Indonesian Journal on Geoscience

7

Authors: Y. Ramadhan, K. Channel, and N. R. Herdianita

The researched area, located in the Dieng Plateau, is included into the Holocene Dieng Volcanic Rock Unit. The regional structure in this area is originated from the major caldera with local fault having orientation of SE - NW. Surface manifestations found in the researched area are hot springs located in Bitingan, Sileri, Siglagah, Pulosari, Kaliputih, and Sikidang. Fumaroles occur in Candradimuka and Pagerkandang and mud pools are located in Sileri and Sikidang craters. Temperatures of the hot springs ranges from 43o to 61o C, pH of 6 - 7, and their conductivity are of 38-78 MeV. The type of hotwater is a mixture of bicarbonate, sulfate, and chloride sulfate deriving from condensation of steam. Based on a relative composition of Cl-Li-B, the hot water is originated from four different reservoirs with different rock associations, while their reservoir temperatures vary from 225o to 300o C. Keywords: geochemistry, water type, reservoir, subsurface temperature, Dieng

11

Authors: A.K Permana, C.R Ward, and L.W Gurba

The Permian coals of the South Walker Creek area, with a vitrinite reflectance (Rvmax) of 1.7 to 1.95% (low-volatile bituminous to semi-anthracite), are one of the highest rank coals currently mined in the Bowen Basin for the pulverized coal injection (PCI) market. Studies of petrology of this coal seam have identified that the maceral composition of the coals are dominated by inertinite with lesser vitrinite, and only minor amounts of liptinite. Clay minerals, quartz, and carbonates can be seen under the optical microscope. The mineral matter occurs in association with vitrinite and inertinite macerals as syngenetic and epigenetic mineral phases. The irregular pattern of the vitrinite reflectance profile from the top to the bottom of the seam may represent a response in the organic matter to an uneven heat distribution from such hydrothermal influence. Examination of the maceral and vitrinite reflectance characteristics suggest that the mineralogical variation within the coal seam at South Walker Creek may have been controlled by various geological processes, including sediment input into the peat swamp during deposition, mineralogical changes associated with the rank advance process or metamorphism, and/or hydrothermal effects due to post depositional fluid migration through the coal seam. Keywords: maceral characteristics, vitrinite reflectance, South Walker Creek coals, Bowen Basin

13

Authors: H.D. Tjia

Gunungsewu Karst (also known as Sewu karst in the literature) is synonymous with morphology of a carbonate terrain dominated by hills crowned by accordant-level tops that developed in a humid tropical environment by comparatively more rapid dissolution and denudation. In addition, the hills are sinoid to cone-shaped. Surface drainage is negligible compared to subsurface water flow. Abandoned channel segments and spatial arrangements of karst hills have been found to correspond with fracture patterns that are genetically associated with the regional compression direction of Jawa Island. Images derived from space platforms show many landform patterns that were neither known from ground-based nor from aerial photograph study. Landforms arranged in ring, multi-ring, spiral, polygonal, and long linear to serpentine patterns are common beside the expected depressions of dolines, poljes, and uvalas. The orientations of the long linear ridges appear to change systematically from those near the coast to those located inland. These linear ridges are interpreted as depositional fronts, most likely representing breaker zones. The youngest depositional ridge fronts, located nearest to the present shoreline, are parallel to the geological strike of Jawa Island. Toward the island’s interior, linear depositional fronts deviate in orientation by as much as 40o. This is now interpreted to have resulted from counterclockwise rotation of the Gunungsewu microplate since the late Middle Miocene. Similar CCW rotations are indicated by the paleomagnetic orientations of igneous rocks located farther east in the southern range of the island. Active tectonics is expressed in stage-wise net uplift of Gunungsewu whereas regional tilting appears negligible. Stacked and often paired river terraces (thus suggesting land uplift) have been used to relatively date paleoarcheological finds. Very recent uplift on the coast show up in lazy-V limestone notch profiles, and occasionally by stacked notches. At the Klayar coast, slight northward tilt of a few degrees are expressed as counter-regional inclination of a subrecent abrasion platform. Keywords: Gunungsewu karst, simple to composite landforms, paleo-breaker lines, active tectonics