CHARACTERISTICS OF PYRITE IN THE MIDDLE CAMBRIAN BURGESS SHALE FORMATION WITH IMPLICATIONS FOR PALEO-REDOX CONDITIONS DURING DEPOSITION

Debate on paleo-redox conditions during deposition of the Burgess Shale is ongoing; anoxic conditions, a fluctuating oxycline, and an exaerobic environment have been suggested. Given that the characteristics of pyrite will vary depending on the environment, and the mechanism by which it formed (e.g., precipitation from anoxic water, microbial sulfate reduction within sediment), a petrographic study was performed to characterize pyrite from fossil-bearing localities in the Burgess Shale Formation at Fossil Ridge and Mt. Stephen. Based on petrographic analysis and supporting major element geochemical data, strata from the Burgess Shale were classified in accordance with their relative proportions of clay (now mica), silt, and carbonate minerals: claystone, silty claystone, calcareous claystone, calcareous siltstone, limestone. Furthermore, pyrite occurrences were categorized based upon both the form of individual grains (framboidal, spherical, polygonal), and the form of aggregates of pyrite grains (disseminated, string, segment, lens, and pod). Strings are short curving chains of pyrite, whereas pods, lenses and segments are homogenous elongated concentrations of pyrite with increasing length to width ratios. Disseminated pyrite occurs as a trace component in all samples, but aggregates of pyrite are more abundant. Pyrite strings only occur in carbonate laminae in association with filamentous calcite structures of similar size, and thus are inferred to have formed in microbial mats. Pods, lenses and segments of pyrite are absent in limestone. The laminae with greater clay and silt content commonly contain pods, lenses and/or segments that are bedding-parallel, and all three aggregate forms may be present in a given sample of claystone or calcareous claystone. Silty claystone and calcareous siltstone commonly contain pods, but lenses and segments are rare or absent. Segments, lenses and pods likely formed during decomposition of buried organic matter of various sizes. The distribution and forms of the vast majority of pyrite in the Burgess Shale indicates that pyrite formed by microbial processes at or within the sediment. This is consistent with an exaerobic environment of deposition.