Salt Dome - Formation

Formation

The formation of a salt dome begins with the deposition of salt in a restricted marine basin. The restricted flow of salt-rich seawater into the basin allows evaporation to occur, resulting in the precipitation of salt, with the evaporites being deposited. The rate of sedimentation of salt is significantly larger than the rate of sedimentation of clastics, but it is recognised that a single evaporation event is rarely enough to produce the vast quantities of salt needed to form a layer thick enough for salt diapirs to be formed. This indicates that a sustained period of episodic flooding and evaporation of the basin must occur, as can be seen from the example of the Mediterranean Messinian salinity crisis. At the present day, evaporite deposits can be seen accumulating in basins that merely have restricted access but do not completely dry out; they provide an analogue to some deposits recognised in the geological record, such as the Garabogazköl basin in Turkmenistan.

Over time, the layer of salt is covered with deposited sediment, becoming buried under an increasingly large overburden. The overlying sediment will undergo compaction, causing an increase in density and therefore a decrease in buoyancy. Unlike clastics, pressure has a significantly smaller effect on the density of salt due to its crystal structure and this eventually leads to it becoming more buoyant than the sediment above it. The ductility of salt initially allows it to plastically deform and flow laterally, decoupling the overlying sediment from the underlying sediment. Since the salt has a larger buoyancy than the sediment above - and if a significant faulting event affects the lower surface of the salt - it can be enough to cause the salt to begin to flow vertically, forming a salt pillow. The vertical growth of these salt pillows creates pressure on the upward surface, causing extension and faulting. (see salt tectonics).

Eventually, over millions of years, the salt will pierce and break through the overlying sediment, first as a dome-shaped and then a mushroom-shaped - fully formed salt diapir. If the rising salt diapir breaches the surface, it can become a flowing salt glacier. In cross section, these large domes may be anywhere from 1 to 10 kilometres (0.62 to 6.2 mi) across, and extend as deep as 6.5 kilometres (4.0 mi).