Page 103 - Sumatra Geology, Resources and Tectonic Evolution

90 CHAPTER 7

SOUTH SUMATRA BASIN, DEVELOPMENT OF STRATIGRAPHIC TERMINOLOGY

MUSPER MARKS SPRUYT DE COSTER 1974 GAFOER et al. 1986
APPROXIMATE 1937 1956 1956 (STANVAC)
AGE (GRDC)
PLEISTO- ! I I III ~ -
QMATER-NARY CENE " II ] I J ~ '
d .
0 Upper N ~ Kasai Tuff ~ Kasai I
Palembang Beds Palembang Mb ~ Formation r~ Palembang Formation c~
Middle Middle ~ ~ Blue Mb ] ~ .~ ~ C~ Middle Muara Enim
Palembang Beds Palembang Mb ~ ~ ~ % Palembang Formation
x~ Brown Mb ~ ,~
m 9 ~ Air Benakat
Lower Lower % ~ Lower Air Benakat
Palembang Beds Palembang Mb e. Sand and Clay ~ Palembang Formation
I/A Formation
o 8 g.
Upper Gumai Shale Telisa Gumai
Telisa Beds Telisa Mb Fomaation Formation Formation
U Telisa
J Limestone ] WelisaMb N ] Lilnestone Fm | Limestone ]Formation
~ Transition Mb ~.~ ~ 9
Wood Lower .- Talangakar Talangakar
Horizon Telisa Mb [2- - - ~ "~ ~ Formation Formation
~= ~
Gritsand Mb ~ ~ ~ m<
z
2E55EEEXEEE
Upper Kikim Tufts
...... ~ Lemat
Tuff-breccia Formation Lahat
Comple~ Formation
Fomaation ...... ~ _ _
Lower Kikim Tufts | "Granite Wash"
~"i !i ! ;
i i I i
i i ijii I~,1 i :1i11: Iii
I ii
.iiii I ~i~ : i ] ] 'I :I~ IKiki KikimTuffs
Fig. 7.4. The development of the straligraphic terminology for the Tertiary of the South Sumatra Basin.

content, containing fresh water gastropods and algae. Although lhe the Lahat and Lemat formations are given as late Mid-Eocene to
ages of all these sediments are poorly constrained, most publications Late Oligocene (NP16-NP24) by Sardjono & Sardjito (1989).
suggest a Late Eocene to Early Oligocene age (e.g. Praptono et al. For an understanding of the regional stratigraphy it is important
1991; Heruyono & Villaroel 1989). to appreciate that at this stage the Barisan Mountains had not yet
In the South Sumatra Basin, rift deposition is represented by been uplifted and there was no separation between sedimentation
the Lahat and Lemat formations which have much in common in the backarc and forearc regions. Grabens of the Horst and
with the Pematang Formation of Central Sumatra. The name Graben Stage cut across the area where the mountains now
Lahat (Series) was proposed by Musper (1937) and descrip- stand. The best studied example of one of these grabens is the
tions are given by Spruyt (1956), De Coster (1974), Hutapea Ombilin Basin near Solok in central Sumatra, which was sub-
(1981), Widianto & Muskin (1989), Hartanto et al. ( 1991) and sequently uplifted and now forms an intramontane basin within
Simandjuntak et al. (1991). The deposits, which outcrop in the the Barisans (Fig. 7.1). The Ombilin Basin, now at an elevation
foothills of the Tigapuluh and Duabelas mountains, include brec- of 500-1100 m above sea level, has a stratigraphy which is
cias, conglomerates and well-bedded greenish-grey sandstones, directly comparable to that of grabens of the Central Sumatra
with volcanic intercalations along the basin margins. In the Basin to the East. In the Early to Middle Miocene, however, this
central areas of the basin, siltstones with tuffaceous shares are basin was still below sea level and receiving marine sediments
encountered in boreholes. The deposits rest unconformably on (Ombilin Formation). In the Late Miocene marine deposition
the basement; conglomerates contain clasts of slate, phyllite, in the basin ceased, indicating that the uplift of the Barisan
metasandstone, marble, basalt, andesite and vein quartz derived Mountains had commenced.
from the basement. Environments of deposition range from Rift sediments in the Ombilin Basin are represented by the Brani
scree, alluvial fan and fluviatile to fresh or brackish water lacus- and Sangkarewang formations. The Brani Formation was defined by
trine in the central parts of the basin. De Coster (1974) used De Haan (1942) from spectacular cliff exposures of red bmccias,
the Lemat Formation as a synonym of the Lahat Formation. He conglomerates and sandstones, to the north of the main Ombilin
distinguishes a coarse clastic member of breccias, conglomerates Basin near Bukit Tinggi. A less well exposed hypo-stratotype,
and sandstones, and a fine grained Benakat Member, composed showing similar lithologies, was later defined by Koesoemadinata
of grey-brown shales, tuffaceous shakes, siltstones and sandstones & Matasak (1981) in the Ombilin Basin. These authors distin-
with occasional thin coals, irregular carbonate bands and guished two members: the Selo Member with sandstone turbidites
glauconitic units. Where beds of coarser grained material occur in lacustrine shales, and a Kulampi Member, composed of
within finer grained units they are described as 'granite wash', upwards fining sequences. The Sangkarewang Formation was also
the erosional product of nearby granites. They are sedimento- defined by Koesoemadinata & Matasak (1981) and described as
logically so immature that outcrops of the transported product dark, grey, laminated shales, rich in plant debris, with fine- to
can often hardly be distinguished from the weathered in situ very coarse-grained intercalations of quartz sandstone. The deposits
granite basement. Finer-grained units occur towards the central commonly show convolute bedding and slumping on a large
parts of the basin and in the upper part of the unit. The ages of scale. Again the environments of deposition of the Brani and