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Weathering products of silicate rocks are particularly useful for evaluating the continental chemical weathering on the Earth's surface and its mechanism. Clay mineralogy and major-element geochemistry of surface sediment samples collected in major rivers of Malay Peninsula and North Borneo in the tropical Southeast Asian region are used to study the present chemical weathering process and its controlling factors of tropical regions. The results indicate that the clay mineral assemblage in Malay Peninsula consists dominantly of kaolinite (average 80%) and minor illite (average 17%), almost without chlorite and smectite, whereas in North Borneo it consists mainly of illite and chlorite, with minor amounts of kaolinite (average 14%) and no smectite. Total contents of illite and chlorite in both Northwest and Northeast Borneo are 84% and 87%, respectively. Major-element geochemical results of both bulk and clay-fraction sediments show intensive chemical weathering degree for both areas. Relatively, the chemical weathering degree is gradually strengthened from Northeast Borneo, Northwest Borneo, to Malay Peninsula, and it is extremely intensive in Malay Peninsula. Our results indicate that, in the tropical Southeast Asian region exampled by Malay Peninsula and Borneo, climatic condition of tropical warm temperature and East Asian monsoon rainfall is the first-order controlling factor on the chemical weathering, resulting in intensive chemical weathering throughout tropical areas, and tectonic activity and lithology of parent rocks are subordinate factors, which still have an important impact on the weathering products, forming completely different clay mineral assemblages between Malay Peninsula and Borneo. clay minerals, major elements, fluvial sediments, chemical weathering, Malay Peninsula, Borneo Citation:Wang H, Liu Z F, Sathiamurthy E, et al. Chemical weathering in Malay Peninsula and North Borneo: Clay mineralogy and element geochemistry of river surface sediments.

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High-resolution 2D acoustic profiles, combined with time slices from a 3D data volume, were used to investigate the paleoincised valleys offshore of the present-day Pahang River, South China Sea. Paleovalleys were formed during the regressive phase of the last glacial cycle. They were submerged and possibly filled during valley formation and postglacial marine transgression. Interpretation of acoustic profiles illustrates that the valleys were incised and infilled during the regression and low stand followed by subsequent deglacial sea-level rise. They were overlain by a transgressive ravinement surface suggesting transitional deposits between fluvial-dominated filling and shallow-marine deposition. This ravinement surface is overlain by Holocene shallow marine deposits. A low-sinuosity low-stand valley system changed to a high-sinuosity meander belt and eventually evolved into a deltaic distributary channel system before the complete submergence of the area. The average Late Pleistocene surface lies between 53 and 64 m below present-day mean sea level in the study area with approximately 16–50 m of valley incision. The Holocene shallow marine cover thickness varies from 5 to 10 m.