West Baram Line Research Articles

Seismic stratigraphy of Late Pleistocene incised valleys and adjacent environments, eastern Central Luconia Province, offshore north‐west Borneo

AbstractIncised valleys are commonly investigated based on outcrop, modern setting and seismic data, which are often limited by data availability, especially for broad (ca >100 km wide) shelf settings. Consequently, few have described complete depositional systems of the incised valleys, especially those linked to their corresponding modern rivers in a source‐to‐sink framework to determine comprehensive controlling factors. This study documents Late Quaternary incised valleys and their characteristics in the Balingian – Central Luconia shelf based on regionally‐extensive three‐dimensional seismic data, two‐dimensional high‐resolution seismic data and boreholes. The three‐dimensional seismic data show that the main Tatau incised valley and tributary Suai incised valley fed the Tatau–Suai shelf‐edge delta. The Tatau incised valley is interpreted as the main incised valley based on its broader width, thicker infill, two‐tier stratigraphic architecture and its larger drainage basin area compared to the tributary Suai incised valley. The Tatau incised valley shifted its direction towards an active tectonic lineament (West Baram Line) and bypassed sediments to the deep‐water via a series of upper slope channels. The Tatau incised valley system comprises the following elements: (i) an upstream segment characterized by rivers dissecting an uplifted hinterland comprising Neogene sedimentary rocks, humid‐tropical vegetation, monsoonal climate, extremely high river discharge and sediment supply; (ii) a midstream segment comprising an emergent, vegetated (tropical rainforest to mangroves) and relatively narrow (ca 150 to 200 km wide) shelf, which was characterized by basinward‐increasing, tectonically‐driven accommodation space, valley incision and deposition; (iii) a downstream segment with increasing tidal‐influence where the Tatau and its tributary valley merged; and (iv) a short (15 to 20 km), narrow (10 to 15 km), tectonically‐controlled shelf‐edge delta and channelized upper slope. This study demonstrates the value of documenting an incised valley system in the context of its broader source‐to‐sink setting, which may provide a useful analogue for ancient incised valleys in comparable geological settings.

Satellite image and outcrop-based structural mapping of Brunei and the contiguous region reveals a new fault, the Brunei Bay Fault

The cause of tectonic subsidence and uplift in the Brunei Bay region of Borneo Island, SE Asia, has remained one of the critical quests for researchers. Previous works have demonstrated that major rivers, including the Padas, Limbang, and Trusan, were captured during the bay formation; however, the causative tectonic event(s) and structures have remained unresolved. The Island scale structural mapping of Brunei and contagious areas using satellite imagery was used to map the structures and interpret the deformation history. The mapping was supplemented with outcrop mapping at selected regions in the Temburong area, which is critical to answering the questions on the existence of Brunei Bay and its tectonic history. The results suggest that faults control the geometry of folds with open synclines and narrow anticlines, and the spacing between two faults decides the width of the synclines. The larger spacing has produced wider synclines, such as the Belait Syncline. The dominant shale lithology was observed and mapped in Labu Syncline, where some outcrops are of slate-grade metamorphism. The Brunei Bay Fault, mapped as a major >80 km long NNW-SSE trending fault, was discovered using satellite, field and bathymetric data. The fault is interpreted to represent a significant crustal-scale left-lateral strike-slip fault that could have been the extension of the Red River-Ailao Shan (RRAS) fault before the opening of the South China Sea. The fault could be the onshore expression of the West Baram Line. Other significant faults mapped are the Batang Trusan Fault (BTF) and the Batang Mitus Fault (BMF), of which the latter is the youngest.

Formation of the Zengmu and Beikang Basins, and West Baram Line in the southwestern South China Sea margin

Formation of the Zengmu and Beikang Basins, and West Baram Line in the southwestern South China Sea margin

Geothermal Gradient And Heat Flow Maps Of Offshore Malaysia: Some Updates And Observations

An update of the geothermal gradient and heat flow maps for offshore Malaysia based on oil and gas industry data is long overdue. In this article we present an update based on available data and information compiled from PETRONAS and operator archives. More than 600 new datapoints calculated from bottom-hole temperature (BHT) data from oil and gas wells were added to the compilation, along with 165 datapoints from heat flow probe measurements at the seabed in the deep-water areas off Sarawak and Sabah. The heat flow probe surveys also provided direct measurements of seabed sediment thermal conductivity. For the calculation of heat flows from the BHT-based temperature gradients, empirical relationships between sediment thermal conductivity and burial depth were derived from thermal conductivity measurements of core samples in oil/gas wells (in the Malay Basin) and from ODP and IODP drillholes (as analogues for Sarawak and Sabah basins). The results of this study further enhanced our insights into the similarities and differences between the various basins and their relationships to tectonic settings. The Malay Basin has relatively high geothermal gradients (average ~47 °C/km). Higher gradients in the basin centre are attributed to crustal thinning due to extension. The Sarawak Basin has similar above-average geothermal gradients (~45 °C/km), whereas the Baram Delta area and the Sabah Shelf have considerably lower gradients (~29 to ~34 °C/km). These differences are attributed to the underlying tectonic settings; the Sarawak Shelf, like the Malay Basin, is underlain by an extensional terrane, whereas the Sabah Basin and Baram Delta east of the West Baram Line are underlain by a former collisional margin (between Dangerous Grounds rifted terrane and Sabah). The deep-water areas off Sarawak and Sabah (North Luconia and Sabah Platform) show relatively high geothermal gradients overall, averaging 80 °C/km in North Luconia and 87 °C/km in the Sabah Platform. The higher heat flows in the deep-water areas are consistent with the region being underlain by extended continental terrane of the South China Sea margin. From the thermal conductivity models established in this study, the average heat flows are: Malay Basin (92 mW/m2), Sarawak Shelf (95 mW/m2) and Sabah Shelf (79 mW/m2). In addition, the average heat flows for the deep-water areas are as follows: Sabah deep-water fold-thrust belt (66 mW/m2), Sabah Trough (42 mW/m2), Sabah Platform (63 mW/m2) and North Luconia (60 mW/m2).

Structures around the Tinjar‐West Baram Line in northern Kalimantan and seafloor spreading in the proto‐South China Sea

AbstractThe Tinjar‐West Baram Line is a NW‐trending trans‐lithospheric fault in northern Kalimantan; its northwestern extension into the South China Sea (SCS) is the West Baram Line. In this paper, we propose that the geodynamic processes of the proto‐South China Sea (PSCS) played a key role in the formation and evolution of the Tinjar‐West Baram Line, based on previous studies of the strata, crustal thicknesses, gravity anomalies and other characteristics of the blocks adjacent to the Tinjar‐West Baram Line and the palaeomagnetic‐based plate reconstructions of the PSCS. The Tinjar‐West Baram Line has a close link to the subduction of the PSCS. Structural restoration reveals that (1) before 35 Ma, the Tinjar‐West Baram Line was a NE‐trending transform fault, which is consistent with the NE‐trending strike‐slip faults widely distributed in the East Asian Continental Margin, in the PSCS. (2) From the perspective of tectonic evolution, the extinction of the PSCS and the spreading of the SCS drove the Luconia Block in the northern SCS to accrete to the western side of the Tinjar‐West Baram Line. This process resulted in a contrast of crustal rocks adjacent to the Tinjar‐West Baram Line; to the east of this line, the Nansha Trough is oceanic crust, whereas to the west of this line the Luconia Block has an affinity with continental crust. The velocity and thickness of the crust show great differences on either side of the Tinjar‐West Baram Line. (3) The kinematic analysis of the Tinjar‐West Baram Line reveals that the collisional orogeny between the Luconia and Kalimantan blocks happened at 45–37 Ma on the west of the Tinjar‐West Baram Line; on its eastern side, the Nansha Trough was subducting south towards Kalimantan Island. During the interval 35–10 Ma, the kinematics of the Tinjar‐West Baram Line shows a feature of dextral strike‐slip faulting. (4) The earlier published palaeomagnetic data show that the Kalimantan Block experienced a counterclockwise rotation of about 50° during the period from 25 to 10 Ma. In addition, the exert counterclockwise rotation of about 20° of the Tinjar‐West Baram Line happened due to the resistance of the Luconia Block. Therefore, the Tinjar‐West Baram Line changed from an early NE‐trending to late NW‐trending structure which results in its present day tectonic framework. Thus, we suggest that the Tinjar‐West Baram Line was originally a NE‐trending transform fault of the PSCS extending to the continental crust. Subsequently, the Tinjar‐West Baram Line became the western border of the PSCS, accompanying the collisional orogeny between the Luconia and Kalimantan blocks. Copyright © 2016 John Wiley & Sons, Ltd.

Reprint of: Nature and significance of the West Baram and Tinjar Lines, NW Borneo

A key consideration in tectonic models for SE Asia and opening of the South China Sea is the role that the West Baram and Tinjar Lines of NW Borneo may have played in accommodating the motion of crustal blocks displaced from Asia following India's collision. There are few studies that focus on these “lines”. Using onshore geological studies and offshore seismic data to address the origin and tectonic significance of these, this paper concludes that rather than a major transform boundary between Luconia and the Dangerous Grounds, the West Baram Line marks the boundary between domains of continental crust that underwent differential extension in the Eocene. The Baram Basin is underlain by hyperextended continental crust on the NE side of the Baram Line. The strong contrast in the geological features across the Tinjar and West Baram Lines likely reflects ancient differences in crustal rheology with Luconia being the more rigid block. Although lack of significant strike slip faulting along the West Baram Line poses problems for tectonic models in which a wide proto-South China Sea is subducted beneath NW Borneo, intra-plate deformation, such as partial inversion of the Dangerous Grounds rift, offers a potential mechanism to mass balance blocks displaced from Asia with the reduced strike slip motion along the West Baram Line.

Nature and significance of the West Baram and Tinjar Lines, NW Borneo

A key consideration in tectonic models for SE Asia and opening of the South China Sea is the role that the West Baram and Tinjar Lines of NW Borneo may have played in accommodating the motion of crustal blocks displaced from Asia following India's collision. There are few studies that focus on these “lines”. Using onshore geological studies and offshore seismic data to address the origin and tectonic significance of these, this paper concludes that rather than a major transform boundary between Luconia and the Dangerous Grounds, the West Baram Line marks the boundary between domains of continental crust that underwent differential extension in the Eocene. The Baram Basin is underlain by hyperextended continental crust on the NE side of the Baram Line. The strong contrast in the geological features across the Tinjar and West Baram Lines likely reflects ancient differences in crustal rheology with Luconia being the more rigid block. Although lack of significant strike slip faulting along the West Baram Line poses problems for tectonic models in which a wide proto-South China Sea is subducted beneath NW Borneo, intra-plate deformation, such as partial inversion of the Dangerous Grounds rift, offers a potential mechanism to mass balance blocks displaced from Asia with the reduced strike slip motion along the West Baram Line.

The North-West Borneo Trough

The North-West Borneo Trough is bordered along its south-east margin by a mélange wedge that has been the subject of disagreement with insufficient discussion. Offshore Palawan it has been interpreted as an accretionary prism that has been preserved in place when subduction ceased in the Middle Miocene. It is unconformably overlain by undeformed Upper Miocene to Holocene draping strata. Farther south-west along the Trough, the seismically identical mélange wedge has been named a Major Thrust Sheet System, which was assumed to have been thrust as a nappe north-westwards over the autochthonous Dangerous Grounds terrane of attenuated continental crust of the South China Sea passive margin. The accretionary prism model is the simplest, resulting in interpretation of the North-West Borneo Trough as a fossil trench. Subduction was halted by the rifted and attenuated continental crust arriving at the Benioff Zone, choking subduction and causing isostatic uplift of the Western Cordillera of Sabah. The subduction system therefore became a collision zone resulting in the term ‘foredeep’, in which case the actual trench position remains obscure and somewhere towards the south-east, but shown in unrestrained geological cartoons. Enigmas remain in the Palawan area of the Philippines. The Trough position is bathymetrically obscure in places and its given position makes it impossible to derive the Calamian micro-continent from continental Asia as required from its stratigraphy. In the south-west, the Trough terminates abruptly at the West Baram Line. The Trough contains several spectacular edifices, formerly suggested to be volcanoes or mud volcanoes but herein shown to be carbonate build-ups that were drowned, indicating that the Trough subsided too fast for build-up continuation, whereas post-rift thermal subsidence of the Dangerous Grounds was slower, allowing the Spratly Islands build-ups to continue active.

Transverse segmentation of the Baram-Balabac Basin, NW Borneo: refining the model of Borneo's tectonic evolution

The West Baram Line separates NW Borneo’s two petroleum systems. Oligocene sandstone and Lower Miocene carbonate reservoirs of the gas-prone Luconia system lie SW of that line. Northeast of the West Baram Line, the oil-rich Baram-Balabac Basin produces from Middle Miocene to Early Pliocene sandstones deposited in a foreland basin on the western side of the mountainous interior of Borneo. On the present-day shelf exploration efforts spanning nearly four decades have focused on the Champion and Baram deltas and associated extensional growth fault structures. Many of these structures have experienced youthful inversion owing to ongoing tectonic shortening. Recent discoveries prove this petroleum system extends into deep water beyond the modern shelf edge where an active fold-thrust belt has formed above autochthonous rifted continental crust of the Dangerous Grounds province in the South China Sea. New regional mapping (c. 100 000 km2) integrating seismic, borehole and gravity data shows that the Baram-Balabac Basin is segmented into four structural domains whose boundary zones trend NW-SE similar to the strike of the West Baram Line. Domain boundaries appear to control the position of the basin’s palaeoshelf edges, turbidite depositional systems, major unconformities and the position of the basin’s largest fields. These observations suggest that the domain boundaries are the expression of deep structures, probably within the underlying rifted continental crust. Two of the domain boundaries can be projected onshore to align with fault systems separating contrasting geological elements indicating they represent features of tectonic interest. The onshore geology of NW Borneo represents the early history of the Baram-Balabac Basin. The onshore geology is reviewed and new outcrop, biostratigraphic and palaeomagnetic data from Sabah are discussed in light of current models for the region’s tectonic evolution. A hybrid model is proposed in which the Baram-Balabac Basin post-dates the Sarawak Orogeny. The Sarawak Orogeny, which appears to be more regionally extensive than previously believed, is attributed to Eocene to Early Oligocene collision of the Dangerous Grounds and Reed Bank with Sabah and Palawan. In the proposed model Oligo-Miocene subduction of oceanic crust under NW Borneo is minimal. The Sabah Orogeny and younger inversion events are related to underthrusting of the Dangerous Grounds driven by both the opening of the South China Sea and by NW-directed subduction beneath SE Sabah in the Semporna-Dent Peninsula. In this context the structural complexity of the Baram-Balabac foreland basin reflects the adjustment of deep crustal blocks to far-field tectonic stress.

Seismic reflection evidence for a Dangerous Grounds miniplate: No extrusion origin for the South China Sea

The collision of India and Asia has caused large strike‐slip faults to form in east Asia, resulting in the “extrusion” of crustal blocks toward the southeast since the Eocene as a result of the indentation of rigid India into Asia. It has been suggested that the South China Sea opened as a result of relative motion between a rigid Indochina (Sundaland) block and China. Alternative models propose that rifting and seafloor spreading were driven by trench forces to the south. We test these competing models by analysis of seismic reflection profiles across the boundary between Sundaland and the southern rifted margin, known as the Dangerous Grounds. We show that the southern boundary of the Dangerous Grounds is a subduction zone that jammed in the middle Miocene. To the west the Dangerous Grounds are bounded by a strike‐slip zone, also active until ∼16 Ma, that becomes diffuse south of the now inactive South China Sea seafloor spreading center. We place the western edge of the Dangerous Grounds just to the east of the Natuna Arch (Lupar Line). The West Baram Line is confirmed as originating as a major strike‐slip fault within the Dangerous Grounds and is continuous with the Red River Fault Zone. Because the Dangerous Grounds were independent of Sundaland until ∼16 Ma, its motion cannot have been constrained by motion of this block, making extrusion impossible as a mechanism to rift the South China Sea. SE motion by both the Dangerous Grounds and Sundaland suggests subduction forces were the primary trigger for plate motions. Our reconstruction places a ∼280 km upper limit on the motion on the Red River Fault and a ∼1400 km width to the paleo–South China Sea.

Marginal basin evolution: the southern South China Sea

The southern South China Sea is divided into contrasting morphology by the West Baram Line. To the west is the Sundaland extinct passive margin in which rifting began in the Eocene (∼46 Ma) and ceased at 19–21 Ma (anomaly 6), where sea-floor spreading began much later than along the shelf of China. The break-up hiatus lasted ∼3–5 Ma marked by the Mid-Miocene unconformity, also preserved on land Sarawak. The post-rift strata date from ∼16 Ma and drape over the rifted topography. To the east is a convergent margin that became a collision zone in the Middle Miocene. The Sunda Shelf is of uniform ∼30 km thickness except in localised deep basins, and extends to a water depth of ∼200 m. The continental slope is narrow. The continental rise ( Dangerous Grounds), is covered by water ranging from 500 m to 3.5 km depth at the continent–ocean transition. Its width ranges from 170 to 330 km. The Rajang Delta extends over the shelf and continental slope. Its post-rift sediments drape over the rifted proximal topography of the Dangerous Grounds. To the east the drape is thinner and has not completely buried the rifted topography. The cuestas appear to have supported the carbonate build-up infrastructures of the Spratly Islands, whose slopes rise abruptly from a sea-floor of 2–3 km depth. The Sabah and Brunei margin is a collision zone that was formerly convergent. The on land geology indicates a Mesozoic ophiolitic basement. The main collision feature is the Western Cordillera, constructed mainly of sandy Eocene to Lower Miocene turbidites, predominantly Oligocene to Lower Miocene in the West Crocker (32–18 Ma), uplifted episodically throughout the Upper Miocene and Pliocene, 14–8 Ma. The 2 km deep Northwest Borneo Trough may be a relict of the convergence phase, but could also be a collisional foredeep. The oil-prolific Baram Delta, resulting from uplift and erosion of the Western Cordillera, has built out as far as the Northwest Borneo Trough. It is suggested that the passive margin continental rise (Dangerous Grounds) has been underthrust beneath Sabah to cause uplift of the Western Cordillera. The West Baram Line accordingly abruptly separates the collision zone from the western extinct passive margin; and is a now extinct major right-lateral transform fault.

Southeast Asia reconstruction with a non-rotating Cenozoic Borneo

A reconstruction of Southeast Asia has been attempted which treats the Sunda block and the Philippine Sea Plate as (relatively) rigid blocks between 50 Ma and 15 Ma. The published reconstructions of Robert Hall (1996 version) have been utilised but modified from Borneo westwards in accordance with geological perceptions and prejudices assembled over thirty-six years of studying the region. Sunda and the Philippine Sea are treated as rigid blocks that have not undergone disruptive internal deformation. In the case of Sunda, this means that Tertiary rift basins have formed but that there has been no oceanic crust developed except in the Andaman Sea on the west and the South China Sea on the east. In this interpretation Indochina has been extruded about 700 km between 35 Ma and 15 Ma (maps are shown at five-Ma intervals, following Hall). Right-lateral movement along the Sumatra Fault! Andaman/Sagaing system is paired with left-lateral movement along the Red River Fault and its precursor, the West Baram Line. The Philippine Sea Plate has moved great distances with an overall left-lateral displacement with respect to Sunda. Smaller blocks between Sunda and the Philippine Sea Plate with less control and with strong internal deformation have been treated in this reconstruction in a manner that accommodates the movements of the major blocks.

Gravity anomalies and deep structural controls at the Sabah-Palawan margin, South China Sea

Abstract The hydrocarbon-rich southeastern margin of the South China Sea is divided by NW-SE lineaments into a series of sharply contrasting segments distinguished by, among other things, abrupt changes in gravity patterns. The Sabah segment is bounded to the SW by the Tinjar or West Baram line and to the NE by the Balabac line at the southwest margin of the Sulu Sea. The most prominent gravitational feature of this segment is the strong freeair gravity low associated with the Sabah Trough which lies about 150 km offshore. Seismic reflection data suggest that loading by prograding sediments and gravity driven thrust sheets has depressed the extended continental crust of the South China Sea below its level of local isostatic compensation, producing the trough as a foreland basin in which sedimentation has failed to keep pace with subsidence. The load masses themselves, supported in part by the rigidity of the underlying crust and lithosphere, are above their levels of local compensation and deep Neogene sedimentary basins lie on the flanks of Bouguer and free-air gravity highs. Gravity values decrease across the Sabah coast so that the Crocker Ranges, including Mt Kinabalu, rest in rough isostatic equilibrium on presumably weaker lithosphere. The free-air gravity anomaly associated with the Sabah Trough is smaller than would have been predicted from the thickness of the water column, suggesting crustal thinning beneath the trough axis. This is not a characteristic of normal foreland basins and can therefore be assumed to predate basin formation. It can be concluded that the NE-SW trending belts of parallel gravity anomaly and geomorphology, of which the Sabah Trough is the most obvious, have been controlled by the pre-existing fabric of the crust and lithosphere of the South China Sea since they are discordant to the Palaeogene geological trends in Sabah. Reconstructions of the Tertiary history of the Sabah segment can be based on this assumption, which also suggests that sediments deposited in rift basins formed during the Palaeogene break-up of the South China margin were the source for much, if not all, of the hydrocarbon reserves of the area.

Tectonic implications of well-bore breakouts in Malaysian basins

Over a hundred well-bore breakout directions in the Malay, Sarawak, Sabah and Sandakan basins show consistent correlation with current and past tectonic stress fields. Breakout directions in the younger layers of the Malay Basin are consistent with shallow-focus earthquake stress trajectories associated with subduction of the Indian Ocean-Australian Plate west. of Sumatra. In older layers, breakouts responded to north-south regional compression. In the Sarawak Basin, one set of breakout directions follows the change of tectonic grain in the Rajang Accretionary Prism. Patterns of other sets of breakout directions indicate major tectonic boundaries (West Baram Line and an unknown N-S line projecting outward from Kuala Balingian) and en bloc rotations of certain tectonic domains. Most of the other sets of breakout directions seem associated with the SW segment of the South China Sea spreading ridge. Breakout directional patterns in the Sabah Basin differentiate a SW and a NE domain which experienced relative clockwise rotation of 25°. The Sandakan Basin experienced compression normal to its axis and also east-west regional compression that is currently active.

Miocene-Pliocene paleogeographic evolution of a tract of Sarawak offshore between Bintulu and Miri

A data base consisting of5,200 km of seismic lines and data from six wells has been utilized to recontruct the evolution of the main environments of sedimentation in a tract of the Sarawak Offshore between Bintulu and Miri. A sequence of six maps shows the progressive shifting through time of the position of the paleo-coastline in the area as well as that of the major deltaic systems. The presence of a morphological paleoescarpment along the so-called West Baram line is evident since at least the Middle Miocene. The data suggest its presence also in older ages but it cannot definitely be demonstrated with our data set. The basin to the northeast of this paleoescarpment has been filled mainly during the Late Miocene-Pliocene by the deposits associated with the progradation of the paleo­ Baram delta system. The spatial distribution of the different environments of sedimentation is at any time controlled by the position of the coastline (input of sediments) and by the presence of the paleoescarpment. This situation is still reflected by the present geography of the area.