The Geology of the Central and South Sumatra Basins

© IPA, 2006 - 3rd Annual Convention Proceedings, 1974

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PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Third Annual Convention, June 1974. THE GEOLOGY

OF THE CENTRAL

AND

77

SOUTH SUMATRA BASINS

G.L. DE COSTER *)

BASIN DESCRIPTION The Central and South Sumatra basins are important oil producing areas on the island of Sumatra in the Republic of Indonesia (Figs. 1 and 2). They are Tertiary structural and depositional basins composed of a Tertiary sedimentary section lying o n an u n c o n f o r m i t y surface of pre-Tertiary metamorphic and igneous rocks. These are two of the three basins located on Sumatra ( t h e third is the North Sumatra basin) aligned northwest-southeast between the Barisan Mountains to the southwest and the Malacca and Karimata Straits and the Java Sea to the northeast and east. T h e rocks exposed in the basins consist almost wholly of Tertiary strata though several uplifted blocks i n the basins (including the Tigapuluh and Duabelas mountains) do expose pre-Tertiary rock at the surface. The rocks exposed in the Barisan Mountains are composed of Paleozoic and Mesozoic metamorphic and igneous rocks and of young Tertiary to Recent volcanics. Analysis of the Central and South.Sumatra basins show that they had very similar and related histories a n d c o u l d be considered as one large basin with many troughs and grabens. The North Sumatra basin, on the other hand, appears to have been separated from the Central Sumatra area throughout most of its h i s t o r y by the Asahan arch and can be treated as a separate basin. For this report the Central and South Sumatra areas will be discussed and described as separate basins. The Central and South Sumatra basins (see Fig. 2) are asymmetric basins bounded on the southwest by faults and uplifted exposures of pre-Tertiary rocks along the m o u n t a i n front of the Barisan Mountains;. on the northeast by the sedimentary or depositional boundaries of the Sunda shelf (site- of the ancestral Sunda landmass); to the south and east by the Lampung high and by an arch that parallels the

east coast of Sumatra; and to the north and northwest by the Asahan arch and the outcrops of pre-Tertiary rocks northwest of Pekanbaru. Both The Asahan and Lampung arches were positive elements throughout much of Tertiary time, separating the Central and South Sumatra basins from the adjoining North Sumatra and Sunda basins, respectively.The two arches were covered by shaUow marine seas only during the .Early and early Middle Miocene time to form temporary connections to the neighboring basins. The northeastern and eastern boundaries of the basins along the Sunda shelf and the Lampung high are difficult to define precisely, but are usually placed where the sedimentary section is less than 1500 feet (460 meters) thick and composed of Plio-Pleistocene and Younger strata lying o n Lower Tertiary or pre-Tertiary rocks. The boundary between the central and south basins is also indefinite due to l a c k of major structural features separating them. This factor is a n o t h e r argument for considering the two areas t o be one basin rather than two. The boundary between the two basins is usually drawn as a northeast-southwest band through the northern part of the Tigapuluh Mountains, joining the axis of a broad arch extending southwest from t h e Sunda landmass to a istructurally complex area in the m o u n t a i n front *) P.T. Stanvac Indonesia Jakarta, Indonesia. The writer wishes to acknowledge his indebtedness to the many geologists and geophysicists presently and fomaerly associated with P.T. Stanvac Indonesia for much of the geologic information add interpretation incorporated in this paper. Information was drawn freely from discussions with colleaguesin P.T.S.I. and from the f'des in the Exploration DepartmenL In addition, he is grateful to .personnel in the exploration and geology • departments of Pertamina and"of other eomp/mies operating in Indonesia for their contributions in general discussions about the regional geology of the Sumatra area.

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80 northnorthwest of Jambi. The Tertiary depositional basins extended farther to the west across portions of the Barisan Mountains and, at various times during the depositonal history of the Tertiary section, were connected by seaways to the open sea of th e ancestral Indian Ocean. There is no evidence for the existence of similar marine seaway connections from the two basins northeastward across the Sunda shelf (the ancestral Sunda Landmass) during the Tertiary Period. The m a x i m u m dimensions of the Central and South Sumatra basins are approximately 510 x 270 kilometers (317 x 168 miles), and 510 x 330 kilometers (317 x 205 miles). The two basins cover approximately 104,000 and 117,000 square kilometers (40,000 and 45,200 square miles); their combined area of 221,000 square kilometers is about the size of the state of Utah, of Great Britain or of the state of Victoria in Australia. STRATIGRAPHY The stratigraphy and generalized lith01ogies of the Tertiary units in the two basins are depicted on three figures - Figure 3 which is the correlation chart for the two basins showing the names used in. this paper and industry equivalent terms; and Figures 4 a n d 5 which are schematic strati~aphic sections for the two basins showing the time (but not the thickness) relationships of the units. The formations in the t w o basins are discussed together in ascending order of age. An important point that must be stressed in the discussion of the stratigraphie nomenclature used in this paper is the distinction between rock-stratigraphic units and time-stratigraphic or time-rock units. Some of the stratigraphie units in the basins are rock-stratigraphic units (i.e. formation, member, etc) that are defined as in the Code of Stratigraphic Nomenclature as bodies of rock characterized by lithologic homogeneity. The unit may contain between its upper and lower limits rock o f one lithologie type, repetitions of t w o or more lithologic types, or extreme h o m o g e n e i t y of constitution which in itself distinguishes the unit. The contacts of such a rock stratigraphic unit are boundaries of lithologic change, and the unit is often diachronous. They are correlated by their lithologie character, and cannot be subdivided

into time-rock slices for tack of faunal control or of seismic boundaries. These units are shown on the correlation charts with time-transgressive boundaries. The other units in the two basins are described and used as depositional sequences or time-rock units. Such a time-rock unit or " s e q u e n c e " is composed of a body of rock, often of varying facies, whose upper and lower boundaries coincide with or are parallel to boundaries of planktonic zones or seismic horizons, or their correlative unconformities. F O R M A T I O N AND SEQUENCES Pre-Tertiary Complex The pre-Tertiary section or " b a s e m e n t " in the two basins is a complex of Mesozoic igneous rocks and of Paleozoic and Mesozoic metamorphics and carbonates. In a few localities, strata tentatively dated as Late Cretaceous to Paleocene-Early Eocene occur beneath the Tertiary sedimentary section and are grouped with the pre-Tertiary. The Paleozoic and Mesozoic m et am o r p h i c and sedimentary rocks were intensely folded and faulted and were intruded by igneous rocks during the Middle Mesozoic orogeny.The complexities of structural relationships of the older rocks observed in outcrop in the Barisan Mountains undoubtedly extend into the subcrop of these same rocks in the two basins. Kikim Tufts and Older Lemat The oldest rocks found in the South Sumatra basin that p o s t d a t e the Mesozoic and Paleozoic strata are thought to be the very tuffaceous sandstones, conglomerates, breccias and clays found in the Lemat-1 and -2 wells, in Tamiang-2, in the Laru wells, and in exposures in the Gumai Mountains southwest of Lahat (1) These rocks are probably a part of the cycle o f continental sedimentation, vulcanism and erosion that accompanied the Late CretaceousEarly Tertiary tectorfism in the South Sumatra basin. (1) wells located as follows: Lemat-1 03 ° 08' 55"S, Lemat-2 03 ° 12' 57"S, Tamiang-2 02 ° 30' 40"S, Laru wells about 03 ° 40°S, Outcrops about -03° 49' S,

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84 Lemat Formation and Benakat Member The term " L e m a t " has been applied in the past to tuffaceous, coarse clastic, continental sedimentary rocks present in the South Sumatra basin that are now interpreted to comprise several distinct units that are lithologically similar but of differing ages. These units included 1) the extensive deposits in the basin of pre-Talang Akar tufts and coarse clastics (the " y o u n g " Lemat), 2) the "old" Lemat in the Lemat wells, and 3) the tufts present in the Laru wells and in outcrop near Lahat. The use of the name Lemat is now restricted to the " y o u n g " Lemat; the term "Kikim Tufts" is used for the "old" Lemat and for the occurrences in the Gumai Mountains and the Laru wells. The coarse clastic member of the Lemat is composed of sandstones, clays, rock fragments, breccias, "granite wash", occasional thin coal beds and tufts, all deposited in a continental environment. The Benakat Member of the Lemat (which is not to be confused with the Air Benakat Formation) occurs in central portions of the basin and is composed of grey-brown shales with some beds of tuffaceous shale, siltstone and sandstone, and occasional thin coal beds, carbonate stringers and glauconite. It was deposited in a fresh to brackish environment and lies conformably on the coarse clastic lower member of the Lemat, locally termed the granite wash member, in the center of troughs. In the proximal portions of the depositional troughs fhe Benakat may grade into the coarser elastic facies of the Lemaf. The relationship, however, is not too clear due to abrupt .thinning and truncation of the Lemat onto the flanks of the Pendopo anticlinorium. The Lemat is normally bounded at its base and its top by unconformities that coincide with seismic horizons. In distal parts of the basin the contact of the Lemat with the Talang Akar is interpreted to be paraconformable. The thickness of the formation is highly variable, ranging from a feather edge to more than 2500 feet (about 760 meters) in south Sumatra, and to as much as 3500 feet (about 1070 meters) (as defined by seismic data) in one fault depression in the central part of the basin. These maximum thicknesses of the unit have not been penetrated by the drill. The Lemat is

probably Paleocene-Early Oligocene and the Benakat Member, Late Eocene-Early Oligocene in age, as determined by spore-pollen and K-Ar age dating of some samples of shale and tuff. Kelesa Formation The Kelesa Formation occurs in the Central Sumatra basin and is composed of conglomerates, coarse quartz sands, variegated shales, coals and tuffaceous material which were deposited in a continental environment. It has a localized distribution in the basin area and was deposited as the initial sedimentary fill in troughs and grabens on the basin shelf. The unit is a sequence whose base is the u n c o n f o r m i t y with the pre-Tertiary that provides a good seismic reflection, and whose top is unconformable with the overlying Lakat Formation. The top is a distinct log pick and seismic reflector, commonly described as an u n c o n f o r m i t y from dipmeter data and the evident change in depositional environment. However, in local areas, continuous deposition undoubtedly t o o k place in a transitional environment. This continental facies is interpreted to grade northwest to the Tapanuli trough into the claystones, sandstones and shales of the pre-Sihapas Formation. The Kelesa attains a maximum thickness of about 4000 feet ( I 2 2 0 meters) in local areas in the southern part of the basin. It has been dated as Oligocene to Early Miocene on palynological evidence. The unit is therefore younger than its depositional cycle equivalent, the Lemat Formation, in the South Sumatra basin. Pre-Sihapas F o r m a t i o n The name Pre-Sihapas is applied by industry to the occurrence of the Kelesa Formation in the northern half of the Central Sumatra basin wh~re it is composed of variegated continental claystone and sandstone, and of massive dark brown lacustrine shale. It grades eastward and southward into the continental, coarse clastics of the Kelesa. The Pre-Sihapas is probably Oligocene but the basal sandstones in the deep portions of the trough may be as old as Upper Eocene. Talang Akar Formation This unit occurs in the South Sumatra basin,

85 overlying the Lemat Formation or the preTertiary section and underlying the Telisa Formation or the Basal Telisa Limestone Member. The Tal~ng Akar is here considered as a sequence which locally onlaps the underlying Lemat or pre-Tertiary and is essentially conformable with overlying strata of the Telisa Formation. Often distinct seismic reflections mark the upper and lower bounding surfaces of the Talang Akar and where the formation is quite thin, the upper seismic horizon is often masked by the reflection from the top of the Basal Telisa Limestone Member. Formerly the Talang Akar was considered to be essentially a sandstone unit with minor amounts of shale, but in this paper it is considered as a time-rock sequence that incorporates important facies and lithic changes within its boundaries. The Talang Akar is composed of delta plain sandstones, siltstones and shales that grade basinward into marginal marine sandstones and shales," and from there farther troughward into marine shales. The contact with the Lemat is unconformable in intermediate and rim parts of the basin and probably paraconformable in trough areas. The contact with the overlying Telisa and Basal Telisa Limestone is conformable over most of the basin, though likely to be disconformable on the basin rim, and is usually placed at the base of the " K " fimestone which is a widespread marker bed. This Talang Akar-Telisa contact is difficult to pick in wells in trough areas because the lithologies of the two formations are generally similar; there, seismic and paleontology information is used to assist in the correlations. The changes in thickness of the formation over the basin occur in a m o r e regallar pattern than it does in the Lemat Formation. The thickness of the Talang Akar varies from a feather edge around emergent highs and basin margins to as much as 1500 to 2000 feet (about 460 to 610 meters) in some of the trough areas in the basin. The thicknesses of the Talang Akar and the Lernat usually vary correspondently in the basin and where one is thick the other is likely to be thick as well. The age of the Talang Akar sequence is Upper Ofigocene and Lower Miocene and probably includes the N.3 (P.22), N.4 and part of the N.5 planktonic foraminiferal zones in the

Banner and Blow P and N Zone system of planktonic foraminiferal zonation. There is a paucity of age dating information for the Talang Akar - few wells have been analyzed for planktonic foraminifera and furthermore most of the wells in the basin were drilled in areas that correspond to delta plain and shelf areas of deposition of the Talang Akar where planktonic. fauna are scarce or absent. No Middle Oligocene rocks have yet been encountered in the basin, although they might occur in the centers of troughs where sedimentation would be expected to be most continuous. Late Oligocene fauna have been identified in beds correlated as the trough facies of the Talang Akar sequence in outcrops in the Air Cawang Kikim section On the southwestern edge of the]~asin and in the Wahalo and Bingintelok wells[2) Samples from the Talang Akar from certain wells in the basin were analyzed as occurring in the Early Miocene Florschuetzia levipoli palynological zone; these samples came from areas where the Talang Akar was deposited in a delta plain environment and where sedimentation probably did not start until Early Miocene. A n o t h e r , informally named, faunal zone, the Black Globi zone, occurs in the trough facies of the upper Talang Akar. In this zone the Globigerinid fauna are dark brown to black, in contrast to the buff and white color of similar fauna in overlying beds, and are associated with pyrite. This suggests a euxenic environment of deposition for the upper Talang Akar in the distal parts of the basin. Lakat Formation This formation in the Central Sumatra basin is c.omposed of relatively clean quartz sandstones and thin interbedded shales deposited in an inner neritic to shoreface environment. The lower contact with the Kelesa was described previously; the upper contact is an electric log and lithology pick. The unit is a poor seismic reflector in the Kampar area anff can only be mapped where, it is tied to well control. The thickness varies with basin position but may be as much as 1100 feet (336 meters) or more in (2) CawangKikim03° 50S, 103 ° 23'E Wahalo-1 02° 59 25"S, 103° 39' 47" Bingintelok-I 02 ° 35'

38"S, 103° 06' 58"

86 certain wells and areas. No paleontologic basin and is composed of calcareous shales information is available for the formation - it interbedded with thin, glauconitic silts and is usually interpreted to be Lower Miocene on • sands, representing the transitional facies from the basis of stratigraphic position. inner to outer neritic marine deposition. The electric log pick is placed at the top of the Si Hapas Group uppermost, relatively thick glauconitic sand below the massive Telisa shale. A widespread The Si Hapas Group is an equivalent term and continuous seismic reflector occurs at the used for the section that includes the Lakat and the Tualang Formations. The lower Si Hapas is top of this unit which is also a sequence the equivalent of the Lakat, and the Upper Si boundary defining the top of the basal wedge Hapas, of the Tualang. The age span for the Si of sedimentation of the Tertiary depositional cycle in the Central Sumatra basin. The Hapas Group is given as N.4 to N.8 in the P and formation is normally about 200 to 400 feet N Zone system of planktonic zonation. (61 to 122 meters) thick in the southern half of the basin and occasionally reaches 500 to 600 Basal Telisa Limestone feet (153 to 184 meters) in some of the wells. T h e Tualang is interpreted to be the This member was deposited in intermediate and shelfal portions of the South Sumatra approximate time-rock equivalent of the Basal basin, on and around platforms and highs. It is Tetisa. Limestone of the South Sumatra basin both a rock unit (member) and a sequence, in and to be of Early Miocene age. It is assigned contact at its base with tlie Talang Akar this age principally on the basis of stratigraphic sequence or with pre-Tertiary rocks and position and the rare occurrence of Spirocoinciding at its top with a strong seismic clypeus in the unit. reflection. The Basal Telisa Limestone is composed of Telisa Formation platform or bank limestone capped in restricted localities by further buildups of detrital, reefal The Telisa Formation is the most widespread and bank limestones. In the distal portions of occurring of tl3e Tertiary units, being deposited the basin the equivalent unit consists of shales during the time of maximum marine transgreswith thin limestone beds that are often mapped sion into the two basins. The Telisa is as a part of t h e undifferentiated Telisa characteristically a fossiliferous, marine shale Formation. The thickness of the lower bank containing occasional thin beds of glauconitic unit is fairly constant over the shelf area, limestone. On the basin rims and shelfal areas it averaging about 200 to 250 feet (about 60 to occurs in a shallow marine facies with siltstones 75 meters) although the thickness variations are and fine-grained sandstones as well as limegreater where the unit lies on the pre-Tertiary stones present with the shale. Sandstones on with more irregular topographic relief to be the basin shelf that heretofore were correlated filled. An additional 200 to 400 feet (about 60 as Talang Akar because they-were sandstones to 120 meters) of limestone are present in those occurring beneath Telisa shales, are now shown areas where the buildups of the upper unit to be sandstones developed within the Telisa. occur, The outcrop of the Baturadja Formation The Telisa lies on Talang Akar in the South in the Garba Mountains is reported to be about Sumatra basin and on the Tualang in the 1700 feet (about 520 meters) thick. The Central Sumatra basin, and on pre-Tertiary member is very fossiliferous and has been dated rocks on most of the basin rim~. It is overlain as Early Miocene. The fauna contained in the by the Lower Palembang (in South Sumatra) Basal Telisa Limestone probably correspond and the Binio ( in Central Sumatra ) Formations mostly to the N. 6 faunal zones but may extend at a contact that is very difficult to select locally into the N.5 and the N~7 zones. consistently over the entire basin areas. The top of the Telisa as picked on lithologic characteristics is diachronous, the characteristic Telisa Tualang Formation lithology persisting until later in time in basin The Tualang occurs in the Central Sumatra centers than on basin edges. Now with-better

87 seismic and regional information available, a more time-constant contact can be Selected, based in part on the occurrence of a seismic reflection within the lower part of the Lower Palembang and the Binio, at the top of or in a Section of sandstones interbedded with shales. Where lithic and seismic data are available, the top of the Telisa is placed at the base of this lower sandstone section in the Lower Palembang or Binio, above the massive shale section of the Telisa. The Telisa on the stratigraphic charts is shown as a sequence (or time-rock unit) with time constant top and base. The thickness of the Telisa varies greatly with basin position and probably is as much as 6000 to 9000 feet (about 1800 to 2700 meters) thick in trough areas. The Telisa can be dated with planktonic fauna. U s i n g the new sequence tops for the Telisa, its zonal ages extend from N.7 or possibly N.6 at the base to as high as into N. 11 at the top. In distal parts of the basin where it lies on the Talang Akar, the Telisa probably extends down as far as into the No.5 zone. The top of the formation may extend into the N.12 or higher zone but those zones in like manner have not been recognized in well samples perhaps because the marine environment has shallowed .to such an extent that few planktonics occur. In the Central Sumatra basin where some paleontologic studies have been made, the age of the Telisa, as well as can be interpreted from the limited data, probably occurs over the time span bracketed by faunal zones extending from approximately N.7 up to N.12 or N.13. It has been reported that the environment of deposition reached its maximum depths during the Globigerinoides bisphericus (G. sicanus) (lower N.8) zone in the South Sumatra basin and in the Globorotalia fohsi fohsi (N. 10-11) zone in the southern half of the Central Sumatra basin. A minor hiatus occurs in the uppermost Telisa and the lower Lower Palembang and lower Binio on the basin edges which is detectable as a disconformity in some of the basin edge wells but is not discernable in wells away from the basin edge nor on the seismic sections. This hiatus may be related to the Intra-Miocene diastrophism and is discussed in further detail in the section on structural and depositional history of the basins.

Lower Palembang (South Sumatra) and Binio Formation (Central Sumatra) These units which are usually considered to be equivalents in the two basins, were deposited during the early stages of the regressive cycle of deposition. They are composed of shales with glauconitic sandstones and occasional limestones, deposited in a neritic environment at the base grading to a shallow marine environment at the top. The base of the unit and its contact with the Telisa were discussed above and is an approximate time constant surface. The upper contact with the Middle Palembang and the Korintji is essentially a lithologic contact, based in South Sumatra on the occurrence of coals, in the Middle Palembang. The Pangadang coals in South Sumatra and the coals in the middle Korintji usually give a strong seismic reflection which is mappable over broad areas. This would suggest that these are fairly synchronous surfaces that can be used in correlations; however, the occurrence of the coals vary in different parts of the basin so that an interpreter cannot be certain that the same coal beds are being mapped on the opposite sides of the larger anticlines which expose Middle Palembang or Korintji strata. There is no diagnostic age dating information available for the two formations. The units have been interpreted in most reports to be mostly Late Miocene in age, though their age range may extend into slightly older and slightly younger stages. The thickness of the units vary considerably with basin position and on how the contacts are selected; thicknesses usually range up to 3300-5000 feet (1000 to 1500 meters). Middle Palembang and Korintji Formation These units were deposited in shallow marine-brackish (at the base), paludal, delta plain and non-marine environments and are composed of sandstones, mudstones and coal beds. They are rock units identified and correlated by lithologic criteria. The lower boundary of the Middle Palembang in the southern part of the basin is usually marked by coal beds, and in the Jambi trough area by equivalent strata where the top contact is picked at the highest glauconitic sandstone of the Lower Palembang. The number of beds and thicknesses of coal decrease from south to

88 north in the South Sumatra basin, l-n South Sumatra strong seismic reflections come from the coal beds as described in paragraphs above. In Central Sumatra the contact with the Binio Formation is picked by lithic and log correlations with the coals (that are locally good seismic reflectors) falling within the Korintji. The upper contact with the overlying Upper Palembang or Nilo probably varies considerably with the basin position, usually being disconformable or unconformable, and is picked at the base of the lowest thick tuffaceous beds in the overlying Upper Palembang or Nilo. The thickness of the Middle Palembang and Korintji vary with basin position and how the contacts are picked; the maximum thickness may reach 1500 to 2500 feet (450 to 750 meters). There are no faunal data from these units that are usable for age dating. The formations are usually interpreted to be uppermost Miocene to Pliocene in age on the basis of stratigraphic position. Upper Palembang and Nilo Formation These units were deposited during the Plio-Pleistocene orogeny and are mostly erosion products derived from the uplifted Barisan and Tigapuluh Mountains and from the uplifted folds being formed in the basins during the orogeny. The formations are composed of tuffaceous sands, clays and gravels, and occasional thin lentils of coal, with great variations of thickness and composition. The basal contact is usually placed at the base of the lowest thick tuffaceous layer. The units occur in the synclines formed during the orGgeny and are absent from the anticlinal folds. A Plio-Pleistocene age is usually assigned to the units based o n their association with the orogeny of that age.

STRUCTURAL GEOLOGY SUMMARY The structural features present in the two basins axe the result of orogenic activity that occurrea in at least three separate episodes the mid-Mesozoic orogeny, the Late Cretaceous-Early Tertiary tectonism and the PlioPleistocene orogeny. The earliest of the major episodes was the mid-Mesozoic orogeny when

the Paleozoic and Mesozoic strata were metamorphosed, faulted, and folded into large structural blocks or belts and intruded by granite batholiths. These belts of metamorphic rock are composed of strata of varying lithologies, of differing degrees of metamorphism and varying intensities of deformation. They are exposed in the Barisan Mountains, are postulated to extend into the basin subsurface, and together form the basic "structural grain" of Sumatra. The second significant tectonic event occ u r r e d probabl~, in Late Cretaceous and Early Tertiary time, when major tensional structures that include grabens and fault blocks were formed in the basins of Sumatra and in the adjoining Sunda basin as well. The general trend direction of these faults and grabens is N-S and NNW-SSE. These tensional features and the remnant structures from the mid-Mesozoic orogeny, coupled with rugged paleotopography developed by differential weathering of the pre-Tertiary rocks, comprise the "old" structural elements of the basin. These elements structured the pre-Tertiary unconformity surface which in turn controlled the deposition of the Lemat, Kelesa and Pre-Sihapas Formations. The other "old" features include the stable platforms in existence prior to Lemat and Kelesa deposition and which remained essentially unaffected by structural movements during the Late Tertiary orogeny. The most prominent structural features in the basins are the northwest trending folds and faults formed during the .Plio-Pleistocene orogeny. The convergence of the Indian Ocean plate against the Sumatra portion of the Southeast Asia plate is postulated to have been the cause of the final uplift of the Barisan Mountains, the development of the major right-lateral wrenching through the length of these mountains, and the formation of the associated fold-fault structures in the basin. In many instances the faulting appears to be partly controlled, their trends interrupted, terminated or offset, and some of their afignments influenced by the boundary faults of the mid-Mesozoic belts of metamorphosed rocks and by the N-S or NNW-SSE faults formed during the Late Cretaceous-Early Tertiary. All these Plio-Pleistocene features are referred to as the '~young" structures of the basins and are

89 easily the most dominant features of Sumatran geology. THE PRE-TERTIARY AND EARLY TERTIARY STRUCTURAL FEATURES The "old" structures in the basin have long been the subject for speculation and interpretation, especially by those exploring for hydrocarbon accumulations that might have been controlled by these older features. Good quality, deep-penetration seismic data and regional and isopach maps of time-rock units have been used in the current studies of these older features. The seismic sections and maps, and the isopach maps of the Lemat and the Kelesa show a considerable amount of topographic relief, some of it fault controlled, to have been present at the time these units were being deposited. Onlap onto the irregular surface that was cut into the pre-Tertiary rocks is common throughout at least the shelf and intermediate portions of the basin where we have data; it is presumed to be present in the deeper parts of the basin as well. These old features are now interpreted to be a combination of paleotopography formed by differential erosion of the pre-Tertiary rocks and of structural elements composed of fault blocks, grabens and stable platform areas formed in Late Cretaceous and Early Tertiary time. The remnants of the belts of Paleozoic and Mesozoic metamorphic and igneous rocks formed during the mid-Mesozoic orogeny also may have comprised part of the regional pre-Tertiary structuring on which the Lemat and Kelesa Formations were deposited. The Late Cretaceous-Early Tertiary tension structures are shown on Figure 6. The major troughs and fault blocks containing thick sections of Lemat, Kelesa and Pre-Sihapas strata include the following: The Benakat "gulley" flanked on the west by faults. This trough, opening to the south into the Lematang trough and to the north into the extension of the Jambi trough, may be a half graben. The Lematang trough, which was probably flanked on the north by a boundary fault separating major blocks of pre-Tertiary rocks.

The Jambi trough. Very little is known about the older Tertiary section in this trough and it is from inference, some seismic fault evidence, gravity data and occurrence of thick sections of Lemat at the northeast end and the west end (in the Barisan mountain-front outcrop) that a trough is postulated here during the time of Lemat deposition. The un-named, north-south trough associated with the Kampar high in central Sumatra. This trough is partially flanked by faults and extends northward onto the shallow basin shelf. The deep graben portion of the Tapanuli trough which received many thousands of feet of Oligocene Sediments. The western flank of the trough may have had intermittent connection to the open sea during the Oligocene. Other faults and smaller grabens of pre- or Early Tertiary age also are present in both basins as shown on Figure 6. In marked contrast to the deep grabens are the platform areas on which no sedimentation occurred until Basal Telisa Limestone and Tualang time. These platforms apparently subsided as stable blocks during the Middle and Late Tertiary and were little affected by the Plio-Pleistocene orogeny. Another broad regional high, partly cut by faults, extended northwest from Palembang toward the basin center. This high, whose surface is composed of hard, massive pre-Tertiary limestone, is flanked by thick sections of L e m a t and Talang Akar clastics and is onlapped b y t h e s e strata from the west and north. " The Tigapuluh and Duabelas Mountains on the other hand, probably were receiving sediments during the deposition of the entire Tertiary section and were not uplifted until Plio-Pleistocene time. Both mountains are rimmed by exposed outcrop of the section from Lemat up to Middle Palembang, 'sugg e s t i n g they were subsiding and receiving sediments during these times. True, the cores of the mountains may have been above wave base or been positive features but there is no way to document this as the Tertiary rock section in the m o u n t a i n areas has been stripped away by erosion.

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91 The finer details of the trends and locations of the old structural features can be reasonably well depicted by the isopach map of the interval between the top of the Talang Akar (or equivalent surface in Central Sumatra) and the pre-Tertiary unconformity (Figure 7). The isopach data on Figure 7 incorporates the effects of pre-Tertiary/Early Tertiary structures, of paleotopography on the pre-Tertiary surface, and of basin, tilting (or differential basin subsidence) during the deposition of the Lemat-Talang Akar/Kelesa-Lahat Formations. Despite the restriction of the effects of basin tilt, the map can be used to analyze the gross features of "old" structure and of relative paleotopography on the pre-Tertiary surface at the time of Lemat and Kel~sa deposition. On the map are seen broad platforms, arches, grabens, fault blocks, numerous isolated mounds and ridges, and many terraces and noses flanking or extending away from regional highs. Most of the faults and troughs are oriented nearly north-south; some are northeast-southwest. The isopach maps and the seismic sections indicate that many of the faults were active during Lemat and Talang Akar deposition. MIDDLE TERTIARY FEATURES

STRUCTURAL

Very little structural movement, other than basin subsidence, seems to have occurred after Lemat and Pre-Sihapas-Kelesa time and before the Plio-Pleistocene orogeny. Some of the "old" faults appear to have been active as late as during early Talang Akar and Lakat deposition but were inactive after that. Some seismic sections indicate some movement along the faults during Telisa deposition b u t these appear to have been of local and only minor significance. Middle Miocene tectonic events interpreted by many geologists from structural evidence in the Barisan Mountains apparently had some effect on the structures in the northern part of the Central Sumatra basin but had httle effect elsewhere in the two basins. There is evidence from some wells located on the northeast edge of the-basins that an hiatus occfirred o n the basin shelfs in the Middle Miocene, possibly a result of eustatic dropin sea level, but there is no indication of structural

movements break.

accompanying this

PLIO-PLEISTOCENE TURES

depositional

STRUCTURAL

FEA-

The most prominent structural features in the basins are those developed during the Plio-Pleistocene orogeny. These features, whose dominant trend is northwest-southeast, are clearly seen on surface geology m a p s and on structure maps of shallow horizons. Figure 8 is a compilation of the axes of Plio-Pleistocene anticlines and synclines and the traces of the major faults formed at the same time. Some of the Plio-Pleistocene features can be seen on structure maps of the Talang Akar and Tualang Formations though earlier events also affect the structuring on this horizon. Structure on the pre-Tertiary unconformity (Figure 9) is the resultant of several events that includes the pre-Tertiary structuring and paleotopography, and differential basin subsidence during the deposition of the Tertiary section, as well as the Plio-Pleistocene folding. This structure map i therefore least representative of pure Plio Pleistocene structuring. Regional analysis of the Sumatra basins highlights some of the significant Plio Pleistocene structural elements that include the following (Figure 8): 1. The Semangko wrench fault, extending the length of the island of Sumatra formed ir the Plio-Pleistocene orogeny as a result o~ the convergent collision of the northeast moving Indian Ocean plate against Sumatra. It is not known if this zone existed as a wrench fault in earlier Tertiary times or iv the Paleozoic or Mesozoic Eras. Evidence against its being present in the Mesozoic i~ that the fault seems to transect the major blocks of pre-Tertiary rocks in the Barisan Mountains that were formed during the mid-Mesoziaic. 2. The northwest trending folds w i t h i n the basin, aligned more nearly parallel to the trace of the Semangko fault than are the blocks of pre-Tertiary rocks in the Barisans. Many of the folds occur in the basins in sets that appear to have right lateral offset. About five sets of folds or anticlinoria are

92

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CENTRAL 8 SOUTH SUMATRA RELATIVE PALEOTOPOGRAPHY AND OIL - G A S FIELDS ISOPACH MAP OF LOWERWEDGE OF T E R T I A R Y

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95 present in the two basins. The northernmost one is aligned northwest, and each succeeding set to the south trends irr-a slightly more westerly dSrection; the southernmost one is nearly east-west. It will be noted that the foldsets are en echelon and slightly rotated to each other, whereas the individual folds within the sets are parallel to each other. Each anticlinorium or fold set terminates at its southeast end against a stable platform or basin margin. 3. The northwest trending faults that are associated with the folds in the basins; both normal and thrust faults are present. Some faults are rejuvenated Early Tertiary features and others were formed contemporaneously with the folding. The alignment and location of ~he PlioPleistocene folds and faults in the basins were controlled by many factors. The plate collision and the accompanying north-easterly vector of compression imposed the dominant northwest trend onto the folds and the faults. Some Plio-Pleistocene faulting of the Tertiary section occurred along the boundary edges of the Underlying blocks of metamorphic and igneous rocks that comprise the pre-Tertiary basement whereas other Plio-Pleistocene faulting consisted of rejuvenation of Early Tertiary faults. Numerous northeast-southwest tension faults are present along the crests of the larger anticlines. The many types of faults that are present in the basins include normal vertical, high angle reverse that steepen to vertical with depth, thrusts that flatten into bedding plane faults a t depth, and normal faults with components of lateral movement. The folds are of various types including straight, curved, sinuous, and straight with abrupt bends at one end. They range in length from a few to more than 90 kilometers. Some are remarkable for their length and for the number of culminations present on their crestal axes. Some folds are of low amplitude; others have steep to vertical flanks with somo degree of flow fold structuring. In places, domes or short north-south folds are present on the crest ,of regional folds. These occur in the form of trapdoor uplifts and are located where the regional fold and the flanking f a u l t bend abruptly to a north-south orientation or where

they are offset in a rigbt lateral sense by north-south faults. STRUCTURAL AND DEPOSITIONAL HISTORY OF THE BASINS The history of the pre-Late Paleozoicevents in Sumatra is still lost to us due to the lack of rocks dating from these periods. The Late Paleozoic a n d Mesozoic events, on the other hand, are partially decipherable from the widespread occurrences of rocks of this age which permit some measure of interpretation to be made for these periods of the geologic histo~- of the area. The history of the Tertiary Period is even better understood because its rock and structural record is much more complete and can be more easily mapped and measured. Moreover, greatest emphasis has been placed on interpreting the more recent geologic events because it is the younger rocks and the mid-Phanerozoic and younger tectonic events that formed the geologic features which are now being explored for hydrocarbons and minerals. The tectonic history of the basin areas from the mid-Mesozoic to the Recent can be subdivided into four major events: 1. the mid-Mesozoic orogeny. 2. the tectonic event of Late CretaceousEarly Tertiary (?) accompanied by tension faulting. 3. tectonic quiescence from the Early Tertiary through the Miocene, accompanied by isostatic subsidence of the basins and deposition of t h e Tertiary sedimentary section. This subsidence was interrupted in the Middle Miocene by diastrophism in the Barisan Mountains and by minor structural movements in the basins, 4. the Plio-Pleistocene orogeny. The tectonic history of Sumatra was profoundly influenced by the movement and recurrent collisions of the Indian Ocean and the" Southeast Asia plates. Reference will be made to these plate tectonic events as they affect the development of the structural and depositional history of the basins. The known depositional history of Sumatra

96 include the following periods of sedimentation: phase is not discussed in this paper because the Late Paleozoic through Early-Middle Meso- areas of tectonism lie far from the basins of primary concern. . zoic Late Cretaceous-Early Tertiary Late Paleocene (+) through Recent EARLY RIODS

AND

MIDDLE

PALEOZO1C

MID-MESOZOIC OROGENY PE-

There is no reference to the presence of rocks of Early and Middle Paleozoic age in Sumatra or in the nearby islands~ The nearest occurrences of older Paleozoic rocks include the Silurian-Ordovician rocks exposed in the western part of the Malay Peninsula northwest of Singapore, and the Devonian rocks reported in Kalimantan. Thus, the record of early geologic time in Sumatra is unavailable or has not yet been revealed. PERMO-CARBONIFEROUS and E A R L Y MIDDLE MESOZOIC DEPOSITION The Late Paleozoic and Mesozoic are well represented by extensive outcrops of PermoCarboniferous, Triassic and Jurassic rocks that occur in the Barisan, Tigapuluh, Duabelas and other mountains in Sumatra, in the islands including Bangka and the Riau and Lingga group lying offshore northeast of Sumatra, and in West Malaysia. These strata are described in considerable detail in published reports and are shown on published geologic maps of the areas. From these occurrences and exposures, it has been interpreted that the vast area now encompassing the Malay Peninsula, the Sunda landmass, and Sumatra was receiving continental margin sedimentation during the Permo-Carboniferous and Early-Middle Mesozoic. The types of sedimentary rock deposited included sandstones, shales, limestones and dolomites that were laid down in shallow to deep marine environments. This sedimentation probably occurred in foreland basins on the margin of the Southeast Asia continental plate, wherever this plate was located in reference to geographic coordinates or in relation to Gondwana Land or to the Eurasia landmass. A history of development of several subduction, zones in the area of the South China Sea, Malay Peninsula, in parts of the Sunda landmass and near Sumatra is postulated by some geologists for the Late Paleozoic and Early Mesozoic periods. This

The mid-Mesozoic orogeny was the first of the four major tectonic events to affect the" history of the basins from mid-Mesozoic to Recent times. This event probably affected all of Sumatra as well as the offshore areas northeast of Sumatra that comprise the Sunda landmass. During this event, the sedimentary strata deposited in Sumatra in the Late Paleozoic and the Early and Middle Mesozoic were uplifted, metamorphosed, faulted and folded into a complex of blocks or zones that form the basic structural framework of the island of Sumatra. These Late Paleozoic and Early-Mid Mesozoic rocks are exposed the length of the Barisan Mountains as well as in tile Tigapuluh, Duabelas and other mountains within the basin areas. In the Barisans, the outcrops have been mapped by zones or belts that include the block mountains of PermoCarboniferous metamorphics, the "slate belt" of Mesozoic metamorphics, the massifs of Late Mesozoic granites, and the other belts as shown on Figure I0. Major faults or zones of weakness probably form the boundaries between these belts of rock. The mid-Mesozoic orogeny was a : m a j o r tectonic event that may have been associated with subduction of an oceanic plate beneath the edge of the Sumatran continental plate. The plate convergence, the initial downbuckling of the crust, and the folding and metamorphism of the Paleozoic and Mesozoic strata in the Barisan Mountains [see Fig. 11) probably occurred about Jurassic time - perhaps Late Jurassic to Early Cretaceous because there are some folded and metamorphosed rocks in the m o u n t a i n blocks that are dated as young as Early Cretaceous (?). The development of the crustal shearing, the underthrusting of the oceanic plate, the uplift of the overthrust plate and the emplacement Of igneous intrusions (see Fig. 12) probably occurred in Middle-Late CretaceOus. The increasing degree of metamorphism and folding thought to be observed from north to south in the blocks of exposed rocks in the Barisan Mountains, and the occurrence of more mafic rocks and of serpentine in association

97

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98

C O M P R E S SION

OCEANIC CONTINENTAL

DOWNBUCKLE Nelson et al (1973) Fig. II

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99 with the metamorphosed Mesozoic rocks found in the most southerly of the blocks, suggested to Nelson e t al (1973) that the sub duction zone may have been located near the south end of Sumatra (see Fig. 10). They also postulated that this zone, in the form of an arcuate band, may have curved eastward and joined a subduction zone interpreted to have been present in central Java and southeast Kalimantan about this time.

LATE C R E T A C E O U S - E A R L Y T E R T I A R Y TECTONIC EVENTS Hardly had the tectonism of the midMesozoic orogeny ceased when the area of the Central and South Sumatra basins was subjected to the next episode of structural movement. During the episode, regional tensile stresses were imposed on the area and grabens, faults, and fault blocks were formed. The significant trend direction was north-south or NNW-SSE but many faults were aligned northeast and some, northwest. These features are depicted on Figure 6 and include the Benakat "gulley" and the Jambi, Lematang and Tapanuli troughs. The stable platform areas shown on the maps are rigid blocks formed by Early Tertiary time and which were not much affected by later orogenic movements. In the Sunda basin, the north-south block fault system perhaps formed about the same time but this has not been proven. Information from the deep basin areas is lacking but evidence that is available indicates the faulting to have b e e n active at least as early as the Oligocene. The basic cause for the development of these major structural features is not known. They m a y have been related to a different phase of plate m o v e m e n t or they may have been associated with a secondary period of mantle upwelling following the mid-Mesozoic orogeny. But whatever the cause, the effect was the widespread development of tension structures in the Sumatra basins. The postulated right lateral wrench fault along the Sumatra coast is a very speculative featiare. We are not sure if there was wrenching nor do we know when the fault originally formed. There is strong evidence, nonetheless, for this feature being a major zone of weakness or a boundary between two crustal blocks and it might have been

originally formed as a wrench fault or a transform fault. The trace of this feature, which we will refer to as the "East Coast Sumatra zone", marks the contact between the Sunda basin-Java block and the Sumatra block whose respective structural trends differ markedly in direction - east-west alignments in Java and northwest in Sumatra. Evidence for .wrenching along the East Coast Sumatra zone is minimal but includes the following: 1) The oroclinal folding of the rocks on Bangka Island into an arcuate pattern that suggests right lateral wrenching. 2) The straight east coast of Sumatra that gives the appearance of marking the trace of a fault. This feature has been noted in many reports, for example by Todd and Pulunggono (1971). 3) An apparent light-lateral offset of Jaw= from Sumatra. 4) T h e abrupt swing or embayment, in a right-lateral sense, of the depth contours on the bathymetric map east of Christmas Island located south of Java. 5) The apparent right-lateral offset of the axis o f the gravity minimum in the Sumatra trench, about 150 kilometers north of Christmas Island. The East Coast Sumatra zone might have extended south from Banka Island through the western tip of Java - w h e r e a fault with mid-Miocene normal movement is mapped f r o m surface o u t c r o p - - thence farther southward to pass near Christmas Island and [hen south in the Indian Ocean. If wrenching did occur, it must have been in post-Jurassic time because the rocks that were folded on Bangka Island are of Triffssic and Jurassic age. The fault might have developed originally as one of a series of north-south transform faults associated with a spreading oceanic ridge located south of Sumatra and Java, perhaps on the east-west trending Cocos ridge. The faulting might also have been contemporaneous with the major o r o g e n i c event of Late Cretaceous to Late Eocene age that formed the Ma Song transform fault near Savu and Flores Island, east of Java (described by Audley-Charles et al (1972). These are the data bearing on this feature; the presence of Late Cretaceous-Early Tertiary faulting and of wrench m o v e m e n t remain to be

100 proven. Accompanying the tectonic activity of the Late Cretaceous-Early Tertiary were episodes of sedimentation in different parts of the basin and vulcanism that produced breccias, tuffs and volcanocl~ts. Shallow basins that received sedimentation may have been created during the Cretacous on the continental shelfs of Sumatra during the uplift of the lip of the overthrust plate• Occurrences of Upper Cretaceous-Lower Tertiary rocks are scarce, however - they may have been removed by erosion that followed soon after their deposition. No marine rocks of this age have yet been found in the basins; the few occurrences of dated rocks from this age include 1) the tuffaceous clastics dated by K-Ar analysis as Late Cretaceous or older from the Lemat-2 well, and 2) the lava tufts with a m i n i m u m age of Late Paleocene-Early Eocene from Tamiang-2 in beds described as having high dip. The tuffs and elastics found in the Laru wells and exposed along the m o u n t a i n front southwest of Lahat, are thought to be of possible Late Cretaceous-Early Tertiary age. DEPOSITION OF THE MENTARY SECTION

TERTIARY

SEDI-

Summary The deposition of the Tertiary sedimentary section in the Central and South SUmatra basins occurred during a period of _tectonic quiescence; the sediments were laid down in isostatically subsiding basins where basin subsidence, erosion of the source areas and deposition in the basins, and changing eustatic sea level controlled the sedimentation. The tectonic quiescence, occurring between periods of tectonic upheaval in" the Late CretaceousEarly Tertiary and the Plio-Pleistocene, may have been the result of reduced sea-floor spreading activity at that time. It is postulated in some published reports, for example, McKenzie and Sclater (1971), that 1) spreading along the I n d i a n Ocean spreading ridge ceased from the time of A n o m a l y 22 (Late Paleocene) to a b o u t A n O m a l Y 17 (Oligocene) and 2) that during the Miocene perhaps both the Indian Ocean and the. Southeast Asia plate were moving northward (2) together or t h a t - t h e Indian Ocean pla'te Was sliding past Sumatra without convergence. Other studies, such as

those by Le Pichon and Heirtzler (1968) and Ewing and Ewing (1967), and others, suggest that plate movement may have halted during the Miocene from the time o f Anomaly 7 or 8 to Anomaly 5. The Tertiary sediments were deposited in the basins in a transgressive-regressive cycle extending from about the Paleocene-Eocene to the Plio-Pleistocene. The sea encroachment into the basins was episodic with important stillstands .and temporary shallowing and deepening effects of the sea occurring periodically- during the major transgressive-regressive cycle. The initial fill of the rugged paleo-topography of the basins occurred from the Paleocene to the Oligocene with the deposition of tuffaceous continental elastics; the initial fill occurred in the Eocene and Oligocene in Central Sumatra. Subsequent deposition in the Late Oligocene to Middle Miocene occurred .in deltaic and marine environments in differentially subsiding basins and troughs. Marine seas encroached into the basin areas and gradually onlapped the topographically high features so that by Early to Mid Miocene time the widest • extent of the basins was receiving sediments being deposited in shallow to deep marine environments. The regressive cycle began in the late Middle Miocene with the slow withdrawal of the sea and the shallowing of the environment of deposition from deeper marine to shal.low marine to paludal and then to deltaic and continental conditions. The close of the sedimentary cycle- occurred i n the PlioPleistocene with the deposition of the tuffaceous elastics that accompanied the PlioPleistocene orogeny.

Deposition of Lemat and Kelesa Formations The first phase o f s e d i m e n t a t i o n in file basins occurred in L a t e Paleocene to Early O!igocene (Eocene to Oligocene in Central Sumatra) in intermontane basin environments (Figure 13). The orogenic activity of the midMesozoic orogeny,, the faulting of the Late Cretaceous and Early Tertiary, and differential erosion of the exposed pre-Tertiary metamorphic and igneous rocks had developed a terrain of substantial toPographic relief that probably was similar to the basin and range p r o v i n c e o f western United States~ The tuffaceous continental sediments of the Lemat .and Kelesa

101

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Generallzed facies map. Paleocene to 011gocene. tions in Central Sumatra basin, Lemat Formation, South Sumatra basin.

Kelesa and Pre-Sihapa|.Formaincl. B~akat N e m b e r , ~n

102 Formations filled the grabens, fault blocks, and topographic lows in these basin areas and.were laii] down as alluvial fan, braided stream, valley fill and piedmont deposits. The sediments were derived for tile most part from local source areas and deposited in the nearby valleys and depressions. The basal unit iff South Sumatra is often very felspathic ("granite wash") suggesting erosion of nearby granite hills. Intermittent vulcanism and erosion of earlier-deposi~ed tufts provided the tuff-ash that is so common aconstituent of sediments laid down at this time. The thin coals in the Lemat in South Sumatra, indicate local swamp conditions. No principal direction of sediment transport can be determined, as gravity flow and water runoff were probably the principal erosion and transport mechanism. Movement continued along many of the faults and deposition in the troughs kept pace with subsidence. The formations occur as thick pods in the downfaulted blocks, are thin or absent on highs and unfaulted blocks, and are characterized by rapid and abrupt thickening • and thinning of section. Many areas showing the Lemat and Kelesa to be absent occur with irregular outline and on no particular trend, indicating where hill,s, knobs, ridges and other topographic highs lay above sedimentation level. In the portions of the basins with little information the isopach maps indicate broad areas having received Lemat or Kelesa sediments. If detailed data were available we would probably see more areas of non-deposition indicating the presence of hills and ridges. Furthermore, additional grabens and fault blocks with Lemat and Kelesa sediments, similar "to the trough between Bangka Island and the Sumatra coast, probably are present in the basins. In the Late Eocene-Early Oligocene a tresh to brackish, lacustrine environment developed in portions of the South Sumatra basin and the Benakat Member shale was deposited in the areas shown on Figure 13. The extensive iake that must have formed at this time may have had intermittent connections with the marine seas through outlets on the west and south -western rim of the basin as indicated .by the occasional limestone, dolomite and glauconite in the Benakat.

Deposition tions

o f Talang Akar and Lakat Forma-

Sedimentation in the t w o basins w a s renewed in Late Oligocene and Early Miocene wffh the deposition of the Talang Akar and Lakat Formations in deep to marginal-shaUow marine, deltaic and fluviatile environments. The distribution of t h e gross facies and environments of deposition Of .these formations is depicted on Figure 14. In South Sumatra, sea-ways t o the Indian Ocean opened, the seas encroached into the basins, and fine muds w e r e laid down in the deeper troughs that were first invaded. The .deposition of fine clastics persisted in these troughs throughout Talang Akar time, sometimes in deep marine to bathyal (?) environments as was determined by wells drilled in these areas. The environment in these troughs was occasionally euxenic as indicated by the presence of dwarf fauna and pyrite, and by the brown coloration of the foraminifera. In Early Miocene time, sedimentation began on the basin.shelfs in the form of alluvial fans and braided streams which filled topographic lows and depressions. As the topography became more constant and level, widespread delta plain sediments, consisting primarily of point bar and braided stream deposits, were laid down over broad areas of the basin. These graded into delta front and marginal marine sands which in t u r n graded into predelta shales laid down in the more distal parts of the basin. As the deltaic sediments prograded toward the basin center , erosion removed some of the deltaic and fluvial sediments deposited on the basin margins and redeposited the material in the new delta plain and delta front. In the southern part of the basin, the principle source direction was from the east and sediment transport was from the southeast toward the n o r t h w e s t . In the Jambi trough the source was l r o m the northeast and sediment tranport was from the east to west and to northwest. The interpretation o f sedimentation on the west flank of the basin is u n c e r t a i n b e c a u s e the Plio-Pleistocene uplift and erosion_of the Tertiary section has removed much of the lithic evidence. It is believed that narrow bands of nearshore marine sands m a y have been deposited around the positive blocks or islands existent within the basin and

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bordering the west side of the basin. The sands around these latter island are tuffaceous, the material probably coming from volcanic sources on or near the islands; many tufts are also present in the overlying Telisa Formation in this part of the basin. During Talang Akar time, the greater part of the Jambi trough was receiving fine muds deposited in a freshbrackish bay to shallow marine environment in the east grading to a marine environment in the west. At- the end of Talang Akar time, the progradation of the delta systems ceased and the deposition of the formation came to a close, as a thin marine shale (the " P e n d o p o " shale) . w a s laid upon, and preserved, the previously laid down fluviatile and deltaic sediments. In Central Sumatra, Lakat and lower Si .ttapas deposition occurred in the Early Miocene. We do not know how much of an hiatus or paraconformity exists between the Kelesa and kakat because there are no foraminiferal nor palynomorph data such as there are in the South Sumatra basin to date the formations. The environment of deposition of t h e Lakat over the Kampar high and .the eastern shelf margin was inner neritic to • shoreface marine, the sediments consisting of relatively clean quartz sands and interbeds of muds. The lower sandstone unit of the Si Hapas Group in the northern half of the Central Sumatra basin is probably identical in type and environment to the Lakat, "but it probably grades westward into marine siltstones and shales in the Tapanuli trough. The faciesenvironment and distribution map (Figure 14) of ,.the Talang Akar and Lakat show many isolated areas of non-deposition which were the hills, knobs, platforms and other topographic highs still emergent during Late Oligocene and Early Miocene. As in the analysis of the Lemat and Kelesa, if detailed information were available from everywhere in the basins, many additional areas of non-deposition would probably be identified. The changes in thickness of these formations over the basins • occur more uniformly than they iflloin the L e m a t a n d Kelesa Formations because there was less topographic relief to be filled with the sediments. The area of the Tigapuluh Mountains is interpreted to have been receiving sedimentation during Lakat time.

Deposition of Basal Telisa Limestone Member and Tualang Formation Deposition in marine environments continued in the two basins in the Early Miocene as the Basal Telisa gin~estone in South Sumatra and the Tualang F o r m a t i o n (Upper Si Hapas) in Central Sumatra were deposited upon the Talang Akar and the Lakat. The distribution of the gross facies and environments of deposition of these units is depicted on Figure 15. In South Sumatra at thi~" time there a p p a r e n t l y - w a s an appreciable reduction in amount of clastics being transported over the shelfal areas of the basin. The same condition appears to have prevailed in tile Sunda basin where a limestone unit was deposited throughout the basin area. The reduced amount of clastics coupled with possible episodic stillstands significantly affected the d e p o s i t i o n a l environments, allowing the limestones to be deposited on the platform and shelfal areas. In the early stages, extensive platform or bank limestone deposits developed; in the later stages, further buildups of detrital, reefal and bank limestone were formed on top of these banks in restricted localities. Many of these buildups were supported by a framework of colonial corals. During the later stages of the buildups some of the uppermost parts were subjected to subaerial leaching. Toward the basin margins, the bank limestone graded into. calcareous clays with fine to medium sands being laid down around topographic highs. While the lime muds were being deposited on the platforms t h e stratigraphic equivalent section, composed of calcareous clays interbedded with thin beds of lime m u d , was being deposited in the deeper troughs. These conditions prevailed over the northern half of the South Sumatra basin and in the troughs and seaways between the platforms in the southern half. These shales are mapped as part of the undifferentiated Telisa F o r m at i o n and not as a separate unit. The deposition of the Basal Telisa Limestone probably ended when marine transgression resumed and quantities of clastics were again transported onto the basin shelves. In Central Sumatra, muds and glauconitic sands and silts of the Tualang (and the equivalent Upper Si Hapas Formation) were deposited over an extensive area in shallow to middle neritic conditions.

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106 _ The islands and their locations, that are depicted on Figure 15 on the western rim of the basins, are conjectural. They have been included on the maps to suggest that the connections to the sea on the west prabably were irregular and restricted in width and were flanked by islands (possibly volcanic) at some localities.

Deposition of Telisa Formation The most widespread marine transgression during the Tertiary occurred in the two basins in the late Early and the early Middle Miocene, and was accompanied by the deposition o f t h e marine shales of the Telisa Formation. Toward the basin margins lime mud, silt and sand also were deposited. The basins were covered to their maximum areal extent by the marine seas and sedimentation occurred throughout the basin area (Figure 16). In South Sumatra the seas are interpreted to have covered the northern extension of the Lampung high and to have joined with marine seas in the Sunda basin. On the northeastern margin sedimentation occurred to the farthest limits of the basins. On the north flank of the Central Sumatra basin the Asahan arch was covered by shallow seas to link with those in the north Sumatra basin. To the west, the Central and S o u t h Sumatra basins were in communication wRh the open sea through broad seaways. Islands (plotted on Figure 16) probably persisted on the west borders of the basins, but their locations and size can not be accurately plotted. They probably were volcanic as the Telisa Formation in the western half of the South Sumatra basin is increasingly tuffaceous from east to west. The fauna in the Telisa suggest deposition in warm neritic environments. Paleontologic analysis of the fauna indicates a m a x im u m water depth of deposition over much of the basinal area to be 600 feet (about 185 meters) or less, but troughs such as Jambi probably were deeper. The maximum water depths occurred in the South Sumatra basin in the Globi. bisphericus (Globi. sicanus) stage and in Central Sumatra during the Globo. fohsi fohsi stage. There is little evidence for major structural m o v e m e n t during Telisa time so we presume that the region was tectonically quiescent with only basin subsidence occurring. A very few

faults in South Sumatra appear to have had some m o v e m e n t during Telisa time but these are of minor magnitude. lntra-Miocene Diastrophism At some point in the Middle Miocene, in late Telisa or early Lower Palembang-Binio time, the environment appears to have shallowed and regional tectonism may have affected the basins in Sumatra. This event may be related to the tectonism whose effects are reportedly observable in the Barisan Mountains and whicbo is described by van Bemmelen (1949) and others as an uplift of the mountains accompanied by vulcanism and by intrusion of diapiric masses and batholiths. This event is recorded on the margins of the Central and South Sumatra basins as an interruption to sedimentation and the development of weathering on an erosion surface, followed by renewed sedimentation. In Central Sumatra, y o u n g Middle Miocene beds lie disconformably on Lower Miocene beds in several wells on the eastern margin of the basin: The occurrence of a seismic horizon within the Binio and the Lower Palembang may be the basinal manifestation of this event which is seen on the basin margins as an hiatus.

Deposition of Lower and Middle Palembang and of Binio and Korintji Formations The regressive cycle of deposition in the two basins began in the Middle Miocene -- about early Lower Palembang-Binio Fo r m at i o n time - and environments of deposition gradually changed from neritic to continental. The Lewer and Middle Palembang in South Sumatra and the Binio and Korintji Formations in Central Sumatra were deposited at this time in shallow-inner neritic to paludal to delta plain environments. The postulated areas of distribution of these units are shown on Figure t7. By middle Palembang-Korintji time widespread areas of swampland and marsh were present throughout the basin areas and vast quantities of organic matter accumulated that were later transformed into coals that are characteristic of these formations. The areal extent of deposition of these formations in the two basins cannot be plotted accurately because the correlations and the identification of contacts between the units are often difficult due to the gradational character

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109 of the units. The areas of distribution o f the units portrayed on Figure 17 are therefore speculative and generalized, but they do depict the withdrawal of the seas from the basins and the decrease in size of the basin areas receiving sedimentation. The barrier islands on the west side of the basins were larger than in earlier stages and seaways between them finally closed by Middle Palembang-Korintji time, leading to a wholly continental environment of deposition. The initial movements of the Plio-Pl~istocene orogeny probably began in the Late Miocene and deposition came to a close before the onset of the orogeny and the deposition of the Plio-Pleistocene sediments. P L I O - P L E I S T O C E N E OROGENY

The trace on the ocean floor of the Benioff zone that plunges beneath Sumatra is interpreted to lie along the axis of the negative gravity anomaly found in the trench. The zone under Sumatra is described by Fitch (1970) and (1972), and Fitch and Molnar (1970) and others as sloping at about 30 degrees and having loci of quake activity down to depths of about 2 0 0 - 2 5 0 kilometers. This is in marked contrast to the Benioff zone under Java which is mapped as sloping at an angle of about 60 degrees and which has loci of activity down to depths of 650 kilometers. The structural features that formed in the basin areas during the orogeny tre~td northwest. It is n o t e w o r t h y that the dominant crustal grain of Sumatra, formed during the mid-Mesozoic, was not appreciably altered by the PlioPleistocene tectonism. The northwest trending blocks of pre-Tertiary rocks in the Barisan Mountains were further uplifted and the extrapolated extensions of these blocks in the basins seem to have continued in their original northwest trend.

The Plio-Pleistocene orogeny is the last of the major tectonic events to have affected the geologic development of Sumatra and it is this event that is responsible for molding most of the geologic features that are so characteristic of Sumatran geology. During this orogenic phase, the Barisan Mountains were uplifted, the right-lateral Semangko wrench fault was deSedimentation occurred in t h e basins conveloped along the length of the Barisans, temporaneously with the folding and faulting. volcanic mountains formed that spewed The Upper Palembang and Nilo Formations, effusive rock over much of the terrain in the composed of tuffaceous continental clastics, mountain area, and the rocks in the basins of were deposited in the synclines and troughs. Sumatra were folded and faulted into their The principal source of sediments in the present-day configuration. Some of the major Plio-Pleistocene were the B arisan Mountains; structural features developed at this time are the secondary source was the erosion from the depicted on Figure 8 and are described above in uplifted folds forming in the basins. Erosion on the section on Structural Geology. The orogeny m a n y of the anticlines cut deep into the was probably the direct result of renewed plate stratigraphic section with Middle and Lower movement of the Indian Ocean oceanic plate Miocene beds c o m m o n l y exposed, and preagainst the Sumatra portion of the Southeast Tertiary rock occasionally exposed, in the core Asia continental plate. Northeast to northof the structures. More than 5000 feet (about northeast m o v e m e n t of the Indian Ocean plate 1500 meters) o f section were removed from was probably resumed in Late M i o c e n e time some of the anticlines. These sediments with the onset of rapid spreading from the stripped from the anticlines together with the Southeast Indian Ocean spreading ridge at a erosion and volcanic products coming from the time identified as Anomaly 5 (refer to Le Baiisan Mountains were deposited in the Pichon and Heirtzler-1968). This m o v e m e n t of synclines and troughs formed in the basins the ocean plate resulted in convergent collision during the-orogeny. As the mountains rose the against the Java, Sumatra, Nicobar and regional terrain slope tilted toward the Andanian Islands that border the Southeast northeast and Sediment trarisport was northAsia plate, and the development of right eastward toward the Sunda shelf. This was a lateral rotational m o v e m e n t between the two complete reversal in direction compared with opposing plates. A subduction zone formed the earlier epochs when the principal source offshore in the Sumatra trench southwest of was fhe emergent Sunda landmass and sediment and parallel tO the offshore islands of Sumatra. transport was generally toward the west and

110 soulhwest into the subsiding basins. The Plio-Pleistocene sediments are characteristically tuffaceous, poorly sorted, poorly consolidated, fine to coarse grained clastics.The similarities in lithologies amongst the Recent, Upper Palembang-Nilo and some of the earlier deposited tuffaceous clastics prese~at p r o b l e m s of identification of the rock units in some areas of the basins -- this is not an u n e x p e c t e d p r o b l e m because the y o u n g e r sediments were derived from the erosion and redeposition of the older rocks. T h e Plio-Pleistocene orogeny is the closing chapter in the geologic history of the Central and S o u t h S u m a t r a basins. 111 fact, however, the orogeny per§ists t o d a y with earthquakes, lateral m o v e m e n t along the S e m a n g k o fault, and vulcanism reflecting the c o n t i n u a t i o n of this major t e c t o n i c event. REFERENCES

CITED

AUDLEY-CHARLES, M.G., C A R T E R , D.J. AND MILSOM, J.S., 1972, Tectonic Development of Eastern Indonesia in Relation to Gondwanaland Dispersal : Nature Physical Sob ence, v. 239. E.WING, J. A N D EWING, M., 1967,Sediment Distribution on the Mid-ocean Ridges with Respect to Spreading of the Sea Floor : Science, v, 156.

F I T C H , T.J., 1970, Earthquake Mechanisms and Island Arc Tectonics in the Indonesian-Philippine Region : BtJIl of Seis Soc Amer, v. 60, no. 2. •F I T C H , T.J. A N D M O L N A R , P., 1970, Focal Mechanisms Along Inclined Earthquake Zones in the Indonesia--Philippine Region : J. G e o phy Res, v. 75. F I T C H , T.J., 1972, Plate Convergence, Transcurrent Faults, and Internal Deformation Adjacent to Southeast Asia and the Western Pacific : J. Geophy Res., v. 77, no. 23. LE P I C H O N , X. AND H E I R T Z L E R , J.R., 1968, Magnetic Anomalies in the Indian Ocean and Sea-Floor Spreading : J. Geophy Res, v. 73, no.6 M C K E N Z I E , D. A N D S C L A T E R , J.G., 1971, The Evolution of the Indian Ocean Since the Late Cretaceous: Geophys J., v. 24, no. 5. N E L S O N , T.H., H A R D I N G , T.P. A N D VAIL, P.R. 1973, PTSI company menaorandum. T O D D , D.F. A N D P U L U N G G O N O , A., 1971, The Sunda Basinai Area :AAPG Presentalion, Houston, Texas 30 March 1971. V A N B E M M E L E N , R.W., 1949, ]'he Geology of Indonesia : The Hague, Governmenl Printing Office and Martinus Nijhoff. W A L S T O N , V.A. A N D J E F F R I E S , T.T., 1973, Exploration Evaluation, Kampar Area -- Central Sumatra :P.T. Slanvac Indonesia Report.