Ketungau Basin in Kalimantan. The Ketangau Basin can be traced into the eastern part of the Mandai Basin to the south
of the Kapuas Me´lange. A further me´lange belt, the Boyan Me´lange occurs further to the south along the southern
margin of the Ketungau and Mandai basins. Williams et al. 1988 have suggested that the Boyan Me´lange was back-
thrust from the northern me´lange zone.
The present authors have carried out a geological survey in southwestern Sarawak to investigate the strati-
graphic relationships among the various formations and to determine the structure of the accretionary complex. The
survey area extended from the outcrop of the Plateau Formation in the south to Sarikei on the outcrop of the
Kapit Member of the Belaga Formation of the Rajang Group to the north Fig. 1.
2. Sediments and structure of the Silantek and Plateau Formations and the Rajang Group
To the south of the Lupar Fault zone, the Silantek and Plateau formations consist of shale and alternations of shale,
siltstone and sandstone with igneous intrusions. The Silan- tek Formation is composed of three members: the Basal
E. Honza et al. Journal of Asian Earth Sciences 18 2000 751–759 753
Fig. 1. Geological boundaries in the area surveyed in western Sarawak. P1–P8 location of profiles illustrated in Fig. 5. The strike of the bedding and angles of dip are shown along the profiles.
Fig. 2. Generalised stratigraphic columns in the area surveyed in western Sarawak. The thickness shown for the Lubok Antu Me´lange is only approximate.
Sandstone; the Temudok; and the Upper Silantek Redbed members. The age of the lower part of the formation is Late
Eocene Haile, 1957. The upper part of the formation prob- ably extends into the Oligocene Tan, 1979. The maximum
thickness of the Silantek and Plateau formations, as calculated from Profile 1, is about 5000 m in the Batu
Lintang area and approximately 4000 m in the Sri Aman area Fig. 2.
The Basal Sandstone Member, the lowermost unit of the Silantek Formation, outcropping along the Lupar Zone,
consists of sandstone and alternations of sandstone and shale. The sandstones are turbidites, with some of the
beds exhibiting grading and sole marks Fig. 3. The Temodok Member consists of coarse to fine grained sand-
stones interbedded with thin beds of siltstone and sandy to silty mudstone. The Upper Redbed Member consists of
mudstone and shale, occasionally interbedded with fine- grained sandstone. This member is overlain conformably
by the Plateau Sandstone Formation.
The Silantek and Plateau Formations were deposited in shallow marine to near-shore environments Haile, 1957;
Tan, 1979. Coal beds in the upper part of the Silantek Formation are exposed repeatedly along the main road to
Sri Aman as the result of folding. In 1995 these coal seams were being mined. The beds are gently folded, the folding
becoming steeper towards the Lupar Fault Zone, with over- turning of the folds near the fault.
The Lubok Antu Me´lange has a matrix of mudstone and shale interbedded with coarse grained sediments. The
me´lange matrix is highly deformed and sheared, faulted and folded. Some beds are overturned, occurring as faulted
blocks, a few to several tens of metres in size. Both native and exotic rock types occur as blocks in the me´lange. Native
blocks include mudstone, shale and sandstone, while exotic blocks are serpentinite, gabbro and basalt, chert, and lime-
stone, or their metamorphic equivalents.
Lower Eocene foraminifera and nannofossils, and reworked Upper Cretaceous coccoliths, occur within
the me´lange matrix Tan, 1979 Fig. 4. Radiolaria obtained from the chert blocks can be grouped into
three assemblages: of upper Tithonian Upper Jurassic; middle Valanginian to Barremian and upper Albian to
E. Honza et al. Journal of Asian Earth Sciences 18 2000 751–759 754
Fig. 3. A measured section in the lowest part of the Silantek Formation to the south of the Lupar Fault Zone near Enkilili. PC: paleocurrent indicator; F: fault.
Cenomanian Lower Cretaceous ages Tan, 1979; Basir Jasin, 1996.
Blocks of unaltered and unsheared calcareous shale and limestone of Early Eocene age occur in a belt along the
southern margin of the me´lange Tan, 1979; Basir Jasin and Taj Madira Taj Ahmad, 1985. The origin of these
blocks has not yet been determined. They may be part of the me´lange, originating as an olistostrome, or may repre-
sent a stratigraphic unit in situ. Haile 1996 designated this zone the Enkilili Formation, and suggested that the Lupar
fault lay to the north of this unit, between the Enkilili Formation and the Lubok Antu Me´lange.
The Lupar Formation consists of turbidites and igneous rocks. The turbidites show rhythmic alternations
of fine-grained, graded sandstones and shale beds. Either sandstone or shale may be dominant in particular
E. Honza et al. Journal of Asian Earth Sciences 18 2000 751–759 755
Fig. 4. Stratigraphy in the area surveyed in western Sarawak. Age data from Haile 1957, Liechti et al. 1960, Tan 1979, Basir Jasin 1996 and Hutchison 1996. Data for the younger formations are from Kirk 1957, Wolfenden 1960, Ho 1978, Almond et al. 1989, Madon 1994 and Banda and Honza
2000.
E. Honza et al. Journal of Asian Earth Sciences 18 2000 751–759 756
Fig. 5. Geological profiles in the surveyed area along the route shown in Fig. 1. The thin vertical bar crossing the profiles marks the boundary of each profile.
sections. Palaeocurrent indicators in the sandstones show that the sediments came from the SW Tan,
1979. Igneous rocks within the Lupar Formation form the Pakong Mafic Complex, composed of gabbro, basalt,
porphyritic basalt and andesite. Tan 1979 distin- guished two zones within the complex: mostly layered
and granular plutonic rocks of the gabbro zone; and fine-grained, massive or pillowed, spilitised volcanic
rocks of the basalt zone Haile et al., 1994. Layering in the gabbros dips steeply towards the north.
The Layar and Kapit members of the Belaga Formation consist of slate and phyllite, interbedded with turbiditic
sandstones and siltstones. Our observations from palaeocur- rent indicators in the Layar Member in the Bukit Tiban area
of central Sarawak show that the sediments were derived from the SW. This direction, as well as the palaeocurrent
directions in the Lupar Member, are oblique to the trend of the outcrops of these stratigraphic units. This relationship
suggests that sediments were supplied from a direction slightly oblique, rather than directly perpendicular to the
subduction trench.
There is very little direct indication of the age of the Belaga Formation. Several benthonic foraminifera from
the Layar Member suggest an Upper Cretaceous age, while the Kapit Member probably extends from the Upper
Palaeocene to the lowermost Eocene Tan, 1979.
3. Imbricated thrusting of the Rajang Group
