Sedimentary and diagenetic markers of the restriction in a marine basin:: the Lorca Basin (SE Spain) during the Messinian

The Lorca Basin (southeastern Spain) is part of a chain of small marginal Neogene basins located in the structurally active Betic area. The Upper Miocene (Messinian) sequence is composed of a thick diatomite-bearing series (Tripoli Unit) overlain by the Main Evaporites, analogous to the classical succession that records the main events during the Salinity Crisis in the Mediterranean region. The shallow restricted conditions of this region amplified the sedimentary responses to local and global forcings. An integrated approach using sedimentology, micropalaeontology, stable isotope geochemistry and organic geochemistry has been applied to the Tortonian/Messinian succession of the Lorca Basin, in order to obtain a continuous record of the environmental changes. The sediments record two major events which affected the whole Mediterranean: (1) high levels of productivity that led to the formation of the diatomite-bearing deposits in the early Messinian (Tripoli); and (2) the Messinian Salinity Crisis with its two major stages, represented by the Halite and Gypsum Units, both mainly precipitated from marine-derived brines. The rapid reflooding of the Mediterranean by normal marine waters at the base of the Pliocene did not reach the Lorca Basin, nor other basins of this part of the Betic area. Instead, continental sediments were deposited as a consequence of the regional uplift of SE Iberia, which started close to the Messinian/Pliocene boundary. The most prominent feature of this basin concerns the record of its restriction by the time of the deposition of the Tripoli Unit, which led to intercalations of precursor evaporitic layers, consisting of Ca-sulphate deposited in sub-aqueous and sabkha conditions, interbedded with diatomites. This alternation of evaporites and diatomites proves that the Lorca Basin was periodically restricted and reflooded by marine waters, a possible cause for this being relative sea-level fluctuations in the Mediterranean. This strengthens evidence of diachronism that suggests that the onset of the first Messinian evaporitic deposition was not synchronous, but was dependent on bathymetry and local tectonics. High productivity during the early Messinian in this basin is demonstrated by the thick deposits of diatomites. However, stagnation episodes may have occurred during this interval, as suggested by the preservation of high amounts of organic matter (organic-rich shales) and the extent of bacterial sulphate reduction which apparently occurred during early diagenesis. The formation of organo-sulphur compounds, replacement of sulphates by carbonates and the high levels of elemental sulphur are by-products of diagenetic processes occurring in a restricted hypersaline environment. (C) 1998 Elsevier Science B.V. All rights reserved.