Hypogene Karst Collapse of the Devonian Prairie Evaporite Basin in Western Canada

Abstract

Regional dissolution trends configured the Middle Devonian Prairie Evaporite basin across Western Canada, resulting in the largest known collapse of a hypogene evaporite karst dominated by halite beds. Dissolution-collapse subsidence resulted from aquifer flows to the north-east within the Alberta and Williston basins, up-structure towards the eastern margin of the salt basin. A 1000-km-long and 150-km-wide salt dissolution trend developed along the eastern evaporite basin margin. The flows occurred as the Western Canada foreland basin deepened in response to the Columbian (Middle Jurassic-Early Cretaceous) tectonism. A second dissolution pattern developed across the southern Saskatchewan area of the Williston basin. Dissolution trends removed up to 250-m-thick beds across south-central Saskatchewan and 100--150-m-thick beds in north-eastern Alberta. These salt dissolution patterns advanced along sets of fault-fracture lineaments that widened and coalesced into larger salt removal areas. The dissolution trends configured the overlying strata as subsidence-collapse troughs tens of km long. In northern Alberta, troughs up to 50 km long configured the Upper Devonian limestone karst palaeotopography, subsequently covered by Lower Cretaceous Athabasca Oil Sands. This bitumen deposit, the largest known, overlies a 300-km-long segment of the dissolution trend that extends along the eastern margin of the Middle Devonian evaporite basin. Syndepositional sand trends up to 25 km long accumulated over collapsed chains of Devonian fault blocks that responded to salt dissolution patterns in the substrate. These sand reservoirs trapped Late Cretaceous-Early Palaeogene oil migrations into the area.