Distinctive hydrocarbon biomarkers from fossiliferous sediment of the Late Proterozoic Walcott Member, Chuar Group, Grand Canyon, Arizona

Abundant extractable hydrocarbons, in association with well preserved kerogen, have been recognised in a dolostone from the Chuar Group (approx. 850 Ma). The biomarkers present include C₁₅-C₃₀, acyclic isoprenoids, C₂₆-C₂₈ steranes, C₂₇-C₃₅ hopanes and extended tricyclic terpanes. Among the striking characteristics of the distributions are the dominance of steranes and methyl steranes lacking side-chain alkylation and the presence of putative C₂₉-C₃₅ neohopanes and gammacerane. The high abundance of steranes is consistent with the presumed eukaryotic affinities of several types of microfossils, including Chuaria, present in associated mudstones. This stratum is the oldest in which gammacerane has been found, and suggests contributions from protozoa to the depositional environment which appears to have been hypersaline. There is a strong correlation between the carbon isotopic composition of the kerogen and that of the various polarity fractions of the bitumens. Chemical degradation and pyrolysis of the kerogens yielded n-alkanes with similar characteristics to the bitumens, but there are differences in the patterns of distribution of the acyclic isoprenoid, sterane and triterpane biomarkers.