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