Abstrak

Bangka Island has primary and secondary tin deposit. However, along the history of tin mining has been done the primary tin deposit domination is less than secondary tin deposit. Secondary tin deposit is formed from Granitic S-type source rock weathering and then transported by gravity. After that, Cassiterite mineral (SnO2) accumulates and forms a placer deposit in the study area. The thickness of sediment cover at Toboali Waters is quite diverse, with a range between 5-20 milliseconds, by knowing the quarter sediment thickness using seismic data interpretation results, is expected to give an idea about tin placer deposit potential in Toboali waters. Based on the Isochron map shows patterns of quaternary sediment thickening to the southwest, south and southeast of Bangka Island.Keywords: Quaternary sediment, placer deposit, isochron map, Toboali WatersPulau Bangka memiliki endapan timah primer dan endapan timah sekunder. Namun, sepanjang sejarah penambangan timah yang telah dilakukan endapan timah primer tidak mendominasi jika dibandingkan dengan endapan timah sekunder. Pembentukan endapan timah sekunder berawal dari pelapukan batuan Granit tipe-S yang kemudian mengalami transportasi akibat adanya gravitasi. Setelah itu, mineral kasiterit (SnO2) terakumulasi dan membentuk endapan plaser di daerah penelitian. Ketebalan sedimen penutup di Perairan Toboali berkisar antara 5-20 milidetik, dengan mengetahui ketebalan sedimen kuarter berdasarkan interpretasi data hasil rekaman seismik diharapkan dapat memberikan gambaran tentang potensi endapan timah placer di Perairan Toboali. Berdasarkan atas peta isokron, memperlihatkan bahwa pola penebalan sedimen kuater ke arah baratdaya, selatan dan tenggara Pulau Bangka.Kata kunci : Sedimen kuarter, endapan placer, peta isokron, Perairan Toboali

Tanjung Berikat Coast in Central Bangka, is a part of the Southeast Asian tin belt. We conducted four Ground-Penetrating Radar (GPR) survey lines and 13 hand auger coring to understand sediment deposition and composition. Two similar units were determined from GPR lines BLG 01–BLG 03: Unit A at the top part, reflected by parallel and continuous reflector configuration, weak–strong electromagnetic wave. Underneath Unit A is Unit B, characterized by subparallel configuration, not continuous–chaotic, weak–medium electromagnetic wave. Unit B is absent in BLG 04. We identify another two units from BLG 04 and BLG 03, Unit C, characterized by subparallel reflector configuration, not continuous– chaotic, weak–strong electromagnetic wave. It exhibits distinctive modulating contact with Unit D. Unit D is characterized by chaotic reflector configuration, relatively stronger electromagnetic wave that might be correlated to the granite intrusion Tanjung Klabat. Sediment deposit is composed of fine–coarse sand, consisting mostly of clastic plutonic and clastic biogenic (coral and mollusk fragments), which increase downward. This indicates marine-fluvial influence, which suggests that sea-level changes strongly influence sedimentation process. Unit A from GPR is correlated to these sediment deposits, the other three units might be correlated to weathering of older insitu deposit.

Bangka Island is one of the islands in Indonesia which is traversed by Southeast Asia granitoid belt. This belt stretches from Burma (Myanmar) to Bangka Belitung. This granitoid has potential as a source rock of mineral that carrying tin and rare earth element. At present, mapping of granitoid rocks to the waters area is rarely published, so acoustic basement mapping is necessary to do in order to determine the distribution of granitoids in Toboali waters. The research method used is a single channel seismic with an energy source of 300 joules. The sound source uses a single plate boomer, so it has a high enough resolution but the penetration is not deep enough. Acoustic basement in Toboali waters varies in depth from 15 - 75 ms or getting deeper south. When viewed from the continuity of the acoustic basement, it is estimated that the granitoid is 7 km from the nearest coastline.

Perairan Tanjung Berikat merupakan salah satu perairan di daerah Pulau Bangka yang dilalui oleh jalur granit Asia Tenggara. Keberadaan jalur granit ini berhubungan erat dengan keterdapatan batuan granit yang berperan sebagai batuan sumber pembawa timah baik di daratan dan perairan. Namun demikian, keterdapatan Granit Klabat yang diketahui pada daerah Tanjung Berikat hanya batuan granit yang terlihat di darat saja. Sementara itu, jika melihat keterdapatan granit dalam bentuk singkapan maupun bongkah-bongkah pada wilayah pantai Tanjung Berikat dan sekitarnya, maka diduga adanya sebaran Granit Klabat ini menerus hingga ke arah laut. Metode yang digunakan ialah seismik saluran tunggal (single channel) dengan sumber suara berupa Boomer Single Plate dengan energi sebesar 400 Joule dan frekuensi 300 -600 Hz. Dari rekaman seismik diinterpretasikan menjadi 4 (empat) unit dimana Unit-1, Unit-2, dan Unit-3 yang berada di atas basement akustik diduga berumur Kuarter. Basement akustik yang berumur Trias Awal kemudian diterobos oleh batuan yang diduga sebagai Granit Klabat. Selanjutnya dilakukan konturing menggunakan metode convergence interpolation. Hasil penelitian ini menunjukkan bahwa sebaran batuan Granit Klabat di daerah penelitian terlihat hingga area ±2,5 km dari garis pantai dengan kedalaman berkisar antara 20-80 milisecond pada bagian tengah-timur daerah penelitian dan 40-80 milisecond pada bagian barat daerah penelitian.

Nongsa Isle belongs to Putri Islands in Batam, is the outermost island that need to be protected either from natural hazards and anthropogenic factor. Therefore, this study was conducted by performing Ground Penetrating Radar analysis, in order to understand the geological condition particularly sedimentology and its process. We used Sirveyor 20 GPR equipment type with MLF antenna frequency 40 Mhz and Radan 5 as processing software, which include time zero correction, spatial filter, deconvolution, migration and adjustment of amplitude and signal gain. Data interpretation was conducted based on radar facies methodology that describes georadar image/radargram. The study result showed differences of sedimentary facies based on three differences of radar facies units, with the first layer (unit 1) is the youngest unit has thicknesses ranging from 3.5 – 5 m that characterized by parallel, strong reflector, high amplitude and continuous reflector configurations, unit 2 from 5 – 11 meter of depth, indicates parallel reflector pattern with medium-high amplitude and continuous, and unit 3 which is the oldest unit with thickness until penetration limit (11 – 20 m), characterized by a configuration of sub parallel – hummocky reflectors that are undulating, low-medium amplitude reflectors. Based on radar facies characteristics such as reflector configuration, reflection amplitude, and reflection continuity, the differencies of depositional facies are changes from fluvial – coastal plain.

Bintan Island is one of the areas traversed by the Southeast Asian granitoid belt which is known to have the potential for heavy mineral placer deposits. Due to the dwindling presence of heavy mineral placer deposits in land areas, it is necessary to look for the potential presence of heavy mineral placer deposits in water areas. Searching for placer heavy mineral deposits accomodation in these waters requires subsurface mapping.The method used in this subsurface mapping is a single channel seismic method with a total of 179 lines in the direction of northeast – southwest and west – east. The results of this seismic recording are then interpreted the boundaries of the seismic facies unit and distributed using the kriging method. Furthermore, the thickness calculates by using the assumption velocity 1600 m/s.Based on the facies unit boundaries that have been interpreted, the quaternary sediments that formed in the study area are divided into 2 types of units, namely: Unit 2 which is estimated to be fluvial – transitional sediment, and Unit 1 which is estimated to be transitional sediment – shallow sea. There is also a difference in thickness patterns in these two units, where unit 2 shows a pattern of sediment thickening that resembles a paleochannel trending northeast – southwest, while unit 1 is relatively uniform.From the results of this study, it can be said that the area that has potential for heavy mineral placer  deposits is in the west - center of the southern waters of Bintan Island. Where the potential for heavy mineral placer deposits should be in the paleochannel deposits that are part of Unit 2.

The Georadar (GPR) as a non-destructive technique has been used extensively for geological research. The study was conducted in order to identify coastal sediment characteristics of northern Obi coastal area by performing GPR radar facies analysis. The model of GPR used was Sirveyor 20 with MLF antenna frequency 40 Mhz, penetration depth was 10 meter, and Radan 5 as processing software which include time zero correction, spatial filtering, deconvolution, migration, adjustment amplitude and signal gain. Data interpretation describes georadar image/radargram based on radar facies methodology. The result showed three differences of radar facies unit. The radar facies unit can be divided into 2 facies units in which there are 2 other sub-units, namely unit 1A, 1B, 2A and 2B. Radar facies characterized by medium-high amplitude, continuous parallel-subparallel, medium-weak reflector. At the top part down to 3.75 m is Unit 1A that intersects with 1B. Below 3.75 – 7.75 m depth interval is Unit 2A, and unit 2B is from 7.75-10 meter depth interval. Unit 1 is interpreted as Quartenary alluvium (Qa) and unit B is Woi Formation (Tmpw). Radar facies characteristic indicated the differences of the geological unit.