Balaguru, A., Nichols, G. and Hall, R.
Southeast Asia Research Group
Royal Holloway, University of London,
Egham, Surrey TW20 0EX

      A characteristic feature of the Neogene sediments of Sabah is that their geographical continuity has been disrupted into subcircular- to elliptical-structures and are faults bounded. The structural setting of these sediments has not been studied before and they have often been simply described as circular- to elliptical-shape ‘basins’. Differential subsidence into muddy melange and diapirism have been proposed as possible mechanism for their origin. Their structural relationship with the regional structural trend has often been simplified or not been considered, and for this reason the tectonic evolution of Sabah is not fully understood. The area is an onland extension of the Tarakan Basin, which is a prolific hydrocarbon province in the NE Kalimantan.

        Integrated detailed outcrop studies and SAR image interpretations carried out in the Malibau, Tidung and Meliau areas of southern Sabah have provided new insights into the structural style and stratigraphy of the area. Two coarsening-upward megasequences of Miocene fluvio-deltaic to shallow marine deposits rest unconformably on the Oligocene deep to shallow marine argillaceous sediments. Miocene palaeocurrent direction of sediments were towards NE and SE. A prominent southeasterly plunging fold belt trends SE-NW consists of asymmetric inclined folds that pass into overturned folds with a SW vergence dominate the Tidung area. Towards NW the fold belt curves NNE with a SSW plunge with tight anticlines and broad synclines. The tight anticlines are associated with faults. These broad synclinal structures were enhanced by erosion and were previously known as Meliau and Malibau ‘basins’. In the Tidung area the fold wavelength are shorter with tight plunging anticlines and broad synclines. Within these apparently broad synclinal structures there are a few open to tight folds. The area has been cut by numerous faults mainly trending NE-SW and NW-SE. These are interpreted as strike slip and normal faults. Relatively less tectonised thick continuous sections are preserved within the Malibau and Meliau Synclines. North of these areas, a series of WNW-ESE trending imbricate fold-thrust belts are clearly dissected by NE-SW trending faults.

        This study has shown that these sediments were not deposited in a series of isolated sub-circular basins, but were deposited in an extensive basin, with sediment input from the W and N. The ‘sub-basins’ or more strictly structural provinces, are the outliers or remnants of a single large basin, which now have their present shape due to the contraction, uplift and erosion.

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