Palynofacies Patterns in Channel Deposits of the Rajang River and Delta, Sarawak, East Malaysia

Abstract

The Rajang River and its delta accumulate siliciclastic sediments and dispersed organic matter within river and tidal channels as rhythmically stratified sand, silt, and organic drapes. These deposits were vibracored and subsampled, resulting in a data set composed of 84 samples taken from 45 cores. Palynofacies preparations of this sediment facies were examined from throughout the alluvial valley and delta to determine if a single organic matter assemblage characterized these deposits. An underlying assumption of palynofacies analyses in ancient transitional settings is that each lithofacies or depositional environment is characterized by a specific organic matter (OM) assemblage. This specific hypothesis is not supported by the present investigation. However, the present investigation has demonstrated that the resultant dispersed OM assemblages within a single sediment facies is heavily influenced by the geochemistry of the system. Three distinct palynofacies assemblages in this data set have been identified using nonparametric and multivariate statistical analyses. One assemblage, characterized by high amounts of Heterogenous and Homogenous (mainly dammar) Unstructured OM, moderate amounts of Structured OM and Finely Dispersed Unstructured OM, and low amounts of Black Indeterminate OM, is restricted to depositional sites that are principally found in freshwater settings. The second assemblage, characterized by the highest quantities of Heterogenous Unstructured OM and Indeterminate Black and the least amount of Finely Dispersed Unstructured OM, is found in channels of the lower delta plain. The distribution of this palynofacies assemblage conforms to the limit of saline influence during the wet season. The third group, characterized by the highest quantitative amount of Finely Dispersed Unstructured OM, is found either in barforms or in black-water tidal channels that are sediment starved. These results indicate that OM assemblages preserved in a single sediment facies characteristic of transitional zones vary in response to the abiotic processes operating within the system.