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

Several Neogene basins in the Eastern Betics contain a record of thick uppe r Miocene evaporites. The inner basins-those located distal to the Mediterr anean 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 lat e Messinian that are linked to the "salinity crisis". The sedimentological and geochemical comparison of the evaporites in the inner and marginal basi ns enable us to show their differences in the modes of deposition under loc al, structural controls and differentiated palaeogeography. In the inner ba sins of the Murcia-Alicante region, a number of evaporite units have been d ifferentiated whose gypsum lithofacies have: (1) variable strontium content s, with mean values (primary gypsum) between 1121 and 2380 ppm; (2) isotopi c compositions of sulphur (delta(34)S) ranging from those characteristic of the Tertiary marine water (+20.3 to +25.5 parts per thousand) to those cha racteristic of the Triassic sulphates (+8.0 to +16.0 parts per thousand), t he latter suggesting recycling of the sulphate anion in meteoric waters; in termediate values (+17.4 to +19.6 parts per thousand) are attributed to mix ed mother brines; and (3) strontium ratios (Sr-87/Sr-86) comprised between 0.70804 and 0.70888; these values range from those of the Tertiary marine w ater to those exceeding the Triassic sulphates, suggesting additional conti nental contributions (from country rocks and hydrothermal solutions). The c omparison between the sulphur and strontium isotopic compositions of the su lphates, together with the study of the biota present in the marls associat ed with the gypsum, allows us to distinguish between marine, continental, a nd mixed units. In the marginal basins of the Eastern Betics, where single evaporite units characterized by selenitic gypsum prevail: (1) the strontiu m contents are more homogeneous, with mean values (primary gypsum) between 493 and 625 ppm; (2) the isotopic compositions of oxygen (delta(18)O) and s ulphur (delta(34)S) are comprised between +12.2 and +14.8 parts per thousan d, and between +22.0 and +23.5 parts per thousand, respectively; and (3) th e strontium ratios (Sr-87/Sr-86) vary from 0.70890 to 0.70895. All these is otopic values reveal a marine origin and the lack of contribution of signif icant continental water during the late Messinian evaporite deposition in t he Betic Strait. These characteristics are similar to those of the Lower Ev aporite. (C) 2000 Elsevier Science B.V. All rights reserved.