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The AAPG/Datapages Combined Publications Database
Williston Basin Symposium
Abstract
SKGS-AAPG
Sixth International Williston Basin Symposium, October 7,
ANOXIC SULPHIDIC DIAGENESIS IN THE ORDOVICIAN WINNIPEG FORMATION OF SASKATCHEWAN
ABSTRACT
Sandstones and shales of the Ordovician Winnipeg Formation from the subsurface of southern Saskatchewan contain remarkable textural and organic chemical evidence of anoxic sulphidic diagenesis. Several different types of sulphide-coated grains occur in the sandstones and are interpreted as having formed a few to several tens of centimetres below the water/sediment interface, near the base of the zone of bioturbation. Sulphide precipitation was brought about by bacterial reduction of sea-water sulphate to H2S. Iron was present in the sediment as detrital oxide grains and probably also as bacteriologically-bound colloidal species. Work in progress shows that the ooids of the Winnipeg Formation contain high amounts of aminoacids, possibly representing partly degraded remnants of the organic matter which mediated the precipitation of the sulphides. Evidence of sulphidic diagenesis in the shales include: 1) abundance of framboidal pyrite; 2) formation of pyrite interstitially to Fe-oxide ooids which formed during oxic sedimentation; 3) replacement of microscopic algae with perfect preservation of chains of cells; 4) pyrite filling of chitinozoan tests; and 5) pyrite replacement of various fossil fragments.
Sulphidization textures that can be considered the precursors of those of the Winnipeg Formation occur in recent sediments of the Venice Lagoon, Italy, and of Long Island Sound. Anoxic sulphidic diagenetic environments must have been widespread in the shallow epeirogenic seas of the early Paleozoic of Canada since pyrite ooids are also found in the Great Bear Lake region and in Newfoundland.
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