Indonesian Journal on Geoscience

Number 7 vol 1, 2012

8
3-D Imaging of Cleat and Micro-cleat Characteristics, South Walker Creek Coals, Bowen Basin, Australia: Microfocus X-ray Computed Tomography Analysis

Authors: A.K. Permana

Abstract

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Number of views: 422

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
Engineering Geological Investigation of Slow Moving Landslide in Jahiyang Village, Salawu, Tasikmalaya Regency

Authors: D. Sarah and M. R. Daryono

Abstract

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Number of views: 374

An understanding of landslide mechanism is imperative to determine the appropriate mitigation method. The slow moving landslide (creeping) which occurred in Kampung Salawangi, Jahiyang Village, Salawu Subregency, Tasikmalaya had caused economical and environmental losses due to the frequent active movement particularly following rainfall events. Engineering geological investigation and slope stability analysis were carried out in the studied area in order to elucidate the mechanism of the landslide. The engineering geological investigation consists of local topographical mapping, geotechnical drillings, hand borings, cone penetration, and laboratory tests. The slope stability assessment of the recent landslide was conducted by a finite element method. The results of engineering geological data analysis show that the studied area is composed of residual soils of soft to firm sandy silt and loose to compact silty sand and base rock of fresh to weathered volcanic breccias with groundwater level varying between 3 - 16 m. The engineering properties of the residual soils indicate that the sandy silt is of high plasticity and the shear strength properties of the sandy silt and silty sand show low value with effective cohesion of 6.0 - 21.74 kPa and effective friction angle of 12.00 - 25.980. The assessment of slope stability shows that the stability of the studied area is largely influenced by the rise of groundwater level marked by the decrease of safety factor and increase of slope displacement. Keywords: slow moving landslide, residual soil, rise of groundwater level, finite element method

11
Tsunami Characteristics along The Coast of Biak Island based on the 1996 Biak Tsunami Traces

Authors: Yudhicara

Abstract

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Number of views: 389

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

14
Evolution of Rajabasa Volcano in Kalianda Area and Its Vicinity, South Lampung Regency

Authors: S. Bronto, P. Asmoro, G. Hartono, and Sulistiyono

Abstract

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Rajabasa Volcano (± 1281 m) and Lampung Tuff, located in the South Lampung Regency, is the main point in order to understand the evolution of Quaternary volcanism in the area. A remote sensing analysis and field geologic work are the methods of the study. The volcanism began with the construction period of the Pre-Rajabasa composite cone which was followed by the destruction period of the cone to form the Pre-Rajabasa Caldera having ca. 25 km in diameter. The present Rajabasa Volcano, along with cones of flank eruptions and monogenesis, has appeared in the Pre-Rajabasa Caldera depression. Those volcanic activities are considered as the second construction period. During the first and the second construction periods, basaltic to andesitic lava flows, pyroclastic breccias, and tuffs were erupted. The Rajabasa eruption points moved in WNW - ESE direction, which were possibly controlled by a subsurface weak zone. The Pre-Rajabasa Caldera erupted voluminous Lampung Tuffs having rhyolite in composition, and they are considered as a combination of pyroclastic falls, flows, and surges, or pyroclastic density currents. Keywords: evolution, Rajabasa, volcano, Kalianda, Lampung, caldera

15
Morphostructure Control Towards the Development of Mahawu Volcanic Complex, North Sulawesi

Authors: S. Poedjoprajitno

Abstract

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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