Depth distribution and chemistry of salts as factors controlling tafoni and honeycombs development

Abstract

Salts play a pivotal role in the processes forming tafoni and honeycombs but only few studies have focused on their areal and depth distribution. An X-ray diffraction, rock leachates chemistry, and evaporation front depth were combined to get a deeper insight into factors controlling salt composition and distribution. Five sites representing various lithological and/or climate conditions were studied. The data show that salt composition is a result of the interaction between chemical compositions of rainwater and lithology. Gypsum was found to be a major salt on all sites with halite dominating in arid sites. On humid sites the high relative air humidity prevents its precipitation. Gypsum and epsomite dominate also on sites where the rock contains a considerable amount of pyrite. Alum-(K) occurs in quartz sandstones when affected by acid rain. Some ions probably occur in residual brines, rather than precipitated salts (CaCl in arid or NaCl in humid sites). On arid sites, the salt content in the tafoni backwalls is 4–20 times higher than in outer walls and rapidly decreases with depth which is consistent with shallow evaporation front in backwalls. The low salt content in the outer walls reflects the dilution effect of surface runoff after rains and the infiltration of water through the outer walls toward depth. In two humid sites, on the contrary, no differences were found between the salt content of the backwalls and the outer walls. At one site the data shows that the whole surface of tafoni serves as the salt precipitation zone and tafoni are thus recently degrading. The honeycomb site differs from all tafoni sites since the honeycombs are mostly protected from rain runoff, which prevents the washing-out of salts even from the outer walls.