The term ‘marginal basin’ has been used for the deep (abyssal plain) part of the eastern South China Sea. The Dangerous Grounds and the Abyssal Plain are both integral parts of the marginal basin.

To the south of the zone of sea-floor spreading lies the Dangerous Grounds Province, of strongly thinned continental crust (continental rise).

The West Baram Line is an old inactive right-lateral fault. West of it is the thick continental crust of the Sunda Shelf.

The plate margin of the Sunda Shelf is a passive margin (rifted continental crust) forming the Dangerous Grounds Province and abyssal plain (sea-floor spreading).

East of the West Baram Line, the plate margin is that of an extinct convergent margin, converted into a collision zone in the Middle Miocene. The foredeep (Northwest Borneo Trough) was filled by post Middle Miocene turbidites resulting in flat-lying ponded sediments.

Ages of events onland and offshore Sarawak and Sabah. ODP drill site 1143.

The elements of a rifted continental (passive) margin. Active rifting extended from Eocene to the Mid-Miocene unconformity.

There are 3 offshore elements:

1. Sunda Shelf of East Asia

2. Continental Rise, known locally as the Dangerous Grounds

3. Abyssal Plain that has sea-floor spreading from Lower Oligocene up to the Mid-Miocene unconformity.

Northeast of the West Baram Line is a convergent plate margin that became inactive in the Middle Miocene, ending up as a collision zone. Related features are:

1. The Northwest Borneo Trough, which is a foredeep

2. The Western Cordillera that resulted from the collision.

Holt (1998) constructed a map of crustal thickness from the gravity derived from satellite measurements.

# Sunda Shelf is clear, with calculated crustal thickness of 25 to 30 km

# There are local areas of thin crust within the Shelf – e.g. Malay Basin, Song Hong – Yingge Hai.

# The continental rise (progressively thinned continental crust) has a thickness ranging from 25 to 8 km.

# The marginal basin underlain by sea-floor basalts forms the abyssal plain. Crustal thickness of oceanic material is from 3 to 9 km.

Sections from the Continental Shelf over the slope onto the Continental Rise (Dangerous Grounds), here deeply buried by 2 seconds of post Mid-Miocene unconformity drape. If the seismic stratigraphy of Idrus et al. (Section B) is correct at depth, then the post unconformity drape is remarkably thick at the front of the Rajang Delta.

Very large throw normal faults; scarp slope may present sub-sea exposures of Pre-Rift continental basement; Post-Rift drape thin on B, thicker on A

Table 1: Pre-rift basement samples from scarp slopes

Sedimentary rocks Metamorphic rocks Plutonic rocks

Light brown-grey siltstone and sandstone containing Clathropteris fern leaves of Upper Triassic–Lower Jurassic age. Vitrinite reflectance: 1.0 to 2.5%.

Biotite-muscovite-feldspar-quartz migmatitic gneiss.

K:Ar date on muscovite (122 Ma) suggests Lower Cretaceous metamorphism

Boulders of dark green diorite composed of plagioclase, clinopyroxene, ilmenite and some quartz. Plagioclase and pyroxene much replaced by epidote, prehnite & chlorite. Age unknown.

Dark grey clay stone containing moulds resembling Upper Triassic Halobia & Daonella

Garnet-mica schist containing sillimanite. One sample contains andalusite. K:Ar date on muscovite (113 Ma) suggests same metamorphic event.

Blocks of intensely altered olivine gabbro. The olivine is almost completely replaced by aggregates of chlorite, talc and montmorillonite. Age unknown.

Gray-green siltstone & fine sandstone containing Upper Palaeocene planktonic foraminifera and coccoliths.

Vitrinite reflectance: 0.4%.

Quartz-phyllite occurs nearby. K:Ar date on muscovite (113 Ma) indicates same Cretaceous metamorphism

Grey-black siliceous shale with radiolarian relicts. Age unknown.

Amphibolite schist. The amphibole gave a K:Ar date of 146 Ma (Jurassic).

Dredge samples that allow us to determine the nature of the rifted basement of the Dangerous Grounds

Table 2: Syn-rift dredged sedimentary samples from scarp slopes

Light grey-green slightly consolidated siltstone containing siliceous sponge spicules, radiolarian and planktonic foraminifera and Middle to Upper Oligocene nannoplankton

Shallow-marine carbonates sampled at 23 sites. They contain Late Oligocene to Lower Miocene (Te) Foraminifera and Nummulites. These well cemented shallow marine carbonates were built-up upon cuestas and show up on regional seismic.

Table 3: Post-rift dredged and drilled formations from upper horizons

Sedimentary formations Volcanic rocks

500 metres of core at ODP Leg 184 drill site. The well bottomed in Upper Miocene calcareous mudstones containing turbidites. The cored section ranges from Upper Miocene to Recent, more calcareous downwards.

Porphyritic basalt; vesicular basalt containing olivine, clinopyroxene and plagioclase gave a K:Ar Pliocene date of 2.7Ma. Vesicular olivine basalt tephra surrounds lumps of Pliocene carbonaceous ooze. The basalt gave a K:Ar date of 0.42 Ma.

Many sites yielded dredges of Pliocene ooze, ranging from grey-green clay to light-grey foraminiferal ooze. Coccoliths indicate a full Pliocene age range. Upper Pliocene ooze fills the outer vesicles of submarine basalt.

Red and green massive dacite. Groundmass contains large plagioclase and small alkali feldspars. Secondary alteration to sericite and chlorite replace clinopyroxene. Age unknown.

Western Dangerous Grounds and transition through the continental slope onto the Sunda Shelf. The G10 is a prominent promontory of the Shelf. To the east lies the Northwest Borneo Trough, across the West Baram Line.

The rifted basement is draped over and completely covered by very thick post Mid-Miocene Unconformity strata. The Drape is provenanced from the Sarawak outcrops of the Belaga Formation (Rajang Group) that is very shaly. The great Rajang River that brings the sediment is accordingly muddy.

The normal faults are shown here to trend approximately North, representing an unresolved conflict with the spreading directions of marginal basin to the north and northeast.

Section from the Sunda Shelf across the Dangerous Grounds. The post Mid-Miocene sedimentary drape is very thick and originates in the muddy Rajang River.

Half grabens and cuestas of the pre-unconformity basement are rather subdued, unlike the region offshore Sabah, especially in the region of the Spratly Islands.

Post-Unconformity thick draping strata (Late Mid Miocene to Recent); draping strata completely buries the rifted sequence; The ‘horizontal ponded strata’ have eroded down into the drape; Example II: an impressive sea-floor cuesta could not be completely buried.

“Ponded strata’ occupy lows in the sea-floor topography.

‘Ponded strata’ are being deposited at the same time as the younger draping strata.

‘Ponded strata’ therefore represent turbidite channels (fairways) directed in-and-out of the cross-section.

Vertical exaggeration of the water layer of ~11 causes a real sea-floor slope of only 7 degrees to appear 54. The draping strata are therefore NOT folded.

The uplift of Sarawak, causing the apparent “rotation” of the palaeo-coastline was related to the end of rifting in the South China Sea and regional uplift known as the Mid-Miocene Unconformity.

The post unconformity strata are found onland ONLY along the coastal zone and they thicken sea-wards e.g. Begrih Formation and Liang Formation.

The Baram Delta begins only after the Unconformity. From Shell data, the future site of the Baram Delta was of deep water, even bathyal condition.

Good outcrops of unconformities are rare in Malaysia. Near Tatau is a spectacular unconformity exposed beside the main road. The overlying Rangsi Conglomerate represents the on land beginning of the post unconformity strata that thicken seawards and are little deformed. The unconformity is actually the Mid-Miocene unconformity (see seismic), between the Setap Shale and the Balingian Fm.

Active (convergent) plate margin that became extinct in the Lower to Middle Miocene. Transformed into a collision zone as the stretched continental crust of the Dangerous Grounds could no longer subduct beneath Sabah. Sediments come from the uplifting Western Cordillera of Sabah. Turbidite ‘fairways’ go perpendicular to the diagram, having been ponded by the anticlinal sea-floor ridges of the Baram toe-front of the Baram Delta.

The turbidites move along one level until they find a way to a lower bathymetric level, and eventually into the Northwest Borneo Trough, which is becoming filled by turbidite meandering rivers. The sediments of the Northwest Borneo Trough have cut down into the draping strata that covers the Dangerous Grounds terrain.

As we proceed northeastwards along the former plate margin, the complexity of a buried seismically unresolved infrastructure appears. It is interpreted as the Crocker Formation, occurring southeast of the Morris Fault of Sabah and the Jerudong Line of Brunei.

Hinz was reluctant to call this an accretionary wedge, and used the term allochthonous wedge, thrust northwestwards over the Dangerous Grounds.

Proposed scenario for Northwest Borneo Trough. Mount Kinabalu remains enigmatic

A TALE OF TWO RIVERS1. The Baram Delta was supplied by a river, or rivers, that did not travel far. The provenance is very sandy formations impressively uplifted near the coastline (Western Cordillera) formed when a convergent plate margin changed into a collision zone. Actually an appropriate name for the Northwest Borneo Trough is a ‘foredeep’. 2. The Rajang Delta continues to be supplied by the great Rajang River. Its source is far from the Delta toe zone. This river is a small-scale Mississippi, flowing over a continent towards a rifted continental margin. Unfortunately for reservoir potential it is a muddy river because it drains the vast outcrop of shaly-slaty Rajang Group Belaga Formation.

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