EVAPORITES AND BRINES 



211 



Warm and arid coastal regions today are mostly in 

 the zones of trade winds (zones of nearly constant 

 wind direction) that lie between about 10° and 35°, 

 both north and south of the equator. The arid coasts 

 are where the trade winds blow from the land to- 

 ward the sea. At least since Precambrian time, 

 trade-wind belts probably have had about the same 

 size and global distribution; they are products of 

 the interaction between solar energy and the spin- 

 ning earth and its atmosphere, and it is reasonable 

 to postulate that the output of solar energy, the 

 earth's spin and the physical properties of the 

 atmosphere have been nearly constant during that 

 period. These 25°-wide trade-wind belts make up 

 about 40 percent of the global surface. If one as- 

 sumes that two landmasses lay astride these belts, 

 each with a 5°-wide (about 300 miles) evaporite- 

 forming basin along the entire shoreline that is in 

 the trade-wind belt, about 1 percent of the globe 

 would be accumulating evaporitss. If, in the geo- 

 logic past, the number of continental landmasses 

 astride the trade-wind belts averaged two, and if 

 the configurations of the coastlines changed every 

 10 million years sufficiently for new portions of the 

 earth's surface to begin accumulating evaporites, 

 there could have been as many as 60 different accu- 

 mulations of this size in the 600 million years since 

 the close of Precambrian time. This means that 

 about 45 percent of the globe could have at one time 

 been the site of evaporite deposition. 



This maximum possible area does not seem to be 

 large enough. Considering the near absence of 

 present-day evaporite-forming basins, it seem likely 

 that on the average, far less than half of the cli- 

 matically favorable sites in the past were likely to 

 be occupied by basins that produced evaporites. 

 Furthermore, very probably at least half of once- 

 formed evaporites have been eroded or dissolved. 

 Finally, one must consider that the area of known 

 evaporites would substantially exceed 8 percent of 

 the earth's surface if none of the evaporite deposits 

 overlapped. Conceivably, other environments than 

 those considered in this model have produced major 

 quantities of marine evaporites, but available geo- 

 logic evidence seems more consistent with this model 

 than any other. For these reasons, the numbers ussd 

 in the above model calculation probably could be 

 changed substantially and still not account for the 

 known areas of marine evaporite deposits. Numbers 

 and assumptions that would lead to the conclusion 

 that areas of undiscovered evaporites are large 

 would require even more drastic departures from 

 what seems geologically reasonable. 



The only alternative assumption that seems to be 



reasonable for this calculation is that the areas of 

 accumulating evaporites were commonly much 

 larger than postulated because in some respects, 

 past environments differed intrinsically from those 

 of the present. It is difficulty to envision larger areas 

 of global aridity caused by wider trade-wind belts, 

 but very possibly areas of inland seas were much 

 larger than now known. Available evidence indi- 

 cates that many of the world's present continental 

 areas were originally part of a single landmass and 

 that the present continents evolved by the slow 

 spreading of individual masses away from this 

 center. As they spread, nearly landlocked inland 

 seas may have evolved to become sites of evaporite 

 deposition that were much larger than any now 

 known. Such a phenomenon could explain the ap- 

 parent overabundance of marine evaporite bodies, 

 but still does not necessarily imply that a large 

 number of evaporite bodies are undiscovered. 



If one accepts the above lines of reasoning, it 

 follows that the likelihood of finding many large 

 new deposits of marine evaporites is small. This 

 means that estimates of resources in known de- 

 posits can be considered nearly equivalent to ulti- 

 mate resources. An estimate of these figures follows. 

 If 25 percent of the world's 57 million square miles 

 of continental area is underlain by evaporite de- 

 posits, and the average deposit contains 300 feet 

 of evaporites, 0.8 million cubic miles of evaporites 

 exist. (The average of the maximum thickness of 

 deposits listed in table 42 is 750 feet; the deposits 

 are estimated to contain 20 percent impurity and 

 to have average thicknesses that are half their maxi- 

 mum thickness.) An independent estimate of this 

 volume by Ronov (1968, fig. 3) concludes that ap- 

 proximately 0.9 million cubic miles of evaporites 

 are in the earth's sedimentary shell, with about 70 

 percent of this volume in relatively near-surface 

 deposits formed in platform environments and the 

 remainder formed in geosynclinal environments. 

 Most evaporites consist of gypsum, anhydrite, or 

 halite; thus, a volume of 0.8 million cubic miles 

 means that about 10" tons of these components 

 exist. 



An estimate by us (based on well-explored por- 

 tions of U.S. deposits) suggests that about 0.002 

 percent of an average marine deposit consists of 

 potassium mineral concentrations, although this has 

 to be a very approximate number. Applying this 

 figure to the 0.8 million cubic miles of evaporites in 

 the world suggests that about 160 billion tons of 

 KCl containing 100 billion tons of K,0 is concen- 

 trated in such bodies. This is very close to the 

 amount inferred to exist in known deposits and 



