The geology and tectonics of the Woyla Group, Natal Area, North Sumatra

Abstract

The Batang Natal River flows across the Western Barisan Mountains of North Sumatra, exposing a section through the Woyla Group, a major unit of Sumatran island arc basement. Detailed field mapping of the section, followed up by thin section petrology, structural analysis and radiometric dating, was undertaken in order to investigate the geology, structure, origin and tectonic evolution of the Woyla Group. The Woyla Group outcropping in the section is divided into eighteen lithotectonic units, each with distinctive lithological characteristics, including basalts, volcanogenic turbidites and debris flows, and esites, spilites, chert-limest?ne breccias, non-marine volcanogenic sediments, quartz-mica schists, metaturbidites, megabreccias, serpentinite, diorite and massive limestones. Most units are deformed to varying degrees of intensity by tight to isoclinal 'F' folding with an associated 'S1 foliation. A later phase of open 'F2' folding deforms the 'S' foliation, as well as other layered units unaffected by 'D'. All the units are bounded and internally disrupted by strike-slip faults, commonly of NW/SE or WNW/ESE trend. Previous models interpreted the Woyla Group as the remnants of a closed marginal basin. The new model proposed here suggests the Woyla Group is a Mesozoic accretionary complex, built up of pelagic and terrigenous sediments, together with fragments of island arc and oceanic material, that were accreted against the Mesozoic Sumatran continental margin. The complex later subsided, to form part of the basement of the Tertiary forearc basin. The Sumatran Fault System, initiated in the late Mesozoic, or possibly earlier, as a consequence of oblique subduction; transected the Sumatran forearc and the magmatic arc. It resulted in major disruption, translation and reshuffling of fault slivers, thereby assembling the allochthonous and parautochthonous terranes now seen in the Woyla Group. The Woyla Group was subaerially exposed by the late Miocene-early Pliocene uplift of the Barisan Mountains.