Figure Captions
 Introduction
Sabah, located in the northern part of
Borneo, lies at an important junction between the Eurasian, Indo-Australian,
Pacific and Philippines Sea plates. It also occupies a central position
between three marginal basins: the Sulu, Celebes and South China Seas (Figures 1
and 9). The evolution of this area is
still far from resolved and makes Sabah an interesting site for geological
investigations. The main tectonic elements of Sabah are a major fold-thrust
belt trending northeast in the west (NW Borneo Trend) and bending to the east
and southeast towards the north and eastern part (Sulu Trend) of Sabah. This
arcuate belt consists of a deep-marine Eocene-Oligocene accretionary complex
(Figure 8).
Five distinct tectono-stratigraphic
provinces can be recognised. Figure 8
shows a
simplified geological map and stratigraphy of Sabah.
(a) An ophiolite complex, which
is considered to form the basement of the sedimentary succession of Sabah;
(b) The Rajang-Crocker accretionary
prism, an arcuate belt consisting of deformed deep-marine, Eocene-Oligocene
sediments;
(c) Broken formations and
melanges which show characteristics of tectonic, sedimentary and diapiric
origin and are thought to have formed in a series of related events in the
Early to Middle Miocene;
(d) Neogene sedimentary rocks,
which are mostly shallow marine to fluvio-deltaic facies deformed into
sub-circular- to elliptical-shaped, fault bounded areas, which are remnants
or outliers known as the 'circular basins' of Sabah;
(e) The Semporna-Sulu Arc, a
region of andesitic to dacitic volcanic activity of Miocene to Quaternary age
in the Dent and Semporna peninsulas.
Tectono-Stratigraphic
Evolution of Sabah
Pre-Cenozoic
The presence of granitic and
metamorphic rocks of possible continental origin (Reinhard and Wenk, 1951;
Leong, 1974) associated with the ophiolitic complex and the interpretation of
gravity data has led to the suggestion that normal continental basement lies
beneath the ophiolite (Holt, 1998; Milsom and Holt, 2001). However, most of
the metamorphic rocks would have had a basic protolith (Hall and Wilson,
2000) and the granitic rocks, which are of very small volume (Hutchison et
al., 2000) could represent arc plutonic rocks intruded into an older ophiolitic
basement (Figure 8).
Paleocene to
Eocene Sedimentation
The Rajang Group is a
widespread association of Late Cretaceous to Eocene deep water mudstones and
turbiditic sandstones which include the Sapulut, Trusmadi and East Crocker
formations (Figure 2). All are thought to
have been deposited in the large NE-SW trending Crocker Basin and all are
highly deformed with tight isoclinal folds and thrusts (Hutchison, 1996). The
Palaeogene was therefore a period of continued deposition of deep marine turbidites.
The strongly deformed turbiditic Rajang Group is interpreted as a part of an
accretionary prism related to southeasterly subduction of the proto-South
China Sea in the NW Borneo.
Late Eocene
Uplift
An unconformity within the
succession of Palaeogene turbidites between the Middle and Upper Eocene is
inferred by Rangin et al. (1990) on the evidence of reworking of nannofossils
and Hutchison (1996) also argues that the West Crocker Formation includes
detritus from uplifted and eroded Rajang Group and East Crocker Formation
rocks. Hutchison (1996) refer to this uplift as the ‘Sarawak orogeny’ and
suggest it was probably driven by collision along the northern Borneo margin
at this time. The unconformity is generally difficult to recognize in outcrop
in Sabah because of similarities in lithologies either side of it and the
strong Neogene deformation. In general the contact is obscure.
Late Eocene-Oligocene Sedimentation
The uplift and erosion of the
Rajang Group accretionary complex provided a source of sediment for the
Borneo trough to the NW and also to the SE where material was deposited in a deep
water setting as the West Crocker, Labang and Kulapis formations (Figure 3). The fold-thrust belt of the West Crocker
Formation, which is well exposed in western Sabah, represents the
accretionary complex related to continued southeasterly subduction of the
proto-South China Sea in the NW Borneo. The Labang and Kulapis formations
exposed in eastern Sabah represent deposition of deep-water clastics in a
forearc basin setting from the Late Eocene through to the Late Oligocene.
During the Oligocene there was widespread regional subsidence. Outcrops of
Labang/Kulapis Formation typically show abundant syn-depositional and
syn-diagenetic extensional faults that suggest active growth faulting
associated with this subsidence.
Early Miocene
Deformation
The early Miocene was a period
of progressive tectonic deformation in the forearc region during subduction
and widespread melange development in Sabah. The inclusion of Labang
Formation sandstone and mudstone clasts in the Gomantong Limestone
demonstrates that uplift and deformation started by the Early Miocene,
between approximately 22 and 20 Ma (Balaguru et al., 2003) (Figure 4). It is therefore likely that deformation
continued for several million years, with the Gomantong Limestone forming on
structural highs during relatively quiescent times in the Early Miocene.
This unconformity separates
the deformed and lithified melange unit of the Kuamut, Garinono, or Ayer formations
from the less tectonised strata (Balaguru, 2001, 2003). The Early Miocene
(22-20 Ma) deformation is interpreted to mark a major tectonic event, causing
formation of the mélanges, major uplift and erosion, which produced the Base
Miocene Unconformity (BMU or Pre-DRU). This tectonic event is related to
subduction and collision of the Dangerous Ground Continental Block to the NW
Borneo and referred as the ‘Sabah Orogeny’ (Hutchison, 1996).
This was followed by a change in depositional
environment from deep-water to a shallow deltaic setting (Balaguru, 2001;
Balaguru et al., 2003; Van Hattam, 2005). The associated major uplift and
erosion provided sediment supply to the deltaic to shallow marine Middle to
Upper Miocene successions of the Meligan and Tanjong Deltas.
Early Miocene Limestone Development
The limestone outcrops in southern Sabah are
correlated with the Early Miocene (Burdigalian) Gomantong Limestone
Formation, which contains clasts of the Labang Formation (Noad, 1998). This
suggests widespread uplift followed by carbonate sedimentation throughout the
central and eastern Sabah. This change in depositional environment is also
recognized offshore NW Sabah. Further south in Kalimantan, carbonate
sedimentation also occurred during this time in the Tarakan Basin (Lentini
and Darman, 1996) and the Kutai Basin (Moss et al., 1997). In eastern Sabah
the Gomantong Limestone outcrops in a ENE-WSW-trending belt stretching at
least 200 km, which suggests that this may have been a zone of uplift along
which localised carbonate sedimentation occurred, isolated from any clastic
sediment influx from the west.
Late Early Miocene to Middle Miocene
Clastic Sedimentation
The end of the period of early
Miocene deformation is marked by the onset of Tanjong, Meligan or Kudat
Formation clastic deposition, the oldest dates for which are late Burdigalian
(NN3/NN4, 18-16 Ma) (Figure 5). The absence
of widespread syn-sedimentary deformation in the Tanjong Formation suggests
that it was deposited during a period of relative tectonic quiescence, which
lasted through the deposition of late Middle Miocene (12-10 Ma) or younger
strata. Continued subsidence created a cumulative thickness of at least 6,000
m. The Middle Miocene subsidence in the central Sabah Basin is possibly related
to coeval development of the Sulu Sea Basin in a back-arc setting (Nichols et
al., 1990) or regional thermal subsidence (Ismail et al., 1995). Facies
trends in the Tanjong, Kalabakan and Kapilit formations indicate that
detritus eroded from uplifted strata of the Rajang and Kinabatangan groups in
the west and was deposited in a deltaic to shallow marine system which
prograded towards the northeast. All the Neogene ‘circular basins’ of eastern
Sabah were part of a single NE-SW trending shallow basin.
Middle Miocene
Deformation and Clastic Sedimentation
Arc-continent collision in
the northern Borneo between the Cagayan Arc and Palawan Continental Block
(Rangin, 1991) created another Middle Miocene Unconformity (MMU, 15.5 Ma)
which marks the Deep Regional Unconformity (DRU) in onshore and offshore
Sabah. This deformation had stopped the extension and caused inversion of the
early Middle Miocene sediments and continued with post-rift sedimentation (Figure 6). There had been continued subsidence
after this tectonic event, which continued to deposit the thick prograding
post-rift sediments. Progradation of a large delta (Champion and Kapilit
deltas) resumed during the time following a very similar pattern to the
underlying Meligan and Tanjong deltas.
Late Miocene
Deformation and Uplift
The Late Miocene (SRU, 8.6 Ma) tectonic event
marks another major folding and uplift which can be correlated as the Shallow
Regional Unconformity (SRU) of this region (Levell, 1987). The Kinabalu
emplacement (10-8 Ma) event plausibly contributed to uplift and supply of
sediments. This was followed by development of the Baram Delta which
prograded to NW Sabah, and Kinabatangan Delta prograded to NE Sabah (Figure 7). The Late Pliocene tectonic event caused
by NW-SE trending strike-slip faulting and transpressional fault movement in
this region resulted in major structural inversion and uplift. This event is
here termed the Meliau Orogeny (Balaguru et al., 2003). All the Miocene
outliers in central Sabah were part of a single shallow basin (Figure 8). The present outcrop pattern of the
Miocene strata is the product of regional strike-slip and transpressional
tectonics and inversion during the Late Miocene (8.6 Ma) onwards and probably
lasted until the latest Pliocene.
The transpressional movement along the major
strike-slip faults in this region would better explain the structural
development in these areas and is possibly related to propagation of
deformation from Sulawesi towards NW Sabah. The Late Pliocene strike-slip
deformation is regionally significant and occurred at a similar time as
important deformation in NE Kalimantan, Sulawesi and NW Sabah. This
transpressional movement is interpreted to be the cause of the major orogenic
deformation, uplift and final structural development in Sabah region and
possibly continued to the present day (Figure 9).
References
Balaguru, A., 2001,
Tectonic evolution and Sedimentation of the Southern Sabah Basin, Malaysia,
Unpub. PhD Thesis, University of London, UK, 420 p.
Balaguru, A, G. Nichols and
R. Hall, 2003, The origin of the "circular basins" of Sabah,
Malaysia, Bull. Geol. Soc. Malaysia, 46, May 2003, p. 335-351.
Balaguru, A., 2006, Orogeny
in action: Tectonic evolution and sedimentation of Sabah, Abstract: Petroleum
Geology Conference 2006, Geological Soc. Malaysia.
Bol, A.J. and B. van Hoorn,
1980, Structural style in western Sabah offshore, Bulletin of the Geological
Society of Malaysia, 12, p. 1-16.
Hall, R., 1996,
Reconstructing Cenozoic SE Asia, in R. Hall and D.J. Blundell (eds.),
Tectonic Evolution of SE Asia, Geological Society of London Special
Publication, 106, p. 153-184.
Hutchison,
C.S., 1996, The 'Rajang Accretionary Prism' and 'Lupar Line' Problem of
Borneo, in R. Hall and D.J. Blundell, (eds.), Tectonic Evolution of SE
Asia, Geological Society of London Special Publication, p. 247-261.
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