The Geology of Indonesia/Halmahera

The Halmahera island group is located in the northeastern part of the Indonesian archipelago. It lies between latitudes 30N and 30S and between longitudes 250E and 30E (Fig. 1). The island is being 180 km from north to south and 70 km from west to east, and is surrounded by the smaller islands of Morotai, Ternate, Bacan, Obi and Gebe. To the west is the Molluca Sea and to the east is the southern part of the Philippine Sea.

Halmahera Island has a remarkable four-armed morphology, resembling the letter K. This shape is similar to Sulawesi Island to the west, but on a smaller scale; its dimensions are about one third of those of Sulawesi and its surface area is about one tenth. The bays between the arms are Kau Bay in the northeast, Buli Bay in the east and Weda Bay in the south.

Generally, Halmahera Island is hilly or mountainous, except the flood plains of some areas: e.g. the mouth of Kobe River in Weda bay and most of the eastern coast of the SW arm. The northeast to southwest trending mountainous ridges alternating with valleys in the NE arm have a relief which varies from 500m to over 1000 m, the highest being 1508m, Bukit Saolat, in the central part of the island. The main ridge in NE Halmahera is composed of structurally complex suite of imbricated ultrabasic, basic and Mesozoic- Paleogene rocks which form the basement. In the NW arm the highest peak is an active volcano (Mount Gonkonora 1700m). The SE arm has a more subdued topography; there is a large area of soft calcareous sediment in the central part of this arm.

The three areas of highest ground in the western and southwestern parts of the island correspond to areas of outcrop of volcanic rocks; these are in the western part of central zone (1170m), north of Saketa (where the hills rise very steeply to 1250m), and north of Paspalele (830m). The only other area of particularly high ground in the SW arm is a ridge east of Maidi formed of conglomerates which rises to 800m.

One of the characteristic features of the rivers of all sizes of Halmahera is that they are generally deeply incised. They may level out into a flood plain close to the coast but through most of their tracts they have steep-sided V-shaped valleys; in the middle and lower parts of their courses the rivers are sinuous.

REGIONAL GEOLOGY

In general, based on physiographic and geologic features, Halmahera can be divided into two province the western province and the eastern province (Fig. 2). The western province is part of a young volcanic belt extending from Morotai, through the northern part of Halmahera, Ternate and Tidore, to Bacan. The area is largely covered by Neogene to Recent sedimentary and volcanic rocks. Basement rocks, exposed in the southern part of Bacan Island, consist of continental crystalline rooks and deformed basic and ultrabasic rocks (Van Bemmelen, 1970, Yasin, 1980, Hall et al. 1988a). The basement rocks of the SW arm of Halmahera are volcanic and volcaniclastic rocks associated with intrusive igneous rocks, They are exposed in the southern part and along the west side of the SW arm, from Paspalele through Oha, from Saketa to Loku, and from Maidi to Lola (Figs.3 k. 4). The eastern province forms an arc extending eastward through the islands of Gebe and Gag towards the northern part of Bird’s Head of Irian Jaya. The area is underlain by an ophiolite complex and Mesozoic deep water sediments, imbricated with Paleogene sediments and overlain by Neogene marine clastics and carbonates (Sukamto et al., 1981; Suriatmadja, 1981). Basement rocks of the SE arm of Halmahera consist of a complex of dismembered basic and ultrabasic rocks, with a variable low grade metamorphic overprint, intercalated with Mesozoic and Eocene sediments.

TECTONIC SETTING

Tectonically, Halmahera lies at the intersection of four rigid plates, the Australian, the Philippine Sea, the Eurasian and the East Mindanao Plates (Hall et al. 1988c). The Australian Plate lies to the south and is bounded to the north by the Sorong Fault system, which is a complex transpressive zone extending eastward from Papua New Guinea more than 1500 km along the northern margin of Irian Jaya and westward some 800 km towards Sulawesi (Fig. 7). This system separates the westward-moving plates to the north from the Australian Plate with a displacement estimated to be as much as 600km (Hamilton, 1979). The Philippine Sca Plate is currently moving westwards with respect to the Australian Plate at about 12cm/year (Moore, 1982). The western boundary of the Philippine Sea Plate (which includes Halmahera) is the Philippine Trench which is linked to the Halmahera Trench.

The Eurasian Plate (Hall, 1987) has its eastern boundary at the Philippine Fault and continues southward into the West Halmahera Thrust of Silver & Moore (1978). The Eurasian Plate in the region of SE Asia and the Philippines is a complex region which includes numerous small ’plates’ which are moving semi-independently. One of these is the East Mindanao Plate which is bounded by the Philippine Fault to the west and the Philippine Trench to east. The East Mindanao Plate is not significant on a global scale; the plate is very narrow and is currently not moving independently of either the Eurasian or the Philippine Sea Plates. The motions of these four plates relative to one another cannot be determined precisely at present. Hall (1987) indicates the Eurasia-Philippine Sea convergence is distributed between the three plates north of the Sorong Fault. Since the motion of the East Mindanao Plate is not determined, the motions of the Philippine Sea, East Mindanao and Eurasian Plates are not known. However, the known relative motion of the Eurasian and Philippine Sea Plates (Ranken et al., 1984) does provide a useful constraint and further constraints are provided by observations of lengths of subducted slabs in the region (Cardwell et al., 1980; Fig. 8) and rate of motion on the Philippine Fault. Hall (1987) proposed that convergence between the Philippine Sea Plate (Halmahera) and the Eurasian Plate (Fig. 9) has occurred by: a) Subduction at the Sangihe Trench and sinistral strike-slip motion at the Philippine Fault which has moved the Eurasian Plate southwards relative to the East Mindanao Plate. b) Subduction in opposite directions at the Philippine Trench and the Halmahera Trench. The Philippine Sea Plate has been subducted westwards beneath the East Mindanao Plate at the Philippine Trench and the East Mindanao Plate has been subducted eastward beneath the Philippine Sea Plate at the Halmahera Trench. It follows that the U-shaped Molucca Sea slab is the subducted equivalent of the East Mindanao Plate.

The relative motion of the three plates north of the Sorong Fault has eliminated the East Mindanao Plate in the Molucca Sea and in this region the Eurasian and Philippine Sea Plates are now in contact. Therefore, as a result of convergence in the Molucca Sea region, the Halmahera forearc (the Philippine Sea Plate) has been very recently over-ridden by the Sangihe forearc (the leading edge of the Eurasian Plate, Hall, 1987). Hall (1987) interpreted the west-dipping thrust observed in the Molucca Sea (the West Halmahera Thrust of Silver & Moore, 1978) as the frontal thrust of the Sangihe forearc (Fig.9) which is regarded as the continuation of the Philippine Fault. The east-dipping thrust on the west side of the Molucca Sea, the East Sangihe Thrust of Silver & Moore, (1978) is considered to be a back thrust developing in the overthrust wedge. The consequence of this collision of the two forearcs, which must have occurred within the last 1Ma, is that the Philippine Sea Plate in the region of southern Halmahera is now under compression and has a vertical load imposed on its leading edge as the Sangihe forearc overthrusts the Halmahera forearc. The Halmahera region therefore flexes, resulting in uplift of the SW arm and subsidence in the Weda Bay region (Hall et al., 1988o).

A consequence of the model (Hall et al., 1988c) is that the nature of the deformation seen in the Halmahera region is a result of the blunt wedge shape of the plate boundary linking the Halmahera Trench to the Philippine Trench. As the Philippine Sea Plate moved west relative to the East Mindanao Plate the blunt wedge was driven into a smaller wedge shaped space. The result has been the development of a system of dextral strike-slip faults, a thickening of the Halmahera wedge, and a westward advance of the Halmahera Trench at a rate faster than subduction is occurring at the Philippine Trench.

STRUCTURAL GEOLOGY

A structural investigation has not formed part of the present study. The summary below of the structure of the area in which the rocks of this study were collected is based on Hall et al. (1988o).

The SW arm is structurally simple. Its present-day topography is highly asymmetrical with a steep west-facing side, clearly fault-controlled, and an eastern surface dipping gently towards Weda Bay. Three cross sections across the SW arm (Fig.6.A-C) show the asymmetry clearly. Shallow wide reefs run ofFshore the eastern coast of the SW arm and the gentle eastward dip of the land surface continues offshore into Weda Bay. Volcanic rocks of the Oha Volcanic Formation form a rigid basement and topographically high terrain on the west side of the SW arm and are overlain by Late Miocene sedimentary rocks of the Superak Formation (Fig.6.A- C). The Neogene sedimentary rocks dip consistently east (Fig.6.A-C). The dip decreases rapidly from the oldest parts of the cover (up to 7(P eastward in the Superak Formation) over an outcrop width of less than 2 km at the base of the sequence. The dip of most of the Neogene cover rocks is less than 300, and declines to approximately 100 at the east coast. The morphology of the western coast of the SW arm is very clearly controlled by steep faults. There are no significant areas of coastal plains or terraces and for long stretches of the western coast there are no substantial beaches at all.

The Central Zone extends across the narrow neck of the Halmahera K. The neck contains a range of low mountains with rivers draining into Kau and Weda Bays. West of the neck the mountains rise to a high dissected plain which descends steeply on its west side into the Molucoa Sea. The steep west coast is evidently controlled by high angle faults. The Central Zone has a basement of Oha Volcanic Formation overlain unconformably by folded Neogene sediments (Fig.6.E). The Neogene rooks are deformed by tight folds with an overall north- south axial trend. To the west it appears that these folded rocks are unconformably overlain by younger lavas which dip gently west. Above the folded Neogene sedimentary rocks is an overthrust sheet of Subaim Limestone Formation and younger rocks emplaced from the east. A cross-section of the Central Fold and Thrust Belt is shown on Fig.6.E and an interpretation of the cross-section is shown on Fig.6.F. The age of the folding and thrusting is constrained by the youngest deformed rocks in the Central Zone. A mid to late Pliocene age (N20/N21) indicates that this deformation event must have occurred no earlier than about 3 Ma.

The SE arm has roughly parallel north and south coasts trending WNW-ESE. It has a low central region which rises to the west, north and east giving the SE arm the form of a broad open half-basin, tilted southwards, with its southern part truncated by the coast. The Ophiolitic Basement Complex forms high mountains at the west end of the SE arm. In the central part of the SE arm are the soft marls and limestones of the Saolat Formation. At the east end of the SE arm is a large basement window cut through by the Gowonli and Paniti Rivers. The Gowonli, Paniti Formations and Ophiohtic Basement Complex within the window are unconformably overlain by the Subaim Limestone Formation whioh dips outward from the window. The Subaim Limestone Formation is separated from the Saolat Formation by a thrust flat with the basement window representing a hanging wall anticline above a ramp in the thrust plane. The thrust ramps up from a flat within the basement at a depth of about 3 km, then follows the base of the Subaim Limestone Formation for several kilometres before cutting up section through the Saolat Formation. The cross-section (Fig.6.D), assuming east- west contraction, implies that minimum shortening is of the order of 20 km.

STRATIGRAPHY

The descriptions of the stratigraphic units in southern and eastern Halmahera are based on field observations and subsequent work undertaken as part of the 1987, 1990 and 1993 GRDC Halmahera Expedition (Fig. 5). The Eastern Halmahera basement includes ophiolitic, metamorphic and sedimentary rocks. The ophiolite is made up of strongly sheared and brecciated mafic and ultramafic rocks including serpentinized peridotite, gabbro, basalt and diabase (Sukamto et al., 1981). Hall et al. (1988a) noted that the basement complex is not dominated by ultrabasic rocks, although the rock types seen vary considerably from area to area, and the basement complex includes about 30% ultrabasic rocks. Basic plutonic rocks are abundant, and are associated with basic volcanic rocks, greenschists, amphibolites and rare blueschists. Deep water sediments include red radiolarian chert and red mudstones. The oldest dated sedimentary rocks are those of the Buli Group (Fig.5). This group includes formations which range in age from Cretaceous to Eocene: the Gau Limestone, Dodaga Breccia, Paniti, Gowonli and Sagea Formations (Hall et al., 1988a; 1988c) of the SE and NE arms. The formations of the group have the following characteristics in common: a) with the older basic and ultrabasic rocks they form the basement to the eastern arms of Halmahera; b) they are all considered to have been deposited in a forearc setting, in varying depths of water; c) they are all more or less deformed, especially in the NE arm where they are imbricated with the Ophiolitic Basement Complex and cannot be mapped as separate units. The oldest rocks in the Buli Group are the Gowonli Formation of the SE arm and the Gau Limestone Formation of the NE arm. The Gowonli Formation is interpreted as the deposit of a basin situated in the forearc of an active arc. The lower part of the sequence is dominated by coarse volcaniclastic material and was deposited in the early stages of basin development in a relatively proximal position. Much of material was probably deposited as debris flows. The basal contact is not observed, but the Gowonli Formation probably rests unconformably on the ophiolitic basement complex. The top of the formation is not seen, but the presence of material derived from the Gowonli Formation as clasts in the basal conglomerates of the Paniti Formation indicates that the upper contact of the Gowonli Formation is an unconformity. The Gau Limestone Formation is interpreted as a deep water carbonate formation deposited m an equatorial ocean basin with subordinate volcaniclastic material derived from active arc volcanism at its margin (Hall et al., 1988a). The formation is interpreted as originally resting unconformably on the ophiolitic basement. The top of the formation is a gradational transition to the Dodaga Breccia Formation.

The oldest rocks known from the SW arm are the probable Cretaceous to Eocene Oha Volcanic Formation which consists of basalts and basaltic andesites (Hakim, 1989). The volcanic rocks are typically calc-alkaline and have trachytic textures, with an alignment of plagioclase feldspar microlites in the groundmass of the rocks, typical of textures in lava flows. The basement volcanics are pervasively altered and alteration minerals include zeolites, chlorite and epidote. The lower contact of these volcanics is not observed and the upper contact is probably an unconformity overlain by Neogene sedimentary rocks. Between the mid Eocene and mid Oligocene there was a major imbrication event which uplifted the Basement Complex. Uplift and erosion is marked by an unconformable contact between the Ophiolitic Basement Complex and late Paleogene and Neogene sedimentary rooks: the Onat Marl Formation, the Jawali Conglomerate Formation and the Subaim Limestone Formation of the NE arm (Hall et al. 1988a); and the Gemaf Conglomerate Formation (Hall et al. 1988c) and the Subaim Limestone Formation of the SE arm.

The contact between the Onat Marl Formation and the Buli Group has not been observed and the formation is interpreted to be overlain unconformably by the Jawali Conglomerate and the Subaim Limestone. (Hall et al., 1988b). In the NE arm the Jawali Conglomerate also rests unconformably on the Basement Complex; it is a conglomerate of fluvial origin (Hall et al., 1988b) and passes up into limestones of the Miocene Subaim Formation. The Subaim Limestone Formation is a massive or well-bedded limestone of reef or reef-derived material with rare clastic intervals. In the SE arm the Gemaf Conglomerate rests unconformably on the Ophiolitic Basement and is conformably overlain by the Subaim Limestone. The Gemaf Conglomerate Formation consists of dark conglomerates, containing well-rounded clasts of ophiolitic debris, and well-sorted dark sands of littoral origin. In the SW arm and the Central Zone, the Subaim Limestone Formation occurs only in small outcrops and as pebbles in younger sediments.

During the Late Miocene subsidence occurred in the SE arm and the Saolat Formation was deposited. The Saolat Formation is a thick sequence of fossiliferous calcareous mudstones and micritic limestones interbedded locally with sandstones and conglomerates containing ophiolitic debris. A transitional stratigraphic contact with the underlying Subaim Limestone Formation is seen. The top of the formation has not been observed in the SE arm, but in the NE arm the Wasile Sandstone Formation was deposited conformably on the Saolat Formation. The Wasile Formation includes turbiditic sandstones and conglomerates considered to represent part of a prograding submarine fan, with the higher beds representing upper-fan channel deposits (Hall et al., 1988b).

A different stratigraphic sequence is found in the SW arm. The Loku Formation was deposited during the Late Miocene; it consists mainly of sandstones, mudstones and conglomerates which are turbidites and debris flows of material derived from a terrain of volcanic arc rocks and reef limestones. The base of the Loku Formation is not seen. The Loku Formation is thought to be overlain unconformably by the Superak Formation of the Weda Group. The Superak Formation rests unconformably on older rocks. The upper part of the formation is considered to pass conformably into the Dufuk and Akelamo Formations. The Superak Formation consists of conglomerates and laterally equivalent shallow water sandstones. It includes channel conglomerates and was probably deposited in a fan delta setting. During the late Miocene to early Pliocene, the Akelamo Formation, consisting mainly of calcareous mudstones rich in organic debris, was deposited. It has a discontinuous distribution in the Central Zone and SW arm. The Akelamo Formation is conformably above, and has a transitional contact with, the Superak Formation and although the upper contact with the Dufuk Formation is not seen, it is believed to be conformable. The Dufuk Formation consists of calcareous sandstones, siltstones, mudstones and conglomerates. The formation is fossiliferous and rich in organic material and was deposited in a shallow marine environment. This formation is conformably overlain by the Gola Formation on the SW arm, while in the Central Zone it is overlain by the Tapaya and Tafonga Volcanic Formations.

The Gola Formation consists of calcareous mudstones and limestones and contains a fully marine fauna indicating an open marine carbonate shelf environment in the Pliocene. The paucity of siliciclastic debris is interpreted as indicating deposition remote from any source of terrigenous clastics. Renewed volcanic activity in the Central Zone is recorded by the Tapaya and Tafonga Volcanic Formations which contain conglomerates, sandstones, tuffs, basalts and andesites, with extrusive volcanics dominating the upper part of the sequence (Hall et al. 1988b). Shallow marine to littoral tu6aceous sandstenes of the Kulefu Formation are the youngest formation of the Weda Group in the SW arm. This formation is probably laterally equivalent to the Tapaya and Tafonga Volcanic Formations.

A period of deformation and uplift followed by erosion occurred prior to deposition of Quaternary reef limestones, alluvium and volcanic rocks which rest unconformably on older rocks.

COMPARISON OF HALMAHERA WITH OTHER AREAS

It is useful to compare the geology and tectonics of Halmahera with the islands of the Philippines situated to the north of Halmahera. The basement rocks of the Halmahera region are Mesozoic ophiolitic rocks and an upper Cretaceous to lower Tertiary arc and forearc sequence. Mindanao has a basement of serpentinites, peridotite, gabbros, diorites, basalts, andesites and the metamorphosed sedimentary rocks (Ranneft et al., 1960). These rocks may be traceable northwards into the eastern Philippines. Karig (1983) interprets eastern Luzon basement rocks as part of an east- facing upper Cretaceous-lower Tertiary arc system; all present-day arcs east of Luzon were formed more recently. An island arc system extended around the western Pacific, the remains of which can be traced from the east Philippines through Halmahera into New Guinea (Hall et al., 1988a).

The Mesozoic and Eocene sediments have notable stratigraphical and petrological similarities to the Marianas forearc and the Eastern Halmahera Basement Complex is interpreted as a pre- Oligocene forearc (Hall et al., 1988a). In contrast, the southern part of the island of Bacan at the southwestern end of the Halmahera group has a basement of high-grade continental metamorphic rocks associated with a deformed ophiolitic complex quite different to the basement of eastern Halmahera (Hall et al., 1988a). Bacan is underlain by continental basement, as indicated by exposures in the Sibela Mountains and by the chemistry of Quaternary volcanic rocks on Bacan which indicate a continental crustal contribution to lavas (Morris et al., 1983). Halmahera on the other hand, is underlain by rocks which formed part of an arc region until the end of the Eocene and has no continental basement. The continental basement of Bacan must have originated in the Australian continental margin exposed in New Guinea (Hamilton, 1979). The continental fragment of Bacan is separated from Halmahera by a splay of the Sorong Fault system which passes through Bacan (Hall et al., 1988a). this is probably the extension of the Molucca-sorong Fault which is one of several splays of the sorong Fault zone identified by seismic reflection work (Letouzey et al., 1983a) between Halmahera and Seram.