A TECTONOSTRATIGRAPHIC FRAMEWORK FOR THE VULCAN GRABEN, TIMOR SEA REGION

Abstract

The Vulcan Graben is a northeast trending intracratonic rift system developed in response to Late Jurassic-Early Cretaceous break-up of the Australian northwest continental margin. A depositional sequence study was undertaken of the Vulcan Graben and the surrounding area, incorporating regional well and seismic data. From this a tectonostratigraphic framework has been established. Three megasequences are defined in relation to the Callovian rift episode; Pre-rift (pre-Callovian), Syn-rift (Callovian to Valanginian) and Post-rift (Valanginian to Present Day), each consisting of a number of discrete depositional sequences. This study has clarified the structural and stratigraphic evolution of the region, enabling development of dynamic depositional models. These models constrain the vertical and lateral facies distribution across the region, thus providing a powerful basis for petroleum exploration, including prediction of reservoir, seal and source rocks.

Significantly, while Late Jurassic rifting was initiated in the Late Callovian, imprinting a NE-SW oriented grain, a second major tectonic episode occurred in the Kimmeridgian trending ENE-WSW, which generated the dominant regional structural architecture. Recognition of this Kimmeridgian event has considerable impact on the successful delineation of structural plays within the region and provided significant control on syn-rift facies distribution and consequently stratigraphic play potential within the Vulcan Graben. Stratigraphic and structural relationships clearly indicate that rifting ceased in the middle Valanginian, followed by post- rift thermal subsidence and consequent transgression and inundation of the Vulcan Graben. The intra-Valanginian event represents a regional disconformity rather than a major tectonic unconformity and defines the boundary between the syn- and post-rift megasequences. This is consistent with the revised Valanginian date for the onset of sea-floor spreading in the Argo Abyssal Plain.

A revised lithostratigraphic nomenclature is proposed for the region which complements the resolution achieved by depositional sequence mapping.