Friday, 23 April 2010

More From the Zechstein Basin

It isn't every day that a specific area of interest comes up in a lecture multiple times. I was already paying quite a lot of attention during this particular sedimentology lecture, but managed to pay even more when the Zechstein sea was brought up. I thought I would share the information I got from it.

The first mention simply came with the mention of commercial uses of evaporite deposits, as salt from the Zechstein sea is mined for industrial purposes and human consumption. Boulby mine in East Yorkshire is apparently the deepest in the UK and extends a mile out to sea.

The Zechstein basin is a saline giant, extensive marine evaporite deposits (deposited when sea water evaporates and leaves minerals) thousands of metres thick in what are known as intracratonic basins (a craton is a part of the Earth's crust that has not been split or merged). There are no modern equivalents of saline giants. Below is a picture of major Phanerozoic evaporite deposits.

Saline giants present a conundrum for geologists as evaporating sea water in the laboratory does not produce natural evaporite minerals and produces them in the wrong volumes (not enough anhydrite, too much sylvite/carnallite). Evaporating a 300m column of sea water only produces less than 5m of salt, yet in Jurassic marine deposits in America the desposits are 3500m thick, which would require the evaporation of a 240km deep marine basin!
On the left is the data predicted compared with the actual amounts found in the Zechstein basin. Below is the same data plotted in a graph. As can be seen, the predictions do not match what is produced.
The explanation for this seems to be that these basins were repeatedly replenished with sea water, possibly before the sea had fully dried out. Doncaster during the Permian was in a sea which was constantly evaporating and being refilled. This required special conditions. There needs to have been a barrier to the open ocean permitting replenishment of sea water (which could be continual or episodic); there needs to have been prolonged subsidence; and there needs to have been prolonged aridity and thermo-haline stratification (this relates to the temperature and salt content of the water).

There are different types of saline giant basins. The Zechstein was a deep barred basin, as shown on the left. These have the same mineralogy across the basin at any given time, overlying anoxic muds, with a straitified water column in 10s to 100s of metres cycles.

Hopefully future lectures will reveal more about the Zechstein basin, though I suspect that I will have to apply information from other examples in order to work out more about it.

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