2000 m above the crater. Altered rock fragments and ash were erupted during this event. The altered rocks show a development of argillic alterations which was formed in the hydrothermal system in depth. The alteration zone is formed along the northeast-southwest and northwest-southeast trend across the three craters. All of the craters are actively discharging solfataric gases, of which sulphur deposits are resulted, and they have been quarried by the local people. The age of the latest magmatic eruption was dated by 14C method from the charcoal sample found in the pyroclastic flow deposits near Bekerah Village. Keywords: Mount Sinabung, phreatic eruption, argillic alteration, 14C method

10

Authors: S. Winardi, B. Toha, M. Imron, and D. H. Amijaya

In western Indonesia, Eocene shale is generally considered as a potential source rock. The Nanggulan Formation outcropping at Kulonprogo-Yogyakarta consists of a shale interval of Eocene in age. Analysis of its organic material content, kerogen type, and maturity level were conducted to know its potential. The laboratory analysis of eleven samples were done to measure its TOC content. Samples with TOC > 0.5% then were analyzed to measure its Rv and TAI. Maturity level was also calculated by TTI from burial history model. The result of analysis shows there are various TOC contents and seven samples of them are categorized into a good-excellent class (TOC > 1%). Kerogen content of those samples is type III (non fluorescene amorphous-humic kerogen). One sample has a good indication of hydrocarbon formation (PY = 9.0 mg HC/g rock). Unfortunately thermal maturity level of the samples is immature (highest Rv 0.39, Tmax 422oC, and TAI 2). Otherwise, TTI calculation result from subsurface burial history modelling indicates that some areas are mature having reached gas window since 0.4 mya, especially in the area which had been influenced by a volcanic intrusion at Oligocene (28.5 mya). Therefore, the Nanggulan Formation shale has a potential capacity as a source rock with some limitation in maturity level. Keywords: potential, Eocene shale, hydrocarbon, source rock, maturity, Nanggulan

14

Authors: L. D. Santy and H. Panggabean

The Ketungau and Melawi Basins, in West Kalimantan, are Tertiary intramontane basins of which the potential for economic conventional oil and gas discoveries have not previously been confirmed. The Ketungau Basin is bordered by the Melawi Basin in the south. Besides non-ideal trapping mechanisms, another major problem in these basins is source rock maturation. Nevertheless, both basins are promising to be explored for oil shale and shale gas energy resources. Therefore, the aim of this paper is to give some perspectives on their source rocks, as an input for the evaluation of the potential of unconventional oil and gas. About twenty samples collected from the Ketungau and Melawi Basins were analyzed using pyrolysis and organic petrographic methods. The results show a poor to good quality of source rock potential. The Ketungau shale, which is the main source rock in the Ketungau Basin, is dominated by type III, immature, and gas prone kerogen. The Silat shale, which is the main source rock in the Melawi Basin, is dominated by type II, immature to early mature, mixed gas, and oil prone kerogen. In the field, Ketungau and Silat Formations have a widespread distribution, and are typically 900 m to 1000 m thick. Both the Ketungau and Silat shales occur within synclinal structures, which have a poor trapping mechanism for conventional oil or gas targets, but are suitable for oil shale and shale gas exploration. This early stage of research clearly shows good potential for the future development of unconventional energy within the Ketungau and Melawi Basins. Keywords: Ketungau and Melawi Basins, oil shale, shale gas, unconventional energy

8

Authors: Asmariyadi, R. Langkoke, A. Maulana, I. Nur, and W. Astaman

This paper is dealing with ore characteristics and fluid inclusion of the Moncong Bincanai, Biringbulu Subregency of Gowa Regency, South Sulawesi Province, Indonesia. The mineralization is a vein type, with the orientation of N170oE /65oSW, hosted in open-space filling within basalt. The mineralization consists of galena, sphalerite, chalcopyrite, and pyrite. Vein thickness ranges from 5 - 17 cm, showing a crustiform banding texture, with a sequence from outer to centre: quartz, carbonate (siderite), sulphide. The quartz displays primary growth textures such as comb, crystalline, saccharoidal, and colloform. Analytical methods applied include AAS and fluid inclusion microthermometry. Chemical composition of the vein indicates an average of Pb = 47.92%, Cu = 1.27%, Zn = 1.02%, and Fe = 9.46%, which shows a significant concentration of Pb. Fluid inclusion microthermometry results indicate a range of formation temperature of 240o - 250oC and salinity of the responsible hydrothermal fluid of 2.1 - 2.5 wt.% NaCl eq. The deposit is categorized into low-sulfidation epithermal deposits, which was formed within a range of 410 - 440 m below paleosurface. Keywords: epithermal vein, base metal, ore mineralization, fluid inclusion

12

Authors: A. Wisanggono, P. Abaijah, K. Akiro, D. Pertiwi, and R. Aditya Sauzy

Gold mineralization at East Asia Minerals’ Binebase Prospect on Sangihe Island displays controls to mineralization typical of other Pacific Rim Intrusion related low sulphidation systems. Better gold grades are localized on EW dilatant structures at the intersection of NW trending structural corridors. Mineralization at Binebase was previously interpreted as being hosted in a high sulphidation style of alteration system. This assumption was based on surface observations of alunite and what was thought to be residual vuggy silica. Recent observations indicate that the alunite is a supergene mineral in nature and that the vuggy silica is the boxworked remnants of silica + pyrite alteration. Precious metal grades decline moving away from feeder structures and there has been significant supergene enrichment of gold in the oxide zone. Exploration drilling completed in 2008 by East Asia Minerals Indonesia resulted in an inferred resource of approximately 835,000 oz of contained gold at a cut off grade of 0.25 g/t Au. This resource included both oxide and hypogene sulphide mineralization. Drilling was reinitiated in 2011 to change the inferred to indicated resource, that drilling programme continued at the time of this publication. Various geophysical techniques were employed at both prospect and regional scales to aid exploration. Ground magnetics defined hydrothermal alteration and magnetite destruction which was coincident with mineralization at both Binebase and Bawone and an Induced Polarisation survey also produced an anomaly which was coincident with disseminated fine sulphide associated with clay alteration within the hydrothermal alteration. The southern part of the island was covered by an airborne geophysical survey in an attempt to understand the regional structural controls on mineralization and to define additional areas of hydrothermal alteration. Keywords: supergene, enriched, intrusion, sulphidation, Binebase, Bowme

15

Authors: U. M. Lumban Batu and S. Poedjoprajitno

Geological features in Labuhan area were studied from the middle of October to the middle of November 2011 covering seventy days. Surface and subsurface data were obtained from interpretation of landsat images and shallow hand-auger boreholes. The geological features are distinctly associated with active tectonics. The stratigraphy clearly indicates at least three phases of tectonic activities since the Late Miocene until Holocene. Tectonics of phase one occurred in the Late Miocene; phase two took place in the period from Pliocene to Late Pleistocene, while tectonics phase three is ongoing in the Holocene. Volcanic activity has intensified since the Early Pleistocene. The landsat images show an irregular outline of the northern coast line. This penomenon is interpreted to be the result of tectonic uplift. On the other hand, the southern coast is linear in plan which is interpreted to correlate with tectonic subsidence. Furthermore, stratigraphic correlation shows that depositional environment changed vertically due to a local subsidence. The northern researched area is occupied by Pleistocene volcanic eruption centres, whilst the younger ones tend to shift southward. This fact tends to indicate that the subduction zone moved southward slowly. Keywords: Quaternary geological phenomena, tectonics, sea- level, Labuhan

7

Authors: N. I. Basuki and S.A. Wiyoga

The carbonate sequence overlies conformably the tuffaceous sandstone unit, and in turn is conformably underlain by the tuff-sandstone unit, both of which are members of the Citarate Formation. The Citarate carbonate rocks were deposited in an open platform back reef environment, which was temporarily drowned by local sea level rise. Regional Middle Miocene deformation formed NNE-WSW trend faults and E-W trend folds in the researched area. This paper discusses the nature of diagenetic alteration of the Citarate carbonate rocks based on petrographic analyses of twenty surface samples. Carbonate rocks from bottom to top comprise algae packstone, packstone-grainstone, coral-algae packstone, and foraminifer wackestone-packstone. Fragments of coral, coralline red algae, and large foraminifera are the dominant bioclasts in most of the observed samples, whereas echinoids and bivalves are less abundant; they are set in a recrystallized micrite matrix. Planktonic foraminifera are abundant only in few samples. Fragments of plagioclase, igneous volcanic rocks, pyroclastic rocks (tuff), and much less abundant quartz are commonly present in all the studied samples. A generalized diagenesis includes early marine cementation by fibrous aragonite, compaction, aragonite dissolution and/or neomorphism, precipitation of equant-grained calcite cement in a phreatic environment, dissolution to form moldic porosities, dolomitization, the formation of stylolites and fractures, and precipitation of late ferroan calcite during burial. Multiple carbonate cements occur as pore-filling phases, with ferroan calcite cementation taking place during later-stage burial. Secondary porosities were formed during different stages in diagenetic processes, such as dissolution, dolomitization, and stylolite and fracture formations. Although precipitation of nonferroan and ferroan calcite cement occluded porosities, porosity enhancement during early selective dolomitization might still be significant. Current observations also revealed the presence of intraparticle, micro-vuggy, and fracture porosities in different samples. Keywords: Citarate Formation, carbonate rocks, diagenesis, porosity

11

Authors: S. Maryanto

This study is focused on the provenance and diagenetic processes affecting the sandstones of Lati Formation cropping out at Berau Area, East Kalimantan Province. Petrographic analysis of twenty-two samples from this formation shows that these sandstones are classified as litharenite, feldspathic litharenite, sublitharenite, feldspathic wacke, and lithic wacke, which are partially calcareous. Preserved diagenetic processes were visible on the petrographic analysis including cementation, replacement, dolomitization, compaction, and dissolution. The provenance of these sandstones is dominated by granitic rocks initiated from tectonic setting of rifted continental margin, transported toward southeast. Keywords: petrography, sandstone, diagenesis, provenance

20

Authors: Bhakti H. Harahap

Sedimentary history and stratigraphy of the Papua and Arafura Sea areas, eastern Indonesia, are gained from surface geological mapping combined with published data from oil companies. Development of some sedimentary units demonstrates that the tectonism have influenced sedimentation of such units comprising a succession of Phanerozoic rocks developing in a stable continental margin. The succession underlain by Cambrian-Silurian-Devonian metamorphic rocks consists of Tuaba, Kariem, Awitagoh, and Kemum Formation, and Modio Dolomite (Pre-Rift Phase). These rocks having been intruded by Late Permian-Middle Triassic granitoids and Carboniferous granite, are unconformably overlain by Late Carboniferous to Cretaceous siliciclastic-rich units comprising Aifam Group and Tipuma Formation (syn-Rift Phase) and Kembelangan Group (Mesozoic Passive Margin Post-Rift). The Aifam Group is separated by a regionally continuous boundary on its top contact from the Triassic-Early Jurassic Tipuma Formation, which filled the block-faulted rift valley subbasins of continentally deposited red beds in the breakup stage. Regionally, developed erosion surfaces of the breakup unconformity have separated these red beds from generally transgressive post-breakup deposits of the Jurassic to Cretaceous marine-sediments of the Kembelangan Group. Beach to shallow marine-glauconitic sandstone and shale of the group pass upward into shelf mudstone. Relative sea level fall related to the tectonic stability of the area led to the development of Eocene to Late Miocene platform carbonates of the New Guinea Limestone Supergroup which occurred in the entire island of Papua and the southern of Arafura that overlie these non-carbonate units (Tertiary passive margin). It is separated from the siliciclastic-rich packages by the Tertiary - Pre-Tertiary boundary. The sea level fluctuation within the group was also recorded during the formation of thin, discontinuous sandstone beds/lenses of Sirga Formation and Adi Member of the Oligocene age (Convergence phase). Turbidite sediments of the Miocene Klasafet Formation was deposited in a deep marine environment at the same time as the eruption of magmatic arc (Compressional phase). The mainland area was exposed above sea level at Late Miocene to Pleistocene (Melanesian Orogeny) and terrigenous detritus deposition began to fill in the basin as molasses type deposits with a marine influence in part (Buru and Steenkool Formations). Keywords: Phanerozoic, tectonostratigraphy, continent, phase, Papua

8

Authors: Sufriadin, A. Idrus, S. Pramumijoyo, I W. Warmada, I . Nur, A. Imai, A.M. Imran, and Kaharuddin

Mineralogical characterization of some garnierite samples from Soroako have been conducted using X-ray diffraction, thermal analysis, and infrared spectroscopy methods. XRD patterns reveal the samples mainly containing the mixture of kerolite (talc-like phase) and serpentine with minor smectite, sepiolite, and silica. Thermal analyses of garnierite samples indicated by DTA curves are in good agreement with patterns that have been reported in literature. Three endothermic peaks normally occur in the ranges between 58º C and <800º C illustrating three steps of weight losses: adsorbed, bound, and hydroxyl/crystal water. One additional weight loss in low temperature region of sepiolite is corresponding to the lost of zeolitic water. Infrared spectra appeared in 3800 - 3200 cm-1 region generally exhibit broad absorption bands, indicating low crystallinities of studied samples and can be assigned to the presence of hydroxyl group bonded to octahedral coordination mainly Mg atom. The bands observed at 1660 cm-1, 1639 cm-1, 1637 cm-1, and 1633 cm-1 in all samples indicate water molecules. FTIR spectra displaying the strong bands at 1045 cm-1, 1038 cm-1, and 1036 cm-1 could be related to the presence of Si-O-Si bonds linking to tetrahedral coordination. The strong absorption bands appeared at 511 cm-1, 505 cm-1, 499 cm-1, and 496 cm-1 in respective samples are attributed to divalent cation bonds (e.g. Mg, Ni-O). Both TG/DTA and FTIR seem to be the powerful tool in diagnosing the crystal chemistry of garnierite which is mainly composed of phyllosilicate minerals. Keywords: infrared, thermal analysis, kerolite, sepiolite

9

Authors: Premonowati, B. Prastistho, and I. M. Firdaus

Lithologically, Punung Formation as a paleoreef comprises coral boundstone rhodolith, algal grainstone, algal packstone, algal wackestone, algal floatstone, and algal rudstone. It is dominated by red algae and had formed a fringing reef in a warmly shallow marine environment. They built seven phases of paleoreef complex. Each paleoreef complex has been bounded by a local unconformity that is characterized by caliche. The Jaten Formation has becomes the base of the Punung paleoreef which build up by an angular unconformity contact on uppermost part. It consists of tuffaceous wacky sandstone with silicate cement. The formation as the reef base indicates two factors. The external factor because of the decrease of a volcanic activity and the internal one was caused by the depositional environment of the Jaten Formation becoming shallower. The subsurface runoff systems in many caves (like: Jaran cave and others) have the same southward direction to the dipping direction of algal grainstone lithofacies of Punung Formation. The vertical caves are formed by a jointing system. Keywords: Punung Formation, paleoreef complex, allostratigraphy, caves

12

Authors: S. Maryanto

Limestone diagenetic records of Sentolo Formation have been studied in order to trace the history of the geological processes after the deposition of the rocks. A stratigraphic measure traverse was made in Hargorejo Village, Kulonprogo District, to identify the petrological characteristics of the Sentolo Formation. Limestone samples were taken along this traverse, and were used for a petrographic analysis. The analysis of thirtyeight limestone samples shows that the type is dominated by bioclastic grainstones which have been affected by various kinds of diagenetic processes after the deposition. The diagenetic processes recorded under the polarization microscope include cementation, replacement, bioturbation, micritization, recrystallization, dolomitization, compaction, fracturing, and leaching. Keywords: grainstone, cementation, bioturbation, micritization, compaction

8

Authors: A.K. Permana

The Permian coals of the South Walker Creek area have a moderately to highly developed cleat system. The cleat fractures are well developed in both bright and dull bands, and generally parallel, inclined or perpendicular to the bedding planes of the seam, with the spaces open or filled by mineral matter, such as clay and carbonate minerals. Microfocus X-ray computed tomography (CT) technique was performed to identify cleat characteristics in the coal seams. This technique allows visualizing of microcleat distribution and mineralization in three dimensional images. Cleat mineralization in the coal seam occurs either as single mineral (monomineralic) or intermixed mineral (polymineralic) masses. The cross cutting relationship was shown by X-ray CT scan analysis. The timing of microcleat formation in the coal seam from early to late is carbonate minerals, clay minerals (kaolinite) plus minor high density (rutile or anatase) phases. Thus, a high resolution of microfocus X-ray CT does not only provides a better visualization, but also could identify microcleat orientation, cleat mineralization, and generation of microcleat. Keywords: cleat characteristics, coal, X-ray Computed Tomography, Bowen Basin, Australia

11

Authors: Yudhicara

Identification on tsunami traces had been conducted along the coast of Biak Island, Papua, to recognize the character of tsunami potential in this region, referring to the 1996 tsunami event. This study is to observe the influence of tsunami to the environment and tsunami character itself which can be learned from tsunami deposit. According to the 1996 Biak tsunami run up distributions, it can be observed that the maximum tsunami run up was found at the coast which has an undulating to steep morphology rather than other places which has a relatively flat one. The 1996 Biak Tsunami arrival times were approximately between 10 and 15 minutes, such as at the northern and southern coasts of Biak, except for the western one which is about 1 to 5 minutes. This was influenced by the local bathymetry and another possible source triggering tsunami such as a local submarine landslide. The number of tsunami waves were between 2 and 4, and the second one was usually the highest. Based on a sedimentological analysis, the 1996 tsunami deposit is characterized by the dominant coarse sand, while grain size distribution curve shows a character of transportation process similar to an ordinary beach process dominated by saltation current. The foraminifera fossil content tends to indicate that the tsunami deposit was derived from > 200 m seafloor depth (bathyal zone). Keywords: 1996 Biak tsunami, tsunami deposit, grain size analysis, fossil

15

Authors: S. Poedjoprajitno

The studied area, situated in northeastern part of North Sulawesi Arm, is dominantly occupied by the Mahawu, Linau, Tompusu, and Kasurutan volcanic rocks. Using remote sensing data, such as landsat image, black and white panchromatic aerial photograph, and IFSAR image, morphology-origin unit and morphology lineament can be interpreted. Four morphology-origin units, those are Mahawu Volcano Complex, Intra-montane Plain structure, Linau Volcano Complex, and Lacustrine Plain are recognized. Furthermore, morphological lineament pattern was statistically processed to find out the general stress direction in the area to determine the probability of the structural morphology occurrence in the Mahawu Volcano Complex. The result shows that generally the development pattern of volcanic cones are irregular, except the Mahawu Volcano Complex showing a linear pattern. This lineament pattern is interpreted as a NW - SE fault pattern controlling the rise of magma. At least, two tectonic and two eruption periods occurred regularly at different time from the Quaternary age till the present. Keywords: morphostructure, Mahawu Volcano cone, fault, lineament, magma, North Sulawesi Arm