Structural Framework of Sunda Shelf

Abstract

The Sunda Shelf is one of the most extensive continental shelves of the world. In June-July 1971 a geophysical survey was conducted over the southern part of the shelf (Java Sea). Water depth, sediment thickness, and the gravity and magnetic fields were measured continuously. Expendable radiosonobuoys permitted seismic refraction measurements. These geophysical data, supplemented by data from earlier studies of the northern Sunda Shelf and geologic data from land areas, provide a comprehensive picture of the structural framework of the entire Shelf. The seismic reflection profiles show that the Sunda Shelf consists of three major units: the northern Sunda Shelf basinal area, the Singapore platform, and the Java Sea basinal area. In the northern unit are two large sedimentary basins (the Brunei and Gulf of Thailand basins) separated by the Natuna Ridge. In the Java Sea are several other basins separated by uplifts. The basins in the western Java Sea are approximately circular and seem to result from tensional forces, whereas those in the eastern Java Sea are narrow and long and appear to be the result of compressional forces. Radiosonobuoys revealed small basement features and resolved many strata having different seismic velocities. Faults are abundant throughout the Sunda Shelf and clearly control the distribution and shapes of the basins. The faults strike north-south in the western Java Sea and northeast-southwest in the eastern Java Sea. A major discontinuity trending north-south (termed here the Natuna rift in the northern Sunda Shelf and the Billiton depression in the western Java Sea) cuts the structures of the entire Sunda Shelf and continues southward across central Java to the deep-ocean floor. Analysis of magnetic anomalies shows that the region can be divided into several distinct magnetic provinces that do not everywhere follow the major structural units mapped by the seismic reflection data. These magnetic provinces coincide with corresponding provinces of lithic units. The gravity field over the central and southern Sunda Shelf averages about +30 mgal. Local gravity anomalies with relative amplitude of 10-25 mgal are superimposed on the regional background level. Although the local gravity anomalies were helpful in resolving the upper crustal structures, the cause for the high regional gravity is unknown. All of the geophysical studies serve to outline the distribution pattern of sediment-filled basins and intervening ridges and platforms beyond the level of understanding reached by early workers through inferences based upon the geology of the land areas, and beyond the scant publications of oil companies within the restricted areas of their concessions. These regional results may be helpful both for understanding the general structure of the Sunda Shelf and for denoting the areas with the greatest future oil potential. The main structural elements are interpreted as the result of past interaction between lithospheric plates.