Abstract

The North Sumatra Basin represents a rift-sag depression along the southwest rim of the west Indonesian craton. The early history of the basin consists of a rift phase from Late Eocene(?) to Early Miocene (PFZ N6), and a sag period from Early Miocene N6 to N8. During rifting, north-south trends produced subsidence south and west of the craton, where coarse clastics (Bruksah Formation) were deposited between rift-highs prior to P22, followed by deposition of widespread marine shales (Bampo Formation) from P22 to N4. During shale deposition, foraminiferal mounds accumulated on the ramps and crests of some of the north-south rifts, associated with specific transgressions within P22 and N4. Marine sands (Belumai Formation) derived from the exposed craton filled in the rift-grabens during the late rift-phase (N4 to N6). At the same time, a series of high-standing structures formed in the southern part of the basin and an unconformity developed above early syn-rift sediments incorporated in these structures.

During the sag-phase, basin-wide subsidence was accompanied by carbonate deposition (Peutu Formation) associated with flooding events at N7 and N8. These carbonates, plus sediments deposited during an intervening lowstand, have been assimilated into a single sequence. On the southern structures, the sequence consists of transgressive platforms (N7) overlain by coral reefs or equivalent deep-water carbonates (N8). On the carton, carbonate mounds and localized buildups overlie thick marine sandstones.

Between these areas, drowning of some of the rifts resulted in deep-water limestones and marls draped over their crests, while others received shallow marine carbonates during N7, followed by marls during N8. The lowstand is expressed as a shelf unconformity in platform/reef sequences, by deposition of sandstone and detrital limestones in shallow settings adjacent to platforms, and by the occurrence of middle shelf limestones within marly N7-N8 sediments in deeper settings.