Controls On the Geometry And Evolution of Humid-Tropical Fluvial Systems: Insights From 3D Seismic Geomorphological Analysis of the Malay Basin, Sunda Shelf, Southeast Asia

Abstract

Abstract High-resolution three-dimensional (3D) seismic reflection data from the Pleistocene-to-Recent succession of the Malay Basin (Sunda Shelf, SE Asia) have been used to evaluate the geometry, dimensions, distribution, and evolution of humid-tropical fluvial channel systems. Based on their seismic expression and variations in sinuosity and depth, we identify six fluvial channel types, which occur in eight, seismically defined, 18–145-m-thick depositional units. Each unit is characterized by a consistent vertical change in channel-body geometry and dimensions as follows (bottom to top): (1) relatively large (300–3000 m wide and 15–45 m deep), straight to low-sinuosity channels and/or large incised valleys at their bases, and (2) smaller (75–250 m wide and 8–23 m deep), highly sinuous channels at their tops. This cyclical stratigraphic architecture is interpreted as a mainly climatically driven change in fluvial discharge and sediment supply, rather than by eustatically or tectonically driven changes in relative sea level based on (i) the study area was located far upstream (ca. 700 km) of the coeval shoreline, and (ii) tectonically quiescent nature of the basin during the Pleistocene to Recent. Although the upstream controls were likely the main controls on the stratigraphic organization (Units 2–4 and 6), the large incised valleys (Units 5 and 7) are interpreted to be formed due a relative sea-level fall during the Last Glacial Maximum (LGM). This study demonstrates how an improved understanding of the temporal and spatial variability of fluvial channel systems can be obtained through detailed analysis of regionally extensive, high-resolution 3D seismic reflection data. In particular, regional constraints on channel geometry, scale, and orientation can be more confidently determined than is typically permitted by deeper, lower-resolution subsurface data. Furthermore, we have provided quantitative constraints on the dimensions, geometries, and paleohydrology of humid-tropical fluvial systems, which form significant, but generally less well-imaged, hydrocarbon-bearing reservoirs, in the more deeply buried parts of the Malay Basin and in many other basins around the world.