Depositional History of an Early Permian Shallow Marine–Paralic Glacial to Post-Glacial System, Northern Perth Basin, Western Australia

The Perth Basin is a major rift basin located along the southwestern margin of Western Australia. Although it is a proven petroleum-producing basin, recently discovered gas flows from organic-rich shale-bearing Lower Permian... [ view full abstract ]

The Perth Basin is a major rift basin located along the southwestern margin of Western Australia. Although it is a proven petroleum-producing basin, recently discovered gas flows from organic-rich shale-bearing Lower Permian formations have revived interest in the Permian depositional record as an unconventional prospect in the basin.

Lower Permian formations sit within a third-order glacially influenced depositional sequence that has been correlated with coeval glacial successions in the Carnarvon and Canning Basins to the north. Direct glacial influence is restricted to coarse siliciclastic facies of the Nangetty Formation deposited in the early syn-rift phase. The upward-fining trend into low energy shelfal deposits of the Holmwood Shale is typical of rift basins where the continued subsidence results in progressive sediment starvation. The overlying High Cliff Sandstone is marked by a sharp basal surface interpreted as a marine erosional surface potentially associated with post-glacial rebound. Sand-rich facies of the High Cliff Sandstone represent progradational shoreface deposits. Conspicuous conglomerates in this formation may have been delivered by glacial meltwaters from a nearby coastal stream and reworked by storm currents. Local concentrations of outsized clasts also support glacial influence during deposition.

Facies of the overlying Irwin River Coal Measures record a significant change in depositional style suggesting major geospatial reorganization along the shoreline. Coal-bearing facies, fluvial channel belts and tidally influenced deposits indicate shoreline retreat and shallowing from initially shoreface settings into subaerial coastal plain environments. Increasing depositional energy upwards associated with the early stages of marine flooding is recorded by changes in bioturbation styles, coarsening-upward packages interpreted as prograding deltaic facies, and thin sandstone beds topped by wave-formed dunes and attributed to washover deposits that may overlie wave ravinement surfaces. Transgressive lags that feature an expression of the Glossifungites Ichnofacies overlie marine flooding surfaces. Full transgression is marked by the deposition of thick mudstone of the Carynginia Formation containing storm-associated facies and local dropstones characteristic of an offshore transition zone setting.

A process-based approach to describe mixed-influence marginal marine systems reveals significant modifications of sedimentary dynamics along the shoreline throughout the Lower Permian, from wave-dominated, fluvial-influenced conditions in the Holmwood Shale and High Cliff Sandstone, to fluvial or tide-dominated settings with minor wave influence in the Irwin River Coal Measures, through to wave-dominated facies in the Carynginia Formation. Changes in shoreline geometry may be attributed to the evolving basin physiography during rifting and fluctuating sea level associated with deglaciation.