2) Palynology of the Sihapas Group, Central Sumatra

KEITH RICHARDS

In: Goodman, D.K, and Clarke, R.T (Eds.)
Proceedings of the IX International Palynological Congress,
Houston, Texas, USA, 1996
American Association of Stratigraphic Palynologists Foundation,
p. 253-262

PALYNOLOGY AND SEQUENCE STRATIGRAPHY OF EARLY MIOCENE-LATE OLIGOCENE DELTAICS IN THE SIHAPAS GROUP, CENTRAL SUMATRA, INDONESIA

This study presents the results of palynological analyses of 55 samples from four wells drilled in the eastern region of the Central Sumatra Basin, Indonesia.

The samples include both ditch cuttings and sidewall cores and are all from the Sihapas Group, which is made up of fluvio-deltaic sediments of Early Miocene-Late Oligocene age. The Sihapas Group is interpreted as an ancient delta system which ceased activity in the Middle Miocene when the marine connection to the Indian Ocean was closed off by mountain uplift.

Most of the samples studied yielded moderate or good recovery of palynomorphs including mixed associations of marine microplankton and pollen and spores from mangrove, backmangrove, freshwater swamp and other terrestrial sources. Samples containing common mangrove pollen also tend to have low to moderate numbers of  marine forms present. The same samples often contain frequent pollen from freshwater swamp (probably peatswamp) forests. These probably represent periods of predominantly high relative sea levels and humid climate, which allowed both mangrove and freshwater swamp vegetation to thrive. Many of the samples which contain low numbers of mangrove pollen have an increased representation of fern spores and pollen from backmangrove vegetation. It is suggested that these represent opportunistic plant communities and may be an indication of  lowering relative sea levels.

A synthesis of all stratigraphic data from the study wells shows that the sediments of the Sihapas delta system accumulated predominantly in a coastal plain or marginal marine setting and periodically in a moderately deep marine shelfal setting. At least five major depositional units can be identified within the Sihapas Group which are broadly referred to as "sequences". Each of these probably includes more than one third order cycle of deposition.

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