Indonesian Journal on Geoscience

9

Authors: Sri Hidayati, Ahmad Basuki, Kristianto, and Iyan Mulyana

Kelud Volcano (+1731 m) in East Java is one of the most active and dangerous volcanoes in Indonesia. A large lake occupies the summit crater. Historical eruptions generally only lasted for a very short time, mostly no longer than a few hours. The outburst is usually accompanied by pyroclastic flows. On August 2007, the activity of the volcano was initiated by the increase of the temperature of lake water and the change of the colour from typical green to yellow. Activities of the volcano are discussed following the swarms of volcano-tectonic (VT) earthquakes on September 10th, September 26th to 29th, and October 24th to November 2nd. On September 26th to 29th, hypocentral distribution of those VT shifted from 5 km deep to just beneath the crater. The highest number of VT earthquakes occurred on November 1st attaining 50 events, then followed by a swarm of B-type events, where the number reached 1437 events in a day. The volcanic activity peaked on November 3rd when seismic records became saturated, which then was preceded by a sharp increase of lake temperature and a sudden deflation of radial tilt. It suggests that the lava extrusion forming a lava dome was taking place. Keywords: Kelud Volcano, crater lake, seismic activity, VT earthquakes, lava dome

13

Authors: R. Heryanto

The Tanjung Formation in Binuang and its surrounding area, situated in the eastern margin of the Barito Basin, overlies the Pre - Tertiary rock basement that consists of metamorphic, igneous, volcanic, and sedimentary rocks. The Tanjung Formation, which are Eocene in age, is unconformably overlain by the Plio - Pleistocene Dahor Formation. The Tanjung Formation consists of coarse-grained sandstone and conglomerate in the lower part, mudstone with interbedded coal seams and sandstone in the middle part, and intercalations of mudstone and fine-grained sandstone showing parallel laminations, wavy - lenticular, and flaser beddings, with some interbedded of medium - to coarse-grained sandstones in the upper part. The rock sequence is overlain conformably by the Claystone Member of the Tanjung Formation. Coal seams in the Tanjung Formation are found to be 50 to 350 cm thick. Megascopically, the coal is black, bright to bright banded, black in streak, conchoidal, and light. Petrographic analysis indicates that the vitrinite, in general, develops within the coal seams in all of the zones, whilst inertinite developes in the Eastern Zone (14.2 - 16.0 %). Vitrinite reflectance of coal samples in the Western Zone varies from 0.43 to 0.47 %, in the Middle Zone is 0.45 %, and in the Eastern Zone is 0.45 to 0.50 %. Rank of the coal seams in all of the zones are subbituminous B, according to the ASTM classification. The depositional environment of the coal bearing sedimentary rocks and coal seams in the Western and Middle Zones was a wet forest swamp facies (backmangrove to fresh water swamp) with upper to lower delta plain environment, in a transgressive condition; whereas in the Eastern Zone was a wet forest swamp (fresh water swamp) in a flood plain fasies, and a transgressive condition. Keywords: coal, maceral, Tanjung Formation, facies, Barito Basin

15

Authors: Febri Hirnawan

Indications exhibiting active tectonic movements in Java occur in many places, characterized by unstable regions, such as active faults, earthquakes, volcanic eruptions, etc. The results of researches conducted at several areas in West and Central Java on tectonic morphometry developments of river basins both in Quaternary deposits and Tertiary sediments exhibit a degree of active tectonic intensity. Such the researches are very important regarding to the spatial development design that is related to a region of active tectonism that should be well understood by planners for decision making mainly through comprehensive approach, in which the phenomena involved explaining the characteristics of the region as part of an active plate margin. Statistic approach as the chosen methodology had been utilized to gain meaningful conclusions through the verification of hypotheses, which are based on valid and reliable tested data obtained from the fields and satellite imagery. Results of regression-correlation tests between azimuths of river segment and lineaments in both chronostratigraphic systems mentioned earlier are significant with several large values of correlation coefficients r of no less than 0.9. On the other hand, results of t-tests are not a significant difference of means of bifurcation ratio (Rb) and drainage density (Dd) from many to river basins. The results of all tests have verified the effect of active tectonic control on the morphometry development process regarding to river segments and density of drainage patterns development in Tertiary and Quaternary deposits. Keywords: tectonic morphometry, degree of tectonic intensity, neotectonic activity, river basin, bifurcation ratio, drainage density

8

Authors: Praptisih, Kamtono, P.S. Putra, and M. Hendrizan

The purpose of the study carried out on the claystone of Batuasih Formation in Sukabumi area is to find out the hidrocarbon potential and its characteristics. The Batuasih Formation, overlying conformably the Walat Formation, consists of claystone, dark gray, shaly, brittle, containing clay ball, calcite veins, with carbonate intercalations. The geochemical analysis conducted on twelve samples shows the TOC values vary between 0.49 – 1.14 % and Tmax of 431 – 434o C. Four samples are categorized to be immature, while the rests are mature. HI values of the Batuasih Formation varying from 77-191 mg HC/TOC indicate that the samples are of C and CD organic facies. Based on those analyses, the source rock is favourably potential to generate a small quantity of hydrocarbon and gas. The hydrocarbon source rock potential level in the investigation area shows a poor to fair organic richness, and its kerogen is included into types II and III. The source rock quality based on the Hydrogen Index (HI) value tends to be a gas prone. Keyword: claystone, source rock, hydrocarbon, Batuasih Formation, Sukabumi

11

Authors: Lambok M. Hutasoit

Significant groundwater level drawdown caused by groundwater abstractions has been reported to occur in Bandung and its surrounding area. One potential method to recover the groundwater condition is artifical recharge, considering high quantity of rainfall in this area. In this research, numerical simulation has been performed in order to predict groundwater condition in the next five years, if: 1) no recovery action is taken (do-nothing) (Scenario 1), and 2) artificial recharge is performed (Scenario 2). Hydrogeological condition reconstruction required for physical model development reveals that the main aquifer in the researched area is Cibeureum Formation, which comprises volcanic fans; the main aquitard is Kosambi Formation, crasisting of lake deposit, whilst the hydrogeologic basement in the research area is the Cikapundung Formation, other Quaternary volcanic rocks, except the Cibeureum Formation, and Tertiary rock units. The recharge area is the areas where the Cibeureum Formation crops out. The result of numerical simulation of Scenario 1 shows that if there is no recovery action taken on the groundwater condition, then in year 2013 the Critical Zone will increase about 116 % and the Damage Zone will increase about 570 %. The result of this scenario also shows that there will be groundwater mining in several areas in the Damage Zone with the total area of 244 km2 or 41 % of the total confined aquifer area. Result of the numerical simulation of Scenario 2 shows that the artificial recharge in the above Critical and Damage Zones will effectively recover groundwater condition in year 2013 which means is, the whole area becomes Safe Zone. The amount of the artificial recharge is about 164 million m3/year and it started from year 2009. The artificial recharge means recharge well, surface reservoir, or recharge ditch where the Cibeureum Formation crops out, or injection well where this formation does not crop out. Keywords: groundwater level, Bandung, artificial recharge, numerical simulation

13

Authors: Igan S. Sutawidjaja

The Batur Caldera, in the northeastern part of Bali Island, is an elliptical collapse structure 13.8 by 10 km in size and another circular composite collapse structure with a diameter of 7.5 km in its centre. Two stages of the collapse were interrupted by silicic andesite lavas and domes. The first collapse was initiated by the eruption of about 84 km3 of the dacitic "Ubud Ignimbrite", about 29,300 years B.P., which caused a steep-walled depression about 1 km deep. The second ignimbrite was erupted from a large crater about the present lake, and it produced about 19 km3 of a similar voluminous dacitic ignimbrite, called the "Gunungkawi Ignimbrite" about 20,150 years B.P. This second eruption triggered a second collapse, which created the central circular caldera, and formed a basin structure. Both the Ubud and Gunungkawi Ignimbrites consist of a similar dacitic composition, white to red (the most abundant nearly 90 %) and dark grey to black dacitic pumice clasts. The large clasts, up to 20 cm in diameter, are in the non-welded ignimbrite, particularly in the upper part of the Gunungkawi Ignimbrite. The intracaldera ignimbrite, called the "Batur Ignimbrite" about 5 km3 in volume is a densely welded ignimbrite and generally shows typical welded features. The ignimbrite comprises at least five different flow units, separated by thin (15 - 40 cm) welded pumiceous airfall deposits, with flattened pumice clasts. Another large eruption occurred about 5,500 years B.P., producing around 0.09 km3 andesitic ignimbrite. This was initiated by phreatomagmatic eruptions, indicated by thick phreatomagmatic and surge deposits, underlying the ignimbrite. The caldera and its vicinity are partly filled, and variably mantled by later eruptive products of dacitic and andesitic phreatomagmatic and airfall deposits. Keywords: 14C dating, ignimbrite, pumice clast, pyroclastic airfall deposit, dacite, phreatomagmatic

10

Authors: S. Poedjoprajitno

Landform evolusion in Manado region has been strongly influenced by tectonic activities beside marine and volcanic ones. Based on surface and subsurface (shallow drilling) geological data, landform evolution of the studied areas can be grouped into four morphological environment units consisting of nine environment sets of morphology, these are: ( a). flood basin, ( b). tidal beach, (c).swamp, (d). river channel, (e).volcanic eruption, (f). flood basin influenced by water, (g).coastal/shore, (h). near shore, and (i). offshore. Collumnar analyses on nine drilling data show that the landform evolution history in the Manado region has undergone more than one tectonic event and three eruptions, proved by the presence of discontinuities on morphological environment units and composition, vertically. Keywords: evolution, morphology, landform, facies, tectonic, volcanism, marine

13

Authors: S. Bronto, S. Mulyaningsih, G. Hartono, and B. Astuti

The Semilir Formation was typically originated from products of a very explosive volcanic activity, i.e. breccias, lapillistones, and tuffs containing abundant pumice. It has a light grey to white colour and high silica andesite to dacite in composition, mainly rich in volcanic glass and quartz. Sedimentary structures of these volcanic rocks are massive, grading, planar bedding, and cross-bedding to antidunes, with grain size varies from ash (≤ 2 mm) to lapilli (2 – 64 mm) to bomb and block (> 64 mm). The formation is widely distributed from the west side (Pleret and Piyungan areas, Bantul Regency, Special Province of Yogyakarta) until Eromoko area in the east (Wonogiri Regency, Jawa Tengah Province). Stratigraphically, the Semilir Formation underlies the Nglanggeran Formation, and overlies the Mandalika Formation in the eastern part and Kebo-Butak Formation in the western part. Geomorphological- and lithological analyses of the Semilir Formation in areas of Parangjoho and Songputri Dams, Eromoko Sub-regency, Wonogiri Regency indicate that the two depressions were alternatively volcanic sources of the Semilir Formation in the Eromoko area. This is proved by the presence of co-ignimbrite breccias(co-ignimbrite lag fall deposits), that descriptively they are polymict breccias. This rock is characterized by a mixing of pumice and various hard rock fragments that primarily are juvenile materials (volcanic blocks, bombs), accessory-, and accidental rock fragments set in pumice-rich volcanic ash and lapilli sizes. The accessory materials came from older volcanic rocks, whereas the accidental ones were originated from basement rocks. During a caldera forming event or a destruction period of an older composite volcanic cone(s), all older rocks resting above the magma chamber were ejected to the surface by a very high magmatic pressure. Since they were heavier than the juvenile material, most accessory and accidental rock fragments were left (lag fall) in caldera rim behind the ash and pumice flow. In the dam areas of Parangjoho and Songputri, the lag fall fragments consisting of andesite, pyroxene andesite, dacite, and pumice, being 10 – 150 cm in diameter are set in pumice-rich lapilli tuffs. Some of the rock fragments are volcanic blocks and bombs, while the older rocks are angular to very angular shape, having prismatic jointing or jigsaw-crack structures. The eruptions in the Parangjoho and Songputri craters were controlled by north-south trending fractures, and they resemble to the Katmaian caldera explosion type. Keywords: Parangjoho, Songputri, Dams, Semilir Formation, co-ignimbrite breccia, volcanic block and bom, pumice

14

Authors: Iskandar Zulkarnain

Five samples consisting of two Permian-Triassic basalts, two Triassic-Jurassic granitic rocks, and a Miocene andesite were collected from the Madina Regency area in North Sumatra that is regionally situated on the West Sumatra Block. Previous authors have proposed three different scenarios for the geological setting of West Sumatra Permian Plutonic-Volcanic Belt, namely an island-arc, subduction related continental margin arc, and continental break-up. Petrographic analysis of the Mesozoic basaltic samples indicates that they are island-arcs in origin; however their trace element spider diagram patterns (Rock/MORB ratio) also show the character of back-arc marginal basin, besides the island-arc. Furthermore, their REE spider diagram patterns (Rock/Chondrite ratio) clearly reveal that they were actually generated in a back-arc marginal basin tectonic setting. Meanwhile, the two Mesozoic granitic rocks and the Miocene andesite reflect the character of an active continental margin. Their spider diagram patterns show a significant enrichment on incompatible elements, usually derived from fluids of the subducted slab beneath the subduction zone. The high enrichment on Th makes their plots on Ta/Yb versus Th/Yb diagram are shifted to outside the active continental margin field. Although the volcanic-plutonic products represent different ages, their La/Ce ratio leads to a probability that they have been derived from the same magma sources. This study offers another different scenario for the geological setting of West Sumatra Permian Plutonic-Volcanic Belt, where the magmatic activities started in a back-arc marginal basin tectonic setting during the Permian-Triassic time and changed to an active continental margin during Triassic to Miocene. The data are collected through petrographic and chemical analyses for major, trace, and REE including literature studies. Keywords: Mesozoic, volcanic-granitic rocks, back-arc marginal basin, active continental margin, magma sources

8

Authors: Kris Budiono

Submarine sliding frequently cause tsunami or a high velocity big wave around the submarine sliding area which enable to reach a long distance. This sliding is not always as a causal factor for hazard and even associate with storm and earthquake. Some of them occur in a narrow area and it’s just formed due to gravitational movement. Different with mechanism of landslide, which is commonly due to water saturated soil, the submarine sliding is a more complex event. The submarine sliding is an important natural process which causes a big volume of sediment mass moves from a shallow area to a much deeper area of seafloor. There are many types and causal factors of seafloor instability, but the sliding terminology is predominantly used for the phenomena. A variety of seafloor materials, environments and sediment masses are some extreme influential factors in creating the types of submarine sliding. The reflector of seismics is usefull to interpret the indications of the occurrence of geological structure and sub-marine slumping In case of the Flores Sea, earthquake is a major causal factor for creating submarine sliding. The bigger magnitude of earthquake, the greater the dimension of submarine sliding. Eventually, it may affect to the more possible occurrence of a potential hazardous tsunami. Keywords: submarine sliding, tsunami, Flores Sea

10

Authors: Amiruddin

An active convergence of continental margin is probably generated in Gondwanaland during Permian to Triassic period which is characterized by the presence of magmatic and volcanic belts and back-arc basins occupied respectively by Permian to Triassic rocks. The magmatic belt is occupied by peraluminous granitic plutons showing characteristics of S- type granite and is considered as tin-bearing granites. The back-arc basins are occupied by the Southern Papua and Galille-Bowen-Gunnedah-Sydney Basins. Those large basins are respectivelly filled by fluvial, fluvio-deltaic to marine Permian-Triassic sediments, which are unconformably overlain by the Jurrassic-Cretaceous marine succession. The paleomagnetic data, confirmed by flora content found in Australia and Papua, indicate that those areas initially belong to the Gondwanaland before part of them were drifted and rotated into the present day position. Tectonically, the presence of those Permian-Triassic magmatic-volcanic belts and back-arc basins in behind, indicates that at the time there were huge compressive activities: convergence of paleo-oceanic Pasific Plate moving westward, collided and subducted into the Southeastern Gondwana Continental Plate, moved relatively eastwards. This phenomenon resembles to the formation of Sumatera Tertiary tectonic zones producing back-arc basins, i.e. South Sumatera, Central, and North Sumatera Basins including the Tertiary Magmatic Arc. Concerning the similarity of Permian-Triassic geological condition of the magmatic arc and back-arc basins in Eastern Indonesia and Eastern Australia including paleoposition, paleotectonic setting, stratigraphic succession, and lithologic composition, it is suggested to carry out an increase in a more intensive tin exploration in the Eastern Indonesia, e.g. Bird Head area and Banggai Sula Island, and also for hydrocarbon target (coal, coalbed methane, oil and gas, and oil shale) in the Southern Papua Basin, East Indonesia. This suggestion is confirmed by cassiterite and hydrocarbon discoveries and exploitation activity in the Eastern Australia and also a new seismic data of the Semai Basin a part of Southern Papua Basin. This seismic record shows a more complete stratigraphic sucession and a number of large structure traps of stratigraphic levels in which the Permian-Triassic units are included within the sequence. Keywords: convergence, Permian-Triassic rocks, Gondwanaland, peraluminous granitic plutons, Eastern Indonesia

18

Authors: Igan S. Sutawidjaja and R. Sukhyar

The Mount Slamet volcanic field in Central Java, Indonesia, contains thirty five cinder cones within an area of 90 sq. km in the east flank of the volcano. The cinder cones occur singly or in small groups, with diameter of the base ranges from 130 - 750 m and the height is around 250 m. Within the volcanic field, the cinder cones are spread over the volcanic area at the distance of 4 to 14 km from the eruption center of the Slamet Volcano. They are concentrated within latitudes 7o11’00” - 7o16’00” S, and longitudes 109o15’00” - 109°18’00” E. The density of the cinder cones is about 1.5 cones/km2. Most of the cinder cones lie on the Tertiary sedimentary rocks along the NW-trending fault system and on radial fractures. The structural pattern may be related to the radial faults in this region. The cone surfaces are commonly blanketed by Slamet air-falls and lava flows. The deposits consist of poorly bedded, very coarse-grained, occasionally overlain by oxidized scoria, and large-sized of ballistic bombs and blocks. There are various kind of volcanic bombs originating from scoriae ballistic rock fragments. The other kind of volcanic bombs are breadcrust bomb, almond seed or contorted shape. All of the cinder cones have undergone degradation, which can be observed from the characters of gully density and surface morphology. By using Porter parameters, Hco is equal to 0.25 Wco, whilst Wcr is equal to 0.40 Wco. The Hco/Wco ratio is higher than Hco = 0.2 Wco reference line. A radiometric dating using K-Ar method carried out on a scoria bomb yields the age of 0.042 + 0.020 Ma. Keywords: Cinder cone, scoriae, ballistic bombs, air-fall, lava flow, radiometric dating.