Giant polygons in the Realmonte mine (Agrigento, Sicily): Evidence for thedesiccation of a Messinian halite basin

The Messinian Realmonte salt deposit consists of a 400-600 m-thick halite s uccession that can be divided into four main lithologic units Chase to tap) : A) cumulates of plates settled out from a stratified water column; B) pla te cumulates in a shallowing upward sequence containing six kainite layers near the tap of the unit; C) cumulates of skeletal hoppers with chevron ove rgrowths precipitated from a nonstratified water body; and D) skeletal hali te and anhydrite. Spectacular vertical fissures cut through the upper part of unit B at its boundary with salt unit C, These fissures are spaced at in tervals of up to 5 m apart, extend down to a depth of 6 m, and are filled b y red mud. The salt beds affected by the fissures are commonly upturned buc kled) and are truncated and onlapped by the succeeding hat-lying halite bed s of unit C, Additionally, the upturned layers are cut by vertical dissolut ion pipes that extend down to as much as a depth of 4 m below the fissured surface. To explain the observed superimposed structures we propose two mechanisms, both induced by the desiccation of the evaporite basin: (1) When the salt surface was exposed, the halite layers were buckled, brok e into polygonal crusts, and formed tepee structures. The buckling was caus ed by a net volume increase due to thermal expansion, and what is more impo rtant, precipitation of new halite induced by evaporative pumping of brines moving up from the groundwater table. The buckled salt layers were then af fected by meteoric dissolution, which created vertical pipes, (2) The tepee structures and the associated zone of dissolution pipes are f urther crosscut by a new network of large polygons. These new polygonal set s are outlined by deep fissures in which red silt collected, blown by the w ind or carried by episodes of surface flooding. This type of deep contracti on crack is known to be produced by volumetric changes induced by annual te mperature fluctuations, The documentation of contraction polygons and other exposure features repre sents the first report of desiccation events during the deposition of the S icilian salt. No geological data are yet available to settle the questions if the salt of Sicily is correlatable with the halite present beneath the d oor of the Mediterranean and if the desiccation of the Realmonte salt basin could have been induced by: a) local tectonic causes (uplift of the basin door by thrust activity, b) simple evaporitic drawdown, c) basinwide drop o f the Mediterranean sea level, or d) complex interaction among these factor s.