Provenance and sediment dispersal in the Rajang River delta/coastal plain system, Sarawak, East Malaysia

Preliminary sedimentologic investigations were conducted in the Rajang River delta/coastal plain system in Sarawak, East Malaysia, to determine the processes and timing of siliciclastic transport and sedimentation in tropical, peat-dominated systems. Methods involved depth profiling and collection and analysis of bottom and suspended sediment load samples from channels and offshore areas, and coring and trenching of subaerial channel, shoreline, and backswamp features. The Rajang delta occurs in an embayment formed by the folded Mesozoic and Cenozoic sediments of the Central Borneo Massif. It covers an area of 11,000 km2 and is physiographically separated into an alluvial valley floodplain, an abandoned tidally flushed delta plain, and an actively accreting rectilinear delta/coastal plain. From 50 to 80% of the surface area is covered by peat 1 to 20 m thick. During the dry season of July/August, 1992, river channels varying from 2 to 45 m deep with discharge rates from < 100 to approximately 3550 m3/s were carrying suspended sediment loads ranging from 0 to 2281 mg/l. At present, approximately 8.2 × 1010 kg/yr of fine-grained sediment is supplied to the system and areal accretion rates are over 1.0 km2/yr. The structural configuration of the uplands/receiving basin and subsequent drainage patterns and tidal influence results in a contradictory distribution of mature, fine-grained sediments in the channels of the active delta/coastal plain, and immature, coarse-grained angular sands and gravels in the channels of the abandoned delta lobe. The contemporaneity of the peat and clastic accumulation is evidenced by the margins of the thick, domed, low ash peat deposits interfingering with and being overlain by root-penetrated siliciclastic sediments. Much of the fine-grained sediment, however, bypasses the delta plain due to in-channel density gradients. Delta/coastal plain accretion occurs by distributary mouth bar buildup, sand bar welding, and ridge and runnel accretion. All subaerially exposed surfaces are quickly vegetated. The aberrant depth and configuration of some channels suggest that drainage is, in part, controlled by contemporaneous faulting. © 1993.