Structures: Folds and Faults in West Kalimantan, Borneo

In West Kalirnantan, the E-W trending fold system is exemplified by the large synclinal structure that involves the Ketungau Basin sediments. It is a symmetrical fold with dips on both flanks around 30 degree. The situation is quite different in the Melawi Basin to the south, were the folding is asymmetric, with a steeply dipping north limb, and the dips decrease to the south. The fold axes extend NWSE. Similar trending folds have also affected Triassic to Cretaceous sedimentary formations to the west of the Melawi Basin.

In the northern part of Kalimantan, a number of thrust and normal faults have been identified trending E-W. The thrust faults may be associated with Late Cretaceous melange formation and were rejuvanated in the Early Miocene. The normal faults were formed after Oligocene time, probably in the Early Miocene.

There are north-trending faults in West Kalimantan, whose nature is ill defined. In Central Jawa, younger faults intersect the Plio-Pleistocene fold axes nearly perpendicularly.

A set of faults in West Kalimantan trends NE-SW, cutting the Cretaceous sediments and Boyan Melange. They are thought to be strikeslip.
 

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.

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.