Diamond in Kalimantan, Borneo, Indonesia

Like gold, diamonds have been known to exist in Kalimantan since the 18th Century. Diamonds have been obtained by panning in the Lardak and Kapuas rivers (Sanggau Regency) since 1836. The largest diamond ever found weighs some 6 carats. All the Kalimantan diamonds are derived from alluvial deposits, and the ultimate source rocks have never been established.

The diamond-bearing alluvial sediments are clastic rocks of 2 to 12 meter thickness, whose clasts are of quartz (yellow to pink color), hornblende, corundum, schist, slate and igneous rock fragments, in addition to magnetite, mica and gold. The slate and schist clasts are known to be pre-Permian. The other rock clasts are Tertiary in age. It was formerly considered that the Pamali Breccia of Southeast Kalimantan represents the primary source of the diamonds. However, Bergman et al. (1987) have shown that this diamondiferous formation is a sedimentary conglomerate of local Bobaris ophiolite provenance.

In 1984 Anaconda Indonesia Inc., together with PT Aneka Tambang, carried out explorations over 4,882,500 hectares of West Kalimantan, under K.P. Number DU 574 by PT Aneka Tambang. Samples were collected from the Landak and Sekayam rivers and examined in Denver, but none showed a positive indication of primary diamond. Only one sample location contained diamonds.

Diamonds are found only in streams that drain the Plateau Sandstone. There is also a correlation between diamonds in the neighboring stream and corundum-bearing rocks occurring as rounded pebbles in the basal conglomerate of the Plateau Sandstone. Paleo-current analyses were made of the Plateau Sandstone, which generally indicated a provenance from the east, but such studies were unsuccessful in locating the primary source of the diamonds. 8 Ma old minette dykes occur at Linhaisai in the northern Barito Province, but they are not considered to be the source of the alluvial diamonds, and the search for the lamproite or kimberlite sources must continue in Central Kalimantan or adjacent Southeast Asia.

It is also possible that at least some of the diamonds were derived from the olivine basalts of Central Kalimantan, and from ophiolites occurring in the Suruk river area in West Kalimantan. The diamonds found in the Tertiary sediments and present day rivers of Kalimantan, Thailand, Burma and Sumatra are characteristically similar. The present geographical locations of the diamond deposits are likely to have resulted from multiple cycles of erosion and sedimentation.

Geology of Sulawesi Island, Indonesia

Geologically, Sulawesi Island and its surrounding area is a complex region. The complexity was caused by con­vergence between three lithospheric plates: the northward-moving Australian plate, the west­ward-moving Pacific plate, and the south-southeast-moving Eurasia plate. The Makassar Strait, which sepa­rates the Sunda Platform (part of the Eurasia Plate) from the South Arm and Central Su­lawesi, formed by sea-floor spreading originat­ing in the Miocene. North of the island is the North Sulawesi Trench formed by the subduc­tion of oceanic crust from the Sulawesi Sea. To the southeast convergence has occurred between the Southeast Arm and the northern part of the Banda Sea along the Tolo Thrust. Both major structures (the North Su­lawesi Trench and Tolo Thrust) are linked by the Palu-Koro-Matano Fault system.

Based on lithologic association and tectonic development, Sulawesi and its surrounding is­lands are divided into 5 tectonic provinces:

• The Tertiary Western Sulawesi Vol­canic Arc
• Quarternary Minahasa-Sangihe Volcanic Arc
• Cretaceous-Paleogene Cen­tral Sulawesi Metamorphic belt
• Cretaceous Eastern Sulawesi Ophiolite Belt and its associ­ated pelagic sedimentary covers
• Paleo­zoic Banda Micro-continental fragments derived from the Australian continent

The contacts between those provinces are faults.

Regional Tectonic of Timor Island, Indonesia

Arc-continent collision between Eastern Sunda Arc (Banda Arc) and Southwest Australia continent formed the southern boundary of the tectonic elements. This collision zone is part of the evolutionary stages of young or early and more to resemble aspects of normal trench-arc system.

In the eastern part of Sumba Island, Indian Ocean crust has a complete experience of intense subduction and Australia now has been raised above the Banda Arc due this. To the west of the collision zone, Sunda Arc moved to the edge of continental Southeast Asia which make up one of the classic collision system features, in which the crust of the Indian Ocean-Australian (about >150 Ma) forces down along the Sunda trench, given that the Indian Ocean Plate, Australia moved northward relative to the Eurasian Plate with a velocity of about 7.5 cm/year (according to Curray, 1989). The island of Java, Bali, Lombok and Sumbawa expected forming this through the formation of the volcanic arc on the southern edge of Sundaland that initially passive. Sumba represents the bedrock that uplifted front of the arc and was trapped in front of the arc current basin (Reed et al., 1986). Some evidence suggests that the development of Sumba geology can be parallel correlated with Doang Borderland located at the end of the edge of the Sunda Shield (Wytze et al., 1991). However, Lombok front arc basin lies to the west of Sumba marked by the opening structures on certain stratigraphy horizon.

Transition zone Sunda-Banda arc is clearly recorded the existence of two straight thrust fault zone, both located in the front of the arc itself. One is represented by Savu fault (thrust type), and the other were behind the arc is called Flores back arc fault (back-arc thrust type). Both systems are connected (Silver and Reed, 1987). Area behind the arc shows laterally discontinuous zones of the back arc fault structures and produced younger accretionary wedges.

Timor Island is located outside the non-volcanic arc islands of Indonesia, between the Australia plates that move toward the north and the outer Banda arc as part of the Eurasian plate. The Timor Island is made by deformation of the northern Australia plate which had being thrust faulted, especially the southern part around Timor Trough.

The Non-volcanic arcs consist of the underwater ridge of Java Trench, Timor Island, Tanimbar, Kei and Seram up to the east. During the Tertiary age, continuous trench system was fairly active in northwestern Sumatra, Java Trench, the Lesser Sunda Islands, Timor, Tanimbar, Kei and Seram, accompanied by active volcanic subduction that can be found on the West Coast of Sumatra, South Coast of Java, Lesser Sunda Islands (Katili, 1990).

Component of plate tectonics collision involved in this, namely the Asian plate lithosphere that appear shaped by the continental crust (Sunda Craton), sea-marginal (marginal sea) of the Banda Sea, and Australian-Irian lithosphere plate (Gondwana), made by the oceanic crust Indian and Australian continental crust include elements from the island of New Guinea, Buru, Obi and others.

Tectonic evolution that began at the age of the Upper End of Perm, Middle Jurassic, Early Cretaceous until the Late Cretaceous and Neogene basin formation resulting from Paleozoic basin which had trending oriented northwest-southeast direction which then formed again (overprinting) by later Mesozoic basin of northeast-southwest trending direction. Meanwhile, the sinistral transform fault was rejuvenated by Neogene normal fault were related both sides (N. Sitompul, S, Wijanto, J., Purnomo, 1993).

Soil in Liwa, Lampung, Sumatra Island, Indonesia

In general, the southern region of Liwa (West Lampung, Sumatra Island) is covered by the residual soil. Soil residues in this region formed by weathering processes are in-situ in the parent rock without experiencing erosion or transportation. Condition tropical regions resulted in the formation of residual soil in Liwa area controlled by the degree of chemical weathering. Climatic and topographic factors also indirectly affect the soil formation process in Liwa area because of these factors helped determine the level of weathering and the thickness of the residual soil. Soil residues from the Liwa area derived from volcanic rock deposition filled up most of volcano hills and valleys. Residual soil weathering is the result of volcanic material such as tuff which has the highest plasticity and high compressibility. It also has a characteristic level of intensive consolidation (Wesley, 1988).

The residual soil in this area can be divided into two types based on their physical appearance. Brown residue on the soil top layer of 0-3 m, whereas below this layer is a red residual soils with a depth> 3 m. They are on the horizon E and B according to the classification of soil profiles according to Soil Survey Staff. Brown residue in the soil on eluviated horizon (E horizon) is characterized by light brown color, many lost their silicate minerals, clay, iron elements, or aluminum due to the washing process and leaving sand or silt particles of minerals resistant. Ground red residues are on illuviated horizon (B horizon) is characterized by red, mineral concentration washing process results in the form of clay minerals, carbon, sesquioxides of iron and aluminum elements.

DETAILS OF GEOTECHNICAL CHARACTERISTIC IN THIS AREA:

·                     Consolidation and compressibility tests showed a tendency to brown residual soil settlement properties (decrease in building construction) are high enough compared with the residual red soil. In accordance with the thickness, consolidation brown residue on the soil is estimated to occur up to 3 m.

·                     Based on the compaction test, natural water content of soil residues (brown and red) are high and are slightly above the optimum water content. As piling material, it is necessary to obtain some degree of drying for the maximum resistance power.

·                     The flow of water occurs vertically from top to bottom through the soil horizons. This process is triggered so that the leaching process of accumulation of minerals often found in the layer B (red residual soil) as enrichment. Lateral water flow at the surface and the erosion is minimal, so that the process of formation of sedimentary soil less than the residual soil widespread in this region.

·                     Soil residues in this region (brown and red) are loaded with halloysite clay minerals, high plasticity value, so that is sensitive to the effects of vibration and changes in pore water pressure. In saturated conditions can cause instability and prone to landslides, especially on steep slopes.

Tectonic and Structure Geology of Sumatra Island

Sumatra Island is located in the path of volcano (NW-SE). Sumatra volcanic arc was formed by the meeting of two plates, the Indo-Australian plate which plunge down into Eurasian plate. The converging between the two plates as more detailed formed tectonic elements as follow:

• Active subduction zone, manifested by the Java-Sumatra Trench.

• Non-magmatic arc as accretionary wedge that formed island of Nias, Simeule Island, Mentawai Islands, etc..

• Fore arc basin, manifested by Sibolga Basin and Bengkulu Basin.

• Magmatic arc, indicated by the Barisan Mountains. Volcanoes located in the Barisan Mountains including Mount Merapi, Mount Kerinci, etc..

• Back arc basin, manifested by the Malacca Straits.

• Continental shelf of Sundaland.

Structure Map of Sumatra Island (Darman & Sidi, 2000)

Important symptoms that occur in Sumatra, in addition to that described above is the presence of horizontal Sumatra fault, known as the Sumatra Fault System (SFS) which divides the island of Sumatra, and following the path of the Barisan Mountains from Aceh to the Sunda Strait. There are two thoughts about SFS:

• Allegedly as a consequence of oblique subduction occurred in Sumatera (Katili, 1985).

• The movement was done by collision between India-Eurasia plate which extruded blocks of Southeast Asia toward Southeast (Tapponier, 1982).

In general, the process of Barisan Mountains uplifting began in Late Miocene, probably reached its peak at the boundary between the Miocene-Pliocene. This uplifting process is not consistently going on until now as estimated by recent geological features followed by the pattern of tectonics in the Early Pleistocene. Tectonic activity along the island formed massive geanticlines that causing the temperature rise related to rapid intrusion of accumulated magma underneath. It is characterized by increasing of both volcanic activity and lateral movement along Sumatra Fault System. All active tectonic activity over the Sumatra region is considered as the main source of recent earthquakes.

Tanjung Redeb, North East Borneo Geology

Structure and Tectonics

Structures found in the Tj Redeb consist of folds, normal faults, strike slip faults and lineaments. Faults trend NW-SE and SW-NE. Folds trend NW-SE and SW-NE forming anticlines and synclines. This are presumed to have four tectonic events. First event inferred during Late Cretaceous time or older. This event made the Bangara Fm. sediments into folding, faulting and low grade metamorphic rocks. Depositions of Early Eocene shallow marine sediment within the Sembakung Fm. (middle and western part of  area) was also formed Tabalar Fm. in the SE mapped in Eocene-Oligocene and followed by the second tectonic event. Deposition of the Bangara Fm. took place in the middle, east, south and west in the Oligo-Miocene where it is locally intruded by Andesitic rocks, which have been altered and mineralized. Oligo Miocene volcanic activity formed the Jelai Volcanic Rocks in the west. After deposition of the Birang Fm. the Latih Fm was deposited. The Latih Fm. sediments were formed surrounding Teluk Bayur during Late Early Miocene up to Middle Miocene.

The third tectonic event seems to have been occurred after the position of the Latih Fm. Deposition of the Labanan Fm. in the SW and Domaring Fm in the east occurred during the Late Miocene up to the Pliocene whereas the Late Miocene sediments of the Tabul Fm was formed in the north and deposition of the Sinjin Fm. (in SW and N of the sheet). After deposition of the Sinjin Fm. the Sajau Fm. was deposited in the Eastern portion of the sheet in the Plio-Pleistocene.

The Late Pleistocene, after deposition of the Sajau Fm. sediments, the fourth tectonic event was presumed to have occurred. This was showing folding and faulting sediments of the Sajau Fm. and older sediments on the lower part to form the recent topography and morphology.

Mineral and Energy Resources

Coal is one of natural resources having a good prospect in the studied area. The coal surveys were carried out since the Netherlands Indies Government and then continue investigating by the Indonesian Government. Coals are found within sediments of the Latih, Tabul, Labanan and Sajau Formations. The coal mining was formerly carried out by the NV Steenkolen Maatschappij Prapatan (SMP).

Previous geologists report 70 coal seams ranging from 20cm to 5.5 M in thickness. There are many varieties of coal grading from bituminous coal to brown coal. The bituminous and sub-bituminous coals have a quality of 6000 calories per gram. The Teluk Bayur coals have 7000 calories per gram. Building materials such as quartz sand and clays are widespread in Teluk Bayur and Labanan areas. Good quality limestone outcrops are found in Tanjung Selor but are limited in area. The limestone also crops out well in Siduung River upstream but it is hard to be mined because of bad transportation. Limited andesite outcrops were also found in the west and they were used by the logging company for building roads.

Situmorang, R.L. and Burhan, G., 1995

Regional Stratigraphy

• Qa – Quaternary alluvium, Mud, silt, sand, cobbles, pebbles and peat, grey to blackish colors, Unit thicknesses up to 40M..
• Ql – QUATERNARY REEF LIMESTONE, Reefal, coralline and brecciated corals, white to grey, brown, crystalline, hollows, containing corals, locally brecciated, deposited in shallow marine environment.
• TQps – SAJAU Fm. Alternations of claystone, siltstone, sandstone, conglomerate, intercalations of coal seams, contains molluscs, quartzite and micas. Shows cross bedding and lamination. Coal seams 20-100CM thick, black to brown. Unit thickness about 775M deposited in fluviatile and delta environments..
• Tps – SINJIN Fm. Alternations of tuff, agglomerate, lapilli, pyroxene andesite lava, silicified tuff, tuffaceous claystone and kaolin. Contains lignite, quartz, feldspar and black minerals. Unit thickness up to 500M.
• Tmpd – DOMARING Fm. Coralline limestone, chalky limestone, intercalations of marl and lignite; deposited in swampy-littoral environment, thickness is about 1000M. Of Late Miocene-Pliocene Age.
• Tmpl – LABANAN Fm. Alternating polymic conglomerate, sandstone, siltstone, and claystone, intercalations of limestone and coal seams (20-150CM thick) deposited in fluvial environment. Thickness is about 450M. Late Miocene-Pliocene age.
• Tmt – TABUL Fm. Consisting of sandstone, claystone, conglomerate and coal seam intercalations. Contains Operculina sp. Unit thickness about 1050M. Deposited in delta, regressive environment. Late Miocene age.
• Tml – LATIH Fm. Quartz sandstone, claystone, siltstone and coal in the upper part. Intercalations of sandy shale and limestone in the lower part. Black and brown coal seams 0.2 to 5.5M thick. Deposited in estuary, delta and shallow marine environments. Unit thickness is about 800M. Early Miocene to Late Miocene age.
• Tomj – JELAI VOLCANICS, Volcanic breccia, tuffaceous sandstone and tuff. Locally intercalated with coal seams, shows graded bedding and cross bedding structures. Andesite cleave intrusive. Unit thickness reached 200M. Oligocene to Miocene age.
• Tomb – BIRANG Fm. Alternations of marl, limestone and tuff in the upper part. Alternations of marl, chert, conglomerate, quartz sandstone and limestone in the lower part. Thickness is about 1100M. Fossils content: Lepidocyclina ephicides, Spiroclypeus sp., Miogypsina sp., Marginopora vertebralis, Operculina sp., Globigerina tripartite Koch, Globigerinita altispira, Globorotalina mayeri Cushman and Ellisor, Globorotalia peripheronda, Globigerinoides immaturus, Globigerinoides sacculifer, Pre-Orbulina transitoria, Uvigerina sp., and Cassidulina sp. Fossils range Oligocene-Miocene Age.
• Teot – TABALAR Fm. Lower part consist of grey marl, sandstone, shale and intercalations of limestone and basal conglomerate. Upper part consists of dolomite and calcarenite and marl intercalations. Deposited in fluvial-shallow marine environment. Thickness is about 1000M. Eocene to Oligocene age.
• Tes –SEMBAKUNG Fm. Claystone, siltstone and sandstone in the lower part. Quartz sandstone, sandy limestone, chert and tuff in the upper part. Contains fossils: Nummulites sp., Discocyclina sp. Operculina sp. Globigerina sp. Reusella sp. Nodosaria sp., Planulina sp., Amphistegina sp., and Borelis sp., Unit thickness up to 1000M. Deposited in marine environment. Eocene age.
• Kbs – BANGARA Fm. Alternations of metamorphic claystone, silicified claystone, black claystone and shale intercalated with laminated tuffs containing radiolaria. Flysch deposit.
• Tomi – INTRUSIVE ROCKS, Andesite, consisting of vitrophyre, prophyllitic andesite and pyroxene andesite lavas.

Geology of Lomblen Island, Indonesia

Regional Geology and Stratigraphy

Regional structure Lomblen Island include of Banda arc Volcanic belt, with structure terms as folding and faulting in NE-SW and SE-NW direction. The oldest rock formation is Kiro Formation (Tmk) in lower Miocene until upper Miocene. This formation consist of lava, breccias, agglomerate implied layered tuff. Kiro formation wedge with Nangapanda formation (Tmn) that consisted of sandy tuff, breccias tuff, and implied by limestone. Those old formation above were infiltrated by granodiorite (Tmd) in upper Miocene. When Pliocen-Plistocene there was volcanic activity such as lava, agglomerate, and tuff.


 

Regional Stratigraphy of Lomblen Island (Noya, Y., and Suwarno, N., 1983)
 

Structure Characteristics

By regional investigation in field, map of topography and interpretation photograph air there are 2 especial structure direction that is: north-south direction and northeast-southwest direction. In this area (Atedai) there are 6 big structure they are lineament, volcano, cauldron, crater, caldera, slide and fault.

Lineament

This lineament structure have the direction NW-SE. This structure is the oldest big structure estimated cut the basement. Alongside this lineament have emerged the volcano network, such as: Watulolo, Atolojo, Watukuba etc.

Cauldron (crater of Atolojo)

This cauldron structure is the result of eruption mount Atolojo, which among other things yield fallout sediment of pyroclastic skoria andesitic. Cauldron diameter 750-1000 m encircling from NE till NW and open toward north.

Caldera Watukuba

This structure is the result of eruption Watukuba yielding dusty sediment of pyroclastic. diameter of caldera Watukuba 2500 m encircle from north direction to west till easterly. In floor of caldera there are geothermal manifestation like hot ground and alteration.

Debris Avalanches/sliding of Wai Teba

This structure represent the slide which have association with the weak area, form like horse poultice that opening eastwards of Watuwawer.

Fault of Wai Kowan

Fault structure have the direction NE-SW. Alongside this structure have attended the hot water source of Wai Kowan, hot ground of Koti and area of alteration Lowo Kebingin.

Fault of Lewoderoma

This structure have the direction NE-SW. As long as this structure have emerged the hot water source of Lewoderoma And hot water of Waiketi.

Geomorfology

Regional area of research by Volcanology Department of Indonesia divided in 5 morphology region:

1. Old volcanic
2. Mount Watuloko
3. Mount Watukuba and Atalojo
4. Debris Avalanches unit
5. Plain morphology unit

· Old volcanic
Set of this distinguished by circular hilly. The relief is smooth until middle. Dale instruct north-south direction. River have parallel semi pattern and sentence sharply form in high stadium erosion. Set of this reside in north formed of old rock volcanic.

· Mount Watuloko
Set of this distinguished by topography form which harsh, precipitous level of inclination and erosion river deeply. Set of this take possession of the middle until north of investigation area limited of old morphology volcanic. Set of this built by lava andesitic which is generally escaped because fault.

· Mount Watukuba and Atalojo
Set of this show the very typical topography form that is volcano crater and caldera. There are a crater with the diameter 750-1000 m ( Mount Atalojo) and a caldera with the diameter 2500 m ( Mount Watukuba). Set of this morphology is formed by fallout of sediment of pyroclastic and lava. The river have pattern radial with the narrow tight dale pattern making dominant vertical erosion.

· Debris Avalanches unit
Set of this have the wavy topography form with middle of level inclination. Generally weak river stream and a little erosion. Set of this formed by rock from landslide.

· Plain morphology unit
Set of this located in coast environment formed by rock alluvial. Set of this distinguished by smooth topography.

North East Borneo, Bulungan Stratigraphy

GENERALIZED BULUNGAN STRATIGRAPHY

Bunyu Beds (Late Pliocene)

Sandy and clayey beds containing numerous intercalation of lignite, lying unconformably on older Tertiary. Either monoclinally dipping 1,5-2^o E, or gently folded. Overlain by young alluvial deposits. No distinctive fossil. Sometimes these beds contain fossiliferous layers with brackish water arenaceous foraminifera and shallow marine Rotalias and Elphidiums.

Sajau-Tarakan Beds (Late Pliocene)

The Sajau Tarakan Formation is composed of two parts: an upper green clay and tuffaceous sandstone member and a lower light blue-green plastic, homogeneous clay member containing some lignite beds. Total thickness about 360 m. The lower division contains some economically important lignite seams in its upper part. These beds are overlain by the Bunyu Beds, and overlie with an angular unconformity the miocene Antjam and Tabul formation.

Antjam Beds (Early Pliocene)

Blue gray firm clay, rich in mollusca and corals, a few limestone beds and thin beds of coal (between half-shiny coal and lignite). Thickness 150 m. They are unconformably overlain by the Sadjau-Tarakan Beds (Group) and in turn disconformably overlie the Kundjang Beds (Group).

Kundjang Beds (Late Miocene)

Reef limestone and fossiliferous marls and clay. Thickness 200 m. The formation is overlain with a slight disconformity by Antjam Beds, and in turn slightly disconformably overlies the Sitam beds. They are correlated with the lower part of the Tabul Beds and the upper part of the Meliat Sandstone Formation, the upper part of the Taballar Limestone Formation and the Menkrawit Beds, etc.

Tempilan Beds (Late Oligocene)

Alternating thin-bedded sandstones, marls and shales, the marls increasing upward, so that the top of the beds consists of pure marl. The thickness is about 1000 m. The larger Foraminifera included in the formation indicate an Upper Oligocene age. The formation overlies the Seilor Beds (Formation) of Lower Oligocene age, conformably in Tidung Districts, disconformably in Salimbatu-Antjam-Bulungan, where the Mankabua marls are intercalated. They are overlain by the Mesalai Marls (Formation) of Lower Miocene age.

Mankabua Marls (Early Oligocene)

Neritic sediments with Camerina fichteli, indicating a Oligocene age. No thickness or composition is mentioned. The beds are disconformably overlain by the Tempilan Beds, and in turn overlie the Seilor Beds. Possibly they are lateral equivalent of at least part of the latter formation.

Marah Beds (Late Eocene)

Some thousands of meters of mica sandstone with intercalations of limestone and marl with larger Foraminifera. The age is Upper Eocene. The Marah Beds can be correlated with the Lower Taballar Marls in the Mankalihat Peninsula, and with the Tulit Beds in the Tidung Districts.

Sungai Orang Beds (Early Eocene)

Several thousands of metres clastic sediments, formed under geosynclinal circumstances, and strongly folded. They consist mainly of mica sandstone with some intercalations of marl and limestone containing larger foraminifera, indicating a lower Eocene age. They may be correlated with the Tikung Beds (formation) in the Tidung districts and the Sandstone Stage (formation) in Mangalihat Peninsula. The S. Orang Beds overlie unconformably the pre-tertiary of Borneo, and they are overlain by the Marah layers (formation) of Upper Eocene age.

Irian Jaya

Irian Jaya terletak pada 1˚-9˚ LS dan 129˚-141˚ BT. Geologi Irian Jaya sangat kompleks melibatkan interaksi antara lempeng Australia dengan lempeng Pasifik. Hampir seluruh evolusi tektonik Kenozoikum merupakan hasil interaksi konvergen antara lempeng Indo-Australia dan lempeng Pasifik (Hamilton, 1979; Dow et al., 1988). Papua Nugini dan Pegunungan Central Range merupakan hasil tumbukan antara kontinen dan bsur kepulauan (Dewey and Bird, 1970). Pegunungan Central Range terbentuk dari batuan Mesozoikum yang terlipat dan tersesarkan serta lapisan Kenozoikum yang terendapkan pada batas Kontinental pasif. Di batasi oleh:

• Utara : Samudra Pasifik
• Timur : Papua Nugini
• Selatan : Laut Arafuru
• Barat : Laut Banda

Irian Jaya, bagian barat dari Pulau New Guinea adalah ekspresi permukaan dari batas utara deformasi blok kontinen Australia dan lempeng Pasifik. Secara topografi, Irian Jaya dianalogikan berbentuk seperti bagian tubuh burung dan di bagi menjadi :

A. Tubuh burung: didominasi struktur berarah barat-baratlaut sepanjang Central Range. Diakhiri sesar mendatar berarah Barat-Timur. Didominasi oleh pegunungan tengah masif dan central range. Daratan di sebelah utara berupa cekungan intramountain yang dinamakan Meervlakte yang dibatasi di bagian utara oleh pegunungan yang dibentuk oleh metamorfisme dengan relief yang sedang.

• Central range: berupa plateau dengan lebar sampai dengan 100 km yang memanjang dari danau Paniai di barat sampai daerah perbatasan Papua Nugini. Dilihat dari peta geologi, terlihat bahwa sebagian besar terdiri dari batuan yang terlipat dan Grup Batuganping Nugini.
• Glasiasi: gejala erosi glasiasi berupa cirques dan lembah berbentuk U. Banyak ditemui moraines di bagian utara main range dan mungkin juga diendapkan di sayap selatan tetapi sudah terpindahkan oleh erosi yang intensif di daerah yang terjal.
• Danau Paniai: dibentuk oleh sesar dan berasosiasi dengan bidang perlengkungan yang membendung air dari sungai Jawee.
• Pegunungan Ofiolit: terletak di antara Central Range dan Meervlakte berkomposisi batuan plutonik basa dan ultra basa sepanjang lebih dari 300 km.
• Meervlakte: merupakan cekungan intramountain dan dataran aluvial sepanjang 300 km dan lebar 50 km yang mengalami subsiden aktif sejak Miosen Tengah sampai sekarang, dengan kecepatan subsiden lebih cepat daripada sedimentasi Umumnya berupa swamp yang disalurkan oleh sungai Idenburg dan meander Ruffaer.

B. Leher burung: ditandai dengan perubahan arah struktur dari barat timur (tubuh) menjadi N-NW (leher).

• Lengguru Fold Belt: punggungan membentuk sabuk yang umumnya tersesarkan dan berupa antiklin.
• Semenanjung Wandamen: adalah bagian utara dekat punggungan batuan metamorf. Punggungan memiliki sistem drainase tertutup mengikuti sayap punggungan.
• Weyland Range: berupa pegunungan masif yang menghubungkan bagian leher dengan tubuh burung.

C. Kepala burung: terdiri dari batuan metamorf dan batuan granit. Bagian batuan metamorf terpotong di bagian utara dan NE oleh lembah linier bidang erosi di Sorong dan sesar Ransiki. struktur sesar berarah barat-timur

Secara geomorfologi di bagi menjadi:

1. Satuan morfologi perbukitan: daerah tengah dan utara, penampakan morfologi: bagian yang bergelombang.
2. Satuan morfologi perbukitan dengan pola kelurusan dan gua-gua: bagian tengah peta, berupa karst.
3. Satuan morfologi dataran: daerah datar hingga agak bergelombang lemah dengan ketinggian kurang dari 100 m dpl.

Geologi Irian Jaya dapat dibagi menjadi 3 mandala geologi utama, yaitu Kontinental, Oceanik dan Transisional.

1. Mandala Kontinental tersusun atas sedimen kraton Australia
2. Mandala Oceanik tersusun atas batuan ofiolit dan kompleks volkanik busur kepulauan sebagai bagian dari Lempeng Pasifik.
3. Mandala Transisional merupakan daerah yang mengandung batuan metamorf regional dan terdeformasi kuat, sebagai produk interaksi antara dua lempeng.

Secara litotektonik, Irian dapat dibagi menjadi 4 mandala, yaitu:

• New Guinea foreland/foreland basin (Arafura Platform): mencakup Laut Arafura dan dataran pantai selatan yang terletak pada Lempeng Australia. Terdiri dari sedimen Pliosen marin dan non-marin yang tidak termetamorfkan dan sedimen Holosen silisiklastik yang menutupi karbonat Kenozoikum dan batuan silisiklastik Mesozoikum.
• Jalur perlipatan dan sesar naik Central Range: tersusun atas jalur orogenik yang memanjang Barat-Timur. Jalur perlipatan dan sesar naik melibatkan batuan Paleozoikum sampai Tersier yang berasal dari benua Australia.
• Jalur metamorfik Ruffaer dan jalur ofiolit: jalur ofiolit Irian Jaya dan jalur metamorfik Ruffaer dipisahkan oleh jalur sesar, jalur ofiolit Irian Jaya ditutupi oleh aluvium yang berasal dari Depresi Meervlakte.
• Kompleks busur kepulauan Melanesia. (Depresi Meervlakte/cekungan pantai utara dan Jalur sesar naik Mamberamo).

Ada 3 model struktur dan tektonisme yang diajukan untuk menjelaskan tentang Irian Jaya:

1. Model pembalikan polaritas subduksi (pembalikan busur) (Dewey and Bird, 1970; Hamilton, 1979; Milsom, 1985; Dow et al. 1988; Katili, 1991)yang menyatakan bahwa lempeng benua Australia menunjam ke arah utara, diikuti tumbukan (collision) dan penunjaman Lempeng Pasifik ke arah selatan pada Palung New Guinea.
2. Model Zippering (Ripper and McCue, 1983; Cooper and Taylor, 1987)yang menyatakan bahwa di bagian timur pulau Irian, terdapat dua subduksi lempeng samudera yang merupakan kemenerusan ke arah barat dari subduksi lempeng Solomon.
3. Model perubahan sudut penunjaman yang menyatakan bahwa subduksi Lempeng Australia berubah sudut penunjaman menjadi vertikal tanpa pembalikan arah subduksi.

Persamaan ketiga model tersebut di atas adalah bahwa semua menyatakan bahwa bagian selatan dari Pulau Irian disusupi oleh batas lempeng pasif utara dari benua Australia yang mengandung sedimen tebal dari sedimen silisiklastik Mesozoikum berubah secara berangsur menjadi lapisan karbonat Kenozoikum.

Sedangkan perbedaan utama yang terjadi adalah peristiwa tumbukan dengan busur kepulauan.

1. Berdasarkan perubahan dari sedimentasi karbonat menjadi sedimentasi klastik yang luas akibat pengangkatan orogenesis, tumbukan berawal sejak Miosen Akhir. (Visser and Hermes, 1966; Dow and Sukamto, 1984; Dow et al., 1988)
2. Berdasarkan umur batuan metamorf pada Papua Nugini, tumbukan berawal sejak Oligosen Awal (Pigram et al., 1989; Davies, 1990)
3. Untuk menjelaskan hal ini, Dow et al., 1988; mengajukan kemungkinan bahwa Irian merupakan hasil dari dua tumbukan yang berbeda antara kontinen dan busur kepulauan, yaitu selama Oligosen dan selama Miosen (Orogenesis Melanesia)
4. Quarles van Ufford, 1996 mengajukan kemungkinan bahwa pada Pulau Irian terjadi dua peristiwa orogenesis yang berbeda secara ruang dan waktu.

• Orogenesis Kepulauan pada Eosen-Oligosen terjadi pada daerah Ekor Burung pada bagian paling Timur dari Pulau Irian (Nugini). Pembentukan dan erosi yang tercatat selama Oligosen dan sedimen klastik yang lebih muda pada Aure Trough.
• Orogenesis Central Range dimulai pada Miosen Tengah dan menyebabkan penyebaran sedimen klastik yang luas. Orogenesis ini dibagi menjadi tahap sebelum tumbukan dan tahap tumbukan. Tahap sebelum tumbukan berkaitan dengan metamorfisme pada sedimen batas pasif, sedangkan tahap tumbukan terjadi ketika pengapungan (buoyancy) litosfer Australia menghentikan subduksi, deformasi melibatkan basement kristalin dari lempeng benua Australia. Dilaminasi tumbukan terjadi antara 7-3 juta tahun yang lalu, menyebabkan aktivitas magma tahap akhir dan pengangkatan pegunungan sebanyak 1-2 km. Proses ini memicu pergerakan sesar mendatar mengiri dengan arah Barat-Timur yang mendominasi tektonik resen pada Pulau Irian bagian Barat.

Secara umum struktur regional Irian Jaya dapat dibagi menjadi 3 zona struktur, yaitu:

1. Tubuh Burung: didominasi oleh struktur berarah Barat-Barat Laut sepanjang Central Range (Jalur Mobil Nugini). Diakhiri oleh sesar mendatar dengan arah Barat-Timur (Zona Sesar Tarera-Aiduna, TAFZ) pada Leher Burung.
2. Leher Burung: didominasi oleh struktur berarah Utara- Barat Laut (Jalur Perlipatan Lengguru, LFB), yang berhenti pada tinggian Kemum pada daerah Kepala Burung.
3. Kepala Burung: didominasi oleh struktur sesar berarah Barat-Timur.

STRATIGRAFI

1. Formasi Awigatoh: tersingkap di Mapenduma dan antiklin Digul. Terdiri dari batuan metabasals, metavulkanik, batugamping, serpih, mudstone. Ditemui struktur lava bantal → endapan laut
2. Formasi Kariem: struktur sedimen: laminasi, ripple, paralel laminasi, cross bedding. Bagian bawah formasi teridi dari 30% siltstone dan 60% mudstone, sedangkan bagian atas >80% batupasir halus. Merupakan endapan turbidit submarine.
3. Formasi Tuaba: terdiri dari batupasir halus s/d kasar, perselingan dengan konglomerat dan serpih, siltstone, mudstone. Diendapkan pada lingkungan marine dekat pantai, dan bagian offshore shelf menuju daerah pasang surut.
4. Formasi Modio: bagian atas didominasi batuan klastik halus dengan struktur cross bedding dan laminasi paralel. Bagian bawah didominasi karbonat. Fosil: gastropoda, Crinoid, Brachiopoda, koral. Endapan daerah pasang susut hingga marine shelf.
5. Formasi Aiduna: terdiri dari batubara, batupasir konglomeratan dengan perselingan batupasir karbonatan, siltstone, serpih. Ditemui struktur cross bedding, ripple, load cast, bioturbasi. Fosil Brachiopoda. Diendapkan pad daerah fluvial s/d delta.
6. Formasi Tipuma: umur Trias – Jura awal. Didominasi mudstone dan siltstone, bagian bawah formasi ditemui perselingan batupasir halus dengan batupasir kasar. Diendapkan pada lingkungan fluvial.
7. Unit Kembelengan: mengandung siltstone karbonan dan mudstone di bagian bawah, batupasir glaukonitan, batupasir berbutir seragam, dan sedikit serpih di bagian top.

• Formasi Kupai
• Formasi Woniwogi
• Pynia
• Formasi Ekmai

Resume sejarah geologi secara singkat pada Tersier:

• Eosen: terbentuk geosinklin dangkal yang mengandung endapan batugamping.
• Oligo – Miosen:pengendapan cekungan.
• Plio-Pliosen:tektonik → konvergen.
  

Darman, Herman dan F.Hasan Sidi. An Outline of Geology of Indonesia. IAGI. 2000.
Dow, Robinson, et al. Preliminary Geological Report: Geology of Irian Jaya. Departemen of Mines and Energy Indonesia & The Australian International Development Assistance Bureau.1988.

Hamilton, Warren. Tectonic of The Indonesian Region. United State Government Printing Office. Washington. 1979.