TRACE ELEMENTS AS PROXIES FOR BOTTOM-WATER PALEO-OXYGENATION IN CAMBRIAN SHALE BASINS

The most commonly employed methods of paleo-redox determination in shale basins are trace fossil indices, organic carbon to pyrite ratios, and size analysis of pyrite framboid populations. All of these methods are potentially problematic when applied to deep-water Cambrian strata: deep-sediment bioturbators had not extensively colonized the deep-marine environment, carbon-pyrite-based methods apply only to strata deposited since the evolution of terrestrial plants, framboids recrystallize readily during low-grade metamorphism. Trace element patterns were obtained for the greenschist-facies Burgess Shale (British Columbia) and the Kinzers Fm (Pennsylvania) in order to test the reported correlation between low oxygen levels and soft-tissue preservation in Burgess Shale-type fossil deposits. Samples from the most prolific fossil sites from the Burgess Shale and Kinzer formations, Fossil Ridge and 22L respectively, yield the following chemical characteristics: Mo < 2ppm, and V/Sc values within the normal range of <9.1. Fossil Ridge shales yield Ni/Co <3.5, indicative of oxic conditions, whereas those of the 22L site yield values <5.6 which is indicative of oxic to dysoxic conditions. In contrast, the Ogygopsis-Byronia fauna occurrences at the Mt. Stephen Fossil Beds (Burgess) and 10C (Kinzers) both yield abnormally high V/Sc (as high as 33.6 in the Kinzers). Ni/Co at Mt Stephen varies between 2.2 to 8.3, indicative of oxic to anoxic conditions. Those at 22L lie between 4.1 and 5.2, indicative of the oxic/dysoxic boundary. Molybdenum at 22L varies between 3 and 10 ppm, whereas those at Mt Stephen are 3 ppm or less. These results suggest that redox-sensitive trace-element values and ratios correlate with Cambrian fossil assemblages. Furthermore, it appears that the Ogygopsis-Byronia assemblage correlates with dysoxic conditions, and that Burgess Shale-type preservation in British Columbia and Pennsylvania are not associated with anoxic bottom waters.