--> Abstract: Structural History Of The Northwest Shelf, Australia: An Integrated Geological, Geophysical And Experimental Approach, by M. O. Withjack and G. Eisenstadt; #90928 (1999).

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WITHJACK, M. O. and G. EISENSTADT
Mobil Technology Company, Dallas, TX

Abstract: Structural History of the Northwest Shelf, Australia: An Integrated Geological, Geophysical and Experimental Approach

We have used regional seismic, well, and experimental data to unravel the structural history of the Northwest Shelf of Australia. The present-day structural geometries and hydrocarbon potential of the Exmouth, Barrow, and Dampier basins on the Northwest Shelf are the product of extension from the Middle Jurassic until the Valanginian. Two factors strongly influenced the style of extensional deformation: the presence of a thick sedimentary cover with numerous detachment levels, and the direction of regional extension relative to the pre-existing basement fabric. Significant igneous activity and at least two episodes of inversion later modified the extensional structural geometries.

Structures on the northwest margins of the Exmouth and Dampier basins resemble those in physical models of extensional forced folding above oblique-slip fault zones. Broad monoclines (e.g., the Madeleine trend) overlie deep-seated basement faults, with shallow, detached faults having a different trend. We propose that E-W regional extension during the Middle Jurassic to Early Cretaceous reactivated deep-seated, NE-SW-trending zones of weakness. The presence of a thick sedimentary cover with numerous detachment levels (e.g., the Lower Jurassic Locker Shale) facilitated the development of extensional forced folds above these zones of weakness and the formation of N-trending, detached normal faults. Igneous activity and inversion, coincident with Valanginian breakup, modified structural geometries in the Exmouth basin. Additional Tertiary-age inversion structures on the Northwest Shelf are similar to those in physical models of distributed inversion where deformation occurs along many small faults.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas