Evaporites modern

Brantley, S. L., D. A. Crerar, N.E. Mpllcr and J. H. Weare, 1984, Geochemistry of a modern marine evaporite, Bocana de Virrila, Peru. Journal of Sedimentary Petrology 54,447-462. 

Latitudinal distribution of the world s modern evaporitic sediments as a percentage of total sediment. Source-. From Warren, J. K. (1989). Evaporite Sedimentology, Prentice Flail, Inc., p. 15. 

Table 17.1 Documented Modern Sea-Margin Evaporites. Sabkha...

As noted earlier, modern evaporites are currently forming in only two deposi-tional environments, sabkhat and salinas. These settings are illustrated schematically in Figure 17.4. The locations of specific examples are given in Table 17.1. 

Modern evaporite settings. Source From Kendall, A. q TTT ------ -------------------... 

Timeiine of marine evaporite deposition during the Phanerozoic. Shown are the volumes of NaCI (halite, dark line) and CaS04 (gypsum and anhydrite, dashed line) deposited over time in km . The arrows mark the current volumes of NaCi and CaS04 contained in modern ocean water. These are approximately 1.8 x 10 and 9 x 10 km , respectively. Source After Holser, W. T. (1984). Patterns of Change in Earth Evolution, Springer, pp. 123-143. 

Dolomites compose a significant portion of ancient evaporites. This mineral is essentially a Mg-rich calcite, having variable Mg content. Its empirical formula is commonly given as MgCa(C03)2. Having no modern examples of dolomite precipitation beyond those of a few salinas and sabkhat, geochemists have been unable to determine conclusively how the ancient deposits must have formed. 

The modern Mediterranean Sea has a water volume of 3.7 x lo km. Net evaporation presently occurs at a rate of 3 3 x 10 km /y. If the Straits of Gibraltar were exposed today, the present-day Mediterranean Sea would dry up in about 1000 y. Assuming an average salinity of 37%o and a seasalt density of 2.165 g/cm, the Mediterranean would have had to been refilled and evaporated at least 15 times to accoimt for the volume of salt in the Messinian evaporites (1 x 10 km ). The evaporites on the margins of the Mediterranean Sea tend to be thinner than the deposits in the middle and deepest parts of the basin, which are up to 1600 m in thickness. In some limited areas on the eastern side of the basin, thicknesses up to 3500 m have been observed ... 

Sabkha Intertidal mudflats in which modern-day evaporites form. Most are located in the Middle East, the northwestern coast of Australia and lagoons along the coast of Texas. 

Last W. M. (1994a) Deep-water evaporite mineral formation in lakes of western Canada. In Sedimentology and Geochemistry of Modern and Ancient Saline Lakes (eds. R. W. Renaut and W. M. Last). Society for Sedimentary Geology (SEPM), Tulsa, pp. 51-60. 

Pan/lacustrine silcretes (Figure 4.5B, C) most commonly develop within, or adjacent to, ephemeral lakes, pans or playas within endoreic basins (Goudie, 1973 Summerfield, 1982 Figure 4.10C). In the majority of modern evaporitic lacustrine environments, silica precipitation is driven by changes in pH and salt concentration (Thiry, 1999), both of which can... 

Butler, G.P. (1969) Modern evaporite deposition and geochemistry of coexisting brines, the sabkha, Trucial Coast, Arabian Gulf. Journal of Sedimentary Petrology 39, 70-89. 

Lowenstein, T.K., Spencer, R.J. Zhang, P. (1989) Origin of ancient potash evaporites Clues from the modern nonmarine Qaidam basin of western China. 

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. 

The sulphite (SO2-) is subsequently oxidized to SOI-. Sedimentary pyrite, formed as a byproduct of sulphate reduction in marine sediments, is a major sink for seawater SO -. The presence of pyrite in ancient marine sediments shows that SO4- reduction has occurred for hundreds of millions of years. On a geological timescale, removal of SO4- from seawater by sedimentary pyrite formation is thought to be about equal to that removed by evaporite deposition (Section 6.4.2). Compilations of pyrite abundance and accumulation rates are used to calculate modern SO - removal by this mechanism and to derive the estimate in Table 6.2. 

Modern models of the Phosphoria Basin generally depict a warm, shallow sea. Based on the presence of contemporaneous evaporites and organic carbon levels as high as 32.9%, Stephens and Carroll (1999) developed a model for a system with both salinity stratified waters and upwelling (Fig. 7). Analysis of 6 0p values have shown water temperatures during phosphogenesis to have been temperate (14-26°C) in the deeper zones of the Phosphoria Sea and between 30-37°C in the shallower waters, which... 

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