VOLCANOSTRATIGRAPHY AND PETROGENESIS OF GEDE MOUNTAIN AND ITS SURROUNDING BOGOR AND CIANJUR DISTRICT, WEST JAVA PROVINCE

• Main Author: Winanto, Erza
• Format: Final Project
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/35954
• Institution: Institut Teknologi Bandung

Gede Mountain, located in Bogor and Cianjur Regencies, West Java Province is an active type-A stratovolcano on the Sunda Arc. Gede Mountain together with its surrounding formed a volcanic complex consist of multiple volcanic centers with complex magmatic histories. This research uses several methods which include remote sensing analysis that covers topographic maps and SRTM imagery and field observations to apprehend geomorphology, volcanostratigraphy, and geological structures. Petrographic and geochemical analysis using XRF conducted to account the activity of magmatism and tectonic conditions of Gede Mountain and its surroundings. The geomorphology of the research area can be divided into 16 units, namely: 1) the Gede- Pyroclastic-Flow Ridge Unit, 2) Megamendung-Volcanic Remnant Unit, 3) Gede-Lava Flow Unit, 4) Sukaresmi-Volcano Foothills Unit, 5) Cipanas-Pyroclastic Plain Unit, 6) Masigit-Caldera Ridge Unit, 7) Geulis-Volcanic Remnant Unit, 8) Cugenang-Pyroclastic Plain Unit, 9) Cisarua-Pyroclastic-Flow Ridge Unit, 10) Joglog-Caldera Ridge Unit, 11) Pangrango-Secondary-Volcanic Cone Unit, 12) Gemuruh-Caldera Ridge Unit, 13) Pangrango-Pyroclastic-Flow Ridge Unit, 14) Joglog-Caldera Plain Unit, 15) Gemuruh- Caldera Plain Unit, and 16) Eruption Crater Unit of Gede Mountain. The volcanostratigraphy of the research area was divided into six crowns, namely: 1) Crown Gede Tua, 2) Crown Geulis, 3) Crown Megamendung, 4) Crown Gegerbentang, 6) Crown Masigit and 7) Crown Gede Muda. The rocks found in the research area are basalts, andesites, pyroclastic breccias, lapilli, lapilli-tuffs, tuffs, and lahar breccias, which are then divided into 26 units based on the source, time of formation, and lithology that are distinguished by size, component abundance, and phenocrysts of rocks. The geological structure found in the research area are divided into primary structure, which is in the form of sheeting and columnar joint, and secondary structures in the form of the Normal Fault, namely Gede Tua Normal Fault, Mount Megamendung Normal Fault, Mount Joglog Normal Fault, Mount Gemuruh Normal Fault, Gede Mountain Normal Fault, and Kasur Sinistral Strike-Slip Fault. Petrographic texture analysis, anorthite content in plagioclase minerals, major element variation, and saturation of magma showed that there are eight phases of magmatism activity in which there is magma differentiation in the form of magma mixing, assimilation of surrounding rocks, and crystal fractionation occurred. The phenocryst abundance analysis, the geochemistry of the major and trace elements elucidates the Gede Mountain magma series and its surroundings. Calc-alkali basalt produced from plate margin within subduction zone forming volcanic arcs with island arc characteristics.