Constraints of stable isotope signatures on the depositional palaeoenvironments of upper Miocene reef and temperate carbonates in the Sorbas Basin, SE Spain

Abstract

Lateral facies changes from upper Tortonian–Messinian marginal carbonates into pelagic marls can be physically traced in two well-exposed sections of the Sorbas Basin, SE Spain. Due to this physical correlation, the foraminifer assemblages from the marls and the oxygen and carbon isotope values of the foraminifer tests can be used to reconstruct the palaeoenvironmental context of deposition of the laterally equivalent carbonates. At the basin margins, an uppermost Tortonian/lowermost Messinian temperate carbonate unit with bryomol and rhodalgal lithofacies is overlain by two successive Messinian reef units comprising coral and Halimeda bioherms and fringing Porites reefs respectively. All these units interfinger basinwards into silty marls and marls. The δ 18O values of both planktonic and benthic foraminifers from the pelagic marls coeval with the temperate carbonates are higher than those from the fine-grained sediments laterally equivalent to the overlying reefs (by 1.91‰ in the planktonic and 1.67‰ in the benthic records). This decrease in δ 18O, together with an increase in the abundance of subtropical/tropical species in planktonic foraminifer assemblages, confirms a rise in sea-surface temperatures in the change from bryomol/rhodalgal to reef carbonate deposition. Carbonate lithofacies in the upper Miocene in the Sorbas Basin are, therefore, primarily controlled by temperature. The low carbon isotope signatures and the benthic foraminifer assemblages in the pelagic sediments coeval with reefs point to poorly oxygenated bottom waters and strong stratification.