f The Sequence Biostratigraphy and Chronostratigraphy of The Malay Basin

Through a comprehensive review of biostratigraphic data from the Malay Basin, in relation to seismic data, it is demonstrated that a large proportion of the microfossil assemblage variation seen in the basin is driven by sequence stratigraphic processes. Sequence biostratigraphic signals fall into two types: 1) signals relating to sea and lake level change, such as abundance and diversity acmes of foraminifera and nannofossils, and acmes of mangrove pollen and lacustrine algae, and 2) palynomorph assemblage changes reflecting climate change, which would have paralleled changes in sea level. Biostratigraphic signals for each systems tract can be distinguished. The maximum flooding surface, separating the transgressive systems tract from the highstand, is generally marked by a foraminiferal (and sometimes nannofossil) abundance and diversity acme, whereas the highstand is generally characterised by pollen signals suggesting a warm and wet climate. The transgressive systems tract is invariably marked by an acme of mangrove (Rhizophora type) pollen, and the transgressive surface by an increase in abundance of marine microfossils. The lowstand is usually characterised by an acme of cool and/or seasonal climate pollen, although some for some lowstands the climate was cool but wet, and these are generally characterised by acmes of pollen from an unusual type of peat swamp. Lacustrine algal signals need to be used differently is sequence interpretation depending on the strength of marine influence. The succession has been divided into 21 sequences based on biostratigraphic signals alone. Fourteen of these are within seismic groups D-M. Sequences within seismic groups A-H can each be independently dated using mainly nannofossils which occur at the maximum flood, but for sequences within seismic groups I – M there are no reliable Malay Basin microfossil occurrences for which the age has been accurately established. However, within groups I through M, terrestrially derived palynomorphs are abundant, and by correlating using palynological zones into the West Natuna Basin, where good nannofossil control is available to the base of the Arang Formation at the base of the Early Miocene (tying the ‘PR’ Malay Basin palynological zones of Azmi et al (1996) into the West Natuna ‘P’ zones of Morley et al (2003), the succession can be accurately dated down to base Group J. The age for Groups K through to M is proposed by sequence correlation with the West Natuna Barat and Gabus Formations, for which the age has been proposed using climate signals alone, through comparison with the global oxygen isotope curve. The study indicates that the age of the Group J/K boundary (previously ~21.4 Ma) is underestimated, and that this boundary should coincide closely with base Early Miocene at 23.09 Ma.