Stepping off the edge: the geological framework of deepwater northern Australia

Abstract

The northern part of deepwater Western Australia and the adjacent Indonesian territory is a vast, underexplored region which includes an area of Triassic geology atypical of the region. Seismic data highlights a substantial elongate Triassic depocentre mappable from the northern margin of the Browse Basin northwards to the Timor Trough. This is informally named the Outer Ashmore Trough. The Outer Ashmore Trough is bounded by an inboard hinge-zone. The axis of the trough extending beneath the Ashmore Reef-1 and North Hibernia-1 wells. The western flank (only partially mappable) is a steeply dipping hinge-zone bounded by an outer high. The trough is mappable northwards, until it is incorporated into the imbricate wedge of the Timor Trough. The trough is floored by thickened Early Triassic shales of the Mount Goodwyn Formation (coeval to the Bedout petroleum system in the vicinity of the Dorado Field). This shale-prone marine interval is succeeded by a thick shallow marine clastic interval incorporated into seaward-dipping gravity-driven faults with an interpreted decollement in the Early Triassic shales. The Norian was dominated by a broad carbonate platform not observed elsewhere on the north-western Australian margin. This interval shares similarities with the outcropping Triassic in Timor Leste. A likely interpretation is that the Outer Ashmore Trough succeeds hyperextended Late Permian rifting. The trough and its adjacent margins offer prospectivity of a potentially significant scale.