Pyrite Ooids from the Winnipeg Formation (Ordovician)
A somewhat different story than told by the Devonian pyrite ooids. We studied pyrite ooids from the Winnipeg Formation, are largely sand-dominated succession in the Ordovician of the Williston Basin. These pyrite ooids are a bit more irregular, but again are primarily found associated with stratigraphic intervals that mark stagnant sedimentation and erosive interludes.
Location map of known Ordovician pyrite ooid occurrences. We used samples from drill cores through the Winnipeg Formation of Saskatchewan.
Location of sampled drill cores and map of sandstone/shale distribution in the Black Island member of the Winnipeg Formation.
A typical stratigraphic section of the Winnipeg Formation with an occurrence of abundant pyrite ooids marked. there are other, less prominent, occurrences lower down in the Black Island member.
Typical sandstone with pyrite ooids. The large pyrite grains are approximately 1-1.5 mm in diameter.
There are different ooid morphologies: (A) large marcasite cores with thin cortexes; (B) marcasite cores with pyrite-marcasite cortexes; (C) thick laminated with quartz inclusions; and (D) thick irregular laminae.
The marcasite cores are the reworked and rounded remains of marcasite mineralized burrows within Winnipeg sandstones (reference below).
Reference for above (marcasite cores):
Schieber, J. 2002, The Role of an Organic Slime Matrix in the Formation of Pyritized Burrow Trails and Pyrite Concretions. Palaios, v. 17, p. 104-109. download PDF file (left click on link and click on "Save as"...)
In contrast to the Devonian pyrite ooids, the Winnipeg cortexes are free of microscopic silicate inclusions and show variable S-isotope ratios from cortex to cortex. Both features are consistent with primary pyrite accretion (one cortex at a time) during formation of these ooids.
see slide text.
A model for formation of the different pyrite ooid morphologies as a result of a combination of erosion-exhumation and/or shallow burial with up-down oscillation of sulfate reduction zone (SRZ).
The initial cores are provided by reworking of pyrite-marcasite mineralized burrows.
Marcasite cortices form when surficial iron sulfide oxidation temporarily lowers pH and raises Fe in pore waters (hydrogen sulfide provided by underlying sediment).
A hypothesis, but there is petrographic evidence of intermittent iron sulfide oxidation (see below).
Alternating cortices of pyrite, marcasite, and pyrite-marcasite mixtures. Minerals identified by electron backscatter diffraction under the SEM.
When cortex boundaries from above are examined in detail, we see corrosive surfaces separating successive cortices. This supports above scenario of intermittent sulfide oxidation, lowering of pH, and marcasite precipitation.
The context of Winnipeg pyrite ooids, and what they can tell us about depositional history. More details in the JSR paper on these grains (below).
Reference for above:
Schieber, J., and Riciputi, L. 2005, Pyrite-marcasite coated grains in the Ordovician Winnipeg Formation, Canada: An intertwined record of surface conditions, stratigraphic condensation, geochemical “reworking”, and microbial activity. Journal of Sedimentary Research, v. 75, p. 905-918. Link