© IPA, 2006 - 3rd Annual Convention Proceedings, 1974
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.
78
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INDEX MAP, CENTRAL AND SOUTH SUMATRA BASINS.
<
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,
104 ° . 1 6 ' 28'*E
104 ° 18' 15"E
103 ° 55" 37"E
103 ° lYE
103 °
20-25'E
81
MM
YRS
AGE
PLE ISTO
2
U N I T S USED IN THIS P A P E R
SOUTH SUMATRA CENTRALSUMATRA
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CORRELATION CHARTS FOR CENTRAL AND SOUTH SUMATRA BASINS SHOWING
INDUSTRY EQUIVALENT UNITS.
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Schematic strotigraphic section for South Sumatra basin~ showing generalized lithologies.
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.
90
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L
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Fig.
6.
late
Creeaceous-Early
Tertiary
seructural
~eaeures.
o.
°
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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ION
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-los
Io5
--v
0
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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
IOOO'-
Contour Interval
r-
Lowerwedge of Terf
Qbs~nt
'"~'~
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Oil
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8 G o s Fields
25
E
5O
KILOMETE R3
I
Fig. 7.
RelatiVe paleotopogra•hy on pre-Tertiary unconformity, illustrated by the isopach map of lower wedge
of Tertiary section (isopach map of top Talang Akar to pre-Tertiary in South, and top Lakat to preTertiary inCentral Sumatra basin).
93
z17
cz
4"
<7
(
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g
w
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Fig. 8.
Pllo-Pleistocene structure features.
94
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PAKANBARU
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CENTRAL 8 SOUTH SUMATRA
"~.
STRUCTURE MAP
T O P OF
PRE-TERTIAR Y
UNCONFORMITY
-5000
~
TO-IO,O00
FT
- I 0 , O 0 0 TO-15,OOO
FT.
x ;'.'~'.'~';~
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KIGOMETERS
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STRUCTURE NAP
ON TOP "OF T H I
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UNCONFORMITY
13
PALEMBANG
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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W
~
~
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o
o
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.'~ i.
C~.-.
I
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Z
o
o
N
CE
F-
i
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!
• Fig.
i0.
Belts of Late Paleozoic and Mesozoic rock formed during Mid-Mesozoic
orogeny.
o
98
C O M P R E S SION
OCEANIC
CONTINENTAL
DOWNBUCKLE
Nelson et al (1973)
Fig. II
I n i t i a l downbuckling of crust resulting
continental plate
U PL
CONTN
IENTAL~~ ~
N
ITRUSO
INS
from convergence of ocean plate a-gainst
I F T
OCEANC
I
Nelson et
Fig.|2
a1(1973)
Shear developmen! in the crust, the underlhrust o! the oceanic plate and the uplift
ol the over'thrust plate and emplacement, of Igneous intrusions;
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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u
o
om
f,
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OZ
oO
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Ow
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Fig.
13.
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
103
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~ ; ~ . t ~
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~ : i j y
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Fig. 14.
I
Environments of deposition during Late Oligocene-Early Miocene (Talang Akar
and Lakat Formations) showing dominant environments and lithologies.
I
~ I
104
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.
105
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~
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m
(/)
b.l
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<~
++M++®
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z
z
.J
Q
t9
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Fig. 15.
8
"%
I
Environments of deposition during Early Miocene (Basal Telisa Limestone and
Tualang
Formation).
0
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
107
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,
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z
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D
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O
[
i:i:i:~:5:iii~i?i:i:i:i~ii:'::i~:i!ii/~Sii:i:i:~:i:!iii!ii
:::::::::::::::::::::::::::::::::::::::::::::::
::i:9:i~:!:{:i:
::~.~:i~:S!:i:
:::!~r~. .
:::::::::::::::::::::::::::::::::::::::::::4:/
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"
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F ig. J6.
DistzJbution of sediments of Telisa Formation
Member).
(excludin 9 Basal Telisa Limestone
108
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C)
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Fig.
17.
Distribution
of 6ediments in Middle and Late Miocene
Palembans and BJnio aria Korint~i Formations)
(Lower
and
Middle
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.
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