Abstract

The West Natuna Basin developed in the manner of many of the Tertiary basins of SE Asia, with a Late Eocene to Oligocene rift phase of fluvio-lacustrine sediments (Belut Group and Gabus Formation), followed by a latest Oligocene to mid Miocene syn-inversion phase of transitional to paralic sediments (Barat, Udang and Arang Formations), and a Late Miocene to Quaternary post-inversion phase of marine shelf sediments (Muda Formation). Developing a chronostratigraphy for the rift and syn-inversion sedimentary successions is problematical since age-restricted marine microfossils are generally of limited occurrence.

This paper demonstrates that the rift and syn-inversion phases are rich in terrestrially derived palynomorphs which reflect: 1) the changing geomorphology of coastal or fluvial plains and 2) successive climatic cycles, each with an initial cool and/or dry climate interval followed by a period with warmer and/or wetter climates. These climate cycles reflect the global climate changes that were responsible for glacio-eustatic sea level variations during the mid-Tertiary, and provide strong evidence for correlation with the global composite oxygen isotope record and the global eustatic sea level curve. The cool/warm climate cycles also permit differentiation of lowstand (cool) from transgressive and highstand (warm) systems tracts.

A total of 15 climate cycles can be interpreted from the Middle Miocene to the mid-Tertiary. Each cycle shows distinct palynological characteristics permitting differentiation: Arang Formation climate cycles reflect mainly very wet climates, but with cool lowstand phases, and warm climate highstands; Barat, Udang and Gabus cycles were characterised by cool and dry lowstands and warm and slightly wetter highstands; whereas in the Belut Group cycles trend from drier to wetter with little temperature change.

Comparison of climate cycle trends with the oxygen isotope record also allows judgement to be made regarding sedimentation rates, and by extrapolation, the time of initiation of sedimentation in the basin.