Commercial grades of potash salts occur in three zones within the upper part of the Middle Devonian Prairie Formation of the Elk Point Group. Sylvinite is the only potash ore in the Regina sub-basin. Observations on mineral composition, texture, and clay content are interpreted to prove the primary origin of the potash ores, and hence the three zones are interpreted to represent major cycles of regression. The zones become successively less widespread upward suggesting the progressive shrinkage of the basin. The importance of widespread solution and recrystallization after burial is discounted; only the uppermost zone appears to be widely leached and contaminated by insolubles. Ores in the lowermost zone have excellent industrial qualities.

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