The Messinian Realmonte salt deposit consists of a 400–600 m-thick halite succession that can be divided into four main lithologicunits (base to top): A) cumulates of plates settled out from astratified water column; B) plate cumulates in a shallowing-upwardsequence containing six kainite layers near the top of the unit; C) cumulatesof skeletal hoppers with chevron overgrowths precipitatedfrom a nonstratified water body; and D) skeletal halite and anhydrite.Spectacular vertical fissures cut through the upper part of unit B atits boundary with salt unit C. These fissures are spaced at intervals ofup to 5 m apart, extend down to a depth of 6 m, and are filled by redmud. The salt beds affected by the fissures are commonly upturned(buckled) and are truncated and onlapped by the succeeding flat-lyinghalite beds of unit C. Additionally, the upturned layers are cut byvertical dissolution pipes that extend down to as much as a depth of 4m below the fissured surface.To explain the observed superimposed structures we propose twomechanisms, 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. Thebuckling was caused by a net volume increase due to thermal expansion,and what is more important, precipitation of new halite inducedby evaporative pumping of brines moving up from the groundwatertable. The buckled salt layers were then affected by meteoric dissolution,which created vertical pipes.(2) The tepee structures and the associated zone of dissolution pipesare further crosscut by a new network of large polygons. These newpolygonal sets are outlined by deep fissures in which red silt collected,blown by the wind or carried by episodes of surface flooding. This typeof deep contraction crack is known to be produced by volumetricchanges induced by annual temperature fluctuations.The documentation of contraction polygons and other exposure featuresrepresents the first report of desiccation events during the depositionof the Sicilian salt. No geological data are yet available to settlethe questions if the salt of Sicily is correlatable with the halite presentbeneath the floor of the Mediterranean and if the desiccation of theRealmonte salt basin could have been induced by: a) local tectoniccauses (uplift of the basin floor by thrust activity, b) simple evaporiticdrawdown, c) basinwide drop of the Mediterranean sea level, or d)complex interaction among these factors.
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|Anno di pubblicazione:||1999|
|Titolo:||Giant polygons in the Realmonte mine (Agrigento, Sicily): evidence for the desiccation of a Messinian halite basin|
|Autori:||S. LUGLI; SCHREIBER B. C.; TRIBERTI B.|
|Appare nelle tipologie:||Articolo su rivista|
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