Page 99 - Sumatra Geology, Resources and Tectonic Evolution

Chapter 7

Tertiary stratigraphy

M. E. M. DE SMET & A. J. BARBER

The purpose of this account is to review the complex terminology of volcanic units in Sumatra is a formidable task. More than 200 stra-
the Tertiary stratigraphic units in Sumatra and propose a revised and tigraphic groups, formations and members have been described
a simplified terminology based on the significance of formations for and defined in the Tertiary of Sumatra; the majority of these
the tectono-stratigraphic development of the island. Formations are names have been introduced as the result of the GSI mapping
classified in terms of Pre-Rift, Horst and Graben, Transgressive, and programme during the past few decades. Fortunately only about
Regressive tectono-stratigraphic stages. 15% of these names are in common use. Often, the regional
The island of Sumatra lies along the southwestern margin of relations of these units are not fully clear due to poor outcrop
the SE Asian continent (Sundaland) beneath which the Indian conditions and the difference in style of definitions used by the
Ocean Plate is currently being subducted at a rate of about various research and exploration groups. Many of the units have
7 cm a-1 in the Sunda Trench (Fig. 7.1). The continental margin been described only from localized areas and were never incor-
of SE Asia is of Andean type, with active and inactive Quaternary porated in the regional picture. A further problem is that names,
volcanoes rising to over 3000 m above a Pre-Tertiary basement, definitions and classifications have been continually altered or
exposed towards the west coast of the island in the Barisan revised as a result of subsequent work, and because of improve-
Mountains. Tertiary sedimentary basins occur both to the SW ments in biostratigraphic age dating. Some of the changes in
and the NE of the mountains and small basins also occur within nomenclature and classification for the backarc, forearc and
the mountain range itself. These basins are described with relation- intra-arc basins are illustrated in Figures 7.2-7.4. Particular pro-
ship to the present-day subduction system as forearc, backarc and blems have arisen where units, which were originally described
intra-arc or intramontane basins (Fig. 7.1 ). The Barisan Mountains and defined from field outcrop, have been adopted by oil compa-
are transected by the Sumatran Fault System, a major dextral nies for time/rock units, defined by reflectors in seismic sections.
transcurrent fault zone which extends along the length of the During this process, facies variations that originally were regarded
island from the Sunda Strait to the Andaman Sea. as separate formations on the basis of lithological data in the
Stratigraphic research in the Tertiary sedimentary basins field outcrops, were incorporated within a single unit in seismo-
commenced in the last decades of the nineteenth century when oil stratigraphy. The ages of the earliest Tertiary sediments in
was discovered in the Telaga Tiga (I 883) and Telaga Said (1885) Sumatra are generally poorly constrained, as the oldest units are
wells near Pangkalan Brandan in North Sumatra. Initially, wildcat commonly terrestrial deposits in which body fossils are exceed-
drills were sited near oil seeps until systematic surface mapping ingly rare and palynological dating has often proved inconclusive.
commenced in the 1880s. Local stratigraphies in the oilfield areas The earliest sediments are generally considered to be of Oligocene
were compiled from field outcrops by geologists working for the to earliest Miocene age, but in the absence of definitive fossil
Bataafse Petroleum Maatschappij (BPM, now Shell) and the Neder- evidence an Eocene age is not precluded, and has been suggested
landsche Koloniale Petroleum Maatschappij (NKPM, later Stanvac) in some areas.
(van Bemmelen 1949). Five large and many small oilfields were During the proliferation of stratigraphic terms for the Tertiary
discovered in Sumatra before World War II. Since the 1970s sediments of Sumatra, attempts have been made to simplify
Sumatra has developed into a major oil and gas province. In the and rationalize the classification by developing hierarchical strati-
post-war period petroleum exploration has been based largely on graphic schemes. Oil companies use their own schemes of groups,
borehole data and seismic reflection profiling. The seismo-strati- formations and members in their concession areas, but these are
graphic units have generally been correlated with the main strati- rarely used consistently, and cannot be easily extended to cover
graphic units which had been previously defined on the basis of broader areas. A scheme of classifying formations into groups
outcrop descriptions and borehole data. and supergroups was developed during the GSI mapping pro-
A systematic compilation and correlation of the Tertiary strati- gramme and is used on the published GRDC maps. The scheme
graphic units throughout Sumatra became possible through follows the recommendations of Hedberg (1976) and Whittaker
the mapping programmes of the Geological Survey of Indonesia et al. (1991). Groups are defined in a vertical stratigraphic
(GSI), by the Geological Research and Development Centre sense, incorporating several successive formations, and are con-
(GRDC) and the Directorate of Mineral Resources (DMR), in fined to the area of a single basin, while Supergroups link together
association with the United States Geological Survey (USGS) units considered to belong to the same tectono-stratigraphic
and the British Geological Survey (BGS) carried out during the stage throughout Sumatra. In principle this may be a sound
1970s and 80s. These programmes were completed in the 1990s method of classification, but in practice the scheme was initially
with the publication of forty-one geological map sheets at the poorly applied, as the Tertiary II Supergroup covers what could
scale of 1:250 000 covering the whole of Sumatra. The maps illus- be more sensibly classified as two distinct tectono-stratigraphic
trate the distribution and extent of the outcrops of the Tertiary stra- stages, awkwardly designated Supergroups IIa and IIb. The
tigraphic units and each map is accompanied by a booklet giving scheme has not proved sufficiently flexible to incorporate the
detailed lithological descriptions and age constraints for the units flood of new data and continually revised interpretations.
shown on the map. This account is up-dated from a study under- In the present account stratigraphic units are considered only
taken on behalf of the University of London Consortium for Geo- at the formation level using the stratigraphic terminology given
logical Research in Southeast Asia (de Smet 1992). in Figures 7.2-7.5. Formations are described in terms of the
tectono-stratigraphic stage that they represent in the history of
the backarc, forearc or intra-arc basin in which they occur. Four
Stratigraphic review distinct tectono-stratigraphic stages have long been recognized
in the Tertiary sediments of the Sumatran backarc basins, and
The review of the stratigraphic terminology which has been this scheme may readily be extended to cover the intra-arc
used over the past hundred years for Tertiary sedimentary and basins within the Barisan Mountains. It may, however, only be