ABSTRACT: The Southern Bonaparte Gulf, Northwest Australia--A Basin Model

M. V. Dauzacker, J. M. Durrant, R. France, T. Nilsen

The Bonaparte Gulf basin Palaeozoic rift is an extension of the Petrel subbasin-Sahul syncline trend, forming one arm of a rift triple junction. Salt tectonics developed in this rift setting are fundamental to a new model of the basin and have influenced the development of three Palaeozoic megasequences defined in the model.

Megasequence I (MS-I), Silurian-Devonian restricted, synrift sedimentation was associated with extensive evaporitic systems which developed rich source rocks and massive halites. Early basinward salt withdrawal influenced sediment distribution and structuring, localizing shallow-water carbonates above deep salt ridges.

Megasequence II (MS-II) Late Devonian-Late Carboniferous sedimentation is restricted to subbasins formed by continued basinward

salt withdrawal. Turbidites were channelled and stacked along subbasin axes during active subsidence (Milligans Formation), and shallow-water carbonate banks continued to develop on basin highs. As subsidence eased, eustatically controlled deltaic sedimentation filled the subbasins, culminating in deposition of a regional shallow-water carbonate (Tanmurra Formation). Continued salt withdrawal in the north produced a carbonate shelf break evidenced by the apparent termination of the Tanmurra Formation seismic reflector, coincident with the carbonate complex development which exhibits continued growth until this period. An overlying unit (Point Spring Sandstone) marks the transition to the next megasequence.

Megasequence III (MS-III) Late Carboniferous-Late Permian marginal marine/fluvial sedimentation is associated with renewed rifting in the northwest. Associated driving subsidence produced northwest tilting, which was reinforced by Mesozoic-Cenozoic passive margin subsidence following the Late Jurassic continental breakup and abandonment of the northwest-southeast Paleozoic rift arm.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990