a regional perspective

This review combines a wide range of onshore and offshore data from Oligocene to Pliocene sediments of northern Borneo to address the following topics: (i) the stratigraphic conditions before and after the Top Crocker Unconformity [TCU], (ii) Early Miocene palaeogeography, (iii) events during later Early to mid Middle Miocene times, including the Deep Regional Unconformity [DRU], and (iv) the waning of the Sabah Orogeny in Early Pliocene times. Emphasis is placed on dating the key events, in order to consistently identify the stratigraphic and tectonic changes observed in the different data sets. The data shows a period of uplift and deformation in the north, perhaps focussed in southern Palawan, during the Oligocene then its sudden cessation at the TCU, with strong contrast in both facies and deformation style of Late Oligocene outcrops on the Kudat Peninsula compared to the Early Miocene sediments drilled offshore. This termination of tectonic deformation coincides with published estimates for the onset of ridge-jump and sea-floor spreading in the South China Sea northwest of Sabah. Following this unconformity the deposition of sediments during the Early Miocene appears to have consisted of a fluvio-deltaic high in the south (from central Borneo) and a broad deep marine basin in the north, from west of the Kudat Peninsula to at least the western part of the Sulu Sea. For the subsequent DRU, its timing and character appears to coincide with large scale sedimentary changes in eastern Sabah and this suggests a pause in regional compression during the early part of the long lasting Sabah Orogeny. The stratigraphic description of these events has important implications in plate tectonic reconstructions for northern Borneo and the South China Sea. The role of subduction on the eastern side of Sabah may be a key component of revised tectonic models and further evaluation of the area from the Dent Peninsula south to the Indonesian border is recommended