Indonesian Journal on Geoscience

7

Authors: L. Arifin

A research on sediments estimated to contain biogenic gas at Saronggi coast Sumenep in Madura, East Java, has been done by using the resistivity method and produced 2-D (two dimensional) cross-sectional along four paths. The results of previous measurements indicate that biogenic gas is present within layers of black shale. It is known that the resistivity values of the black shale bearing biogenic gas is between 0.9-1.3 Ohm m. These resistivity values are used as references to determine the black shale layer bearing gas in four resistivity profiles. The depth of the black shale layers are varies: in profile 1 the depth is between 26.3 and 44.6 m, in profile 2 is between 30 and 45 m, in profile 3 is between 44.6 and 64.8 m, and in profile 4 is between 8,75 and 44,6 m. Key word: biogenic gas, resistivity, black shale, Saronggi

13

Authors: E. Sunardi

Palaeomagnetism has a role to make a new stratigraphic nomenclature based on the earth magnetism of a nonperiodic reversal polarity phenomenon. Since this phenomenon has a global impact, it can be used as a potential correlation, known as the Magnetic Polarity Chronostratigraphy Unit. Remanent magnetism direction having reversal polarity against present earth magnetic field has long been known since early palaeomagnetic studies. Modern development of geomagnetic polarity time scale (GPTS), initiated in early 1960’s has followed the advancement of radiometric dating which is more accurate. A palaeomagnetic record obtained in the past of 3 Ma of rock in the Sangiran Area, East Java, has contributed in making a correction of previous magnetostratigraphy researches as well as a new level stratigraphy boundary and a new event of geomagnetic polarity. Based on the correlation record of secular variation in Mojokerto and Sangiran, the magnetostratigraphy of formations of Pleistocene age at Mojokerto has been arranged. The geomagnetic polarity stratigraphy of the past 4 Ma of the rocks in the Bandung Basin and its surrounding areas can be used as a guidance of isochronous position in the stratigraphy correlation. Keywords: paleomagnetism, magnetic polarity chronostratigraphy, isochron, stratigraphic correlation

19

Authors: Z. Zakaria

Scientists, governments, employers, and communities have the same view on landslide disasters, that is the need to cut losses and to avoid loss of men. To deal with landslide disasters, an understanding of an area is required, especially for mitigation ( limitations and stabilization). To handle the slope - prone to landslides, an integrated approach is needed. A stabilization and integrated design (Starlet model) is a proposal in the handling landslide - prone slopes the integration among: (1) mapping system, (2) slope stability analysis, (3) simulation design of stable slope, and (4) guidance with environmental management along with monitoring. In addition, this also requires the participation of scientists, government officials, communities, and employers in facing these avalanche disasters. Citatah landslides formed a system with the largest to the smallest sizes that is a unity. The landslide in the western part of Pasir Pabeasan (Pasir Pabeasan - Citatah) is a complex one with the type of lateral spread. There are other landslides in the surrounding area, i.e: rock fall, rock toppling, slump, debris slide, and slide. The landslide shows the system and dimensions of landslide from the smallest to the largest one. Slopes in 22.29o to 44.28o should be noted because in general they are critical. They need stabilizations. Landslide area mapping system should consider the genetic region. A terrain genetic map will show areas of terrain genetic unit (TGU) which strongly supports the zoning maps of land movements. Based on terrain genetic unit (TGU), the Citatah landslides are common at symbol TGU 2331, 2232, locations between 2231 and 2331, locations between 2232 and 2331, and locations between 2233 and 2331, around the intersection of two faults (Cimandiri thrust fault cut by dextral strike - slip fault). Keywords: model of stabilization, slope design, mitigation, landslides, terrain genetic unit, Citatah Landslide