The Structure
and Stratigraphy of Deepwater Sarawak, Malaysia: Implications
for the Tectonic Evolution of the NW Borneo Continental Margin
Madon, Mazlan1, Kim Cheng Ly2,
Robert Wong1 (1) PETRONAS, Kuala Lumpur, Malaysia (2) Petronas Carigali, Kuala Lumpur, Malaysia
The structural-stratigraphic history of
the Sarawak deepwater is related to
the tectonic history of the South China Sea. Seismic interpretation
reveals a regional unconformity separating rifted basement from overlying
undeformed bathyal sediments. Well data indicate that the unconformity is
caused by uplift/erosion at ~16 Ma. This Mid-Miocene Unconformity (MMU) occurs
over the entire NW Borneo margin, and is related to the Dangerous Grounds-Sabah
collision during Mid-Miocene. The post-MMU sediments are marine muds and
turbidites derived from the Sarawak shelf are ponded in
depositional lows.
The pre-MMU section comprises
half-grabens filled with fluvial and coastal-plain sediments. The synrift is
correlated with the Paleocene-Eocene (pre ~32 Ma) rifting of the South China margin. The 32-Ma event
is regarded as the break-up unconformity marking the initiation of sea-floor
spreading. In Sarawak, this event is probably
related the uplift of the Rajang foldbelt in the Late Eocene, following the
collision of Luconia Block with Borneo. That collision
resulted in a foreland basin to the north of the Rajang foldbelt. An interval
of transparent seismic facies above the synrift is interpreted as
transgressive, deep marine shale, representing the “flysch” phase of the
foreland basin sedimentation. The bathyal marine shale interdigitates with, and
passes upward into, fluvial and coastal plain sediments (the “molasse” facies)
derived from the uplifted Rajang turbidites. The foreland basin sedimentation
(Oligocene to Mid-Miocene) was interrupted by the deformation event at the MMU,
after which the Sarawak basin evolved essentially as a passive margin
post-Mid-Miocene times. The present-day NW Sabah margin is a useful analog for
the pre-MMU foreland basin of Sarawak.