Indonesian Journal on Geoscience

10

Authors: AGUNG PRIBADI, EDDY MULYADI, INDYO PRATOMO

Maninjau is a large collapsed caldera that was resulted from a large eruption of silicic pyroclastic material (220-250 km3), and was distributed to more than 75 km away from the source. Field observations have provided a good understanding to the eruptive mechanism of the Maninjau caldera formation, in respect to their stratigraphy, sedimentology and geochronology of the eruptive products. Two formations have been identifi ed, those are: the Maninjau Formation, comprising a sequence of ignimbrite and surge units erupted from the Maninjau Caldera, and the Malalak Formation as a prominent andesitic fall unit likely derived from the Singgalang - Tandikat Volcano that overlies the Maninjau Formation. The variable velocity of the fl ows suggests that the Maninjau eruption initiated by violent. The later phases of the eruption became more violent and associated with caldera collapse. Keywords: caldera collapse, eruptive sequence, ignimbrite, eruptive mechanism, Maninjau

12

Authors: HERMAN MOECHTAR

In the studied area, the sedimentary Quaternary facies consists of fl uvial channel separated by fl oodbasin, swamp, and fl oodplain deposits. Changes in channel style from channel 1, 2, and to 3 are interpreted as the result of a change in the type of river discharge from low to high sinuosity channels. Vertical changes in the character of these fl uvial channels can be related to changes in humidity. The lateral and vertical succession of the fl oodbasin and swamp environments shows evidence of decreasing and increasing of these facies. They are the result of changes in climate. Vertical changes of the Quaternary deposit successions can be related to changes in climate. It is concluded that the subinterval facies I.a to I.c as form of channel 2 and fl oodplain facies (subinterval facies I.b) refl ects a continuosly increase in climate from minimum to climatic maximum. Whereas, from the subinterval facies I.c to II.c during deposition of the subinterval facies II.b by the occurrence of fl oodbasin facies 2, the humid climate decreased from climatic maximum to minimum. Probably, this can be called as astrostratigraphy or orbital stratigraphy. Keywords: facies, depositional environment, climate, stratigraphy

14

Authors: AGNÈS MAZOT, ALAIN BERNARD, and IGAN SUPRIATMAN SUTAWIDJAJA

Papandayan is a strato volcano situated in West Java, Indonesia. After the last magmatic eruption in 1772, only few phreatic explosions have been occurring. At the present time, the activity is centered in the northeast crater manifested by the presence of fumaroles and hot springs. In November 2002 an explosive eruption occurred and ejected ash and altered rocks. Study of the altered rocks revealed that an advanced argillic alteration took place in the hydrothermal system by an interaction between acid fl uids and rocks. Four zones of alteration have been formed as a limited extension along faults or across permeable structures at different levels beneath the active crater of the volcano. Two types of acid fl uids are distinguished in the crater of the Papandayan Volcano: (1) acid sulphate-chloride water with pH values between 1.6 and 4.6, and (2) acid sulphate water with pH values between 1.2 and 2.5. The samples collected after the eruption revealed an increase in the SO4/ Cl and Mg / Cl ratios. This evolution is likely explained by an increase in the neutralization of acid fl uids which tends to show that water-rock interactions were more signifi cant after the eruption. The changes in chemistry observed in 2003 were the consequence of the opening of new fractures where unaltered or less altered volcanic rocks were in contact with the ascending acid water. The high δ34S values (9-17‰) observed in the acid sulphate-chloride water before the November 2002 eruption suggest that dissolved sulphates were mainly formed by the disproportionation of magmatic SO2. On the other hand, the low δ34S values (-0.3-7 ‰) observed in acid sulphate-chloride water sampled after the eruption suggest that the origin of dissolved sulphates for these waters is the surfi cial oxidation of hydrogen sulphide. Keywords: Papandayan Volcano, phreatic eruption, hydrothermal system, fl uid geochemistry, advanced argillic alteration