The sedimentology of Miocene shallow marine clastics of the Sandakan Formation of eastern Sabah

Abstract

The Sandakan Basin lies on the east coast of Sabah and contains some excellent exposures, resulting in part from the steep topography of the region and from the recent development of housing estates and road construction. These often spectacular outcrops allow examination of the Miocene succession, which has not previously been feasible. A total of 4 months has been spent collecting sedimentological data throughout the c. 25 km by 15 km basin. Over 70 measured sections have been logged across the basin and this , in conjunction with numerous macrofossil, microfossil and ichnofacies studies, has allowed the differentiation of the interbedded sands and clays, previously described as a marginal marine succession, into facies associations ranging from continental to shallow shelfal in origin. Seven main facies groups are identified which range from mangrove deposits cut through by fluvial channels, through delta top with channels into facies deposited in a lagoonallinterdistributary bay environment. These pass into shoreface trough or planar crossbedded sands with common Ophiomorpha or Skolithos trace fossils and finally into offshore marine muddy facies. Palaeoseismicity is suggested by synsedimentary deformation, often confined in discrete horizons which can be traced over several kilometres. Detailed palaeontological studies ofthese rocks will allow development ofthis facies scheme and further division of these systems into sub-environments . These well exposed marginal marine facies present an exceptional opportunity to develop a sequence stratigraphic framework of the Sandakan Basin through the Miocene. Examination of the eustatic/ tectonic control of the system will then be applied to hydrocarbon-bearing strata in adjacent basins. Furthermore this study will constrain palaeogeographic reconstructions of the region.