Evaporites sequence

Thus, the Mediterranean Sea must have been refilled in an episodic fashion such that conditions favoring shallow-water evaporite deposition were rapidly reattained. Some geologists have proposed that this was achieved via periodic inflows of seawater from the Atlantic Ocean over the exposed Gibraltar Sill into a nearly dry Mediterranean Sea basin. This must have taken the form of a waterfeU hundreds of meters in height The episodic nature of this process is reflected in the repeating evaporite sequences foimd throughout the Messinian deposits. 

The marine sedimentary rocks of the Codroy Group host the evaporite sequence of interest and are divided in ascending order into the Ship Cove... 

Figure 8.24. Summary of composition of nonstoichiometric dolomite. A. Data for 246 samples from continental exposures of dolostones and dolomitic limestones. B. Data for 153 samples from continental exposures of dolomites related and not related to evaporite sequences. C. Ranges of Holocene dolomite compositions. D. Data for 35 samples of deep-marine dolomite from DSDP cores. E. Histogram of 245 samples of deep-marine dolomite E evaporite-associated, O organic origin, L samples of Lumsden (1988) above, D detrital, and U uncertain. (After Lumsden, 1988.)...

The geologic record is characterized by a dearth of Precambrian evaporitic sequences, including their sulfate facies. Stratiform barites do exist, but they may be, at least in part, of hydrothermal origin. The scarcity of evaporites is... 

There is evidence that the major ion composition of seawater has varied over many millions of years, linked to very long term geochemical cycling. Evidence from ancient marine evaporite sequences (Box 6.2) sets limits on the possible extent of that variability. 

Morad, S., Al-Aasm, I.S., Longstaffe, F.J., Marfil, R., De Ros, L.F., Johansen, H. Marzo, M. (1995) Diagenesis of a mixed siliciclastic/evaporitic sequence of the Middle Muschelkalk (Middle Triassic), the Catalan Coastal Range, NE Spain. Sedimenlology, 42, 749-768. 

Figure 62. Changes in the calcium and sulphate concentration of a water-based drilling fluid during drilling of a complex evaporite sequence. Scales Ca2+ 0-16 mM SO/ 0-40 mM API (static) fluid loss 10-20 mL. (Reproduced with permission from reference 178. Copyright 1992.)...

Main evaporite sequence MASSIVE SALT ARGILLITE D2 EVAPORITE D2 No clear correlation MASSIVE SALT... 

An important feature of salts with diffuse dissolution regime is their capability to dissolve quickly at the excess of moisture and precipitate easily at its deficit. In conditions humid climate these salts are easily removed from the surface. An exception is salt karst associated with young diapirs in Rumania. On the surface and outside water bodies they are preserved only in conditions of arid climate (Iran, Israel, Chile). Their main mass is associated with evaporite sequences in the subsurface. In conditions of high pressure and temperature halite, sylvinite and some other soluble salts acquire plasticity and because of this become impermeable. That is why at depth they play role of a aquifuge and are subject to diapirism. 

Common sources of potassium are the feldspars and micas of the igneous and metamorphic rocks. Potash minerals such as sylvite occur in some evaporitic sequences, but their contribution is not important. Although the abundance of potassium in the Earth s crust is similar to that of sodium, its concentration in groundwater is usually less than a tenth that of sodium. Most groundwater contains less than 10 mg As with sodium, potassium is highly soluble and therefore is not easily removed from groundwater except by ion exchange. 

Figure 1.18 Summary of stratigraphic units in Searles Lake evaporite sequence (Smith, 1979).

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