Indonesian Journal on Geoscience

8

Authors: Untung Sudarsono and Indra Budi Sudjarwo

On 29th May 2006, there was a huge mud outpouring in the Porong Sub-regency, Regency of Sidoarjo, East Jawa and for one year, it had flooded the area of more than 5 km2 including Porong, Tanggulangin and Jabon Sub-regencies. The mud known as Lumpur Sidoarjo. The impact of the mud outpouring was the presence of a subsidence around the main outpouring for the width of 6.3 km2 in ellipse stretching to the north. The subsidence area covered Tanggulangin Sub-regency: Kedungbendo Village, Porong Sub-regency: Siring, Jatirejo, Mindi, and Renokenongo Villages, and Jabon Sub-regency: Pejarakan and Besuki Villages. The rate of the subsidence is approximately 2 cm/day. Keywords: outpouring, Sidoarjo, subsidence, mud, Porong

11

Authors: Gendoet Hartono, Adjat Sudrajat, and Ildrem Syafri

This paper discusses the study on the basalt volcanic rocks and the volcano morphology indicating the existence of an ancient submarine volcano in Tawangsari-Jomboran sub-regency, Sukoharjo-Wonogiri, Central Java. In general, this basalt volcanic rocks were identified as andesite breccia which might be grouped into the Mandalika Formation of Oligosen-Miosen age (Surono et al., 1992). The origin of the Mandalika Formation in relation to the classic sedimentation process and the submarine volcanism is still needed to be evaluated. The present study was based on the detailed descriptions of the rocks both in the field and in the laboratory. The autoclastic basalt outcrops consisting of breccias show the characteristics of the igneous rock fragment component embedded in the groundmass with the same composition, namely igneous rock, dark grey to black in colour; porphyritic texture, rough surface, brecciated; pillow structures, massive, fine vesicularities, amygdaloidal filled with calcite, and radial fractures; calk-alkaline andesite composition ( SiO2 = 54.71% , K2O = 1.15% ). This rock body attains the dimension of 2 - 5 m length, and 40 cm - 1 m in diameter with the direction of the deposition varies following the direction of the eruption source. Brecciated structures on the surface was controlled by the high cooling rate and the low flow, while the interior of the rock is massive because it was not in a direct contact to the cooler mass outside. Autoclastic basalt breccias and or the pillow basalt lava was interpreted to be formed by the undulating low gradient of morphology with the average angle of <10o. On the other hand, the low basaltic magma viscosity produced the effusive eruption related to the formation of the low angle morphology. The distance between the hills generally composed of pillow basalt is between 500 m - 1 km. The typical pillow structure of the igneous rock as described above is interpreted to be the product of the lava flow related to the effusive eruption from a submarine volcano located under or close to the seawater surface. Keywords: submarine volcano, pillow lava, effusive eruption, autoclastic breccias

11

Authors: Priatna and Eka Kadarsetia

Volcanic gases, collected from Gendol and Woro solfatara fields, the summit of Merapi Volcano during 1990-1994,show an increase in chemical composition of H2, CO, CO2, SO2, and HCl prior to the volcanic events, on the contrary to the drastic decreasing water vapour. The carbon/sulfur ratio of the volcanic gases lies between 1.5 and 5.7 which means that they were derived from the fresh magma. The Apparent Equilibrium Temperature (AET) which is calculated from chemical compositions of volcanic gases using reaction of SO2+3H2 = H2S+2H2O showed an increasing value prior to the volcanic events. The Merapi activities lasted during August 1990 to November 1994 showed a significant increase in ratio SO2/H2S prior to the November 1994 pyroclastic flow. The isotopic composition of volcanic gas condensates indicates that water vapour in Gendol is directly derived from the fresh magma. On the other hand, the contamination and cooling by the subsurface water occurred around the Woro field at a shallow part. Keywords: volcanic gas, eruption, Merapi Volcano, 1990-1994

15

Authors: Nana Suwarna and Bambang Hermanto

To asses the characteristics of the Early to Middle Miocene Berau coal in the Berau Basin, leading to interpretation of coal depositional environments, some fresh outcrop and subcrop samples and also drill cores of the coals have been analyzed microscopically. Coal petrographic analysis was performed on twenty four coal samples from the Middle Miocene Lati Formation. Vitrinite, present in a high value, and ranging between 66.2 - 96.2%, is dominated by vitrinite B. On the other hand, inertinite and exinite, showing a similar value, exist in a low to moderate amount. Vitrinite reflectance, present in a low value, varies from 0.40 - 0.58%. Low mineral matter content is dominated by clay minerals (0.4 - 6.6%) with minor pyrite. Transitions from wet and very wet forested swamps to drier conditions with lower tree density are indicated by the higher content of vitrinite B, whilst a reverse trend is indicated by the lower content of vitrinite A. Petrographic indices obtained from facies diagnostic macerals show that an accumulation of the ancient peats under prevailing relatively wet limited influx clastic marsh to very wet forest swamps or moors is considered. The composition of the coal samples supports the interpretation of a system of fluvial to meandering streams in an upper delta plain environment. The original peat-forming vegetation was composed mainly of cellulose rich, shrub-like plants, tree ferns, herbaceous plant communities, with minor amount of trees. Thereby, the organic facies concept is thus applicable in basin studies context and has potential to become an additional tool for depositional environment interpretation. Keywords: Berau Coal Measures, Miocene, East Kalimantan, petrography, depositional environment

9

Authors: Danny Zulkifli Herman

Taliwang conservation forest, West Nusa Tenggara, is particularly covered by volcanic rocks consisting of alternated breccia and tuff. The breccia is composed of andesitic – dioritic clasts and matrix of fine-grained to lapilli tuff which had partially been hidrothermally altered and invaded by gold bearing quartz veins. Outcrops of quartz veins are commonly limonitized, trending nearly east-west and north-south with respectively dip of 70o and 85o. Identification of quartz veins originated from people mining’s waste leads to a prediction that there are quartz stockworks beneath the earth surface (50 – 100 m depth) in the conservation forest area, from which quartz veins penetrated the illite-paragonite-calcite-siderite-nacrite altered country rock. Textures of quartz veins were identified such as comb with some coarse- grained euhedral crystals, sugary/saccharoidal/fine grained crystalline quartz and ghost-bladed. Veins and host rocks generally contain disseminated and spotted pyrites. Evaluation of quartz textures, altered rocks analysis (PIMA method), fluid inclusion studies and chemical analysis (AAS method) of selective altered rocks/quartz vein samples exhibits that the alteration and mineralization processes might occur in an epithermal system, connecting with a change of hydrothermal fluids from near neutral into acid conditions at a temperature ranging from 231 to 185oC. Alteration of illite-paragonite-kalsit-siderite is suggested as a result of reaction between host rock and a near neutral fluid, whilst nacrite (kaolin group) or argillic is a result of reaction between host rock and an acid fluid, within a mixing zone of meteoric fluid and condensed acid gas released during boiling process of hydrothermal fluid in the depth. On the basis of salinity ranging from 0.9 to 2.2 equivalent wt.% NaCl of fluid inclusion,it is predicted that the deposition of gold bearing quartz and associated ore minerals takes place in a depth ranging from 293 to 120 m beneath paleosurface. Keywords: hydrothermal alteration, textures, quartz veins, epithermal gold

13

Authors: Herman Moechtar, Indyo Pratomo, Herman Mulyana, and Soemantri Poedjoprajitno

Studies of stratigraphy on Quaternary deposits in the southwest region of Lake Tondano, Tomohon Regency, South Sulawesi, revealed six facies of deposition environments. These facies consist of Tondano tuff, young volcanic eruption, lake, sand lake, tidal lake, and swamp deposits. Based on the correlation of the lateral and vertical variation of the Quaternary deposits, three unit of the depositional facies were recognized (UFP I-III). Each of the depositional facies unit is characterized by variation of the depositional environment changes which were controlled by tectonics, and was followed by the activity of Sonder main fault. The Sonder main fault activity, which controlled the depositional environment changes was recorded as an UFP. Probably, the character of this stratigraphy could be called as cyclo-tectonostratigraphy. The study was based on analyses of sedimentology and stratigraphy of six borehole information obtained along the E-NW traverse which is approximately parallel to the shorelines of the southwestern part of Lake Tondano. The penetration of the borehead varied from 1.20 to 8.7 m. Keywords: sedimentology, stratigraphy, tectonic, geology structure, Lake Tondano

9

Authors: Danny Zulkifli Herman

Most alluvium with valued heavy minerals originated from regions of continental shelf, from which recycle process for stable minerals play an important role in transportation and final concentration formation of the minerals. Kalimantan Island as part of continental shelf has a great opportunity to provide condition or environment for placer deposition with zircon content due to: comprising particularly basement rock of calc-alkali to alkali series (granite, granodiorite, tonalite and monzonite) which were suggested as the main supplier sources of zircon; having a long period of regional stability as main contribution in continuation processes of weathering, transportation and forming deposition environment of alluvium and located within a tropical climate with high humidity from which mechanical and chemical weathering processes play an important role in releasing zircon from its source rocks. Continental placer deposit of fluvial sub-environment category is suggested to have possibility to form deposition zone lying between the mountains and the seas, reaching over wide areas and forming huge volume reservoirs with content of zircon. Based on zircon identification (associated with gold or diamond) of placer pan concentrates at some areas of West Kalimantan and Central Kalimantan, these have proved how widespread of zircon deposition within fluvial sub-environments in both regions. This important information inspires an opportunity to do exploration of placer zircon deposits within fluvial sub-environments in entirely Kalimantan Island, without neglecting to the transition placer of strandline sub-environment category and marine environment which enable to have an economic deposit of zircon. Keywords: zircon, placer, continental, transition and marine environment

13

Authors: Ony Kurnia Suganda, Dina Anggreni Sarsito, Hasanuddin Zaenal Abidin, dan Aris Kusnandar

Papandayan volcano is located in the southern part of Garut regency, about 70 km southeast of Bandung, West Java. Monitoring the activities of Papandayan volcano has been done using various methods both continously and periodically, one of them is deformation method using repeated GPS (Global Positioning System) survey. GPS survey method is basically used to obtain the pattern and speed of the deformation body of the volcano, both in horizontal and vertical directions and also could be used to determined the location and size of the strain source of deformation based on Mogi model. By describing the shallow seismic activities before and after the eruption, this could show us the correlation of deformation characteristic and its seismic activities. By the result of eight GPS campaigns show that the deformation acceleration is running rapidly, where the fluctuation of shallow seismic activities are directly followed by inflation and deflation of volcano body. Pressure source movement running up and down to southwest-northeast direction. This correlation will lead us to the more comprehensive phenomena of a volcanic eruption, especially in Papandayan volcano. Keywords: GPS survey, strain, correlation, deformation, seismicity

11

Authors: SUTIKNO BRONTO and SRI MULYANINGSIH

Three maars are well identifi ed in the Muria Peninsula, i.e. Bambang Maar, Gunungrowo Maar, and Gembong Maar. The maars were formed by monogenetic volcanic eruptions due to the interaction between heat source (magma), groundwater and calcareous basement rocks. This interaction is able to produce very high pressure of gas and steam causing phreatic explosions, followed by phreatomagmatic- or even magmatic explosions and ended by a lava extrusion. Satellite image analyses have recognized twelve circular features, comprising Bambang Maar, Gunungrowo Maar, and Gembaong Maar. Phisiographically, these maars are characterized by circular depressions which are surrounded by hills that are gently sloping down away from the crater or having a radier pattern morphology. Outcrops and drilling core in the circular areas that are considered as volcanic maars are lava fl ows, pyroclastic breccias, lapillistones, and tuffs, located far away from the eruption centres of Muria and Genuk Volcanoes. One of the circular features, i.e. Jepara Circular Feature, is also supported by negative anomaly (<30 mgal) showing a circular pattern. In the future, a maar volcano could possibly erupt depending on the tectonic reactivity in the region. Keywords: maar volcano, Muria Peninsula, circullar feature, phreatic explosion, phreatomagmatic explosion

12

Authors: HAMDAN Z. ABIDIN and BHAKTI H. HARAHAP

Bonjol gold prospect, known as Old Dutch Gold mine, consists of several ore bodies (Malintang, Balimbing, Lubang Sempit, Lubang Belanda and Lubang Perak). The deposit hosts within the altered volcanic rocks known as Gunung Amas Formation of Early Miocene age (9.3 ± 0.4 - 11.9 ±1.0 Ma). This formation consists of various rock types such as rhyolitic tuff, volcanic breccia, dacitic tuffs and rhyolites. These rocks are moderate to strongly alter. Mineralogy of the deposit consists of gold and silver with minor pyrite, sphalerite and galena. Besides this, hematite, jarosite and manganese are also present as supergene minerals. Ore minerals are found within quartz veins ranging from few centimetres to tens of metres thick. The veins are characterized by crustiform, comb, vuggy, botroyidal, layering and bladed. Quartz is a dominant mineral as hydrothermal alteration in addition to illite, dickite, monmorillonite, kaolinite, chlorite, smectite, natrolite, nontronite, calcite, halloysite, palygorskite, muscovite, sepiolite, analcime, heulandite, clino-chlor, zircon, zoisite, laumontite, alunite, biotite and erionite. The presence of these secondary minerals could be classifi ed into prophylitic, argillic and advanced argillic types. Analytical result of gold–bearing quartz vein indicates higher content of gold (0.3%) and silver (400 ppm). In contrast, the content of sulphide minerals (Cu, Pb, and Zn) is very low (< 100 ppm). Combined geology, mineralogy, textures and alteration minerals, it is concluded that gold deposit in the area shows an indication of a low sulphidation epithermal type within Gunung Amas Formation. Keywords: gold, low sulphidation, epithermal type, gunung amas, Bonjol