Principal Features of Evaporite Karst in Canada

Abstract

Outcrops of sulfate and mixed sulfate-carbonate rocks are common everywhere in Canada outside of the Shield province. Interstratal salt deposits are abundant in the interior lowlands. Types of karst that occur are determined chiefly by relations between (i) formation thickness and purity, (ii) regional topography and hydraulic gradient, (iii) effects of receding Wisconsinan and earlier glaciers, and (iv) extent of modern permafrost. Exposures of bare karst on thick, pure sulfate formations are comparatively rare. Two principal landform types found on them are: (1) high-density polygonal karst (micro-sinkhole densities of thousands per km²), where hydraulic gradients are high and tills are thin; (2) hills and ridges of blocks uplifted and fractured by hydration (anhydrite) tectonics at paleo-icefront positions where hydraulic gradients are low. Deeply till-mantled karst dominated by collapse and suffosion sinkholes in the mantling detritus is well developed in southwestern Newfoundland and in central and northern Nova Scotia. Covered karst is abundant on sulfates conformably overlain by carbonate or clastic strata; collapse sinkholes are the principal landform. Very large breccia pipes (up to 25 x 15 km) are associated with deep subrosion of salt during glacier recessions. Syngenetic breccia karst is a fourth, distinct category created in some formations of thin, interbedded dolostones and sulfates. Where these are exposed to high hydraulic gradients, deep calcite-cemented breccias were formed in a first generation, upon which sinkhole and pinnacle karsts and dissolution drape topographies were able to develop rapidly in late-glacial and post-glacial conditions.