ABSTRACT

Paleokarst in Mississippian strata of southeastern Saskatchewan most commonly occurs in the coated grain rocks of the Frobisher and Alida beds. The karstic intervals are generally proximal to the sub-Mesozoic unconformity; however dissolution features in strata well below that exposure surface are possibly related to intra-Mississippian unconformities. The dissolution features can be categorized into four basic types: 1) paleo-epikarst, 2) protokarst, 3) large and small dissolution cavities and 4) intrastratal karst. The paleo-epikarst includes both small features recognizable in core and one 400 m wide, 600 m long and 8 m deep depression of the paleotopography in the Melrose area that is interpreted to be a polje. The larger cavities are up to 10 m deep and have characteristics indicative of paleocaves. The criteria for their recognition include: rotated or tilted blocks of host rock, cave crusts, flowstone and dripstone, internal sediment, crackle breccia, breakdown breccia and allochthonous sediment. In contrast, the smaller cavities are less than 2 m deep and commonly only rotated or tilted breakdown breccia blocks are observed. Intrastratal karst has no apparent relationship to exposure surfaces.

Timing of the solution process and anhydrite plugging, and determining the source of sufficiently large volumes of water for dissolution are problematic. Studies on Mississippian rocks of Wyoming and Montana suggest that karsting occurred at the sub-Absaroka unconformity during a 34 m.y. period that preceded deposition of the Pennsylvanian rocks which commonly inlay the paleokarst. There are three possible origins for the solvent waters: 1) downward percolating meteoric water, 2) upward migrating formation water and 3) mixing zone meteoric and marine waters. One of the first two appears most likely to have been responsible for dissolution features proximal to the sub-Absaroka exposure surface. The third may have produced paleocaves associated with intra-formational exposure surfaces. Any open spaces left in the accumulation of breakdown blocks and allochthonous sediment were occluded by anhydrite. The origin of the anhydrite is not well understood. It may have been emplaced by downward percolation of sulphate-rich brines from the Watrous, but there is some evidence to suggest the introduction of hydrothermal sulphate-rich waters from the deeper portion of the basin. Karsting of the Alida apparently reflects semi-arid to humid climatic conditions but the evaporites in the succession from the Kisbey to Poplar Beds indicate the onset of aridity.