Replacement anhydrite porphyroblasts are abundant in Mississippian carbonates from south-eastern Saskatchewan and have caused deterioration in oil field reservoir quality. Previously they were interpreted to have formed at the time overlying Mississippian bedded anhydrites were deposited. Porphyroblasts are here reported to be absent from a late diagenetic altered zone, composed of dolomitized and anhydrite-plugged beds, that occurs immediately beneath the Mississippian unconformity. This absence from strata that normally contain them indicates that anhydrite porphyroblast growth occurred after the formation of the sub-unconformity altered zone. This zone is believed to have been rendered impermeable, or otherwise made unsuitable for anhydrite replacement, before introduction of fluids that elsewhere precipitated the anhydrite porophyroblasts. The altered zone is dated as post-Watrous redbeds (post-Triassic); consequently the growth of replacement anhydrite post-dates this period. The occurrence of included bitumen in many porphyroblasts indicates that some replacement occurred after or during the introduction of oil into the Mississippian reservoirs (Late Cretaceous?).An early diagenetic origin for pre-replacement-anhydrite joints, stylolites and dolomite need not be invoked now that replacement anhydrite has been shown to be a late diagenetic feature.