Marine to lacustrine evolution in an evaporitic environment: the late miocene Lorca Basin, Spain

Abstract

The Lorca Basin, in the eastern sector of the Betic Range (SE Spain), is an intramontane basin recording an evaporitic succession (La Serrata Formation), of up to 300 m thick, with a ~ 235 m thick saline unit within. Altogether, the evaporitic record was originally interpreted as Messinian (Geel, 1976) and later assigned to Tortonian (Krijgsman and others, 2000). The detailed geochemical study provides relevant paleogeographic information at local scale and highlights the importance of hydrochemical changes taking place in coastal evaporite basins changing between marine and non-marine conditions without lithological variations. A stratigraphic framework is proposed correlating the outcropping gypsum beds (Gypsum Mb of La Serrata Fm) and the subsurface saline succession (Halite Mb) by means of strontium and sulfate isotopes (fig. 1). In the lower part of the Gypsum Mb the isotopic trends suggest that gypsum formed from marine waters while in the upper part, with Triassic isotopic signals, gypsum formed in a coastal lake mainly fed by non-marine waters. In the Halite Mb, the textures indicate precipitation in a very shallow, often dried, environment. Fluid inclusion compositions and bromine contents in salt show an evolution from normal marine brines, to brines resulting from the recycling of previously precipitated halite essentially by means of non-marine waters in a coastal lake setting. The overlying Laminated Pelite Mb (Geel, 1976) consists in its lower part of a number of non-marine gypsum beds intercalated between marine marls suggesting post-evaporitic refilling events of the Lorca Basin by the Mediterranean Sea before its final continentalization during the Pliocene. Biostratigraphic studies in progress are expected to refine age allocation within the evaporitic unit and therefore improve our understanding of the relationship to the “Messinian Salinity Crisis”. © 2015 The Authors