Indonesian Journal on Geoscience

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

5

Authors: D. Kusnida, Subarsyah, and B. Nirwana

This paper presents the result of marine magnetic survey in Tomini Basin, Central Indonesia. On the basis of marine magnetic modeling, the main structural and geological elements of the basement of Tomini Basin are identified. At the centre of the basin, the up-doming feature points to an elevated magnetic susceptibility value. A geological model indicates that the entire basement of Tomini Basin is characterized by an oceanic-like crust with a basin axis at the centre nearly an east-west direction and suggests a rift-related graben. Keywords: Tomini Basin, magnetic, susceptibility, basement

9

Authors: Hasanuddin Z. Abidin, H. Andreas, I. Meilano, M. Gamal, I. Gumilar, and C.I. Abdullah

The Yogyakarta earthquake of 27 May 2006 occurred at 05:54 WIB with magnitude of 6.4 Mw. It shaked the region of Bantul, Yogyakarta, Sleman and Klaten for about 60 seconds. A week after the earthquake, i.e. 4-8 June 2006, a GPS survey was conducted on 48 GPS points belonging to the 2nd order national cadastral control network located in the earthquake affected region. The 2nd survey was conducted on 21-26 January 2008. The surveys were conducted using 14 dual-frequency geodetic type receivers and the Bernese 5.0 scientific software was used for data processing. The results of GPS surveys show that horizontal components of the co-seismic deformation of earthquake are generally about 10-15 cm or smaller. The GPS-derived displacement vectors and depths of aftershocks suggested the existence of left-lateral fault, with strike and dip angles of about 48o and 89o, located at about 5-10 km east of Opak Fault which is usually drawn along the Opak River. GPS surveys also estimate that horizontal components of the post-seismic deformation of Yogyakarta earthquake are about 0.3 to 9.1 cm between June 2006 and June 2008. While the co-seismic deformation shows the sinistral displacement, the post-seismic deformation indicates the dextral displacement of the eastern region of Opak Fault (Gunung Kidul area) which is relative to a more stable western region. Keywords: earthquake, Yogyakarta, deformation, co-seismic, post-seismic, GPS survey, Opak Fault

14

Authors: Rus Abdissalam, Sutikno Bronto, Agung Har ijoko, and Agus Hendra tno

Karangtengah area in the southeastern part of Wonogiri Regency, is part of the Southern Mountains which is wholly composed of Tertiary volcanic rocks. Nevertheless, the eruptive centre of these volcanic rocks is still unknown. Based on an integrated study that comprises geomorphology, stratigraphy, structural geology, petrology, alteration, and mineralisation the existence of Karangtengah paleovolcano can be identified. The paleovolcano was formed below sea water, basaltic in composition, and it was part of a volcanic island arc during the time. Volcanogenic minerals are found in the central facies containing Fe, Cu, Pb, and Zn. Keywords: paleovolcano, central facies, Wonogiri, Karangtengah, mineralization

8

Authors: H.G. Hartono dan S. Bronto

Generally, Tertiary volcanisms in the Southern Mountains, Central Jawa were started with the formation of pillow lavas having basalt to basaltic andesite in composition. This initial stage volcanism developed into a construction period of composite volcanoes that consist of alternating basaltic to andesitic lava flows, breccias, and tuffs. The construction period could be followed by a destructive phase, producing pumice-rich pyroclastic breccias, lapillistones, and tuffs of high silica andesite to dacite, or even rhyolite in composition. A stratigraphic measuring section at Bulu area, Sukoharjo Regency, presents an alternating fine-grained andesitic volcaniclastic material and some limestones, with the total thickness is 143.33 m. The thickness of bedded volcaniclastic material tends to be thickening upward from 35 m until 90 m. The grain size of the volcaniclastic material also tends to be coarsening upward from clay size through silt and fine sand to coarse sand and granules. Paleontological analysis on fossils contained in the limestone gives an age of Early Miocene (N7 - N9). The volcaniclastic rocks is conformably overlain by the Mandalika Formation, comprising alternating andesitic breccias, lavas, and tuffs. These data imply that the fine-grained volcaniclastic material is an initial product of the construction period of Gajahdangak Volcano in the area, that formed the Mandalika Formation. This Formation is overlain by the Semilir Formation, composed of pumice-rich pyroclastic breccias and tuffs with dacitic composition. This associated volcanic rock reflects a product of a caldera explosion or a destructive phase. Based on the characteristics of lithology of volcanic products from the initial stage, to a construction and destruction period, and compiled age data, the Southern Mountains represent formal volcanic rock units that are able to be divided into many formations. Keywords: volcaniclastic rocks, composite volcano, caldera explosion, construction period, destruction phase, stratigraphy, Southern Mountains

10

Authors: S. Maryanto

Diagenetic process records could be observed based on the petrography and XRD mineralogy laboratory analyses from thirty-two samples of limestone taken from the Rajamandala Formation at Pawon Cave Section, West Bandung. These laboratory analyses show that the rocks have partially been affected by a dolomitization. The dolomitization recorded at the limestone is a fabric selective dolomitization of the rock matrix, continued to the whole components of the limestone. The dolomite crystals are generally idiotopic to xenotopic mosaic rhombohedral in shapes with fine to moderate crystal sizes. Dolomite mainly consists of magnesium ion initiated from water formation trapped soon after the rock deposited. The middle part of the Rajamandala Formation, which is commonly affected by dolomitization is generally associated with meteoric water dissolution and creates several caves. Keywords: dolomite, fabric selective, petrography, meteoric dissolution, cave

12

Authors: M.H. Hermiyanto and N. Sudini Ningrum

Organic petrologic data of the DOM of Talangakar and Muaraenim Formations show that the organic matter consisting mainly of vitrinite group is essentially composed of telocollinite (1.0 – 70.8 %) and desmocollinite (0.8 – 66.6 %) with minor telinite (0.6 – 9.4 %), detrovitrinite (0.6 – 6.0 %), and corpocollinite (0.6 – 2.0 %). Minor exinite (0.4 – 7.8 %) and inertinite (0.4 – 8.0 %) are also determined. However, mineral matter varies from 0.6 – 99.44 %. Downwards, the increase in vitrinite reflectance (0.33 – 0.48 %) is concomitant with the depth of each formation. Furthermore, based on Rock-eval pyrolysis, TOC value of the Talangakar Formation ranges from 0.09 – 15.38 %, Gumai 0.34 – 0.39 %, Airbenakat 0.32 – 4.82 %, and Muaraenim between 0.08 – 15.22 %. Moreover the PY (Potential Yield) value variation of the Talangakar, Gumai, Airbenakat, and Muaraenim Formations are between 0.04 – 36.61 mg HC/g rock, 0.53 – 0.81 mg HC/g rock, 0.1 – 4.37 mg HC/g rock, and 0.07 – 129.8 mg HC/g rock respectively. Therefore, on the basis of those two parameters, the four formations are included into a gas - oil prone source rock potential. However, the Talangakar and Muaraenim Formations are poor to excellent category, whereas the Air Benakat tends to indicate a poor – fair category and Gumai Formation are only within a poor category. Tmax value of the Talangakar ranges from 237 – 4380 C, Gumai 316 – 3590 C, Airbenakat 398 – 4340 C with exceptions of 4970 C and 5180 C, and Muaraenim Formations 264 – 4250 C. The Talangakar Formation contains kerogen Type II dan III, with the HI (Hydrogen Index) value varies from 45.16 – 365.43. However two samples show value of 0. The organic content of the Gumai and Air Benakat Formations are included into kerogen type III, with HI value ranges from11.87 – 40.82, and 19 – 114 respectively. Moreover the Muaraenim Formation has two category of kerogen type and HI value, those are type III with the HI value of 1 and kerogen type I with HI value of 821.29. The diagram of Tmax vs. HI shows that the organic thermal maturation of the four formations are included into an immature to mature level. Keywords: organic petrology, rock-eval pyrolisis, organic matter, South Sumatra Basin