GEOLOGICAL PAPERS. 85 



NOTES ON THE THEORIES OF ORIGIN OF GYPSUM 



DEPOSITS. 



By R. S. Shekwin, University of Oklahoma, Norman, Okla. 

 Read before the Academy, at Ida, December 30, 1901. 



SEVERAL different theories have been given to account for the 

 origin of gypsum beds, but the one which is most generally ac- 

 cepted for the formation of the gypsum of Kansas and Oklahoma is 

 that of evaporation of water in an inland sea. When sea- water is 

 evaporated gypsum is deposited, and if the evaporation is continued 

 a heavier deposit of salt is obtained. 



The substance of the argument in favor of this theory is that the 

 gypsum is of^en more or less closely associated with salt, and that the 

 composition in many cases is similar to that of the material obtained 

 by evaporation of sea-water. 



Some idea of the amount of water which must have been evapo- 

 rated during later Permian times iu Kansas and Oklahoma, according 

 to this theory, can be obtained by calculations based on the composi- 

 tion of sea- water. It is improbable that sea- water ever contained a 

 larger percentage of gypsum than it does at present ; so the estimates 

 of the amounts of water are not likely to be too high. 



According to the most reliable analyses,^ sea- water contains 3.5 per 

 cent, of mineral matter, of which 3.6 per cent, is calcium sulphate. 

 Ordinary rock gypsum contains about 21 per cent, of water of crys- 

 tallization, as expressed by the formula CaS04 + 2H20. Taking this 

 into account, the amount of gypsum contained in sea-water is about 

 0.16 per cent. The specific gravity is given"-^ at from 2.28 to 2.31. 

 Using the lower figures, the thickness of the layer of pure rock gyp- 

 sura which could be deposited from a body of water one foot in depth 

 is a little less than .0007 foot. It is stated that the process must 

 have been continuous in Kansas while at least eight or nine feet of 

 rock gyysum was being formed. Taking the lower estimate, the 

 amount of water which must have been concentrated during the 

 period of continuous deposits of gypsum was the equivalent of a sea 

 or lake more than 11,000 feet deep, and covering the area over which 

 the deposits were made. 



As it is unlikely that an inland sea with a depth of two miles was 

 cut off' and evaporated sufficiently to deposit all of its gypsum, some 

 hypothesis must be brought forward to show how this enormous 

 amount of water could have been supplied. It is possible to conceive 



1. Challenger Reports, Encycl. Britt., article "Sea."' 



2. Univ. Geol. Surv, of Kan., vol. V, pp. 22, 80. 



