Page 102 - Sumatra Geology, Resources and Tectonic Evolution

TERTIARY STRATIGRAPHY 89

CENTRAL SUMATRA BASIN, DEVELOPMENT OF STRATIGRAPHIC TERMINOLOGY

MUSPER 1937 MERTOSONO
APPROXIMATE VAN BEMMELEN DE COSTER 1974 & NAYOAN 1974 CAMERON etal. PRAPTONO etal.
AGE 1949 (STANVAC) (PT CALTEX) 1983 1989
CENE
NARY
QUATER" PLEISTO" i I ! ! ~-~J~" ~ , ~ i ~J~-- i ! ~
!
d J ~ p e r Nilo ~ Minas -~ Minas J Minas
Palembang Beds Formation Formation Formation Formation
w
,,, Middle Korinci
Palembang Beds Formation
Petani Petani Petani
Formation Formation Formation
w m Lower Binio
m zm m Palembang Beds Formation
Telisa Telisa Telisa Telisa
Formation Formation F o r m ~
~ Sihapas
~ i ~ ~ I Formation
Bangko Fm ! <
Mica (restr. marine) ~ (with several ~ Transition Formation
Sandstone - ~ ~ members) ~ Menggala
m Formation r Formation

g ,,,
Pematang Pematang Brown Shale/ PEMATANG
Formation Formation _Fro_ - / GROUP
Breccia

i i I

Fig. 7.3. The development of the stratigraphic terminology for the Tertiary of the Central Sumatra Basin.

rock derived from the nearby basement. Lake sediments from this local green tuffaceous quartz arenite and coarse tuff. Sandstones
stage reach thicknesses of several kilometers, often indicating are commonly cross-bedded and may contain thin coal stringers
euxinic bottom conditions, and play a major role as source rocks and mussel bands. The Bruksah Formation varies greatly in thick-
in the Sumatran petroleum province. ness and is probably highly diachronous. It is interbedded with,
The age of sediments of the Horst and Graben stage is every- and overlain by the Bampo Formation, which consists of poorly
where problematic as due to their terrestrial origin, age-diagnostic bedded, black, pyritic mudstone, locally interbedded with micac-
fossils are exceedingly rare. Palynological schemes have been eous and carbonaceous sandstone and siltstone with a sparse
used for stratigraphic correlation (e.g. Morley 1991) but due to fauna. Limestone nodules are locally abundant and tuffaceous inter-
reworking, age-dating based on palynology has often proved calations also occur. Environmental conditions were ftuviatile,
inconclusive. The age of the Horst and Graben sediments is paralic and restricted marine. Pyritic mudstones indicate that
constrained at a regional scale by underlying Eocene marine water circulation to the open ocean was restricted by a barrier
platform limestones and by overlying Early to Mid-Miocene towards the west, allowing the development of euxenic conditions.
marine shales. Published stratigraphic schemes show a range in In Central Sumatra rift sediments are represented by the
age for the Horst and Graben deposits from Late Eocene to earliest Pematang and Kelesa formations. The Pematang Formation has
Miocene. Age interpretations are rarely supported by biostrati- sometimes been regarded as a 'Group' and subdivided into for-
graphic data other than by the age of the overlying marine mations (e.g. Williams et al. 1985; Longley et al. 1990; Praptono
shales. There may also be regional variation in the age of et al. 1991), and as a formation it has been divided into a series
formation of the grabens but, for reasons mentioned above, this of 'Members' (e.g. Lee 1982; Cameron et al. 1983). However
is difficult to prove. In the present account it is assumed that classified, the sediments include a variety of coarse red, green
graben formation in Sumatra commenced in the latest Eocene grey and black breccias and conglomerates, with medium- to fine-
and ceased in the Late Oligocene (Figs 7.6-7.8). grained sandstones, claystones and shales, intercalated with coal
In the North Sumatra Basin the rift sediments comprise the seams. Environments of deposition are mainly continental: scree,
Bruksah and Bampo formations (Cameron et al. 1980) (Figs 7.2 alluvial fan, fluvial and lacustrine with locally euxenic conditions
& 7.6). Graben deposits from North Sumatra form an exception and minor marine incursions. The euxinic shales have a high
to the rule that most sediments from the Horst and Graben Stage organic content and include the Pematang Brown Shale, which is
are terrestrial in origin. Before the NW displacement of the considered to be a good petroleum source rock. Deposition was,
forearc area along the Sumatran Fault System, commencing in the at least locally, interrupted by erosion, weathering and soil develop-
Mid-Miocene, the northern Sumatra area lay along the margin of ment, giving several internal unconformities within the succession.
Sundaland and subject to marine influences (see Chapter 14). The The Kelesa Formation was defined by De Coster (1974) and is used
Bruksah Formation rests unconformably on the Pre-Tertiary base- in Stanvac publications for the southern lateral extension of the
ment and commences with thick basal breccio-conglomerates, Pematang Group. It includes a similar range of lithologies to the
representing alluvial fans, followed by light to dark grey, micac- Pematang Formation, with the addition of tuffaceous shales, and
eous, poorly sorted quartz sandstone, siltstone and mudstone, with in the Bengkalis Trough lacustrine shale with a high organic