Redox-sensitive trace elements document chemical depositional environment and postdepositional oxidation of the Ediacaran Biri formation, southern Norway.

摘要

The Ediacaran Biri Formation comprises carbonate and silisiclastic facies including a ~ 70 m thick organic-rich shale facies exposed 9 km west of Biri, Norway in a steep bedrock stream channel at Djupdalsbekken. This outcrop is overlain by ~ 30 m of coarse-clasitic conglomeritic facies of the Ring Formation, deposited in the southern and western portion of the Hedmark basin as prograding subaerial and submarine delta fans. Concentrations and distributions of some redox-sensitive trace elements, specifically molybdenum and uranium, within the Biri Formation shale indicate deposition under sub-oxic to anoxic conditions. Pyrite framboid size distribution corroborates trace element evidence and suggests that sulfidic conditions existed within the sediment with a chemocline at or near the sediment-water interface.;An attempt to date the Biri Formation shale by Hannah et al. (2007) found disturbed Re-Os isotope systematics from samples in the first 8 meters of the exposure, while data obtained from samples further down section were undisturbed. Here, an attempt to understand these disturbed and undisturbed sections using redox-sensitive trace element chemistry suggests the disturbed data was a result of post-depositional re-oxygenation within the upper few meters of the Biri shale. This is indicated by concentration peaks in trace element profiles that result from remobilization and subsequent re-fixation of these elements at different locations in the shale. A well constrained hypothesis constructed using uranium and molybdenum as proxies for rhenium shows that rhenium was likely remobilized after deposition of the Biri Formation and either subsequently re-deposited, or flushed out of the system. In this scenario, the post-depositional remobilization of rhenium (and most likely osmium also) resulted in disturbed Re-Os isotope systematics described by Hannah et al. (2007).;Trace element geochemistry, petrographic, and delta13C and delta18O stable isotope evidence document post-depositional re-oxygenation of the Biri Formation shale. Re-oxygenation occurred either synchronous to deposition of the overlying Ring Formation or during a later event, the Caledonian orogeny (~ 440 Ma) being the most likely candidate. While the geochemical evidence does not preclude one time period or the other, disturbed Re-Os isotope systematics and resulting dates given by Hannah et al. (2007) can only be supported by re-oxygenation of the Biri Formation shale during the Caledonian orogeny.