Orogeny and the collapse of the Devonian Prairie Evaporite karst in Western Canada: impact on the overlying Cretaceous Athabasca Oil Sands

Abstract

The Middle Devonian Prairie Evaporite Formation accumulated up to 200 m of halite-dominated beds across Western Canada in a salt basin extending from northern Alberta southeastwards into southern Saskatchewan and western North Dakota. A 1000 km long salt dissolution trend along the eastern basin margin resulted in the removal of 100–150 m of halite–anhydrite beds. A second salt dissolution trend removed up to 200 m of section across southern Saskatchewan. The removal of the halite-dominated beds and the collapse of the overlying strata responded to regional aquifer and vertical water flows driven by compaction up-structure to the NE towards the eastern margins of the foreland Alberta and intracratonic Williston constituent basins of the Western Canadian Sedimentary Basin. Flows resulting in dissolution trends in the deepening Alberta foreland basin were responses to Middle Jurassic–Early Cretaceous Columbian orogenic tectonism, in contrast with the multiple Paleozoic and Mesozoic dissolution stages across the southern Saskatchewan area of the northern Williston basin. Water flows along the Devonian Keg River strata migrated into the overlying Prairie Evaporite salt beds. The dissolution fronts advanced along multi-kilometre long NW- and NE-oriented sets of fault–fracture lineaments resulting from orogenic Precambrian block movements propagated into the overlying Devonian strata. The Athabasca Oil Sands accumulated above a 300 km long segment of the dissolution trend along the eastern margin of the evaporite basin. An unusually low 1:2 thickness ratio of removed salt beds to overlying strata resulted in significant structural controls on the deposition of the overlying Cretaceous McMurray Formation point bar complexes. Fluvial point bars up to 6 km long and tens of metres thick collinearly aligned along fairways tens of kilometres long, following the halite dissolution trends 200 m below. These sand reservoirs trapped Laramide oil migration into the area. Biodegradation followed, resulting in a trillion barrel resource. A final stage of salt removal occurred with the influx of subglacial meltwaters, rejuvenating 10 km2 Cretaceous collapses across southern Saskatchewan.

Alignment of saline springs with evaporite karst structures in northeast Alberta, western Canada: analogue for cretaceous hypogene brine seeps to the surface

“…The Athabasca River Valley in northeast Alberta overlies a segment of the Assiniboia PaleoValley, which developed parallel to the underlying salt removal trend (i.e. the salt scarp) in Middle Devonian substrate (Broughton 2016a, b, 2017a, 2018a, Grobe 2000Hauck et al 2017). Catastrophic floods of 13,000 and 10,000 years ago emanated from the glacial Lake Agassiz, which extended across most of southern Saskatchewan (Smith & Fisher 1993).…”

“…The 400 km long Presqu'ile barrier reef developed at the northern seaward end of the shallow inland sea dur-ing the Givetian Stage of the Lower-Middle Devonian. This resulted in a barred evaporite basin (Holter 1969;Meijer Drees 1986;Broughton 2017aBroughton , 2018aBroughton , 2020. Reconstruction of the Middle Devonian paleogeography suggests the barred inland sea of the Elk Point Basin was positioned at an equatorial latitude of 10° to 20° south.…”

“…AND COLLAPSE STRUCTURES Craton deformations controlled the development of two distinctive salt dissolution trends in the Alberta Basin foreland and the northern Williston Basin (Fig. 1) (Broughton 2017b(Broughton , 2018a. The earliest salt removal patterns developed across the northern Williston Basin, impacting southern Saskatchewan and adjacent areas of eastern Montana and western North Dakota (DeMille et al 1964;Christensen 1967;Holter 1969;Christensen et al 1982;Christensen & Sauer 2002;LeFever & LeFever 2005).…”

“…Subsidence-collapse structures resulted from the removal of up to 130 m of salt section prior to and during the deposition of the Lower Cretaceous McMurray Formation (Fig. 2), the principal reservoir rock of the Athabasca oil sands in northeast Alberta (Broughton 2013(Broughton , 2015(Broughton , 2018a. The resulting dissolution-collapse structures vary from sinkholes and breccia pipes to 10s km long troughs that consist of differentially subsided fault blocks of Upper Devonian strata infilled with deposits of the McMurray Formation.…”

Recent advances in karst research: from theory to fieldwork and applications

“…Linked to the issue of palaeokarst reservoirs, the first paper in this volume, by Broughton (2017), reviews the origin and distribution of the regionalscale dissolution patterns that reconfigured the Middle Devonian Prairie Evaporite Formation salt basin in northeastern Alberta and southern Saskatchewan and interprets the impact of continental-scale tectonism on these dissolution processes. The dissolution trends in the formation strata accumulated across Western Canada resulted in the largest known hypogene halite karst collapse and subsidence structures.…”