Giant polygons in the Realmonte Mine (Agrigento, Sicily); evidence for the desiccation of a Messinian halite basin

Abstract

The Messinian Realmonte salt deposit consists of a 400-600 m-thick halite succession that can be divided into four main lithologic units (base to top): A) cumulates of plates settled out from a stratified water column; B) plate cumulates in a shallowing-upward sequence containing six kainite layers near the top of the unit; C) cumulates of skeletal hoppers with chevron overgrowths precipitated from a nonstratified water body; and D) skeletal halite 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 intervals of up to 5 m apart, extend down to a depth of 6 m, and are filled by red mud. The salt beds affected by the fissures are commonly upturned (buckled) and are truncated and onlapped by the succeeding flat-lying halite beds of unit C. Additionally, the upturned layers are cut by vertical dissolution 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, broke into polygonal crusts, and formed tepee structures. The buckling was caused by a net volume increase due to thermal expansion, and what is more important, precipitation of new halite induced by evaporative pumping of brines moving up from the groundwater table. The buckled salt layers were then affected by meteoric dissolution, which created vertical pipes. (2) The tepee structures and the associated zone of dissolution pipes are further crosscut by a new network of large polygons. These new polygonal sets are outlined by deep fissures in which red silt collected, blown by the wind or carried by episodes of surface flooding. This type of deep contraction crack is known to be produced by volumetric changes induced by annual temperature fluctuations. The documentation of contraction polygons and other exposure features represents the first report of desiccation events during the deposition of the Sicilian salt. No geological data are yet available to settle the questions if the salt of Sicily is correlatable with the halite present beneath the floor 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 floor by thrust activity, b) simple evaporitic drawdown, c) basinwide drop of the Mediterranean sea level, or d) complex interaction among these factors.

The Messinian salinity crisis in Cyprus: a further step towards a new stratigraphic framework for Eastern Mediterranean

“…This tectonic activity, that could have been related to an earlier phase of collision of the Cyprus Arc against the Eratosthenes microplate, could have resulted in the tectonic uplift of the Troodos Massif and in the reduction in the paleodepth of the surrounding basins, leading locally to conditions close to emersion, as suggested by the bird foot tracks recognized on top of the clastic evaporites in the Tokhni quarry. The local development within MSC stage 2 deposits of sedimentary features indicating short‐lasting episodes of emersion (as found in the halite of Sicily; Lugli et al ., ; Roveri et al ., ) is likely related to a combination of base‐level drop, tectonic uplift and sedimentation rate of evaporites. It is worth noting that the ephemeral development and preservation of such shallow water environments in an uplifted area of marginal basins appears to be incompatible with a deep desiccation of the Mediterranean.…”

Paleoclimate reconstruction during the Messinian evaporative drawdown of the Mediterranean Basin: Insights from microthermometry on halite fluid inclusions

“…Their halite samples were taken from salt below the desiccation surface identified by Lugli et al . [] (Figure ). The Th measurements by Rigaudier et al .…”

“…[] range between 21 and 42°C, with an average Th of 34 ± 5°C, which are significantly higher than the values we obtained from this work (11.2–22.7°C, with an average value of 15.9°C). There are negligible differences in the halite facies below (Unit B) and above (Unit C) the desiccation surface of the Realmonte mine [ Lugli et al ., ]. In both the halite units, chevron crystals with growth bands, indicating overgrowth at the bottom of a shallow nonstratified water body, are clearly present.…”

“…Even before the onset of the MSC, the sedimentary filling consisted of subaerial clastic deposits, shallow marine sands, patch reefs, and subaerial hiatuses [ Decima and Wezel , ; Keogh and Butler , ; Roveri et al ., ]. Moreover, Lugli and Lowenstein [] suggesting that the Realmonte salt deposit formed by precipitation from a shallow nonstratified water column based on fluid inclusions data in halite crystals from the Realmonte mine.…”

“…In addition, during the evaporative event that brought the salt precipitation stage of the MSC, the Mediterranean Basin experienced a huge drawdown, which led to an almost complete desiccation of the basin, as documented by the exposure surface found within the halite of the Realmonte salt mine [ Lugli et al ., ].…”