Abstract

Seismic stratigraphic interpretation techniques are used to document the structural and stratigraphic evolution of the southwest Sumatran coast (Bengkulu shelf) between 4°00' and 5°00' S latitude. A Paleogene basinal area located under the Bengkulu shelf which, has previously been interpreted as a marine embayment, is interpreted in this paper to be a continuation of the south Sumatran graben system.

A large northeast-trending high basement block with adjacent graben to the east are the prominent structural features. The western side of the graben has been down-dropped by a series of high-angle normal faults. Extension began during the Paleogene and was controlled by the same tectonic mechanisms that influenced the Eocene rift basins in the Sumatran back-arc area. Over 10,000 feet of Paleogene sediments, similar in composition to the Paleogene section in south Sumatra, accumulated in the rapidly-subsiding graben. A mid-Oligocene unconformity truncates the basement high and signfies a possible change in the tectonic configuration of the region. A key aspect of this change is the switch of rapid subsidence from the east side of the basement high to the west side with the initiation of the present Sumatran forearc. Right-lateral slip along the Sumatran fault began during the Middle Miocene with the onset of the collision of the Australian-Indian plate with the Asian plate. Restoring approximately 150 kilometers of offset along the Sumatran fault causes this graben to line up with the Benakat Gully in the south Sumatra basin.

After the mid-Oligocene unconformity truncated the uppermost graben fill sequences, the first Neogene transgressive cycle began with the deposition of the Early Miocene Baturaja carbonates. The Middle Miocene Parigi carbonate serves as a boundary between previous fine-grained siliciclastic sediments and a younger regressive sequence of deltaic deposits. Erosion of the Barisan Mountains to the east, provided the sediment load necessary to build a series of Plio-Pleistocene deltaic/slope deposits which prograde onto the eastern flank of the Sumatran forearc basin.