An Ancient Ocean Beneath the Prairie
Beneath the plains of Saskatchewan lies the world's largest known potash deposit, the geological remnant of a vast inland sea from the Middle Devonian period, around 400 million years ago. At that time, the region was located near the equator and covered by the Elk Point Sea, a shallow body of water connected to the open ocean. A hot, arid climate led to intense evaporation. Over millions of years, reefs grew and restricted the flow of water from the ocean, causing the inland sea to repeatedly evaporate and recharge. This process concentrated dissolved salts, which precipitated out and settled on the seabed. For every meter of salt accumulation, an estimated 60 meters of seawater had to evaporate. The resulting salt deposit, known as the Prairie Evaporite Formation, reaches a thickness of over 200 meters in some areas.
The potash minerals are concentrated in the upper 60 meters of this formation. This geological deposit consists mainly of halite (common salt), sylvite (potassium chloride), and carnallite. The mixture of sylvite and halite, called sylvinite, is the primary ore. Geologists have identified three main potash-bearing layers, or "members," within the formation: the Esterhazy, Belle Plaine, and Patience Lake Members. After their deposition, these soluble salt layers were buried under more than 1,000 meters of sediment, which formed a protective caprock that shielded them from being dissolved by fresh water over the ensuing eons.