Indonesian Journal on Geoscience

4

Authors: Suryantini and H. Wibowo

Analytical Hierarchy Process (AHP) is a Multi Criterion Decision Making (MCDM) Technique. It can handle any complex, multicriterion, and multiperson problems. In AHP, the problems are decomposed into a hierarchically structure and are given the weights according to its importance. Thus, the strength with which one alternative dominates another with respect to a given criterion can be identified. The output is a priority ranking indicating the overall preference for each decision alternative. This paper describes the application of AHP to select a geothermal prospect in Sulawesi Island to be developed in the near future. The alternatives consist of three geothermal prospects. i.e., Suwawa, Pulu, and Marana. Three major criteria are used and applied into those three prospects: Geoscience, Infrastructure, and Social-culture aspects. Under each of these major criteria, there are several subcriteria. Geoscience criterion which consists of Resources, Geothermal System, and Geological Risk Subcriteria is given the highest weight with the assumption that if resources are large and can be developed commercially, then there is no reason not to be exploited; the technology and other infrastructure aspects are no longer an obstacle. The result shows that Suwawa Prospect is the best option to be developed in the near future. The second option or rank is Marana Prospect, and the third is Pulu Prospect. This result is in agreement with the future plan of the development of Sulawesi Island. If the regulation and plan of development were suddenly changed, the goal of this AHP might not be appropriate anymore, and the second or third option might replace the first rank. The benefits of using AHP are (1) the facts and reasons behind the decision are well documented, (2) able to handle quantitative and qualitative inputs, (3) able to accommodate environmental, social and other influences, and (4) able to handle subjective judgments of individuals. Lessons learned from AHP application for geothermal prospect selection could be extended into multi criterion decision making at a group level. Keywords: AHP, MCDM, geothermal prospect, geological risk

6

Authors: I. A. Sadisun, Bandono, H. Shi mada, M. Ichinose, and K. Matsui

In order to gain insights into the detailed physical disintegration characteristics of various types of mudrock, a series of static slaking exposure and immersion tests were carefully carried out in this study. The intent of this paper is to exhaustively describe and discuss the results of the tests, including slaking mechanism, mode and rate/intensity. In general, it can be obviously identified that there are significant different susceptibilities of each mudrock tested to slake-disintegration. These differences can not only be identified from the results of slaking exposure test but also from slaking immersion test. It seems that there is also an agreement in both testing results, which show that the most resistant mudrock to slaking was Ikeshima shales, and was comparatively, followed by Ombilin siltstones, Tanjung Enim mudstones-claystones, and Subang claystones as the worst slaking characteristic. The detailed differences in fundamental characteristics of physical disintegration characteristics will further widely be discussed in this paper. Keywords: disintegration, mudrock, exposure, immersion, slaking test

14

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

The Bunikasih vein system in the Pangalengan district of West Java is a low-sulfidation, adularia sericite epithermal gold deposit. It is hosted by Late Miocene andesitic volcanic and volcanoclastic rocks occurring in the south western margin of Malabar Volcano complex. Gold ore and alteration minerals related to deposition of gold in Bunikasih deposits superimposed on Late Tertiary-Quaternary andesitic formation that were altered and mineralized by some hydrothermal events. The veins consist almost entirely of quartz, with small amounts of adularia, bladed calcite, pyrite, and gold. Gold ore shoots are vertically restricted and are more continuous horizontally. The veins display complex and multi episodic filling with texture characteristics of open space precipitation such us colloform, lattice bladed, crustiform banding, vugs, breccia, and cockade and comb texture. The presence of bladed calcite and silica pseudomorph after bladed calcite suggests that the hydrothermal fluids boiled. In the Cibaliung section of the area, anomalous gold is related to veins trending northeast - southwest, milky quartz with dark grey to black manganese staining is found intermittently for a length of about 800m. The mineralized andesite ore bodies exhibit broad alteration patterns adjacent to mineralization, passing from fresh rock into anargillic, chlorite zone, and then sericite-silica close to mineralization. An argillic assemblage composed of kaolinite with fine-grained pyrite bulb is present in the upper portions and surrounding of the quartz vein system. The veins range from centimeter to meter in size. Of 24 vein samples collected, gold averages up to 0.3 grams per tone ("g/t"), to a high of 24.6 g/t. The Bunikasih epithermal gold deposit was mined by people for more than 10 years, mainly for the gold ore. Keywords: alteration, vein texture, gold mineralization, Bunikasih

8

Authors: K. Budiono, Handoko, U. Hernawan, and Godwin

The Ground Penetrating Radar (GPR) survey has been carried out around the Sidoarjo mud eruption. The aim of the survey is to see how far the development of shallow subsurface geological condition related to the eruptions that frequently occur around the main eruption. Ground Probing Radar is one of the geophysical method which is developed as a tool for a relatively shallow and detailed subsurface geological survey. The use of GPR method is not different with the seismic reflection method. The penetration depth of GPR method depends on electric properties of subsurface geological condition such as electric conductivity and dielectric constant. Both of these properties are related to physical properties of soil or rock such as water content and salinity. The result of the survey shows that at a shallow depth the geological structure such as fold, fault, and joint is frequently seen. Based on these results, the GPR method is very useful to decide the indication of potential area of small mud intrusion, so that the result can be used to help the mitigation plan. Keywords: Ground Probing Radar, shallow geologcal structure, Sidoarjo

10

Authors: H. Th. Verstappen

The horizontal configuration and vertical dimension of the landforms occurring in the tectonically unstable parts of Indonesia were resulted in the first place from plate tectonics. Most of them date from the Quaternary and endogenous forces are ongoing. Three major plates – the northward moving Indo-Australian Plate, the south-eastward moving SE-Asian Plate and the westward moving Pacific Plate - meet at a plate triple-junction situated in the south of New Guinea’s Bird’s Head. The narrow North-Moluccan plate is interposed between the Asia and Pacific. It tapers out northward in the Philippine Mobile Belt and is gradually disappearing. The greatest relief amplitudes occur near the plate boundaries: deep ocean trenches are associated with subduction zones and mountain ranges with collision belts. The landforms of the more stable areas of the plates date back to a more remote past and, where emerged, have a more subdued relief that is in the first place related to the resistance of the rocks to humid tropical weathering Rising mountain ranges and emerging island arcs are subjected to rapid humid-tropical river erosions and mass movements. The erosion products accumulate in adjacent sedimentary basins where their increasing weight causes subsidence by gravity and isostatic compensations. Living and raised coral reefs, volcanoes, and fault scarps are important geomorphic indicators of active plate tectonics. Compartmental faults may strongly affect island arcs stretching perpendicular to the plate movement. This is the case on Java. Transcurrent faults and related pull-apart basins are a leading factor where plates meet at an angle, such as on Sumatra. The most complicated situation exists near the triple-junction and in the Moluccas. Modern research methods, such as GPS measurements of plate movements and absolute dating of volcanic outbursts and raised coral reefs are important tools. The mega-landforms resulting from the collision of India with the Asian continent, around 50.0 my. ago, and the final collision of Australia with the Pacific, about 5.0 my. ago, also had an important impact on geomorphologic processes and the natural environment of SE-Asia through changes of the monsoonal wind system in the region and of the oceanic thermo-haline circulation in eastern Indonesia between the Pacific and the Indian ocean. In addition the landforms of the region were, of course, affected by the Quaternary global climatic fluctuations and sea level changes. Keywords: mega landforms, morphostructures, plate tectonics, Indonesia

19

Authors: C. Ansori

Opal is a beautiful precious gemstone that is equal or more valuable than diamond. In Indonesia, precious opal is found at the Lebak Regency, Province of Banten. Banten’s opal widely has been recognized due to its beautiful opalescence. This paper is a review of the last research; preliminary study of Banten’s opal, characteristics of opal-CT and determining of opal type from geochemical data, added by new data to compile concept and to make mineralization model. In order to fulfill these targets, field geology research and analysis of mineralog/gemology, petrography, X-RD, and major and trace element geochemistry have been done. The Banten’s opal is opal-CT showing opalescence (play of colour), weathering, and leaching silica from volcanic glass by dark grey claystone hosted. Mineralization model is divided into three periods; at Early Pliocene volcanic clastic sediments rich in volcanic glass occured as fluvial sediments. Afterwards, at Late Pliocene - Pleistocene folding, weathering and leaching of silica took place. Intensive jointing, faulting, and folding quickened weathering and leaching processes to formed opal at limb of anticline through Holocene. The prospecting area of Banten’s opal is in tuff unit with intercalation of conglomerate or pumiceous breccia, at limb of anticline. The host rock of opal is dark grey claystone which underlies polimict conglomerate/pebbly sandstone sequence with cross stratification, imbricated, and erossional stucture; more than 8 m deep. Keywords: opal, genetic, mineralization model, Banten

5

Authors: S.Y. Cahyarini

Sr/Ca content ratio in coral is a promising tool for a sea surface temperature reconstruction. In this study, Sr/Ca was analyzed from the top core of fossil coral from Central Mentawai, Indonesia (BLS). The dating of U/Th is used for the age determination of the fossil coral. Based on the U/Th dating, the fossil coral used in this study was deposited in 14th century which approximately has a time range 11 years. The result of this study shows that in the 14th century, the sea surface temperature increased by 3.20 C during 11 years period. Keywords: coral, Sr/Ca, sea surface temperature

8

Authors: N.C.D Aryanto and H. Kurnio

The abundance of sea floor zircon mineral in the studied area was found at the southwest of Kabung Island (SKWL-56) which was accumulated in silty sand sediments at 23 m water depth. Based on the grain mineragraphy analysis, this mineral has various characteristics: rose, white, and brownish yellow colours, elongated prismatic crystals, and translucent to transparent appearances. The distribution profile lines have shown that the distribution of seafloor zircon tends to decrease from Kabung and its surrounding areas to Lemukutan, and from Kabung to the Land of Kalimantan. Considering the abundances, distribution, crystal shape, and mineral association, the zircon mineral in the studied area is a placer deposit that was produced by transport processes characterized by the changing form of crystal. Some samples observed, showed corrosion marks due to a transportation activity, while the transparent level change of these minerals was suggested to be due to much penetration into cavities in the mineral, which lead to the presence of gangue material during the deposition process. Keywords: sea floor zircon, cross line profile, abundant intensity, Singkawang waters

16

Authors: B.H. Harahap

Volcaniclastic rocks exposed in the Tanjung Balit area, West Sumatra, are red in color, loose, soft, and fractured. The rocks are strongly altered to be clay with calcium (CaO) and natrium (Na2O) contents extremely low (0.02 and maximum 0.30wt% respectively), and high loss of ignition (LOI) (4-15wt%). Mobile minor elements such as strontium (Sr), rubidium (Rb), and lanthanum (La) are also changed as their values do not fit to a normal contents range of fresh volcanic rocks in general. Geochemical analyses of the rocks resulted in the following oxide values: silica (SiO2) ranges from 58 – 68wt%, alumina (Al2O3) 15,90 – 21,46wt%, magnesium (MgO) 1, 36 – 1.61wt%, iron oxides (Fe2O3) 4,8-6,9wt%, potassium (K2O) 2,73 - 4,23wt% and titanium (TiO2) 0,60 - 0,70wt%. In general, the element contents especially the immobile ones (Ti, Mg and Fe) show that the rocks were originated from magmatic activity, which is also supported by petrographical characteristics that are still showing felspar and quartz minerals relicts. Based on SiO2 versus K2O diagram, these rocks are classified as high K-calc-alkaline series and belong to an intermediate composition. The rocks of the above criteria such as high alumina, very low magnesium and less than 1% titanium are one of the evidences that they are a magmatic arc product. The trace element of the rocks is summarized on the spider diagram; here their patterns strongly resemble to the typical of magmatic arc as well with enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and heavy REE (HREE). Trace element pattern and REE suggest they were originated from a high K- calc-alkaline magma source. Keywords: volcaniclastics, calc-alkali, petrography, arc magma, Tanjung Balit

7

Authors: M.K. Adisaputra and D. Kusnida

The sample studied was derived from the base of core MD 982156, one of the boreholes obtained during the IMAGES Expedition. The expedition was carried out as a research cooperation between Indonesia and French Governments using Marion Dufresne Research Vessel, belonging to IFRTP (French Institute for Polar Research). Within this sample, the phillipsite minerals were firstly found by the first author. Using SEM (Scanning Electron Microscope) at the magnification of x1,000 for group and up to x20,000 for individual photos, the nannoplanktons were shown to accumulate on the phillipsite minerals as a binding matrix. Different directions and velocity of the bottom current when the phillipsite minerals were originated, might have contributed in binding of the various mineral shapes. The calcareous nannoplankton in the studied area is preserved within the sediments between 3880 m to 3914 m depths below sea level (bsl). This interval possibly located between Lysocline and Carbonate Compensation Depth (CCD), is indicated by the presence of well preserved calcite minerals and calcareous nannoplankton within sediments. Discoaster multiradiatus, the dominant fossil within the sediments in the studied area, is Paleocene in age. Therefore, the binding of nannoplakton accumulation as a matrix of the phillipsite minerals occurred as a postgenetic deposition. Keywords: Paleocene, postgenetic, nannoplankton, phillipsite, Roo Rise, Indian Ocean

16

Authors: F. Hasibuan

The interregional and global correlation of the Triassic biota of Indonesia was based on the review of previous workers and the author himself. Scythian Epoch (Early Triassic) in Timor is subdivided into Early Scythian with Ophiceras demisso, Meekoceras sp., Pseudomonotis subaurita, Gervillia subpannonica, and Myophoria sp., whilst Late Scythian is indicated by the presence of Owenites egrediens and Sibirites sp. The presence of Anisian Stage (Middle Triassic) in Misool is indicated by ammonite Beyrichites and bivalve Daonella lilintana. In Timor, this stage is pointed out by the presence of Joannites cymbiformis, Monophyllites wengensis, Protrachyceras archaelus, Daonella indica, Tracyceras cf. aon, Brochidium timorense, and Lima subpunctatoides. Terebellina mackayi found above Beyrichites-bearing bed in Misool has an age range from Anisian to Ladinian. It is concluded that the boundary between Anisian and Ladinian lies between beds with Beyrichites and Terebellina mackayi. Early Carnian Stage (Late Triassic) in Timor is indicated by the presence of Joanites cymbiformis, Waldhausenites sp., Miltites sp., and Halogyra cipitiensis; whereas Late Carnian is indicated by the presence of Cladicites crassestriatus and Tropites subbulatus. The presence of Halobia verbeeki, Pinacoceras parma, Neobetites sp., Parabetites sp., Malayites sp., Amarassites sp., and Halorites sp., indicates the Early Norian Stage, whilst the presence of Cladiscites tornatus, Cyrtopleurits malayicus, and Trachypleuraspidites sp. implies the Late Norian. The Rhaetian Stage in Timor contains Choristoceras indoaustralicum, whereas in Misool it contains Choristoceras sp. and Cochloceras sp. Keywords: Triassic, biota, macrofossil, Scythyan, Anisian, Carnian, Norian, Rhaetian

17

Authors: D. Setiady

Based on the depth contour, the sea bottom morphology in southern part of the researched area is very steep, while in the northern area it is sloped slightly. It shows that sedimentation process is from south to the north and continues to the west. It is supported by the current process in studied area. Rock slope stability of hilly morphology in the Cimandiri River area is related to weathering, erosion, and transportation process in coastal and nearshore areas. The presence of mineral in the studied area caused by those processes, was accumulated in the mouth of Cimandiri River, coastal, and nearshore areas. Those minerals were deposited in Cibelendung to Karangbeureum nearshore area by the longshore current. Magnetite and pyroxene minerals are dominant along the coastal and offshore areas of Pelabuhan Ratu Bay. The presence of augite and diopside shows that the source rock is basic igneous rocks (basalt), while the presence of hornblende and biotite minerals tend to indicate that the source rock is intermediate igneous rocks (andesite). Kata kunci: augite, diopside, basalt, hornblende, biotite, Pelabuhan Ratu