Marine and continental water contributions to a hypersaline basin using diatom ecology, sedimentology and stable isotopes: an example in the Late Miocene of the Mediterranean (Hellin Basin, southern Spain)

Abstract

A multidisciplinary approach including diatom ecology, petrography and stable isotopes of carbonates and sulfates is applied to reconstruct the conditions of evaporitic deposition in the Hellin Basin. The Hellin Basin is one of many similar restricted basins that evolved in this region just after the emplacement of the Betic thrust complex. These studies show that continental conditions prevailed in a restricted shallow basin during the evapoitic deposition. Episodic marine flooding, however, is indicated by diatom assemblages of diatomite layers intercalated in thick continental gypsum deposits which represent the last stage of concentration, controlled by arid climate. Isotope data indicate a continental origin of the sulfate ions, which were probably supplied to the basin by continental waters enriched in sulfates from the weathered Triassic evaporites. So, during this time, the basin evolved into a continental drainage system with hypersaline magnesian conditions (dolomite dominant). Aqueous sulfate from Triassic sources is affected by bacterial sulfate reduction, but was further reoxydized as sulfate in oxydized fresh water. The importance of sulfate bacterial reduction indicates abundant organic matter and high organic productivity; it indicates anoxic condition in the bottom sediment and possible episodes of water stagnation. This total sedimentologic and diagenetic picture, together with excellent paleoenvironmental (diatom) control reflects the complexity of the basin filling within the intramontane basins of the Betic Chain.