## 2 Study Area  
The study area is located in south-western Sicily, between the small towns of Siculiana and Montalegro (Figure 1). It geologically corresponds (Figure 2a; [37]) to the south to southeast-vergent Sicilian Fold and Thrust Belt (FTB, [38]) bordered by the Gela thrust front and Kabilian-Calabrian thrust front. The FTB is an element of the collisional complex of Sicily, which also includes a late Pliocene-Quaternary foredeep (Gela foredeep), onlapping the frontal sector of the thrust belt in the southern part of the island, its offshore area, and the Pelagian-Iblean foreland with its African crust. In particular, the southern and central parts of the FTB (Figure 2b), are characterised by the outcropping of the most complete evaporitic succession of the \"Gessoso-Solfifera\" formation of the Messinian age [39]. The evaporitic deposits (Figure 3) mainly consist of limestone, gypsum, salt, and numerous intercalations of clays, marls, and carbonates. Gypsum strata are composed of various kinds of selenite (branching, banded, and massive) and detrital gypsum layers alternating with marl and carbonate. The Messinian units overlie diatomic laminates (Tripoli formation [Fm. ]: TRP) and clay deposits of late Tortonian to Early Messinian age, and the succession is overlain by Pliocene marly calcilutites (Trubi Fm. : TRB). The whole sequence [41] is locally overlain by middle-upper Pliocene marly clays (Monte Narbone Fm. : NRB). Sandy clays and arenites (Montallegro Fm. : MNT), clays breccias (BRC), and turbiditic calcarenites (Agrigento Fm. : GRG) are the more recent Pleistocene units. Figure 1: Geographic setting and general map of the study area. In greater detail, a first and a second sedimentary cycle [42] are distinguished in the Messinian evaporites, which are separated by an angular unconformity. The first cycle comprises [43]: carbonate deposits (Calcare di Base Fm. : BEC), massive selenite (Cattolica Fm. : CTL), and a salt unit (Clastic Ca-sulfates, Mg and K salts, and halite unit: SLT). The younger second cycle is mainly characterised by thin gypsum layers (balatino and selenite: GPQ) and marks layers (upper evaporites, Pasquasia Fm. : GPQ) levels interbedded with detrital mud, silt, sandstones, and conglomerates overlain by siliciclastic sediments (mainly silty clays: Arenazzolo Fm. : RNZ), characterised by marked organic matter levels. The complexity of the geological setting of the area is increased to be a consequence of the Plio-Pleistocene tectonic phases that generated the fold and thrust belts and high-angle faults with subsequent lateral contacts between the Messinian gypsum units and the older clayey deposits. In addition, the wide distribution of soluble rocks in this sector of Sicily is responsible for intense karstic processes, producing a great variety of either epigeous and hypogeous landforms [41]. Figure 2: **(a)** Mt. Etna edifice and main elements of the collisional complex of Sicily: Kabylian-Calabrian thrust basement—KCB; fold and thrust belt—FTB; Gela foredeep—GF; Pelagian-Iblean foreland—PIF; **(b)** structural map of Sicily (modified from [40]) highlighting the spatial distribution of the evaporite outcrops in Sicily (after [39]): (1) Plio-Pleistocene cover, (2) volcanics, (3) evaporites (Messinian), (4) upper Miocene-lower Pliocene deformed foreland deposits, (5) upper Oligocene-lower Miocene deformed foreland deposits, (6) Kabilian-Calabrian crystalline units, (7) Sicilide units, (8) Meso-Cenozoic carbonate platform deformed units (Sicilian-Maghrebian shallow-water units), (9) Meso-Cenozoic slope-to-deep-basin deformed units (Sicilian-Maghrebian deep-water units), and (10) Meso-Cenozoic carbonate platform not-deformed units (Sicilian-Maghrebian foreland). From a morpho-structural standpoint, the landscape is characterised by a system of gentle anticlines and synclines extending parallel to the NW-SE trend of the FTB and large blocks of evaporitic rocks encircled by clayey deposits, with frequent relief inversion produced by differential erosion phenomena [39].