struktur pada batuan serpentinit di sepanjang sempadan jalur tengah dan jalur barat semenanjung...

Convention Bandung 2004 (CB2004) The 33rd Annual Convention & Exhibition 2004

Indonesian Association of Geologist Horizon Hotel, 29-30 Nov, 1 Oct 2004, Bandung

Geo-Science 1

STRUKTUR PADA BATUAN SERPENTINIT DI SEPANJANG SEMPADAN JALUR

TENGAH DAN JALUR BARAT SEMENANJUNG MALAYSIA

Jatmika Setiawan1), Ibrahim Adullah2), Zaiton Harun2)

1) UPN “Veteran Yogyakarta 2) Universitas Kebangsaan Malaysia

Abstrak Di Sepanjang Sempadan (batas) Jalur Tengah dan Jalur Barat Semenanjung Malaysia, yang dikenali sebagai garisan Bentong-Raub atau sutura Bentong, terdapat beberapa singkapan jasad serpentinit yang berjajaran pada arah antara N340oE-N350oE. Singkapan-singkapan tersebut dipercayai mewakili batuan ultramafik yang dahulunya merejah batuan Devon Bawah dan kemudiannya telah berubah menjadi serpentinit. Batuan serpentinit tersebut tersingkap di Bukit Rokan Barat, Kampung Selaru, Petasih (Negeri Sembilan); Cheruh-Bentong (Negeri Pahang) dan Kelantan Barat (Negeri Kelantan). Seterusnya kajian struktur di batuan serpentinit dapat digunakan untuk menjelaskan sejarah canggaan (tektonik) yang telah berlaku di sepanjang sempadan Jalur Barat dan Jalur Tengah Semenanjung Malaysia (Rajah 1). Hasil kajian struktur di batuan serpentinit dapat digunakan untuk menjelaskan sejarah canggaan (deformasi) yang telah berlaku di sepanjang sempadan Jalur Tengah dan Jalur Barat Semenanjung Malaysia. Observasi lapangan pada singkapan-singkapan serpentinit dijumpai struktur foliasi, telerang, jalur ricih dan sesar seperti ditunjukkan dalam Tabel 1. Foliasi umumnya berjurus utara-baratlaut (UBL), kecuali di Petasih dan Cheruh yang berjurus barat-baratlaut (BBL). Deformasi pertama berarah arah NNW hingga NNE, menghasilkan sesar songsang (naik) geser kanan (SSGK) dengan jurus sesar SE-NW dan kemiringan antara 70o-80o. Deformasi kedua dengan tegasan berarah NE-SW, menghasilkan sesar SSGK berjurus antara N340oE-N350oE atau N160oE-N170oE dan kemiringan antara 70o-85o. Deformasi ketiga dengan tegasan kompresi berarah NEE-SWW, menghasilkan sesar mendatar kanan normal (SMKN) dengan jurus dan kemiringan N20oE/70o atau N130oE/70o. Deformasi terakhir dengan tegasan timurlaut-baratdaya (TL-BD), menghasilkan sesar songsang geser kiri (SSGK) dengan Jurus dan kemiringan N270oE/85o. Hasil kajian petrografi dari semua sayatan nipis mendapati bahawa batuan asal ultramefik telah terubah menjadi mineral serpentin. Kecuali di daerah Cheruh-Bentong masih terlihat batuan asalan serpentinit adalah batuan igneus Peridotit.



Geo-Science 2

ICHNOLOGICAL CHARACTERISTICS IN THE MODERN MAHAKAM DELTA,

EAST KALIMANTAN

Ery Arifullah 1), Andang Bachtiar 2), Djuhaeni 3) 1) Master Student, Department of Geology, ITB:

2) GDA Consultant 3) Department of Geology, ITB

Abstract Detailed analysis of Modern Mahakam Delta sediments concentrated on identifying chnological and sedimentological characteristics of five deltaic sub-environments. These include: 1) distributary channel lower delta plain in the southern part, which are tipically displaying deep penetrating Skolithos, Psilonichnus, Planolites, Arenicolites, Ophiomorpha, escape burrow, resting burrow , 2) fluvio-estuarine channel lower delta plain in the northern part which are tipically displaying Arenicolites, Skolithos, Teichichnus, Planolites, Siponichnus, Chondrites, Ophiomorpha, Monocraterion dan Gyrolithes, 3) interdistributary area which are tipically displaying Arenicolites, Conichnus, Ophiomorpha, Scaubcylindrichnus, Planolites, Rosselia, Teichichnus dan escape burrow, 5) mouth bar sediments are displaying Ophiomorpha, Skolithos, Planolites dan Rhizocorallium. Over all the sediments are bioturbated in maximum in 40 % with ichnological variation of diversity level between northern and southern part of Mahakam Delta. Ichnological research in the Modern Mahakam Delta potentially improve our understanding of deltaic facies sedimentologi. Our findings suggest that ichnological types and distribution is controlled by sedimentological process, sedimentary textures and salinity. Abstrak Analisis detil pada sedimen-sedimen Delta Mahakam Modern dikhususkan pada identifikasi karakteristik ichnologi dan sedimentologi pada lima sub lingkungan pengendapan. Sub lingkungan itu meliputi: 1) distributary channel lower delta plain bagian selatan yang secara tipikal dicirikan dengan kehadiran Skolithos dengan penetrasi yang dalam, Psilonichnus, Planolites, Arenicolites, Ophiomorpha, escape burrow, resting burrow , 2) fluvio-estuarine channel lower delta plain bagian utara secara tipikal dicirikan dengan kehadiran Arenicolites, Skolithos, Teichichnus, Planolites, Siponichnus, Chondrites, Ophiomorpha, Monocraterion dan Gyrolithes, 3) interdistributary dicirikan dengan kehadiran Arenicolites, Conichnus, Ophiomorpha, Scaubcylindrichnus, Planolites, Rosselia, Teichichnus dan escape burrow, 5) mouth bar dicirikan dengan



Geo-Science 3

kehadiran Ophiomorpha, Skolithos, Planolites dan Rhizocorallium. Secara umum sedimen terbioturbasi maksimum 40 % dengan variasi tingkat keragaman ichnologi yang berbeda antara bagian utara dan selatan Delta Mahakam. Penelitian ichnologi di Delta Mahakam Modern memungkinkan bagi kita untuk lebih memahami fasies sedimentologi delta. Penilaian kami menunjukkan bahwa distribusi dan tipe ichnologi dikontrol oleh proses sedimentologi, tekstur sedimen dan salinitas.



Geo-Science 4

THE TIDAL FLAT ENVIRONMENT OF THE

NGRAYONG SANDSTONE IN THE WESTERN PART OF THE MADURA

ISLAND

MAC. Endharto Geological Research and Development Centre, GRDC

Abstract This study attempts to analyse the depositional system of the Ngrayong Sandstone into the basin pattern with using the basin analysis approached, concerning the hydrocarbon explorations in Madura Island. The study area is divided into three main zones which initiated with the western segment of the island. There are some geological key traverses e.g., Gunung Geger, Gujug Laut, Waterfall, Gujug village, Tanah Merah, Torjun-Somber, Sapulu and Kombangan. Along the Gunung Geger-Gujug Laut-Water Fall section (south margin-western portion) is remarked by the appearance of the depositional classic progradation which characterized by the sedimentary structures (i.e. lenticular bedding, flaser bedding etc), implicate that the Ngrayong Sandstone was deposited within the tidal sand flat (open coast) system subdivision, from supratidal-salt mars to the shallow portion of the subtidal. environment. At the open sand quarry outcrop near Gujug Laut Village, well exposed of the basal portion (2m thick) of the Ngrayong Sandstone sequence, dark grey silty sand and shale demonstrate ripple cross lamination and wavy bedding underlie bioclastic lithic arenite contains tabular cross bedding indicate the intertidal sand flat, lower portion of tidal flat under condition low current speed. Tabular cross bedding occurs in the bioclastic lithic arenite interpreted as the sand flat in the headward portion of the macrotidal estuaries. Shelf transgression of marine condition is shown by the occurrences of the sandy limestone, upper part of the sequence. The presence of the hexagonal gypsum crystal formed within the calystone indicates the evaporation process in subaquaceous of supratidal zone. Paleocurrents are measured from planar and trough cross beddings display southwest direction (200o-190o). Based on the sedimentary structures and trace fossils found in the Ngrayong Sandstone unit, along the Torjun-Somber section.



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Along the Sapulu traverses (northern part of the west segment), the sand unit of the Ngrayong Sandstone was precipitated and more influenced by fair-weather wave base to outer shelf-transition zones, the lithology is characterised by the presences of the coarsening upward of the sandy limestone and alternating mudstone and shalestone. Change gradually from low to high energy to the landward is also present in this area, which is evidenced by the occurrences of the packstone. The Kombangan traverse exhibits subtidal type of sediment marked by trace fossil of Cruziana (Planolites.). Marine condition as shallow lower shoreface to deep upper shoreface usually associates with Skolithos and Cruziana.

Abstrak Sistim pengendapan Batupasir Ngrayong ke dalam tataan pola cekungan dengan menggunakan pendekatan analisa cekungan, kaitannya dengan esplorasi hydrokarbon di Pulau Madura. Daerah studi dibagi menjadi 3 (tiga) zona segmen regional, dimulai dengan segmen barat P. Madura. Lintasan geologi kunci di segmen barat ini diwakili oleh beberapa lintasan yaitu Gunung Geger, Gujug Laut, Air-terjun, Desa Gujug, Tanah Merah, Torjun – Somber, Sapulu dan Kombangan. Di lintasan Gunung Geger-Gujug-Airterjun (zona selatan-segmen barat) dijumpai singkapan tipe endapan progradasi klasik yang dicirikan oleh struktur sedimen (a.l., lapisan lenticular, flasser dll), mengimplikasikan Batupasir Ngrayong ini diendapkan di lingkungan dataran pasang naik/surut dan sekitar daerah gundukan daerah pasang naik/surut, zona antar-pasang, dengan pendangkalan ke arah zona supratidal-rawa-rawa payau. Di Desa Gujug tempat penambangan pasir terbuka tersingkap bagian bawah dari runtunan Batupasir Ngrayong mengindikasikan lingkungan/zona dataran pasir antar-pasang (the intertidal sand flats) di bawah kondisi kecepatan arus rendah. Kondisi ini biasanya menghasilkan laminasi gelembur gelombang. Pendangkalan ke atas berasosiasi dengan lapisan tipis lanau akan menghasilkan lapisan bergelombang pasir lanauan dan lanau. Struktur silang-siur bersusun (tabular cross bedding) terdapat pada batupasir bioklastik litik mengindikasikan satuan ini diendapkan di daerah dataran pasir, tepatnya dibagian hulu estuary makro-tidal. Kondisi laut (trangressi) ditunjukkan oleh hadirnya batugamping klastik. Evaporasi/penguapan juga terjadi di daerah ini yang merupakan zona supratidal dengan indikasi banyaknya kandungan kristal gypsum-hexagonal di dalam satuan batuan lempung. Struktur sedimen silang-siur planar dan mangkok pada batupasir kuarsa menunjukkan arah arus purba ke arah barat-daya (200o-190o). Berdasarkan aspek struktur sedimen dan jejak-fosil di dalam satuan Batuanpasir Ngrayong, untuk lintasan Torjun-Somber, diendapkan di daerah dataran pasir dan asosiasinya (sand flat-mix flat) dan



Geo-Science 6

zona antar pasang naik/surut (intertidal) merupakan bagian atas lingkungan muka-pantai (upper shoreface). Di lintasan Sapulu (zona utara-segmen barat) lebih didominasi oleh tipe endapan yang dipengaruhi oleh bagian dasar gelombang sedang (fairweather wave base) sampai ke zona lepas pantai-transisi dicirikan oleh batugamping pasiran mengasar ke atas dan perselingan antara batulumpur dan lanau. Di lintasan ini juga dijumpai adanya perubahan secara gradasi dari energi rendah ke energi tinggi ke arah daratan, yang dicirikan oleh endapan batugamping packstone pasiran. Lintasan Kombangan merupakan endapan bagian dari pasang naik/surut (subtidal) dicirikan oleh hadirnya jejak fosil Cruziana (Planolites). Kondisi lingkungan laut seperti bagian bawah muka pantai (lower shoreface) sampai ke bagian atas muka pantai (upper shoreface), kondisi ini biasanya berasosiasi dengan adanya jejak fosil jenis Skolithos dan Cruziana.



Geo-Science 7

RECENT OSTRACODA (MICROCRUSTACEA) FROM A BANDA -

TO - TIMOR SEA DEPTH TRAVERSE: IMPLICATIONS FOR PALEOBATHYMETRIC

STUDIES

K.T. Dewi1, P.Frenzel 2, A. Muller3, D. van Harten4 1. Marine Geological Institute, Bandung, INDONESIA

2. Marine Biology Department, University of Rostock, Germany 3. UFZ Centre for Environmental Research, Magdeburg, GERMANY

4. Department of Geology, Free University, Amsterdam, The NETHERLANDS

Abstract Twenty-three box-core top samples from a transect off Timor Island between the Banda and Timor Seas have been used for an ostracod study. These box-core samples were collected during the SNELLIUS EXPEDITION II (1984) in water depths between 150 and 3070m. More than one hundred species have been identified. Generally, the number of species is diverse (2-143 species) and the number of specimen is between 2 and 4942 individuals per 100 cc of sediment sample. The distribution of ostracod species clearly correlates with water depth. Abundance and diversity of ostracod fauna tend to decrease with increasing water depth, except for one anomaly in station G5-6-132. This sample was collected in the deepest part (3070m) of the study area and shows moderate abundance of ostracods. The unusually high number of ostracod species in the Lower Abyssal zone is mainly caused by the high occurrence of shallow water ostracods. It may be explained by the influence of deep-water masses that cause downslope contamination. The highest numbers of individuals (>1000) and species (>100) occur in stations with water depth less than 210 m. This zone of water depth is dominated by Paracytheridea, Hemiparacytheridea, and various species of Bairdiidae.. The bathyal zone is characterized by Cytheropteron, Saida, Bradleya; Pelecocythere, Henryhowella, Krithe and Parakrithe The results of this study clearly show that ostracods are an important component of marine sediments regarding the identification of paleoenvironments, and particularly paleobathymetry. They provide a new and promising tool for future studies of paleobathymetry based on ostracod distribution at continental margins.



Geo-Science 8

STUDI FASIES BATUGAMPING DI DAERAH TASIKMALAYA DAN

SEKITARNYA, JAWA BARAT

Praptisih, Safei Siregar dan Kamtono Puslit Geoteknologi LIPI

Abstract The limestone which exposed in Tasikmalaya area is the part of Kalipucang Formation and Sukaraja Member, Bentang Formation. In general are brownish yellow, white and gray in color. Some samples from the limestone of this formation have been taken and analysed in the laboratory. Petrographic analyses showed that the limestone can be devided in two type facies, these are coral boundstone facies and packstone facies. The coral boundstone faciesit self is separated as platy coral subfacies and branching coral subfacies. Based on lithologic characteristic and fossil content the carbonate rock in this area is interpreted as “Patch Reef”. Abstrak Batugamping yang terletak di daerah Tasikmalaya merupakan bagian dari Formasi Kalipucang dan Anggota Sukaraja Formasi Bentang. Pada umumnya berwarna kuning kecoklatan, putih dan abu-abu. Sejumlah conto batugamping ini telah diambil dan dianalisis di laboratorium. Analisis petrografi menunjukkan bahwa batugamping di daerah ini dapat dibagi menjadi 2 type fasies yaitu Coral Boundstone Facies dan Packstone Facies. Dimana coral Boundstone Facies dibagi menjadi 2 Sub Fasies yaitu Platy Coral Facies dan Branching Coral facies. Berdasarkan sifat-sifat litologi serta fosil yang dikandungnya batugamping ini diendapkan pada terumbu yang yang berbentuk “Patch Reef”.



Geo-Science 9

ZONA SESAR CIMANDIRI-BARIBIS

Iyan Haryanto Jurusan Geologi-Universitas Padjadjaran

Abstract The Cimandiri and Baribis fault zones are appropriately well defined and developed in the present day West Java Region, and making up a structural frame of the region. According to previous work, the thread of fault which defined the overall zones are came from two distinct and different tectonics periodes. Landsat imagery interpretation and many related treverses, has revealed the variation of geometrical and directional properties along fault strike. In the northern tip, the Baribis fault zones has E-W trending direction and extend to the SE, running along the conspicuous major lineament of Citanduy river drainage while the Cimandiri fault zones has E-W and NE to SW trending direction and extend running to the south trough the southern part of Pelabuhan Ratu coastline and Bay and Ciletuh Bay. The result of the current study, incorporating information obtained from structural lineaments, rock formations and stratigraphic analysis of West Java Region as well may show that Baribis and Cimandiri fault zones are a single fault zones of the same origin, with various kinematic and geometric nature along strike, developed in Plio-Plistosen still stand up to today as an active fault. The onset of kinematic and the movement mechanism of the Cimandiri and Baribis fault are characterized by the presence of an extensive N-S compresional regime effectively prevailed at the Plio-Plistosen. The compresional regime has driven the former rock formations shifted northward laterally through the NE-SW trending major slip (Cimandiri fault) and NW-SE lateral slip (Baribis fault). Abstrak Sesar Cimandiri dan Baribis merupakan sesar regional yang berkembang di Jawa Barat. Hasil peneliti terdahulu, meyakini bahwa kedua sesar tersebut berbeda jenis serta terbentuk pada periode tektonik yang berlainan. Berdasarkan hasil interpretsi citra indraja daerah Jawa Barat, diketahui beberapa hal yang menarik yaitu jalur sesar Baribis di bagian utara berarah relatif barat-timur dan diperkirakan menerus hingga ke tenggara melalui kelurusan lembah Citanduy. Kemudian sesar Cimandiri yang berarah barat-timur hingga timurlaut-baratdaya, diperkirakan menerus ke selatan melalui kelurusan pantai Teluk Pelabuhan Ratu hingga Teluk Ciletuh. Di daerah Subang kedua jalur sesar regional ini diperkirakan bergabung.



Geo-Science 10

Hasil kompilasi data kelurusan struktur, umur batuan dan stratigrafi Jawa Barat, diketahui bahwa sesar Baribis dan Cimandiri merupakan satu jalur sesar yang sama, terbentuk pada Kala Plio-Plistosen dan masih aktif hingga sekarang. Mekanisme gerak sesar Cimandiri-Baribis sendiri diawali oleh adanya peningkatan tegasan kompresi utara-selatan yang efektif terjadi pada Kala Plio-Plistosen. Tegasan ini menyebabkan batuan yang telah terbentuk bergerak ke utara secara lateral melalui sesar mendatar yang berarah timurlaut-baratdaya (sesar Cimandiri) dan baratlaut-tenggara (sesar Baribis), sedangkan di bagian utara membentuk sesar naik.



Geo-Science 11

REMBANG-MADURA-KANGEAN (RMK) FAULT ZONE, EAST JAVA BASIN : THE NATURE AND ORIGIN OF A GEOLOGIC

BARRIER

Awang H. Satyana 1), C. Prasetyadi 2) 1) Exploration Division, Badan Pelaksana Migas, Jakarta

2) Geology Department, University of Pembangunan Nasional Veteran, Yogyakarta

Abstract A major wrench zone, left-lateral slip in nature, strongly deformed a series of Late Oligocene to Pliocene rocks presently occupying the northern coastal line of East Java. The fault zone trends west – east forming a deformed zone 15 to 40 kms wide as long as 550 kms from Rembang area in the west through Madura Island to Kangean islands in the east. The fault zone develops within the van Bemmelen (1949)’s physiographic Rembang Zone. The RMK Fault Zone accommodated the inversion and reactivation of pre-existing structures started in the Late Miocene and continued into the Pleistocene. A strike-slip movement along steeply dipping normal faults are observed within the fault zone to cause rotation of fault blocks. Several uplifted blocks are recognized, such as the Tuban and Kangean Highs. Flower structures are definitely identified on seismic sections crossing the fault zone, showing basement-involved, deeply-rooted vertical master faults splaying into sedimentary covers. In map view, these splays are mapped as fold and fault belts trending west-east, west northwest-east southeast, and west southwest-east northeast; the axes are positioned to be en echelon or parallel relative to the basement fault trends. The movement within the fault zone had subsided the Paleogene rifted blocks such as the Central Deep and Tuban-Camar Trough and also uplifted and rotated the rifted blocks located presently in onshore East Java (such as Purwodadi and Cepu Highs). Shale diapirism is commonly observed to the south of the fault zone. Based on the regional setting of East- and Southeast Sundaland, it is known that the RMK Fault Zone occurred within the hinge belt or slope area of geologic transition from stable Eastern Sunda Shelf to the north to deep-water area of Randublatung Zone - South Madura Sub-Basin - Lombok Sub-Basin to the south. The stable Eastern Sunda Shelf overlies the expected micro-continent called Paternoster-Kangean. Therefore, the RMK Fault Zone developed at the southern margin of the micro-continent. The fault zone is considered to develop contemporaneously with the collision of micro-continents to the east of Sulawesi



Geo-Science 12

in the middle to late Miocene (Buton-Tukang Besi and Banggai-Sula). The collision of Australia with Timor and rotational bending of the Banda Arc during the Mio-Pliocene is considered to enhance the left-lateral slip of the RMK Fault Zone. At its western end, the fault zone buttressed the old massive Karimunjawa-Bawean extended continental crust trending southwest-northeast parallel with the Meratus Trend. The paper examines the nature and origin of the RMK Fault Zone and its role to the history of the Neogene stratigraphy and structural style.



Geo-Science 13

FASIES KARBONAT FORMASI PACIRAN DAN MODEL TERUMBU

Premonowati1), R.P. Koesoemadinata2), Harsono Pringgoprawiro3)

and Wahyoe Soepri Hantoro4) 1) UPN Veteran Yogyakarta.

2) Guru Besar (Emiritus) Departemen Teknik Geologi, ITB. 3) Guru Besar (Emiritus) Departemen Teknik Geologi, ITB.

4) Wahyoe Soepri Hantoro, Puslitbang Teknik Geologi LIPI Bandung

Abstrak Formasi Paciran terbentuk pada umur 4 juta tahun lalu atau Zona N19 hingga sekarang dijumpai 9 mikrofasies dalam fasies boundstone, 5 Mikrofasies dalam Rudstone, 3 mikrofasies dalam Floatstone, 8 mikrofasies dalam Grainstone, 7 mikrofasies dalam Packstone, 2 mikrifasies dalam Wackestone, 5 mikrofasies dalam Mudstone dan dolomit/dolomitan. Fasies Boundstone didominasi oleh koral hermatypic laut dangkal membentuk lingkungan reef framework dan dominasi ganggang membentuk fasies Rhodolit. Fasies Rhodolit membatasi kelompok batuan praterumbu dengan terumbu sebagai konglomerat alas. Dominasi Rhodolit perlu ditambahkan ke dalam klasifikasi batuan karbonat menurut Koesoemadinata (1983). Model terumbu terbentuk mulai lingkungan laguna, reef framework, reef flat, reef crest, reef slope, proximal talus, dan distal talus. Abstract The Paciran Formation has been formated since 4 million years ago (N19 zone). It is found 9 microfacies in boundstone, 5 microfacies in rudstone, 3 microfacies in floatstone, 8 microfacies in grainstone, 7 microfacies in packstone, 2 microfacies in wackedstone, 5 microfacies in mudstone and dolomite. Boundstone facies is dominated by hermatypic corals in shalow sea which form reef framework environment and dominated algae which form rhodolite facies. Rhodolite facies as basal conglomerate. The dominated Rhodolite is necessary added in carbonate classification of Koesoemadinata (1983). The reef setting succesive from lagoon, reef framework, reef flat, reef crest, reef slope, proximal talus, and distal talus.



Geo-Science 14

STUDI PETROLOGI BASAL SEBAGAI INDIKASI VOLKANISME DI

DAERAH GRUMBULPRING, SANGIRAN – JAWA TENGAH

S. Bronto 1), R. Ciochon 2) , Y. Zaim 3), R. Larick2), A. Wulff 4), Y.

Rizal3), S. Carpenter4), A. Bettis4), R. Morley5), Sudijono1) & Suminto 1)

1) Puslitbang Geologi, Indonesia, 2) Dept. of Anthropology, Iowa University, USA, 3) Dept. Teknik Geologi, FIKTM-ITB, Indonesia, 4) Dept. of Geography, Iowa University, USA,

5) Palynova-PT Eskindo Pratama, Bogor-Indonesia Abstract There is a basaltic rock extending east-west direction (ca. 1.5 km) and 200 m wide at Grumbulpring village and Watuburik River. The rock has a porphyritic texture with phenocrysts of plagioclase, olivine and clinopyroxene set in groundmass of volcanic glass and microcrystals of olivine, pyroxene, opaque and felspar. The outer part of the igneous body is blocky and shows highly vesiculer structure, but inward gradually changes to massive one. This is followed by glass content that decreases and becoming micro crystals rich in the groundmass. The basalt is interpreted as a lava plug produced by in situ fissure eruption of a monogenetic volcano or an excentric one. Phreatic explosions occurred in the early phases to yield hydroclastic breccias. In the following stages phreatomagmatic explosions ejected scoria and basaltic blocks together with older rocks. The eruption was ended by a basaltic lava extrusion in the fracture zone. Further studies are required in order to understand the relation between this volcanic activity and lahars and mud volcano in Sangiran area. Key words: basalt, eruption, excentric, fissure, grumbulpring, monogenetic, sangiran,

volcano, watuburik. Abstrak Batuan beku basal yang tersingkap memanjang timur-barat lk. 1,5 km dengan lebar 200 m di Dusun Grumbulpring dan Kali Watuburik diteliti dalam kaitannya dengan geologi daerah Sangiran. Batuan mempunyai tekstur porfiri dengan fenokris plagioklas, olivin dan piroksen yang tertanam di dalam masa dasar gelas dan kristal mikro olivin, piroksen, mineral opak dan felspar. Bagian luar tubuh basal berstruktur bongkah dan berlubang kasar, namun ke arah dalam secara berangsur berubah menjadi masif. Perubahan secara bertahap itu juga terjadi pada komposisi gelas yang semakin berkurang dan menjadi kaya kristal



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mikro di dalam masa dasar. Basal ini diinterpretasikan sebagai sumbat lava hasil erupsi celah secara in situ suatu gunungapi monogenesa yang sekaligus juga gunungapi eksentrik. Erupsi diawali dengan letusan freatik yang menghasilkan breksi hidroklastika, disusul dengan letusan freatomagmatik yang mulai melontarkan bom skoria dan bongkah basal, serta diakhiri dengan ekstrusi lava basal di dalam rekahan memanjang. Penelitian lebih lanjut diperlukan guna mengetahui hubungan kegiatan gunungapi itu dengan endapan lahar dan mud volcano di daerah Sangiran. Kata kunci: basal, erupsi, eksentrik, celah, grumbulpring, monogenesa, sangiran,

gunungapi, watuburik.



Geo-Science 16

PENTARIKHAN JEJAK BELAH TERHADAP BATUAN TEROBOSAN DIDAERAH G. MIING, BATULICIN,

KALIMANTAN-SELATAN

H. Utoyo, I. Saefudin dan H.Z. Abidin Puslitbang Geologi Bandung

Abstract Intrusive rocks that exposed in G. Miing, are dykes, intruding the Haruyan Formation. The rocks consist of andesite, hornblende andesite, pyroxene andesite, diorite and hornblende diorite.

The results of fission track dating of diorite and crystall tuff show 44.1 ± 2.5 and 43.1 ± 2.5 m.a (Eocene), while andesites display 14.4 ± 1.0, 12.7 ± 1.0 and 12.8 ± 1.0 m.a (Middle Miocene). These data indicate two periods of magmatic activities i.e: Eocene and Middle Miocene epochs. Abstrak Batuan terobosan, yang tersingkap di G. Miing berupa retas-retas (dykes), menerobos litologi Formasi Haruyan. Batuan terobosan tersebut terdiri dari andesit, andesit horenblenda, andesit piroksen, diorit dan diorit horenblenda.

Hasil pentarikhan Jejak Belah terhadap batuan diorit dan tuf kristal menunjukkan umur 44,1 ± 2,5 dan 43,1 ± 2,5 juta tahun (Eosen), sedangkan batuan andesit menunjukkan umur 14,4 ± 1,0 , 12,7 ± 1,0 dan 12,8 ± 1,0 juta tahun (Miosen Tengah). Hasil pentarikhan tersebut menunjukkan aktifitas magmatisme sebanyak 2 kali, yaitu pada Kala Eosen dan Miosen Tengah.



Geo-Science 17

PRELIMINARY STUDY OF PETROGRAPHY FOR THE ROCKS IN PASAMAN AND

SURROUNDINGS AREA, WEST SUMATERA

Sri Indarto, Iskandar Zulkarnain dan Sudarsono

Pusat Penelitian Geoteknologi-LIPI

Abstract The investigation of lithology in Pasaman and surrouinding, West Sumatera had been worked in the Tambang Pambaluan River, Simpang Dingin River, Batang Sumpur River and in Talamau Mountain surrounding. The lithology is very in variaties. The metasandstone, some time altered and minerallized interbedded with slates and some time intruded by quartz vein, the outcrop is exposed in Tambang Pambaluan River and Simpang Dingin River. Chlorite schists and marble has been exposed in Sinabuan River. The metamorphic rocks in estimated as Woyla Group Member which Late Jurassic to Early Cretaceous. The granite rock, exposed in Sinabuan River and Simpang Dingin River, as intrusion in metasandstone and slate, can be intrepreted as Kanaikan Formation Member and formed in Eocene-Oligocene age. Tuffs exposed in the down stream part of Sinabuan River, an olivine basalt lavas exposed in Batang Sampur River (Panti), the both as Undifferentiated Volcanic Member and formed in Miocene. The volcanic rocks of pyroxene andesite, basaltic andesite, andesitic basalt founded in surrounding of Talamau Mountain and which formewd in Quaternary. Based on the rocks mentioned can be indicated in Pasman and Surrounding area had been formed with some processes. Abstrak Penelitian litologi di daerah Pasaman dan sekitarnya Sumatera barat dilakukan sepanjang S. Tambang Pambaluan, S. Sinabuan, S. Simpang Dingin, S. Batang Sumpur (Panti) dan di sekitar G. Talamau. Batuan yang didapatkan sangat bervariasi. Batupasir meta, sering terubah dan termineralisasi berselang-seling dengan batusabak dan kadang-kadang diterobos oleh urat-urat kuarsa, tersingkap di S. Tambang Pambaluan dan S. Simpang Dingin. Sekis khlorit dan marmer, tersingkap di S. Sinabuan. Batuan malihan tersebut diduga sebagai Anggota Kelompok Woyla berumur Jura Akhir-Kapur Awal. Batuan granit, tersingkap di S. Sinabuan dan S. Simpang Dingin, sebagai intrusi pada batupasir meta dan batusabak, diduga termasuk Anggota Formasi Kanaikan berumur Eosen-Oligosen. Batuan tufa yang tersingkap di bagian



Geo-Science 18

muara S. Sinabuan, dan lava berkomposisi basalt olivin tersingkap di S. Batang Sumpur (Panti), dua-duanya diduga sebagai Anggota batuan volkanik tak terbedakan berumur sekitar Miosen. Batuan volkanik dari jenis andesit piroksen, andesit basaltik, dan basalt andesitik didapatkan di sekitar G. Talamau, berumur Kuarter. Berdasarkan batuan tersebut dapat menunjukkan bahwa daerah Pasaman pernah mengalami sejumlah proses pembentukan batuan yang berbeda-beda.



Geo-Science 19

ELEPHAS MAXIMUS DARI KAMPUNG RANCAMALANG,

KABUPATEN BANDUNG : SEBUAH STUDI PENDAHULUAN

Julianty, T. Suwarti, dan S.R. Sinung Baskoro

Geological Museum, GRDC Bandung Abstract A mandible fossil of Elephas was discovered in Kampung Rancamalang, Desa Margaasih, Kecamatan Margaasih, Kabupaten Bandung. It was found in a well in a depth of 5 – 6 metres. The fossil was deposited in a loose conglomerate layer, poorly sorted with volcanic rocks fragments. The dentitions have a very close resemblance to that from Cipeundeuy area, and it probably belonged to an Asian elephant (Elephas maximus). The age is approximately Late Pleistocene. There are 1 intact molar series from right side, and 1 broken molar series of left side. The left molar is broken into three parts and there are 3 individual plates of distal part of it. The distal part of the right series had been lost. The ramus mandibularis sinistra is broken up into two parts proximally, and the proximal part is broken laterally. The dentitions is all covered by black greyish matrix, either on buccal, lingual, or occlusal surface. The anrerior roots of both sides are well preserved. This finding is known to be the most complete fossil elephas found in Indonesia especially in Java, and it has increased the data of elephant’s evolution in West Java particularly, and in Java generally. Abstrak Fosil rahang bawah gajah modern (Elephas) telah ditemukan di Kampung Rancamalang, Desa Margaasih, Kecamatan Margaasih, Kabupaten Bandung. Fosil tersebut ditemukan di kedalaman 5-6 m di dalam sumur. Proses fosilisasi belum sempurna dan tidak merata. Secara stratigrafi fosil ini terdapat pada lapisan konglomerat yang belum padat, terpilah buruk dengan fragmen utama batuan gunungapi. Gigi-geligi pada fosil memiliki kemiripian dengan yang ditemukan di daerah Cipeundeuy dan kemungkinan besar milik gajah Asia (Elephas maximus). Fosil ini diperkirakan berumur Plistosen Akhir.



Geo-Science 20

Rahang tersebut terdiri dari dua seri molar sisi kiri dan sisi kanan. Seri molar kiri patah menjadi tiga di bagian distal, tiga lempeng di antaranya terpisah secara individual. Ujung distal seri molar kanan tidak ada. Ramus mandibularis sinistra patah terbagi dua di bagian proksimal dan bagian proksimal patah di arah lateral. Ramus mandibularis dextra lengkap. Seluruh permukaan molar baik di sisi buccal, lingual, maupun oklusal tertutup matriks pasir berwarna hitam. Akar gigi bagian anterior pada kedua molar terawetkan dengan baik. Fosil ini merupakan fosil gajah terlengkap yang pernah ditemukan di Indonesia khususnya di Jawa, dan penemuan fosil rahang bawah ini telah menambah data perkembangan gajah di Jawa Barat khususnya dan Jawa pada umumnya.



Geo-Science 21

CRYSTAL SIZE DISTRIBUTION (CSD) AND PETROLOGICAL EVOLUTION OF

PLINIAN AND VULCANIAN ERUPTIONS FROM MERAPI VOLCANO, JAVA :

A COMPARATIVE INVESTIGATION

S. ANDREASTUTI, B. VOIGHT* 1) Volcano Technology Research Center, Directorate of Volcanology and Geological

Hazard Mitigation, 2) Department of Geosciences, Pennsylvania State University,

Abstract The types of eruption at Merapi volcano range from gentle lava dome effusions to energetic explosions, with these styles commonly related to open and closed vent conditions, respectively. Through its eruption history, the type of Merapi eruptions can be devided into pre-1800 AD which is dominated by explosive eruptions ranged from sub-plinian and plinian to vulcanian of closed vent type; and post-1800 AD eruption mainly of open vent type, have been primarily related to dome collapse events, with occasional vulcanian explosions. The change in activity from plinian events to dome collapses and collapse-triggered explosions is suggested to occur about the same time as the Kepuhharjo tephra (c. 250 yrs. ago). Geochemical characteristics of tephra units from pre-1800 AD Merapi eruptions show a range of medium to high K2O (1.1 to 2.5 wt%), high Al2O3 (19 to 21wt%) and low MgO (1.9 to 3.6 wt%). Silica compositions varied from basalt to basaltic andesite (51-57 wt%).

Plinian and vulcanian eruptions of Merapi Volcano have been observed from Crystal Size Distribution (CSD) and petrology aspects to understand its magmatic processes. Plinian eruption of Tegalsruni tephra (1960 yrs BP) and Temusari tephra (about 1800 yrs BP) were compared to vulcanian eruptions of Kepuhharjo tephra (250 yrs BP) and the 1872 eruption. In general the Merapi tephras show an increase in crystallinity from the onset to the end of each eruption, reflecting the zonation tapped in the magma reservoir; in the Temusari tephra, this crystallinity change accompanies an increase in SiO2. Hornblende is more abundant in larger (plinian) eruptions than in dome collapse (vulcanian) eruptions. Percentage of plagioclase content is variable throughout the Tegalsruni eruption but increases through the Temusari eruption, during which time average plagioclase number densities decrease.



Geo-Science 22

ANALYSIS STRUKTUR DAN TEKTONIK DAERAH GARBA DAN SEKITARNYA

SUMATERA SELATAN SERTA IMPLIKASINYA TERHADAP POTENSI

HYDROKARBON

Agus Guntoro, Denny S. Djohor, Arista M.Bayu N, Tazar I dan Aditya A.

Jurusan Tehnik Geologi Fakultas Teknologi Mineral Universitas Trisakti

Abstrak Daerah Garba terletak pada cekungan busur belakang dan merupakan bagian dari Cekungan Sumatera Selatan sub Ckungan Palembang. Dari studi literatur diketahui bahwa Cekungan Sumatera selatanini setidaknya telah mengalami 4 fase tektonik (Hartanto, drr, 1991), yaitu : Fase pertama terjadi pada waktu Mesozoik Tengah dan membentuk pola – pola struktur yang dikenal sebagai pola Sesar Lematang dengan arah N 300o E (Pulunggono drr, 1992) Sebagai hasil tumbukan dan penunjaman dari Lempeng Indo – Australian. Fase kedua terjadi pada Akhir Kapur hingga Awal Tersier membentuk pola utara – selatan. Pada awal Tersier, sesar – sesar ini menjadi sesar normal dan mengalami proses penenggelaman dan mulai terbentuk pusat-pusat pengendapan. Fase ketiga terjadi pada Miosen Tengah membentuk pola-pola struktur yang dikenal sebagai pola Barisan yang berarah baratlaut – tenggara. Fase terakhir pada Plio-Pleistosen menyebabkan terbentuknya peremajaan dari sturktur – struktur yang sudah ada sebelumnya atau disebut sebagai struktur inversi. Proses deformasi yang terjadi dalam beberapa dengan regim tektonik yang berbeda menyebabkan terbentuknya tatanan struktur yang sangat komplek. Hasil dari pemetaan geologi di daerah Garba memperlihatkan perkembangan tektonik yang mengalami fase inversi menunjukkan orientasi struktur yang sulit untuk ditafsirkan. Di dalam upaya untuk memahami pola dan karakteristik dari perkembangan struktur inversi ini maka data-data lapangan yang telah dikumpulkan dilakukan analisis struktur melalui pemodelan serta pendekatan melalui pemodelan Sand Box modelling untuk struktur inversi.



Geo-Science 23

Dilihat dari perkembangan tektonik ini, maka secara umum daerah Garba telah mengalami fase tektonik peregangan, yang memungkinkan terbentuknya endapan sedimen yang tebal dan fase kompresi yang menyebabkan teremajakannya struktur-struktur tua menjadi terangkat. Pada banyak cekungan di dunia dengan pola yang demikian terbukti merupakan daerah yang prospek terhadap hidrokarbon (Dronkers and Matozek 1991). Untuk daerah Garba diperkirakan pengaruh struktur inversi ini menyebabkan lapisan-lapisan yangs ekarang memiliki sejarah Burial History yang memungkinkan pematangan dari batuan induk untuk menjadi hidrokarbon



Geo-Science 24

SANDBOX MODELLING OF THE SYN-RIFT DEPOSITIONAL SYSTEM

Benyamin Sapiie, Ivan Nugraha & Sayentika

Department of Geologi, FIKTM, ITB

Abstract The western margin of Sundaland is obviously presented by the existence of Sumatera. The tectonic system of the rifting phase (Eocene-Early Miocene) in the western margin of Sundaland has been an interseting issue for many years. Many author had different opinions about this issue. Some thought that the rifting system in Sumatera is purely extensional, caused by the mantle upwelling during the Miocene orogeny and some other thought that the rifting system in Sumatera is more a strike-slip regime, Possibly caused by the difference of plate movements from Pre-Tertyari to eocene. The existence of anomaly thermal on several faults that has parallel orientation with the subduction zone has made some believe that Sumatera is a back arc basin during the rifting phase. The weakness of this theory is because there are a lot of other different graben orientatios with those that are parallel to the subduction zone. On the other hand, theavailable strike-slip theory has not clearly explained the different graben orientations on each of the basin in Sumatera. Even thought the rifting in Sumatera caused by the strike-slip regime, this different orientations have made it seems that each basin in Sumatera is developed by different tectonic system during the Eocene-Early Miocene. From our experiments, we discovered that the rifting in Sumatera is mainly controlled by the basement structure yhat already existed before Tertiary rather than regional strain. During the Eocene-Early Miocene, lateral dextral movements in Semangko (Great Sumatera) Fault System that already existed since 75 Ma and Red River Fault Systems has developed the strike-slip system in western margin of Sundaland. The combination of this movementa and the suture orientation of the Mutus, Mergui and Malacca “ Basement” terrain in Sumatera hgas successfully explained all of the structure orientations in Sumatera during the rifting phase.



Geo-Science 25

WEST JAVA BASIN DELINIATION USING 2-D GRAVITY MODELING

Eko Widianto*, Djoko Santoso**, Iwan Tachjudin Taib**, Wawan Gunawan Abdul Kadir**

* JOB Pertamina – ConocoPhillips (Sakakemang) Ltd. ** Institut Teknologi Bandung

Abstract The determination of boundary between Northwest Java and the Bogor Basins, in relation with the oil and gas exploration play concept of in this area is very important, since the both basins are very different in their characteristics and prospectivity. The existing seismic data is unable to identify that boundary. Gravity study that covered West Java from latitude 5020’00’’ S to 7050’00” S and longitude 105015’00’’ E to 108040’00’’ E, and the 2-D modeling has delineated the structural configuration of the West Java Basins. There are two vertical discontinuities of density at depths of 31 km and 4 km that probably associated with the earth mantle and the average depth of the West Java Tertiary basin respectively. The West Java basement is grouped into 2 types. In the southern part of Bogor Basin is dominated by mélange with density ranging between 2.93 – 2.97 gram/cc, while in the northern part by metamorphic and intrusive rocks with density average of 2.74 gram/cc. Based on the gravity data, the Bogor Basin is interpreted as a Rift Basin with the northern boundary has relatively moved to the south compared to the boundary that was determined in the previous study. This condition will have implications for the oil and gas exploration on both basins.



Geo-Science 26

DEBRIS AVALANCHE OF MOJOSONGO, IS IT RELATED TO A LARGE ERUPTION?

Zaennudin, Supriyati Andreastuti, Isa Nurnusanto, Dewi Sayudi,

Wartono Rahardjo* Balai Penyelidikan dan Pengembangan Teknologi Kegunungapian, Direktorat Vulkanologi dan Mitigasi Bencana Geologi, Jogyakarta

*) Jurusan Geologi, Universitas Gajah Mada

Abstract Debris avalanche deposits were found in Mojosongo, Karanganyar, Solo, with irregular structures and mega blocks as fragments within those collapsed outcrops. These deposits still contain original stratification of original rocks, forming wavy morphology of mega blocks. According to its stratigraphic position, the avalanche deposit was underlain subsequently, toward older deposit were: lahar deposit that was characterized by large fragments within it, and by greenish grey pyroclastic deposit. All these deposits included in Notopuro Formation of Pleistocene Age. Faults were found at the location type of debris avalanche deposit and showed NE-SW (20oC) and NW-SE (N340oC) directions. They were formed within two time periods. The first fault was formed and cut through the pyroclastic deposit. On the other hand the second fault cut younger lahar deposit than mentioned before. Therefore, it is suggested that the first one was formed after a large eruption that produced greenish grey pyroclastic deposit, and the second was formed during the collapse. Abstrak Endapan debris avalanche ditemukan di daerah Mojosongo, Karanganyar, Solo, dengan struktur tak beraturan, dan mega blok sebagai fragmen dalam longsoran. Endapan-endapan tersebut masih memperlihatkan struktur perlapisan batuan asli. Blok-blok longsoran ini membentuk morfologi bergelombang. Berdasarkan posisi stratigrafinya, endapan debris avalanche, didasari oleh berturut-turut ke arah yang lebih tua, endapan lahar dengan fragmen bongkah batuan dan endapan piroklastik abu-abu kehijauan. Endapan-endapan tersebut termasuk dalam Formasi Notopuro yang berumur Plistosen. Struktur sesar yang ditemukan di daerah lokasi tipe debris avalanche menunjukkkan arah timurlaut-baratdaya (N20oE) dan baratlaut-tenggara (N340oE). Sesar-sar ini terbentuk dalam 2 periode waktu. Struktur sesar pertama terbentuk dan memotong endapan piroklastik. Sedangkan sesar kedua memotong endapan lahar yang lebih muda dari yang disebutkan sebelumnya. Sehingga diperkirakan sesar pertama terbentuk sesudah terjadinya letusan yang menghasilkan endapan surge, sedangkan sesar kedua terbentuk pada saat terjadinya longsoran.

struktur pada batuan serpentinit di sepanjang sempadan jalur tengah dan jalur barat semenanjung...

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