Seasonal sediment transport and deposition in the Rajang River delta, Sarawak, East Malaysia

The Holocene Rajang River delta plain, which covers an area of 6500 km(2), has developed in a tropical, ever-wet climatic setting. Feat deposits, up to 15 m thick, occur in this delta plain. The tributary system to the delta is about 50,000 km2 in area. Elevations exceed 2000 m in the drainage basin and hill slopes are steep. Rainfall in the region exceeds 370 cm/year, with highest rainfall levels or the wet season being coincident with the December-March monsoon. The monthly drainage-basin discharge is calculated to average about 3600 m(3)/s, and the discharge normally ranges from 1000 to 6000 m(3)/s. Spring tides in coastal areas range from 2.9 to 5.8 m. Tide data indicate that the tides are semidiurnal with a noticeable diurnal inequality. Vibracores recovered from bar forms in tidally influenced distributary channels contain laminated silts and sand-silt couplets that show evidence of rhythmic heterolithic stratification. Grain-size data indicate that these preserved delta plain siliciclastic sediments are the result of estuarine depositional processes that occur during intervals of reduced rainfall or the dry season (April-November). The number of laminae preserved per neap-spring cycle is the highest (congruent to 18-20), and the average thickness is the greatest in the middle part of the delta plain. Distributary channels in this region normally contain low-salinity brackish water to freshwater. Vibracores recovered from delta front and prodelta sediments show evidence of heterolithic stratification, but rhythmicity is absent. Grain-size data indicate that preserved delta front and prodelta sediments are implaced by wet season processes (December-March) when fluvial flux and delta-plain erosion are at their maxima. Individual silt laminae and/or silt and sand interbeds are sometimes many centimeters thick, but average about 1 cm. These silt laminae and silt and sand interbeds or varves represent annual sedimentation events. These varves demonstrate that about 24 million MT of sediment produced by the drainage basin is deposited in the delta front and prodelta region annually. (C) 2000 Elsevier Science B.V. All rights reserved.