f Characterization of Peat-forming Environments of Miocene Coal Using Biofacies in the Malay Basin, Malaysia

Coals are common in many Miocene paralic sedimentary successions in the Malay Basin. Four types of peat-forming environment associated with coal precursors can be recognized by using high resolution biofacies analysis, and each has a different significance in terms of depositional environment and sedimentary facies interpretation. They are ‘basinal’, ‘watershed’ or ‘kerapah’, ‘brackish/marine’ and ‘freshwater alluvial’ peats. Basinal peats occur behind mangrove swamps, and form during the period of stable sea level and everwet climate. They begin as topogeneous peats and develop into domed, ombrotrophic peats at a later stage. Currently, they represent the most widespread type of peat in the Southeast Asian region, occurring widely in Sarawak, Brunei, Malay Peninsula, Sumatra and Kalimantan and frequently form thick coals in Malay Basin successions. Watershed or Kerapah peats form on low lying watersheds and other poorly drained areas where mineral influx minimal. They form when the climate is everwet and rainfall exceeds runoff. Unlike basinal peats, they develop independently of sea level change and thus can form at any time during eustatic sea level cycle provided the climate remains everwet. Today such peats occur locally in Sarawak and Central Kalimantan. In the Malay Basin they were probably more common during periods of sea level lowstand and coals thought to be from Kerapah peats may have occurred commonly on low lying interfluves. ‘Brackish/marine’ peats are very rare at present, but were probably common in the Miocene. Today they form in brackish settings which are subject to sediment starvation and limited nutrient availability, such as on carbonate substrates. However, a thick, and widespread coal formed at the end of Malay Basin Seismic Group E (about 9.0 Ma) on clastic sediments, and biofacies analysis suggests that this formed as a peat on exposed low relief area subject to subtle interaction of brackish water, probably at a time when sea levels fell. Freshwater alluvial peats could occur within alluvial plain settings such as abandoned fluvial channel and flood plains. These peats may be considered as ephemeral compared the previous three types, and thus coals derived from freshwater alluvial peats tend to be much thin and limited areal<br>distribution. The means of differentiating these four coal types, and their significance to depositional interpretation, will be discussed.