The salt deposits of the Middle Devonian Elk Point Group extend from North Dakota to Canada's Northwest Territories. Subsurface knowledge is best in Saskatchewan. Here the most wide-spread of these deposits, the Prairie Formation, reaches a maximum thickness of 670 feet and contains the extensive potash ores now being developed. The Prairie salt beds overlie thin carbonates containing reefs. These sediments form part of a depositional sequence starting with thin shales and grading upward to carbonates, followed by evaporites climaxing in potash salts and capped with red beds. This sequence is overlain by similar depositional units in which, however, evaporites are less well developed.
Because of the excellent subsurface control in this vast gently dipping shelf area, it is possible to recognize structural features that might go unnoticed in more complex areas. Structural highs due to salt flowage are unknown. There are, however, many structural lows which are due to removal of salt by subsurface leaching.
The Rosetown low, covering about 144 sections, is one of the larger features of this type.
The Hummingbird trough is a complex structure. Seismic and subsurface information indicate that there have been repeated episodes of salt leaching accompanied by downwarping and compensation in overlying strata.
It is impossible to escape the conclusion that the Prairie evaporites have been severely modified by subsequent solution. The unusual structures resulting from this phenomenon may be present but unrecognized in other salt basins.