Abstract

The paleogeographic investigations discussed in this paper are limited to an area east of the Barisan Range, approximately between the Tamiang River to the North and the Toba-Asahan River to the South. However, this study also needs to consider the geological evolution of adjacent areas.

Since Eo-Oligocene time, sediments which are probably about 5000 to 7000 meters thick were deposited in N.E. Sumatra, in a basin configuration different from the present basinal structure which developed during the Mio-Pliocene orogenic phase.

The Oligocene "Basal, Sequence" is found westwards up to Nias Island. It was deposited in a continental - to paralic environment during the initial stage of the development of the Tertiary Sumatran Geosyncline. The Oligo-Miocene "Transitional Series", deposited in a continental, paralic to littoral environment, exhibits various facies, ranging from paludal, shelf, deltaic to continental.

The environment of the Middle - to Late Miocene Baong Formation has a neritic - to sublittoral character.

The lithologic and faunal associations of the Eo-Oligocene sediments reveal a paleogeographic pattern, consisting of islands and islets surrounded by shallow water. During the Oligo-Miocene, subsidence of the backdeep basin continued, but connection with the open sea (fore-deep) was still restricted by a barrier (ridge) which probably consisted of an island are approximately along the present western coast of Sumatra.

During Middle - to Late Miocene, the emergent part of the basin subsided below sea level. Baong clay and marls were deposited in a neritic and sublittoral environment.