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

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