Evaporites : Sediments, resources and hydrocarbons / Эвапориты: осадки, ресурсы и углеводороды

I define an evaporite as a salt rock that was originally precipitated from a saturated surface or nearsurface brine by hydrologies driven by solar evaporation (Figure 1.1). This simple definition encompasses a wide range of chemically precipitated salts and includes alkali earth carbonates (Table 1.1). Some workers restrict the term evaporite to those salts formed directly via solar evaporation of hypersaline waters at the earth's surface. In order to emphasise the highly reactive nature of evaporites in the sedimentary realm I think of such evaporites as primary, that is, precipitated from a standing body of surface brine and retaining crystallographic evidence of the depositional process (e.g. bottom-nucleated or current-derived textures). Outside of a few Neogene examples, there are few ancient evaporite beds with textures that are wholly and completely “primary” (Figure 1.1).

Almost every subsurface evaporite texture is secondary; it is diagenetically altered, frequently with fabrics indicating per¬ vasive early recrystallisation and perhaps some later crossflow of basinal waters. Under this definition of a primary versus secondary salt, even the nodules in Holocene mudflats, includ¬ ing those in the type area for sabkhas the Arabian (Persian) Gulf, are secondary textures. They are a porewater overprint superimposed on a primary matrix of mud and sand. Ancient bedded evaporites are typically dominated by secondary tex¬ tures, although beds may still retain “ghosts” or partial relicts of primary textures such as laminae, aligned anhydrite nodules after growth-aligned gypsum, or patches of aligned halite chevrons floating in a matrix of mosaic halite spar. Even when evaporite beds are extensively recrystallised, most retain laminae made up of impurities that define primary depositional laminae. <...>