Stratigraphic Evolution of the Shoalhaven-Illawarra Group Boundary, Southern Sydney Basin

The Shoalhaven-Illawarra Group boundary of the Late Permian southern Sydney Basin is poorly exposed in the Wollongong region. Consequently, geochemical and facies variations across the boundary are poorly understood. The lack of information has produced a gap in the understanding of the transition between the marine sedimentary rocks of the Broughton Formation and the non-marine Pheasants Nest Formation. An understanding of this transition would give the context for the changes leading to the deposition of the economically important Illawarra Coal Measures. Application of geochemistry (XRF), petrography (XRD, microscopy) and statistical methods (ANOVA, principal component analysis, cluster analysis), have been used to describe the palaeoenvironmental changes of the sequence and produce an evolutionary model. The study demonstrates that the deposition of the Pheasants Nest Formation was a result of the development of an extensive barrier complex during the time of the Shoalhaven-Illawarra Group boundary. Rapid sedimentation, consistent aggradation and a gradual regression in the underlying Broughton Formation aided barrier formation. This was predominately controlled by the amount of volcaniclastic sediment supplied from a northeast orientated volcanic chain that was located in the current offshore. Petrographic, geochemical and sedimentological analyses indicate that volcanic influence was persistent throughout the entire Broughton and Pheasants Nest Formation, with sporadic pulses of sediment accounting for increased Si, Ca, Fe and K concentrations. These sporadic pulses represent increased periods of volcanic activity distributing sediment into a shallow marine basin. Longshore currents (northeast trending) transported the detritus into the Wollongong area where it was deposited. Following the marine regression, fluvial processes operated on a coastal plain, passing laterally into a floodplain environment. Sediment was probably transported by seasonal meltwater floods. Statistical analyses (principal components, cluster analysis) reduced an extensive elemental data set (n =162) into two main factors. The analyses reveal that the Broughton and Pheasants Nest Formations are strongly influenced by aluminosilicate and heavy mineral concentration. Heavy mineral concentrations are commonly used as a measure of sediment immaturity. In this case, the volcaniclastic detritus has undergone limited recycling and weathering. The depositional features of the Shoalhaven-Illawarra Group boundary are significant as it provides a coherent explanation for the deposition of the Pheasants Nest Formation; thus clarifying the palaeoenvironmental changes that led to coal formation in the Sydney Basin