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).

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.