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.
 

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.

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.

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.