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The Sabah Basin, Eastern Malaysia is a Neogene trench-associated sedimentary basin filled with progradational cycles of marine and coastal sediments that display spectacular arrays of facies changes. The strata also show a decreasing intensity of deformation resulting from an interplay of subsidence due to local extension and compression due to crustal convergence. Consequently, age determinations and correlation of these strata create various problems that reqwre special biostratigraphic methods. More reliable age-determinations of Miocene strata in the Sabah Basin, especially in turbiditic sequences, have been achieved through quantitative nannofossils analysis. Foraminiferal assemblages in this type of sequences in the basin commonly indicate unreliable ages which seemingly reflect , undetected floods of older, allochthonous forms masking lesser preserved younger autochthonous type. The analyses of nannofossils from sediments in five wells also show mixtures of nannofossils of different ages. Analysis of nannofossils abundance allow the recognition of the more common occurrence of indigenous forms as compared to fewer reworked older types. The abundance pattern thus reflects the nature of sedimentary facies and support the turbiditic depositional model. The dominant indigenous assemblages, as differentiated from the reworked assemblages through quantitative analysis are then quite useful in determining the ages of this problematic Miocene sequence of the Sabah Basin.

The Malay Basin is a prolific hydrocarbon producing basin located off the east coast of the Malay Peninsula (Fig. 1). It started as an extensional / rift basin during the Oligocene, followed by thermal subsidence in the Early Miocene. Compression started sometime in the Middle Miocene which resulted in localized inversion. Maximum basin inversion was recorded during the late Middle Miocene The stratigraphic scheme generally used for the Malay Basin is shown in Fig. 2. Understanding transgressive-regressive cycles, and their controlling factors, have important influence on Malay Basin sedimentation, and hence directly impacts exploration efforts in the basin. Seismic data and biostratigraphic analysis carried out since the 1980's showed evidence of periodic marine transgression as early as Late Oligocene (Group L). Although several marine transgression events were recorded (Fig. 3), the biostratigraphic data indicates that during much of Miocene the basin did not become fully marine until the Pliocene. The limited occurrence of a fully marine faunal assemblage indicates a restricted marine condition. In many instances the presence of common to abundant freshwater algae further indicates intermittent development of 'lacustrine' conditions within the Miocene (Fig. 4). In the Malay Basin the transgressive-regressive cycles are believed to be the result of interplay of tectonic and sea level fluctuations (Mazlan, 2011), particularly during the Miocene. Evidence of tectonic deformation includes the presence of basement highs in the southern end of the basin (Fig. 5), and these would have exerted a significant impact on the depositional environment, and consequently the sedimentation patterns. This basement high is thought to behave as a sill which prevents the Malay Basin from being fully connected to the sea. The long term uplift of the basement high in the south would have been superimposed with fluctuation of sea level (Mazlan, 2011; Fig. 6). There would be instances where sea level rises exceeded the rate of tectonic uplift. During such time the sill or 'gate' would have opened up and caused marine flooding into the basin. At other times, the basin would be disconnected from the sea and lacustrine conditions would be established.