This paper, the first of three reviews on the evaporite-base-metal association, defines the characteristic features of evaporites in surface and subsurface settings. An evaporite is a rock that was originally precipitated from a saturated surface or near-surface brine in hydrological systems driven by solar evaporation. Evaporite minerals, especially the sulfates such as anhydrite and gypsum, are commonly found near base-metal deposits. Primary evaporites are defined as those salts formed directly via solar evaporation of hypersaline waters at the earth's surface. They include beds of evaporitic carbonates (laminites, pisolites, tepees, stromatolites and other organic rich sediment), bottom nucleated salts (e.g. chevron halite and swallow-tail gypsum crusts), and mechanically reworked salts (such as rafts, cumulates, cross-bedded gypsarenites, turbidites, gypsolites and halolites). Secondary evaporites encompass the diagenetically altered evaporite salts, such as sabkha anhydrites, syndepositional halite and gypsum karst, anhydritic gypsum ghosts, and more enigmatic burial associations such as mosaic halite and limpid dolomite, and nodular anhydrite formed during deep burial. The latter group, the burial salts, were precipitated under the higher temperatures of burial and form subsurface cements and replacements often in a non-evaporite matrix. Typically they formed from subsurface brines derived by dissolution of an adjacent evaporitic bed. Because of their proximity to 'true' evaporite beds, most authors consider them a form of 'true' evaporite. Under the classification of this paper they are a burial form of secondary evaporites. Tertiary evaporites form in the subsurface from saturated brines created by partial bed dissolution during re-entry into the zone of active phreatic circulation. The process is often driven by basin uplift and erosion. They include fibrous halite and gypsum often in shale hosts, as well as alabastrine gypsum and porphyroblastic gypsum crystals in an anhydritic host. In addition to these 'true' evaporites, there is another group of salts composed of CaSO4 or halite. These are the hydrothermal salts. Hydrothermal salts, especially hydrothermal anhydrite, form by the subsurface cooling or mixing of CaSO4- saturated hydrothermal waters or by the ejection of hot hydrothermal water into a standing body of seawater or brine. Hydrothermal salts are poorly studied but often intimately intermixed with sulfides in areas of base-metal accumulations such as the Kuroko ores in Japan or the exhalative brine deeps in the Red Sea. In ancient sediments and metasediments, especially in hydrothermally influenced active rifts and compressional belts, the distinction of this group of salts from 'true' evaporites is difficult and at times impossible. After a discussion of hydrologies and 'the evaporite that was' in the second review, modes and associations of the hydrothermal salts will be discussed more fully in the third review
Permian bedded salt is widespread in the Anadarko Basin of western Oklahoma and the Texas Panhandle, where partial or total dissolution of the shallowest salt in some areas has resulted in subsidence and/or collapse of overlying strata. Groundwater has locally dissolved these salts at depths of 10–250 m. The distribution (presence or absence) of salt-bearing units, typically 80–150 m thick, is confirmed by interpretation of geophysical logs of many petroleum tests and a few scattered cores. Salt dissolution by ground water is referred to as “salt karst.”Chaotic structures, collapse features, breccia pipes, and other evidence of disturbed bedding are present in Permian, Cretaceous, and Tertiary strata that overly areas of salt karst. The dip of Permian and post-Permian strata in the region normally is less than one degree, mainly towards the axis of the Anadarko Basin. Where strata locally dip in various directions at angles of 5–25 degrees or more, and underlying salt units show clear evidence of dissolution, these chaotic dips must result (mostly, if not totally) from subsidence and collapse into underlying salt-dissolution cavities.Gypsum karst and resultant collapse of overlying strata have been proposed in many parts of the Anadarko Basin. However, the gypsum beds typically are only 1–6 m thick and more than 100 m deep, and cannot contribute to disruption of outcropping strata—except where they are within 10–20 m of the surface.Typical areas of disturbed bedding comprise several hectares, or more, with outcrops of moderately dipping strata—as though large blocks of rock have foundered and subsided into large underground cavities. Other examples of disturbed bedding are small-diameter breccia pipes, or chimneys, that extend vertically up from salt-karst cavities, through several hundred meters of overlying strata. The best evidence of these chimneys are collapsed blocks of Cretaceous strata, chaotically dropped some 50 m, or more, that are now juxtaposed against various Permian formations on the north flank of the Anadarko Basin. Any study of surface or shallow-subsurface geology in the Anadarko Basin must consider the influence of subsurface salt karst on the structure and distribution of overlying rocks
