# The Geology of Indonesia/Arafura Sea

The Arafura Sea is a continental shelf area situated between Irian Jaya (western New Guinea) and the northern part of the Australian continent. To the north, it is bordered by the Tertiary collision zone between the Australian craton and the northern Irian Jaya island arc, while to the south, it adjoins the stable Australian craton. The westerly extension of this area is vague, but can be taken approximately to the eastern margin of the Tanimbar Trench in the south and Aru Trough in the north. To the east, the Arafura Sea forms a stable continental shelf which has suffered little from the intense compressional events at the margins. Episodes of rifting and basin formation are recorded in Paleozoic and Mesozoic sequences, the effective boundary is provided by a Precambrian structural high, the Wessel Rise (Figure 1). The bathymetry of the Arafura Shelf exhibits depths of between 50 and 80 m (160 and 260 ft), but deeper parts down to more than 600 m (1,970 ft) occur at the edges. The east-southeast trending Merauke Ridge across the Aru Islands separates the Arafura depression from a narrower foreland basin in the north, so called Akimeugah foredeep.

## Geological setting

Tanimbar Trench and Aru Trough
Previous investigators favor the idea that these two troughs represent the eastward continuation of the Sunda-Timor subduction zone that was bent by the westward-advancing Pacific plate. The Benioff zone dips toward the Asian continent, and, consequently, establishes a chaotic wedge of imbricated sediments and crystalline basement on the northern side (Katili, 1975; Hamilton, 1979).
Cardwell and Isacks (1978) challenge this so-called one-plate model, distinguishing two separate Benioff zones. One zone extends northward beneath Timor to a depth of 600 km (370 mi) and the other, separated by the Tarera-Aiduna transform fault, extends southward beneath Seram to a depth of about 200 km (125 m). This two-plate model could perhaps accommodate the loop shape arc without challenging the concept of rigid plate injection. However, the two-plate model for the Banda Sea is difficult to reconcile with the results of field investigations by Audley-Charles et al (1979), because their findings indicate that the stratigraphy and structure of the Seram show remarkable resemblance to the Timor. They argue that whatever hypothesis describes the tectonic evolution of Timor would be equally applicable to Seram, implying a preference for a one-plate model for this region.
Schlüter (1983), on the other hand, does not separate the Seram and Tanimbar Trench along the Tarera-Aiduna fault, but instead contends that the Tanimbar Trench terminates suddenly east of Kai Island while the Seram Trench begins west of Kai island. The Tanimbar Trench is considered to be the eastern extension of the Timor Trough, while to the north, it deepens and merges laterally into the 3,500 m (11,480 ft) deep Aru trough. Because the north-northeast-to-south-southwest trending contour lines turns abruptly northwestward between the Tanimbar and Kai Islands, Schlüter (1983) assumes a major fault in this area, separating the Tanimbar Trench from the Aru basin (Figure 2).
According to Bowin et al (1980), the tectonic history of this area is very complicated because the Aru Trough shows crustal extension rather than compression (Figure 4). Schlüter (1983) contends that the most prominent difference between the Tanimbar and Kai segments of the Banda arc is that vertical tectonic movement predominates along the eastern Aru Trough rather than along the Tanimbar Trench. Large vertical offsets (up to 1.8 s) on normal faults downthrown on the basinward side occur along the eastern flank of the Aru Trough, but only small vertical offsets (up to 0.15 s) can be observed along the southeastern flank of the Tanimbar Trench. North - South step faults (Figures 5 and 6) characterize the eastern margin of the Aru Trough. More precisely, they are found offshore northwest and southwest of the main island and onshore on its western part. This type of tectonics could provide traps against faults or by draping over fault blocks.

## The Aru Archipelago

On the Aru Islands, Neogene to Quaternary marls and shallow-water limestones predominate, although there is evidence for older terrigenous material that is presumably derived from arkosic (granitic?) outcrops (Fairbridge, 1951).
Results of previous seismic refraction investigations (Curray et al, 1977; Jacobson et al, 1979; and Bowin et al, 1980) support the idea that the Aru Archipelago is an old, peneplaned platform covered by Neogene to Recent sediments.
Southwest of Aru Island, numerous step faults are present, delineating very narrow blocks about 2.5 km (1.6 mi) in width, with an average throw of up to 300 milliseconds (Figure 6). The pattern both north and south of the Aru Archipelago suggests lateral strike-slip movements in addition to step faulting. The step faulting south and north of Aru is not accompanied by thickening of the Mesozoic and Tertiary sedimentation sections (Schlüter, 1983). Tjia (1977) concludes that the N 105° E directed, maximum principle stress derived from lineaments and fracture analyses in the island of Aru appears to relate to warping of the Aru Island.
More recent investigations (Schlüter, 1981) reveal that the Aru Archipelago is an eastward-tilted block resulting from tectonic interaction of the Banda arc system and the Arafura Platform.

## The Arafura Platform

The Arafura Platform belongs to the Australian-Irian continental part of the Gondwana lithospheric plate. The platform, lying almost entirely within the limits of the 200 m bathymetric contour, is a northward, offshore extension of the Australian continent. Schlüter (1983) distinguishes seven sequences in the Arafura Shelf, of which the top sequence is interpreted as Neogene, becoming thinner toward the shelf (Figure 8). The sequences form part of the chaotic wedge of highly disturbed rocks along the outer Banda Island arc. The underlying sequences are of Paleogene to Neogene age. They extend as much as 35 km beyond the Tanimbar Trench and the Aru Trough, northwestward, and mark the base of the chaotic wedge. Based on the evidence of seismic reflection results (Balke, Crostella, and Halse, 1973; Lofting et al, 1975), the shields area can be subdivided into several intra-cratonic rises and depressions, such as the Merauke Rise, the Arafura basin, the Money Shoal graben, and the Malita Calder graben. The intra-cratonic grabens presumably consist of thick Precambrian to Paleozoic strata that are draped by relatively thin Mesozoic-Tertiary sequences. Only the Money Shoal graben contains Jurassic to Cenozoic fluvio-deltaic deposits of more than 2 km thickness (Brown, 1980). The largely fluviatile and paralic nature of the Jurassic and Cretaceous strata in this graben, along with the presence of many small unconformities, indicates that a land area was present in the vicinity of the present-day Darwin Shelf shoreline during most of the Mesozoic (Balke and Burt, 1976).
Nicols (1970) contends that pre-Mesozoic structural deformation is present in the Money Shoal area, where Paleozoic prospects might exist. Immediately south of the international border between Indonesia and Australia, the large, northeast oriented depression present is known as the Malita-Calder graben. Contrary to the Money Shoal graben, which is a pre-Mesozoic graben situated in a stable block, Balke et al (1973) place the Malita-Calder graben into the mobil zone where drape folding over a deepseated fault block, formed during Jurassic time, is predominant. It is sharply bounded on both sides and, in particular, fault hinged to the Darwin Shelf, a part of the Arafura Platform. This graben should extend into Indonesian waters, southeast of Tanimbar, and represents a Mesozoic-Tertiary sequence where thick marine sediments have accumulated in front of the descending Australian lithosphere (Nicols, 1970).

## Akimeugah Foredeep

The Akimeugah Foredeep is located north of a basement high (the Merauke Ridge) which separates it from the Arafura Basin to the south. The basin is a foreland basin located partially onshore and offshore eastern Irian Jaya. The Quaternary (Buru Formation) exceeds 3500 m thickness in the basin which was formed following uplift during the Melanesian Orogeny in post-Miocene times. Poorly delineated, foreland sediments may overlie Paleozoic or Mesozoic platform sequences with the Jurassic-Cretaceous Lower Kembelangan Formation thinning across the Merauke Ridge.
Onshore geomorphology can be subdivided into the foothills area, with elevation ranging from 150-600 m above sea level, and the alluvial plain of the southern coast of Irian Jaya. The foothills are is gently folded, with the main folding axis trending approximately east to west and gradually changing to east-southeast as it extends eastward. The geomorphologic unit consists of young Tertiary sediments, with Quaternary clastics up to 6000 m thick. The base of the sediments is the older Jurassic Kambelangan Formation, which was located in the southern foothills of the Jaya Wijaya range.
Several exploratory wells have been drilled offshore. The ASM-1X for example penetrated 1100 m of Kebelangan Formation which thins out gradually eastward and is interpreted to be absent to the high block of Kumbai-1, west of Jaosakor-1 well. The formation top is 1700 m deep in this well but it can reach 5000 m in the deepest part of the Pliocene molasse foredeep, south of the foothills. The Miocene New Guninea Limestone Group is 1128 m deep in ASM-1X, with some frequent erosional evidence at the top of this formation below the molasse-type Buru Formation.