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 t o 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, wit h highest rainfall levels or the "wet" season being coincident with the Dec ember-March monsoon. The monthly drainage-basin discharge is calculated to average about 3600 m(3)/s, and the discharge normally ranges from 1000 to 6 000 m(3)/s. Spring tides in coastal areas range from 2.9 to 5.8 m. Tide dat a indicate that the tides are semidiurnal with a noticeable diurnal inequal ity. 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 the se preserved delta plain siliciclastic sediments are the result of estuarin e depositional processes that occur during intervals of reduced rainfall or the "dry" season (April-November). The number of laminae preserved per nea p-spring cycle is the highest (congruent to 18-20), and the average thickne ss is the greatest in the middle part of the delta plain. Distributary chan nels in this region normally contain low-salinity brackish water to freshwa ter. Vibracores recovered from delta front and prodelta sediments show evid ence of heterolithic stratification, but rhythmicity is absent. Grain-size data indicate that preserved delta front and prodelta sediments are implace d by " wet" season processes (December-March) when fluvial flux and delta-p lain erosion are at their maxima. Individual silt laminae and/or silt and s and 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 reserv ed.