Palaeogroundwater evolution in playa–lake environments

The Palaeogene history of the Almazán basin is that of a non-marine basin sediments derived from fluvial distributary systems and lateral playa–lakes. The coeval uplifting of the Cameros Massif and the Aragonian Branch of the Iberian Chain during Alpine compression exposed Triassic evaporites, which became the main source of sulphates to the playa–lake system, as recorded by the Bordalba Formation. This unit exhibits the classical facies distribution of gypsum-precipitating playa–lakes, which can be grouped into two main settings: 1) distal alluvial/mud flat and 2) saline mud flat/ephemeral lake. The stable isotope data for gypsum from the distal alluvial/mud flat areas (12.2 < δ 18O SMOW < 16.5; 10.8 < δ 34S CDT < 13.0‰) lie within the range of values of neighbouring Triassic gypsum (or are slightly lower for δ 34S CDT). This agrees with palaeogeographical interpretations suggesting that the Triassic evaporites were the source of gypsum for the Almazán basin. The slightly lower values could be explained in terms of a minor contribution through the oxidation and dissolution of Palaeozoic-derived sulphides. The isotopic values for gypsum of the saline mud flat/ephemeral lake environments (19.8 < δ 18O SMOW < 23.82; 15.6 < δ 34S CDT < 19.9‰) are higher than those of the distal alluvial/mud flat. The higher δ 18O SMOW in the saline mud flat/ephemeral lake environments is interpreted as being a consequence of the progressive evaporation of shallow groundwaters from proximal to distal areas. The heavier values for the δ 34S CDT in the saline mud flat/ephemeral lake environments are interpreted as reflecting the activity of sulphate-reducing bacteria, which is evidenced by spheroidal dolomite aggregates on gypsum crystals and by the grey colours of the gypsum facies.