Strontium geochemistry of Miocene primary gypsum; Messinian of southeastern Spain and Sicily and Badenian of Poland

Abstract

ABSTRACT The strontium contents of primary marine gypsum facies were studied in Messinian (Upper Miocene) basins of SE Spain and Sicily, and in the Badenian (Middle Miocene) Carpathian foreland basin of Poland. The results obtained were interpreted on the basis of the findings reported for the gypsum facies of recent salinas in the western Mediterranean coast. The strontium profile obtained from the lower part of the gypsum deposit of the Sorbas basin (SE Spain) has homogeneous strontium contents, reflecting deposition under conditions of hydrological stability and relatively deep waters, but the upper part has a more variable strontium content related, in accordance with other sedimentological criteria, to a shallowing of the basin. This is consistent with data reported from other Messinian basins of SE Spain (San Miguel de Salinas and Palma de Mallorca). In the Caltanissetta basin (Sicily), the section studied in Eraclea Minoa (Upper Evaporite) is made up of 6-8 gypsum cycles that have a facies evolution from fine-grained laminar gypsum at the base of each cycle to selenites at the top, accompanied by increasing strontium values. This evolution is attributed to increasing salinity conditions and is consistent with geochemical data reported in the literature. In the Badenian basin the six gypsum sections studied are from different subbasins. The gypsum deposits comprise two members. In the lower member, which is mainly selenitic, two parts can be differentiated and a significant strontium increase (attributed to a rise in salinity) was recorded in the transition from the lower to the upper part. In the upper member, which is mainly clastic gypsum, a general strontium decrease indicates a fall in salinity linked to a deepening or freshening of the platform. In the Badenian selenitic layers a lateral variation in the strontium contents exists: higher and less homogeneous strontium contents were recorded in the marginal zones, suggesting variable salinity conditions, whereas in the deeper areas the gypsum layers show more homogeneous strontium values, expected from more stable and less concentrated brines. This distribution is in agreement with data reported from the Messinian basin of San Miguel de Salinas (SE Spain). The strontium contents in both the Messinian and Badenian selenitic samples are lower than those in the selenitic crusts of the recent salinas. These lower contents may be a result of a lower salinity of the mother brines at the steady state achieved during the precipitation of the thick selenitic sequences, with respect to the highly concentrated (with no reflux) brines of the recent salinas. Our findings highlight the relevance of studying strontium contents in primary gypsum facies of marine formations as a tool for interpreting the paleogeography and the paleosalinity of the evaporitic enviroents and for correlating cyclic selenite formations. This method could be especially helpful when combined with other geochemical indicators (i.e., isotopic and/or fluid inclusions composition).