Depositional environment, organic matter sources, and variable 17α(H)-diahopane distribution in Early Permian samples, southern Sydney Basin, Australia

Early Permian strata exposed in the southern part of the Sydney Basin, NSW, Australia, include the Pebbley Beach Formation, the Snapper Point Formation and the Wandrawandian Siltstone. These formations were deposited during the Late Paleozoic Ice Age, when Australia was close to the South Pole. The Pebbley Beach and Snapper Point formations were deposited in shallow marine to coastal environments, while the Wandrawandian Siltstone was deposited in deeper water following a marine transgression. However, opinions differ on the precise environmental and climatic conditions that prevailed during their deposition. Sixteen outcrop samples from these formations were collected and analysed to determine their depositional conditions and the source(s) of their organic matter.

The biomarker and aromatic hydrocarbon distributions show that the rocks are in the early to mid-oil window. Some aromatic parameters show that the stratigraphically younger Wandrawandian Siltstone is more thermally mature than the older Pebbley Beach Formation, which is interpreted to be due to the geographic position of these outcrops in the Sydney Basin. The samples have low to intermediate pristane/phytane ratios (from 1.4 to 3.4), showing variation from oxic to suboxic depositional conditions. Most of the mudstones and siltstones sampled were deposited in a marine environment, with some fluvial-deltaic or estuarine influence, and contain mixed type II and type III organic matter. The Pebbley Beach formation is characterised by terpane distributions dominated by the C₂₄ tetracyclic terpane, and to a lesser extent the C₁₉ tricyclic terpane. The Snapper Point Formation has a mix of C₁₉ and C₂₃ tricyclic terpanes and C₂₄ tetracyclic terpane, while the Wandrawandian Siltstone is dominated by C₁₉, C₂₁, and C₂₃ tricyclic terpanes. The sterane distribution follows a C₂₉ > C₂₈ > C₂₇ pattern in the majority of the samples, including in the marine deposited Wandrawandian Siltstone. The three formations have very variable relative amounts of diahopanes and other rearranged hopanes, with the highest C₃₀*/C₃₀ αβ hopane ratio (6.9) in the Wandrawandian Siltstone. This formation also has high amounts of diasteranes compared to the other two formations, but the relative amounts of diasteranes are not as high as for the diahopanes. Classically, this suggests a clay-rich sediment in an oxic or suboxic, acid-catalysed depositional environment, with enhanced diagenetic alteration of the biomarkers. The Wandrawandian Siltstone was deposited in suboxic conditions, not unusual to many rocks that contain much less rearranged hopanes. Unusual characteristics of the Wandrawandian Siltstone include significant slumping and soft sediment deformation, and deposition under periglacial-glacial conditions. The occurrence of the slumps on the continental slope led to significant sediment overturn, and this may have resulted in enhanced diagenetic and catalytic rearrangement reactions, leading to the elevated levels of diahopanes and other rearranged hopanes. The disequilibrium between the relative abundances of diahopanes and diasteranes suggests different formation mechanisms for these biomarkers.