The Primary Lower Gypsum in the Mediterranean: A new facies interpretation for the first stage of the Messinian salinity crisis

The detailed facies and physical stratigraphic analysis of the Primary Lower Gypsum in the Mediterraneanindicates a surprising bed-by-bed correlation at basin-scale (Spain, Italy, Hellenic arc and Cyprus arc), that istuned to the orbital calibration for the first stage of the Messinian salinity crisis from 5.96 to 5.61 Ma ago. Atotal of 16, precessionally-controlled, gypsum cycles were deposited rapidly in less than 350 ka, formingsequences up to 300 m thick. The lack of subaerial exposure features and the common facies associations andstacking pattern for sections located thousands of kilometers apart in different geological settings indicates amodest depositional depth, not extremely shallow. Selenite deposition occurred only at the bottom ofrestricted marginal basins less than 200 m deep, while no gypsum could precipitate in the deeper euxinicMediterranean portions where only thin and barren shale/dolostone couplets formed. The lowermostselenite beds pass laterally to dolomite-rich limestones interbedded with barren euxinic shales in poorlyoxygenated settings, indicating that the gypsum sedimentation was diachronous and did not necessarilymark the onset on the Messinian salinity crisis.Evaporite facies sequences (EF1 to 8) within individual gypsum beds show small-scale, subaqueoussedimentary cycles that mimic regressive–transgressive cycles: a) initial evaporite precipitation at relativelylow supersaturation produced the massive selenite (facies EF3) in a relatively deep setting; b) continuousevaporation and drawdown by oscillating brine level formed the banded selenite (EF4) at the aridity acme ofthe precessionally-controlled cycle; c) general progressive brine level rise with strong brine flow led to theformation of large selenite supercones branching laterally (supercones in Spain and branching selenite, EF5,in the rest of the Mediterranean); and d) flooding by undersaturated continental water terminated gypsumprecipitation with the deposition of argillaceous sediments (EF1, Northern Apennines), and/or limestone(EF2, Sicily and Spain) during the humid phase in the precession climate cycle.The stacking pattern and selenite facies associations suggest an overall shallowing-upward trend with abasin-wide hydrologic change starting from the 6th cycle (5.84 Ma), which is marked by the appearance ofthe branching selenite facies (supercones) in Spain and indicates that the brines became current-dominated.The Sr-isotope stratigraphy suggests that in the Northern Apennines the brines were strongly modified bycontinental waters (87Sr/86Sr=0.708893 to 0.708998), and received direct pulses of Atlantic seawater (87Sr/86Sr=0.70900 to 0.709024) only in the upper part of the section. In areas away from the mainland, such asSicily, the continental input was by far less important.