Abstract

. . . . the causes we know everything about depend on causes we know very little about, which depend on causes we know absolutely nothing about. — Tom Stoppard, Tavesties.

In northern Sumatra four major volcano-sedimentary sequences, separated by unconformities, are recognised. Three are pre-Tertiary in age and the other is Tertiary to Recent.

The oldest rocks belong to the Late Palaeozoic Tapanuli Group which is primarily clastic and probably largely of glaciomarine origin. Similar rocks occur in northwestern Malaysia and southern Thailand. Two periods of deformation, accompanied by magmatism and metamorphism, preceded the deposition of the overlying Peusangan Group. This group is related to an eastward dipping palaeo-subduction zone and it consists of two main components a Late Permian volcanic arc assemblage and a Middle to Late Triassic back-arc succession which is equivalent to the Semanggol Formation of Malaysia. The Late Mesozoic Woyla Group includes a volcanic arc assemblage and portions of a dismembered ophiolite together with a cover sequence which formed in a back-arc basin. Subsequent closure of this basin in the Late Cretaceous and the low angle of plate convergence during the Tertiary has resulted in severe deformation of the ophiolitic portion of the Woyla Group.

Since at least the Late Eocene northern Sumatra has been the locus of periodic volcanic arc activity with sedimentation concentrated in the adjacent fore-arc and back-arc basins. A marked unconformity in the Late Oligocene and a tectonic event initiated in the late Middle Miocene have been used to divide the Tertiary into three supergroups (1, 2 & 3). The later event, comtemporary with the commencement of sea-floor spreading in the Andaman Sea, led to the rise of the Barisan Mountains by the Pleistocene and the growth of the dextrally transcurrent Sumatran Fault System Serpentinites from the Woyla Group ophiolite were emplaced from the latest Miocene.