Indonesian Journal on Geoscience

9

Authors: Asep Kurnia Permana, Aris Kusworo, and Andi Heri Prastian

The Triassic rocks of the (West) Timor Basin mainly deposited in the marine environment have been identified. The fine-grained clastic and carbonate rocks of this Triassic marine facies are considered to be the most promising source rocks potential in this basin. Geochemical and petrographic data from outcrop samples of the Triassic carbonate of Aitutu Formation are presented in this paper, in terms of the organic maturation, kerogen type, and the origin of the organic matter. Some representative selected samples were subjected to Rock-Eval Pyrolysis, vitrinite reflectance measurement, and thermal alteration index determination as well as bitumen extraction. The extracts were then analyzed by the GC-MS (Gas Chromatography-Mass Spectrometry). The samples were collected from a marine deposit of the Triassic sequence. The TOC (Total Organic Carbon) values of the analyzed samples range between fair and rich level (0.51% - 9.16%, wt.%, TOC), whilst the kerogen is dominated by Type II with minor Type III. The organic matter was considered to be predominantly oil/gas prone with lesser gas prone potential. The thermal maturity assessed from Tmax, TAI, and vitrinite reflectance shows an immature to early peak mature stage. The biomarkers indicate mainly that the organic matter was derived from mixed source rocks facies containing alga debris and higher plant terrestrial origin.

13

Authors: Benyamin Sapiie and Meli Hadiana

Unstable water table would lead to moist condition in the uppermost layer of the ombrotrophic peat, favoring fungi to grow. This is confirmed by the higher abundance of sclerotinite maceral in samples from the upper part of the coal core. Sandbox experiment is one of the types of analogue modeling in geological sciences in which the main purpose is simulating deformation style and structural evolution of the sedimentary basin. Sandbox modeling is one of the effective ways in conducting physically modeling and evaluates complex deformation of sedimentary rocks. The main purpose of this paper is to evaluate structural geometry and deformation history of oblique convergent deformation using of integrated technique of analogue sandbox modeling applying to deformation of Seram Fold-Thrust-Belt (SFTB) in the Seram Island, Eastern Indonesia. Oblique convergent strike-slip deformation has notoriously generated area with structural complex geometry and pattern resulted from role of various local parameters that control stress distributions. Therefore, a special technique is needed for understanding and solving such problem in particular to relate 3D fault geometry and its evolution. The result of four case (Case 1 to 4) modeling setting indicated that two of modeling variables clearly affected in our sandbox modeling results; these are lithological variation (mainly stratigraphy of Seram Island) and pre-existing basement fault geometry (basement configuration). Lithological variation was mainly affected in the total number of faults development. On the other hand, pre-existing basement fault geometry was highly influenced in the end results particularly fault style and pattern as demonstrated in Case 4 modeling. In addition, this study concluded that deformation in the Seram Island is clearly best described using oblique convergent strike-slip (transpression) stress system.

8

Authors: Tubagus Solihuddin

Astract - Rising sea levels since the Last Glacial Maximum (LGM), some ~20,000 years ago, has drowned the Sunda Shelf and generated the complex coastal morphology as seen today. The pattern of drowning of the shelf will be utilized to assess likely timing of shoreline displacements and the duration of shelf exposure during the postglacial sea level rise. From existing sea level records around Sunda Shelf region, “sea level curve” was assembled to reconstruct the shelf drowning events. A five stage drowning model is proposed, including 1) maximum exposure of the shelf at approximately 20,500 years Before Present (y.B.P.), when sea level had fallen to about -118 m below present sea level (bpl.), 2) melt water pulse (MWP) 1A at ~14,000 y.B.P. when sea level rose to about -80 m bpl., 3) melt water pulse (MWP) 1B at ~11,500 y.B.P., when sea level was predicted around -50 m bpl., 4) Early-Holocene at ~9,700 y.B.P, when sea level was predicted at about-30 m bpl, and 5) sea level high stand at ~4,000 y.B.P., when sea level jumped to approx. +5 m above present sea level (apl.). This study shows that the sea level fluctuated by more than 120 m at various times during LGM and Holocene. Also confirmed that sea level curve of Sunda Shelf seems to fit well when combined with sea level curve from Barbados, although the comparison remains controversial until now due to the considerable distinction of tectonic and hydro-isostatic settings. Keywords: Last Glacial Maximum, sea-level changes, transgression, drowning shelf

10

Authors: Komang Anggayana, Basuki Rahmad, Agus Haris Widayat

Abstract - Fifteen samples were taken ply by ply from a 33 m thick drill core of Muara Wahau coal seams for in- terpretation of depositional environments. Generally, lithotype variation in the bottom part of the coal seams has a lower frequency than in the upper part. Petrographical analysis was performed to determine the maceral composition, groundwater index (GWI), and gelification index (GI). The samples from lower sections show much higher GWI-GI values and lower variation frequency than from the upper section. This characteristic is interpreted as the result of development of mesotrophic to ombrotrophic peats during the deposition of lower to upper parts of the section, re- spectively. During the development of the mesotrophic peat, water was more abundant and relatively stable in budget. However, during the development of ombrotrophic peat, water was less abundant and relatively not stable in budget. The latter is related to the water supply depending only on rain, resulted in the more sensitive water table in the om- brotrophic peat. The unstable water table is thought as the reason of higher variation frequency of lithotype, GWI, GI, as well as maceral composition in the upper part of the core. Unstable water table would lead to moist condition in the uppermost layer of the ombrotrophic peat, favoring fungi to grow. This is confirmed by the higher abundance of sclerotinite maceral in samples from the upper part of the coal core. Keywords: coal seam facies, Muara Wahau, Kutai Basin

10

Authors: Arifudin Idrus, Sukmandaru Prihatmoko, Hill. Gendoet Hartono, Fadlin Idrus, Ernowo, Franklin, Moetamar, and Iwan Setiawan

Abstract - This paper discusses characteristics of some key features of the primary Buru gold deposit as a tool for a better understanding of the deposit genesis. Currently, about 105,000 artisanal and small-scale gold miners (ASGM) are operating in two main localities, i.e. Gogorea and Gunung Botak by digging pits/shafts following gold-bearing quartz vein orientation. The gold extraction uses mercury (amalgamation) and cyanide processing. The field study identifies two types/generations of quartz veins namely (1) Early quartz veins which are segmented, sigmoidal, dis- continous, and parallel to the foliation of host rock. The quartz vein is lack of sulfides, weak mineralized, crystalline, relatively clear, and maybe poor in gold, and (2) Quartz veins occurred within a ‘mineralized zone’ of about 100 m in width and ~1,000 m in length. The gold mineralization is strongly overprinted by an argillic alteration zone. The mineralization-alteration zone is probably parallel to the mica schist foliation and strongly controlled by N-S or NE- SW-trending structures. The gold-bearing quartz veins are characterized by banded texture particularly colloform following host rock foliation and sulphide banding, brecciated, and rare bladed-like texture. The alteration types consist of propylitic (chlorite, calcite, sericite), argillic, and carbonation represented by graphite banding and carbon flakes. The ore mineralization is characterized by pyrite, native gold, pyrrhotite, and arsenopyrite. Cinnabar, stibnite, chalcopyrite, galena, and sphalerite are rare or maybe absent. In general, sulphide minerals are rare (<3%). Fifteen rock samples were collected in Wamsaid area for geochemical assaying for Au, Ag, As, Sb, Hg, Cu, Pb, and Zn. Eleven of fifteen samples yielded more than 1.00 g/t Au, in which six of them are in excess of 3.00 g/t Au. It can be noted that all high-grade samples are originally or containing limonitic materials, that suggest the role of supergene enrichment. Interestingly, most of the high-grade samples contain also high grade As (up to 991ppm), Sb (up to 885 ppm), and Hg (up to 75 ppm). Fluid inclusions in both quartz vein types consist of four phases including L-rich, V-rich, L-V-rich, and L1-L2-V (CO )-rich phases. Mineralizing hydrothermal fluid is typified by CO -rich fluid, moderate temperatur of 300 - 400 ºC and a typical low salinity (0.36 to 0.54 wt.% NaCl eq). Based on those key features, gold mineralization in Buru Island meets the characteristics of LS epithermal or orogenic gold deposit types; however, it tends to be fitter with orogenic gold deposit rather than another type. Keywords: gold mineralization, metamorphic rock, Buru Island, Indonesia

13

Authors: Sigit Maryanto

Limestones of the Baturaja Formation occur at Air Rambangnia traverse, South Ogan Komering Ulu, South Sumatra, and they are used as objects for microfacies studies. The microfacies studies are based on a detailed petro- graphic analysis of thirty four limestone samples, taken from the traverse. Four types of the limestones are identified such as wackestone, packstone, grainstone, and floatstone. At least five microfacies form the limestone succesion of the Baturaja Formation. They are interpreted as sedimentary facies of very restricted bay and pond, back reef local slope, slope and shelf edge, winnowed platform edge sand, and reef flank facies. Keywords: Baturaja, limestone, petrography, microfacies, sedimentary facies