ABSTRACT

The Nottingham North Alida Beds Unit is located on the northern margin of the Williston Basin in southeast Saskatchewan. Lower Mississippian shallow shelf carbonates of the Mission Canyon Formation display transgressive-regressive cycles in an overall shallowing upwards sequence. Cyclic beds average 32 m in thickness and are laterally continuous across the unit.

A cycle includes three facies in ascending order: (1) Unconformity-subaerial exposure facies (MC-2, MC-4) displaying a clay-rich and dolomitized weathered zone, (2) Intertidal Oncolite Boundstone Algal facies (Algal of MC-3, Lower MC-5) - early marine cementation accounts for preservation of primary interparticle porosity, (3) Normal Marine facies (MC-3, Upper MC-5) containing only secondary non-fabric selective leached porosity. The facies include crinoid ossicles, brachiopods, foraminifera, ostracodes, calcareous algae, solitary and colonial corals, and bryozoa.

Two fluid systems are present within each cycle: a) a thin finely laminated mudstone (often dolomitized) trapping fluids within the Intertidal Oncolite Boundstone (facies 2); and b) the Unconformity weathered zone (facies 1) trapping fluids in the Normal Marine facies (facies 3).