PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION
Third Annual Convention, June 1974. 77
T H E G E O L O G Y O F T H E C E N T R A L A N D S O U T H S U M A T R A B A S I N S
G.L. DE COSTER *)
BASIN DESCRIPTION
The Central and South Sumatra basins are i m p o r t a n t oil producing areas o n 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 o n Sumatra ( t h e third is the North Sumatra basin) aligned northwest-southeast between the Barisan Mountains to the south- west 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 m o u n t a i n s ) 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 y o u n g Tertiary to Recent volcanics. Analysis of the Central and South.Su- matra basins show that they had very similar and related histories a n d c o u l d be considered as o n e large basin with m a n y troughs and grabens. The North Sumatra basin, o n the o t h e r h a n d , appears to have been separated from the Central Sumatra area t h r o u g h o u t 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 S o u t h Sumatra basins (see Fig. 2) are asymmetric basins b o u n d e d o n 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 L a m p u n g arches were positive elements t h r o u g h o u t 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 c o n n e c t i o n s to the neighboring basins. The northeastern and eastern boundaries of the basins along the Sunda shelf and the L a m p u n g high are difficult to define precisely, b u t 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 b o u n d a r y 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 t h a n two. T h e b o u n d a r y between the two basins is usually drawn as a northeast-southwest band through the n o r t h e r n part of the Tigapuluh Mountains, j o i n i n g 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.
© IPA, 2006 - 3rd Annual Convention Proceedings, 1974
Disc Contents
Contents
Search
I
|
I
I I
I
I
l
Se.
io • .
g o a 91~e i o o o
I , , I ,
0 •
&I4DAMANo
0
• • NICOeAR
~* ISLANDS
\ X N O R T H '
%
C E N T R A L A N D S O U T H
•
.'C':-~-'~ SU.AT~ Sl-RUCTU~L
B A S I N S
S C A L E I : 15,OO0,OOO 0 wO 300 4 5 0 6 0 0 - 1t'50
Kt L,DMi~T f, l ~
z
10~ o i
. . . . ..~ (" ....:
, I $
~, C A M B O D I A "~IETN AM~
M A L A Y A
I10 e I
N
~oJ
0
# , 4 W ,4 S E A
0
,;o. , ~ . ,;o.
- i o
FIG. I I N D E X M A P C E N T R A L A N D S O U T H S U M A T R A B A S I N S
| !
• .oo
g,, •
. ~ $ .. q # • - - <
< ~ , - ~
~ ( F\s" ' J i
<c
/
/
k
I FIG 2. INDEX MAP, CENTRAL AND SOUTH SUMATRA BASINS.
northnorthwest o f Jambi. The Tertiary deposi- tional 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 o f t h 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. T h e m a x i m u m dimensions o f the Central and S o u t h 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.
S T R A T I G R A P H Y
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 s t r a t i ~ a p h i c sections for the two basins showing the time (but not the thickness) relationships of the units. T h e formations in the t w o basins are discussed together in ascending order of age.
An i m p o r t a n t p o i n t that must be stressed in the discussion o f t h e stratigraphie n o m e n c l a t u r e used in this paper is the distinction between rock-stratigraphic units and time-stratigraphic or time-rock units. S o m e of the stratigraphie units in the basins are rock-stratigraphic units (i.e. formation, m e m b e r , etc) that are defined as in the C o d e of Stratigraphic N o m e n c l a t u r e as bodies o f rock characterized by lithologic homogeneity. The unit may contain between its upper and lower limits rock o f one l i t h o l o g i e type, repetitions of t w o or m o r e lithologic types, or e x t r e m e h o m o g e n e i t y o f constitution which in itself distinguishes the unit. The contacts o f such a rock stratigraphic unit are boundaries o f lithologic change, and the unit is often diachronous. T h e y are correlated by their lithologie character, and cannot be subdivided
into time-rock slices for tack o f faunal control or of seismic boundaries. These units are shown on the correlation charts with time-transgressive boundaries.
The o t h e r 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 o f a body o f rock, often of varying facies, whose upper and lower boundaries coincide with or are parallel to boundaries o f planktonic zones or seismic horizons, or their correlative unconformities.
F O R M A T I O N AND S E Q U E N C E S Pre-Tertiary C o m p l e x
The pre-Tertiary section or " b a s e m e n t " in the t w o basins is a c o m p l e x of Mesozoic igneous rocks and of Paleozoic and Mesozoic m e t a m o r p h i c s and c a r b o n a t e s . In a few local- ities, strata tentatively dated as Late Cretaceous to Paleocene-Early Eocene occur b e n e a t h the Tertiary sedimentary section and are grouped with the pre-Tertiary. The Paleozoic and Meso- zoic m e t a m 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 o f structural relation- ships o f the older rocks observed in o u t c r o p in the Barisan Mountains u n d o u b t e d l y e x t e n d into the subcrop o f these same rocks in the two basins.
Kikim Tufts and Older Lemat
The oldest rocks found in the South Sumatra basin t h a t 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 G u m a i Mountains southwest o f Lahat (1) These rocks are probably a part o f the cycle o f continental sedimentation, vulcanism and ero- sion that accompanied the Late Cretaceous- Early 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
A G E
PLE ISTO PLIOC
Lu L
Z
uJ I U M O~E E
bJ Z bJ ¢J O (.9 ._1 O
W
z L
IJJ
¢J 0 bJ
MM YRS
U N I T S USED IN THIS P A P E R SOUTH SUMATRA CENTRAL SUMATRA
UPPER
2 ~ E M B A N G ~ Z ~ . ? ~ N I L O 6 - 7 MIDI~LE
- ~ _ ~ _ ~ . G _~.__~.~?_~ .
I 0 LOWER 1 2 - PALEMBANG
16 T E L l S A
~ 7 F M , S -20 ~T~_ L~3--'
TALANG
-
"2'5.5 AKAR
J
BENAKAT
37.5 ~
f~
40 LEMAI~'FM
,~'?-- ~
M
GRNT
50 WASH" l
E
/
57 ,r"~
Z u W 0
W 6 0
J
K I K I M Y U ' ~ F S "~'~
÷ OTHER
MESO-
ZOIC
PRE - TERT.-t- ROCKS
P A L E O - SED, M E T A ,
ZOIC 8~ IGN
FIG. 3
81 E Q U I V A L E N T UNITS
=
SOUTH SUMATRA CENTRAL SUMATRA K A S A I £t~]Z,I~ZI~.~.~'~Z~I~I
~ E i ~ 2 t ~ U F F
? F M
K O R I N T J I MUARA ENIM
B I N I 0 A I R P E T A N I B E N A K A T
FM. F M
T E L l S A G U M A I " -z. ~--
-..W ( FM TELl SA FM
T U A L A N G ( /
/
R A O J A tL A T A L A N G
H A pA S,~--~,
s,)
A K AR [ PRE-
' S I H A P A S
r
• F M
K EL ES?A r r
L A H A T j / ( FMo
L
PRE
-
TERT. P R E - T E R T . PRE - T E R T .ROCKS ROCKS ROCKS
SED, META, SED, META, SED, M E T A ,
8 IGN & IGN 8 IGN
CORRELATION CHARTS FOR CENTRAL AND SOUTH SUMATRA BASINS SHOWING INDUSTRY EQUIVALENT UNITS.
~ z
D 0
I--- N - -
I 0 0 I~ ¢0
Z
_J
~-> Z
~- - -
-J 133
~ l ~
0
I]¢
Z
b.I I--
3AISS380SNV~£
',~ ,,,,,,,,,, ,,, 0,:,o,°~
• O
0 Q.
'1' I l l
• I - . l i 8 ! 1 1 i ! i - i !
'i!l
~ "-'.I',.! ~'ili,,' ;'i'i' i,i
~ o ~ o
~- ~=~ ~
-I-."
I -
i i .,,
i
i
_ . - : -
°
b .
"i,i:: ~ "~". . I
~,~ ..'." z ~ ~ ..~ ~ I.-
i I ~
iJ
>!!:;~"'>}
F~
> ;; "l
! v ~ ¸
"'J I "-J
Q. O.
s,3~dl °, 1 ~1
Z u.I
w ._.1 ~
- J w _ J W
3 N 3 0 0 I N 0 9 1 - 1 0
- 3 0 3
F i g . 5 Schematic strotigraphic section for South Sumatra basin~ showing generalized lithologies.
Lemat F o r m a t i o n and Benakat Member
The term " L e m a t " has been applied in the past to tuffaceous, coarse clastic, c o n t i n e n t a l sedimentary rocks present in the South Sumatra basin that are now interpreted to comprise several distinct units that are litho- logically 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 " o l d "
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 T u f t s " is used for the " o l d " 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 c o n t i n e n t a l environment. The Benakat Member of the Lemat (which is not to be confused with the Air Benakat F o r m a t i o n ) 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 e n v i r o n m e n t 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 n o t too clear due to abrupt .thinning and truncation of the Lemat o n t o the flanks of the P e n d o p o anticlinorium. The Lemat is normally b o u n d e d 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 thick- ness 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 ( a b o u t 1070 meters) (as defined by seismic data) in one fault depression in the central part of the basin.
These m a x i m u m thicknesses of the unit have n o t been penetrated by the drill. The Lemat is
probably Paleocene-Early Oligocene and the Benakat Member, Late Eocene-Early Oligocene in age, as determined b y spore-pollen and K-Ar age dating of some samples of shale and tuff.
Kelesa Formation
The Kelesa F o r m a t i o n occurs in the Central Sumatra basin and is composed of conglo- merates, coarse quartz sands, variegated shales, coals and tuffaceous material which were deposited in a c o n t i n e n t a l 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 u n i t is a sequence whose base is the u n c o n f o r m i t y with the pre-Tertiary that pro- vides a good seismic reflection, and whose top is u n c o n f o r m a b l e with the overlying Lakat F o r m a t i o n . The top is a distinct log pick and seismic reflector, c o m m o n l y 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, c o n t i n u o u s deposition u n d o u b t e d l y t o o k place in a transitional e n v i r o n m e n t . This c o n t i n e n t a l facies is inter- preted to grade northwest to the Tapanuli trough i n t o the claystones, sandstones and shales of the pre-Sihapas F o r m a t i o n . The Kelesa attains a m a x i m u m thickness of a b o u t 4000 f e e t ( I 2 2 0 meters) in local areas in the southern part of the basin. It has been dated as Oligocene to Early Miocene o n palynological evidence.
The u n i t is therefore y o u n g e r than its depositional cycle equivalent, the Lemat F o r m a t i o n , 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 F o r m a t i o n in the n o r t h e r n half of the Central Sumatra basin wh~re it is composed of variegated c o n t i n e n t a l claystone and sandstone, and of massive dark brown lacustrine shale. It grades eastward and southward i n t o the c o n t i n e n t a l , coarse clastics of the Kelesa. T h e Pre-Sihapas is probably Oligocene b u t the basal sandstones in the deep portions of the trough may be as old as Upper Eocene.
Talang Akar F o r m a t i o n
This unit occurs in the South Sumatra basin,
overlying the Lemat F o r m a t i o n or the pre- Tertiary 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 b o u n d i n g 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 u n i t with m i n o r amounts of shale, b u t in this paper it is considered as a time-rock sequence that incorporates i m p o r t a n t 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. T h e 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 conform- able 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 i n f o r m a t i o n 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 p a t t e r n t h a n 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 i n f o r m a t i o n 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 expect- ed 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 e n v i r o n m e n t and where sedimentation probably did n o t start u n t i l 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 e n v i r o n m e n t 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 sand- stones 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 u n i t 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 b u t 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"
certain wells and areas. No paleontologic information is available for the formation - it is usually interpreted to be Lower Miocene o n • the basis of stratigraphic position.
Si Hapas Group
The Si Hapas Group is an equivalent term used for the section that includes the Lakat and the Tualang Formations. The lower Si Hapas is the equivalent of the Lakat, and the Upper Si Hapas, of the Tualang. The age span for the Si Hapas Group is given as N.4 to N.8 in the P and N Zone system of planktonic zonation.
Basal Telisa Limestone
This m e m b e r was deposited in intermediate and shelfal portions of the South Sumatra basin, on and around platforms and highs. It is both a rock u n i t (member) and a sequence, in contact at its base with tlie Talang Akar sequence or with pre-Tertiary rocks and coinciding at its top with a strong seismic reflection.
The Basal Telisa Limestone is composed of platform or b a n k limestone capped in restricted localities by further buildups of detrital, reefal and bank limestones. In the distal portions of t h e basin the equivalent u n i t consists of shales with thin limestone beds that are often mapped as a part of t h e undifferentiated Telisa Formation. The thickness of the lower bank unit is fairly constant over the shelf area, averaging a b o u t 200 to 250 feet (about 60 to 75 meters) although the thickness variations are greater where the u n i t lies o n the pre-Tertiary with more irregular topographic relief to be filled. A n additional 200 to 400 feet ( a b o u t 60 to 120 meters) of limestone are present in those areas where the b u i l d u p s of the upper u n i t occur, The outcrop of the Baturadja F o r m a t i o n in the Garba Mountains is reported to be a b o u t 1700 feet (about 520 meters) thick. The member is very fossiliferous and has been dated as Early Miocene. The fauna contained in the Basal Telisa Limestone probably correspond mostly to the N. 6 faunal zones b u t may extend locally into the N.5 and the N~7 zones.
Tualang Formation
The Tualang occurs in the Central Sumatra
basin and is composed of calcareous shales interbedded with thin, glauconitic silts and sands, representing the transitional facies from inner to outer neritic marine deposition. The electric log pick is placed at the top of the uppermost, relatively thick glauconitic sand below the massive Telisa shale. A widespread and c o n t i n u o u s seismic reflector occurs at the top of this unit which is also a sequence b o u n d a r y defining the top of the basal wedge of sedimentation of the Tertiary depositional cycle in the Central Sumatra basin. The formation is normally about 200 to 400 feet (61 to 122 meters) thick in the southern half of the basin and occasionally reaches 500 to 600 f e e t (153 to 184 meters) in some of the wells.
T h e Tualang is interpreted to be the approximate time-rock equivalent of the Basal Tetisa. Limestone of the South Sumatra basin and to be of Early Miocene age. It is assigned this age principally on the basis of stratigraphic position and the rare occurrence of Spiro- clypeus in the unit.
Telisa Formation
The Telisa F o r m a t i o n is the most widespread occurring of tl3e Tertiary units, being deposited during the time of m a x i m u m marine transgres- sion into the two basins. The Telisa is characteristically a fossiliferous, marine shale containing occasional thin beds of glauconitic limestone. O n the basin rims and shelfal areas it occurs in a shallow marine facies with siltstones and fine-grained sandstones as well as lime- stones present with the shale. Sandstones on the basin shelf that heretofore were correlated as Talang Akar because they-were sandstones occurring beneath Telisa shales, are now shown to be sandstones developed within the Telisa.
The Telisa lies o n Talang Akar in the South Sumatra basin and on the Tualang in the Central Sumatra basin, and on pre-Tertiary rocks on most of the basin rim~. It is overlain by the Lower Palembang (in South Sumatra) and the Binio ( in Central Sumatra ) F o r m a t i o n s at a contact that is very difficult to select consistently over the entire basin areas. The top of the Telisa as picked on lithologic characteris- tics is diachronous, the characteristic Telisa lithology persisting until later in time in basin centers than on basin edges. Now w i t h - b e t t e r
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 Palem- bang 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 f o r m a t i o n 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 maxi- mum depths during the Globigerinoides bisphe- ricus (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 m i n o r 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 n o t 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 o f the basins.
Lower Palembang (South Sumatra) and Binio F o r m a t i o n (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 lime- stones, deposited in a neritic environment at the base grading to a shallow marine environ- m e n t 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 u p p e r 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 S o u t h 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 o f the basin so that an interpreter cannot be certain that the same coal beds are being mapped on the opposite sides o f the larger anticlines which expose Middle Palembang or Korintji strata. There is no diagnostic age dating i n f o r m a t i o n 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 F o r m a t i o n These units were deposited in shallow marine-brackish (at the base), paludal, delta plain and non-marine environments and are composed o f sandstones, mudstones and coal beds. T h e y 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 J a m b i 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
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 m a x i m u m 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 Creta- ceous-Early Tertiary tectonism and the Plio- Pleistocene 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 meta- morphism and varying intensities of deforma- tion. 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 oc- c 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 r e m n a n t structures from the mid-Mesozoic orogeny, coupled with rugged paleotopography developed by differential weathering of the pre-Tertiary rocks, comprise the " o l d " structu- ral elements of the basin. These elements structured the pre-Tertiary u n c o n f o r m i t y sur- face which in turn controlled the deposition of the Lemat, Kelesa and Pre-Sihapas Formations.
The other " o l d " features include the stable platforms in existence prior to Lemat and Kelesa deposition and which remained es- sentially unaffected by structural movements during the Late Tertiary orogeny.
The most p r o m i n e n t 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 m o u n t a i n s , and the formation of the associated fold-fault structures in the basin. In m a n y instances the faulting appears to be partly controlled, their trends interrupted, terminated or offset, and some of their afignments influenced by the b o u n d a r y 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
easily the most d o m i n a n t features of Sumatran geology.
THE PRE-TERTIARY AND EARLY TER- TIARY STRUCTURAL FEATURES
The "old" structures in the basin have long been the subject for speculation and inter- pretation, 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 a m o u n t of topo- graphic relief, some of it fault controlled, to have been present at the time these units were being deposited. Onlap o n t o the irregular surface that was cut into the pre-Tertiary rocks is c o m m o n 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 com- bination of paleotopography formed by dif- ferential 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 F o r m a t i o n s 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 n o r t h by a b o u n d a r y fault separating major blocks of pre-Tertiary rocks.
The Jambi trough. Very little is k n o w n 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 m o u n t a i n - f r o n t outcrop) that a trough is postulated here during the time of Lemat deposition.
The un-named, north-south trough associat- ed 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 i n t e r m i t t e n t c o n n e c t i o n 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. A n o t h e r broad regi- onal high, partly cut by faults, extended north- west 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 n o t uplifted u n t i l Plio-Pleistocene time. Both m o u n t a i n s are rimmed b y exposed outcrop of the section from Lemat up to Middle Palembang, 'sug- g e s t i n g they were subsiding and receiving sediments during these times. True, the cores of the m o u n t a i n s may have been above wave base or been positive features but there is n o way to d o c u m e n t this as the Tertiary rock section in the m o u n t a i n areas has been stripped away by erosion.
0 L k-
L - I m k-
y- ~ ~- FAUST
.:~ ~ x ~
, ~
~ " ~ -
• !
F i g . 6 . l a t e C r e e a c e o u s - E a r l y T e r t i a r y s e r u c t u r a l ~ e a e u r e s .
bSSIiBLE Z O N E
0
(r)
•
°o .
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 u n c o n f o r m i t y (Figure 7). The isopach data on Figure 7 incorporates the effects of pre-Tertiary/Early Tertiary struc- tures, 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 m a n y terraces and noses flanking or extending away from regional highs. Most of the faults and troughs are oriented nearly north-south; some are north- east-southwest. The isopach maps and the seismic sections indicate that many of the faults were active during Lemat and Talang Akar deposition.
M I D D L E T E R T I A R Y S T R U C T U R A L FEATURES
Very little structural m o v e m e n t , other than basin subsidence, seems to have occurred after Lemat and Pre-Sihapas-Kelesa time and before the Plio-Pleistocene orogeny. Some of the
" o l d " faults appear to have been active as late as during early Talang Akar and Lakat deposition b u t were inactive after that. Some seismic sections indicate some m o v e m e n t along the faults during Telisa deposition b u t these appear to have been of local and only m i n o r significance. Middle Miocene tectonic events interpreted by m a n y geologists from structural evidence in the Barisan Mountains apparently had some effect on the structures in the n o r t h e r n part of the Central Sumatra basin b u t had httle effect elsewhere in the two basins.
There is evidence from some wells located o n the northeast edge of the-basins that an hiatus occfirred o n the basin shelfs in the Middle Miocene, possibly a result of eustatic d r o p i n sea level, b u t there is n o indication of structural
movements accompanying break.
this depositional
PLIO-PLEISTOCENE STRUCTURAL FEA- TURES
The most p r o m i n e n t structural features in the basins are those developed during the Plio-Pleistocene orogeny. These features, whose d o m i n a n t trend is northwest-southeast, are clearly seen on surface geology m a p s and on structure m a p s 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 F o r m a t i o n s though earlier events also affect the structuring on this horizon. Structure on the pre-Tertiary u n c o n f o r m i t y (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 n o t k n o w n 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.
A b o u t five sets of folds or anticlinoria are
f
( 2 - - " " . =
\
- l o s
0 ..-
\ .
- - v
!I
N
m
I O N
0 °
Io5
CENTRAL 8 SOUTH SUMATRA
R E L A T I V E PALEOTOPOGRAPHY AND OIL - G A S FIELDS ISOPACH MAP OF LOWERWEDGE
OF T E R T I A R Y
I O O O ' - Contour Interval
Lowerwedge of Terf Qbs~nt
~ ' q b Oil 8 G o s Fields
.0 25 5 O
KILOMETE R3
Fig. 7.
r -
' " ~ ' ~ • )4
E
I
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 pre- Tertiary inCentral Sumatra basin).
z17
w o
f I Q / g
/
/
Fig. 8. P l l o - P l e i s t o c e n e structure features.
&
<7
9
O9
0
cz 4 "
(
<
94
%
c2N
PAKANBARU
+.\
~ z 4O • x
-PS I
~'~.~ x " \ "~"
\ .
. . . ~ x I
I
CENTRAL 8 SOUTH SUMATRA
S T R U C T U R E MAP
T O P OF P R E - T E R T I A R Y
UNCONFORMITY
- 5 0 0 0 T O - I O , O 0 0 FT
~ - I 0 , O 0 0 T O - 1 5 , O O O FT.
~ . ~ - Is,OOO FT
KIGOMETERS
FtQ.II STRUCTURE NAP ON TOP "OF T H I
\
t\ \
\ I
\
x ; ' . ' ~ ' . ' ~ ' ; ~
x
""~:-".'--~-~-:':'.'
" ~ ,
%
O
~ T
P~E - TERTIARY UNCONFORMITY
"~. 13
I ° S
PALEMBANG
present in the two basins. The n o r t h e r n m o s t one is aligned northwest, and each succeed- ing 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 Plio- Pleistocene folds and faults in the basins were controlled by many factors. The plate collision and the accompanying north-easterly vector of compression imposed the d o m i n a n t northwest trend o n t o 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 con- sisted 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 n u m b e r 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 p r e s e n t 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 w h e r e
they are offset in a rigbt lateral sense by north-south faults.
S T R U C T U R A L AND DEPOSITIONAL HIS- TORY 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 Cretaceous- Early Tertiary (?) accompanied by tension faulting.
3. tectonic quiescence from the Early Tertiary through the Miocene, ac- companied 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 o f Sumatra was p r o f o u n d l y influenced by the m o v e m e n t and recurrent collisions of the Indian Ocean and the"
Southeast Asia plates. Reference will be m a d e to these plate tectonic events as they affect the development of the structural and depositional history of the basins.
T h e k n o w n depositional history of Sumatra
include Late . zoic Late Late
the following periods of sedimentation:
Paleozoic through Early-Middle Meso- Cretaceous-Early Tertiary
Paleocene (+) through Recent
EARLY AND MIDDLE PALEOZO1C PE- RIODS
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 P e n i n s u l a northwest of Singapore, and the Devonian rocks reported in K a l i m a n t a n . Thus, the record of early geologic time in Sumatra is unavailable or has n o t 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 Permo- Carboniferous, Triassic and Jurassic rocks that occur in the Barisan, Tigapuluh, Duabelas and other m o u n t a i n s 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 con- tinental margin sedimentation during the Permo-Carboniferous and Early-Middle Meso- zoic. The types of sedimentary rock deposited included sandstones, shales, limestones and d o l o m i t e s that were laid d o w n in shallow to deep marine environments. This s e d i m e n t a t i o n probably occurred in foreland basins on the margin of the Southeast Asia c o n t i n e n t a l plate, wherever this plate was located in reference to geographic coordinates or in relation to Gond- wana Land or t o the Eurasia landmass. A history of development of several subduction, zones in the area of the S o u t h China Sea, Malay Peninsu- la, in parts of the Sunda landmass and near Sumatra is postulated by some geologists for the Late Paleozoic and Early Mesozoic periods. This
phase is n o t discussed in this paper because the areas of tectonism lie far from the basins of primary concern.
M I D - M E S O Z O I C OROGENY
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 m o u n t a i n s within the basin areas. In the Barisans, the outcrops have been mapped by zones or belts that include the block m o u n t a i n s of Permo- Carboniferous 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 m a y have been associated with s u b d u c t i o n of an oceanic plate beneath the edge of the S u m a t r a n c o n t i n e n t a l 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 a b o u t 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 y o u n g as Early Cretaceous (?). The development of the crustal shearing, the u n d e r t h r u s t i n g of the oceanic plate, the uplift of the overthrust plate and the emplacement O f igneous intrusions (see Fig. 12) probably occurred in Middle-Late CretaceOus.
The increasing degree of metamorphism and folding t h o u g h t to be observed from n o r t h to south in the blocks of exposed rocks in the Barisan Mountains, and the occurrence of more mafic rocks and of serpentine in association
I
~ W
- ~. ~ ~
0 I E W U
I I ~ III IiI IiI
0 . .
I
o o ,¢
I
0 1 , ~ ' ~ ) ' 1 .'~ i.
C~.-.
I
I
(
~C
• F i g . i 0 .
Z
CE F-
f
CO! Belts of Late Paleozoic and Mesozoic rock formed during Mid-Mesozoic o r o g e n y .
o
o o N
i
o
C O M P R E S S I O N
C O N T I N E N T A L
O C E A N I C
D O W N B U C K L E
Fig. II I n i t i a l downbuckling of crust resulting c o n t i n e n t a l p l a t e
Nelson et al (1973) from convergence of ocean plate a-gainst
U P L I F T
CONTINENTAL~ ~ ~ OCEANIC
INTRUSIONS
Nelson et a 1 ( 1 9 7 3 ) Fig.|2 Shear developmen! in the crust, the underlhrust o! the oceanic plate and the uplift
ol the over'thrust plate and emplacement, of Igneous intrusions;
with the m e t a m o r p h o s e d 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 Kali- mantan 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 mid- Mesozoic orogeny ceased when the area of the Central and S o u t h Sumatra basins was subjected to the n e x t episode of structural m o v e m e n t . 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 " g u l l e y " 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 m u c h 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. I n f o r m a t i o n 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 d e v e l o p m e n t 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 o f mantle upwelling following the mid-Mesozoic orogeny.
But whatever the cause, the effect was the widespread d e v e l o p m e n t 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 w h e n 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 z o n e " , 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 e m b a y m e n t , in a right-lateral sense, of the depth contours on the b a t h y m e t r i c map east of Christmas Island located south of Java.
5) The apparent right-lateral offset of the axis o f the gravity m i n i m u m in the Sumatra trench, about 150 kilometers n o r t h of Christmas Island.
T h e East Coast Sumatra zone might have extended south f r o m Banka Island through the western tip of Java - w h e r e a fault with mid-Miocene normal m o v e m e n t 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 c o n t e m p o r a n e o u s 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
proven.
A c c o m p a n y i n g 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 Creta- ceous-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 T E R T I A R Y SEDI- MENTARY SECTION
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 Cretaceous- Early 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)
