Indonesian Journal on Geoscience

8

Authors: H.G. Hartono dan S. Bronto

Generally, Tertiary volcanisms in the Southern Mountains, Central Jawa were started with the formation of pillow lavas having basalt to basaltic andesite in composition. This initial stage volcanism developed into a construction period of composite volcanoes that consist of alternating basaltic to andesitic lava flows, breccias, and tuffs. The construction period could be followed by a destructive phase, producing pumice-rich pyroclastic breccias, lapillistones, and tuffs of high silica andesite to dacite, or even rhyolite in composition. A stratigraphic measuring section at Bulu area, Sukoharjo Regency, presents an alternating fine-grained andesitic volcaniclastic material and some limestones, with the total thickness is 143.33 m. The thickness of bedded volcaniclastic material tends to be thickening upward from 35 m until 90 m. The grain size of the volcaniclastic material also tends to be coarsening upward from clay size through silt and fine sand to coarse sand and granules. Paleontological analysis on fossils contained in the limestone gives an age of Early Miocene (N7 - N9). The volcaniclastic rocks is conformably overlain by the Mandalika Formation, comprising alternating andesitic breccias, lavas, and tuffs. These data imply that the fine-grained volcaniclastic material is an initial product of the construction period of Gajahdangak Volcano in the area, that formed the Mandalika Formation. This Formation is overlain by the Semilir Formation, composed of pumice-rich pyroclastic breccias and tuffs with dacitic composition. This associated volcanic rock reflects a product of a caldera explosion or a destructive phase. Based on the characteristics of lithology of volcanic products from the initial stage, to a construction and destruction period, and compiled age data, the Southern Mountains represent formal volcanic rock units that are able to be divided into many formations. Keywords: volcaniclastic rocks, composite volcano, caldera explosion, construction period, destruction phase, stratigraphy, Southern Mountains

10

Authors: S. Maryanto

Diagenetic process records could be observed based on the petrography and XRD mineralogy laboratory analyses from thirty-two samples of limestone taken from the Rajamandala Formation at Pawon Cave Section, West Bandung. These laboratory analyses show that the rocks have partially been affected by a dolomitization. The dolomitization recorded at the limestone is a fabric selective dolomitization of the rock matrix, continued to the whole components of the limestone. The dolomite crystals are generally idiotopic to xenotopic mosaic rhombohedral in shapes with fine to moderate crystal sizes. Dolomite mainly consists of magnesium ion initiated from water formation trapped soon after the rock deposited. The middle part of the Rajamandala Formation, which is commonly affected by dolomitization is generally associated with meteoric water dissolution and creates several caves. Keywords: dolomite, fabric selective, petrography, meteoric dissolution, cave

12

Authors: M.H. Hermiyanto and N. Sudini Ningrum

Organic petrologic data of the DOM of Talangakar and Muaraenim Formations show that the organic matter consisting mainly of vitrinite group is essentially composed of telocollinite (1.0 – 70.8 %) and desmocollinite (0.8 – 66.6 %) with minor telinite (0.6 – 9.4 %), detrovitrinite (0.6 – 6.0 %), and corpocollinite (0.6 – 2.0 %). Minor exinite (0.4 – 7.8 %) and inertinite (0.4 – 8.0 %) are also determined. However, mineral matter varies from 0.6 – 99.44 %. Downwards, the increase in vitrinite reflectance (0.33 – 0.48 %) is concomitant with the depth of each formation. Furthermore, based on Rock-eval pyrolysis, TOC value of the Talangakar Formation ranges from 0.09 – 15.38 %, Gumai 0.34 – 0.39 %, Airbenakat 0.32 – 4.82 %, and Muaraenim between 0.08 – 15.22 %. Moreover the PY (Potential Yield) value variation of the Talangakar, Gumai, Airbenakat, and Muaraenim Formations are between 0.04 – 36.61 mg HC/g rock, 0.53 – 0.81 mg HC/g rock, 0.1 – 4.37 mg HC/g rock, and 0.07 – 129.8 mg HC/g rock respectively. Therefore, on the basis of those two parameters, the four formations are included into a gas - oil prone source rock potential. However, the Talangakar and Muaraenim Formations are poor to excellent category, whereas the Air Benakat tends to indicate a poor – fair category and Gumai Formation are only within a poor category. Tmax value of the Talangakar ranges from 237 – 4380 C, Gumai 316 – 3590 C, Airbenakat 398 – 4340 C with exceptions of 4970 C and 5180 C, and Muaraenim Formations 264 – 4250 C. The Talangakar Formation contains kerogen Type II dan III, with the HI (Hydrogen Index) value varies from 45.16 – 365.43. However two samples show value of 0. The organic content of the Gumai and Air Benakat Formations are included into kerogen type III, with HI value ranges from11.87 – 40.82, and 19 – 114 respectively. Moreover the Muaraenim Formation has two category of kerogen type and HI value, those are type III with the HI value of 1 and kerogen type I with HI value of 821.29. The diagram of Tmax vs. HI shows that the organic thermal maturation of the four formations are included into an immature to mature level. Keywords: organic petrology, rock-eval pyrolisis, organic matter, South Sumatra Basin