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Journal of the Geological Society

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

^{1 1}University of Bern, Institute of Geological Sciences, Baltzerstrasse 1, CH-3012 Bern, Switzerland
^{2 2}Present address: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, 77 Massachusetts Ave., Cambridge, MA 02139, USA (e-mail: stefan@erl.mit.edu)
^{3 3}Appalachian State University, Department of Geology, 195 Rankin Science Building, Boone, NC 28608, USA
^{4 4}Petroleum Development Oman, P.O. Box 81, Muscat, PC 113, Sultanate of Oman

Terminal Neoproterozoic and Cambrian evaporite rocks worldwide record a marked sulphur isotope excursion to very heavy ³⁴S isotopes that reflect a period of significant burial of reduced sulphur. New ³⁴S 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 ³⁴S_anhydrite of 39.4 ± 2.6 (1; range 32.4–46.4; n=61), and a mean ³⁴S_pyrite of 1.4 ± 7.3 (1; 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 ³⁴S_anhydrite values overlap with data from other terminal Neoproterozoic to Cambrian basins, but the mean ³⁴S_anhydrite is highest in Oman. This observation cannot be attributed solely to the specific anoxic conditions and strong bacterial sulphate reduction in Oman, because such conditions were widespread in most evaporite basins of the terminal Neoproterozoic–Cambrian Periods. Instead, slight age differences between these basins can introduce a bias in the sulphur isotope curve. Excellent preservation of Ara Group deposits and in particular the precise age constraints make the Ara Group representative for the seawater sulphur isotopic composition at the terminal Neoproterozoic–Cambrian boundary. As a result of the inherent variability of bacterial sulphate reduction, ³⁴S values at the lower end of the spectrum should be most representative for the global signal. Based on results of this study, the global signal is estimated to be approximately 35–40.

KEYWORDS: Neoproterozoic, Cambrian, Oman, sulphur, isotope geochemistry

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