Nature, origin and evolution of a Late Pleistocene incised valley-fill, Sunda Shelf, Southeast Asia
File(s)
Published version
Author(s)
Alqahtani, FA
Johnson, HD
Jackson, CA-L
Som, RB
Type
Journal Article
Abstract
Shallow (<500 ms TWT/c. 500 m sub-sea) 3D seismic reflection data from a large (11500 km2), ‘merge’ survey, supplemented with high-resolution site survey data, have enabled identification and characterisation of a large (up to 18 km wide by 80 m deep) Late Pleistocene incised valley on the Sunda Shelf, offshore eastern Malaysia. The valley-fill displays five large-scale stratigraphic features: (1) it is considerably larger than other seismically-resolvable channel forms and can be traced for at least 180 km along its length within the available 3D seismic volume; (2) it is located in the axial part of the Malay Basin with a_NNW-SSE orientation, which is parallel to the maximum thickness of the underlying Oligo-Miocene succession (the main fill of the Malay Basin); (3) the youngest fill of the valley-fill is dominated by a large (up to 800 m wide and 23 m deep), high-sinuosity channel, with well-developed lateral accretion surfaces, point bars and meander loop cut-offs; (4) the immediately adjacent interfluves contain much smaller, dendritic ‘tributary’ channel systems, which drain into the larger incised valley system, and (5) a ca. 16 m thick, shell-bearing, Holocene age clay caps the valley-fill. Site survey borehole data from the abandonment part of the large meandering channel at the top of the incised valley-fill display the following: (1) pebbles, gravels and cobbles form a basal channel lag above a prominent, seismically-defined erosional surface; .(2) interbedded fine-grained sandstones and claystones, with shell fragments and terrestrially-derived organic material, form the middle part of the channel-fill; and (3) inclined heterolithic stratification (IHS), visible on high-resolution site survey seismic data, is the dominant macro-scale sedimentary structure. The interval adjacent to the incised valley comprises muds and silts with dispersed plant material and shell debris. These data document two major depositional environments: (1) a meandering channel that preserves a very high-energy channel lag, which was followed by a channel abandonment succession, with evidence of brackish water conditions suggesting an estuarine depositional setting, and (2) an interfluve comprising a mud-dominated, lower coastal plain environment with periodic marine influence. This incised valley is interpreted as part of a large axial drainage system, which ran along the length of the Malay Basin, connecting the Gulf of Thailand in the north with the South China Sea in the south-east (>1200 km long). We interpret this as the valley of a major trunk river that dissected the Sunda Shelf and named the palaeo-Chao Phraya-Johore River. Incision of the valley occurred during the Last Glacial Maximum (LGM), when a sea-level fall of ca. 120 m exposed the low-gradient (<0.01º) Sunda Shelf. Transgressive infill of the valley was associated with rising sea-level during the Holocene, which caused drowning of the Sunda Shelf and deposition of the modern shelf mud blanket. The valley size, orientation and palaeo-hydrology suggests close interaction between the regional tectonic framework, low-angle shelf physiography and a humid-tropical climatic setting. The drainage basin area palaeo-Chao Phraya-Johore River is estimated to have been ca. 1,350,000 km2, which makes it larger than most of the major drainage basins in the Southeast Asia and comparable in size to those of the world’s present-day largest rivers. It was probably the largest drainage system in the Sunda Shelf area.Estimates of the bankfull discharge, based on empirical relationships derived from modern rivers, suggests values ranging from ca. 36,740 to 46,550 m3/s for the large meandering river. This is larger than many modern rivers in Southeast Asia and is comparable to the Yangtze (31,900 m3/s) and Irrawaddy rivers (36,000 m3/s). These high discharge values are consistent with both the estimated drainage basin area and the humid tropical climate of the palaeo-Chao Phraya-Johore River.
Date Issued
2015-04-17
Date Acceptance
2014-12-19
Citation
Sedimentology, 2015, 62, pp.1198-1232
ISSN
1365-3091
Publisher
Wiley
Start Page
1198
End Page
1232
Journal / Book Title
Sedimentology
Volume
62
Copyright Statement
© 2014 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists.
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
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Publication Status
Published