Marine to non-marine sedimentation in the upper Miocene evaporites of the Eastern Betics, SE Spain: sedimentological and geochemical evidence

Abstract

Abstract Several Neogene basins in the Eastern Betics contain a record of thick upper Miocene evaporites. The inner basins—those located distal to the Mediterranean coast—include evaporite units ranging from the late Tortonian to the early Messinian, whereas the marginal basins—those located in the vicinity of the Mediterranean coast—include evaporite units corresponding to the late Messinian that are linked to the “salinity crisis”. The sedimentological and geochemical comparison of the evaporites in the inner and marginal basins enable us to show their differences in the modes of deposition under local, structural controls and differentiated palaeogeography. In the inner basins of the Murcia–Alicante region, a number of evaporite units have been differentiated whose gypsum lithofacies have: (1) variable strontium contents, with mean values (primary gypsum) between 1121 and 2380 ppm; (2) isotopic compositions of sulphur ( δ 34 S) ranging from those characteristic of the Tertiary marine water (+20.3 to +25.5‰) to those characteristic of the Triassic sulphates (+8.0 to +16.0‰), the latter suggesting recycling of the sulphate anion in meteoric waters; intermediate values (+17.4 to +19.6‰) are attributed to mixed mother brines; and (3) strontium ratios ( 87 Sr/ 86 Sr) comprised between 0.70804 and 0.70888; these values range from those of the Tertiary marine water to those exceeding the Triassic sulphates, suggesting additional continental contributions (from country rocks and hydrothermal solutions). The comparison between the sulphur and strontium isotopic compositions of the sulphates, together with the study of the biota present in the marls associated with the gypsum, allows us to distinguish between marine, continental, and mixed units. In the marginal basins of the Eastern Betics, where single evaporite units characterized by selenitic gypsum prevail: (1) the strontium contents are more homogeneous, with mean values (primary gypsum) between 493 and 625 ppm; (2) the isotopic compositions of oxygen ( δ 18 O) and sulphur ( δ 34 S) are comprised between +12.2 and +14.8‰, and between +22.0 and +23.5‰, respectively; and (3) the strontium ratios ( 87 Sr/ 86 Sr) vary from 0.70890 to 0.70895. All these isotopic values reveal a marine origin and the lack of contribution of significant continental water during the late Messinian evaporite deposition in the Betic Strait. These characteristics are similar to those of the Lower Evaporite.