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Initiation of a Volcanic Island Arc Journal of the Geological Society, London, Vol. 145, 1988, pp. 577-590, 12 figs Printed in Northern Ireland Late Palaeogene-Quaternary geology of Halmahera, Eastern Indonesia: initiation of a volcanic island arc R.HALL,l M. G. AUDLEY-CHARLES,' F. T. BANNER,' S. HIDAYAT2 &L S. L. TOBING~ Department of Geological Sciences, University Collcge London, Gower Street, London WClE 6BT, UK Geological Research and Development Centre, Bandung, Indonesia Abstract The Late Palaeogene-Quaternary stratigraphy of Halmahera is described, and new forma- tion names are proposed, based on recent field investigationsof the NE and central partof the island. This stratigraphic information provides new insights into the Neogene history of Halmahera and the development of the present island arc. The Late Palaeogene and younger rocks rest unconformablyon an ophiolitic Basement Complex which formed part of a Late Cretaceous-Early Tertiary fore-arc. After volcanic arc activity ceased in the Eocene the former fore-arc terrane was uplifted and deeply erodedin the Late Palaeogene. Some of theLate Palaeogene-Early Miocene river valleys are currently being re-excavated by the present rivers. Slow subsidence began in the mid-late Oligocene and by theend of theMiocene all easternHalmahera was thesite of shallow-watercarbonate deposition. There is no evidence for arc volcanism in central Halmahera at this time and the reported Oligo-Miocene volcanism in nearby regions is interpreted as volcanism related to the Sorong Fault system. The Miocene shallow water region subsided rapidly in the Early Pliocene and the sedimentary basin formed was filled with mark succeeded by siliciclastic turbidites, with increasing amounts of calc-alkaline volcanic debris from a Pliocene volcanic arc built on the western arms of Halmahera, probably on the eroded Early Tertiary arc. This phase of rapid subsidence in the Pliocene back-arc region resulted from the initiation of subduction of the Molucca Sea lithosphere eastwards beneath Halmahera. Differential subsidence on NW-SE and NE-SW sets of faults in the region immediately behind the active arc led to the formation of deep sediment-filled basins adjacent to the eastern arms. A major deformation event in the Pleistocene resulted in folding and local thrusting at the junction betweeneastern and western Halmahera and volcanism ceased in thePliocene arc. Thethird Halmahera arc, the Quaternary arc, currently active in the northern partof the islands, began activity within the last 1 Ma and is built upon the deformed and partly eroded Pliocene arc. The Pleistocene deformation event and shift in positionof the arc are interpreted as the result of the interaction of the eastward-dipping Molucca Sea plate with adjacent plates, either with a fragment of the Australian continent in the Sorong Fault zone and/or with the Philippine Sea plate beneath northern Halmahera. At the end of the Eocene a major plate reorganization event 1000 km east of Sulawesi beforesubduction began. As occurred in the western Pacific which is recognizable over a Molucca Seasubduction proceeded, Australia moved verywide region (Hayes & Lewis1984) along the Pacific obliquely northwards with respect to the Pacific but regional margin and in SE Asia. On Halmahera (Fig. 1) this event geological and palaeomagnetic evidence is not yet sufficient led to imbrication of ophiolites,metamorphic rocks and to lix the past position of Halmahera relative to Australia. sediments which hadformed part of aCretaceous-Early The history of the region after the Late Eoceneis at present Tertiaryfore-arc terrane (Hall er al. 1988) traceable very poorly known since the geology of the regions around northwardsinto the Philippines at least as faras east the complex knot of Halmahera has not yet been Mindanao. This fore-arc terraneis the Basement Complex of investigated in detail. Marine geophysical studies of these theNE and SE arms of Halmahera.The present and regions provideimportant constraints on the recent plate recently active volcanic arc built on the NW arm and islands tectonic history but information from land-based studies is off westernHalmahera is situatedabove aneast-dipping essential to extend our knowledgeback beyond afew subduction zone (Fig. 2). Thus the Early Tertiary fore-arc millionyears. The geological setting of Halmaheraand terrane, which forms the east Halmahera basement, is now previousknowledge of the region is summarized in our situated in a back-arc position relative to the present arc. On earlier paper dealing with the Basement Complex (Hall et the eastside of the Molucca Seaeastward subduction al. 1988); here we describe the cover rocks to the Basement beneath Halmahera has formed the Halmaheravolcanic arc, Complex and relate new field geological information to the and on the west side oceanic lithosphere has been subducted development of the Halmahera volcanic arc. westwards beneathnorth Sulawesi, forming the Sangihe volcanic arc (Fig. 1). Studies of recent seismicity show that ,theMolucca Sealithosphere has an inverted U-shaped Late Palaeogene and Neogene stratigraphy of configuration (Fig. 3, Hatherton & Dickinson1969; Halmahera Cardwell et al. 1980)with aminimum of loo0 km of subductedlithosphere. Therefore, the eastHalmahera Halmahera is covered by tropical rainforest and therefore BasementComplex must havebeen situated at least field geological investigations are accomplished by geologists 577 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/577/4889510/gsjgs.145.4.0577.pdf by guest on 28 September 2021 578 R. HALL ET AL. E 120° 130° I I I 0 km 500 j P Fig. 1. Location of Halmahera, Bacan and principal bathymetric features of the adjacent regions after Mammerickx ef al. (1976). and teams of porters, carrying equipment and food, making tion of Halmahera is still at a reconnaissance level and the traversesin areas selected by aerialphotographic study. difficulties of fieldwork preclude the establishment of These traverses have a duration of several days, are entirely detailed measured type sections; the localities from which on foot, follow rivers where possible but are modified en the new formations have been describedare shown in Fig. 4. route by geology, terrain,weather, time, difficulty and This work forms part of a joint project between UCL and supplies. Details of the traverses and localities referred to in the GeologicalResearch andDevelopment Centre the text are shownin Fig. 4. Inorder to describe the (GRDC),Bandung, Indonesia, and asthe investigation succession Late Palaeogene and Neogene rocks have been proceeds we expectthat changes will berequired tothe assignednew formationnames. Some stratigraphical stratigraphydescribed here, to take account of new terminologyhas been introduced in earlier publications discoveries andstratigraphical variation within the large (Apandi & Sudana 1980; Supriatna 1980; Sukamto et al. areaunder investigation. One aim of the project is to 1981) and comparison of the stratigraphy described here and produce a new geological map of the islands but it is not yet that of previous authors is shown in Fig. 5. As a result of possible to show the distribution of the formations described our work we have been able to date much of the Neogene here. We have therefore modified the published geological sequence more precisely and subdivide it lithostratigraphi- maps (Apandi & Sudana 1980; Supriatna 1980; Yasin 1980) cally ingreater detail. This stratigraphy provides new to summarizepresent knowledge of the geology and insights intothe Neogene history of Halrnaheraand the structure of Halmahera (Figs 6 & 7) which incorporates the development of the present island arc. However, the new results of the project so far. Sample numbers referred to in formation names are necessarily provisional since investiga- the text and shown in Fig. 4 are stored in the Department of Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/577/4889510/gsjgs.145.4.0577.pdf by guest on 28 September 2021 LATECENOZOIC GEOLOGY, HALMAHERA, INDONESIA 579 PHILIPPINE Fig. 2. Principal tectonic features of the Hal- BANDA SEA mahera region after Hamilton (1979) and Silver (1981). Solid triangles represent active volcanoes of the Halmahera and Sangihe arcs. Slip rates AUSTRALIAN along the Philippine Trench from Ranken er al. (1984). GeologicalSciences, University College London(UCL). established in the Early Miocene (see below) but the NE Fauna1 identifications were based on thin section determina- part of the NE arm must have been the site of carbonate tions. In certain cases the ages of samples have been given reefs by the Late Oligocene since high energy, inner shelf in the text using the P and N zones of Blow (1969,1979) and limestones sampled in theOnat Rivercontain reworked the East Indies Letter Stages and the approximate positions coralgal material of Late Oligoceneage (Tel-4) with no of these stage boundaries are shown in Fig. 5. evidence of stratigraphic admixing (HA72A: Lepidocyclina (Nephrolepidina), Rotalia gr. trochidiformis, Borelis pygmaeus, Halkyardia sp., with abundantdebris of Onat Mar1 Formation (mid-late Oligocene) rhodophyticcoralline algae, rarer bryozoa and molluscs). The Neogene sequence exposed on Halmahera shows some West of the Onat River the Onat Marl Formation has not variation across the island reflecting the history of uplift, been recognized although in the Subaim Fault zone there erosionand re-submergence following the major event in are slices of redeposited calcisiltites and calcilutites
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