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Sebaran Granit Klabat Berdasarkan Interpretasi Seismik Saluran Tunggal Di Perairan Utara Tanjung Berikat

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

MULTI-MODEL VARIATION OF THE ENHANCED ASIAN RAINFALL AND CONTINENT-OCEAN THERMAL GRADIENT FROM PRE-INDUSTRIAL TO MID-HOLOCENE

Rainfall over the Asian continent during the mid-Holocene was higher than today as shown by the rainfall proxy records. During the mid-Holocene, increased rainfall over the Asian Continent has been suggested to be associated with the strengthening of the Asian Summer Monsoon (ASM) following a sharper continent-ocean thermal gradient. This study examined multi-model variation between changes of the continent-ocean thermal gradient and the increased rainfall over Asia during the mid-Holocene as compared to the pre-Industrial. We analyzed surface temperature, precipitation, and wind at 850mb from nine Global Climate Models (GCMs) which are all obtained from the database of the Paleoclimate Modeling Intercomparison Project Phase-3 (PMIP3). Multi-model analysis shows that changes in a continent-ocean thermal gradient has a positive correlation with ASM wind. However, a negative correlation occurs between changes in the continent-ocean thermal gradient with Asian rainfall. Models that simulate large changes in the continent-ocean thermal gradient produced the smallest increase in the Asian rainfall and vice versa. Such inverse relation is likely due to the cooling of Indian Ocean SST since its correlation scores with Asian rainfall is much higher than the one with the warming of the Asian continent. Thus, multi-model variation of the increased rainfall over the Asian continent between mid-Holocene and today is mainly related to the multi-model variation of the cooling in the Indian Ocean SST

ANALISIS SEBARAN SEDIMEN BERDASARKAN HUBUNGAN ANTARA KECEPATAN ARUS DENGAN UKURAN BUTIR DI PERAIRAN PANTAI SIGANDU BATANG, PROPINSI JAWA TENGAH

Pantai Sigandu di Kabupaten Batang merupakam pantai yang sangat dinamis dimana proses abrasi, akresi dan suplai sedimen dari beberapa sungai yang bermuara di perairan pantai ini telah mengakibatkan bentuk pantai yang berubah – ubah. Perubahan bentuk pantai ini diikuti dengan pola sebaran sedimen dasar laut. Parameter hidrooseanografi yang berpengaruh secara langsung terhadap proses-proses sebaran sedimen permukaan dasar laut yang terjadi di laut adalah arus, gelombang dan pasang surut. Parameter-parameter ini akan berpengaruh terhadap pergerakan sedimen di laut, sehingga perlu dianalisi hubungan antara aspekhidrooseanografi dan sebaran sedimen permukaan dasar laut di Pantai Sigandu. Tujuan dari makalah ini adalah untuk mengetahui jenis sedimen yang sangat dominan dan pengaruh kecepatan arus terhadap sebaransedimen pada Perairan Pantai Batang. Pengambilan data lapangan dilakukan menggunakan metode kuantitatif dan metode deskriptif yang bersifat eksploratif yang meliputi pengukuran dan pengambilan data pasang surut, pemetaan karakteristik pantai, data sampel permukaan dasar laut, dan kompilasi data arus. Hasil penelitian menunjukkan bahwa ukuran butir sedimen perairan Pantai Batang dominan lanau dengan kandungan presentasinya mencapai 96,4 %, lempung 3,1 %, dan pasir 0,5% dan mendapat nilai korelasi sebesar 0,0109 di mana nilai tersebut sangat kecil korelasinya, dan arus tidak memiliki pengaruh terhadap distribusi besar ukuran butir sedimen. Berdasarkan hasil penelitian dapat disimpulkan bahwa jenis sedimen yang paling dominan adalah lanau dan arus yang memiliki pengaruh terhadap distribusi besar ukuran butir sedimen.Berdasarkan analisa sebaran sedimen selama penelitian diketahui jenis sedimen di Perairan Pantai Sigandu Batang didominasi oleh sedimen lanau dan lanau pasiran dengan pola sebaran sedimennya sejajar garis pantai, dimana sedimen jenis lanau pasiran berada pada perairan yang dangkal sedangkan sedimen jenis lanau dan lempung berada pada perairan yang dalam. Hal ini terjadi karena pengaruh arus laut yang didominasi oleh pasang surut yang semakin lemah

PB RATIO ANALYSIS OF FORAMINIFERA TO OBSERVE PALEOCEANOGRAPHIC CHANGES DURING HOLOCENE IN ARAFURA SEA

Arafura Sea is influenced by several climatic dynamics, it is also a part of the coral triangle that provides most of marine organism diversity of the world. Therefore, this area is an important  waters that impact the climatic dynamic so its paleoceanographic changes need to be understood. For that, we analyzed the foraminiferal PB ratio from marine sediment core ARAFURA-24 with a core length of 179 cm, collected from 47.4 m water depth, combined with that of Aru-07, taken from 276 m water depth (core length 152 cm). Both sediment cores were collected from the Arafura Sea using a gravity corer on board Geomarin III. ARAFURA-24 was sub-sampling in every 20 cm interval, while Aru-07 had been prepared in every 10 cm interval. PB Ratio values from ARAFURA-24 and Aru-07 ranged from 0,56% - 7,43% and from 29,89% to 82,66%, respectively. The age model was reconstructed by 14C radiocarbon dating derived from organic sediment, combined with tie points of PB ratio records. The result indicates that ARAFURA-24 has been sedimented since the last 9.7 kyr BP. PB ratio records reveal three maximum sea level rises, which are before 7.4 kyr BP, at 5.86 kyr, and after 3 kyr BP (approximately at 2 kyr BP at Aru-07). From the age model reconstruction, sedimentation during the last 3 kyr BP was relatively slower than that in the older period. It can be concluded that the foraminiferal PB ratio during Late Holocene was not significantly impacted by sedimentation rate (hence detrital influence), in contrast, during Mid-Holocene detrital influence had more impact on the PB ratio record

SEDIMENTATION RATES AND CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY OF THE NANGGULAN FORMATION, KULON PROGO, INDONESIA

The Nanggulan Formation is the oldest sedimentary rock of Paleogene age that was deposited in the eastern part of the Southern Central Java Basin. A total of 103 nannofossil samples were taken from two traverses in the study area, i.e., the Watupuru and Jetis Routes. Based on the biodatum identified from the nannofossil samples, the biostratigraphy of the rock formation is divided into five zonations, namely the upper part of Zone NP16, Zone NP17, the lower part of Zone NP18, the upper part of Zone NP22, and the lower part of Zone NP23, expanding from 41.1 Ma to 32.2 Ma of age (Middle Eocene to Early Oligocene). Only Zone NP17 is identified as a complete zone, whereas the other four are observed as partial. The fluctuation of global sea level is believed to be an influence on the deposition of the Nanggulan Formation. The sedimentation rate and the change of nannofossil species shows a decrease of oligotrophic (Sphenolithus) and an increase of eutrophic (Reticulofenestra) taxa, especially in small reticulofenestrids (Reticulofenestra spp.). This occurrence suggests a shift in the environmental conditions from an oligotrophic condition around 41.1 Ma to a eutrophic one, particularly after 40.40 Ma. The enhanced eutrophication in the Watupuru and Jetis Routes was caused by an increasing terrigenous input in 40.40 Ma and after, consequently providing nutrient availability on the water surface. This interpretation is supported by the increase in the sedimentation rate when sea level slightly decreased in 40.40 Ma

Litofasies Dan Lingkungan Pengendapan Pada Formasi Elat, Kecamatan Kei Besar, Kabupaten Maluku Tenggara, Maluku

Daerah Kei Besar merupakan bagian timur busur banda tepatnya pada zona lengkungan sistem busur banda bagian timur yang didominasi oleh batuan karbonat. Informasi mengenai fasies dan lingkungan pengendapan Formasi Elat berdasarkan batuan yang tersingkap masih sangat sedikit sehingga diperlukan penelitian yang lebih akurat. Metode penelitian yang digunakan adalah observasi 31 stasiun pengamatan dan pengukuran penampang stratigrafi pada 5 lintasan penelitian. Hasil penelitian menunjukkan Formasi Elat didominasi oleh litologi kalkarenit dan batulempung.yang digolongkan menjadi 6 litofasies yaitu alternating calcarenite & shale, amalgamated calcarenite, blocky calcarenite, structurless mud, slump deposit dan coarsening upward calcarenite. Asosiasi fasies berdasarkan 5 lintasan penelitian menunjukkan lingkungan pengendapan daerah penelitian berdasarkan Wilson (1975) adalah Foreslope, Deep Shelf Margin dan Open Sea Shelf. Sedangkan berdasarkan Christopher G St C Kendall (2012) lingkungan pengendapan daerah penelitian adalah Lower Slope, Middle Fan dan Lower Fan. Sejarah pengendapan Formasi Elat dimulai dengan mengendapnya litofasies alternating calcarenite & shale pada Channel Complex, setelah itu terbentuk litofasies amalgamated calcarenite, blocky calcarenite dan coarsening upward calcarenite pada lingkungan Middle Fan dan pada akhirnya pada Lower Fan terbentuknya litofasies structurless mud dan alternating calcarenite & shale. Proses sedimentasi Formasi Elat termasuk dalam siklus sedimentasi yang berada pada fase low stand system tract

MARINE GEOMAGNETIC ANOMALY BELT AND ITS RELATIONSHIP TO THE REMNANT ARCS IN THE NORTHWESTERN JAVA SEA, INDONESIA

The continuous marine geomagneticsurvey within a timeintervalof1-secondsampling and a precision of 0.1 nT was conducted in the northwestern Java Sea to identify and interpret the general trend of total marine magnetic anomalies and the possibility related to thegeological resourcepotential. These magnetic data were then processed according to the formula corrected and applied to marine magnetic data. The total marine magnetic anomalies of the northwestern Java Sea indicate a well-defined lateral trend belt of anomaly contours. Anomalies are divided into four delineation zones: Zones I, II, III, and IV. A preliminary analysis of these anomalies led to the interpretation, reflecting the residual of a slightly east-west trending geological body underneath.Examination of magnetic anomalies suggests Zone I and IV characterize a basinalarea, Zone II depicts a granitic belt, and Zone III describes a Cretaceousmagmatic arc system in the east that extends from Middle Java across the Java SeathroughSouthern Kalimantan. These magnetic anomalies seem to coincide with the free air gravity anomalies data derived from TOPEX satellite data. 

ASH LAYERS FROM SOUTH ANDAMAN SEA: PROBABLY SOURCED FROM TOBA CALDERA

Deep Sea sediment core PC-1 from the South Andaman Sea (7 ̊19.85' N; 94 ̊ 39.26' E; in East Andaman Basin) below the water depth of 3144 m contain discrete ash layers at various depths. According to morphological study, these ash layers contain glass shards of different varieties i.e. Type-I, Type-II, Type-III, Type-IV and Type-V and it is comparable to glass shards of Toba volcanic reported from other parts of the world. This observation is also supported on the basis of relative biostratigraphic datum observed in the core PC-1. The Layer-A (56 cm thick) at 210 cm bsf is just above the biostratigraphic datum of ca. 0.12 Ma, correspond to Youngest Toba Tuff (YTT), followed by Layer-C belongs to Middle Toba Tuff (MTT) and Layer-D inferred as Oldest Toba Tuff (OTT). This interpretation is further supported by the geochemical data obtained from the EDX analysis, which suggest high silica and alkali contents of rhyolitic composition. Hence, geochemical composition, morphology and biostratigraphic data of these discrete tephra layers show identical characteristics to the products of Toba eruptions, including YTT, MTT and OTT

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Front Cover Vol. 36 No.