In the sedimentary basins of the northern United States and western Canada large amounts of rock, broadly described as evaporitic, are either petroleum bearing or so situated in their geologic association with petroleum that evaporite geology bears directly on the petroleum occurrence.
The evaporites are members of facies assigned to two distinct groups—a coastal-shoal/salt-flat suite and a reef/salt-basin suite, illustrated by Devonian and Mississippian examples.
(1) Desert-zone coastal salt flats (sabkhas) have the peculiarity of transforming calcareous sediment to microdolomite containing quantities of gypsum and anhydrite. The permeability changes thus effected make stratigraphic traps for oil and gas. Fossil sabkhas among Late Devonian strata of the Western Canada basin and the Mississippian rocks of the Williston basin hold large reserves of sulfur and petroleum.
(2) Middle Devonian reefs in the Elk Point evaporite basin were built up in stages. The first reef stage (Winnipegosis-Black Creek) forms part of an association of sedimentary units deposited in the following order: (1) platform; (2) reef, ending with laminar dolomite on both reef and basin floor; (3) bituminous calcitic and anhydritic laminites on the basin floor (not on reef sites); and (4) laminar halite filling the interreef tracts nearly to reef-top level.
The reefs are not in stratigraphic-facies relation to the salt, for during calcitic laminite and salt sedimentation they were exposed subaerially (not implying any change in general sea level outside the basin). Much diagenetic activity affected the laminite deposits, some of it apparently occurring independently (perhaps yearly) in each lamina.
Some anhydrite is nodular-dolomitic, resembling primary anhydrite of present-day sabkha capillary-zone type, though most of it is associated so intimately with laminar organic matter that it can be described as mineralized algal mat.