GEOLOGICAL SOCIETY OF THE PHILIPPINES
CENOZOIC TECTONIC EVOLUTION OF EAST CENTRAL SABAH, NORTH BORNEO
Delphine Roques and Jon Noad
Research School of Geological and Geophysical Sciences,
Birkbeck College and University College London, Gower Street, London WC1E 6BT, UK.
Geology Department, Royal Holloway, Egham, Surrey TW20 0EX, UK
The Late Palaeogene to Neogene tectonic evolution of Central East Sabah has been studied in recent fieldwork. The aim was to determine the mechanisms of formation of the Central Sabah basin and the later so called 'circular basins' in order to link them within the regional tectonic framework of North Borneo.
The deep water/oceanic sediments of the Labang Formation (Eocene to Lower Miocene) are extensively affected by tight folds trending approximately E-W. The Gomantong limestones (Upper Oligocene to Lower Miocene), which overlie the Labang Formation, show thrusts to the NE which are compatible with the previous folds. On top of these formations are chaotically disrupted rock units (melanges), of disputed origin. Tectonic, diapiric and mass flow origins have all been considered. However, the tight folds, thrusts and melange emplacement could be related to a single compressive event at the end of the Lower Miocene.
Unconformably resting on top of the Labang Formation, is the Tanjong Formation (late Lower Miocene to Middle Miocene) of the Bukit Garam Basin which is slightly folded. This formation mainly exhibits a set of N-S to NNE-SSW fractures. To the northeast the Sandakan Formation (Middle to Upper Miocene) unconformably rests on the Labang Formation and is characterised by beds which dip gently towards the west. At the scale of the outcrop, synsedimentary NE-SW to E-W normal faults have been recognised, as well as a set of NW-SE fractures with no obvious sense of movement. In the Sandakan Basin the Sandakan Formation is cross-cut by a series of steep NNE-SSW scarps interpreted as normal faults. These are associated with intense fracturing with the same trend. Minor folding and faulting and subsequent erosion of a previously more extensive basin explain the development of the now isolated Bukit Garam and Sandakan basins.
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