A NEW STRATIGRAPHY FOR THE ISLANDS OF THE SUMATRAN FORE-ARC, INDONESIA

The Sumatran Forearc of western Indonesia contains a number of islands where extensive exposures of basement rocks and their sedimentary cov er may be examined. The islands, such as Nias and the Batu Islands, ar e located along the outer-edge of the Sumatran Forearc, whilst others, including the Banyak Island Group and Pini Island, lie within the for earc basin. Detailed sedimentological, palaeontological and palaeobath ymetric data from the Tertiary strata from the forearc region require a new stratigraphy, as previous stratigraphic schemes have not explain ed the variations across the region adequately. This stratigraphy, dev eloped initially from detailed data collected on Nias and the Banyak I slands, can fully account for the successions of sedimentary rocks on the Banyak and Batu Islands and Siberut and explains many of the appar ent inconsistencies between previous stratigraphies. A basement comple x and six new formations are formally defined in this paper; important sedimentological differences between these formations represent key s tages in the evolution of the outer part of the Sumatran Forearc. Stud ies 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 continent al basement in others. Such heterogeneity is to be expected in a long livid 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. Det ailed biostratigraphic analyses and structural and geochronological st udies indicate a major Early Miocene unconformity in the western (Lahe wa Sub-basin) and parts of central Nias (Mujoi Sub-basin). This unconf ormity was developed as a direct result of a period of basin inversion that affected western parts of Nias. Whilst sub-aerial erosion occurr ed in parts of western Nias, conformable deposition of the Gawo and Ol odano Formation continued in the Gomo and eastern parts of the Mujoi S ub-basins. The shallow marine sedimentary rocks of the Olodano Formati on tended to accumulate on intra sub-basinal highs whose position was controlled by active faults that transected the sub-basins. The sedime ntary record reveals that the Lower and Middle Miocene phases of diffe rential uplift and subsidence had ceased by the Late Miocene. A massiv e influx of Himalayan derived Bengal Fan sediments reached the Sunda T rench in the Sumatra area in the late Middle Miocene. Continued additi on of Bengal Fan material to the accretionary wedge south-west of Nias resulted in steady plate deflection and subsidence through flexural p rocesses in forearc basin areas. The flexural consequence of increased load added to the prism, and associated subsidence history is documen ted by the sedimentary record on Nias where the shallow marine Olodano Formation passes up into the neritic to upper bathyal Lahomie Formati on. The Pliocene unconformity which is observed over all areas studied in the forearc is well constrained by structural, biostratigraphic an d sedimentological studies. The unconformity represents the initiation of a major phase of uplift and deformation that continues to the pres ent day. Rapid uplift of the outer are ridge and deformation of the pr ism during the Pliocene led to increased subsidence landward of the de formation. The rapid subsidence of the forearc basin landward of the o uter-are ridge has contributed greatly to the ''apparent'' differences between the forearc basin and the outer-are ridge at the present day: two areas with remarkably similar pre-Pliocene histories now have rem arkably different physiographies. (C) 1997 Elsevier Science Ltd.