ABSTRACT: The sequence stratigraphic evolution of Tertiary passive margins: an example from the Barrow-Exmouth Sub-basin, Western Australia

Young, Hamish C. , National Centre for Petroleum Geology and Geophysics, Adelaide, Australia

A high resolution, regional sequence stratigraphic interpretation covers the Tertiary section of the Barrow - Exmouth Sub-basin, on the North West Shelf of Australia. The study examines the evolution of a passive margin with subsidence across a hinge zone. Twenty chronostratigraphic events were identified using biostratigraphic, wireline and seismic data. Correlations on sequence boundaries were extended across several Australasian, Atlantic and Indian Ocean margins. Changes in lithofacies, sedimentation rates and shelf geometries were investigated.

Carbonate-dominated margins share similar stratal architectures to their siliciclastic counterparts. Whilst both lithofacies are dominated by hydrodynamic processes, the proportion of carbonate can be traced to palaeogeographic changes.

The sedimentation rates on passive margins around the world vary significantly in magnitude, but display similar trends. During the Palaeocene - Eocene, sedimentation rates were relatively low, then increased significantly in the Middle Miocene and remained high during the Pliocene.

During the Tertiary, shelf geometries changed from a ramp to a rim profile, reflecting a reduction in accommodation space rather than an increase in sedimentation rate or sea level rise. Changes in stratal architecture appear to be largely governed by load-induced subsidence. In the examples investigated, deposition on the outer shelf creates accommodation by flexure. Deposition on the more rigid inner shelf results in less flexure and reduced accommodation.

Significant changes in shelf geometry can be initiated and globally-synchronised via sea level variations, shown by a Late Eocene - Early Oligocene sea level rise which pushed sedimentation onto the inner shelf, developing a progradational, rimmed margin. Sea level fall during the Middle Miocene moved sedimentation to the outer shelf, developing an aggradational, rimmed margin at the beginning of the Pliocene.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia