Exceptionally high contents of rearranged hopanes in a late Permian section, southern Sydney Basin, Australia

Abstract

A suite of 19 late Permian–early Triassic outcrop samples from the southern Sydney Basin were analysed by gas chromatography-mass spectrometry. Three series of rearranged hopanes were identified in the samples: 18α(H)-neohopanes, 17α(H)-diahopanes, and the early-eluting 9,15-dimethyl-25,27-bisnorhopane homologues. There is very large variability in the relative abundance and distribution of rearranged hopanes compared to the 17α(H)-hopanes between samples. There is one group of samples, associated with the “dead zone” after the Permian-Triassic mass extinction event, and deposited above the uppermost coal seam, where rearranged hopanes are not present, while in other samples their abundance is very high, and the C₃₀ diahopane/C₃₀ αβ hopane ratio reaches 93. No dependency on thermal maturity or depositional water salinity were observed, and there is no correlation between the presence and distribution of rearranged hopanes and diasteranes. There is a positive correlation between the presence of monomethylalkanes, interpreted to be associated with cyanobacterial input, and a high relative abundance of rearranged hopanes. The samples with the highest relative abundances of rearranged hopanes and monomethylalkanes are from the Tongarra Coal at Austinmer Beach, where three coal seams are separated by laterally-continuous pale-coloured ash-fall tuffs. The high abundance of terrigenous organic matter, deposited under fresh water sub-oxic conditions, together with extensive volcanic activity, may be the main factors contributed to the formation and enrichment of these exceptional amounts of rearranged hopanes in the Sydney Basin.