Abstract

The Salawati area constitutes the oil producing western extremity of Irian Jaya, Indonesia. It lies in the complex area of interaction of three major crustal plates: The Pacific oceanic plate to the north, the Australian continental plate to the southeast, and the Asian continental plate to the southwest.

Geometric, geomorphologic, geological and geophysical evidences strongly suggest that Salawati island was attached to the Irian Jaya mainland during the time of Miocene-lower Pliocene reef development, and that it has drifted away in middle Pliocene to Pleistocene time, opening the Sele Strait rift zone. The island moved 17.5 km (11 miles) southwestward after an initial counterclockwise rotation of 13°.

The drift is subsequent to the creation of the large left-lateral Sorong fault zone, that is part of the transitional area separating the westward moving Pacific plate from the relatively stable Australian plate. The motion was triggered during a widespread magmatic intrusion of the Sorong fault zone, when the basalt infiltrated a right-lateral fault system located in the area of the present Sele Strait.

Drifting took place along three parallel major left-lateral strikeslip faults, that can be traced from the Sele Strait to the southern part of Salawati island. The amount of relative displacement increases from the SE fault to the NW fault.

These faults became later the site of important down-to-the-NW, normal faulting to accomodate the subsidence due to the load of Pliocene-Pleistocene deposits derived from the high northern basaltic mountains.

Pliocene-Pleistocene diastrophism has thus defined several zones of varied structural characters: the Sele Strait, the Irian Jaya western, Salawati island, and their respective surrounding.

Once Salawati Island is placed into its former Irian Jaya frame, and the northern compartment of the left-lateral Sorong fault zone is moved back east, the Miocene landscape appears to be characterized by a widespread carbonate development with reefs thriving at the edge of an early New Guinean landmass facing an open sea to the west. The original distribution of reefs is somewhat different from the present arrangement.