Secondary halite deposits in the Iranian salt karst: General description and origin

Abstract

This paper summaries 12 years of documentation of secondary halite deposits in the Iranian salt karst. A variety of secondary halite deposits was distinguished and classifed into several groups, on the basis of the site and mechanism of their origin. Deposits formed: i) via crystallization in/on streams and pools, ii) from dripping, splashing and aerosol water, iii) from evaporation of seepage and capillary water, and iv) other types of deposits. The following examples of halite forms were distinguished in each of the above mentioned group: i) euhedral crystals, foating rafts (raft cones), thin brine surface crusts and flms; ii) straw stalactites, macrocrystalline skeletal and hyaline deposits, aerosol deposits; iii) microcrystalline forms (crusts, stalactites and stalagmites, helictites); iv) macrocrystalline helictites, halite bottom fbers and spiders, crystals in fuvial sediments, euhedral halite crystals in rock salt, combined or transient forms and biologically induced deposits. The occurrence of particular forms depends strongly on the environment, especially on the type of brine occurrence (pool, drip, splashing brine, microscopic capillary brine, etc.), fow rate and its variation, atmospheric humidity, evaporation rate and, in some cases, on the air fow direction. Combined or transitional secondary deposits can be observed if the conditions changed during the deposition. Euhedral halite crystals originate solely below the brine surface of supersaturated streams and lakes. Macrocrystalline skeletal deposits occur at places with rich irregular dripping and splashing (i.e., waterfalls, places with strong dripping from the cave ceilings, etc.). Microcrystalline (fne grained) deposits are generated by evaporation of capillary brine at places where brine is not present in a macroscopically visible form. Straw stalactites form at places where dripping is concentrated in small spots and is frequent suffcient to assure that the tip of the stalactite will not be overgrown by halite precipitates. If the tip is blocked by halite precipitates, the brine remaining in the straw will seep through the walls and helictites start to grow in some places. Macrocrystalline skeletal deposits and straw stalactites usually grow after a major rain event when dripping is strong, while microcrystalline speleothems are formed continuously during much longer periods and ultimately (usually) overgrow the other types of speleothems during dry periods. The rate of secondary halite deposition is much faster compared to the carbonate karst. Some forms increase more than 0.5 m during the frst year after a strong rain event; however, the age of speleothems is diffcult to estimate, as they are often combinations of segments of various ages and growth periods alternate with long intervals of inactivity. Described forms may be considered in many cases as the analogues of forms found in the carbonate karst. As they are created in a short time period the conditions of their origin are often still visible or can be reconstructed. The described halite forms can thus be used for verifcation of the origin of various carbonate forms. Some of the described forms bear clear evidence of the paleo-water surface level (transition of the skeletal form to halite crystals and vice versa). Other kinds of deposits are potential indicators of the microclimate under which they developed (humidity close to the deliquescence relative humidity).