GIPPSLAND, A COMPOSITE BASIN-A CASE STUDY FROM THE OFFSHORE NORTHERN STRZELECKI TERRACE, GIPPSLAND BASIN, AUSTRALIA

Abstract

Detailed interpretation of reflection seismic and well data from the northern Strzelecki Terrace constrain the effect of Southern Margin and Tasman Sea rifting on the evolution of the Gippsland Basin. A new model is proposed which divides the basin into two structurally distinct provinces (East and West Gippsland Basin), separated by a broad zone of accommodation which is referred to in this paper as the 'Kingfish/Tuna Transition Zone'. This zone is a distinct region across which structural styles change within the basin due to the interaction of extensional forces resulting from both Southern Margin and Tasman Sea rifting. No evidence has been found, however, for the existence of transfer zones within the northern margin of Gippsland Basin as previously suggested by other authors.

The Gippsland Basin is observed to have a composite history; a younger 'Tasman Rift' Basin (a Tasman Sea aulacogen) overlying a regionally more extensive 'Strzelecki Basin' (the result of rifting along Australia's Southern Margin). Both basins have formed as half graben with opposing asymmetry. Re-evaluation of the Cretaceous palynology in conjunction with reflection seismic data from selected wells have enabled division of the Cretaceous section of the northern Strzelecki Terrace into three tectonically distinct sedimentary units: the Lower Strzelecki, Upper Strzelecki and Golden Beach Megasequences. The Lower Strzelecki Megasequence exhibits considerable thickening towards a south-bounding master fault, and is inferred to have been deposited during a phase of active rifting. It is separated from the overlying Upper Strzelecki Megasequence by a pronounced late Aptian age angular unconformity. The Upper Strzelecki Megasequence is a thick sedimentary unit which shows less syn-sedimentary faulting and is inferred to be deposited during a period of tectonic quiescence, possibly during a sag phase following active rifting. The Golden Beach Megasequence shows renewal of rifting with growth towards a north bounding fault system and is differentiated from the underlying Strzelecki Megasequences by a distinct change in seismic character across a subtle early Campanian age angular unconformity.