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Late Tertiary (Neogene) sediments deposited in the Sunda forearc reveal the stratigraphic and structural evolution of this active plate margin. A dense grid of multichannel seismic reflection data and information from 17 exploratory wells were used to establish a detailed seismic-stratigraphic framework of the forearc region from 0° to 6°N. Several important tectonic cycles are recognized: Paleogene orogeny, Neogene subsidence, and late Tertiary tectonism. Superimposed on these regional tectonic events are three major transgressive-regressive cycles of sedimentation related to changes in sea level and provenance.
Paleogene and older metasedimentary and metamorphic rocks comprise basement beneath the landward (inner) margin of the forearc basin. Both basement rocks and lower Tertiary sedimentary rocks were deformed and eroded approximately 25 to 30 Ma. The continental shelf was exposed to subaerial erosion, and basin deposits were restricted offshore, coincident with a worldwide lowstand of sea level in the Oligocene. The Paleogene orogeny probably occurred prior to the erosional event that cut the prominent angular unconformity on the shelf.
The Neogene history of forearc basin development is characterized by subsidence and nearly continuous sedimentation. A basal transgression began in latest Oligocene time and culminated in the middle Miocene. Alternating sequences of limestone and shale comprise two second-order cycles of sedimentation that are superimposed on an overall transgressive event. A major regressive sequence followed in the Pliocene, owing to an influx of siliciclastic clay, silt, and sand derived from Sumatra. Sedimentation rates were high, and large volumes of terrigenous material were deposited in deltaic systems on the shelf. The shelf-slope break prograded basinward nearly 10 km (6 mi) through lateral accretion and aggradation during a relative highstand or stillstand of sea level.
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