TECTONICS AND SEQUENCE STRATIGRAPHY IN MESSINIAN BASINS, SICILY - CONSTRAINTS ON THE INITIATION AND TERMINATION OF THE MEDITERRANEAN SALINITY CRISIS

Deposits within the Caltanissetta basin of central Sicily have been im portant for developing the desiccating deep basin model for the Messin ian salinity crisis in the Mediterranean. Linked structural and strati graphic studies show that the Sicilian depocenters are synclines relat ed to underlying thrust structures of the frontal part of the Maghrebi an chain, Prior to the salinity crisis these basins were hydrodynamica lly linked through the foredeep to the Mediterranean, The precursor se diments (Terravecchia Formation) formed a delta, sourced from the nort h, Early Messinian regression acted on a range of paleobathymetries, s hallow in the north and progressively deeper in the south, locally com plicated by active thrust structures, Initial draw-down of Mediterrane an base level is marked across the thrust belt by first cycle carbonat es and evaporites on the structural highs and lows, respectively. Vast accumulations of halite and potassium salts (up to 1200 m) are restri cted to growing thrust synclines. The different evaporite signatures m ay be related to different water conditions reflecting various meteori c and marine circulations across various subbasins, with cyclicity ref lecting high-frequency variations in sealevel. A sequence stratigraphi c model is developed to explain fractionation of evaporite facies betw een different paleobathymetric settings and to predict the temporal ev olution of the successions in different subbasins. Thrusting provides accommodation space for evaporites and also controls the water pathway s into the desiccating basins. The intra-Messinian unconformity separa ting first and second cycle evaporites is a ''type 1 sequence boundary '' related to the forced regression associated with the acme of Medite rranean desiccation, an interpretation supported by local ravinement a nd incised valley fills. The overlying second cycle evaporites are a c ombination of detrital, reworked first cycle material and primary gyps um formed under brackish water. Regional onlap relationships and bed c ontinuity suggest that this water body was of regional extent with a s ystematically rising base level. These interpretations have several im plications for Messinian correlations in the Mediterranean: The deep b asin evaporites correlate with an intra-Messinian unconformity that se parates the two cycles on Sicily and postdate the first cycle deposits ; regional base level was largely restored before the end of the salin ity crisis.