Continental formations of Miocene age outcropping in southeastern Spain include beds of diatomites (in economic quantities) irregularly alternating with carbonate beds. The minerology and isotope geochemistry of the alternating carbonates and diatomites indicate variable depositional conditions associated with an overall deepening of a lake. Anhedral dolomite associated with sulfate minerals characterizes the first depositional stage and has heavy oxygen- and light carbon-isotopic values which are consistent with a formation in a restricted environment with highly evaporated waters. Increasing dilution of the water is reflected in the presence of abundant aragonite near the base of the assemblage which passes to calcite near the top. The gradual upward decrease of both delta 18 O and delta 13 C in these minerals indicates a parallel decrease of the evaporation/inflow ratio within the depositional basin. Also, seasonal variations of composition and volume of the water affect both the formation and diagenesis of carbonate minerals.