A 16,000-km series of seismic and geomagnetic traverses in the Gulf of Thailand, northern Sunda Shelf, and the adjacent deep-sea floor was completed during 1969. The results show the presence of three large sediment-filled basins separated by swells or ridges. Westernmost is the Gulf of Thailand basin that extends almost from Bangkok to Singapore. A buried ridge separates this basin from two other basins on the northeast, the Mekong and the Brunei-Saigon. Both of the latter basins reach the adjacent land areas, and the Brunei-Saigon basin appears to be the main part of the Northwest Borneo geosyncline. The Mekong and Brunei-Saigon basins are limited on their northeastern sides by a narrow ridge that lies beneath the outer continental shelf and the upper continental slope, a feature termed the "Peripheral Ridge." Sediments in all three basins are thicker than 2 km and they may be thicker than 4 km. They contain tectonic folds, unconformities, faults, and at least a dozen structures believed to be diapiric intrusions. The deep-sea floor in the northeast beyond the continental slope contains an abyssal plain and also a broad comparatively level plateau that is dotted with seamounts, some of which are capped with coral atolls. Sediments in this deep region, the China basin, are of two distinct ages separated by a widespread unconformity.

Inferences from geologic studies on land suggest that the basin in the Gulf of Thailand was formed during Late Cretaceous time, and the present form of the ones beneath the Sunda Shelf was acquired subsequently. Sediments have accumulated in them ever since, with interruption by tectonic activity and erosion. Prospects for oil and gas appear to be favorable, as also is indicated by the intense exploration and test drilling off Borneo and in the southern part of the Gulf of Thailand by international companies. Other parts of the sediment-filled basins deserve attention as well.