Eighty (80) vibracores were recovered from across the Rajang River delta, Sarawak, East Malaysia during field seasons in 1992 and 1993. Thirteen (13) sediment types were identified within the alluvial plain, delta plain, and delta front. Based on a visual estimation of organic matter (OM) content, all sediment facies were divided into two groups. Sediment types poor in OM include silt, rooted silt, sand, sandy silt, silty sand, silt with sand, alternating sand and silt, and ball clay facies. Sediment types with a high OM content represent sapric peat, muck, clayey peat, hemic peat, and organic drape facies. The present study concentrates on microscopic and statistical analyses of dispersed OM assemblages recovered from twelve (12) sedimentary facies (the alternating sand and silt facies studied by Feng [1994], was excluded).

Two hundred and twenty samples were processed following standard palynofacies preparation techniques: HCL and HF treatment of sediment with subsequent mounting of the organic residue onto slides. Prepared slides were point counted (300 particles per slide) following the Amsterdam Palynological Organic Matter Classification scheme. Fifteen (15) OM types identified during point-counting were grouped into four categories which include Structured debris, Palynomorphs, Unstructured debris, and Indeterminate debris. Microscopic analysis showed that palynofacies assemblages are dominated by terrestrial-derived OM.

Unstructured debris were found to be the most common in the analyzed samples. Structured organic particles are the second most common type of palynodebris in the sample suite. The quantitative share of both Palynomorphs and Indeterminate particles is very low.

The non-parametric Mann Whitney statistical test was used to evaluate point-count data to determine the presence/absence of differences in the amounts of dispersed OM among the twelve sedimentary facies. UMPGA Cluster analysis was conducted to check whether OM composition varied within the sand, silt, sandy silt, and organic drape facies depending on the sample location within the delta.

The results of Mann Whitney analyses showed that there are significant statistical differences between the organic-poor and organic-rich facies in the relative distribution of Structured and Unstructured debris. These differences stem from a greater percentage of Structured debris and a smaller percentage of Unstructured debris in the organic-rich facies relative to the organic-poor facies. Since the bulk of dispersed OM in the delta is interpreted to be of terrestrial origin, variations in the rate of decomposition of terrestrial-derived plant material is considered to be the most important factor responsible for the differences in sample suites. Several observations may account for these variations. Among these are differences in the acidity of pore waters in the burial or accumulation site, differences in the taphonomic nature of OM accumulations (autochthonous vs. allochthonous assemblages), different accumulation patterns within organic-poor and organic-rich facies, and the presence of a temporal hiatus between the peat/muck facies and underlying silt/rooted silt facies.

The results of the Mann Whitney analyses also showed that most sedimentary facies do not differ from each other statistically in the relative amount of Palynomorphs. Exceptions to this pattern are the differences in palynomorph abundance between the sand vs. silt facies and the sand vs. rooted silt facies. These variations stem from a greater quantitative share of palynomorphs in the silt and rooted silt facies. Variations in grain size and rates of accumulation between the sand vs. silt/rooted silt facies, as well as differences in the quantity of palynomorphs per liter of water during the monsoonal and dry seasons, are invoked as possible reasons to explain these observations.

There is no definite pattern in the distribution of Total indeterminate debris between sedimentary facies. Only two facies, the sand facies and silty sand facies (distributed mainly in the delta front), contain a significantly greater percentage of Black indeterminate debris (pyrite) relative to that of all other facies analyzed. The higher percentages of pyrite probably resulted from bacterial activity where a greater amount of sulphate was available within the pore-waters. This is due to marine influences operating in the delta front.

Based on the results of Mann Whitney analysis, it is concluded that the Rajang River delta can be characterized by two consistent types of dispersed OM assemblages (palynofacies) originating from organic-poor and organic-rich sedimentary facies. The palynofacies suite recovered from the organic-poor facies preserves relatively low percentages of Total structured and relatively high percentages of Total unstructured debris. Palynofacies from the organic-rich facies have relatively high percentages of Total structured and relatively low percentages of Total unstructured debris. The quantitative shares of Palynomorphs and Indeterminate particles are analogous in both facies groups. The homogenous nature of OM assemblages recovered from organic-poor facies (silt, rooted silt, sand, sandy silt, silty sand, silt with sand, and ball clay) has important paleoenvironmental implications. Characteristics typical of these OM assemblages can potentially serve as an additional aid in distinguishing deltaic depositional environments in the stratigraphic record.

UMPGA Cluster analyses indicated that the sandy silt facies is the only facies in which vibracore clusters are distributed in a discernable geographic pattern throughout the Rajang delta. Vibracore clusters identified in the silt facies and sand facies appear to reflect quasi-patterns, while no discernable pattern was found in the distribution of the organic drape facies.

The vibracores from cluster #1 in the sandy silt facies are found within the area influenced by fluvial sedimentation. They contain lower percentages of particles clustered in OM group #1 (Structured indeterminate, Heterogenous unstructured, Fungal hyphae, and Black indeterminate) and higher percentages of particles from OM group #2 (Sporomorphs, Cuticles, Fungal palynomorphs, Finely dispersed, Colorless indeterminate, and Woody debris). The vibracores from cluster #2 are located within tide-dominated areas. The opposite trend is typical of group #2. Higher percentages of particles occur in OM group #1, whereas lower percentages are found in OM group #2. Factors resulting in the observed OM variations between two vibracore clusters may involve differences in the proximity of fluvial and tidal regimes to the source vegetation (riparian vegetation within the alluvial and delta plains), as well as differences in the hydrodynamics of these depositional subenvironments. A higher proportion of Finely dispersed debris was also observed in the sandy silt facies recovered from the vibracores within fluvial-dominated area. Low sedimentation rates associated with this facies may explain this phenomenon.

The results of cluster analyses indicate that differences in OM composition in the sandy silt facies is a function of geographical location within the Rajang River delta. These differences may serve as an additional tool for distinguishing fluvial- and tide-dominated areas within ancient deltas.