Abstract Abstract: Offshore of northwest Borneo, the occurrence of distinct submarine mass failures on the upper continental slope poses a substantial challenge to deepwater operations for the energy industry. These features are part of a complex of mass-transport deposits (MTDs) that occur in the near-surface interval across most of the upper continental slope, including a large area undergoing field development for hydrocarbon production. In the study area, the shallowest and most prominent feature discernible on conventional 3D seismic data is MTD 1, which has a profound influence on the present-day seafloor topography. This feature has a distinctive fan-like outline in plan view, a maximum strike dimension of approximately 6 mi (10 km), a dip extent up to 24 mi (40 km), and a maximum thickness up to 570 ft (176 m). The fan-like external form and the presence of a dip-oriented erosional keel suggest that the depositional process was a less coherent debris flow, with little to no original internal stratigraphy preserved. The less coherent nature of this feature is further supported by a key observation that this MTD overran an area of substantial high bathymetric relief, which is located in the area considered for a field development. Locally overlying MTD 1 are a series of younger near-seafloor features, termed “canyon-to-fairway” corridors that display a confined updip to less confined downdip plan-view morphology. These unique features locally erode and smooth the rugose top surface of the near-surface MTD 1 and can be interbedded with the lower intervals of the usually overlying draped sediments. Development of these late Pleistocene canyon-to-fairway corridors suggests that these features probably formed during a period of sea-level fall or at a lowstand. A blanket of three distinct intervals of draped sediments cap this entire sequence, composed mostly of muddy turbidites grading upward into hemipelagic deposits. The present hummocky seafloor topography mimics the rugose top surface of the shallowly buried MTD 1, except along its northeast lateral margin and where smoothed by canyon-to-fairway corridors. Internally within MTD 1, physical properties probably vary substantially both laterally and vertically, because draped sediments, turbidites, and occasional channelized sediments were incorporated in the failed matrix of this feature. Some of the geohazards, potentially affecting a field development, are a direct a result of the ubiquitous occurrence of MTD 1 in the study area. These potential geohazards include local steep slopes, seafloor scarps, and variable near-seafloor soil conditions. Understanding the impact of each of these potential geohazards, caused primarily by the presence of MTD 1, on a field development is vital input for selection of production well-site locations and placement of subsea infrastructure.

Abstract Detrital garnet compositions can be used to help determine the provenance of sedimentary rocks. In this study a garnet compositional database consisting of more than 2500 wet chemical and electron microprobe analyses was compiled from the literature. For the garnets in the database the six principal garnet end-member compositions (pyrope, almandine, spessartine, uvarovite, grossular and andradite) were calculated. A multi-stage methodology was devised to match garnet compositions to source rocks, and a series of garnet provenance fields on ternary plots were identified. The method was tested using compositional data from detrital garnet studies in several areas where provenance has already been identified, with good results. The methodology was then used to assess the provenance of detrital garnets from Neogene sandstones of northern Sabah, Borneo for which provenance is unknown and in a region where there are few garnet analyses for comparison. Comparison of garnet compositions from possible sources on Palawan, Philippines and in Borneo combined with the ternary plots excludes some known garnet-bearing rocks as potential sources and suggests derivation of metamorphic and igneous garnets from Palawan during the Early Miocene. Supplementary material: The compositional database and garnet data plotting spreadsheets are available at www.geolsoc.org.uk/SUP18649 .

Remote-sensing analysis of high-resolution satellite imagery of modern carbonate platforms in the Celebes Sea, east of Sabah, Borneo, Malaysia, was used to map geomorphology and sediment. Unsupervised classification of satellite images was interpreted in the context of environmental facies of seven isolated carbonate platforms and calibrated using analyses of surface sediments. In total, 140 sediment samples were collected and analyzed for grain-size and sorting. The grain-size analysis showed that sediment varied among the geomorphic elements, which included island, island/volcano, reef complex, carbonate sand shoal, grass-covered sand shoal, shallow lagoon, and deep lagoon. To generate carbonate sediment texture maps, the proportion of mud and the grain-size attributes (mean grain size and sorting) of each sediment sample provided a basis to classify samples into rock-equivalent textures. Integration of remote-sensing, field, and sedimentological data provided a means to characterize texture distribution maps and depositional facies maps. These maps suggest that mudstone to wackestone occurs mainly in the deep lagoon; wackestone to packstone is dominant in the shallow lagoon; the carbonate sand shoal is characterized by packstone to grainstone; and the reef complex is made up of boundstone to rudstone. These results facilitate estimates of the proportions of potential reservoirs on these platforms and the heterogeneity in facies distribution, based on the size of various recent carbonate platforms. Diagenesis notwithstanding, ancient analogs indicate the Selakan and Maiga platforms could be potential reservoirs, whereas the Selakan and Gaya platforms display more facies classes and represent poor potential reservoirs.