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

Abstract

The Holocene Rajang River delta plain, which covers an area of 6500 km 2 , has developed in a tropical, ever-wet climatic setting. Peat deposits, up to 15 m thick, occur in this delta plain. The tributary system to the delta is about 50,000 km 2 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 (ഡ18-20), 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. ᭧

Statistical analysis of the temporal and spatial controls on tidal signal preservation in late-Holocene tidal rhythmites, Romney Marsh, Southeast England

“…The alternate higher and lower semi-annual peaks in the smoothed layer thickness plots, more particularly from Boulderwall, is also seen in the Dungeness tide station data at this wavelength and is typical of other sedimentary sequences in which long tidal rhythmite records have preserved semi-annual and annual periodicities (Martino and Sanderson 1993;Kvale et al 1994Kvale et al , 1995Kvale et al , 1999Tessier 1998). A seasonal influence on the clarity of the tidal signal preserved in the sedimentary record (Van den berg 1981; Brown et al 1990;Dalrymple et al 1991;Kvale et al 1994;Park et al 1995;Allen and Duffy 1998a, b;Li et al 2000;Allen 2004;Fan et al 2004;Staub et al 2000;Dark and Allen 2005) such as storm incidence, biological activity or river discharge cannot, of course, be entirely ruled out. For example, periods when sand and mud layer thickness patterns tend towards being in-phase with each other perhaps suggest that the longer period elements of the tidal signal were occasionally modified by some other factor.…”

“…However, even in relatively long and (apparently) perfectly preserved laminated sequences the number of layers recorded over any particular depositional cycle, such as the neap-spring fortnight, does not match the number of tides theoretically capable of inundating the site, and neither is this number constant in time or space (Brown et al 1990;Williams 1991;Martino and Sanderson 1993;Greb and Archer 1998;Staub et al 2000;Choi et al 2001Coueffe et al 2004). For each high tide, a range of depositional outcomes may be envisaged: at the low energy extreme a site not being flooded at all or non-deposition, to the accumulation of mud only (perhaps over several tides), to the preservation of a well-defined sand-mud couplet, to erosion of intervening mud layers allowing amalgamation of sand layers from two or more tides, to extensive erosion of some or all of a previously deposited sequence (Dalrymple and Makino 1989;Dalrymple et al 1991;Martino and Sanderson 1993;Archer and Johnson 1997;Kvale et al 1995Kvale et al , 1999Archer 1998).…”

Shell-gathering from mangroves and the seasonality of the Southeast Asian Monsoon using high-resolution stable isotopic analysis of the tropical estuarine bivalve (Geloina erosa) from the Great Cave of Niah, Sarawak: methods and reconnaissance of molluscs of early Holocene and modern times

“…The northeast monsoon brings the heaviest rainfall of the year (Fig. 2a) and consequent maxima in river runoff levels in Sarawak at this time (Blaber et al, 1997;Staub et al, 2000). This pattern is complicated by El Niñ o events, however, which cause drought in the region (Nakagawa et al, 2000).…”

Active tectonic deformation along rejuvenated faults in tropical Borneo: Inferences obtained from tectono-geomorphic evaluation

“…Soils in both basins are poorly developed, representing the mineral composition of the parent materials (Scott, 1985). The average monthly sediment discharge rate for the Rajang River basin is 3600 m 3 s − 1 (Staub et al, 2000) while the Baram basin delivers at an average, 1445 m 3 s −1 of sediments monthly to the South China Sea (Hiscott, 2001).…”