Chronostratigraphy of Miocene turbiditic sequence of Sabah Basin from nannofossil assemblages

Abstract

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.