MODELING THE SULFUR AND OXYGEN ISOTOPIC COMPOSITION OF SULFATES THROUGH A HALITE-POTASH SEQUENCE - IMPLICATIONS FOR THE HYDROLOGICAL EVOLUTION OF THE UPPER EOCENE SOUTHPYRENEAN BASIN

The isotopic composition of sulfates sampled throughout a complete evaporite sequence has contributed to the understanding of the evolution of the Southpyrenean Upper Eocene basin. The delta(34)S and delta(18)O values appear to be constant in thick segments of the sequence. This pattern is clearly different from the continuous decrease in delta values predicted by the evaporation processes in a closed system. The observed trend of the delta(34)O and delta(18)O values can be successfully modeled by an evaporation process in an open system, where steady state conditions are reached, separated by periods of significant changes in the hydrological regime of the basin. Moreover, in the segments of the sequence where they are constant, the delta(34)S and delta(18)O values help to constrain the hydrological parameters of the model. A first marine stage of the evolution of the basin corresponds to the formation of the Basal Anhydrite and Lower Halite Units. The mineral sequence corresponds to evaporation in a basin with a progressive degree of restriction and evaporation compensated by equal inflow of major marine origin. The sharp decrease in the delta(34)S and particularly the delta(18)O values, recorded at the end of the Basal Anhydrite Unit, illustrates a limited reservoir effect produced by the increase of restriction leading to the halite precipitation. The modeling of the constant isotopic values through the Lower Halite Unit confirms the isotopic composition of Eocene marine dissolved sulfate (delta(34)S = +20.0 parts per thousand; delta(18)O = + 8.7 parts per thousand) deduced from the average values of marginal gypsum. The second continental stage of basin evolution corresponds to the formation of the Potash Unit and the Upper Halite Unit. The mineral sequence and lateral extension indicate that the basin evolved with a decreasing brine volume after disconnection from the sea. The dilution of the residual brine stopped the precipitation of carnallite, and the basin then evolved as a perennial endorreic lake with continental recharge replacing evaporation. The sharp decrease in the delta(34)S and delta(18)O values in the sylvite member is consistent with the massive precipitation of sulfate occurring during the decrease in the volume of water. The delta values of the sulfates intercalated in the carnallite and the Upper Halite Unit indicate the influence of the recycling of both the already deposited Upper Eocene sulfates and the Upper Triassic sulfates carried by continental recharge. The constant delta(34)S and delta(18)O values from the uppermost half of the Upper Halite Unit is used to constrain the sulfate source as 80% recycled Upper Triassic and 20% recycled Eocene gypsum.