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salt dome
A salt dome is a type of structural dome formed when a thick bed of evaporite minerals (mainly salt, or halite) found at depth intrudes vertically into surrounding rock strata, forming a diapir. It is important in petroleum geology because salt structures are impermeable and can lead to the formation of a stratigraphic trap.
==Formation==
The formation of a salt dome begins with the deposition of salt in a restricted marine basin. Because the flow of salt-rich seawater into the basin is not balanced by outflow, much to all water lost from the basin is via evaporation, resulting in the precipitation and deposition of salt evaporites. The rate of sedimentation of salt is significantly larger than the rate of sedimentation of clastics,〔Schreiber, B.C. and Hsü, K.J. (1980) Evaporites. In Developments in Petroleum Geology, Vol. 2 (Ed. G.D. Hobson), pp. 87–138. Elsevier Science, Amsterdam.〕 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 geologic 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 - the salt can begin to flow vertically, forming a salt pillow.〔RGD 1993. Geological Atlas of the subsurface of The Netherlands: Explanations to map sheet IV Texel-Purmerend (1:250,000). Rijks Geologische Dienst (Haarlem): 127 pp.〕 The vertical growth of these salt pillows creates pressure on the upward surface, causing extension and faulting.〔Dronkert, H. & Remmelts, G. 1996. Influence of salt structures on reservoir rocks in Block L2, Dutch continental shelf. In: Rondeel, H.E., Batjes, D.A.J., Nieuwenhuijs, W.H. (eds): Geology of gas and oil under the Netherlands, Kluwer (Dordrecht): 159–166.〕 (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 across, and extend as deep as .
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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