Abstract

The Lower and Middle Miocene sediments in the North Sumatra Basin comprise several distinct lithologic units which are related laterally by facies changes. In the southeastern region of the Basin the transgressive sequence of sediments of early to late Lower Miocene age was deposited unconformably upon pre-Tertiary Strata. In the northwestern part dark shales were deposited. During Lower Miocene time the basin deep was located in the northwestern region. In late Lower Miocene time, the southeastern region reached stable shelf conditions as indicated by quartzose sandstones known as Belumai Sandstone, occasionally intercalated with sandy calcarenites and calcilutites of the Telaga formation. During Middle Miocene time subsidence took place accompanied by deposition of great thicknesses of open marine shales of the Baong formation. In the vicinity of Arun, a considerable thicknesse of limestones of probably shelf origin was developed, indicating that the area had been subjected to a continuous uplift. This movement which is apparently the beginning of the major Barisan uplift, was also recorded in the south-eastern region by the occurence of some sand intercalations in the Baong shale formation. In the offshore area, deposition of Tertiary sediments started during Lower Miocene time. Shelf carbonates known in the vicinity of NSB area as Malacca Limestone, occasionally overlie the basal quartz sandstones. The clay and sand sequence of Pliocene to late Miocene age overlie unconformably the Lower Miocene sequence. During most of the Lower and Middle Miocene time., the majority of offshore area was initially in the stable shelf condition, followed by periods of non deposition.