A record of the Messinian salinity crisis in the eastern Ionian tectonically active domain (Greece, eastern Mediterranean)

This integrated study (field observations, micropalaeontology, magnetostratigraphy, geochemistry, borehole data and seismic profiles) of the Messinian–Zanclean deposits on Zakynthos Island (Ionian Sea) focuses on the sedimentary succession recording the pre‐evaporitic phase of the Messinian salinity crisis (MSC) through the re‐establishment of the marine conditions in a transitional area between the eastern and the western Mediterranean. Two intervals are distinguished through the palaeoenvironmental reconstruction of the pre‐evaporitic Messinian in Kalamaki: (a) 6.45–6.122 Ma and (b) 6.122–5.97 Ma. Both the planktonic foraminifer and the fish assemblages indicate a cooling phase punctuated by hypersalinity episodes at around 6.05 Ma. Two evaporite units are recognized and associated with the tectonic evolution of the Kalamaki–Argassi area. The Primary Lower Gypsum (PLG) unit was deposited during the first MSC stage (5.971–5.60 Ma) in late‐Messinian marginal basins within the pre‐Apulian foreland basin and in the wedge‐top (<300 m) developed over the Ionian zone. During the second MSC stage (5.60–5.55 Ma), the PLG evaporites were deeply eroded in the forebulge–backbulge and the wedge‐top areas, and supplied the foreland basin's depocentre with gypsum turbidites assigned to the Resedimented Lower Gypsum (RLG) unit. In this study, we propose a simple model for the Neogene–Pliocene continental foreland‐directed migration of the Hellenide thrusting, which explains the palaeogeography of the Zakynthos basin. The diapiric movements of the Ionian Triassic evaporites regulated the configuration and the overall subsidence of the foreland basin and, therefore, the MSC expression in this area.


INTRODUCTION
The Messinian salinity crisis (MSC; Selli, 1954), the greatest Mediterranean palaeoenvironmental perturbation, inspired researchers for more than fifty years and still offers a fascinating story tale. Despite the fact that certain aspects were investigated to great extent, major questions remain open (Roveri et al., 2014a). The scope of the present study is to describe, in detail, the Messinian-Zanclean deposits and their inclusive fauna on Zakynthos Island (Ionian Sea), a pivotal area located at the transition between the eastern and the western Mediterranean. The MSC-associated environmental changes are revealed through high-resolution integrated stratigraphy. Furthermore, a tectonosedimentary evolution model is proposed, which incorporates new palaeontologic, sedimentologic, biostratigraphic, palaeomagnetic and geochemical data.

THE MESSINIAN SALINITY CRISIS
Since the beginning of the Messinian (7.251 Ma), the gradual restriction of the Mediterranean-Atlantic marine connection was combined with long-term orbital forcing (Hilgen et al., 2007) to significantly alter the palaeoenvironmental conditions in the entire basin, through: (a) a reduction of deep-water ventilation at 7.15 Ma, which is recorded by diatom-rich and opal-rich sediment deposition between 7.15 and 6.7 Ma, (b) a sudden drop in calcareous plankton diversity at 6.7 Ma, (c) the intensification of bottom-water stagnation and stratification, (d) carbonate precipitation between 6.3 and 5.97 Ma and (e) the complete disappearance of planktonic foraminifers during summer insolation minima (Sierro et al., 1999(Sierro et al., , 2003Kouwenhoven et al., 1999Kouwenhoven et al., , 2003Bellanca et al., 2001;Blanc-Valleron et al., 2002;Kouwenhoven & van der Zwaan, 2006;van Assen et al., 2006;Manzi et al., 2007Manzi et al., , 2011Manzi et al., , 2013Gennari et al., 2013).

GEOLOGICAL SETTING
Western Greece is dominated by the external zones of the Hellenide fold-and-thrust belt, namely the pre-Apulian (or Paxos), the Ionian and the Gavrovo zones ( Fig. 1A and inset map of Fig. 2). From the Triassic to the Upper Cretaceous, western Greece was part of the Apulian continental block on the southern passive margin of the Tethys. At a regional scale (9100 km) the alpine belt can be considered as the margin of the Tethys Ocean, inverted by the collision of Apulia with Europe. On a smaller scale (910 km), in the Hellenic realm, the subbasins of the Hellenic Tethyan margin were inverted to produce the main Hellenic thrust sheets or folded zones. This occurred progressively from the innermost (eastern) to the more external (western) zones (Karakitsios, 1995(Karakitsios, , 2013. The pre-Apulian zone consists of Triassic-Lower Jurassic evaporites, followed by Middle Jurassic to Oligocene deposits, mainly neritic-pelagic carbonates overlain by Neogene and Quaternary marly limestones, marls and sands intercalated by late Miocene evaporitic beds (Karakitsios & Rigakis, 2007;Karakitsios, 2013). The Ionian zone comprises sedimentary rocks ranging from Triassic evaporites to Jurassic-late Eocene carbonates and minor cherts and shales, which are overlain by an Oligocene flysch (Karakitsios, 1995(Karakitsios, , 2013. The thrust boundary between the Ionian and pre-Apulian zones is marked by intrusive evaporites (Fig. 1A). This suggests that contraction deformation was the most important structural control on orogenesis in western Greece. Although halokinesis was important along the boundary faults during the Mesozoic extension, thrusting has overprinted the Mesozoic extensional structures to such an extent that the latter are almost impossible to distinguish (Karakitsios & Rigakis, 2007;Karakitsios, 2013). Field observations of the relationship between the pre-Apulian and Ionian zones emphasize the close association between Hellenide thrusts and folds and areas of evaporite exposure (evaporite dissolution-collapse breccias and gypsum; Karakitsios & Pomoni-Papaioannou, 1998;Pomoni-Papaioannou et al., 2004), even where the precise location of the thrust is unclear. Evaporites crop out along the leading edges of thrust sheets as well as in the onshore geological section and the offshore seismic profiles (Fig. 1). This location, together with their occurrence in tectonic windows above deformed flysch (observed in many places of Akarnania), . Zakynthos onshore and south Zakynthos offshore sections: a) Google Earth map with the location of the sections, b) sections symbols, c) onshore Zakynthos section ZS, d) interpreted offshore seismic profile Z-207, and e) interpreted offshore seismic profile Z-141. Both seismic profiles (d & e) extend through the Ionian (Ion) and the pre-Apulian (P-A) zones (after Kokkalas et al., 2013, modified). suggests that the evaporites represent the lowest detachment level of individual overthrust sheets in the external Hellenides. Furthermore, the absence of pre-evaporitic units from outcrops in western Greece, the great thickness of the evaporites (greater than 3 km in boreholes in the Ionian zone: IGRS-IFP, 1966;BP, 1971), and the Geological map of Zakynthos Island. The regional location is indicated in the inset map. Boreholes: Z1 (Zakynthos-1); Z2 (Zakynthos-2), Z3 (Zakynthos-3); LA1 (Laganas-1); KB101 (Keri-well), AK1 (Agios Kyrikos-1), KY1 (Kypseli-1). probable incorporation of Permian basement into the thin-skinned orogenic wedge east of the Pindos thrust all support the idea that the evaporites form a moderate to major d ecollement level throughout the external Hellenides, rather than widespread diapirism (Underhill, 1989;Karakitsios, 1995Karakitsios, , 2013. Thus, the role of the evaporites resembles that in the thin-skinned thrust belts of Western Europe (Karakitsios, 2013).
The Zakynthos Island's alpine sequence belongs (Fig. 2) to the pre-Apulian and partly to the Ionian zone separated by the Ionian thrustwhose emplacement took place during the early Pliocene (BP, 1971;Sorel, 1976;Nikolaou, 1986;Underhill, 1989;Karakitsios & Rigakis, 2007). The pre-Apulian zone of Zakynthos comprises Upper Cretaceous to Pleistocene sediments, whereas the Ionian zone is represented by Triassic breccias and microcrystalline gypsum corresponding to the lower stratigraphic unit of this zone. Cretaceous to Oligocene carbonates outcrop on Marathonisi Islet (2.5 km east of Keri; Fig. 2) and probably represent a transitional facies between the Ionian and pre-Apulian domains (Nikolaou, 1986). Numerous reverse faults cut the pre-Apulian sequence along the NW-SE direction, exposing late Miocene sandstones and marls along the coastal sections between Agios Sostis and Keri (Fig. 2); these are significantly folded, indicating a post-Miocene age for the Ionian thrust emplacement (Underhill, 1989;Karakitsios, 2013).
The Ionian sequence is unconformably overlain by Neogene and Quaternary deposits outcropping mainly in south-eastern Zakynthos. These are similar to those of the pre-Apulian zone, but thinner and characterized by unconformities (Dermitzakis, 1977;Nikolaou, 1986), and they consist of early Miocene clastic sediments outcropping in the Skopos area, followed by late-Messinian marls and shales (30-100 m thick) intercalated with gypsum (Fabricius et al., 1998). The early Pliocene deposits include calcareous pelagic marls (Trubi limestone) and sandstone intercalations (Kontopoulos et al., 1997). The late Pliocene to Pleistocene deposits comprise a transgressive clastic sequence recording sea-level changes concurrent with intense tectonic movements (Tsapralis, 1981;Triantaphyllou, 1996;Triantaphyllou et al., 1997;Duermeijer et al., 1999;Zelilidis et al., 1998;Agiadi et al., 2010). The average depositional depth in this area exceeded 450 m during the Pliocene and the early Pleistocene (until 1.66 Ma), gradually diminished to 200-400 m (1.25-0.97 Ma), until the final tectonic uplift of the south-eastern part of the island took place certainly after 0.97 Ma (Agiadi et al., 2010).

METHODS Field geology, borehole and seismic data
Field observations were combined with detailed sampling along the Kalamaki-Argassi and Agios Sostis areas in the south-eastern part of Zakynthos Island. A main concern was to distinguish the Ionian zone Triassic gypsum from the Messinian gypsum, both expressed in this area. The Ionian thrust's cartographic position and the Neogene deposits' distribution were based on new tectonic, stratigraphic and sedimentologic observations, in conjunction with a reinterpretation of the available borehole and onshore seismic data.

Planktonic Foraminifera
In the Kalamaki and Agios Sostis sections, 198 samples were collected. In the Kalamaki section, 175 samples were obtained at 0.05-to 0.50-m intervals in the west (pre-MSC) and the east subsection (Messinian-Zanclean), representing 37.1 m of sediment thickness (Fig. 3). In the Agios Sostis section, 23 samples were collected at 0.05-to 1-m intervals for a total thickness of 20 m. Fourteen samples (AS 1-14) were studied from the 16.5 m of the pre-evaporitic sequence (sampling step about 0.30 to 1 m), and nine samples were examined from the 20 m of the post-evaporitic sequence. Three samples (15)(16)(17) were obtained from the first 1.20 m of the post-evaporitic sequence, which are rich in gypsum crystals, six samples (AS 18-23) were taken from the last 1.5 m of the Agios Sostis section (Fig. 4).
The samples were washed through a 63-lm sieve. Planktonic foraminifer biostratigraphy was based on the presence of index species, their abundance, and the coiling ratio of Neogloboquadrina acostaensis. The bioevents were determined on the planktonic foraminifer assemblage observed in the fraction larger than 125 lm, following the biozonal scheme of Iaccarino et al. (2007) and through correlation with astronomically calibrated successions of the Mediterranean area (Krijgsman et al., 1999b;Sierro et al., 2001;Iaccarino et al., 1999;Gennari et al., 2008). The palaeoecological remarks were based on the qualitative and semiquantitative analysis of the assemblage with respect to the identified species' modern ecological affinities (B e & Tolderlund, 1971;Hemleben et al., 1989).

Calcareous nannofossils
Smear slides were prepared following the standard technique described by Perch Nielsen (1985) and Bown & Young (1998). The taxonomy of the determined calcareous nannofossil species was in accordance with Aubry (1984Aubry ( , 1988Aubry ( , 1989Aubry ( , 1990Aubry ( , and 1999 and Perch Nielsen (1985). The biostratigraphic analysis was based on the standard biozonal scheme of Martini (1971). The biochronology and the numerical ages of the biozone boundaries were assigned according to Lourens et al. (2004), Raffi et al. (2006) and Backman et al. (2012).
The nannofossil biostratigraphy incorporated qualitative and semiquantitative analyses. Qualitative methods are traditionally based on the estimation of index-species presences, obtained through a rough scan of the sample. Concerning semiquantitative methods, these were mainly Fig. 3. Kalamaki composite section (west subsection: 37°44 0 11.95″N, 20°54 0 13.14″E; east subsection: 37°44 0 12.62″N, 20°54 0 19.09″E) with lithology, samples' location, macrofossils of the pre-evaporitic sequence, the evaporitic unit (the PLG facies description is referred in the key), the top of the evaporitic unit, and the post-evaporitic sequence, and two Sr isotope values from the base and the top of the gypsum unit). M/P: Miocene/Pliocene boundary, pre-MSC: pre-Messinian Salinity Crisis part of the section, MES: Messinian Erosional Surface, samples KAL 1 to KAL 173 with their position in the section. used to overcome the very terrigenous and relatively shallow nature of the studied samples, in which nannofossils are rather scarce. To obtain accurate biostratigraphic estimations, at least 100 fields of view were investigated per slide, using a Leica DMLSP optical polarizing light microscope at 12509 magnification. The identified taxa were categorized based on their semiquantitative abun-dances as follows: A, abundant, more than one specimen in every field of view; C, common, one specimen in every ten fields of view; R, rare, one specimen in every 50 fields of view; P, present, one specimen in more than 100 fields of view; and RW, reworked specimens.

Fish otoliths
The same samples collected for the biostratigraphic analysis were also examined for the fish otolith content. In addition, several 25 kg bulk samples were obtained throughout the pre-evaporitic interval and the upper part (Trubi Formation) of Kalamaki section, as well as from each of the marl beds alternating with the gypsum (Fig. 3). The aims were (1) to identify the fish fauna in the area before, during and after the MSC; (2) to determine the timing of the reestablishment of a purely marine fauna after the crisis (Carnevale et al., 2006); and (3) to estimate the palaeoenvironmental conditions especially just before the onset of evaporite deposition. Fish otoliths were described according to the criteria set by Nolf (1985) and systematically identified based on the scheme of Nelson (2006). The methodology of Nolf & Brzobohaty (1994), as readjusted by Agiadi et al. (2010), was applied to estimate the palaeodepth. The palaeoecological analysis was based on the fish's present-day distribution and ecological data acquired through the FishBase database (Froese & Pauly, 2014).

Magnetostratigraphy
The magnetostratigraphy of the Kalamaki section was based on a subset of the same samples used for biostratigraphy, 61 samples in total (Figs 3 and 5). Core samples were obtained with a water-cooled diamond-head corer installed on an electric driller. The standard specimens were thermally demagnetized in an ASC oven and the natural remanent magnetization (NRM) was measured on a 2G-Enterprises DC SQUIDS cryogenic magnetometer, in a magnetically shielded room at the ALP Laboratory of Peveragno (Italy). Samples were first heated at 90°C, and then, successive 30°C steps were applied up to a maximum of 560°C. The magnetic susceptibility was monitored during heating to detect mineralogical changes. The NRM data set was processed using the Remasoft software (Chadima & Hrouda, 2006) to calculate the direction of the characteristic remanent magnetization (ChRM) and the virtual geomagnetic pole (VGP) for each sample.

Oxygen and carbon stable isotopes
One hundred and six (106) bulk rock samples from the pre-evaporitic and 26 samples from the post-evaporitic sequence were analysed in terms of oxygen (d 18 Ο carb ) and carbon (d 13 C carb ) stable isotopes (Fig. 6). Moreover, stable isotopes were additional analysed on 45 samples from pre-evaporitic and 37 samples from the post-evaporitic sequence on a combination of species (Turborotalita multiloba and Orbulina universa for the Miocene; Globigerinoides obliquus for the Pliocene) depending on their availability. More explicitly, the isotopic analyses were performed on: T. multiloba (d 18 Ο Τ.multiloba . d 13 C O.universa ) shells on samples from the base of the section (0-7 m), O. universa (d 18 Ο O.universa . d 13 C Τ.multiloba ) shells for the remaining of the Messinian pre-evaporitic interval (9.6-19.5 m), and G. obliquus (d 18 Ο G.obliquus . d 13 C G.obliquus ) shells for the Pliocene Trubi Formation ( $ 15 m thick). The isotopic composition of bulk sediments and planktonic foraminifers, in combination with the all the other data presented here, focused on investigating the environmental conditions before the commencement of evaporite deposition, as well as the system inertia after the MSC. Both bulk samples and foraminifers were analysed to test whether and/or how the fauna responded to the environmental changes as these were imprinted in the sediment. The d 18 O results from the bulk sediment and the foraminifera were comparable in the pre-evaporitic sequence. Therefore, they can be used interchangeably. The lower part of the post-evaporitic sequence (Lago Mare facies) was barren of foraminifera. Consequently, only bulk sediment isotopic analysis was possible. On the other hand, the Trubi sediments were very rich in foraminifera, which were used for the isotopic analysis. The measurements on the bulk samples were conducted with a mass spectrometer VG SIRA 9, whereas a KIEL-IV carbonate device coupled online to a Delta V Advantage IRMS (Thermo Scientific) was used for the isotopic analyses on the planktonic foraminifers. The CO 2 extraction was attained through reaction of powder samples (50-100 mg) with anhydrous orthophosphoric acid, at 50°C. The values were expressed in per mil relative to the Vienna PDB (V-PDB) standard reference.

Strontium isotopes
Two gypsum samples (Fig. 3) were analysed for strontium isotope content to distinguish the stage 1 PLG deposits from the stage 3 UG Roveri et al., 2014b). Strontium isotope analyses were carried out at the Scottish Universities Environmental Research Centre in East Kilbride, Scotland (SUERC). Samples were leached in 1 M ammonium acetate prior to acid digestion with HNO 3 . Strontium was separated using Eichrom Sr Spec resin. Matrix elements were eluted in 8 M HNO 3 and 3 M HNO 3 , before elution of Sr in 0.01 M HNO 3 . The total procedure blanks for Sr samples prepared using this method was b200 pg. In preparation for mass spectrometry, Sr samples were loaded onto single Re filaments with a Ta-activator similar to that described by Birck (1986). Sr samples were analysed with a VG Sector 54-30 multiple collector mass spectrometer. A 88 Sr intensity of 1V (1 A~10-11A) AE10% was maintained. The 87 Sr/ 86 Sr ratio was corrected for mass fractionation using 86 Sr/ 88 Sr = 0.1194 and an exponential law. The mass spectrometer was operated in the peak-jumping mode, with data collected as 15 blocks of 10 ratios providing an internal uncertainty of b0.000020 (2 S.E.). For this instrument NIST SRM 987 gave 0.710249 AE 0.000008 (1 S.D., n = 17) during the course of the present study.

Kalamaki-Argassi area
The Neogene sequence of the Kalamaki-Argassi area ( Fig. 2) lies unconformably over the Ionian zone basement. In the Kalamaki area, the lower 14.5 m consist of alternating massive and laminated marls with rare calcareous marl and calcarenite intercalations. Bivalves, Discospirina, pteropods, echinoids and molluscs are observed from 5 to 13 m, whereas some slumps and chaotic horizons occur between 14.5 and 17.5 m. The rest of the pre-evaporitic sequence is partially covered by Quaternary detritus (Fig. 7a).
The previous unit is conformably followed by a 108-mthick evaporite succession in the Kalamaki area (Figs 3 and 8). The base of this succession is partially covered by Quaternary debris, which conceals the shales' cyclicity below the first selenite bed. Eight gypsum-marl cycles are identified (A to H), exhibiting different gypsum depositional facies: massive, massive stratified, banded, laminite and branching selenite (Lugli et al., 2010). Thin gypsrudite and gypsarenite intercalations appear in the upper one-third of the succession (Figs 7c-f, and 8) and may be related to the occurrence of small irregular floods entering in the evaporitic basin. These evaporitic facies and their conformable superposition on the pre-MSC succession allow ascribing them to the Primary Lower Gypsum unit (PLG; Roveri et al., 2008a), which accumulated under precessional control during the MSC stage 1 (5. 971-5.60;CIESM, 2008;Manzi et al., 2013). Unfortunately, the lack of good exposure at the base of the gypsum does not allow a bed-to-bed correlation with the PLG reference sections. The uppermost eight metres above the PLG succession are terrigenous deposits, comprising laminated greenish marls; interrupted, after the fourth metre of the succession, by a dm-thick gypsarenite bed (cycle H of depositional gypsum type; Fig. 3). These sediments originated from the hanging wall of a small normal fault separating the lower from the uppermost part of the succession. There are no important facies changes, erosional, or angular discordance in the lower boundary of this succession. In this section, reworked evaporites are present also in association with the primary facies. Therefore, this portion can be included in the PLG unit.
In the Argassi area ( Fig. 2), only the evaporitic unit was described. The PLG unit (Fig. 9a,b) comprises eight gypsum-marl cycles that can be correlated with those of the Kalamaki section (Fig. 10). However, in the Argassi area, it is not possible to observe the stratigraphic transition of the evaporitic succession to the overlying and the underlying formations, due to tectonics and the Quaternary debris cover.
The uppermost part of the gypsum unit in the Kalamaki area is capped by an angular unconformity ( Fig. 7b) that can be correlated with the MES and marks the passage to the post-evaporitic unit (CIESM, 2008;Roveri et al., 2014a). The unconformity is sealed by a four-mthick unit consisting of alternating greenish and discontinuous calcareous marl beds, which conformably pass upward to a dark bed and to the Pliocene deposits of the Trubi Formation. The latter begins with three metres whitish massive marly limestones, overlain by an alternation of decametric marly limestones and laminated marl beds. The Trubi Formation can be subdivided into nine lithological cycles (Figs 7b and 11) corresponding to laminated marls and massive 'trubi-like' whitish carbonate couplets.

Agios Sostis area
The Neogene succession of Agios Sostis area lies over the pre-Apulian zone sequence. The lowermost part of the pre-evaporitic unit consists of decametre-thick shale and sandstone alternations, exhibiting upward decrease in clastic input (Figs 4 and 9c). These deposits are overlain  by 3.5 m of thin sandstone beds with marl intercalations, which in turn are capped by 3 m of laminated green shale and marl alternations.
In the Agios Sostis area, two parallel linear gypsum outcrops are observed, the Ploumari outcrop and the Panagia (Machairado) outcrop (Fig. 2). The western outcrop (Ploumari) consists of primary bottom-growth selenite (PLG) deposited in shallow-water settings (less than 200 m; Lugli et al., 2010) and is exposed for about three km. The PLG succession is here incomplete and represented only by one or two gypsum beds consisting of very large crystals that are typically found in the two lowermost PLG cycles (Lugli et al., 2010).
Conversely, clastic gypsum deposits that derive from the resedimentation of older PLG deposits mainly characterize the eastern outcrop (Panagia). Moving from Panagia to Agios Sostis in a NNW-SSE direction (Fig. 2), we observe a down-slope transition of different gravity-flow deposits: from chaotic deposits including dm-thick resedimented gypsum blocks to gypsum turbidites alternated with primary gypsum cumulate. These deposits are well exposed in the Agios Sostis section.
This suite of evaporite-bearing gravity-flow deposits suggests that the Panagia sediments derived from the erosion of PLG located to the west, like those outcropping in Ploumari. Unfortunately, we could not observe the preevaporitic/evaporitic transition in this sector. Thus, it cannot be excluded that the PLG deposits are actually blocks that slid downslope from a more western position. Westward, these PLG deposits were completely eroded during the late Messinian, as indicated by the angular unconformity between the early Pliocene conglomeratessandstones and the early-middle Miocene marls, which was first observed by Nikolaou (1986) in the Perlakia area (about one km north of the Keri village; Fig. 2).
Along the Agios Sostis shoreline, a 16-m-thick gypsum unit lies above an erosional or sliding surface developed on top of the pre-evaporitic succession (Figs 4 and 9c). It consists of several alternations of primary (cumulate) and clastic gypsum (gypsrudite, gypsarenite and gypsiltite; Figs 4 and 9d, f), commonly showing high-angle cross and convolute lamination (Figs 4 and 9e). This succession is mostly characterized by clastic gypsum deposits without the in situ shallow-water selenite, which is found in the Kalamaki-Argassi area. The Agios Sostis exposed evaporite corresponds to a mass flow, which slid from the west to the east, following the dipping of the slope between the forebulge and the foredeep of the Alikanas basin. Therefore, the Agios Sostis evaporitic unit can only derive from the dismantlement and resedimentation of the western PLG deposits. Thus, it is ascribed to the RLG unit, which was deposited during the 2nd stage of the salinity crisis (5. 60-5.55;CIESM, 2008;Roveri et al., 2008a). The siliciclastic component increases upward in  the RLG succession, which is divided into four units; each unit is topped by a centimetre-thick marl bed. Approximately two metres of hybrid sandstones are capping the clastic evaporite unit. The presence of the urban fabric of Agios Sostis Community (harbour) hinders further observations. Nevertheless, the section's continuity is observed in the Agios Sostis islet (Fig. 2), which is sep-  two metres is observed (lower level 6 of Fig. 4). The last seven metres of the section (levels 46-53 m) are well observed in the rocky elevation of the Agios Sostis harbour, some 130 metres NNW of the islet (upper level 6 and level 7 of Fig. 4). They comprise a six-m-thick succession of graded sandstone and marl alternations, followed by 1.5 m of whitish marls belonging to the postevaporitic Trubi Formation.

Borehole and seismic reflection data
Correlation of the available borehole logs (Fig. 12) over the pre-Apulian basement does not provide a clear idea about the thickness distribution of the Neogene sediments of the Zakynthos (Alikanas) basin. The Neogene sediment thickness ranges from 800 m, in the west, to 1350 m, in the east, but there are no stratigraphic layers in the well logs, which could be used as stratigraphic datums (marker beds) to recognize the thickness distribution. Indeed, apart from the western borehole (Kb101), the other boreholes, towards the east, do not penetrate the entire Neogene sequence, and yet the boreholes do not begin from the same stratigraphic level. Only the two western logs allow observing the reduced thickness of the lower Miocene sequence (Kb101) and its gradual thickening towards the immediately eastern log (Z2). Nevertheless, the Zakynthos (Alikanas) basin onshore seismic profile performed in the eastern half of this basin (Figs 2 and 13) shows clearly the eastward thickening of the Miocene-Pliocene sequence (Messinian gypsum included) towards the depocentre of the basin. Furthermore, the seismic profile suggests that the Ionian thrust is located west of the western Ionian Triassic diapir (Agia Dynati diapir; Fig. 2), more to the west than previously considered. In addition, in the Agios Kyrikos-1 well ( Fig. 12; AK1), at the Alikanas basin's depocentre, an increase in the salinity of the drilling mud was recorded, when the drilling penetrated the Messinian evaporites. This fact, combined with the observation of a simultaneous speed reversal of the seismic waves at the same stratigraphic level (C. Nikolaou, pers. comm., 1986), is an indirect indication of the presence of halite in the Messinian evaporitic unit at the basin's depocentre zone.

Planktonic Foraminifera
Kalamaki section The abundance of planktonic foraminifers varies throughout the section. In the pre-evaporitic sequence, several samples are barren (KAL 16,17,19,33,34,53,82,88,Fig. 12. Zakynthos (Alikanas) basin wells (their location is indicated in Fig. 2). The borehole logs are projected to a line parallel to the dipping of the Alikanas monocline (Figs 1 and 2) to enable correlating the Miocene-Pliocene deposits' lateral evolution. However, we cannot recognize the thickness distribution from the well logs, because there are no available stratigraphic datums. Therefore, we use only the two western logs to observe the reduced thickness of the lower Miocene sequence (Kb101) and its gradual thickening towards the immediately eastern log (Z2). 92-94, 96, 97, 19, 110, 118, 123-130, 136-139) or contain only benthic foraminifers (KAL 13,30,32,(36)(37)(38)47,48,(50)(51)(52)103). The preservation is generally good in the lower part of the section, up to 14 m from the base, and it becomes moderate upward. Planktonic foraminifers within the Trubi Formation are very abundant and well preserved.
In the pre-evaporitic part of the Kalamaki section ( Fig. 3), the following species are identified: Turborotalita multiloba, Turborotalita quinqueloba, Globigerinita glutinata, Globigerina bulloides, Globigerina obesa, Orbulina universa, Neogloboquadrina acostaensis (dextral and sinistral forms) and Globorotalia scitula. In the lowermost two metres of the section, Turborotalita multiloba (0.6-7.2 m; samples KAL5-41) is the only species present (0-2 m, KAL7-10) or occur together with rare specimens of Globigerinoides spp., O. universa and T. quinqueloba. A paracme interval, within its abundance distribution, is recognized between 1.95 and 4.3 m (KAL 11-KAL 23), where the species is very rare or absent. The paracme end of this interval (bioevent 3a, Fig. 14) is also recognized in Sorbas Basin, in the upper part of cycle UA23, and in Caltanissetta Basin, just above the influx of G. scitula (Manzi et al., 2011. The distribution of T. multiloba is similar in Kalamaki and Falconara sections and is probably related to the palaeoecological requirements of the species and the basin's configuration during this period. Above the paracme end, T. multiloba becomes the main planktonic foraminifera faunal component up to 7.2 m, although its distribution is discontinuous upward, as it exhibits only two significant peaks at 9 and 10.5 m (KAL 56 and KAL 64) corresponding to its highest abundance. Turborotalita quinqueloba and G. glutinata are rare in the lower part of the section and dominate the assemblages, in the thick massive marl interval from 7 to 8.5 m (KAL 39 through KAL 49). Neogloboquadrina acostaensis is generally rare in the section, displaying a relative abundance fluctuation between sinistral and dextral specimens. Sinistral forms are more abundant from the base up to 1.5 m (KAL 9), whereas dextral representatives are prevalent in the 2.7-to 8.4-m interval (KAL 15-KAL 49). Sinistral forms prevail again in the interval between 8.8 and 9.2 m (KAL 54-KAL 57; 60-70%) and at 10.5 m (KAL 64; 40%). Globorotalia scitula is present with two short influxes at 3.

Agios Sostis section
The samples from the pre-evaporitic sediments of Agios Sostis coastal section are barren of foraminifera. The complete sequence is characterized by the presence of gypsum crystals, possibly of diagenetic origin; gypsum was also recovered from the sieved residual. The lower 1.2 m of the post-evaporitic sequence, in the coastal elevation of Agios Sostis harbour, are barren of foraminifera, whereas the uppermost 1.5 m contains a rich planktonic fauna. Samples AS 18 and 19 yield specimens of Globorotalia conomiozea (Fig. 4). This species disappeared from Mediterranean during the pre-evaporitic phase of the MSC, at 6.52 Ma (Krijgsman et al., 1999a, b;Sierro et al., 2001). Therefore, its presence in the postevaporitic sequence of Agios Sostis may be assumed due to reworking. The last four samples of the section (AS 20-23), collected one metre below the Trubi Formation, are characterized by the presence of G. obliquus, G. trilobus, G. decoraperta, G. nepenthes, G. bulloides, G. falconensis, T. quinqueloba, Orbulina sp., Sphaeroidinellopsis spp. and N. acostaensis.

Calcareous nannoplankton -Kalamaki section
The lower part of the Kalamaki sequence (KAL 10-50) features an abundant nannofossil assemblage in good preservation state (Fig. 3). The prevailing species are Helicosphaera carteri, Calcidiscus leptoporus, Umbilicosphaera jafari and small Reticulofenestra spp. (R. haqii, R. minutula). Six-rayed discoasters are very rare, and Discoaster quinqueramus is practically absent. Nearly monospecific assemblages of Sphenolithus abies are observed upward. The upper part of the sequence is characterized by the sporadic presence of Amaurolithus delicatus, A. primus, A. tricorniculatus and rare Nicklithus amplificus.
The upper part of the PLG in Kalamaki section (KAL 114-125; Fig. 3) is characterized by intense reworking and contains rare nannofossil specimens, in bad preservation, mostly reworked assemblages of Oligocene-Miocene age also including Cretaceous taxa.
The Lago Mare facies unit (KAL 120-134), beneath the Trubi Formation of Kalamaki section (Fig. 3), displays rare to common well-preserved small Gephyrocapsa spp. coccoliths (1.5-3.0 lm) within a rich but reworked assemblage. Just below the Trubi Formation (KAL 134), the rare presence of Ceratolithus acutus together with specimens of Reticulofenestra zancleana is recorded.

Fish remains
Sixteen fish taxa are identified in the pre-evaporitic sediments of Kalamaki section based on the otolith remains (Table 1; Fig. 15). The samples obtained from the marls in between the evaporites, as well as from the upper part of Kalamaki section, including the Trubi Formation, did not yield otolith remains. The Kalamaki pre-evaporitic teleost fauna resembles those identified by Girone et al. (2010) in the pre-evaporitic successions of northern Italy. Indeed the great diversity of the benthic-benthopelagic group is asserted here as well. Gadiculus labiatus is the most frequent benthic species in the assemblages, although the overall abundance of gobiids is higher. However, the gobiid specimens are very small and indeterminable to the specific, or even generic, level. The pelagic realm is dominated by myctophids, particularly those belonging to the genus Diaphus, which is much diversified; five different species are identified, with high abundance in the studied samples. Notably, Diaphus rubus and Myctophum coppa, two fossil species first reported by Girone et al. (2010) in the pre-evaporitic sequence of  (Cohen et al., 1990). Diaphus cf. pedemontanus, D. rubus and Myctophum coppa are absent in the so far studied Pliocene and Pleistocene assemblages of the eastern Mediterranean (Agiadi et al., 2011Agiadi, 2013). During the time interval preceding the onset of the MSC, the Kalamaki area presents a well-diversified fish fauna, comprising both abundant pelagic and benthic-benthopelagic fish. The continuous presence of gobiids throughout the basal part of the section and the occurrence of Buglossidium sp. in sample KAL 31 suggest rather shallow-water depths before the onset of gypsum deposition. In fact, the occurrence of benthopelagic Physiculus aff. huloti (bulk sample KAL base) indicates depths less than 320 m (OBIS, 2006). However, the great diversity and abundance of the genus Diaphus and the presence of gadids (Gadiculus argenteus and Gadiculus labiatus) in the upper part of the basal sequence (just prior to the gypsum deposits, samples KAL 27, KAL 28, KAL 38, KAL 39, KAL 45), suggest the area was not secluded, and depths certainly greater than 50 m may be expected. In addition, Maurolicus muelleri is present in almost all the samples examined and in great abundances. Although this bathypelagic species has a large bathymetric distribution today, between 0 and 1524 m according to Wheeler (1992), it is usually found between 300 and 400 m depth (Mauchilne, 1988). Combining the above results, the palaeodepth is estimated up to 300 m for the basal sequence of Kalamaki section.
Considering the modern ecological data for the extant species present in the Kalamaki fish assemblage (Table 1)

Magnetostratigraphy
The thermal demagnetization patterns and the trend of the magnetic susceptibility with increasing temperature clearly show two different behaviours in the pre-evaporitic unit and in the Trubi Formation of Kalamaki section (Fig. 3). The demagnetization patterns of both groups indicate that a low-temperature component, probably of viscous origin, is sometimes demagnetized at 120°C. A second, normally oriented component is demagnetized up to 210-330°C; it generally contributes with 70-80% to the initial magnetization in the pre-evaporitic interval and with 50-70% in the Trubi Formation. This component is interpreted as an overprint of the present-day field; in fact, in geographic coordinates, its mean declination is 0.7°N and the inclination is 54.5°( N = 52; A95 = 2.7°), which are values very close to the present-day field direction in the area of Zakynthos (3.79°N of declination and 54.05°of inclination). Samples collected in the pre-evaporitic sequence have a weak magnetization, showing a mean normal remanent magnetization (NRM) of 4.07 E-04 A/m (with a standard deviation of 6.02 E-04). Usually, an abrupt increase in magnetic susceptibility to 350-420°C is detected, concomitant with an increase of the remanent magnetization, both indicative of authigenic sulphides. Besides the low-temperature components, a higher temperature component is rarely isolated in this group above 210°C. In several samples, the characteristic component (ChRM) is isolated between 230 and 350°C. This component exhibits both normal and reversed polarities (Fig. 5)

Stable oxygen and carbon isotopes
In the pre-evaporitic sequence of Kalamaki section, bulk sediment and planktonic foraminifer (including both species analysed) d values vary widely (À2.71<d 18 O carb &<7.45; À4.32<d 13 C carb &<1.94; À1.44<d 18 O foram &<3.73; À2.90<d 13 C foram &<0.95), with heavier values measured in the massive and calcareous marls (Fig. 6). The d 18 O carb is generally negative and ranges between À0.5 and À2& in the laminated marls, whereas it displays several positive peaks up to 6 and 8& in the massive marls, with an overall upward increasing trend. These peaks may record 'pulses' of higher salinity/evaporation that are more intense near the evaporitic unit. The d 13 C carb value from the base of the section up to 14 m is quite steady around À1.5&, only showing three positive peaks, up to 0, 2 and 1& at the base of the section (ca. 1 m), at the upper part of the massive marls (at 8.5 m) and at the top of the calcareous marls (13 m), respectively. In the interval from 15 m until the top of the pre-evaporitic sequence, the d 13 C carb values are highly variable (ranging from À5 to À0.5&). The d 18 O multiloba and d 13 C multiloba remain approximately constant up 7 m, with average values around 3-4& and À2.5&, respectively. From 9.6 up to 19.5 m, the d 18 O universa and d 13 C universa exhibit high-frequency and low-amplitude fluctuations ranging from À1 to 1& and À0.5 to 1&, respectively. These are similar to the d 18 O G.obliquus and d 13 C G.obliquus values measured at the basal Trubi Formation; the latter, however, fluctuate more widely, ranging from À2 to 1& and from 0.5 and 1.4&, respectively. In the upper part of the PLG (cycle H, one metre in the eastern section), the d 18 O carb values range between À5 and 8&, whereas d 13 C carb values mostly fluctuate around À4& (Fig. 6). The most prominent change, linked to environmental phenomena, seems to be the positive d 18 O shift (ca. 8&) recorded around the gypsarenite bed, revealing high salinity that is consistent with evaporite deposition (cycles A-G); therefore, the gypsarenite of cycle H is probably related to the penecontemporaneous reworking of primary facies belonging to the PLG unit. The d 18 O carb values of ca. À3& are considered indicative of brackish conditions (Pierre & Rouchy, 1990). Sharp d 13 C carb negative and positive shifts characterize rapid changes in the hydraulic budget and in the ventilation of the basin, from lacustrine to saline conditions (Shackleton et al., 1995).
The M/P boundary is marked by an excursion towards higher d values (d 18 O G.obliquus = À0.75&, d 13 C G. obliquus = 0.91&) with respect to those of the post-evaporitic unit (Fig. 6). The d 18 O G.obliquus values remain negative in the black shale just below the Trubi, although they still fall within the range of the Pliocene values. This development could reflect the rapid transition to marine conditions at the onset of the Pliocene. Upward from the base of the Trubi, the d 18 O G.obliquus variability correspond to the ten cycles (I to X), which are probably related to the precessional oscillations (Shackleton et al., 1995;di Stefano & Sturiale, 2010 (Fig. 6).

Strontium isotopes
The results of the strontium isotope analyses in the Kalamaki section gypsum unit are presented in Fig. 3. Selenite samples yield 87 Sr/ 86 Sr values of 0.708993 for (2r = 0.0014) and 0.709007 (2r = 0.0015). These values correspond to PLG deposits from stage 1 of the salinity crisis (Roveri et al., 2014a,b). It is worth to notice here that the 87 Sr/ 86 Sr values of the worldwide Triassic marine evaporites are lower, ranging between 0.7076 and 0.7082 (Ort ı et al., 2014).

DISCUSSION Chronostratigraphic framework
The observed distributions of planktonic foraminifer and calcareous nannofossil species were used to recognize astronomically tuned bioevents that are already defined in lower Messinian Mediterranean sections (Fig. 14;Krijgsman et al., 1999a,b;Sierro et al., 2001;Blanc-Valleron et al., 2002;Manzi et al., 2011Manzi et al., , 2013. The first peak of T. multiloba is recognized one metre from the base of the western Kalamaki subsection and can be correlated with its first abundant occurrence dated at 6.415 Ma in the Perales section (Sierro et al., 2001). The first abundant occurrence of dextral N. acostaensis, identified also in the Perales section and dated at 6.339 Ma, is recorded at 2.7 m. This biostratigraphic event is considered more reliable than the N. acostaensis coiling change as no sinistral specimens were observed below this level. The presence of G. scitula at 3.7 m can be correlated with its first influx and dated between 6.291 and 6.287 Ma. The interval between 8.8 and 9.2 m, dominated by sinistral N. acostaensis, is correlated with the relevant interval found in the Metochia section (Gavdos Island; Drinia et al., 2007) and in the Perales composite section (Sierro et al., 2001;Manzi et al., 2013), and it is dated between 6.140 and 6.108 Ma. The influx of T. multiloba at 9 m can be correlated with a relevant influx of this species at Perales section, which falls in the upper part of cycle UA28, dated at 6.121 Ma. The influx of G. scitula at 9.3 m correlates well with the second known influx of this species, dated between 6.105 and 6.099 Ma (Sierro et al., 2001). The dominance of sinistral N. acostaensis identified at 10.4 m can be correlated with the second influx of sinistral N. acostaensis dated between 6.078 and 6.08 Ma, approximately at the same level of the influx of T. multiloba at 10.5 m, which could correspond to the last prominent influx of the species in the Perales section. The oligotypic assemblage recorded below the slumped interval and the high abundance of planktonic foraminifera at 14.3 m were also observed in the Sorbas Basin and in the Northern Apennines (Sierro et al., 2003;Gennari et al., 2013) preceding the last eccentricity maximum centred at ca. 6.01 Ma, before the onset of the MSC.
Based on this biostratigraphic framework, the normal polarity at the lower part of the Kalamaki pre-evaporitic sequence (up to 0.68 m) is correlated with subchron C3An.2n (6.733-6.436 Ma after Lourens et al., 2004), below the first abundant occurrence of T. multiloba. The reverse magnetic signal at 6.53 m falls within subchron C3An.1r. This interval is confirmed by the position of the N. acostaensis dextral-coiling specimens' abundant occurrence and the G. scitula first influx. The normal polarities between 8.58 and 12.03 m correlate with subchron C3An.1n (6.252-6.033 Ma) and the reversal polarities at 14.02-20.15 m fall within the lower part of subchron C3r.
As for the calcareous nannofossil, the typical specimens of D. quinqueramus, usually recorded in other pre-evaporitic deposits (e.g. Gavdos Island; Triantaphyllou et al., 1999), are not found in the Kalamaki sequence, probably due to the establishment of a semi-closed, neritic, littoral environment (Wade & Bown, 2006). However, the presence of N. amplificus supports the biostratigraphic assignment of the Kalamaki pre-evaporitic unit within the NN11 biozone, dated between 6.82 and 5.98 Ma (Raffi et al., 2006;Backman et al., 2012).
Reworked planktonic foraminifera and nannofossil specimens characterize the upper part of the PLG succession and the Lago Mare deposits in the Kalamaki section. Gephyrocapsa spp. specimens, although suggesting an age within the early Pliocene (sub-bottom of Gephyrocapsa spp. at 4.33 Ma; Lourens et al., 2004), were also observed in the late-Messinian pre-evaporitic diatomites of Gavdos . Thus, their presence in the Kalamaki sequence contributes to the limited knowledge on the precise stratigraphic and geographic distribution of these early representatives of the genus in the SE Mediterranean region.
Just before the Trubi Formation (Fig. 16), the planktonic foraminifera assemblages in both Kalamaki and Ag. Sostis sections are assigned to the Transitional Unit identified in the ODP sections by Iaccarino et al. (1999). In sample KAL 134, the rare presence of calcareous nannofossil Ceratolithus acutus, together with specimens of Reticulofenestra zancleana, implies an age within the base of the NN12 biozone, in the early Zanclean (5.36 Ma;di Stefano & Sturiale, 2010;Backman et al., 2012). The M/P boundary (5.33 Ma) is defined by the distribution of dextral and sinistral forms of N. acostaensis and the Sphaeroidinellopsis Acme Base, further supported by the increasing of the oxygen and carbon isotope values, and it is placed at the base of the Trubi Formation. The recognized bioevents and the polarity reversal identified in the lower part of Trubi Formation of Kalamaki section are correlated with the upper part of subchron C3r and the normal polarity at the C3r/C3n boundary is placed in the 9.6-to 10.4-m interval mid-point (event III; Fig. 16) dated at 5.235 Ma (Lourens et al., 2004), below the Sphaeroidinellopsis Acme End (5.21 Ma). The sinistral shifts of N. acostaensis at 7.8 and 8.52 m are ascribed to the first and second N. acostaensis sinistral shifts that are reported from several Mediterranean Lower Pliocene sections (di Stefano et al., 1996;Lourens et al., 1996;Iaccarino et al., 1999;Pierre et al., 2006;Gennari, 2007;Drinia et al., 2008), dated at 5.330 and 5.281 Ma (Lourens et al., 2004) and correlated with the base of cycle 2-top cycle 1 and the base of cycle 2-top cycle 3, respectively. The base of the Sphaeroidinellopsis Acme Zone, recorded at 8.3 m, and its top, at 11.3 m, correspond to the cycle 2 and the base of cycle 6, respectively. The first common occurrence of Globorotalia margaritae (FCO) is recorded in KAL 171 and marks cycle 10, dated at 5.08 Ma (Lourens et al., 2004). Consequently, the studied Trubi Formation sediments belong to the MPL1 biozonelower part of the MPL2 biozone; its base may be correlated with the base of the Zanclean, as defined in the Eraclea Minoa GSSP at 5.33 Ma (van Couvering et al., 2000). The presences of Sphenolithus spp. (abundance >5%), Reticulofenestra pseudoumbilicus (abundance 1-2%, possibly representing the R. pseudoumbilicus Pliocene Paracme Zone; di Stefano & Sturiale, 2010), rare Reticulofenestra zancleana, and several discoasterid species (D. brouweri, D. pentaradiatus, D. surculus, D. intercalaris, D. variabilis), in conjunction with the common occurrence of Amaurolithus spp., document the biostratigraphic correlation with the nannofossil biozone NN12 (Martini, 1971), or MNN12a-b . Therefore, the Trubi sediments of Kalamaki section (Fig. 16) have an early Zanclean age, ranging between 5.36 and 5.0 Ma.

Kalamaki palaeoenvironmental reconstruction
Pre-evaporitic sequence The fish assemblage identified in the pre-evaporitic Messinian of Kalamaki section (Fig. 3) is indicative of a coastal marine area, with easy access to the open ocean, not exceeding 300 m in its deeper parts. The great diversity of the fauna, along with the fact that it incorporates deep-water pelagic species, such as Maurolicus muelleri, Gadiculus argenteus and Physiculus aff. huloti, as well as shallow-water inhabitants, such as gobiids, suggests that fossils from different adjacent underwater domains were combined during deposition. This phenomenon is commonly observed in the otolith faunas and results from the small-distance transportation of the otoliths during deposition, which is attributed to the high bathymetric gradient . Thus, the Kalamaki area palaeogeographic scheme proposed here is further supported by this observation. Several palaeoenvironmental changes can be recognized by the variations of the faunal and isotopic markers resulted to a stepwise progression towards the evaporitic conditions. Interval 6.45-6.121 Ma (0-9 m). In this interval (KAL 10-50; Fig. 3), the nannofossil assemblage of H. carteri, C. leptoporus, U. jafari, R. haqii and R. minutula is mostly represented by whole coccospheres, indicating high productivity and freshwater input to the surface waters (e.g. Triantaphyllou et al., 2009a,b). In addition, the impressive monospecific assemblages of Sphenolithus abies in the upper part of this interval (samples KAL 45-55) suggest marine mesotrophic environments (Wade & Bown, 2006) and were observed in others coeval Mediterranean sites (Cyprus, Piedmont, Lozar et al., 2010 andnorth-eastern Apennine, Manzi et al., 2007).
In this basal part of the pre-evaporitic sequence, the bulk sediment d 18 O values range between 0 and 2.1&, reflecting marine conditions with slightly increased salinity and/or temperature (Fig. 6). The positive trend of d 13 C carb reveals high productivity related to cold waters. The oligotypic planktonic foraminifera record is characterized by levels with 60-100% dominance of T. multiloba (KAL 7-KAL 10). This species is related to progressive isolation of the Mediterranean, and it is thought to be tolerant to increased salinity (Sierro et al., 2001). Its occurrence together with T. quinqueloba and G. glutinata in this part of the section is related to cold and nutrient rich surface waters (B e & Tolderlund, 1971;Sierro et al., 2003). This is confirmed by its isotopic signature, showing d 18 O multiloba values greater than 3&, which could record the upwelling of deep-intermediate cold water, possibly characterized by slightly raised salinity. According to the position of the Kalamaki section within the Mediterranean, this water mass could represent the deepintermediate water mass formed in the eastern Mediterranean and flowing to the west and to the north into the proto-Adriatic . The peaks of T. multiloba in the Mediterranean have been correlated to the insolation minima (Sierro et al., 2003), and they coincide with the late Miocene glacial cycles . The low planktonic foraminifera diversity 2-4 m from the base is associated with the d 18 O bulk negative excursions, nannofossil assemblages indicative of high productivity and freshwater input, and suggests a temporary restriction of the Mediterranean-Atlantic connection (e.g. during a sea-level fall, or excess precipitation) at this time, leading to a reduction of the sea-surface salinity. Within this barren interval, at 3.7 m, the return to normal marine conditions is inferred by the oxygen isotope positive values and the influx of the deep-dwelling (B e & Tolderlund, 1971) species G. scitula. Maximum d 18 O bulk values are observed between 7 and 9 m and correspond to hypersalinity episodes. The increased d 18 O bulk values coincide with higher abundance of the small-sized microfaunas (G. glutinata and T. quinqueloba) which are related to cold-eutrophic surface waters and/or high salinity. Relevant horizons characterized as 'aplanktonic zones' or 'nearly aplanktonic zones' have been recorded in Upper Abad Section and in Red Sea related to high salinity and cold surface waters. These conditions were unfavourable for planktonic foraminifera to survive (Sierro et al., 1999;Hemleben et al., 1996). However, climatic variability towards a cooling phase is recorded around 6.121 Ma by Van der Laan et al. (2005) and Gennari (2003). At the upper part of this interval around 9 m, the d 18 O carb values average around 0& indicating periods when normal marine conditions prevailed.
Interval 6. . The nannoflora assemblages in the upper part of the basal Kalamaki sequence are indicative of a marine mesotrophic environment. Towards the top and before the deposition of the gypsum, the d 18 O carb varies between À4 and 8&, suggesting rapid salinity fluctuations between highly evaporated and diluted conditions (Bellanca et al., 2001). Additionally, there is a simultaneous decrease in d 13 C carb values and an increase in d 18 O carb values at the same stratigraphic level. Wacey et al. (2008) suggested that such a phenomenon could be the result of isotopically light carbon influence from bacterial sulphate reduction, due to increased stagnation at the beginning of sulphate deposition. Similar trends in both carbon and oxygen isotopes were described in the Legnagnone section (Northern Apennines, Italy) by Gennari et al. (2013). As O. universa proliferates in mixed-layer surface waters during surface-water stratification, the d 18 O O.universa reflects slight variations of temperature, freshwater input, or trophic content (Sierro et al., 2003). There is a 4& difference between the d 13 C values in the upper and lower part of the basal Kalamaki interval. This probably reflects the contrast between nutrient-depleted waters of the stratified summer mixed layer, recorded in the upper part of the basal sequence, and the increased nutrient availability of the basal interval (d 13 C T.multiloba ). A cooling phase punctuated by hypersaline conditions (evidenced by a remarkable similarity between the d 18 O carb and the d 18 O O.universa ) is centred at 12 m, and may be delimited by the levels of increased freshwater input and the warmer phases evidenced by the fish fauna and planktonic foraminifera assemblages. Indeed, the fish fauna is dominated by warm-water species, notably Diaphus taaningi and Physiculus aff. huloti in the intervals below and over this cooling phase, whereas these taxa are completely absent in the cooling phase, which is characterized by the presence of Maurolicus muelleri mostly preferring temperate conditions. The onset of MSC is accompanied by the disappearance of plankton assemblages, heavier oxygen isotopic values and a sharp negative carbon isotope shift.

Post-evaporitic sequence
The establishment of rather eutrophic conditions may be hypothesized, based on the presence of small placoliths, along with the siliciclastic nature of the sediments at the uppermost part of the Kalamaki Lago Mare unit, just before the onset of Trubi Formation (e.g. Young, 1994). The samples from approximately one metre below the Agios Sostis Trubi Formation (Fig. 4) contain (AS 20-23) moderately preserved nannofossil assemblages and more reworked material with respect to the Kalamaki samples, indicating greater land proximity.
In the interval 5.6-5.33 Ma (1.4-7.5 m), from the top of the gypsum unit to the beginning of the Trubi Formation, the bulk sediment d 18 O values vary widely from 8& in the gypsum to À6& in the upper part of the sequence. The d 13 C carb values fluctuate between À6 and À1& (Fig. 6). These isotopic values reflect wide variations of the palaeoenvironmental conditions from hypersaline conditions to fresh and brackish water.
The total nannofossil content in the interval just above the Miocene/Pliocene (M/P) boundary in the Kalamaki section implies warm surface-water conditions in a pelagic environment. In particular, the presence of discoasterids, Rhabdosphaera spp., Sphenolithus spp., Scyphosphaera spp., Amaurolithus spp. is indicative of warm subtropical conditions (e.g. Rio & Sprovieri, 1986). Relatively increased productivity is inferred by the presence of Helicosphaera carteri, a species preferring warm waters and moderately elevated nutrient levels. The faunal assemblages as well as the isotope analyses reveal an open marine, stable and well-ventilated environment, settled at the beginning of the Zanclean, due to the restoration of normal marine conditions. The concurrent d 18 O G.obliquus increase and d 13 C G.obliquus decrease are possibly related to sea-surface temperature variations and reveal that the glacial cycles (Sprovieri et al., 2006) also influenced the eastern Mediterranean during the early Pliocene.

Zakynthos Neogene palaeogeographic and structural evolution
Field observations show that the Neogene sediments in the main part of Zakynthos Island, west of the Ionian thrust, were deposited over the pre-Apulian zone formations, whereas those in the south-eastern part, east of the Ionian thrust, were deposited over the Ionian zone. The correlation of the field observations with the available borehole data and the onshore seismic section (Figs 2, 12 and 13) reveals that the Neogene formations over the pre-Apulian zone sediments, east of the Vrahionas anticline (Fig. 2), form a monocline dipping approximately 30°E SE. Onshore seismic section (Figs 2 and 13) and boreholes stratigraphy (Fig. 12) indicate that the Neogene deposits (gypsum included) increase their thickness towards the dipping monocline.
The Neogene sediments exhibit their maximum thickness (800-1350 m) within the largest depocentre zone, extending eastward from the Agios Sostis area. The westernmost gypsum outcrops (Ploumari) consist of transported PLG blocks, whereas the easternmost ones (Panagia) contain gypsum turbidites and gypsum cumulates (Fig. 2). Consequently, the PLG deposits accumulated in a shallow marine basin, which was located in the western part of the Zakynthos (Alikanas) basin (west of the Ploumari outcrops; Fig. 2), and they were almost completely eroded during late Messinian. Indeed, about one km north of the Keri village (Fig. 2), in the Perlakia location, early Pliocene conglomerates, and sandstones lie over the early-middle Miocene marls through an angular unconformity. Thus, the eroded PLG deposits provided the clastic material that was deposited as gypsum turbidites (RLG) along the Messinian basin slope (Panagia outcrops) and in its depocentre. In addition, dismantled PLG blocks slid towards the slope and were incorporated in the RLG deposits.
On the contrary, Neogene sediments over the Ionian zone formations in the south-easternmost part of the island are considerably thinner (200-300 m). Gypsum deposits in Kalamaki and Argassi areas correspond to MSC stage 1 PLG unit; to the east (Skopos area, Fig. 2), only some rare and small gypsum outcrops can be observed, mainly reworked gypsum and occasionally gypsum turbidites. These differences suggest that the Zakynthos Neogene basin was not uniform, but subsidence in the monocline area (over pre-Apulian basement) was more rapid as compared with the eastern area (over Ionian basement).
The above observations are explained by a simple model of continental foreland-directed migration of the Hellenide (Alpine) thrusting, during the late Neogene and Pliocene. The Ionian thrust foreland basin can be modelled according to the foreland basin systems proposed by de Celles & Giles (1996), although it is necessary to take into consideration the particular role of the Ionian Triassic evaporites diapiric movements ( Fig. 1; Karakitsios, 1995). The Neogene formations over the pre-Apulian zone correspond to the foredeep, the forebulge and flank between them; those overlying Ionian zone rocks correspond to the wedge-top (Fig. 17). The wedge-top is more extensive in this foreland basin due to the extensive diapirism of the Triassic evaporites as compared to a foreland basin without an evaporitic base. At the same time, the diapirism induced tectonic activity, which limited the width of the foredeep zone. The greater subsidence characterizing the foredeep and flank-to-forebulge areas is reflected in the increased thickness of the clastic Neogene formations over the pre-Apulian zone, which partially derived from the eroded pre-Apulian forebulge-backbulge. Simultaneously, the diapiric movement of the Ionian Triassic evaporites, mainly halite and anhydrite (Karakitsios & Pomoni-Papaioannou, 1998;Pomoni-Papaioannou et al., 2004), prevented substantial subsidence to occur. As a result, the eastern part of the Neogene basin over the Ionian zone basement corresponds to a slowly subsiding wedge-top. This is reflected in the observed thin clastic Neogene sediments derived from the Ionian orogen's erosion.
During the Messinian, the shallow depths in the wedge-top and forebulge-backbulge area of the basin, together with the upper Messinian overall negative hydrological balance (Mertz-Kraus et al., 2009) led to the deposition of the PLG. The subsequent PLG erosion produced clastic gypsum that was deposited through gravity flows in the slope and the depocentre, in analogy with the situation observed in Belice section (Sicily; Roveri et al., 2006;Lugli et al., 2010), although the contact between the RLG and Lago Mare was not observable in Agios Sostis section. In addition, PLG mass flows were accumulated in the distal Zakynthos foreland slope. Therefore, the Agios Sostis evaporites correspond to a mass flow, which slid from the west to the east, following the dipping of the slope between the forebulge and the foredeep of the Zakynthos (Alikanas) basin. Consequently, the Agios Sostis evaporitic unit can only be interpreted as derived from the dismantlement and resedimentation of the western PLG deposits. It would not be possible for them to originate from the Triassic diapirs, which are located to the east. Furthermore, during the Messinian, the Triassic diapirs were largely covered by the Neogene sediments (Fig. 17A). By the end of the Messinian, the PLG unit was completely eroded at the forebulge-backbulge zone, probably due to subaerial exposure. The MES (Fig. 17) is located directly above the pre-evaporitic sequence west of the Ploumari gypsum outcrops (Fig. 2), and it passes over the remaining PLG in the proximal slope (Ploumari gypsum outcrops). In the distal slope and the foreland depocentre, the MES becomes a correlative conformity.
The wedge-top zone was characterized by shallower depths, mainly due to the diapiric uplift of the Ionian Triassic evaporites (Karakitsios, 1995). Indeed, the sea depth in the Kalamaki area, prior to the PLG deposition, is estimated up to 300 m, based on the fish assemblages. Consequently, the Kalamaki area, placed over the external Ionian Triassic diapir (Agia Dynati diapir; Figs 2 and 17), received PLG deposits. Eroded gypsum material from the wedge-top PLG also supplied clastic gypsum to the foredeep RLG unit, which was located to its west (Fig. 17A). Possibly, the last cycle of the PLG deposits in Kalamaki section (clastic gypsum in cycle H) derived from the remained eastern wedge-top primary gypsum deposits, which were exposed and eroded at that time.
In the Argassi area, PLG was also deposited over the Agios Ioannis diapir; the upper PLG part was subsequently eroded. Quaternary erosion obscures the continuation of the deposited sediments of this area. Possibly the Argassi and Kalamaki sections during PLG deposition belonged to the same evaporitic basin that was separated by an early activation of the Agios Ioannis diapir (Fig. 17A).
The proposed palaeogeographic scheme for the last period of the Trubi Formation's deposition is presented in Fig. 17B. The Lago Mare unit lies unconformably over the MES in the wedge-top (Kalamaki area), whereas in the distal slope and in the depocentre it was deposited with a correlative conformity. The Trubi Formation succeeds the Lago Mare unit, in stratigraphic continuity. The onset of the Trubi Formation at 5.33 Ma, corresponding to the Pliocene flooding of the Mediterranean basin, is recorded well on Zakynthos Island.
The present results indicate that the Messinian gypsum units of Zakynthos Island were deposited in a Mediterranean peripheral/marginal basins context (sensu Roveri Fig. 17. Palaeogeographic reconstruction. (a) During the Messinian, the Zakynthos (Alikanas) foreland basin was formed in front of the Ionian thrust. The Neogene formations deposited over the pre-Apulian domain corresponded to the foredeep and the flank between foredeep and forebulge, whereas those overlying the Ionian zone corresponded to the wedge-top, which was uplifted due to the diapiric movements of the Ionian Triassic evaporites. In the 1st MSC stage, PLG was deposited on both forebulge and wedge-top. In the 2nd MSC stage, PLG was totally or partially eroded (creating the Messinian Erosional Surface; MES) and redeposited with correlative conformity mainly as gypsum turbidites (RLG unit) in the depocentre of the foreland basin. (b) During the end of the Messinian, in Kalamaki-Argassi wedge-top area the Lago Mare sediments were unconformably overlying the Primary Lower Gypsum above the MES, whereas in the depocentre of the foreland basin they were conformably deposited over the Resedimented Lower Gypsum. The Zanclean flood was recorded on both areas by the conformable deposition of the Trubi Formation. With red raffled and continuous line the unconformity and the correlative conformity of the MES, respectively. et al., 2014a), formed during the late Messinian in the pre-Apulian foreland basin, before the early Pliocene main deformation phase responsible for the tectonic emplacement of the Ionian zone over the pre-Apulian zone.
The distribution of the Messinian evaporites on Zakynthos Island clearly agrees with the CIESM (2008) stratigraphic scheme, which was described in many western (Apennines; Manzi et al., 2005;Sicily, Roveri et al., 2008a;Manzi et al., 2011;Spain, Omodeo-Sale et al., 2012;Matano et al., 2014) and eastern (Cypsus, Manzi et al., 2015;Israel, Lugli et al., 2013) Mediterranean areas. It is characterized by primary bottom-grown selenite (PLG) deposited in marginal shallow-water settings and by their subsequent deep erosion and resedimentation in the deepest parts of the basin as submarine mass waste and turbidites (RLG). The main particularity of the Zakynthos foreland basin is that the Triassic evaporitic diapirs controlled its configuration and subsidence.

CONCLUSIONS
Our integrated study provides a detailed reconstruction of the geological, stratigraphic, tectonic, palaeogeographic and palaeoenvironmental evolution of the Zakynthos Island during the Messinian. The development and distribution of the depositional environments before the major Ionian thrust activation was mainly influenced by the Triassic evaporites diapiric movements in the foreland basin and has profound effects on the distribution of the Messinian evaporites.
• Shallow-water (<200 m) bottom-grown selenite evaporites belonging to the PLG were accumulated in the wedge-top basin on top of Ionian alpine basement and in the backbulge-forebulge zones developed on top of the pre-Apulian units. • During the MSC stage 2, the PLG unit was deeply eroded at the shallow basins and was resedimented as clastic evaporites in the deeper foredeep, which was developed at the contact of the two tectonic units; minor gypsum cumulate primary facies were also deposited. • In the pre-Apulian area, the clastic evaporites change laterally from mass-wasting Messinian gypsum, in the west, to gypsum turbidites indicating a deeper environment, in the east. • In the wedge-top, the MES is overlapped by the Lago Mare unit, over which the Trubi Formation lies in stratigraphic continuity.
Planktonic foraminifera and calcareous nannoplankton biostratigraphy, magnetostratigraphy and correlation with astronomically tuned Mediterranean sections provide the chronostratigraphic framework for the studied sequences.
• The pre-evaporitic sequence in Kalamaki section is dated between 6.415 and 6.01 Ma. Nine planktonic foraminiferal bioevents are distinguished.
• In Kalamaki section, PLG was deposited during the first MSC stage, between 5.971 and 5.60 Ma.

• The resedimented PLG deposits observed in Agios
Sostis area correspond to the second MSC stage (5.6-5.55 Ma). • The M/P boundary is placed at the base of the Trubi Formation, dated at 5.33 Ma. The Kalamaki section Trubi Formation, corresponds to the interval between 5.33 and 5.08 Ma, in which five planktonic foraminiferal bioevents are distinguished.
The presented palaeoenvironmental reconstruction of the pre-evaporitic Messinian in Kalamaki can be summarized as follows: • 6.45-6.2 Ma (0-7 m)marine conditions prevailed, with slight salinity and temperature variations. The positive d 13 C carb trend and the oligotypic planktonic foraminiferal assemblage, dominated by T. multiloba, indicate high-productivity cold waters • 6.2-6.14 Ma (7-9 m)high d 18 O carb corresponding to hypersalinity episodes alternate with normal salinity conditions with decreased food availability and low temperature • 6.122-5.97 Ma (9-22 m)marine mesotrophic conditions are well established; the isotopic record suggests nutrient-depleted waters of the stratified summer mixed layer • around 6.05 Ma (12 m)a cooling phase punctuated by hypersalinity episodes is indicated by both the planktonic foraminifera and the fish assemblage • In the upper part of the pre-evaporitic sequence, the observed synchronous decrease in the carbon isotopic ratio, and increase in the oxygen isotopic ratio may be caused by isotopically light carbon input from bacterial sulphate reduction, due to increasing stagnation at the beginning of sulphate deposition.
After the MSC, eutrophic conditions are indicated at the uppermost part of the Lago Mare facies unit, just before the onset of Trubi limestone Formation.

SUPPORTING INFORMATION
Additional Supporting Information may be found in the online version of this article: Appendix S1. Oxygen and carbon stable isotope measurements in the Kalamaki section. Table S1. Nannofossil abundances in Kalamaki section sediments. Table S2. Planktonic foraminifera abundances in Kalamaki section sediments.