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How did partially enclosed ancient seas create very thick beds of rock salt?
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Salt crystal magnified.
Extremely thick beds of rock salt, or halite, formed in the geologic past through the evaporation of seawater within partially enclosed basins. The creation of these massive deposits required specific environmental conditions and geological mechanisms:12
- Aridity: The primary requirement was a climate where the rate of evaporation significantly exceeded the total inflow of water from rainfall and rivers, allowing brines to concentrate to the point of mineral precipitation.34
- Basin Isolation: These basins were partially isolated from the open ocean by a barrier or sill on the seafloor. This barrier prevented the concentrated, dense brines from flowing back out to the sea, trapping them within the basin.14
- Replenishment: To achieve inordinately thick sequences, the basin required periodic replenishment. As evaporation concentrated the water, the barrier was occasionally breached—due to global sea-level changes or crustal downwarping—allowing fresh seawater to enter and provide a new supply of salt to be evaporated.4
Over long periods, this cycle of isolation, evaporation, and replenishment resulted in the accumulation of salt beds that can reach hundreds of feet in thickness.1