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The Rajang River delta is a peat-accumulating coastal-deltaic system located in an embayment
formed by the folded Mesozoic and Cenozoic strata of the Central Borneo Massif on the western
coast of Borneo. The Rajang River cuts a relatively straight path westward after emerging from
the upland area. The river begins to bifurcate in an 180o pattern beginning at the approximate
position of Sibu, resulting in five main distributary channels and the development of a Holocene
delta, covering an area of 6,500 km2. It is divided into an "abandoned" tidally flushed delta plain
and an actively accreting rectilinear delta/coastal plain.
Tropical vegetation provides biomass to accumulating sediments. Riparian vegetation in the distal
reaches is composed of marine to brackish water-fed mangroves and Nipa; the proximal parts are
fringed by dipterocarp forests and cultivated lands. Peat swamps, dominated by dipterocarps in
which several ecological catenas have been identified, occur adjacent to riparian vegetation. Peats
greater than 1 m thick cover about 50% of the delta plain surface, 80% of the adjacent coastal
plain, and 75% of the alluvial valley. Maximum peat thickness exceeds 20 m, and all peats have
accumulated within the past 7,000 to 7,500 years.
Subfossils of macrofloral elements are rarely encountered in vibracores extracted from throughout
the regime. Macrofloral elements are restricted to mineral-substrate soils found beneath peat
(mainly roots), isolated lenticular pods within peat swamps (primarily leaves), and channel bars
(mixed assemblages) where they accumulate in troughs of large-scale bedforms. Subfossil
assemblages recovered from these sites exhibit a wide range of preservational states, with both
well- and poorly-preserved phytoclasts in each assemblage. It is not possible, therefore, to use
preservational state as a taphonomic criterion to interpret autochthonous vs allochthonous
assemblages in the fossil record.
Citation: International Organization of Paleobotany Conference V- Santa Barbara, CA.,1996, Abstracts Volume, p. 33.
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