Sequence stratigraphy of the Middle Miocene-Pliocene southern offshore Sandakan Basin, East Sabah

Abstract

A sequence stratigraphic study was performed in the southern offshore Sandakan Basin with the aim of deriving a workable chronostratigraphic scheme and identifying plays, including stratigraphic traps. The data set includes 1800 km of high quality 1990 seismic and 8 wells. The study area can be divided into a structurally complex landward northern province and a relatively undisturbed basinward southern province. Three chronostratigraphic units were identified, each bounded by Type 1 third-order sequence boundaries. Each unit consists ofthird to fourth-order sequences which can be correlated from landward coastal plain facies to basinward bathyal facies. Unit 1 (Middle Miocene-early Upper Miocene) is characterised by moderate progradation, moderate aggradation; Unit 2 (middle Upper Miocene) by high progradation, low aggradation; and Unit 3 Oate Upper Miocene-Pliocene) by high aggradation, low progradation. . The positions of the prominent shelf edges in Unit 1, Unit 2 and lower Unit 3 indicate a southeastward progradation from Middle to Upper Miocene. Pliocene progradation was probably eastwards as suggested by N-S growth faults in the northeastern part of the study area, and is interpreted to have occurred within a ramp setting, as prominent shelf edges are lacking. The study led to the recognition of two play-types in the southern province, both associated with lowstand systems tract sediments. The fIrst is a slope fan play in Unit 1. The second is a basin floor fan play in Unit 2. Comparison of the locally derived onlap chart and the global onlap chart of Haq et al. (1987) highlighted some differences. The main difference is major Upper Miocene progradation in the study area versus Upper Miocene aggradation in the global chart, testifying to the importance of the interplay of local tectonics and sedimentation in the Sandakan Basin.