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Abstract
The angular unconformity at the base of the Grant Group forms a distinctive surface evident both at outcrop and in the subsurface on seismic and well logs. Detailed analysis of a 2D seismic dataset, integrated with available well data, suggests a complex suite of processes controlling the formation, deformation and resultant morphology of this surface. They can be subdivided into three categories depending on whether the mechanisms pre-dated, were coeval with, or post-dated deposition of the Grant Group. Pre-Grant faulting during the Devonian–Lower Carboniferous Pillara Extension resulted in development of significant paleotopography that influenced the morphology of the Base Grant Group unconformity. The pre-Grant interval contained thick salt, which was locally removed in the Silurian, with the development of isolated bodies of thickened Worral Formation. Later after deposition of the Grant group, the remainder of the Mallowa salt underwent dissolution/withdrawal, resulting in collapse of the flanks of earlier Worral pods of sediments, forming a ‘sombreros’ structure and folding the Base Grant Group unconformity. The main syn-Grant processes include regional glacial erosion, and extensive development of paleo-tunnel valleys that formed beneath and at the margin of the Permian–Carboniferous ice sheet as it advanced into the Fitzroy Trough. This created significant relief on the Base Grant Group unconformity, with tunnel valleys characterised by steep-sided, often U-shaped valleys up to 525 m deep and 12 km wide. Post Grant deposition, the unconformity surface was subsequently modified during the Triassic–Jurassic Fitzroy Movement, resulting in fault reactivation and development of a series of en-echelon anticlines associated with a regional wrench system.