Petrographic and geochemical data suggest that dolomite cement in shallow-marine sandstones of the Triassic Buntsandstein in the Ateca horst of the Iberian Range, Spain, has precipitated in the zone of mixing between phreatic meteoric and marine waters. Dolomite or rhombohedric crystals consisting of concentric bands of alternating syntaxial dolomite and calcite have filled isolated voids (350 μ-12 mm in diameter). Subsequently, limpid dolomite followed by sparry calcite precipitated in void center as the latest cements. The carbonates have low δ13CPDB (-9.5 to -6.0%) and δ18OPDB (-8.1 to -3.2%.) values and are enriched in radiogenic strontium (87Sr/86Sr = 0.710112 to 0.720293) indicating a dominantly meteoric component in the porewaters. The overall sequence of carbonate cementation suggests precipitation from porewaters that evolved in the mixing zone with successively increased meteoric influence, and finally to fully meteoric probably due to a major sea-level fall. © 1992.