Stratigraphy and environmental conditions of the terminal Neoproterozoic-Cambrian Period in Oman: evidence from sulphur isotopes.

Terminal Neoproterozoic and Cambrian evaporite rocks worldwide record a marked sulphur isotope excursion to very heavy δ34S isotopes that reflect a period of significant burial of reduced sulphur. New δ34S data from anhydrite and pyrite of the Ara Group in Oman extend this record to Arabia. Depositional history of the Ara Group spans c. 8 Ma across the terminal Neoproterozoic Cambrian boundary. Therefore, these rocks provide a detailed record of sulphur isotopic evolution in this important time interval. Ara Group sulphur isotope values include a mean δ34Sanhydrite of 39.4‰ ± 2.6‰ (1σ range 32.4-46.4‰; n = 61), and a mean δ34Spyrite of 1.4‰ ± 7.3‰ (1σ; range -10.8‰ to 11.4‰; n = 12). The isotope signatures were not altered significantly during diagenesis and are representative of the isotopic composition of parent waters in the basin. Marine bromine concentrations in halite and marine fossils in associated carbonate rocks indicate that these waters had a marine origin. Bacterial sulphate reduction was the dominant process leading to the segregation between isotopically heavy sulphur incorporated in evaporite rocks and isotopically light sulphur buried in pyrite and organic matter. Strong sulphate reduction operated in a hypersaline and partly anoxic basin: these environmental conditions were also conducive to the preservation of organic material and make the Ara Group a prolific hydrocarbon province. Significant variation of the observed values is attributed to the inherent variability of bacterial sulphate reduction, including partially closed-system behaviour, disproportionation processes, limited diagenesis, and possibly secular variation of sulphur isotopes within the basin. Ara Group δ34Sanhydrite values overlap with data from other terminal Neoproterozoic to Cambrian... [ABSTRACT FROM AUTHOR]