Nature, origin and evolution of a Late Pleistocene incised valleyfill, Sunda Shelf, Southeast Asia

Understanding the stratigraphic fill and reconstructing the palaeo-hydrology of incised valleys can help to constrain those factors that controlled their origin, evolution and regional significance. This condition is addressed through the analysis of a large (up to 18 km wide by 80 m deep) and exceptionally well-imaged Late Pleistocene incised valley from the Sunda Shelf (South China Sea) based on shallow three-dimensional seismic data from a large (11 500 km), ‘merge’ survey, supplemented with site survey data (boreholes and seismic). This approach has enabled the characterization of the planform geometry, cross-sectional area and internal stratigraphic architecture, which together allow reconstruction of the palaeo-hydrology. The valley-fill displays five notable stratigraphic features: (i) it is considerably larger than other seismically resolvable channel forms and can be traced for at least 180 km along its length; (ii) it is located in the axial part of the Malay Basin; (iii) the youngest part of the valley-fill is dominated by a large (600 m wide and 23 m deep), high-sinuosity channel, with well-developed lateral accretion surfaces; (iv) the immediately adjacent interfluves contain much smaller, dendritic channel systems, which resemble tributaries that drained into the larger incised valley system; and (v) a ca 16 m thick, shell-bearing, Holocene clay caps the valley-fill. The dimension, basin location and palaeohydrology of this incised valley leads to the conclusion that it represents the trunk river, which flowed along the length of the Malay Basin; it connected the Gulf of Thailand in the north with the South China Sea in the south-east. The length of the river system (>1200 km long) enables examination of the upstream to downstream controls on the evolution of the incised valley, including sea-level, climate and tectonics. The valley size, orientation and palaeo-hydrology suggest close interaction between the regional tectonic framework, low-angle shelf physiography and a humid-tropical climatic set-