Evaporites precipitation sequence

Figure 9.7 Simplified chart showing the evaporite precipitation sequence from waters of various compositions. Alkaline and neutral pathways are shown. The alkaline pathway is common in salinas, playas and apolyhaline lakes. The neutral pathway can be divided into two different sequences according to the ratio of the concentrations of HC03 to (Ca2+ + Mg2+). The main ions still in solution in the residual brines are given at the end of the sequence (grey frames). On the right-hand side, the average salinity of the water is provided as an indicator of total ion concentration (g L-1).

G) Palustrine micritic limestone infilled by a dark secondary micrite associated with gypsum crystals. Simplified chart showing the evaporite precipitation sequence from waters of various compositions. 

Calculations demonstrating the actual limits on changes in the major ion chemistry of seawater imposed by the evaporite precipitation sequence are beyond the scope of this book. However, some simple... 

TABLE 3.3 Precipitation Sequence of Seawater Evaporitic Salts... 

The order of precipitates is the same as that seen in modern marine evaporites and can be reproduced by experimental evaporation of seawater. This sequence of salt precipitation sets limits on the possible changes of major ion compositions in seawater, since changes beyond these limits would have resulted in different sequences of salt formation. 

Evaporative lakes are common in midcontinent regions in Australia, the western United States, southwest Canada, and Africa, for example. Depending upon the extent of evaporation and the amounts and compositions of inflowing waters, the first precipitates are calcite and, perhaps, dolomite, which may be followed by gypsum/anhydrite, then halite, and finally more complex sulfate, carbonate and halide salts (cf. Holland 1978 Berner and Berner 1987 Faure 1991). Similar sequences of mineral precipitates may accumulate in restricted evaporite basins in equatorial areas (Berner and Berner 1987). 

Rock units involved in salt-dissolution studies in western Oklahoma and nearby areas are mainly of early Guadalupian (Permian) age. These strata make up a thick sequence of red beds and evaporites deposited in and near a broad, shallow inland sea that extended north and northeast of the carbonate platform that bordered the Midland Basin (Fig. 1) (Mills, 1942 Clifton, 1944 Ham, 1960 Johnson, 1967). Evaporites, mainly salt (halite) and gypsum (or anhydrite), were precipitated from evaporating seawater as layers on the sea floor or grew as coalescing crystals and nodules in a host of mud just below the depositional surface. Thick red-bed shales, siltstones and sandstones were deposited around the perimeter of the evaporite basin, and some of these also extended as blanket deposits across the basin. Many thin red-bed clastic units are interbedded with the evaporites. 

Thermocatalytic sulfate reduction probably is the main source of H2S in the deep subsurface (25.81). The reactions in Table II, based on the evolution of H2S and CO2 by sulfate reduction of hydrocarbons in clastic sequences where there is available SO42- (i.e. sequences with evaporite beds or cements), illustrate the importance of this process to diagenesis. Not only does it provide a diagenetic sink for Fe via pyrite, but depending on the relative availability of reactants, can either cause precipitation or dissolution of calcite and alter feldspars to clay minerals. 

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. 

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