SOUTH SUMATRA BASIN

SOUTH SUMATRA BASIN

The geology of the South Sumatra Basin is a result of tectonic activities that are closely related to the subduction of the Indi-Australian Plate, which moves north to northeast to the relatively silent Eurasian Plate. The subduction zone of the plate covers the area west of Sumatra and south of Java. Some small plates (micro-plates) that are between the interaction zones also move and produce convergence zones in various shapes and directions. The subduction of the Indi-Australian plate can affect rock conditions, morphology, tectonics and structures in South Sumatra. The tectonic collisions of plates on Sumatra Island produce a front arc, magmatic, and rear arc.

The South Sumatra Basin is formed from depression which is surrounded by Pratersier rock heights. The removal of the Barisan Mountains takes place at the end of the Limestone accompanied by block faulting. Besides the mountains

The ranks as block mountains are some of the old rock heights that are still exposed on the surface are in the Thirteen Mountains, Twelve Mountains, Lingga Island and Bangka Island which are remnants of the "Sunda Landmass" high, which is now a Sunda Shelf. The South Sumatra Basin has undergone three orogenesis processes, the first being in the Middle Mesozoic, the second in the Late Cretaceous to the Early Tertiary and the third in the Plio-Plistocene. Plio-Plistocene Orogenesis produces geological structural conditions as seen at this time. Tectonics and geological structures of the South Sumatra Basin can be divided into three groups, namely, the Semangko Fault Zone, the folding zone trending northwest-southeast and the fault zones that are closely related to Pratersier folds and faults undergoing rejuvenation.

Physiographically, the South Sumatra Basin is a Tertiary basin with a northwest-southeast direction, which is bounded by the Semangko Fault and Bukit Barisan in the southwest, the Sunda Shelf to the northeast, Tinggian Lampung in the southeast which separates the basin from the Sunda Basin, and the Twelve Mountains. and the Thirty Mountains in the northwest that separates the South Sumatra Basin from the Central Sumatra Basin. The position of the South Sumatra Basin as a back arc basin (Blake, 1989)

Regional Tectonics

Blake (1989) states that the South Sumatra Basin area is a Tertiary-aged back arc formed as a result of interactions between Sunda Shelf (as part of the Asian continental plate) and the Indian Ocean plate. This basin area covers an area of ​​330 x 510 km2, of which the southwest is bordered by the Pre-Tertiary outcrops of Bukit Barisan, to the east by the Sunda Shelf (Sunda Shield), to the west bordered by the Thirteen Mountains and to the southeast by Tinggian Lampung.

According to Salim et al. (1995), the South Sumatra Basin was formed during the Tertiary (Eocene-Oligocene) Early when the graben series developed as a reaction to the angular subduction system between the Indian Ocean plate beneath the Asian Continent plate. According to De Coster, 1974 (in Salim, 1995), it is estimated that there have been 3 episodes of orogenesis that form the structural framework of the South Sumatra Basin namely orogenesa Middle Mesozoic, Late Cretaceous tectonics - Early Tertiary and Plio-Plistocene Orogenesa.

Interaction between Sunda Exposure (as part of the Asian continental plate) and the Indian Ocean plate

Sumatra Tectonic Map

The first episode, Paleozoic and Mesozoic deposits are metamorphosed, folded and broken into chunks of structure and intruded by granite batolites and have formed archetypal basins. According to Pulunggono, 1992 (in Wisnu and Nazirman, 1997), this phase forms a fault trending northwest - southeast in the form of shear faults.

The second episode of the Late Cretaceous in the form of an extension phase produces tensional motion which forms graben and horst in the north-south general direction. Combined with the results of the Mesozoic orogenesa and the results of weathering of rocks - Pre-Tertiary rocks, these tensional motion forms an old structure that controls the formation of the Pre-Talang Akar Formation.

The third episode was in the form of a compression phase in Plio - Plistocene which caused the precipitation pattern to turn into a regression and play a role in the formation of folding and fault structures so as to form the present geological configuration. In this tectonic period there was also the removal of the Bukit Barisan Mountains which produced a Semangko horizontal fault that developed along the Bukit Barisan Mountains. Horizontal movements that occur from Early Plistocene to the present affect the conditions of the South and Central Sumatra Basin so that newly formed faults in this area have almost parallel developments with the Semangko fault. As a result of this horizontal movement, the orogenesa that occurs in Plio - Plistocene produces folds that traverse northwest - southeast but the faults formed are directed northeast - southwest and northwest - southeast. The type of cesarean contained in this basin is a rise in fault, horizontal fault and normal fault.

The appearance of the dominant structure is the structure trending northwest-southeast as a result of the Plio-Plistocene orogenesis. Thus the structure pattern that occurs can be distinguished by the old pattern trending north - south and northwest - southeast as well as a young pattern that traverses northwest - southeast which is parallel to Sumatra Island.

Regional Stratigraphy South Sumatra Basin

The stratigraphy of the South Sumatra Basin area has been widely discussed by previous geologists, especially those working in the petroleum environment. Initially the discussion focused on Tertiary sediments, generally never published and only applied in their own environment.

 South Sumatra Stratigraphy

Previous researchers have compiled the general stratigraphic sequences of the South Sumatra Basin, including: Van Bemmelen (1932), Musper (1937), Marks (1956), Spruyt (1956), Pulunggono (1969), De Coster 2 (1974), Pertamina (1981). Based on previous researchers, the South Sumatra Basin Stratigraphy is divided into three groups: Pre-Tertiary rock groups, Tertiary rock groups and Quaternary rock groups.

1. Pre-Tertiary Rocks

Pre-Tertiary Rocks The South Sumatra Basin is the basis of the Tertiary sedimentary basin. This rock is found as igneous, metamorphic and sedimentary rock (De Coster, 1974) Westerveld (1941), dividing Paleozoic (Permocarbon) rocks in the form of slate and Mesozoic (Yurakapur) in the form of volcanic facies series and deep sea facies series. These Pre-Tertiary rocks are thought to have undergone intensive folds and faults in the Middle Cretaceous to the Late Cretaceous times and were intruded by igneous rocks since the orogenesis of Middle Mesozoic (De Coster, 1974).

2. Tertiary rocks

Based on previous studies the Tertiary sedimentation sequence in the South Sumatra Basin is divided into two stages of deposition, namely the stage of oceanic and sea shrinkage stages. The sediments formed at the stage of the oceanic rock are called the Telisa Group (De Coster, 1974, Spruyt, 1956), from the age of the Early Eocene to the Middle Miocene consisting of the Lahat Formation (LAF), Talang Akar Formation (TAF), Baturaja Formation (BRF) , and Gumai Formation (GUF). Whereas those formed in the marine shrinkage stage called the Palembang Group (Spruyt, 1956) from the Middle Miocene - Pliocene age consist of the Air Benakat Formation (ABF), Muara Enim Formation (MEF), and Kasai Formation (KAF).

a. Lahat Formation (LAF)

According to Spruyt (1956), this Formation is located inconsistently above the bedrock, which consists of thin layers of andesitic tuffs which gradually turn upwards into tuffaceous clay stones. In addition, andesite breccia intersects with andesite lava, which is located at the bottom. Tufan claystone, fresh green and weathered purple to purplish red. According to De Coster (1973) this formation consisted of tuffs, agglomerates, claystones, tufan sandstones, conglomerates and breccia that were from the Late Eocene to the Early Oligocene. This formation is deposited in freshwater. Thickness and lithology varied greatly from one place to another because of irregular basin forms, then at the Eocene to Early Miocene age, volcanic activity produced andesite (Westerveld, 1941 vide of side katilli 1941), this activity peaked at the age of Late Oligocene while the rocks are referred to as "old Andesite Lava" rocks which also intrusion rocks deposited in the Early Tertiary Period.

b. Talang Akar Formation (TAF)

The name Talang Akar originating from Talang Akar Stage (Martin, 1952) other names that have been used are Houthorizont (Musper, 1937) and Lower Telisa Member (Marks, 1956). Talang root formations in several places in direct contact are not in harmony with Pre-Tertiary rocks. This formation in several places is in harmony with the Lahat Formation (De Coster, 1974), the relationship is called stratigraphic overlap, it also interprets the stratigraphic relationship between the two formations, especially in the middle, obtained from the Limau well drilling data located in the Southwest of Prabumulih City ( Pertamina, 1981), the Talang Akar Formation is divided into two, namely: Members of the "Gritsand" consist of sandstones, which contain quartz and the grain size at the bottom is rough and the top is finer. At the top of the sandstone it turns into a conglomerate sandstone or breccia. Sandstones are white to grayish brown and contain mica, sometimes there are alternating brown claystone with coal, in this member there are remnants of plants and coal, the thickness is between 40 - 830 meters. These sediments are fluviatile to delta deposits (Spruyt, 1956), also according to Spruyt (1956) the transitional members at the bottom consist of alternating fine to medium sized quartz sandstones and claystone and coal seams. Sandstones at the top intersect with thin limestones and side sandstones, marl, claystone and shale. This member contains fossils of Molluscs, Crustaceans, remaining large foram fish and small forams, deposited in the paralis, litoral, delta environment, until the edge of the sea is shallow and gradually toward the open sea towards the basin. This formation is from the Late Oligocene to the Early Miocene. The thickness of this formation in the southern part of the basin reaches 460 - 610 meters, while in the northern part of the basin has a thickness of approximately 300 meters (De Coster, 1974).

c. Baturaja Formation (BRF)

According to Spruyt (1956), this formation was deposited in harmony above the Talang Akar Formation. Consisting of reef limestone and sandstone sandstone. Gumai mountains are exposed from the bottom up in succession of tufan marbles, layers of coral limestone, white gray sandstone, these limestones contain large forams, including Spiroclypes spp, Eulipidina Formosa Schl, Molusca and so on. Thickness between 19 - 150 meters and Early Miocene age. Environment Precipitation is shallow sea. The naming of the Baturaja Formation was first put forward by Van Bemmelen (1932) as "Baturaja Stage", Baturaja Kalk Steen (Musper, 1973) "Crbituiden Kalk" (vd Schilden, 1949; Martin, 1952), "Midle Telisa Members" (Marks, 1956 ), Baturaja Kalk Sten Formatie (Spruyt, 1956) and Telisa Limestone (De Coster, 1974). The location of the Baturaja Formation type is in the Baturaja cement plant (Van Bemelen, 1932).

d. Gumai Formation (GUF)

This formation is deposited after the Baturaja Formation and is the result of sediment deposition that occurs when the sea trough reaches its peak. The relationship with the Baturaja Formation at the edge of the basin or the area in the shallow basin is harmonious, but in some places in the basin centers or in deep part of the basin sometimes run with the Baturaja Formation (Pulonggono, 1986). According to Spruyt (1956) This formation consists of tufaan napal with bright gray to dark gray. Sometimes there are hard, tuff, glauconite sandstone layers, tuff breccias, shale clay and a thin layer of limestone. Sediment deposits in this formation contain a lot of Globigerina spp, and marl is hardened. Westerfeld (1941) states that the Telisa layers are a monotonous series of shales and marl containing Globigerina sp with tufa interlude as well as glauconite sand layers. The age of this formation is Early Middle Miocene (Tf2) (Van Bemmelen, 1949) whereas according to Pulonggono (1986) the Early Miocene to Middle Miocene (N9 - N12) age.

e. Air Benakat Formation (ABF)

According to Spruyt (1956), this formation is the initial stage of the deposition cycle of the Palembang Group, namely at the beginning of the deposition of marine shrinkage. This formation is from the Late Miocene to the Pliocene. The lithology consists of tuffaceous sandstones, a little or a lot of tuffaceous clays that intersect with the limestone limestone or the sandstones are getting more and more the glauconite content decreases. In this formation Globigerina spp is found, but many contain Rotalia spp. At the top there are many molluscs and plant remains. In Limau, in the investigation of Spruyt (1956) found clay flakes which were blue to gray brown, flakes of sandy clay and tuffaceous sandstones. In the Jambi area found in the form of bluish clay, marl, sand flakes and sandstones containing Molluscs, glauconite is sometimes coarse. Deposited in the lower neritic depositional environment and gradually shallow upper sea level (De Coster, 1974). The thickness of this formation ranges from 250-1550 meters. The location of this type of formation, according to Musper (1937), is located between Air Benakat and Air Benakat Kecil (approximately 40 km north-northwest of Muara Enim (Lembar Lahat). The other name is "Onder Palembang Lagen" (Musper, 1937), " Lower Palembang Member "(Marks, 1956)," Air Benakat and en Klai Formatie "(Spruyt, 1956).

f. Muara Enim Formation (MEF)

According to Spruyt (1956) this formation is in fact aligned above the Air Benakat Formation. This formation can be divided into two members "a" and members "b". Members of "a" are also called Brown Members consisting of brown claystone and sandstone to gray brown, fine to medium sized sandstones. In Palembang there are also coal seams. Members "b" are also called Blue Green Members consisting of sandstone clay and tufaan claystone which are green in blue, some layers of coal are dark red, fine coarse sandstones are white to light gray. In members "a" sometimes found the content of Foraminifera and Mollusca in addition to coal and plant residues, while in members "b" other than coal and plant residues are not found fossils except brackish water foram Haplophragmoides spp (Spruyt, 1956). The thickness of this formation is around 450-750 meters. The member "a" is deposited in the litoral environment which gradually changes the environment of brackish and land water (Spruyt, 1956). The location of the type is located in Muara Enim, Kampong Minyak, Lembar Lahat (Tobler, 1906)

g. Kasai Formation (KAF)

This formation ends the ocean shrinkage cycle (De Coster and Adiwijaya, 1973). At the bottom consists of tufan sandstones with several intervals of tuffaceous claystone, then there is a conglomerate alternating layers of loose tufan claystone and sandstone, at the top there is a pumice tuff layer containing residual plants and cracked wood with a sedimentary cross structure, lignite as lenses in tufan sandstone and claystone (Spruyt, 1956). Tobler (1906) discovered Viviparus spp and Union spp freshwater mollusks, whose age was thought to be Plio-Plistocene. Deposition environment for brackish water to land. This unit is thrown wide in the eastern part of the sheet and reaches a thickness of 35 meters.

3. Alluvial Deposition Unit

The distribution of this unit includes the river area and the banks of large rivers in the form of meanders in the middle and on the banks of the river. The thickness of the alluvial deposits varies, and this unit consists of the results of frozen rags, sedimentary rocks, loose metamorphic rocks measuring fine sand to the crust.

Reference:

• Barber, A.J., Crow, M.J., Milsom, J. S., 2005, “Geology, Resources and Tectonic Evolution”, Geological Society London, Geological Society Memoirs No.31.
• Bishop, M.G., 2001, “South Sumatra Basin Province, Indonesia: The Lahat, Talang Akar-Cenozoic Total Petroleum System”. USGS Field Report 99-50-S.
• Eubank, R.T., Makki, A.C., 1981, “Structural Geology of The Central Sumatra Back Arc Basin”. Proceedings Indonesia Petroleum Association, Tenth Annual Convention.
• http://muhammadafit.blogspot.com/2013/11/geologi-indonesia-cekungan-sumatra.html
• http://pswtkertas.tumblr.com/post/32869528255/geologi-regional-cekungan-sumatera-selatan.