Lithological Variations and Influences on the Stratigraphy of the Patchawarra Formation within the Patchawarra and Nappamerri Troughs of the Cooper Basin, South Australia
Jordan Sheehan - 2013
Honours Degree of Bachelor of Science (Petroleum Geology and Geophysics)
Australian School of Petroleum
The University of Adelaide
The Early Permian Patchawarra Formation, Cooper Basin, hosts a diverse preservation of coal bearing sequences as well as sand and shale-rich lithologies. However, variation throughout its stratigraphy is poorly understood, particularly on a regional scale, providing a challenge for subsurface applications. This study integrates a multifaceted approach to capture trends and better understand the main controls on gross lithological variation within this formation throughout the Patchawarra and Nappamerri Troughs, and investigate whether the structural movement of the Gidgealpa-Merrimelia-Innamincka (GMI) Trend was influential during deposition.
Lithostratigraphic correlation divided the Patchawarra Formation into three broad facies assemblages throughout the Patchawarra Trough. These assemblages appeared laterally consistent throughout the Moomba Region, however, became poorly developed throughout the northern Nappamerri Trough, which was characterised by an overall sandier succession with minimal coal preservation, and a unique coal-depleted upper facies assemblage.
Seismic interpretation across the GMI Trend displayed variable uplift histories, with onlap and erosion primarily occurring during middle to late Patchawarra deposition. It was recognised that gross lithological variation throughout the Patchawarra Formation was most likely controlled by the rates of sedimentation and relative subsidence between both Troughs. In particular, thick sand and coal deposits throughout the Patchawarra Trough were interpreted to reflect more stable tectonic conditions, whereas poor coal preservation and variable depositional character throughout the northern Nappamerri Trough was indicative of subsidence rates unfavourable for peat growth and dissimilar to conditions experienced elsewhere.
This study also determined the GMI Trend was unlikely to have influenced early deposition. Instead, this trend most likely acted as a structural barrier to late deposition within the Nappamerri Trough. In association with accommodation space, the timing and magnitude of relative uplift was critical in determining what thinned, eroded, onlapped and thus what lithological variation was retained along the GMI Trend and other structural highs throughout the region. The findings of this study gives insight into the regional depositional history of the Patchawarra Formation, and provides a stepping stone for other avenues of research to better understand the influences on the variation throughout the Patchawarra Formation.