Salt Dissolution and Pointers to Vanished Evaporites: Karst, Breccia, Nodules and Cement

Abstract

Given enough time all subsurface evaporites eventually dissolve. Even in the subsurface sedimentary realm there are probably more intervals of dissolution residues than there are beds of salts. The rate of evaporite dissolution changes with temperature and rate and volume of crossflowing undersaturated pore waters (Fig. 7.1) Solubility of the chloride salts consistently increases with increasing temperature (prograde solubility), with the sulphates and the sodium carbonates it initially increases, but can then decrease at higher temperatures (retrograde solubility) Dissolution typically begins in the shallow subsurface as the edges of salt beds that are flushed by meteoric or marine waters and continues deeper in the subsurface, wherever and whenever bed edges are flushed by undersaturated basinal brines. Partial dissolution, whereby crystals of more saline salts (typically halite and bitterns) are flushed, leaves behind residues of the less saline salts (typically gypsum-anhydrite, as in many caprocks).