Morphogenesis of a new landform responsive to superimposed sulphate and carbonate buried karsts in western Canada

This case study documents the morphogenesis of a new karstic landform consisting of a calcareous tufa mound with a gypsum caprock that developed at La Saline Lake, an oxbow of the Athabasca River in northeast Alberta, Canada. The development of this novel tiered carbonate-sulphate mound architecture resulted from emplacement above the largest known buried salt dissolution collapse-subsidence karst system, which was superimposed by an overlying buried carbonate karst. Dissolution of halite-anhydrite beds of the Middle Devonian Prairie Evaporite, 200 m below, resulted in extensive subsidence and brecciation of the overlying Upper Devonian carbonate karst. The development of the calcareous tufa mound resulted from meteoric-driven groundwater mixed with glacial meltwater directed along the shallowly buried karstic carbonate terrain of Middle-Upper Devonian limestone, which floors the McMurray Formation, the main reservoir unit of the Lower Cretaceous Athabasca Oil Sands. The flow of the carbonate-saturated groundwater responsible for the emplacement of the tufa mound at the surface was abruptly redirected along a deeper pathway in response to dissolution collapse-subsidence adjustments within the buried salt karst of the Prairie Evaporite. The redirected groundwater flow deeper encountered an anhydrite-dominated segment of the salt dissolution trend within the Prairie Evaporite Formation, 200 m below. The migration up-section of sulphate-saturated brine onto the surface of the previously formed tufa mound resulted in the emplacement of a gypsum caprock as a new landform.