This research aimed to reveal the petrogenesis of granitic rocks of Bayah Complex starting from magma differentiation to exposing event, this research also intended to determine the tectonic environment. The methods carried out in this research include field observation, petrographic analysis using polarized light microscopy, and geochemical analysis using X-Ray Fluorescence (XRF) and Inductively Coupled Mass Spectrometry (ICP-MS). Petrographic analysis shows that Bayah granitic rocks are composed of quartz, plagioclase, and K-feldspar while the rest are amphibole, biotite, sericite, chlorite, epidote, and opaque. Based on its major oxide concentrations, Bayah granitic rocks classified as granite and diorite-quartz which have high-K calc-alkaline magma. 4 samples of granitic rocks showed the A/N+K+C > 1 molar ratios belonging to the peraluminous S-type granite index while the remaining 1 sample showed a molar ratio of A/N+ K+C < 1 and A/N+K > 1 which classified as metaluminous I-type granite. Accordingly, Bayah granitic rocks are S-type granite which crystallized from sediment-derived magma, the sediments itself estimated sourced from continental especially Malay Peninsula, Indonesian Tin Island, and Schwaner Mountains. During differentiation, the magma undergone crustal contamination reflected by the increase in both SiO2 0.51 wt% and Al2O3 1.95 wt%, and decrease in Fe2O3 + MgO 0.61 wt% from the pure composition of sediment-derived magma. Furthermore, the occurrence of crustal contamination also recognized from high concentrations of Rb and Ba which indicate the interaction of magma with the materials of continental crust. Regard to the exposing event, Bayah granitic rocks approximated to be exposed due to regional tectonic activity which caused Orogenesa I in the Early Oligocene to the Late Oligocene. Moreover, based on the plot of trace elements especially Rb, Y, Nb, Ta, and Yb on Harker and tectonic discriminant diagrams, Bayah granitic rocks are formed on volcanic-arc active continental margins in accordance with regional tectonic setting.

Abstract. In order to reveal the physical condition of the aquifer, the pumping test using Cooper-Jacob (1946) principle has conducted at well SM5. The observation data of the test then processed to generate various value of hydraulic properties i.e. 3.241x10-4 cm 2 /sec for transmissivity (T), 8.103x10-6 cm/sec for conductivity (K), 0.05055 for storativity (S), and 3.852x10-3 ft -1 for specific storage (Ss). These data show that the aquifer composed of unconsolidated sedimentary rocks ranged from coarse sand to silt. In addition, also performed the feasibility test of groundwater by using Multimeter which produces chemical parameter data. The chemical parameter of eight well samples have average values of 6.62, 766.25 μs/cm and 376.25 mg/L for pH, electric conductivity (EC), and total dissolved solid (TDS) respectively, while physical observation shows no turbidity and odor.

The research aimed to reconstruct the geological structures and tectonics of the Bayah complex area. The structures found that grouped into regional structural patterns used to determine the ages and the events that responsible to its formation. The methods used in this research include field and studio method. Field method carried out to map the outcrops and record geological structures data using geological compass, GPS, tape measurement, and geological hammer, while studio method performed to process and analyze data using software such as Win Tensor, Dips, MapInfo Professional 10.5 and CorelDraw X4. The geological structure of the Bayah has varying patterns and ages. The fracture patterns show N-S and E-W direction which is belong to Sundanese and Java Pattern formed in range of the Early Eocene to Pliocene. While the faults that have direction of SW-NE and E-W are classified into Meratus and Java Pattern. However, metamorphic rock foliations show NW-SE and N-S direction that belonging to the Pre-Tertiary Sumatra Pattern. The three faults of this research are estimated to be formed by the effect of orogenesis that occurring in different events and ages. JSA-014 fault is predicted to form due to orogeny I or orogeny II in the Early Oligocene - Middle Miocene, this fault classified as the 2nd order right lateral wrench fault. JSA-034 fault is formed by orogeny I in Early Oligocene - Middle Miocene, this fault is also classified as the 2nd order right lateral wrench fault. While JSA-080 fault has relatively young age that formed due to orogeny III in the Middle Miocene - Pliocene and belonging to the 3rd order left lateral wrench fault.