Messinian evaporite facies associations in the western Mediterranean

The detailed facies analysis of the Messinian evaporites in the western Mediterranean reveals the presence of many different depositional settings and associated deposits. Here we illustrate the characteristics of the most important facies associations in the framework of the age model of Roveri et al. (2008).Primary Lower Gypsum (PLG), 1st stage (5.96–5.6 Ma)The Primary Lower Gypsum consists of up to 16 gypsum/euxinic shales or carbonate lithological cycles. The basal cycles (1st-5th) are thicker massive selenite grading into banded selenite. The upper ones (6th-15th) consist of thinner beds showing a basal massive and banded selenite portion, followed by clusters of selenite crystals that grew laterally, grouped in branches projecting outward from a nucleation zone: the “branching selenite”.The PLG shows an impressive similarity in term of number of cycles, facies and stacking pattern, across the entire western Mediterranean (Lugli et al., 2006), allowing bed-by-bed correlations.The PLG cycles were probably deposited in less than 200 m-deep peripheral sub-basins and pass laterally in shallower settings into laminated limestone/organic-rich shales cycles (Gennari et al., this volume). In deeper poorly oxygenated settings the PLG equivalent deposits are dolomitic limestones interbedded with euxinic shales (Roveri et al., 2008; Manzi et al., 2007; this volume).Resedimented Lower Gypsum (RLG), 2nd stage (5.6–5.55 Ma; TG12-TG14)The clastic gypsum facies shows three main facies associations ranging from coarser- to fine-grained (Manzi et al., 2005; 2007; Roveri et al., 2006; 2008): i) chaotic deposits, “proximal” poorly evolved gypsum-shale flow deposit, including primary evaporite slabs, boulders and mountain-size blocks, debris flow and hyper-concentrated flow deposits. ii) lobe deposits from high to lows density gravity flows, made up of gypsarenites, silt and shales forming tabular or lenticular bodies. iii) drape deposits as the ultimate flow evolution products, consist of laminated gypsum interbedded with shales. The accumulation of such a wide spectrum of gravity-driven deposits was probably related to the formation of large submarine collapse and glide structures triggered by tectonically-induced gravitational instability.Calcare di base (CdB), 2nd stage (5.6–5.55 Ma; TG12-TG14)The “Calcare di Base” of the Caltanissetta basin is considered a calcareous evaporitic and microbial deposit belonging to the Lower Evaporites laterally equivalent of the primary selenite, that for some authors, in turn, is lateral equivalent of halite (Rouchy and Caruso, 2006). Its brecciated texture was related to in situ collapse produced by halite/gypsum dissolution (Decima et al., 1988). Our studies reveal that CdB is never associated with the primary selenite, but exclusively with clastic and laminated gypsum (RLG), suggesting deposition from mixed gravity flows. Individual carbonate beds commonly show low lateral persistency, are characterised by pinch-out terminations and show widespread bed gradation, erosional bases, load structures and clay chips, suggesting a clastic origin and moderate distance transport through high- to low-density gravity flows (Manzi et al., this volume).Halite, 2nd stage (5.6–5.55 Ma; TG12-TG14)Our new data suggest common sedimentary facies for the various Sicilian halite bodies (Lugli et al., 1999; Roveri et al., 2006; 2008). The halite deposits can be divided into two units: 1) a lower with halite, minor kainite and carnallite cumulite layers deposited in a relatively deep (below wave base) stratified water body with a strong shallowing upward trend, and 2) an upper halite one precipitated from a non-stratified, relatively shallow water body. The transition between these two units is marked by several mud/halite cycles that experienced meteoric dissolution and giant thermal contraction polygons by annual temperature fluctuations in subaerial conditions (Lugli et al., 1999). These features reveal the progressive rapid infill of relatively deep basins due to high halite sedimentation rates, culminating with complete desiccation. Then, a new halite phase developed in shallow water settings. The deposition of at upper part of the halite unit in the depocenters may be correlated to a gypsum cumulite horizon which underwent dehydration at the margins (Roveri et al., 2008; Manzi et al., 2009).Upper Evaporites, 3rd stage (5.55–5.33 Ma)The Upper Evaporites show a well-developed cyclic pattern of 9-10 lithological cycles (Manzi et al., 2009) starting with thin, laminated gypsum cumulates and gypsarenites developed on top of an alternation of fluvio-deltaic sandstone and shelfal shales. The top of seven of these cycles consists of primary selenitic gypsum, locally forming large domal structures. The Primary gypsum beds do not show branching selenite which is common in the LE. Banded selenite have been recognised only in the lowermost cycle. Calcareous stromatolites have never been found associated with these primary deposits. On the other hand, laminated gypsum, which always is present in the lower part of UE cycles, is not present in the LE.The UE overall thickness is commonly less than the LE. The UE unit is characterized by a thinner basal bed of banded selenite, overlain by a cluster of 5 thicker gypsum beds. A 7th bed, just below the “Arenazzolo” unit, is usually separated from this cluster by an argillaceous interval, locally more than 50 m thick and containing up two sandstone horizons; a Lago-mare faunal assemblage is commonly present in this interval.ReferencesCIESM, 2008. Workshop Monographs, 33, Monaco.Decima A., Mckenzie J.A., Schreiber B.C., 1988. Journal of Sedimentary Petrology, 58, 256-272.Lugli S. et al., 2006. Acta Naturalia De “L’Ateneo Parmense”, 42-2, A31 - RCMNS IC Parma 2006. Lugli S. et al., 1999. Journal of Sedimentary Research, v. 69, p. 764-771.Manzi V. et al., 2005. Sedimentology, v. 52, p. 875-902.Manzi V. et al., 2009. Sedimentology, doi: 10.1111/j.1365-3091.2009.01063.xManzi V. et al. 2007. Palaeogeography, Palaeoclimatology, Palaeoecology, 251, 470-499.Rouchy J.-M. & Caruso A., 2006. Sedimentary Geology, 188-189, 35-67.Roveri M. et al., 2006. Acta Naturalia De “L’Ateneo Parmense”, 42-1, 125-199.Roveri M. et al., 2008. Terra Nova, 20, 483-488.