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Locally in southeastern Saskatchewan, Mississippian nodular anhydrites (after subaqueous gypsum) beneath an unconformity have been altered to limestone--limestones that are commonly porous and oil-bearing. Such carbonates are commonly intergrown with pyrite and celestite, and thus are difficult to interpret from logs. At localities with calcitized anhydrite, the unconformity is overlain by Jurassic red beds in which pigments have been reduced to green hues. In the region, carbonates beneath the unconformity are normally overlain by red beds and have been completely dolomitized and plugged with anhydrite to form an impermeable caprock. Mississippian anhydrites subcrop at the unconformity surface and reveal little evidence of alteration--even to gypsum.
Textures in replaced anhydrites indicate that calcitization involved both creation of porosity and in-situ (small-scale) replacement leading to retention of anhydrite (and later gypsum) fabrics. Celestite formed as strontium was released from anhydrite during replacement by gypsum and calcite. Sulfur in associated pyrite is isotopically lighter than the anhydrite, suggesting anhydrite-alteration involved the activities of sulfate-reducing bacteria. Evidently, H2S liberated during the reaction migrated across the unconformity to reduce overlying red beds.
Limestones of this type do not appear to have been reported previously. Stratigraphic and petrographic evidence indicates replacement, although spacially related to the unconformity, was not a weathering phenomenon. It occurred after the unconformity was buried.
Unexpectedly heavy ^dgr13C and ^dgr18O values (+ 1.22 to 1.54, and -1.0 to -3.7) obtained from the replacement limestones seem to preclude the utilization of organic carbon in the reaction. The
source of carbonate and of the energy required for sulfate-reducing bacterial activity is therefore problematic.
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