August 13. Permian salt

I’ve indicated a couple times that the Earth’s climate was changing pretty dramatically from the Carboniferous into the Permian. This was partly a result of (or a cause of) the end of the Carboniferous rainforests, and partly a result of increased glaciation in the southern hemisphere. It was also to at least some extent a consequence of the land masses combining into the supercontinent of Pangaea – vast areas were distant from shores, and depending on wind circulation patterns, vast areas were also in the rain shadows of some pretty large mountain ranges. By the middle part of the Permian, arid conditions were common, and despite the glaciers in the southern hemisphere, the tropics were pretty hot.

One result of the hot arid climate was extensive deposits of salt. In what is now Kansas and Oklahoma, more than 500 feet of Permian salt lies in the subsurface. Salt mines around Hutchinson, Kansas, led to its nickname, The Salt City. The salt in Kansas and Oklahoma, while more than 500 feet thick in total, is really a package of individual beds that are a few to many tens of feet thick, separated by zones of mudstone and shale. It appears that the area was a shallow, evaporative sea, probably an arm of the restricted basins of West Texas and New Mexico that we’ll talk about later this month.

To an extent, the shallow salt sea in Kansas during the Permian represents the last bit of sea, left over from the seaway that extended through what is now the Great Plains for much of the Paleozoic Era.

Zechstein salt in northern Europe

In northern Europe, a similar basin formed in which the Zechstein salt was deposited. The restricted basin extended from what is now Poland, west across northern Germany, the Netherlands, and beneath extensive areas of today’s North Sea. It formed fairly late in Permian time, about 270 to 260 million years ago. Because it was later in the period, it’s possible that it began at least in part as a result of deglaciation in the southern hemisphere – melting ice flooded shallow areas in the tropics, where evaporation exceeded the rate of rise so salts precipitated out of the water. The Zechstein salt serves as the impermeable seal for some significant oil accumulations in older Permian rocks in the North Sea area. In some places, the buried salt has risen buoyantly to form salt domes that help trap oil and natural gas as well.

Permian salt is found all over the world in the former tropical areas, including northern South America, central Russia, and smaller basins in Australia and Eurasia. Most of southern South America, Africa and Antarctica appear to have been well within or near the Antarctic Circle. 

—Richard I. Gibson

Zechstein salt

Map by San Jose and Drdoht at the German language Wikipedia used under GFDL