A new stratigraphy for the islands of the Sumatran Forearc, Indonesia

Abstract

Abstract The Sumatran Forearc of western Indonesia contains a number of islands where extensive exposures of basement rocks and their sedimentary cover may be examined. The islands, such as Nias and the Batu Islands, are located along the outer-edge of the Sumatran Forearc, whilst others, including the Banyak Island Group and Pini Island, lie within the forearc basin. Detailed sedimentological, palaeontological and palaeobathymetric data from the Tertiary strata from the forearc region require a new stratigraphy, as previous stratigraphic schemes have not explained the variations across the region adequately. This stratigraphy, developed initially from detailed data collected on Nias and the Banyak Islands, can fully account for the successions of sedimentary rocks on the Banyak and Batu Islands and Siberut and explains many of the apparent inconsistencies between previous stratigraphies. A basement complex and six new formations are formally defined in this paper; important sedimentological differences between these formations represent key stages in the evolution of the outer part of the Sumatran Forearc. Studies of the basement rocks across the forearc area suggest the basement is inhomogeneous; large intact sections of ophiolitic material occur in some areas, whilst there is evidence for both oceanic and continental basement in others. Such heterogeneity is to be expected in a long lived obliquely convergent margin. In Oligocene-earliest Miocene times extension of the heterogeneous basement is inferred through indirect evidence. Palaeobathymetric data from the Oyo Formation indicates that the initial deposition in the newly formed extensional sub-basins on Nias was, in most areas, deep marine, in many cases below the CCD. Detailed biostratigraphic analyses and structural and geochronological studies indicate a major Early Miocene unconformity in the western (Lahewa Sub-basin) and parts of central Nias (Mujoi Sub-basin). This unconformity was developed as a direct result of a period of basin inversion that affected western parts of Nias. Whilst sub-aerial erosion occurred in parts of western Nias, conformable deposition of the Gawo and Olodano Formation continued in the Gomo and eastern parts of the Mujoi Sub-basins. The shallow marine sedimentary rocks of the Olodano Formation tended to accumulate on intra sub-basinal highs whose position was controlled by active faults that transected the sub-basins. The sedimentary record reveals that the Lower and Middle Miocene phases of differential uplift and subsidence had ceased by the Late Miocene. A massive influx of Himalayan derived Bengal Fan sediments reached the Sunda Trench in the Sumatra area in the late Middle Miocene. Continued addition of Bengal Fan material to the accretionary wedge south-west of Nias resulted in steady plate deflection and subsidence through flexural processes in forearc basin areas. The flexural consequence of increased load added to the prism, and associated subsidence history is documented by the sedimentary record on Nias where the shallow marine Olodano Formation passes up into the neritic to upper bathyal Lahomie Formation. The Pliocene unconformity which is observed over all areas studied in the forearc is well constrained by structural, biostratigraphic and sedimentological studies. The unconformity represents the initiation of a major phase of uplift and deformation that continues to the present day. Rapid uplift of the outer arc ridge and deformation of the prism during the Pliocene led to increased subsidence landward of the deformation. The rapid subsidence of the forearc basin landward of the outer-arc ridge has contributed greatly to the “apparent” differences between the forearc basin and the outer-arc ridge at the present day: two areas with remarkably similar pre-Pliocene histories now have remarkably different physiographies.