Abstract

A new approach combining the structural framework with sequence stratigraphic concepts is used to better understand the Middle Baong Sandstone stratigraphy of the Aru onshore area, North Sumatra. The approach is based on tectonic concepts, expressed in terms of half-graben rifting, and the isochore mapping of depositional units, using sequence boundaries and marine flooding surfaces. The last rifting episode took place during Oligocene to Early Miocene time. Extensional fault geometries have a north-south dominant trend, with a rift shoulder lying on the Yang Besar High and the down-flank area on the Tamiang Deep. A period of accelerated subsidence, which resulted in deposition of a thick marine shale of Lower Baong age, was followed by one of decreasing rates. This post-rift tectonic episode was probably produced by transpressional stress generated by the northward movement of India, which caused northwest-southeast right-lateral movement along the Tamiang Deep.

This tectonic remnant topography controlled the spatial distribution of subsequent depositional systems. The rift shoulder on the east combined with transpressional uplift on the west to form a structural valley that localised the incisement and infilling of the Middle Baong Sandstone during the rapid eustatic fall of 10.5 Ma (N14). The post-rift incised valley fill sequence was masked by Upper Baong shale during the ensuing rapid eustatic rise. Isochore mapping identifies two tributary channels that flowed from the south and southwest. These channels served as conduits that transferred sediments predominantly from the Barisan uplift to the valley complex. This mapping also supports the existence of the valley edge on the eastern and western parts, where isochore contours become zero and the marine flooding surface merges with a sequence boundary. Well-to-well correlations show that aggradational and retrogradational fourth order sequence sets become thinner near those areas.