Indonesian Journal on Geoscience

7

Authors: Ungkap M. Lumumbanbatu dan Suyatman Hidayat

The investigated area is a region prone to liquefaction hazards since it consists of loose, saturated Quaternary deposits. The Quaternary deposits were deposited in several sediment environments such as swamp, estuary, near shore, off shore, and fluvial (flood plain, flood basin and paleo channel). Among those Quaternary deposits, the paleo channel and estuary deposits which mainly consist of fine - to medium - sand have high susceptibility to liquefaction. An earthquake-source zone which is able to trigger earthquake and to cause liquefaction hazards is known to be thrust fault and subduction zones. Earthquakes produced by thrust fault activities are classified as shallow earthquakes while earthquakes produced by a subduction zone are deep earthquakes. The investigated area is divided into low liquefaction susceptibility, moderate susceptibility and high liquefaction susceptibility zones. This division has been made based on physical characteristic of the sand deposits, grain size, thickness of sand and its position to groundwater level as well as the physical character of overburden layer. Keywords: liquefaction susceptibility, earthquake source, Kendal, fault

12

Authors: Rachmat Heryanto

The Bengkulu Basin is known as a typical fore-arc basin, situated in the southwest of Sumatera Island. The basin was occupied by the Eo-Oligocene equivalent Lahat Formation that is unconformably overlain by the Oligocene-Miocene volcanic rock of Hulusimpang Formation. It is then succeeded by siliciclastics and few carbonates of Early-Middle Miocene Seblat Formation. Unconformably overlies the Seblat Formation is siliciclastics of the Middle-Late Miocene Lemau Formation, followed by the Late Miocene-Pliocene Simpangaur Formation. The deposition was ended by a typical volcanic rock of the Bintunan/Ranau Formation. The results of sedimentary rock and organic petrographic, scanning electron microscopic and geochemistry (TOC & rock-eval and gas chromatography) analyses conducted on several selected surface outcrop samples and some oil seepage samples collected during the field work, indicate that hydrocarbon is possibly present in the Bengkulu Basin. A source rock possibly occurs in fine-grained carbonaceous sediments of the Seblat and Lemau Formations. The reservoir rock is possibly the sandstone bed of the Lemau Formation and the sandstone and limestone beds of the Seblat Formation. The caprock is a claystone bed present within the Seblat Formation as well as in the Lemau Formation. The hydrocarbon trap probably occurs as stratigraphic and structural types. Keywords: source rock, reservoir rock, hydrocarbon, Bengkulu Basin, Sumatera

15

Authors: Gendoet Hartono and Sutikno Bronto

Physiographically, the Gunung Sewu Subzone is predominantly composed of limestone of the Wonosari Formation, but in Wediombo area volcanic rocks of the Wuni Formation is exposed. The Wediombo volcanic rocks contain lava flows and volcanic breccias associated with Batur intrusive rock, in which all the rocks have andesitic composition. The problem is whether the Wediombo volcanic rocks originated from far distance area which then was intruded by local magma after its deposition, or it is an association of the Wediombo volcanic and the Batur intrusive rocks which both are the remnants of a paleovolcano in the area. To identify the central facies of the paleovolcano, the geological principle that “the present is the key the past” is used as a geological approach. Wediombo area forms a high landscape showing an elevation of about 280 m above sea level with dips of the outer slopes 20 - 40%. The drainage pattern developing in the area follows the existing circular structure to form subradial - radial patterns. The high landscape shows a circular structure with a diameter of less than 2 km and it opens to the Indian Ocean. Besides the Mount Batur intrusive rock, there are some other dikes which have been already hidrothermally altered, with some mineralization in the circular structure. Meanwhile, the outer slopes are composed of alternating lava flows and fragmental volcanic rocks of 22 - 25o in dip forming a concentric pattern. Petrologically, the Batur intrusive rock is light to dark grey in color, hypocrystalline porphyritic texture, with phenocryst ranges from 1.2 - 2.2 mm in size, subhedral-euhedral crystals, fine vesicular structures. The rock comprises plagioclase, pyroxene, hornblende and opaque minerals. Geochemically, the rock indicates an island arc tholeitic magma (SiO2 = 60.38 – 64.53 wt%, K2O = 0.63 – 0.85 wt%). Those data indicate that the circular structure was the central facies and the outer slope was the proximal facies of the Wediombo paleovolcano in Gunungkidul Yogyakarta. Keywords: Wediombo, Gunungkidul, paleovolcano, central facies, proximal facies