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Reservoir Geometries and Sand Composition in the Holocene Rajang River Delta, Sarawak, East Malaysia

Staub, James R.1
1 Department of Geosciences, The University of Montana, Missoula, MT.

The Rajang River drainage basin is dominated by Cretaceous to Eocene sediments of accretionary complex and forearc basin origin. Rock types are folded and faulted siliciclastics and carbonates as well as igneous intrusives and extrusives. Many sedimentary rocks have undergone low-grade metamorphism. The drainage basin covers about 50,000 km2 and relief exceeds 2000 m. Rainfall is in excess of 370 cm/year. The delta of the Rajang River covers 6500 km2, tides in coastal areas range from 2.9 to 5.8 m, and tidal influence extends 120 km inland into the alluvial valley. Waves from the northeast monsoon dominate from December to March (wet season) and from the southwest monsoon (dry season) during the middle months of the year. Average wave heights are on the order of 2 m. The delta is up to 80 km wide and its arcuate coast is 160 km long. Five distributaries with channel mouth widths of 2 to 12 km separate sand beaches up to 35 km in length. Distributary mouth sand bodies are typically elongate tidal-ridged, on the order of 5 to 7 m thick, and have maximum dimensions of 3 by 10 km. Shoreline sands are also on the order of 5 to 7 m thick and ridge and swale topography is present at the surface or exposed along the margins of channels for up to 25 km inland from the coast. Sand cuts obtained from grab samples (n = 185) and cores (n = 239) were analyzed on a whole-sand basis (>63 µm) to determine grain shape, grain-size variation, and composition. The geographic distribution of samples is from the alluvial valley to offshore coastal areas. Grab sample results indicate that from the alluvial valley to the coast sands transition from angular to subrounded, mean grain-size increases by almost one phi, from very fine sand to fine sand, and quartz content increases from about 55% to over 95% at some coastal locations. Chert, a rock fragment, represents about 3% of the whole-sand fraction in coastal areas. Grab sample results are mirrored by core samples representing the same environments that cover a time span of 7.5 ka. Changes in the whole-sand fraction occur because fine-grained rock fragments, most prevalent near the alluvial valley, are smaller that the quartz grains. With increasing distance, quartz content increases, the ratio of mono- to polycrystalline quartz is constant, and rock fragments decrease. It is primarily the breakdown of sand-sized metamorphic rock fragments that causes the overall sand size and quartz content increase.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009