Principles of Oceanic Salt Deposition and Metamorphism

Abstract

Oceanic salt deposits result from evaporation processes in epicontinental sea basins which lack appreciable drainage either into or out of them. Optimal evaporation occurs in arid regions. Deposits are found generally after orogenic periods in the foreland or backland of strongly folded and faulted geosynclinal areas of germano-type tectonics.[1][1] Most of the evaporites discussed in this paper are from the upper Zechstein rocks of Germany. Primary crystallization of these deposits proceeds under dynamic-polythermal conditions, with successive precipitates of dolomite, gypsum, halite, some epsomite, and carnallite, the most important primary minerals. Secondary variations in lateral mineral successions can be observed within distances of meters, and are connected with metasomatic replacement processes. Because of retrograde metamorphism, potassium-magnesium-sodium sulfates are formed in equilibrium with the high-sulfate content of the metamorphic solutions. This is in strong contrast to the sulfate deficiency in ordinary brines. The initially dilute solutions of retrograde metamorphism may be of foreign origin. However, the frequency of secondary replacement processes, especially in the potash beds, is essentially explained by the fact that most accompanying anhydrite beds have been deposited as gypsum. Thus, by geothermal heating, about 0.5 cubic meters of dilute CaSO4 solutions for each cubic meter of gypsum are set free. The salt deposits show a rhythm of true annual varves, even reflecting the 11-year solar period. The level of the ocean surface was the constant base level during deposition of the entire series of Zechstein 1 to 4. Communication channels of the English-German and Russian basins were restricted to, but not completely interrupted with the northern “Scandic” ocean, even during the stages of potassium salt formation. In anorogenic times and in regions with germano-type tectonics, sinking of the earth's crust is likely to occur at a rate of 0.1 to 1 mm per year. Thus, sinking of the sea bottom could not have been rapid enough to allow the contemporaneous deposition of several hundred meters of salts. “Descendent” processes and desert salt-pan conditions are not important for the successions of Zechstein deposits 1 to 4. [1]: #fn-1