Abstract

The nature of the crust underlying Cenderawasih (Geelvink) Bay has an important bearing on its oil potential but unfortunately very little geophysical work has been done over the bay, and the sporadic outcrops of rocks near the margins offer only tantalising glimpses of the nature of the underlying crust.

The northern margin of the bay is formed by the major east-west-trending Yapen Fault Zone. The rocks caught up in the fault zone and exposed on Yapen and Num Islands are early Tertiary island are volcanics unconformably overlain by Neogene sediments and belong to the pacific Plate.

The southwestern margin of Cenderawasih Bay is formed by a very complex northwest-trending fault zone called the Wandamen Fault Zone. It can be described generally as a series of fault wedges of Paleozoic crystalline basement rocks and overlying Mesozoic and early Tertiary platform and troughtype sediments that are strongly folded along narrow tectonic zones. Most of the fault zone is covered by water but the geology of the sporadic islands along the zone shows that rocks of continental affinities extend over 50 km into the Bay.

However, the available evidence suggests that beyond this limit the crust is made up of island arc volcanics originating in the Pacific Plate. This evidence comprises :

(a) At the northwestern extremity of the Wandamen Fault Zone, early Tertiary island arc volcanics of the Pacific Plate are upfaulted against Palaeozoic metamorphics of Kepala Burung.

(b) Very high gravity values at the eastern limit of the Wandamen Fault Zone indicate the presence of oceanic crust.

(c) The rocks of the southern apex of Cenderawasih Bay have been thrust 30 km southwards over the Australian continental margin and include a belt of ultramafic rocks and associated early Tertiary island arc volcanics.

The hydrocarbon potential of the rocks of the bay would therefore be limited to the overlying Neogene sediments and as thick carbonate is developed at the base of the sedimentary successions in Yapen, Biak and Waigeo islands similar buildups could occur in the bay and provide suitable reservoir rocks. However, the rest of the sediments are mostly poorly sorted, immature, terrigenous sediments with limited potential as oil source rocks.